Macromolecular Metal Carboxylates and Their Nanocomposites (Springer Series in Materials Science, 138)

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Springer Series in

materials science

138

Springer Series in

materials science Editors: R. Hull C. Jagadish R.M. Osgood, Jr. J. Parisi Z. Wang H. Warlimont The Springer Series in Materials Science covers the complete spectrum of materials physics, including fundamental principles, physical properties, materials theory and design. Recognizing the increasing importance of materials science in future device technologies, the book titles in this series ref lect the state-of-the-art in understanding and controlling the structure and properties of all important classes of materials.

Please view available titles in Springer Series in Materials Science on series homepage http://www.springer.com/series/856

A.D. Pomogailo G.I. Dzhardimalieva V.N. Kestelman

Macromolecular Metal Carboxylates and Their Nanocomposites With 113 Figures

123

Prof. Dr. Anatolii D. Pomogailo Russian Academy of Sciences Inst. Problems of Chemical Physics Acad. Semenov av. 1 142432 Chernogolovka Moscow region, Russia Email: [email protected]

Prof. Dr. Vladimir N. Kestelman KVN International Inc. Jamie Circle 632 19406 King of Prussia Pennsylvania, USA Email: [email protected]

Dr. Gulzhian I. Dzhardimalieva Russian Academy of Sciences Inst. Problems of Chemical Physics Acad. Semenov av. 1 142432 Chernogolovka Moscow region, Russia Email: [email protected] Series Editors: Professor Robert Hull University of Virginia Dept. of Materials Science and Engineering Thornton Hall Charlottesville, VA 22903-2442, USA

Professor Jürgen Parisi Universität Oldenburg, Fachbereich Physik Abt. Energie- und Halbleiterforschung Carl-von-Ossietzky-Straße 9–11 26129 Oldenburg, Germany

Professor Chennupati Jagadish Australian National University Research School of Physics and Engineering J4-22, Carver Building Canberra ACT 0200, Australia

Dr. Zhiming Wang University of Arkansas Department of Physics 835 W. Dicknson St. Fayetteville, AR 72701, USA

Professor R.M. Osgood, Jr. Microelectronics Science Laboratory Department of Electrical Engineering Columbia University Seeley W. Mudd Building New York, NY 10027, USA

Professor Hans Warlimont DSL Dresden Material-Innovation GmbH Pirnaer Landstr. 176 01257 Dresden, Germany

Springer Series in Materials Science ISSN 0933-033X ISBN 978-3-642-10573-9 e-ISBN 978-3-642-10574-6 DOI 10.1007/978-3-642-10574-6 Springer Heidelberg Dordrecht London New York Library of Congress Control Number: 2010931466 c Springer-Verlag Berlin Heidelberg 2010  This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilm or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer. Violations are liable to prosecution under the German Copyright Law. The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Cover design: eStudio Calamar Steinen Printed on acid-free paper Springer is part of Springer Science+Business Media (www.springer.com)

Preface

This book is devoted to the single functional group, metal derivatives of unsaturated carboxyl ion RCOO , where R is a radical with multiple bonds. This field embraces a huge number of chemical compounds, among which are new types of monomers and polymers with interesting structures and properties and unusual chemical transformations. This field includes both natural and artificial polymers but mainly various synthetic materials. Macromolecular metal carboxylates are currently the object of extensive studies due to their unique catalytic, magnetic, optical, and other properties as well as perspective precursors of novel nanocomposite functional materials. These complexes and nanocomposites have attracted scientific interest both from a fundamental point of view and their potential applications. Reactivity of unsaturated metal carboxylates containing metal atoms in immediate proximity to a polymerizable bond is closely related to their molecular structure. It is of essence to reveal the peculiarities of their behavior, which is determined by the metal on the one side and the polymeric backbone on the other. In this book, the main representatives of unsaturated carboxylic and corresponding polymeric acids as well as the methods of synthesis of metal carboxylates are analyzed. There are no analogs of such monographs devoted to various aspects of synthesis, polymerization, and properties of the monomeric and macromolecular metal carboxylates and nanocomposites in the literature. Structure of monomer and macromolecular metal carboxylates, the type of coordination of carboxylate ion, the electronic and valence state of metal, and specificity of metal–organic ligand bond were also considered. We want to note the role of kinetic and stereochemical effects on the main stages of polymerization and copolymerization of such metal-containing monomers. Knowledge of these peculiarities allows one to effectively control the structure and properties of metallopolymers. An alternative way to produce of macromolecular metal carboxylates by the interaction of polymeric acids with metal compounds is also discussed. In this book, the features of complexation of carboxylic macromolecular ligands, the effects of a polymer chain, the constants of formation and stability of macrocomplexes formed are considered. Special chapters of the book are devoted to applications of metallopolymer and nanocomposites as well as polymer-assisted synthesis of metal nanoparticles. v

vi

Preface

We think that this book is the first comprehensive analysis of this field of science. We tried to consider the problem as exhaustively as possible, and we hope that missed questions are not principal. Who is our potential reader? Chemistry of carboxylates, as any interdisciplinary field of science and technique, rapidly develops, and intensive accumulation of experimental data in this field embarrasses not only beginners but also experienced researchers working in this field. First of all, this book can be useful for a wide range of scientists and engineers of research institutes and industry. Then, it can serve as a handbook for students, postgraduate students of universities and colleges that are interested in this field of science. After 25 years of our own researches in this field and analysis of literature, we believe in the necessity of appearance of this book generalizing accumulated data on all aspects of monomeric and polymeric metal carboxylates. Section 9.2.1 was written together with Professor Aleksander S. Rosenberg who to our great regret deceased untimely. Chernogolovka, Russian Federation King of Prussia, PA May 2010

Anatolii D. Pomogailo and Gulzhian I. Dzhardimalieva Vladimir N. Kestelman

Contents

1

Introduction .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . References .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .

1 4

2

Monomeric and Polymeric Carboxylic Acids . . . . . . . . . . . .. . . . . . . . . . . . . . . . . 2.1 Mono- and Polybasic Unsaturated Carboxylic Acids: Characteristic and Polymerization . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . 2.1.1 Monobasic Carboxylic Acids with One Double Bond.. . . . . . . . 2.1.2 Unsaturated Dicarboxylic (Dibasic) Acids . . . .. . . . . . . . . . . . . . . . . 2.1.3 Unsaturated Carboxylic Acids with Triple Bond (Acetylenic Acids) . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . 2.2 Peculiarity of Polymerization of Unsaturated Carboxylic Acids and their Polymers Structure .. . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . 2.3 Stereoregular Polyacids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . 2.4 Cross-Linked Polyacids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . 2.5 Graft- and Block-Copolymers with Carboxyl Fragments.. . . . . . . . . . . . . 2.6 Natural Polyacids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . 2.6.1 Polysaccharides.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . 2.6.2 Humic Acids .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . References .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .

7

3

Synthesis of Unsaturated Carboxylic Acid Salts . . . . . . . . .. . . . . . . . . . . . . . . . . 3.1 Reaction of Unsaturated Carboxylic Acids with Metal Hydroxides, Oxides, and Carbonates . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . 3.2 Reactions of Acetates and Other Salts with Unsaturated Carboxylic Acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . 3.3 Ligand Exchange Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . 3.3.1 With Metal Halides .. . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . 3.3.2 With Metal Alkoxides .. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . 3.3.3 Other Exchange Reactions .. . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . 3.3.4 Synthesis of Bimetallic Compounds . . . . . . . . . .. . . . . . . . . . . . . . . . . 3.4 Sol–Gel Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . 3.5 Other Reactions .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . 3.6 Synthesis of Cluster Containing Unsaturated Carboxylates . . . . . . . . . . . References .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .

7 7 9 10 10 17 18 19 22 22 22 24 27 27 29 30 31 32 33 34 34 35 37 52 vii

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Contents

4

Spectral Characteristics and Molecular Structure of Unsaturated Carboxylic Acid Salts . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . 57 4.1 Metal (Meth)acrylates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . 57 4.1.1 IR Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . 58 4.1.2 Magnetic Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . 63 4.1.3 Electron Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . 64 4.1.4 Molecular Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . 65 4.2 Metal Dicarboxylates .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . 69 4.2.1 Monomeric Salts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . 69 4.2.2 Coordination Polymers .. . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . 74 4.2.3 Ferromagnetic Properties of Metal Dicarboxylates .. . . . . . . . . . . 83 4.3  -Complexes of Metal Carboxylates . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . 85 4.4 Unsaturated -Oxo Multinuclear Metal Carboxylates .. . . . . . . . . . . . . . . . 88 4.4.1 IR-Spectroscopy .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . 89 4.4.2 Mass-Spectrometry .. . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . 91 4.4.3 Molecular Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . 92 4.5 Cluster-Containing Unsaturated Carboxylates . . . . . . . .. . . . . . . . . . . . . . . . . 94 4.6 Metal Carboxylates with Unsaturated Ligands of Acetylene Type .. . . 96 References .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .100

5

Polymerization and Copolymerization of Salts of Unsaturated Carboxylic Acids . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .105 5.1 Types of Initiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .106 5.2 Kinetic and Stereochemical Effects .. . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .109 5.2.1 Radical Polymerization of Alkali and Alkaline Earth Metal Salts of Unsaturated Carboxylic Acids . . . . . . . . . . .109 5.2.2 Radical Polymerization of Transition Metal (Meth)acrylates .. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .112 5.2.3 Regulation of Stereochemistry of Radical Polymerization of Metal Carboxylates . . . . . . . .. . . . . . . . . . . . . . . . .117 5.3 Solid Phase Polymerization of Unsaturated Metal Carboxylates.. . . . .121 5.3.1 Thermal Polymerization of Unsaturated Metal Carboxylates .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .122 5.3.2 Solid State UV and Radiation Initiated Polymerization . . . . . . .123 5.3.3 Reactivity of Unsaturated Metal Carboxylates in Solid Phase. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .125 5.4 Copolymerization and Terpolymerization .. . . . . . . . . . . .. . . . . . . . . . . . . . . . .128 5.4.1 The Main Principles of Copolymerization of Alkali and Alkaline Earth Metal Salts . . . . . .. . . . . . . . . . . . . . . . .129 5.4.2 Reactivity of Tin-Containing Carboxylates . . .. . . . . . . . . . . . . . . . .131 5.4.3 Copolymerization of Transition Metal Salts . .. . . . . . . . . . . . . . . . .133 5.4.4 Kinetic Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .134 5.4.5 Terpolymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .138 References .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .141

Contents

ix

6

Polymer-Analog Transformations in Reactions of Synthesis of Metal Macrocarboxylates .. . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .145 6.1 Complexation of Metal Ions with Macromolecular Ligands . . . . . . . . . .146 6.2 Metal Ion Binding by Polyacids . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .150 6.3 Metal Ion Binding by Stereoregular Polyacids . . . . . . . .. . . . . . . . . . . . . . . . .159 6.4 Peculiarities of MXn Binding by Cross-Linked Polyacids . . . . . . . . . . . .161 6.5 Formation of Macrocomplexes with Grafted Polycarboxylic Fragments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .162 6.6 Bimetallic Polycomplexes .. . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .166 6.7 Formation of Organic–Inorganic Composites . . . . . . . . .. . . . . . . . . . . . . . . . .168 6.8 Binding of MXn by Natural Carboxyl Group Containing Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .171 References .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .174

7

Molecular and Structural Organization of Metal-Containing (Co)Polymers .. . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .179 7.1 Ionic Aggregations and Multiplets . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .179 7.1.1 Ionomers Synthesis .. . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .179 7.1.2 Morphology and Structure of Ionomers .. . . . . .. . . . . . . . . . . . . . . . .180 7.2 Morphology and Topological Structure of Metal-Containing Polymers .. . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .191 7.2.1 Three-Dimensional Network Polymers . . . . . . .. . . . . . . . . . . . . . . . .192 7.2.2 Interpenetrating Polymer Networks . . . . . . . . . . .. . . . . . . . . . . . . . . . .194 7.2.3 Hybrid Supramolecular Structures . . . . . . . . . . . .. . . . . . . . . . . . . . . . .198 7.3 Basic Types of Units Variability in Metal-Containing (Co)Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .205 7.3.1 Units Variability, Caused by Elimination of Metallogrouping During Polymerization .. .. . . . . . . . . . . . . . . . .207 7.3.2 Units Variability, Caused by Various Oxidation Rate of d-Metals .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .208 7.3.3 Anomalies in Metal-Containing Polymers Chains Caused by a Variety of Chemical Linkage of a Metal with a Polymerized Ligand.. . . . . . . . . . . . . . .209 7.3.4 Extracoordination as One of the Types of Anomalies (Spatial and Electronic Structure of a Polyhedron) .. . . . . . . . . . .210 7.3.5 Unsaturation of Metal-Containing Polymers and Their Structurization.. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .211 References .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .213

8

Properties and Basic Fields of Application of Metal-Containing Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .217 8.1 Improvement of the Polymeric Materials Properties Based on Cross-Linking Action of Monomeric and Polymeric Salts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .217

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8.2 Radiation Resistance, Photophysical and Optical Properties of Metal-Containing (Co)Polymers .. . . . . . .. . . . . . . . . . . . . . . . .226 8.3 Water-Absorbing and Sorption Properties of Metal-Containing (co)Polymers . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .232 8.4 Sorption Properties of Metal-Containing (co)Polymers . . . . . . . . . . . . . . .238 8.5 Catalysis by Macromolecular Metal Carboxylates .. . .. . . . . . . . . . . . . . . . .245 8.5.1 Catalytic Reactions of Oxidation of Hydrocarbons .. . . . . . . . . . .246 8.5.2 Reactions of Peroxidase Decomposition . . . . . .. . . . . . . . . . . . . . . . .249 8.5.3 Other Catalytic Reactions . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .251 References .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .252 9

Monomeric and Polymeric Metal Carboxylates as Precursors of Nanocomposite Materials . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .257 9.1 Formation and Stabilization of Nanoparticles at Presence of Macroligands with Carboxyl Functional Groups . . . . . .257 9.2 Basic Obtaining Methods of Metal-Containing Polymeric Nanocomposites on the Basis of Monomeric and Polymeric Carboxylates . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .263 9.2.1 Thermal Conversions of Metal-Containing Carboxylated Precursors . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .263 9.2.2 Polymer Carboxylate Gels and Block Copolymers as Reactors for Nanoparticles .. . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .273 9.2.3 Sol–Gel Methods in the Obtaining of Oxocluster Hybrid Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .277 9.2.4 Metal-Containing Polymeric Langmuir–Blodgett Films . . . . . .279 9.3 Metal-Containing Polymeric Nanocomposite Materials of the Carboxylated Type. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .281 References .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .284

10 Conclusion . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .289 Index . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .293

Chapter 1

Introduction

At present, there are three main ways of production of metal-containing polymers on the basis of carboxyl precursors [1]: (I) the interaction of metal compounds MXn with linear functionalized (carboxyl-containing) polymers, when the main polymer chain remains untouched (so called polymeranalogous transformations), (II) the polycondensation of proper precursors, when metal ions are incorporated into and removed from the main chain leading to polymer destruction, (III) the recently developed method, polymerization and copolymerization of metalcontaining monomers. I ~ CH2 – CH – CH2 – CH – CH2 – CH~ + MXn COOH

COOH

COOH

~ CH2 – CH – CH2 – CH – CH2 – CH~ COOH

C

O

COOH

O MXn–1

II R–L–R HCOO

+ MXn

OCOH

III CH2 = CH

initiation

–2HX

~ OCOR – L – RCOO – MXn–1~

~ CH2 – CH ~

C=O

C=O

O

O

MXn–1

MXn–1

Metallopolymers on the basis of transition metals, obtained by method I, as a rule are characterized by low content of bond metal and are used mainly for ionexchange extraction, concentration, and isolation of metals. Condensation method II as a rule uses dicarbonic acids, including element – substituted (L), for instance carboran-containing acids [2, 3]. Production of metallopolymers by method III and investigation of their structure and properties is the main content of this book. Metal carboxylates are widely used in science and technology. They are a part of polynuclear coordination compounds (in catalytic and biomimetic systems) and

A.D. Pomogailo et al., Macromolecular Metal Carboxylates and Their Nanocomposites, Springer Series in Materials Science 138, DOI 10.1007/978-3-642-10574-6 1, c Springer-Verlag Berlin Heidelberg 2010 

1

2

1 Introduction

metal-proteins, intermediate compounds of many metabolic processes. Biochemical behavior of metal-enzymes and antibodies is determined in many respects by their carboxyl ate function [4, 5]. Carboxyl ion can serves as mono-, bi-, and even tridentant ligand of metal ions with numerous types of coordination. For instance, 18 structural functions of carboxyl group were found for monobasic carboxylates of transition metals [6] and 15 diverse types of coordination were observed by X-ray diffraction study for homolog of maleate anion C2 O4 2 [7]. Oxalate ions bind metal ions by only one or two of four oxygen atoms, and as a rule five-member metallocycles are realized, i.e., potentially tetradentant anion C2 O4 2 serves usually as bidentant cyclic ligand. In general case, the type of coordination depends on a large number of factors: the nature of metal atoms and outersphere cations, the system of hydrogen bonds, the presence of competing acidic or electroneutral ligands L0 . This permits to consider these compounds as so called “smart” materials. This can be illustrated by the following example. It was shown by IR1 and EXAFS that coordination of zinc ion in Zn (II)-neutralized ethulenemethacrylic acid ionomer depends on temperature, the presence of adsorbed water, pressure applied to melt at 130 ıC [9, 10]. In vacuum, Zn(II) carboxylate has mainly a hexacoordinated structure, which gives IR peaks as (COO / at 1,624 and 1,538 cm1 , but at atmospheric pressure (P D 0:1 MPa) a tetracoordinated structure is formed with as (COO / at 1,585 cm1 . Donor–acceptor properties of carbonic acids and their anions in aqueous solutions are characterized by the basicity constant pKa . The value of pKa can be calculated quantum-chemically [11]. The energy of decoupling of double bond electrons of acrylic acid and its cobalt salt, as well as the ways of formation of the transition state, differs substantially [12]. In principle, the problem of metal carboxylates can be divided into two unequal parts: the larger and long-developed problem of salts of saturated carbonic acids, and the smaller and recently developed problem of unsaturated carboxylates. Fundamental data on synthesis and properties of saturated carboxylates of metals and their application are rather well considered in reviews and books, for example monograph [13], comprehensive in 1983 and still of current importance, and rather complete old and recent reviews [14–17]. Studies of unsaturated carbonic acids are 1

IR spectroscopy is widely used for study of structure of these complexes, because valence vibrations iC D O are sensitive to geometry of COO group and its surrounding [8]. In this group the double bond is delocalized and the valence vibration CO splits in asymmetric high-frequency .as / and low-frequency symmetric .s / vibrations. Intermediate symmetry is possible depending on the type of coordination (this will be considered in detail in Chap. 4). Other methods of study of structure of metal-containing group are determination of the values of charges on oxygen atoms of carboxyl and hydroxyl groups and the values of chemical shifts of 13 C NMR of carbon of carboxylate group, estimation of ionization energy (especially for thermochemical calculations), and others.

1 Introduction

3

practically not generalized. Scrappy data on methods of their synthesis, structure, chemical transformations and numerous applications are dispersed in scientific and patent literature. Sometimes, analysis of some unsaturated carboxylates can be encountered in reviews and chapters in monographs, but they do not give complete presentation of the state of problem. At the same time, this type of compounds was intensively studied in last years by methods of high-molecular compounds with the aim of obtaining new types of metal-containing materials. Although many attempts of generalization of synthesis methods and polymerization transformations of some specimens of this type of metal-containing monomers are known (see, for example, [18–20]), among them dissertations (for example, [21, 22]), it is enigmatic why unsaturated carboxylates were not thoroughly analyzed like their saturated analogs. This task is more difficult because the multiple bond affects all aspects – synthetic and structural chemistry (for instance, in many cases the multiple bond can be involved in formation of carboxylate unit), reactivity of these compounds, polymerization ability. Maybe, this can be explained by interdisciplinary character of the problem. On the one hand, synthetic and structural part of carboxylates belongs to inorganic and coordination chemistry where unsaturated ligands are considered traditionally as “ugly ducklings”. On the other hand, methods of synthesis and investigation of these promising exotic monomers are rarely developed in high-molecular chemistry. The interests of specialists in these two fields of science are rarely intersected in this promising and rapidly developing field of chemistry, therefore one of the aims of this book is to draw together these specialists. Among vast diversity of salts of unsaturated carbonic acids, derivatives of acrylic, methacrylic, crotonic, oleic, fumaric, maleinic, acetyldicarbonic, vinylbenzoic, and some other acids, which are virtually typical metal-containing monomers containing multiple ready-to-open bonds and metal atoms chemically bond to organic part of the molecule [18]. Unsaturated bond affect coordination of carboxylate ligand. Intensive development of this field in last years is caused by practical value of obtained products, polymers with ion metals in each chain. This improves many properties of polymers and their composites. In subsequent chapters we plan to analyze thoroughly transformations of unsaturated metal carboxylates in the course of synthesis, as well as their polymerization and copolymerization with conventional monomers. Here we only give one example of such transformations. Photopolymerization of diacetylene acid (CH3 (CH2 /11 CCCC(CH2 /8 COOH was studied in [23] on the interphase air–water in the presence of divalent metal ions Ba (II) (pH 7.7), Cd (II) (pH 6.8), and Pb (II) (pH 6.0). It was found that in the course of the photopolymerization carboxylate group of acetylene acid in monolayer in subphase of ions Ba (II) and Pb (II) changed its coordination from bridge to bidentant, whereas for Cd (II) the bidentant structure was unchanged at the decrease of molar square from 0.8 to 0.18 nm2 /molecule, i.e., the polymerization stimulates more compact packing of carboxyl groups in monolayers. Experimental data and theoretical calculations show that the change of the type of coordination, so called carboxylate

4

1 Introduction

shift, is a low-energy process. This plays an important role in catalytic cycles of metal enzymes [24]: O µ−1, 1

O M

O M

µ−1, 2

M O

O M

M

O M

The carboxyl shift of coordination from bidentant-chelate (1, 1) to bidentantbridge (  1, 2) is easily observed by 1 H  13 C [25] NMR. Diversity of functions, symmetry of ligands, metal–ligand coordination, various types of bonds in their molecules determine unique possibility for construction of promising materials on their basis. Metal oxo-clusters with unsaturated carboxylate ligands are very promising as nanostructural elements for organic–inorganic hybrid nanocomposites [26]. First of all, these are high-organized objects with strictly determined size and shape which are remained unchanged in final material. Therefore, their distribution in matrix is homogeneous and mono-disperse nanostructures are formed. In other words, mono- and polycarboxylates of metals are objects of supramolecular chemistry, and their polymer films are characterized by improved mechanical [27, 28], adhesion [29], optical [30], electric [31], and other properties. This book is devoted to a wide range of problems, embracing methods of synthesis, structure, and properties of unsaturated metal carboxylates, features of their polymerization transformations, morphology, as well as properties and characteristics of formed metallopolymers, including polymeranaloguous transformations. The interest to the problem increased substantially when it was found that these materials are effective precursors of metal–polymer nanocomposites [32], in which carboxylate matrix or products of its transformation serve as stabilizing agents and prevent aggregation of nanoparticles of metals or their oxides [33].

References 1. D. W¨ohrle, A.D. Pomogailo, Metal Complexes and Metals in Macromolecules. Synthesis, Structures and Properties (Wiley-VCH, Weinheim, 2003) 2. V.A. Sergeev, N.I. Bekasova, M.A. Surikova, E.A. Baryshnikova, Ya.V. Genin, N.K. Vinogradova, Dokl. Akad. Nauk. 332, 601 (1993) 3. V.A. Sergeev, N.I. Bekasova, M.A. Surikova, E.A. Baryshnikova, N.M. Mishina, T.N. Balykova, Ya.V. Genin, P.V. Petrovskii, Vysokomol. Soedin. A. 38, 1292 (1996) 4. C. He, S.J. Lippard, J. Am. Chem. Soc. 120, 105 (1998) 5. W. Ruttinger, G.C. Dismukes, Chem. Rev. 97, 1 (1997) 6. M.A. Porai-Koshits, Zh. Strukt. Khim. 21, 146 (1980) 7. V.N. Serezhkin, M.Yu. Artem’eva, L.B. Serezhkin, Yu. N. Mikhailov, Zh. Neorg. Khim. 50, 1106 (2005)

References

5

8. K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th edn. (Wiley, New York, 1986) 9. H. Hashimoto, S. Kutsumizu, K. Tsunashima, S. Yano, Macromolecules 34, 1515 (2001) 10. S. Kutsumizu, M. Nakamura, S. Yano, Macromolecules 34, 3033 (2001) 11. S.Yu. Monakhov, T.A. Stromnova, Zh. Obshch. Khim. 77, 1841 (2007) 12. T.S. Zyubina, G.I. Dzhardimalieva, A.D. Pomogailo, in Proceedings of the Russian Conference ‘Present-day state and tendency of development of organometal catalysis’ (IPCP RAS, Chernogolovka, 2009, p.94) 13. R.C. Mehrotra, R. Bohra, Metal Carboxylates (Academic Press, London, 1983), p. 396 14. M.A. Porai-Koshits, in Krystallokhimiya (Itogi nauki i tekhniki) [Crystal Chemistry (Advances in Science and Crystal Ingeneering), vol. 15, ed. by E.A. Gilinskaya (VINITI, Moscow, 1981), p. 3 15. G.B. Deacon, R.J. Phillips, Coord. Chem. Rev. 33, 227 (1980) 16. A.P. Pisarevskii, L.I. Martynenko, Koordin. Khim. 20, 324 (1994) 17. M.A. Kiskin, I.L. Eremenko, Usp. Khim. 75, 627 (2006) 18. A.D. Pomogailo, V.S. Savostyanov, Metallcontaining monomers and their polymers (Khimiya, Moscow, 1988) 19. G.I. Dzhardimalieva, A.D. Pomogailo, Russ. Chem. Rev. 77, 259 (2008) 20. U. Schubert, Chem. Mater. 13, 3487 (2001) 21. R.F. Schlam, Structure and Reactivity of Metal Carboxylates. Thesis Dr. PhD (Brandeis University, UMI, Ann Arbor, 1998) 22. G.I. Dzhardimalieva, (Co)polymerization and thermal transformations as a way for synthesis of metallopolymers and nanocomposites. Doct. Sci. Chem. Thesis (ICPC RAS, Chernogolovka, 2009) 23. G. Ohe, H. Ando, N. Sato, Y. Urai, M. Yamamoto, K.J. Itoh, Phys. Chem. B. 103, 435 (1999) 24. D.D. LeCloux, A.M. Barrios, T.J. Mizoguchi, S.J. Lippard, J. Am. Chem. Soc. 120, 9001 (1998) 25. A. Demˇsar, J. Koˇsmrlj, S. Petriˇcek, J. Am. Chem. Soc. 124, 3951 (2002) 26. L. Rozes, N. Steunou, G. Fornasieri, C. Sanchez, Monatsh. Chem. 137, 501 (2006) 27. Y.C. Chen, S.X. Zhou, H.H. Yang, J. Appl. Polym. Sci. 995, 1032.(2005) 28. M.N. Xiong, S. Zhou, L. Wu, B. Wang, L. Yang, Polymer 45, 8127 (2004) 29. T.P. Chou, G.Z. Cao, J. Sol-Gel Sci. Technol. 27, 31 (2003) 30. Y.Y. Yu, C.Y. Chen, W.C. Chen, Polymer. 44, 593 (2003) 31. C.R. Kagan, D.B. Mitzi, C.D. Dimitrakopoulos, Science 286, 945 (1999) 32. A.D. Pomogailo, A.S. Rozenberg, I.E. Uflyand, Metal Nanoparticles in Polymers (Khimiya, Moscow, 2000) 33. A.D. Pomogailo, V.N. Kestelman, Metallopolymer. Nanocomposites (Springer, Berlin, Heidelberg, New York, 2005)

Chapter 2

Monomeric and Polymeric Carboxylic Acids

Our goal was not to analyze known unsaturated carboxylic acids (this problem itself is unrealizable), but only to give a general idea about unsaturated acids and their polymers more often used for obtaining metal carboxylates. Basic attention was paid to those representatives which are a priori capable of polymerization. As data on unsaturated carboxylic acids are dispersed in numerous researches, directories, and catalogs, many of which are not always accessible, their most important characteristics are given below. Other unsaturated heteroacids and their polymers (for example, vinylsulfonic and vinylbenzoic sulfonic acids, thio-, phosphonic, amino-, and other acids) are not analyzed in this book. More detailed information can be found in other available literature [1–4].

2.1 Mono- and Polybasic Unsaturated Carboxylic Acids: Characteristic and Polymerization These types of monomers traditionally form the material basis of high-molecular compounds chemistry. Polycarboxylic acids and polymers based on its derivatives are large-tonnage products. Unsaturated carboxylic acids are used to a great extent for the preparation of polyethers and polyesters, polynitriles, polyamides, etc.

2.1.1 Monobasic Carboxylic Acids with One Double Bond The brightest representatives of monobasic unsaturated acids are acrylic and methacrylic acids (and their derivatives) – the extremely important products in high-molecular compounds chemistry. The most widespread commercial syntheses of acrylic acid are oxidative carbonylation of ethylene, vapor-phase oxidation of propylene, butylene, and acrolein, hydrolysis of ethylene cyanohydrin, hydrolysis

A.D. Pomogailo et al., Macromolecular Metal Carboxylates and Their Nanocomposites, Springer Series in Materials Science 138, DOI 10.1007/978-3-642-10574-6 2, c Springer-Verlag Berlin Heidelberg 2010 

7

8

2 Monomeric and Polymeric Carboxylic Acids

of “-propiolactone, etc. The basic method for obtaining the acrylic acid is the preparation from acetylene, carbon oxide, and water: 4 CHCH C 4H2 O C Ni .CO/4 C 2 HCl ! 4 CH2 DCHCOOH C NiCl2 C H2 (2.1) The reaction proceeds with a high yield both at standard pressure (in this case CO is engaged as nickel tetracarbonyl) and at 30 atm and 170ı C with gaseous nickel tetracarbonyl in the presence of catalytic quantities of nickel salts. Methacrylic acid, CH2 DC.CH3 /COOH, is obtained by gaseous-phase oxidation of isobutylene, by catalytic gaseous-phase oxidation of methacrolein, and through an intermediate formation of acetone cyanohydrin, etc. Many homologs of acrylic acid exist in geometrical stereoisomeric forms caused by a different arrangement of substituents at a double bond, for example, crotonic (trans-) and isocrotonic (cis-) acids, CH3 CHDCHCOOH. Crotonic acid, contained in the croton oil, is a crystal substance, b.p. 180ıC and m.p. 72ı C. Isocrotonic acid (b.p. 169ıC, m.p. 72ı C) is a less stable form and it is transformed partly into crotonic acid by heating up to more than 100ıC. Angelic (trans-) and tiglic (cis-) acids, CH3 CHDC.CH3 /COOH, are isomers. The first acid is the labile form (b.p. 185ı C, m.p. 45ı C), the second is the stable form (b.p. 198ıC, m.p. 64:5ı C). .C/-Cytronellic acid, .CH3 /2 CDCHCH2 CH2 CH(CH3 /CH2 COOH (b.p. 152ıC at 18 mm Hg) is an optical active compound. Undecylenic acid, CH2 DCH.CH2 /8 COOH, is formed at vacuum distillation of castor oil, b.p. 213ıC at 100 mm Hg, m.p. 24ıC. Ricin acid, CH3 (CH2 /5 CH(OH)CH2 CHDCH(CH2 /7 COOH, is also used comparatively often. Palmitooleic acid, CH3 (CH2 /7 CHDCH(CH2 /7 COOH, is an oily liquid, b.p. 223ıC at 10 mm Hg, m.p. C14ı C. Erucic (b.p. 225ıC at 10 mm Hg, m.p. 34ı C) and brassidic (b.p. 256ı C at 10 mm Hg, m.p. 65ı C) acids (CH3 (CH2 /7 CHDCH(CH2 /11 COOH) are geometrical isomers. 4-Vinylbenzoic acid has received the most expansion among vinylbenzoic acids. From unlimited number of characterized polyunsaturated fatty acids with two and three isolated ethylenic bonds in a molecule, used for obtaining the carboxylates, the following acids have been used. Sorbic acid, CH3 CHDCHCHDCHCOOH, is synthesized by sorbic aldehyde oxidation prepared by condensation of three molecules of acetic aldehyde. Geranic acid is obtained from 2-methylpentene-2-one-6. ’- and “-Eleostearic acids with three double bonds (CH3 (CH2 /3 CH2 CHDCHCH2 CHDCHCH2 CHDCH(CH2 /4 COOH) are also interesting: ’-isomer is a low-melting form (m.p. 47ı C) and it rearranges into high-melting “-isomer (m.p. 67ı C) at UV-irradiation. Thus, they are cis–trans-isomers having especially high abilities to “exsiccation” as well as all acids with three ethylenic bonds. Linolenic acid, CH3 (CH2 CHDCH)3 (CH2 /7 CO2 H, is also one of the “exsiccant” fatty acids (b.p. 229ı C at 16 mm Hg and 184ıC at 4 mm Hg, density 0.905 g/cm3 .20ı C/, it is quickly oxidized and solidified in air). Linolenic acid and

2.1 Mono- and Polybasic Unsaturated Carboxylic Acids: Characteristic and Polymerization

9

many unsaturated arachidonic acids are the vital fatty acids. Dehydrogeranic acid, (CH3 /2 CDCHCHDCHC(CH3 /2 DCOOH, (m.p. 185–186ı C) also should be noted.

2.1.2 Unsaturated Dicarboxylic (Dibasic) Acids Unsaturated dicarboxylic acids can be mono- or polyunsaturated. The most important representatives of “-dicarboxylic acids are the first members of this row, maleic (m.p. 130ı C) and fumaric (m.p. 287ı C) acids, HOOCCHDCHCOOH, differed by a spatial structure. Maleic acid has cis- and fumaric acid has trans-configuration. Both acids are obtained by heating of malic acid but at different temperatures. In industry, maleic acid (as maleic anhydride) is prepared under catalytic oxidation of benzene by the oxygen in the air. When two electron-seeking carbonyl groups are conjugated with an olefinic system, acceptor character of CDC bond especially increases. Maleic anhydride has the best acceptor properties among derivatives of ’, “-unsaturated dicarboxylic acids. Maleic acid is stronger than fumaric acid: hydrogen atom of the first carboxyl group dissociates more easily, than in case of fumaric acid, and conversely for the second carboxyl group. Ionization constants at 18ı C are: – For maleic acid pK1 D 2:0, pK2 D 6:23 – For fumaric acid pK1 D 3:03, pK2 D 4:38 For comparison we shall note that for oxalic acid (saturated analog of maleic and fumaric acids), pK1 D 1:46 and pK2 D 4:40. Citraconic, methylmaleic (m.p. 91ı C), mesaconic, and methylfumaric (m.p. 202ıC) acids have the same relationship among themselves as well as with maleic and fumaric acids: the first of them is a cis-form, second is a trans-form. CH2=CCO2H CH2CO2H

(1)

CH3CCO2H HCCO2H

(2)

CH3CCO2H HO2CCH

(3)

Itaconic, 2-methysuccinic (1), acid and their isomers, citraconic (2) and mesaconic (3) acids, are more often than other acids used for the binding of metal ions as well as their polymeric analogs. Besides, itaconic acid is the perspective candidate for obtaining the high-functionalizated copolymers. It is connected with the low cost of itaconic acid received from renewable sources under fermentation by Aspergillus terrus microorganisms. Unsaturated tribasic propene-1,2,3-threecarboxylic (aconitic) acid, HOOC  CH2 C.COOH/DCH COOH, is obtained by water elimination from citric acid. It is rather distributed in flora and contained in sugar-cane and beet; it is extracted from Aconitum poisonous plants of the buttercup family. Unfortunately, these acids have not yet found practical application in the metal carboxylates synthesis.

10

2 Monomeric and Polymeric Carboxylic Acids

2.1.3 Unsaturated Carboxylic Acids with Triple Bond (Acetylenic Acids) Interaction of sodium derivatives of acetylenic hydrocarbons with carbon dioxide facilitates the preparation of acetylenecarboxylic acids in which triple bond is localized near a carboxyl group as depicted in the following scheme: Cn H2nC1 C  CNa C CO2 ! Cn H2nC1 C  CCOONa

(2.2)

This type of acids has received the name of propiolic acid series because of the simplest representative of this series, propiolic acid, HCCCOOH. Propiolic acid is a liquid with a pungent smell (b.p. 83ı C at 50 mm Hg, m.p. 9ı C). The peculiarity of propiolic acid (as will be shown in the subsequent chapters) gives the possibility of the replacement of hydrogen atom by metal not only in the carboxyl group but also in the acetylenic residue. Methyl-propiolic (CH3 CCCOOH, b.p. 203ı C), 2-octynoic acid (CH3 (CH2 /4 CCCOOH), and phenyl-propiolic ((C6 H5 /CCCOOH) acids are the most commonly used for the preparation of corresponding carboxylates among numerous higher homologs of propiolic acid. Carboxylic acids in which triple bond is far from the carboxyl group can be synthesized from the corresponding dibromodirevatives of fatty acids by hydrogen bromide elimination upon alkali, for example, stearolic acid, CH3 (CH2 /7 CC(CH2 /7 COOH, and its isomer – 6-octadecynoic acid, CH3 (CH2 /10 CC(CH2 /4 COOH. A set of strongly unsaturated acids containing acetylenic and ethylenic bonds was extracted from plants and prepared synthetically. Derivatives of acetylenedicarboxylic acids are less important for the problem under consideration, although 10,12-penta-cosadiynoic acid (CH3 (CH2 /11 CC CC(CH2 /8 COOH) forms Langmuir–Blodgett films easily [5]. Some properties of unsaturated carboxylic acids considered are summarized in Table 2.1. The composition and structure of unsaturated carboxylic acids determine the basic approaches to their carboxylates, on the one hand, and to their polymerization, on the other hand.

2.2 Peculiarity of Polymerization of Unsaturated Carboxylic Acids and their Polymers Structure Unsaturated carboxylic acids can be classified as polymerized (most often by the radical mechanism) ionized monomers as well [8]. In turn, obtained linear watersoluble polymers are ionomers – ion-containing polymers with a carbon-containing main chain and relatively small number of partly or completely ionized acidic groups of carboxylic, sulfonic, phosphoric, and other acids in a side chain [9–11].

Monobasic unsaturated carboxylic acids Acrylic acid CH2 DCHCOOH Methacrylic H2 CDC(CH3 /COOH (2-methylpropionic) acid Crotonic CH3 CHDCHCOOH (trans-2-butenoic)) acid 2-Ethylacrylic acid H2 CDC(C2 H5 /CO2 H 2-Pentenic (trans-2 C2 H5 CHDCHCO2 H pentenic) acid 4-Pentenic CH2 DCHCH2 CH2 COOH (3-vinylpropionic, allylacetic) acid 2-Propylacrylic acid CH3 (CH2 /2 (DCH2 /CO2 H 2-Octenoic acid CH3 (CH2 /4 CHDCHCO2 H 3-Vinylbenzoic acid H2 CDCHC6 H4 CO2 H 4-Vinylbenzoic H2 CDCHC6 H4 CO2 H (styrene-4-carboxylic) acid 2-Carboxyethyl-acrylate CH2 DCHCO2 .CH2 /2 CO2 H C6 H5 COCHDCHCO2 H trans-3-Benzoylacrylic (4-oxo-4-phenyl-2butenoic) acid 2-Bromoacrylic acid H2 CDC(Br)CO2 H 2-Bromomethyl-acrylic CH2 DC.CH2 Br/COOH acid 103ı C/19 mm Hg

62–65ı 70–73ı

94–97

1.214 .25ı C/

(continued)

1.441 1.4588

0.951 .25ı C/ 0.944 .25ı C/

5–6 91–95 142–144

1.428

0.981 .25ı C/

22:5

83–84/12

165–188 154/22

1.437 1.452

9–11

1.027 .25ı C/ 0.986 .25ı C/ 0.99 .25ı C/

4.69(25)

176 106ı C/20

1.4242; 1.4185 1.431; 1.4288

nD 20

71.5 .70–72ı C/

1.051; 1.045 .25ı C/ 1.015

d 4 20 (g/mL)

185(760) 180–181ı C

4.25(25) 4.66

pKa .ı C/

13 12–16

M.p. .ı C/

139;142/760 163

Table 2.1 Composition and characteristics of unsaturated carboxylic acids Acid Formula B.p. (ı C/mm Hga )

2.2 Polymerization of Unsaturated Carboxylic Acids and their Polymers Structure 11

Table 2.1 (continued) Acid Ricinoleic acid, (R)-12-hydroxy-cis-9octadecenoic, 12-hydroxyl-oleinic acid 10-Undecenoic acid cis-5-Dodecenoic acid Palmitoleinic (cis-9-hexadecenoic) acid trans-Oleinic (trans-9-octadecenoic, trans-Elaidic) acid cis-Oleinic (cis-9-octadecenoic, elanoic) acid cis-11-Eicosenoic (gondoic) acid Nervonic (cis-15-Tetra-cosenoic) acid ’-Linoleic (cis, cis, cis-9,12,15Octadecatrienoic acid 11

CH3 (CH2 CHDCH)3 (CH2 /7 CO2 H

230–232/1

42–43

13–14

CH3 (CH2 /7 CHDCH(CH2 /13 COOH

194–195/1.2

CH3 (CH2 /7 CHDCH(CH2 /7 COOH

42–44

23–24

288/100

CH3 (CH2 /7 CHDCH(CH2 /7 COOH

0.5

23–25

M.p. .ı C/

CH3 (CH2 /7 CHDCH(CH2 /9 CO2 H

137/2 135/0.4 162/0.6

B.p. (ı C/mm Hga )

CH2 DCH.CH2 /8 COOH CH3 (CH2 /5 CHDCH(CH2 /3 CO2 H CH3 (CH2 /5 CHDCH(CH2 /7 COOH

Formula CH3 (CH2 /5 CH(OH)CH2 CHDCH(CH2 /7 COOH

pKa .ı C/

(continued)

1.480

1.4606

0.883 .25ı C/

0.914 .25ı C/

1.459

1.449 1.454 1.457

nD 20

0.887 .25ı C/

0.912 .25ı C/ 0.906 .25ı C/ 0.895

d 4 20 (g/mL) 0.940

12 2 Monomeric and Polymeric Carboxylic Acids

Table 2.1 (continued) Acid ”-Linolenic acid (cis, cis, cis-6,9,12Octadecatrienoic) acid cis-5,8,11,14,17Eicosapenta-enoic acid Acetylenic carboxylic acids Propynoic (Acetylenecarbo-xylic, Propinoic) acid 2-Butynoic (tetrolic, 1-Propynecarboxylic, 3-Methyl-propiolic) acid 2-Pentynoic acid 4-Pentynoic (Propargylacetic) acid [6, 7] 2-Hexynoic acid 2-octynoic (2-Octyn-1-oic) acid Phenylpropynoic acid 54  53

18; 9, 16–18

144/760; 83/50; 102/200

203/760

110/30

230 148–149/19 135–137

CH3 (CH2 CHDCH)5 (CH2 /3 CO2 H

HCCCOOH

CH3 CCCO2 H

CH3 CH2 CCCO2 H CHCCH2 CH2 COOH

CH3 (CH2 /2 CCCO2 H CH3 (CH2 /4 CCCO2 H

C6 H5 CCCOOH

137

2–5

47–53 54–57

78–80

M.p. .ı C/

Formula B.p. (ı C/mm Hga ) CH3 (CH2 /3 CH2 CHDCHCH2 CHDCHCH2 CHDCH(CH2 /4 COOH

2.23(25)

2.50

1.84 (25)

pKa .ı C/

1.431

1.138 .25ı C/

(continued)

1.460 1.46

1.4977

0.943 .25ı C/

0.992 .25ı C/ 0.961 .25ı C/

nD 20

d 4 20 (g/mL)

2.2 Polymerization of Unsaturated Carboxylic Acids and their Polymers Structure 13

a

1 mm Hg D 133.322 n/m2

Table 2.1 (continued) Acid Unsaturated dicarboxylic acids Fumaric (trans-1,2Ethene-dicarboxylic acid Maleic (2-Butenedioic, cis-1,2Ethylene-dicarboxylic, Toxilic) acid Itaconic (2-propene-1,2dicarboxylic; Succinic acid, methylene-) acid cis, cis-Muconic (cis, cis2,4-2,4-Hexadienedioic) acid Acetylendicarboxylic (2-Butynedioic) acid 2-Acetamido-acrylic acid; Acetyl-dehydroalanine Maleic acid monoamide (maleamic) acid 194–195 180–187 (pazl.) 185–186 (pazl.) 158–161

HOOCCHDCHCHDCHCOOH

HOOCCCCOOH

CH2 DC.NHCOCH3 /COOH

H2 NCOCHDCHCO2 H

298–300ı C (cybl)1659 cybl.)/1.7 137–140

M.p. .ı C/

165–168

B.p. (ı C/mm Hga )

HO2 CCH2 C(DCH2 /CO2 H

HOOCCHDCHCO2 H

HOOCCHDCHCOOH

Formula

3.85, 5.45

1.92, 6.23

3.02, 4.38

pKa .ı C/

1.573 .25ı C/

1.59 .25ı C/

d 4 20 (g/mL)

nD 20

14 2 Monomeric and Polymeric Carboxylic Acids

2.2 Polymerization of Unsaturated Carboxylic Acids and their Polymers Structure

15

Among carboxylic acids, polyacrylic (PAA) and polymethacrylic (PMAA) acids have found an application as macroligands for the binding of the metal ions. PAA is a weak polymeric acid and it is similar to the polybasic saturated acids in its chemical properties. The average value of pKa in aqua solutions (concentration 0.1 mol/L, alkali titration, 25ı C) is equal to 6.4. Polyacids are obtained usually in water solutions in the presence of potassium, sodium, or ammonium persulfate or by initiating systems of “ammonium persulfate – ascorbic acid” type – and also under the action of metal chelates .M   50; 000/ [12]. As a rule, these monomers exist in a form of cyclic or linear dimers, in which double bonds are considerably removed from each other. CH3 CH2

C

C

O OH

CH3

HO O

C

C

CH2

CH2

C

C

CH3

O OH

HO O

C

C

CH2

CH3

PAA, synthesized in the presence of peroxide initiators, is characterized by branching and rather low molecular weights; the reason is the reactions of chain transfer to a monomer or to a polymer due to hydrogen atoms of CH2 group. Thus, the ratio of the growth rates to the termination rates of polymer chains upon bulk polymerization of methacrylic acid .44:1ı C/, initiated by azobis(isobutyronitrile), is equal to kp =kt 0:5 D 0:278; H D 13:5 kcal=mol. [1] Kinetics of free-radical polymerization of the nonionized methacrylic acid in water solutions has a lot of peculiarities (see, for example [13]). Unsaturated carboxylic acids can enter into the polymerization reaction both in protonic (below pKa / and in deprotonic (anionic) (over pKa / forms: pKa

RCOOH ! RCOO C HC

(2.3)

Deprotonation results in the appearance of electrolytic repulsion between polymerized groups. It depends on many factors, the main of which are solvent nature, pH, and an ionic strength of a solution. They determine also molecular-mass (MM) characteristics of the polymers formed [14, 15]. Concentration of the ionized carboxyl groups [COO ] is ˛c, where ˛ is the average dissociation degree, cis the total concentration of carboxyl groups. The curve of the potentiometric titration of the polymeric acids is described by the Henderson-Hasselbach equation: pH D pKa0  m lg Œ˛=.1  ˛/ ;

(2.4)

where pKa0 is the characteristic constant, equal to pH value at ˛ D 0:5; m is the empirical parameter considering influence of electrostatic effect – deviation of system behavior from the law for low molecular weight analogs (for polyelectrolytes m > 1, whereas for monomeric electrolytes m D 1). The ratio of the ionization constant of a polymeric acid to the ionization constant of an analogous monocarboxylic acid is approximately equal to 104 . It is important that the acidity of the carboxyl

16

2 Monomeric and Polymeric Carboxylic Acids

group having two nonionized acidic groups in the neighborhood should be more than the acidity of the carboxyl group with one or two ionized carboxyl groups; the neighboring groups should not be necessarily the same type. It is illustrated by this typical example. Carboxyl group and phenolic fragment influence mutually on their acidic properties because of the intramolecular hydrogen bond formation. It is very important under the action of ribonuclease [16]. Values of pKa0 and m of polyacids depend on the ionic composition of the solution. Thus, pKa0 of carboxyl groups in polymeric acids can be approached to pKa0 of their monomeric analogs with an increase in the ionic strength of the solution; pKa0 and mcan change from 6.17 to 4.60 and from 2.0 to 1.44 [17]. In other words, PAA is a weak polyelectrolyte and pH increasing induces the rise of the number of negative charges. Besides, pKa0 value is essentially influenced by the neighboring groups and by the cross-linking degree of polymer chains, degree of a coil convolution. Characteristic viscosity Œ of the ionized acids is higher than that Œ of the initial acids because of the electrostatic repulsion between ionized groups and extension of polymer chains. It confirms the rod-like form of the short chains of the ionized PAA in water. The chain length and the solvent nature determine the solution concentration at which polymeric coils start to interact. Polymeric coils can be considered as relatively isolated in a good solvent of 1–2 mass% concentration and at the molecular mass of PAA 100,000. PAA macromolecule is unfolded in water to a greater extent than in the organic ™-solvent (dioxane). Its hardness is characterized by the value of Kuhn segment ˚ and can be compared with the flexibility of the noncharged polyequal to 17 A mers. Hydration numbers are equal to 4.9–5.4 at 25ı C and 5.6–6.0 at 35ı C per one PAA unit. PMAA is the nearest chemical analog of PAA but it has a series of anomalous properties in aqueous and alcoholic solutions which are given below. The first property is the more compressed and compact structure stabilized by hydrogen bonds which results in the formation of the cyclic secondary structures. The second one is the hydrophobic interactions of methyl groups (at ˛ < 0:15). Hydrophobic areas stick together aspiring to avoid contacts with water (like surface-active substances (SAS) which consist of polar and nonpolar groups and form micelles at dissolution in water. Nonpolar groups in the micelles are turned inside). Besides, replacement of CH3 : : : CH3 to H3 C : : : H2 O contacts induces additional structuring of water, decreasing the solvent entropy and exceeding the entropy increase of macromolecules at their structure destruction. Addition of organic solvents or ionization induces the cooperative polymer unfolding, and methanol forms the intramolecular hydrogen bonds worse than water. It is also necessary to take into account that PMAA itself shows pH-induced transfers especially in the diluted aqueous solutions at pH equal to 4–6 [18]. The contact structures formed in polyethylacrylic acid molecules are more stable than PMAA structures. It is seen from the comparison of free energy of their conformational transfer (F D 0 for PAA, F D 150 for PMAA and F D 1; 000 cal/mol for PEAA) [19]. Polymonomethylitaconate behaves similarly to PMAA.

2.3 Stereoregular Polyacids

17

Among other polyacids, we shall note poly-4-carboxystyrene, prepared by polymerization and more often by copolymerization of 4-vinylbenzoic acid and polymaleic acid and polymers based on polymaleic anhydride. These polyacids can also be obtained by polymeranalogous reactions. Formation of the copolymer of maleic acid and vinyl alcohol under copolymerization of maleic acid with vinylbutyl ester at 60ı C followed by hydrolysis of the precipitate formed [20] can be given as an example. Copolymerization of unsaturated carboxylic acids is also characterized by a lot of peculiarities. Without going into details we shall note only that kp =kt0:5 value decreases, as a rule, with an increase in their concentration in a monomeric mixture (by the example of copolymerization of styrene with itaconic acid [21]). Lastly, we shall note that many representatives of carboxyl containing polymers are the so-called “smart” polymers having such a feature as the temperature influence on the chains conformation [22]. Thus, the ability of the linear macromolecules of a heat-sensitive poly(N -vinylcaprolactam-co-methacrylic acid) to swell is the function of pH and temperature of a solution and MAA unit’s quantity [23]. 2-Carboxybenzoyl- and 3-carboxyl-2-naphthoyl-substituted derivatives of styrene and 4-vinylbenzoyl-20-benzoate [24] are able to form polychelate complexes due to O,O-functional knots. CH2 CH

n

CH2 CH

C

m

O C

OH

O

The number of such examples can be increased essentially without doubt.

2.3 Stereoregular Polyacids Stereoregular polyacids, especially PAA and PMAA, are the most interesting macroligands. Isotactic PAA is obtained by hydrolysis of polyisopropenylacrylate which, in its turn, is synthesized at 78ı C with use of BrMgC6 H5 as a catalyst [25]. Isotactic PMAA is prepared by methods of polymeranalogous reactions such as hydrolysis of isotactic PMMA (it is the methyl methacrylate polymerization initiated by ethylmagnesiumbromide). Isotacticity degree of this polymer is approximately equal to 90% and molecular weight is equal to 4.8  104 [26]. Radiation polymerization of acrylic acid (initiated by ”-irridation of 60 Co at 78ı C in polar solvents) gives syndiotactic PAA as a product [27]. Another approach is the transformation of syndiotactic anhydride of PAA, obtained by cyclopolymerization, into syndiotactic

18

2 Monomeric and Polymeric Carboxylic Acids

PAA. After that syndiotactic PAA is transformed into isotactic polyanhydride (at heating with Py) and into isotactic PAA [28]. The isotactic polyelectrolyte has a local spiral conformation (degree of helicity is equal to 0.72) because of strong electrostatic repulsion between the fixed charges, while atactic and syndiotactic chains have a flat zigzag conformation. Flexibility of PAA depends on tacticity and nature of a solvent and is in the following order: isotactic > atactic > syndiotactic (in organic mediums) and syndio- > iso > atactic (in water). Iso-PMAA also has a local spiral conformation, and syndio-PMAA has a flat zigzag conformation. The flat zigzag conformation is favorable for the formation of contacts between hydrophobic methyl groups that realized in the nonionized molecules of PMAA in water; formation of CDO: : :.HO hydrogen bonds between the neighboring monomer units are preferable in the organic solvents. Stereoregular structures influence essentially on the nature of the conformational transfer from compact globules to more unfolded solvated chains [29]. Addition of the organic solvents containing nonpolar groups weakens the interaction of methyl groups and promotes the transfer of a macromolecule into more unfolded conformation. It will be shown in the subsequent chapters that stereostructure of polyacids influences essentially on their ability to carboxylate formation.

2.4 Cross-Linked Polyacids Cross-linked polyacids have been produced by the industry of ion-exchange resins for several decades. Usual subacid cation-exchange resins include groups of aliphatic carboxylic acids and contain 3.5–5 mg-eqv of an acid per 1 g of a material. Cationites consisting of the cross-linked PMAA, obtained directly under suspension copolymerization of MMA with a mixture of divinylbenzenes, contain a high number of carboxyl groups (9–10 mg-eqv/g) that correspond to the polymer in which almost 100% of side groups are acidic. The comprehensive description of such synthesis for obtaining the ion-exchange resins is given in numerous guides. Most often, ion exchangers are converted into necessary forms: deprotonated .˛ ! 0/, completely protonated .˛ ! 1/ and partly protonated .1 > ˛ > 0/. For obtaining a deprotonated form, an ion exchanger is treated with 5% aqueous solution of NaOH. A protonated form is prepared under washing out an ion exchanger with a solution of 1 N HNO3 . A partly protonated ion exchanger is formed under the treatment of the protonated and deprotonated forms by the calculated quantity of an alkali or an acid. The most typical examples are saponified copolymer of methylmethacrylate and divinyl benzene and aminated dimethyl ester of iminodiacetic acid and chloromethylated styrene copolymer with divinyl benzene; the other ion exchanger is obtained under condensation of pyridine, polyethylenepolyamine, and epichlorohydrin, modified by a chloracetic acid.

2.5 Graft- and Block-Copolymers with Carboxyl Fragments

19

Micronetwork polymers are divided into microporous (pores size less than 2 nm), mezoporous (pores size is 2–5 nm) and macroporous (pores size more than 5 nm). These polymers provide more successful isolation of carboxyl groups than gel polymers under the same conditions. However, micronetwork polymers also have some drawbacks. For example, concentration of carboxyl groups approx. equal to 1 mmol/L can be considered as the limiting concentration. At the concentration of carboxyl groups, more than 1 mmol/L effects of intrapolar interaction are revealed, and significant amount of anhydride cycles is formed. The nature of these particles and also their pores elasticity allow them to be used both in gaseous- and liquid-phase reactions in aqueous and in nonaqueous mediums, maximal operating temperature for subacid resins being about 125ı C. Significant attention to this class of polymers is given in literature (see, for example, monography [30]) because of wide spread and systematical researches of carboxylic cation exchangers (the saponified copolymer of methylmethacrylate and divinyl benzene type) and ampholytes.

2.5 Graft- and Block-Copolymers with Carboxyl Fragments Copolymers on the basis of graft and block-copolymers satisfy the basic requirements for designing the macroligands of the new type with polymer-bearing functional groups. And though these types of copolymers are used for obtaining composite materials with the improved physicochemical properties and imparting new properties to the modified polymers [31, 32], such “bilayer” materials with carboxyl groups appeared to be an interesting object for the formation of the macromolecular metallocomplexes – metal carboxylates. Graft- and block-copolymers are macroligands and their properties are determined in many respects by the type of a polymer-substrate, by the quantity and length of a graft carboxyl fragment, and by the character of their distribution in a material: whether they localize only on a polymer-substrate surface forming an external covering, form a layer with some diffusive extension into the depth of a polymer to which they are grafted, or distribute evenly in the whole volume of the polymer (Fig. 2.1). The general scheme of obtaining such carboxylcontaining macroligands in a reductive view can be shown as follows [33, 34]. The polymer to which carboxylated fragments are grafted (most often, PE, PP, CEP, PVC, PTFE, PS, cellulose, etc.) is subjected to mechanical, chemical (induced initiation, ozonolysis, oxidationreduction systems, etc.), and radiochemical (”-irradiation of 60 Co; accelerated electrons; low-temperature gas-discharge plasma of low pressure; plasma of glow low-, high-, and ultrahigh-discharge; corona discharge; UV-irradiation, etc.) initiation in the presence of a grafted acid (or by the post-effect). Such initiation results in the formation of active centers (free radicals, ion-radicals, ions on which graft polymerization takes place) on the surface or in the near-surface layer of the initial polymer. Graft polymerization of unsaturated acids can be homophase (graft is in a solution of polymers) or heterophase (suspension or gaseous-phase). The last type of processes can be presented by Scheme 2.1.

20

2 Monomeric and Polymeric Carboxylic Acids

a

b

c L

L L

L

L L

L

L

L

L

L

L

L L

L

L

L

L

L

L

L

L

L

L

L L L

L L

L

L L L

L

L L

L L

50 nm

d

e

f

L

L

L L

L

L

L

L

L L L

L

50-80 mm

L

L

L L L

200 mm

L

L L

L L

L

L L

L

L L

L

L

L L

L

L L

L

L L

L

L

L

L

L L

L

L

L

L

L

L

L L L L L L L L L L L L L L L L L L L L L L

L L

L L

L

L

L L

L

L

L

L

L

L

L

L

100 mm

L

Fig. 2.1 Schematic diagram of the distribution of functional groups in polymers of various types. The type of polymer: (a) a linearic or branched polymer, (b) a slightly cross-linked (swelled) polymer, (c) a highly cross-linked polymer (macroporous) polymer, (d) a polymer with a grafted functional layer, (e) a polymer with microencapsulated particles, (f) the material of hybrid type m(CH2 =

Initiation Polymer surface

CH) COOH

(CH2 – CH-)m COOH

Scheme 2.1 Graft polymerization of unsaturated acids on the surface of polymer

The most effective technique of graft polymerization of unsaturated acids is the gaseous-phase graft polymerization of acrylic and methacrylic acids on the surface of HDPE under plasmochemical treatment. For example, formation of a monolayer from carboxyl groups on a polymer-powder surface (Ssp D 10 m2 /g) is equivalent to a graft of 1% mass of acrylic acid. As a rule, thickness of a grafted layer does not exceed 10–30 nm. For acrylic or methacrylic acids graft polymerization on the surface of HDPE not only penetrating radiation, but also low-temperature HF-gas-discharge plasma can be used (Fig. 2.2) A contribution to the total action of such plasma is introduced by electrons, ions, radicals, excited particles, and electromagnetic radiation; elementary act of monomer insertion into polymer structure is catalyzed by electron-ion bombardment of this surface (Table 2.2) [35].

2.5 Graft- and Block-Copolymers with Carboxyl Fragments Fig. 2.2 Scheme of the setup for high frequency grafting of monomers into a surface of polymer (powder): 1 – reservoir for polymer (powder), 2 – vacuum seal, 3 – quartz discharge tube, 4 – high frequency generator, 5 – reactor with stirring, 6 – reservoir for a grafting monomer

21 1 2 3 4 5

Vacuum

6

Table 2.2 Graft polymerization of acrylic and methacrylic acids on HDPE initiated by high frequency dischargea Degree of grafting Polymer substrate Grafting monomer wt% 104 mol/g PE CH2 DCHCOOH 2:0 2:8 PE The same 9:1 12:6 PE The same 13:5 18:8 PE CH2 DC.CH3 /COOH 2:7 3:7 PE The same 7:0 9:7 PE The same 12:0 16:7 CH2 DCHCH2 OH 2:0: 2:76 PEb a Power 1 W/cm3 , residence time in discharge is 1 s, the temperature for monomer and substrate 20ı C b To compare the data of grafting of allylic alcohol are given

From the peculiarities of gaseous-phase graft polymerization of AA and MAA to a powdered HDPE, we shall note high values of radiochemical yield under initiation by ”-irradiation (GM more than 2,000 molecules/100 eV of absorbed energy at 20ı C) and also high effectiveness of high frequency grafting (Table 2.2): achievement of 5–10 mass% is not difficult experimentally because of sufficiently high vapor pressure of these monomers at graft temperature. A characteristic feature of graft polymerization of AA on the oriented PE-films is a formation of stereoregular (isotactic) structures in graft fragments [36]. As the thickness of a graft layer increases, ordering in a graft PAA connected with oriented character of monomeric molecules in adsorbed layer, has become apparently worse. The most important factors determining stereoregularity of graft copolymers is the structure of those areas on which graft occurs: pore size, supramolecular structure, etc. By an example of gaseous-phase graft copolymerization of vinylidene chloride and acrylic acid grafted on stretched polyamide fibers of nylon-66, the opportunity of the matrix synthesis of macromolecules with monomers distribution specified by a substrate was found [37]. The effect of matrix copolymerization is caused by a selective sorption of acrylic acid molecules on peptide groups of a fiber. It results in the formation

22

2 Monomeric and Polymeric Carboxylic Acids

of a sorption layer on a polymer-substrate; composition of this layer reflects an alternation of structural elements of polymer-substrate. This order also remains in macromolecules of a copolymer forming in the sorption layer. In case of a graft of acrylic acid to a powdered HDPE, stereoregular structures were not revealed [38]. Graft polymers are copolymers with the peculiarity in a reactive groups’ location; almost all reactive groups are on the surface and are accessible for the reagents (including metal salts) at suspension technique of binding together. By a graft of acrylic and methacrylic acids ion-exchange membranes are obtained (see, for example, [39]).

2.6 Natural Polyacids 2.6.1 Polysaccharides In the last years, special attention has been paid to modification of natural polymers properties including imparting a functional carboxyl groups to them. Especially it has been referred to the most widespread natural polymer, cellulose, which forms the basis of cell walls of the highest plants. Sufficient mechanical strength, good rheological properties, and possibility of application in fibers, filters, membranes, powders, or woven materials expand the areas of use of a macroligand which connects ions of various metals. Chemical properties of cellulose are determined by presence of one primary and two secondary OH-groups in each elementary unit and also by acetal (glucoside) bonds between elementary units. High reactivity of cellulose allows to carry out numerous chemical transformations with the purpose of obtaining various macroligands, including carboxyl groups (see, for example [40])), on the basis of cellulose. Among other polysaccharides suitable for these purposes, we shall note starch, dextrans, chitin, their dezacetylated derivatives, chitozane and pectins, and also alginic acids. Alginic acids are polysaccharides of algae which consist of D -mannuric acid residues [41]. Carboxymethyl cellulose (CMC) is the most often used polymer for obtaining the metal containing polymers among natural polymers. CMC is a homogeneous powdery fine-dispersed polymer containing only carboxyl groups (up to 5  103 mol/g) which can participate in ionic binding at moderate pH (up to 10).

2.6.2 Humic Acids Humic and fulvic acids are the most important natural macroligands. These acids are the main organic producers of biogeocomplex, they are a mixture of the same type of macromolecules of variable composition (Fig. 2.3) [42].

2.6 Natural Polyacids

23

Al2O3, Fe2O3

(CH3)2CHCH2CHNH2COOH

CaO

(C6H10O5)2

SiO2, P2O5

-(COOH)n, -(OH)n

Mineral components

-(NH2)n, -(CH2)nO OH

Hydrolised components

H

H H

H H H

C

C C

O C C C H CH2

R1 C

O

N H

CH2

H R3 N C H

Core segment

C OH

C O O

CH2

CH2

C O HO C

(CH2)n-

H

R2 H N

C

R C

H

O

H

O H

N

C

H

C

OCH3 -O-C6H11O5 -O-C6H9O6

O

O

N

C

R H

O

OOO

R C

O C O P

C O Ca O P

OO-

Complexes, sorption

Lateral segment

Fig. 2.3 Formula of a structural unit of humic acid by D.S. Orlov (cit. on [42])

They are the most widespread complex substances determining migration and fastening of metal ions in soils. Distribution of metal ions in various physicochemical phases renders a determining influence on their mobility and bioaccumulation. In this connection, the charged macromolecular ligands such as humic acids play a key role in localization and accumulation of metal ions in natural objects. Metal-binding properties of humic and fulvic acids have been intensively investigated in the last years [42–44].

24

2 Monomeric and Polymeric Carboxylic Acids

These acids are obligatory and are the main unit in soil formation process and they form a specific bioelements “depot,” which regulates a nutrition pattern of plants depending on environmental conditions. The problem of complexation of heavy metals with these macroligands is important for the binding of their mobile forms. At last, complexation of metal ions with humic acids plays an important role in the processes of migration and delivery of biogenic metals into biological systems, in the ore formation processes, and for the solution of environmental problems. Macromolecules of humic acids contain various functional groups differed by an acidity degree (see Fig. 2.3), each group is the potential center of metal binding. Many kinetic regularities of humic acids complexation are similar to their synthetic analogs: centers formed under ionization of weaker acidic groups enter the reaction with an increase in pH; an increase in ionic strength induces an increment of their acidic properties, however much, on the contrary, it influences the stability of metallocomplexes formed. Thus, carboxyl groups are widely spread in numerous synthetic and natural objects. As it will be shown below, their metal-binding properties depend on many factors and, first of all, on composition and structure of a macroligand and its prehistory. Many of the necessary properties can be operated at the designing stage of macroligand and also by optimization of methods of binding of metal ions. We do not analyze in this chapter substituted mono- and polycarboxylic acids capable of participating in condensation processes, for example, m-carboranedicarboxylic acid forming oligomerous salts with divalent metals, etc.

References 1. L.S. Luskin, Acrylic acid, methacrylic acid and the related esters, in: Vinyl and diene monomers Part 1, ed. by E.C. Leonard (Wiley, New York, 1971), pp. 105–262 2. N.A. Plate, E.V. Slivinskii, The Bases of Chemistry and Technology of Monomers (Nauka, Moscow, 2002) 3. Handbook of Chemistry and Physics, 76th edn., ed. by D.R. Lide (CRC Press, Boca Raton, New York, London, Tokyo 1995) 4. E.A. Bekturov, V.A. Myagchenkov, V.F. Kurenkov, Polymers and Copolymers of Styrene Sulfonic Acid (Nauka, Alma-Ata, 1989) 5. C. Ohe, H. Ando, N. Sato, Y. Urai, M. Yamamoto, K. Itoh, J. Phys. Chem. B 103, 435 (1999) 6. D. Bouyssi, J. Gore, G. Balme, Tetrahedron Lett. 33, 2811 (1992) 7. V.V. Vintonyak, M.E. Maier, Org. Lett. 9, 655 (2007) 8. V.A. Kabanov, D.A. Topchiev, Polymerization of Ionizing Monomers (Nauka, Moscow, 1975) 9. Coulombic Interactions in Macromolecular Systems. ACS Symposium, Series 302, ed. by A. Eisenberg, F.E. Bailey (American Chemical Society, Washington, DC, 1986) 10. J. Choi, M.F. Rubner, Macromolecules 38, 116 (2005) 11. M. Law, J. Goldberg, P.D. Yang, Annu. Rev. Mater. Res. 34, 83 (2004) 12. A.F. Nikolaev, G.I. Okhrimenko, Water Soluble Polymers (Khimiya, Leningrad, 1979) 13. S. Beuermann, M. Buback, P. Hesse, R.A. Hutchinson, S. Kukuckova, I. Lacik, Macromolecules 41, 3513 (2008) 14. C. De Stefano, A. Gianguzza, D. Piazzese, S. Sammartano, J.Chem. Eng. Data 45, 876 (2000) 15. C. De Stefano, A. Gianguzza, D. Piazzese, S. Sammartano, React. Funct. Polym. 55, 9 (2003) 16. Polymer-Supported Reactions in Organic Synthesis, ed. by P. Hodge, D.C. Sherrington (Wiley, Chichester, 1980)

References

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17. G. Morawets, Macromolecules in Solutions (Mir, Moscow, 1970) 18. L. Ruiz-Perez, A. Pryke, M. Sommer, G. Battaglia, I. Soutar, L. Swanson, M. Geoghegan, Macromolecules 41, 2203 (2008) 19. F. Fichtner, H. Schonert, Colloid Polym. Sci. 255, 230 (1977) 20. P.L. Dublin, U.P. Stauss, J. Phys. Chem. 74, 2842 (1970) 21. M. Abdollahi, A.R. Mahdavian, H.R. Buanzadeh, J. Macromol. Sci. Part A Pure Appl. Chem. 43, 1597 (2006) 22. Handbook of Polyelectrolytes and Their Applications, ed. by S.K. Tripathy, J. Kumar, H.S. Nalwa (American Scientific, Stevenson Ranch, CA, 2002) 23. E.E. Makhaeva, H. Tenhu, A.R. Khokhlov, Macromolecules 35, 1870 (2002) 24. Y. Ueba, K.J. Zhu, E. Banks, Y. Okamoto, J. Polym. Sci. Polym. Chem. Ed. 20, 1271 (1982) 25. P. Monjol, C. R. Acad. Sci. 265, 1426 (1967); 266, 81 (1968) 26. E.C. Kolawole, M.A. Bello, Eur. Polym. J. 16, 325 (1980) 27. A. Chapiro, T. Sommerlatte, Eur. Polym. J. 5, 725 (1969) 28. J.P. Jones, J. Polym. Sci. 33(126), 15 (1958) 29. I.B. Lando, J.L. Koing, I. Semen, J. Macromol. Sci. B7, 319 (1973) 30. K.M. Saldadze, V.D. Kopylova-Valova, Complexing Ionites (Complexites) (Khimiya, Moscow 1980) 31. G. Batterd, D.U. Treger, Properties of Grafted and Block-Copolymers (Khimiya, Leningrad, 1970) 32. A. Noshei, J. McGrat, Block-copolymers (Mir, Moscow 1980) 33. A.D. Pomogailo, D.A. Kritskaya, A.P. Lisitskaya, A.N. Ponomarev, F.S. Dyachkovskii, Dokl. Akad. Nauk SSSR 232, 391 (1977) 34. D.A. Kritskaya, A.N. Ponomarev, A.D. Pomogailo, F.S. Dyachkovskii, J. Polym. Sci. Polym. Symp. 68, 23 (1980) 35. Plasmachemical Reactions and Processes, ed. by L.S. Polak (Nauka, Moscow, 1977) 36. B.L. Tsetlin, A.V. Vlasov, I.Yu. Babkin, in Radiation chemistry of polymers (Nauka, Moscow, 1973) 37. B.L. Tsetlin, V.N. Golubev, Dokl. Akad. Nauk SSSR 201, 881 (1971) 38. A.D. Pomogailo, Thesis PhD, Doct. Chem. (ICP AN SSSR, 1981) 39. E.A. Hegazi, N.B. Al-Assy, A.M. Rabie, I. Ishigaki, J. Okamoto, J. Polym. Sci. Polym. Chem. Ed. 22, 597 (1984) 40. B.N. Stepanenko, Chemistry and Biochemistry of Polysaccharides (Vysschaya shkola, Moscow, 1978) 41. A.A. Muzzarelli, Natural Chelating Polymers (Pergamon Press, Oxford, 1973) 42. Sh. Jorobekova, Macrolig and Properties of Humic Acids (Ilim, Frunze, 1987) 43. J. Butfle, Complexation Reactions in Aquatic Systems (Ellis Horwood Ltd, Chichester, 1989) 44. K. Kydralieva, Sh. Jorobekova, Metal Ions in Enzyme-Inhibitory Systems (Ilim, Bishkek, 2002)

Chapter 3

Synthesis of Unsaturated Carboxylic Acid Salts

Prospective metal carboxylates of this type as subjects of various investigations, especially as potential monomers for the preparation of metallopolymers, attract the attention of a large number of researchers to the development of methods for their synthesis. These efforts in particular are aimed at the design of geometrical, electronic, and other characteristics of the compounds to achieve the specified properties. The major distinctions between methods for the synthesis of unsaturated carboxylic acids salts are in the type of the precursor metal compound employed and the corresponding method for introduction of a ligand.

3.1 Reaction of Unsaturated Carboxylic Acids with Metal Hydroxides, Oxides, and Carbonates Neutralization reaction is widely used for the synthesis of unsaturated carboxylic acids salts. The essence of the method is dissolution of a metal oxide, hydroxide, or carbonate in a solution (commonly aqueous or aqueous-alcoholic) of the corresponding acid. The target product is isolated by evaporation of the resulting solution until crystallization starts or by filtration of the precipitate if the metal carboxylate is insoluble or limitedly soluble in water [1–8]. Stoichiometric amounts of the starting materials or slight excess of an acid is adequate for use in the preparation of salts of the group I and II s-elements [9–13]. In view of the high polymerizing ability of alkali metal (meth)acrylates, the reaction is usually carried out in dilute solutions at reduced temperature, and often in the presence of special compounds that inhibit polymerization [14–16]. In many cases, if the salts formed are readily hydrolyzed, an excess of the acid has to be used or water has to be removed by either azeotrope distillation or by binding it with another substance. Thus, anhydrous calcium acrylate and methacrylate were synthesized at 40–100ıC in hydrocarbon solvents followed by azeotrope distillation and drying [17]. Also, there have been patent reports on the preparation of (meth)acrylates of lanthanoids [18] or Zn [19] in two-phase aqueous-organic or organic media. Acrylates and methacrylates of d -elements are synthesized according A.D. Pomogailo et al., Macromolecular Metal Carboxylates and Their Nanocomposites, Springer Series in Materials Science 138, DOI 10.1007/978-3-642-10574-6 3, c Springer-Verlag Berlin Heidelberg 2010 

27

28

3 Synthesis of Unsaturated Carboxylic Acid Salts

to this method in alcoholic or hydrocarbon (benzene, toluene) suspensions [20–25]. The use of nonaqueous media decreases the probability of formation of basic salts and favors the preparation of the purer reaction products in high yields (higher than 95% in the case of Zn(II), Co(II), Ni(II), and Cu(II) with double bond content higher than 94%). In the case of triple charged metal cations, for example, Fe(III), Cr(III), and so on, depending on the reaction conditions and the reactant nature, either normal salts [26–29] or trinuclear oxo-carboxylates are formed (see Sect. 2.3). Metal dicarboxylates appear to behave similarly. Thus, maleic acid upon reaction with zinc oxide in an aqueous medium reacts as monobasic acid giving rise to a mixed complex, ZnH(OOCCHDCHCOO)(OH)H2 O, whereas under the same conditions in methanol it behaves as a dibasic one [13]. The complexation of metal ions with dicarboxylic acids was studied by potentiometric titration [30–34] and spectrophotometric analysis [32, 35]. For example, it was found that 1:1 complexes are formed at pH 4.9–5.2 in the copper(II) – maleic acid system [32] while in the thallium(III) – fumaric (or maleic) acid system at pH 2.0–3.5 [30]. Neutral M.C6 H4 O4 /C and protonated MH.C5 H4 O4 /2C complexes coexist in the 4f -element metal ion – itaconic acid system at pH 3–4 [33], which was confirmed by preparative isolation of the neutral itaconates M2 .C5 H4 O4 /3 nH2 O .n D 3  6/ and the protonated complexes MH.C5 H4 O4 /2  nH2 O .n D 1; 2/ [36–38]. Stability constants for neutral complexes are higher than lg K values for the protonated complexes, thus correlating with the disassociation constants for the acid .K1 D .1:62 ˙ 0:19/  104 ; K2 D .4:39 ˙ 0:18/  106 / (Table 3.1). An interesting modification of this method consists of preparation of the metal hydroxide in situ [39], if one of the products is poorly soluble, the reaction proceeds to completion [40, 41]. Thus, the reaction of beryllium sulfate, maleic acid, and barium hydroxide in a 1:2:1 molar ratio in water results in immediate precipitation of barium sulfate. BeSO4 C Ba .OH/2 C C4 H4 O4 ! BaSO4 C Be .C4 H2 O4 / 2H2 O This is an example of the successful preparation of beryllium(II) carboxylate complex in an acidic medium. Monocarboxylic ligands are involved in the beryllium coordination sphere only in alkaline or neutral media to give oxo-carboxylates (RCOO)6 (Be4 O) [42]. Table 3.1 Logarithms of the constants of stability (lg K) of itaconate complexes for the compositions of MLC and MHL2C Ion of 4f-element lg KML C lg KMHL 2C Ion of 4f-element lg KML C lg KMHL 2C Lanthanum 2.52 ˙ 0.20 1.56 ˙ 0.20 Terbium 3.05 ˙ 0.22 1.87 ˙ 0.12 Cerium 2.78 ˙ 0.24 1.78 ˙ 0.13 Dysprosium 2.90 ˙ 0.25 1.82 ˙ 0.14 Praseodymium 3.02 ˙ 0.19 1.91 ˙ 0.22 Holmium 2.66 ˙ 0.28 1.64 ˙ 0.17 Neodymium 2.95 ˙ 0.24 1.98 ˙ 0.12 Erbium 3.18 ˙ 0.05 1.77 ˙ 0.20 Samarium 3.10 ˙ 0.23 1.96 ˙ 0.11 Thulium 2.45 ˙ 0.17 1.57 ˙ 0.16 Europium 3.33 ˙ 0.18 1.98 ˙ 0.09 Ytterbium 2.62 ˙ 0.18 1.61 ˙ 0.17 Gadolinium 3.06 ˙ 0.38 1.91 ˙ 0.16 Lutecium 2.80 ˙ 0.24 1.65 ˙ 0.14

3.2 Reactions of Acetates and Other Salts with Unsaturated Carboxylic Acids

29

3.2 Reactions of Acetates and Other Salts with Unsaturated Carboxylic Acids This method is well known for the synthesis of saturated metal carboxylates [43]. Availability of the starting materials, volatility of the acids evolved, and the fact that carboxylic acids could be taken in stoichiometric amounts are advantages of the method. The drawbacks include frequent contamination of the products by traces of acetic acid and, in the case of salts of strong acids, side reactions, in particular polymerization. To prevent undesired processes, proton acceptors, such as calcium or sodium carbonates [44–46], are usually employed. Alternatively, the reaction is carried out under inert atmosphere and moderate temperature [47]. The reaction under discussion appeared to be very efficient for the synthesis of oxopolynuclear Mn12 (meth)acrylates. These important substances exhibit molecular magnet properties and are monomeric precursors for the polymeric analogs [48–50]. Equilibrium of the reaction is shifted by vacuum removal of the formed CH3 COOH, and the repeated reaction with excess of unsaturated acid yields full substitution by (meth)acrylate ligands. ŒMn12 O12 .CH3 COO/16 .H2 O/4  C 16CH2 C.CH3 /COOH $ ŒMn12 O12 .CH2 C.CH3 /COO/16 .H2 O/4  C 16CH3 COOH There are no principal limitations for the synthesis of heterometallic polynuclear complexes like [Mn10 Fe2 O12 (CH2 C(CH3 )COO)16 (H2 O)4 ] according to this scheme [50]. Hydrothermal syntheses, which are of interest for the design of new materials with unusual structures, have been intensely developed in recent years [51]. These approaches are applicable mainly for dicarboxylate ligands that have low reactivity toward homopolymerization. Thus, the reaction of copper(II) acetate monohydrate with fumaric acid under mild conditions gives air-stable copper(I) fumarate with high density .d D 3:24 g cm3 / [52]. The essence of the method is as follows: starting materials are placed in a autoclave and kept there for 1.5 days at 150ı C. The crystals formed are isolated, washed with water, and dried. This method typically gives rise to more condensed products with two- or three-dimensional spatial structure (Fig. 3.1) [53]. For example, lanthanoid fumarate complexes, [Ln2 (OOCCHDCHCOO)3 (H2 O)4 ]3H2O (Ln D Sm(III) [54] and Eu(III) [55]), contain less H2 O molecules than compounds [Ln2 (OOCCHDCHCOO)3 (H2 O)4 ]8H2 O [56], obtained in solution at room temperature, while both have the same Ln/fumarate ratio (2:3). Tetranuclear Zn(II) fumarate complex, [Zn4 (OH)2 ((OOCCHDCHCOO)3 (4; 40 bipy)2 ] [57], that was synthesized in an analogous way exhibit rather high thermal stability (up to 380ı C). It should be noted that products of the hydrothermal synthesis are often studied by supramolecular chemistry due to a combination of complex structures and unusual properties.

30

3 Synthesis of Unsaturated Carboxylic Acid Salts

Fig. 3.1 Changing of the phases of cobalt succinate, from low temperature (60ı C, far left) to high temperature (250ı C, far right)

Metal acetylacetonates are often utilized as starting materials. This method is especially efficient for the preparation of complexes with different ligands. Mixed lanthanoid complexes (Nd3C , Eu3C , Gd3C , and Yb3C ) with benzoylacetone [58] or acetylacetone [59, 60] and unsaturated acids (acrylic, methacrylic, fumaric, and maleic) were synthesized by reaction of the acid with benzoylacetonate or acetylacetonate of an f -element in dioxane. Interaction of terbium tris(acetylacetonate) with maleic acid [61] is exothermic over the concentration range of (3–6)  102 and 0.5 M for the metal salt and the acid, respectively (H D  8:2 ˙ 0:4 kJ=mol; G D 17:0 ˙ 0:8 kJ=mol; S D 29:4˙ 1:6 J=K mol; lgK D 2:97 ˙ 0:06, dioxane, 298 K). The change in the enthalpy can be attributed to a number of factors, including the formation of a stronger bond of the ligand with the Tb3C ion upon substitution of the acetylacetonate for the maleate anion. Examples of such stabilizing effect of the maleate ligand are rather frequent. The kinetic studies [62] for the interaction of Pd.H2 O/4 2C with maleic acid revealed that the reaction proceeds with a stoichiometric ratio 1:1 according to a complex mechanism via a series of sequential and parallel reactions of the A $ B ! C type. In the first stage, an intermediate product, cation [Pd(H2 O)3 OOCCHDCHCOOH]C , is formed with stability constant K D 205 ˙ 40 M1 . Two parallel reversible reactions with the participation of maleic acid and hydromaleate anion, respectively, give rise to the formation of the cation. In the next step, a slow intramolecular cyclization (rate constant 0:8 ˙ 0:1 s1 ) or a nucleophilic attack of the intermediate B by the carboxylate or the acid molecule leads to a creation of the olefin complex [Pd(H2 O)2 OOCCHDCHCOOH.

3.3 Ligand Exchange Reactions These reactions are the most common for synthesis of the complexes and are used for the exchange of inner-sphere as well as outer-sphere ligands. Not only substitution of solvent molecules but also substitution of other ligands by the desired

3.3 Ligand Exchange Reactions

31

ligand can take place upon this transformation. The method is often employed for the exchange of group I s-elements for f -elements and is particularly efficient when no stable soluble metal salts are available.

3.3.1 With Metal Halides Mono- and disubstituted unsaturated acyl derivatives of bis(cyclopentadienyl) titanium were first obtained by A.N. Nesmeyanov et al. [63] by the reaction of bis(cyclopentadienyl)titanium dichloride with salts of methacrylic acid. Monomeric compounds of this type are readily soluble in benzene, acetone, DMF, pyridine, partially soluble in MMA [64]. The general scheme for their synthesis could be presented as follows (Scheme 3.1). Similar routes can be used for the preparation of titanium dicarboxylate derivatives [65–69]. The main drawback for the method is formation of large amounts of finely dispersed chlorides that are difficult to remove. A promising way to solve this problem is to carry out synthesis of the alkyl orthotitanates Ti.OR/4n .OR0 /n , in a system of two immiscible solvents, one of which dissolves the resulting salt and the other dissolves the alkyl orthotitanate [70]. There are known examples of using liquid ammonia as the solvent, as it dissolves ammonium chloride but does not dissolve alkyl orthotitanate. This method ensures good yields but requires elevated pressure in each stage of the process.

H2C

C C O Ti

C2H5O

O C C CH2

O

H3C

O

O Ti

O C C O

CH3

CH2

CH3

= H2

2C

K

H O

Ti

K

O

=

C

C

H

H C COOH

HOOC C

Cl

C

C COOH

Cl

O

C O

=

C C O

Ti

O

O

K

O

O

C H

)C

3

2 =

C

CH

H C

C(

O

O C C

C 2

H

O

Ti

O

C C

CH

Ti

H HOOC C

O

O

C

H O

C

H2

O

C

Ti

(C

C

)C H3

2H 5O

C C(

O

O

O

)C

H3

Cl

Ti

O C CH O

CH2

H2C C C O

Ti

Cl

H 3C O

Scheme 3.1 The general scheme of synthesis of unsaturated acid derivatives of bis(cyclopentadienyl) titanium (IV)

32

3 Synthesis of Unsaturated Carboxylic Acid Salts

3.3.2 With Metal Alkoxides Metal carboxylates are often prepared by the reactions of metal alkoxides with organic acids or acid anhydrides. Titanates Ti.OR/3 OCOC.CH3 / D CH2 (R D t-butyl, t-pentyl, t-ethylhexyl) were obtained by nucleophilic substitution from titanium(IV) alkoxide and methacrylic acid taken in a stoichiometric ratio [71]. In a similar way dibutoxybis(butylmaleinate)titanium was synthesized upon treatment of Ti(OBu)4 with maleic anhydride in a 1:2 molar ratio [72]. If the released alcohol has higher boiling temperature than the acid, or if the precursor metal tetraalkoxide is unavailable, the reaction is carried out in two steps: CH3

CH3 CH2 = C

CH2 = C

+ Ti(OR)4 COOH

+ ROH COOTi(OR)3

CH3 CH2 = C

+ 3 HOCH2CH(CH2CH3) – (CH2)3 – CH3 COOTi(OR)3 CH3

CH2 = C

+ 3 ROH COOTi(OCH2CH(CH2CH3) – (CH2)3 – CH3)3

The released alcohol is removed by continuous evaporation under reduced pressure. Note that these syntheses were carried out without a solvent. Very high yields (up to 97%) of the target products attest to a low contribution of side processes, for example, polymerization transformations, that are possible under these conditions. In general, employment of acid trans-esterification requires special care as a few other side reactions, such as esterification of the precursor acid with the released alcohol, hydrolysis of the titanium ester by the latent water, formation of titanium oxide compounds like TiOx (OOCR)42x (x D 0:5 or 1) can take place [73]. To avoid these complications utilization of the acid anhydrides is more efficient for the introduction of unsaturated acyl residues into metal alkoxides: Ti(OR)4 + n(CH2 = C–CO)2O CH3 (CH2 = C–COO)n Ti(OR)4 – n + nCH2 = C–COOR CH3

n = 1; 2

CH3

Employment of metal alkoxides as a precursor is particularly important when coordination of a water molecule need to be avoided, as it is known to cause luminescence quenching for the compounds of f -elements. Therefore, an alternative method for the synthesis of anhydrous europium(III) methacrylate from europium triisopropoxide in organic medium has been proposed, which differs from standard methods for the preparation of f -element complexes in aqueous solutions [74]. This method is also convenient for the preparation of hetero-ligand complexes (Scheme 3.2) [75].

3.3 Ligand Exchange Reactions

Eu2O3

HCl / NH4Cl

EuCl3

33 Na(OiPr) / iPrOH / benzene

CH3 Eu(OCHCH3)3

CH

H2C

R1 = CH3, R2 = CH3

C β−diketone / AA / iPrOH / benzene

O R1 C HC

O Eu

O O

O O C

C R2

R2

R1

R1 = CH3, R2 = R1 =

C CH

, R2 =

R1 = CF3, R2 =

S

Scheme 3.2 Synthesis of the monomeric hetero-ligand europium(III) complex

According to this scheme, in the first stage, a reactive isopropoxide of europium(III) is formed. Its reaction with acrylate and “-diketonate ligands in the mixture of organic solvents results in the formation of monomeric europium(III) complex without coordinated water molecules.

3.3.3 Other Exchange Reactions Liquid crystal monomeric complexes of Mg(II) and Zn(II) were prepared by interaction of chlorides or sulfates of the metals with sodium salt of the corresponding unsaturated acid in water [76]. MgCl2  6H2 O C 2NaO2 CR ! Mg.O2 CR/2  xH2 O C 2NaCl C .6  x/H2 O .R D .CH2 /11 OCOCH D CH2 I C6 H3 .O.CH2 /11 OCOCH D CH2 /2 I x D 0; 2/ Target products are separated by filtration or by extraction with chloroform or acetone. Mn(II) (meth)acrylate [77] and dicarboxylate [78] complexes were synthesized according to the same scheme using methanol as a solvent. Employment of ammonia in exchange reaction of this type often promotes formation of soluble carboxylate complexes, in particular this is common for copper(II) [79]. In some cases precursor carboxylate of an s-metal is prepared in situ. Complexes of europium and terbium with cinnamic acid and other ligands of formula Ln.OOCCHD CHC6 H5 /nDxH2 O, where Ln D Eu(III), Tb(III), D D 1,10-phenantroline (phen), 2; 20 -dipyridine (2; 20 -dipy), benzotriazol (bta) .n D 2; x D 0/, triphenylphosphinoxide (tphpho) .n D 1; x D 2/ were synthesized according to this route [80]. Complexes of varied composition can be synthesized depending on the reaction conditions. For example, binuclear copper hydromaleinate solvated with ethanol molecules is formed when exchange reaction is conducted in organic medium, whereas the presence of water fosters transformation of the complex into a mononuclear hydrate, Cu(OCOCHDCHCOOH)2 4H2 O [46]. Tendency of copper(II) ions toward hydration diminishes upon increase of reaction temperature.

34

3 Synthesis of Unsaturated Carboxylic Acid Salts

Binuclear cromium(II) acrylate was synthesized by treatment of cromium(II) chloride with sodium acrylate under inert atmosphere [81]. This compound appeared to be extremely sensible to oxygen and self-ignited when exposed to air.

3.3.4 Synthesis of Bimetallic Compounds One of the variations of exchange reactions is a combined synthesis of heterometallic carboxylates. In a typical procedure, to a solution of metal M1 carboxylate a salt of another metal M2 is added followed, if necessary, by a carboxylate and an accompanying ligand. The target complex is isolated by filtration or crystallization. Heteronuclear carboxylates containing ions of d - and f -elements were synthesized using this approach [82–84]. The bimetallic maleinate Cux Zn1x C4 H2 O4 2H2 O .x D 0:06/ was isolated from solutions of copper and zinc maleinates upon slow evaporation at 60ı C.45 In a similar way heterometallic trinuclear crotonates were synthesized with yields higher than 80% [85].

3.4 Sol–Gel Reactions Sol–gel synthesis techniques are promising for the preparation of metal carboxylates of the type under consideration. These techniques are based on hydrolysis of alkoxides M(OR)4 in an organic medium followed by condensation of the resulted products leading to gel formation [86, 87]. The presence of carboxylic group in the alkoxide molecule allows regulating its reactivity due to the formation of latent water to conduct controlled hydrolysis and growth of the carboxylate-substituted metal oxo clusters. There are numerous examples of this type of reaction. Thus, the reaction of zirconium(IV) or hafnium(IV) alkoxide with excess methacrylic acid in propyl alcohol affords polynuclear oxo-carboxylate [88,89]. Note that in an attempt to replace the chelating methacrylate groups by acetylacetonate (AcAc) groups, the zirconium oxo cluster Zr4 O2 .OOCC.CH3 /DCH2 /12 prepared by the method described above were converted into a mononuclear complex [90]: Zr4 O2 .OOCC.CH3 / D CH2 /12 C 8AcAc  H ! 4Zr.AcAc/2 .OOCC.CH3 / D CH2 /2 C 4CH2 D C.CH3 /COOH C 2H2 O This example attests that the subsequent modification of the resulting oxo cluster molecule seems to be impeded. It was shown that the clusters Zr6 O4 (OH)4 (OOCR)12 and [Zr6 O4 (OH)4 (OOCR)12 ]12 (RCOO – methacrylate [91] or acrylate [92]) do not interconvert [93], although they are structurally related and rather labile in solution due to carboxylate ligand exchange. Using exchange reactions, it is possible to replace all or some methacrylate ligands in Zr6 O4 (OH)4 (OOCR)12 by other carboxylate ligands, for example, propionate or isobutyrate [94]. The complex Zr6 O4 .OH/4 .O2 CC.CH3 /DCH2 /8 .O2 CCH.CH3 /2 /4 (BuOH) was also synthesized

3.5 Other Reactions

35

directly by the reaction of Zr(OBu-n)4 with a mixture of methacrylic and isobutyric acid. There have been reported examples of metal alkoxides modification by unsaturated ligands, such as itaconic acid anhydride [95, 96], acetoacetoxyethyl methacrylate [96, 97], p-vinylbenzoic, and p-vinylphenylacetic acids [96]. The key factors used to efficiently control the composition and the size of the formed oxo clusters include the molar ratio of an organic acid and a metal alkoxide and the nature of the metal alkoxide. For example, the reaction of tin(IV) isopropoxide with different acids including methacrylic at an equimolar reactant ratio results in the formation of a dimer with six-coordinated tin atoms, while at the reactant ratio between 1.4 and 2 the reaction gave rise to compounds [Sn(2 -Oi Pr)(Oi Pr)(O2CR)2 ]2 (R D (Me)CDCH2 , C6 H5 , CH3 ) [98]. However, no polynuclear oxo complexes were obtained in the reaction even when the molar ratio of RCOOH to Sn(OiPr)4 was greater than 2, unidentified polymers being the final products of the reaction. It was found that the highest molar ratio of an acid to titanium(IV) alkoxide leading to formation of the oxo cluster Ti6 O4 (OEt)8 (OOC(Me)DCH2 /8 is 1.33 [99]. At higher ratios polymeric or oligomeric structures are formed, as, for example, in the case of Ti9 O8 (OPr)4 (OOC(Me)DCH2 /16 [100]. The attempts to prepare yttrium oxo-carboxylate complexes failed. Only anhydrous yttrium methacrylate Y(OOCC(Me)DCH2 /3 was isolated after esterification and characterized by X-ray crystallography [101]. However, mixed oxo complexes with different compositions and structures based on yttrium(III) and titanium(IV) with methacrylate ligands were synthesized in quantitative yields [102]. Similar synthetic approaches are applicable for the preparation of heteronuclear complexes [103–105]. Thus, upon variation of the ratio of starting alkoxides complexes of the varied composition could be obtained, for example, for 1:1 and 1:2 ratios of titanium and zirconium alkoxides, the complexes Ti4 Zr4 O6 .OBu/4 .CH2 D C.CH3 /COO/16 and Ti2 Zr4 O4 .OBu/2 .CH2 D C.CH3 /COO/14 , respectively, were obtained [106].

3.5 Other Reactions Some fumaric acid salts are obtained by catalytic isomerization of maleic anhydride followed by the reaction of the resulting acid with a metal compound. For example, iron(II) fumarate was prepared using maleic anhydride in the presence of thiourea [107] or hydrochloric acid [108]. Subsequent transformations of the fumaric acid can be carried out in accordance to one of the above mentioned methods. Unusual trans addition reaction of HCl to triple bond of the unsaturated ligand leading to formation of copper(II) chlorofumarate f[Cu(OOCCHDCClCOO)(H2 O)2 ]H2 Ogn was observed in the aqueous solution of acetylenedicarboxylic acid and CuCl2 [109, 110]. Metal complexes of Shiff bases are convenient reactants for the preparation of carboxylate complexes. Thus reaction of Fe(III) complexes with the tetradentate ligands N; N 0 -bis(salicylidene)ethylenediamine (salenH2 ) or bis(salicylidene)-o-phenylenediamine (salophH2 ) with acetylenedicarboxylic acid

36

3 Synthesis of Unsaturated Carboxylic Acid Salts

solution in BuOH resulted in the formation of the binuclear Fe(III) complexes [fFe(salen)g2(OOCCCCOO)] and [fFe(saloph)g2(OOCCCCOO)] with dicarboxylate bridges [111]. Examples of employment of organometallic compounds as starting reagents are not rare [112–115]. For example, the reaction of pentaphenylantimony with maleic acid at room temperature results in the cleavage of the MC bond to yield Sb(V) acyl derivatives [112]. Ph5 Sb C HO.O/C  CH D CH  C.O/OH ! Ph4 Sb  O.O/C  CH D CH  C.O/OH C PhH Change in the molar ratio of the reagents allows synthesizing disubstituted carboxylates. It should be noted that the resulting products are moisture sensitive and are easily hydrolyzed. Antimony (or bismuth) carboxylates can also be prepared in one step upon oxidation of triphenylantimony (or triphenylbismuth) with t-butylhydroperoxide or hydrogen peroxide in the presence of acrylic acid according to the following scheme [116]: Ph3 MCROOHC2CH2 DCHCOOH ! Ph3 M.CH2 D CHCOO/2 CROHCH2 O M D Sb; BiI R D t  Bu; H Reaction proceeds smoothly in ether at room temperature with yields ranging 50–90%. An unexpected product was obtained in the reaction of equimolar amounts of trimethylstannanol with maleic anhydride [113]. Irrespective of the reaction conditions, bis(trimethylstannyl) maleate is formed rather than the expected monosubstituted derivative: O

H H3 C

O

H3C Sn H3C

CH3 Sn CH3

+ O

CH3

O

CH

CH

C

C

–H2O O

HC HC

O

C

O

Sn(CH3)3

C

O

Sn(CH3)3

O

H

This reaction route is apparently related to the dimeric structure of the original tin reagent. When aryl tin hydroxides are involved in the reaction, one aryl group is eliminated giving rise to cyclic organotin maleate: O CH Ar3SnOH + O

C

CH C

O

O

HC

C

HC

C

Ar

O Sn

O

O

+ ArH Ar

3.6 Synthesis of Cluster Containing Unsaturated Carboxylates

37

Organolead maleates were prepared in a similar way. Organotin (organolead) maleates are solid compounds, soluble in organic solvents, and crystallized as needles or plates. In some cases alkylmetal hydroxide for the above reaction is prepared in situ [117]. For example, synthesis of the dimethylthallium(III) propionate is carried out according to the following scheme: H3C H3C

Tl

I

H3C

Ag2O H2O, 2 h, –AgI

H3C

Tl

OH

CH

H3C

C COOH

H3C

Tl OOC C CH

A peculiar reactivity of allyltitanocene compounds toward certain substances, such as carbon dioxide can result in the formation of carboxylates [118, 119]. The reaction with CO2 proceeds as an insertion into the Ti  ˜3 – allyl bond: R CO2

(C5H5)2Ti

R

(C5H5)2Ti O

C

O O

(C5H5)2Ti

(C5H5)2Ti

R

O C

R C

O

O

It has been demonstrated by theoretical [120, 121] and experimental [122–125] studies that the key step in the catalytic reaction of CO2 with ethylene is also the formation of mono- and binuclear acrylate complexes, including hydrido acrylate forms:

H

Mo

O

O

PMe2Ph

PMe2Ph

O Mo

PhMe2P

O O

PMe2Ph

P P

O W

H PMe3

3.6 Synthesis of Cluster Containing Unsaturated Carboxylates Synthesis of cluster containing carboxylates, that are molecular compounds having metal–metal bonds and ligands capable to polymerize, seems to have prospective. There are two main approaches developed for the synthesis of cluster containing monomers. One approach is based on the introduction of a ligand capable to polymerize into a polynuclear complex, for example, substitution of the existing ligand with an unsaturated one, their oxidative addition, or addition to double

38

3 Synthesis of Unsaturated Carboxylic Acid Salts

MM bond under mild conditions and so on. Another approach is building up the corresponding ligands with clusters [14]. Carboxylates were synthesized with high yields using as precursors trinuclear carbonyl clusters Os3 (CO)12 and its derivatives Os3 (CO)11 (CH3 CN), (H)Os3 (CO)10 (-OR) (R D H, Ph) [126, 127]:

(CO)3Os

Os(CO)4 Os(CO)4 H H CH2 = CHCOOH Os(CO)3 (CO)3Os Os(CO)3 80°C, benzene, 14 h O O O C Ph CH = CH 2

Outer sphere substitution principle was utilized in a series of consequent syntheses of Mo6 cluster carboxylates [128, 129]. CF3 COO

.Bu4 N/2 Œ.Mo6 Cl8 /Cl6  ! .Bu4 N/2 Œ.Mo6 Cl8 /.CF3 COO/6  CH2 DCHCOO

! .Bu4 N/2 Œ.Mo6 Cl8 /.CF3 COO/6n .CH2 D CHCOO/n  The main problem of carrying out this type of reaction, namely the problem of substitution of all six outer sphere chlorine atoms with other outer sphere ligands, has been solved in recent years. This was achieved by the employment of intermediate triflate groups CF3 COO , which could be easily substituted under mild conditions with acrylate groups. The number of triflate groups substituted the acrylate groups was found by the ratio of integral intensities for the protons N.Bu4 /C (NCH2  group) and groups CH2 DCHCOO in the 1 H NMR spectra to be between 1 and 3 [130]. The methods discussed are applicable for the design of monomeric carboxylates based on the heteropolynuclear clusters as well [131]. Therefore, it can be concluded that methods of synthesis of unsaturated carboxylic acid salts are fairly diverse. In most cases, they are similar to the methods and procedures used to synthesize saturated metal carboxylates, except for the fusion technique. The last mentioned technique is widely used in industry to obtain anhydrous saturated carboxylates in good yields; however, it has limitations for unsaturated carboxylates due to possible polymerization even during the synthesis of the monomer. It should be emphasized that readily hydrolysable carboxylates, for example, acyl derivatives of metal alkoxides are synthesized in nonaqueous media. As it has been mentioned above, characteristic features of the reaction chosen determine the peculiar properties of the product obtained, i.e., the carboxylates with the specified structure and composition can be obtained by purposeful change in conditions for the synthesis. The summary table can be presented as follows (Table 3.2). Practically all toolbox of the preparative inorganic and organometallic chemistry methods is utilized for the synthesis of unsaturated metal carboxylates, yet in some cases development of special techniques is required.

MAA

MAA

MAA

Li.CH2 D C.CH3 /COO/

Na.CH2 D C.CH3 /COO/

Cu2 .CH2 D C.CH3 /COO/4  2H2 O .CuOH/2 CO3

Na2 CO3

Li2 CO3

Table 3.2 Synthesis and some characteristics of unsaturated metal carboxylates Starting reagents Metal carboxylates Acids Metal compounds Interactions of metal (hydro)oxides and carbonates with unsaturated carboxyl acids Metal (meth)acrylates Na.CH2 D CHCOO/ AA NaOH Ba.CH2 D MAA Ba(OH)2 C.CH3 /COO/2  H2 O Ba(II) Ba.CH2 D C.CH3 /COO/2 methacrylate hydrate

Methanol, 24 h, stirring, then boiling 5 h, crystallization at 20ı C, 12 h

as .COO/ D 1;572I s .COO/ D 1;423 cm1 as .COO/ D 1;558I s .COO/ D 1;419 cm1 eff D 1:35 B (298 K), max D 722, 364 nm, M D 483 (osmometry, in benzene)

(continued)

[25]

[10]

[10]

[132]

Dehydration, vacuum, 50ı C or recrystallization in dry methanol Methanol, 15ı C, hard stirring, precipitation with diethyl ether. The same

Ref.

[9] [132]

The characteristics

pH D 7.0 ˙ 0.1 Hot H2 O, crystallization

Reaction conditions

3.6 Synthesis of Cluster Containing Unsaturated Carboxylates 39

AA

AA

MAA

ŒM3 O.CH2 CHCOO/6  3H2 OOH; M D Fe.III/, Cr(III), V(III)

Fe.CH2 D C.CH3 /COO/3

Starting reagents Acids

M.CH2 D CHCOO/2 nH2 O; M D Zn.II/, Co(II), Ni(II), Cu(II)

Cu2 .CH2 D C.CH3 /COO/4 2(4-VPy)

Metal carboxylates Cu2 .CH2 D C.CH3 /COO/4 2Py

Table 3.2 (continued)

NaHCO3 , FeCl3

Metal hydroxides

Metal hydroxides, (hydro)carbonates

Metal compounds

Methanol, ethanol, stirring 3–5 h, precipitation with diethylether H2 O; 40ı C

Methanol, DMFA, benzene, toluene, stirring 5 h, precipitation with diethylether, acetone

Reaction conditions

The characteristics eff D 1:39 B (298 K), max D 749, 384 nm, M D 614 (osmometry, in benzene) eff D 1:45 B (298 K), max D 746, 386 nm, M D 689 (osmometry, in benzene) Double bonds > 94%, as .COO/ D 1;520–1;575 cm1 ; s .COO/ D 1;360–1;370 cm1

[40] (continued)

[22, 23]

[20, 21]

[25]

Ref. [25]

40 3 Synthesis of Unsaturated Carboxylic Acid Salts

Be.OOCCH D CHCOO/2H2 O

Metal dicarboxylates Co.HOOCCH D CHCOO/2  5H2 O Ni.OOCCH D CHCOO/2H2 O Cu.OOCCH D CHCOO/H2 O ZnH.OOCCH D CHCOO/.OH/  H2 O

Metal carboxylates Cu2 ŒCH2 D C.CH3 /COO4 .H2 O/2

Table 3.2 (continued)

(CuOH)2 CO3 ZnO

MalA MalA

BeSO4 4H2 O, Ba(OH)2 8H2 O

NiCO3

MalA

MalA

CoCO3

Metal compounds Cu2 .OH/2 CO3

MalA

Starting reagents Acids MAA

H2 O, stirring 17 h, boiling 3 h, pH D 3, crystallization

The same H2 O, stirring, 60–70ı C

Hot H2 O, stirring, pH D 7 The same

Reaction conditions Boiling 2 h, methanol

IR: 3,400, 3,575, 1,190–1,125, 1,310– 1,350 cm1 1 H-NMR (D2 O, 20ı C): ı D 6:05, s, 2H , CH ; 13 Cf1 Hg NMR: ı D 133:3, 2C, C H; 9 Be NMR: ı D 1:21, s, 1Be

The characteristics as .COO/ D 1;572 cm1 ; s .COO/ D 1;415 cm1

(continued)

[41]

[2] [13]

[44]

[44]

Ref. [24]

3.6 Synthesis of Cluster Containing Unsaturated Carboxylates 41

ItA

LnH.CH2 D C.COO/CH2 COO/2  nH2 O; n D 1; 1.5; 2; Ln D La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Lu, Er

CdCO3

MalA

Rare earth elements carbonates

CdCO3

(CuOH)2 CO3

MalA

MalA

Ag2 CO3

Metal compounds 25% NH4 OH, BeSO4 4H2 O, Ba(OH)2 8H2 O

MalA

Starting reagents Acids MalA

Cd.OCOCH D CHCOO/H2 O

AgOOCCH D CHCOOH CuOOCCH D CHCOOHH2 O Cd.OCOCH D CHCOO/H2 O

Metal carboxylates .NH4 /2 ŒBe.OOCCH D CHCOO/

Table 3.2 (continued)

(continued)

[36]

[134]

[133]

H2 O suspension, excess of carbonate H2 O suspension, carbonate: acid D 1:2 (mol) H2 O, conc. acid solution

[5]

Ref. [41]

[4]

as .COO/ D 1;540–1;580 cm1 ; s .COO/ D 1;300–1;335 cm1 , product of solubility – 108 , Kstability D 6:0  103  1:25  104

The characteristics 1 H-NMR (D2 O, 20ı C): ı D 6:02, s, 2H, CH ; 13 Cf1 Hg NMR: ı D 134:6, 2C, C H, 171.2, 2C, C OO; 9 Be NMR: ı D 2:02, s, 1Be

The same

Hot H2 O, suspension

Reaction conditions H2 O, stirring 16 h, boiling 5 h, pH D 5.5, crystallization

42 3 Synthesis of Unsaturated Carboxylic Acid Salts

Ca.CH2 D CHCH2 COO/2  H2 O

Cu.OCOCH D CHOCO/ Other salts Li.CH3 CH D CH  CH D CHCOO/

Cu.OOCCH D CHCOOH/2 4H2 O

Metal carboxylates

Table 3.2 (continued)

CaCO3

LiOH

Sorbite acid

3-butenoic acid

Cu2 (OH)2 CO3

Metal compounds (CuOH)2 CO3 or Cu(OH)2

FA

Starting reagents Acids MalA Reaction conditions

H2 O, 3 h

(continued)

[12]

[11]

H-NMR: ı D 1:6 (d, 6-H), 5.6 (d, 2-H), 6.0 (m, 4-H, 5-H), 6.8 (dd, 3-H) IR: 3,448, 3,081, 2,987, 1,582, 1,547, 983, 912 cm1 ; 1 H-NMR: ı D 5:94 (1H, ddt, J D 14:2, 10.2, 7.0 Hz), 5.13 (1H, br d, J D 14:2 Hz), 5.10 (1H, br d, J D 10:2 Hz), 2.97 (br d, 2H, J D 7:0 Hz)

Hot H2 O, pH D 8

[46]

Ref.

[3]

1

The characteristics IR: 1,660, 1,700, 835 cm1 ; eff D 1:97 B (298 K)

Methanol, boiling 3 h, pH 3–4

H2 O solution of the alA ( 250ı C FTIR: 1,589, 1,522, 1,437, [75] 1,385, 922 cm1 ; UV-vis: 297 nm; Mp > 250ı C FTIR: 1,595, 1,558, 1,531, [75] 1,487, 1,452, 1,387, 962 cm1 ; UV-vis: 311, 336 sh, nm; Mp > 250ı C (continued)

48 3 Synthesis of Unsaturated Carboxylic Acid Salts

Cu(NO3 /2 3H2 O

Mn and Mg crotonates

ŒCo2 M.OCOCH D CHCH3 /6 .C9 H7 N/2 M D Mn, Mg

Starting reagents Acids Eu(Oi Pr)3

ŒCuLa.OCOC.CH3 / D CH2 /5 .phen/ .C2 H5 OH/2

Metal carboxylates Eu.DBM/2 .CH2 D CHCOO/DBM – dibenzoylacetone

Table 3.2 (continued)

La.OCOC.CH3 / D CH2 /3  2H2 O

Metal compounds AA, DBM

H2 O, boiling, 3–4 h, crystallization

H2 O-ethanol solution, pH D 4.1; crystallization

Reaction conditions 2-propanol:benzene(1:1), 4.5 h, boiling

The characteristics FTIR: 1,597, 1,552, 1,522, 1,479, 1,456, 1,442, 941 cm1 ; UV-vis: 316, 341 sh, nm; Mp > 250ı C as .COO/ D 1;550 cm1 ; s .COO/ D 1;419 cm1 ; .C D C/ D 1648 cm1 I eff D 1:95 B as .COO/ D 1;530, 1,572 (Mn), 1,546, 1592 cm1 (Mg) s .COO/ D 1;401 (Mn), 1,400 cm1 (Mg); max D 525, 550, 574 (Mg), 526, 555, 580 nm (Mn)

(continued)

[85]

[80]

Ref. [75]

3.6 Synthesis of Cluster Containing Unsaturated Carboxylates 49

.C6 H5 /4 SbOCOCH D CHOCOSb.C6 H5 /4

Other reactions .C6 H5 /4 SbOCOCH D CHCOOH

Metal carboxylates Eu.OOCCH D CHC6 H5 /2(phen) phen – 1,10-phenantroline Eu.OOCCH D CHC6 H5 /2(dipy) dipy – 2,20 -dipyridile Eu.OOCCH D CHC6 H5 /2(bta) bta –benzenetriazole Eu.OOCCH D CHC6 H5 /2(tphpho)2H2 O Tphphotriphenylphosphineoxide Tb.OOCCH D CHC6 H5 /H2 O

Table 3.2 (continued)

NaOOCCH D CHC6 H5 NaOOCCH D CHC6 H5

NaOOCCH D CHC6 H5

Eu(NO3 /3 6H2 O

Eu(NO3 /3 6H2 O

Tb(NO3 /3 6H2 O

MalA

(C6 H5 /5 Sb

(C6 H5 /5 Sb

NaOOCCH D CHC6 H5

Eu(NO3 /3 6H2 O

MalA

Metal compounds NaOOCCH D CHC6 H5

Starting reagents Acids Eu(NO3 /3 6H2 O

1:2 (mol), dioxine, 60ı C

Benzene-dioxine (5:1), 24 h, 20ı C

H2 O, ethanol, pH D 6–7, yield 82–90%

H2 O, ethanol, pH D 6–7, yield 82–90%

H2 O, ethanol, pH D 6–7, yield 82–90%

H2 O, ethanol, pH D 6–7, yield 82–90%

Reaction conditions H2 O, ethanol, pH D 6–7, yield 82–90%

[80]

[80]

[80]

mp D 215ı C mp D 218ı C

mp D 290ı C

(continued)

[112]

[112]

[80]

mp D 227ı C

.C D O/ D 1700, 1620 cm1 , m.p. 165ı C, yield 87% .C D O/ D 1640, 1620 cm1 , m.p. 232ı C (dec), yield 99%

Ref. [80]

The characteristics mp D 220ı C

50 3 Synthesis of Unsaturated Carboxylic Acid Salts

.C6 H5 /2 Pb.OCOCH D CHOCO/ ŒfFe.salen/g2 .OOCC  CCOO/ salenH2 – N; N 0 -bis(salicylidene) ethylenediamine ŒfFe.saloph/g2 .OOCC  CCOO/ salophH2 – N; N 0 -bis(salicylidene)-ophenylenediamine

Metal carboxylates .CH3 /3 SnOCOCH D CHOCOSn.CH3 /3

Table 3.2 (continued) Reaction conditions Benzene, 6 h, 80ı C, recrystallization with acetone Benzene, 4 h, 80ı C BuOH, boiling 3 h

BuOH, boiling 3 h

Metal compounds (CH3 /3 SnOH

(C6 H5 /3 PbOH [fFe(salen)g2 ]

[fFe(saloph)g2 ]

Maleic anhydride

Acetylenedicarboxylic acid

Starting reagents Acids Maleic anhydride

IR: 2,084, 1,626, 1,596, 1,542, 1,440, 1,382, 968 cm1 IR: 1,632, 1,604, 1,578, 1,552, 1,534, 1,440, 1,372, 920 cm1

Yield 70%

The characteristics Yield 82%

[111]

[111]

[113]

Ref. [113]

3.6 Synthesis of Cluster Containing Unsaturated Carboxylates 51

52

3 Synthesis of Unsaturated Carboxylic Acid Salts

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54

3 Synthesis of Unsaturated Carboxylic Acid Salts

80. I.V. Kalinovskaya, V.E. Karasev, A.N. Zadorozhnaya, L.I. Lifar, Koordinats. Khim. 27, 551 (2001) 81. G.I. Dzhardimalieva, I.N. Ivleva, Yu. M. Shulga, E.N. Frolov, A.D. Pomogailo, Izv. Akad. Nauk, Ser. Khim. 1145 (1998) 82. B. Wu, W.-M. Lu, X.-M. Zheng, Chin. J. Chem. 20, 846 (2002) 83. B. Wu, W.-M. Lu, X.-M. Zheng, J. Coord. Chem. 55, 497 (2002) 84. B. Wu, Y. Guo, Acta Crystalogr. E 60, m1356 (2004) 85. W. Clegg, P.A. Hunt, B.P. Straughan, J. Chem. Soc. Dalton Trans. 1127 (1989) 86. A.D. Pomogailo, Russ. Chem. Rev. 69, 60 (2000) 87. A.D. Pomogailo, Kolloid. Zh. 67, 726 (2005) 88. G. Kickelbick, U. Schubert, Chem. Ber. 130, 473 (1997) 89. S. Gross, G. Kickelbick, M. Puchberger, U. Schubert, Monatsh. Chem. 134, 1053 (2003) 90. B. Moraru, G. Kickelbick, M. Battistella, U. Schubert, J. Organomet. Chem. 636, 172 (2001) 91. G. Kickelbick, U. Schubert, Chem. Ber./Recueil. 130, 473 (1997) 92. G. Kickelbick, P. Wiede, U. Schubert, Inorg. Chim. Acta 284, 1 (1999) 93. M. Puchberger, F.R. Kogler, M. Jupa, S. Gross, H. Fric, G. Kickelbick, U. Schubert, Eur. J. Inorg. Chem. 3283 (2006) 94. F.R. Kogler, M. Jupa, M. Puchberger, U. Schubert, J. Mater. Chem. 14, 3133 (2004) 95. Ch. Barglik-Chory, U. Schubert, J. Sol-Gel Sci. Technol. 5, 135 (1995) 96. U. Gbureck, J. Probst, R. Thull, J. Sol-Gel Sci. Technol. 27, 157 (2003) 97. C. Sanches, M. In, J. Non-Cryst. Solids 147/148, 1 (1992) 98. E. Martinez-Ferrero, K. Boubekeur, F. Ribot, Eur. J. Inorg. Chem. 802 (2006) 99. U. Schubert, E. Arpac, W. Glaubitt, A. Helmerich, C. Chau, Chem. Mater. 4, 291 (1992) 100. G. Kickelbick, U. Schubert, Eur. J. Inorg. Chem. 159 (1998) 101. H. Fric, M. Jupa, U. Schubert, Monatsh. Chem. 137, 1 (2006) 102. M. Jupa, G. Kickelbick, U. Schubert, Eur. J. Inorg. Chem. 1835 (2004) 103. B. Moraru, G. Kickelbick, U. Schubert, Eur. J. Inorg. Chem. 1295 (2001) 104. J. Mendezz-Vivar, P. Bosch, V.H. Lara, J. Non-Cryst. Solids 351, 1949 (2005) 105. A. Albinati, F. Faccini, S. Gross, G. Kickelbick, S. Rizzato, A. Venzo, Inorg. Chem. 46, 3459 (2007) 106. B. Moraru, G. Kickelbick, U. Schubert, Eur. J. Inorg. Chem. 1295 (2001) 107. J. Novrocik, J. Pecha, M. Novrocikova, Czech. P., Zh. Khim. 11, 53 (1988) 108. K.M. Khurshid Alam, F. M. Kaniz, A. Gulzar, Pak. J. Sci. Ind. Res. 707 (1987) 109. H. Billetter, I. Pantenburg, U. Ruschewitz, Acta Crystallogr. E 62, m 881 (2006) 110. H. Billetter, I. Pantenburg, U. Ruschewitz, Acta Crystallogr. E 61, m1857 (2005) 111. P. Kopel, Z. Sindelar, R. Klicka, Trans. Met. Chem. 23, 139 (1998) 112. V.V. Sharutin, O.K. Sharutina, A.P. Pakusina, V.K. Belsky, J. Organomet. Chem. 536–537, 87 (1997) 113. V. F. Mishchenko, Z.M. Rzaev, V.A. Zubov, Biostable Tin-Containing Polymers (Khimiya, Moscow, 1995) 114. M. Kamal, A.K. Srivastava, React. Funct. Polym. 49, 55 (2001) 115. M. Kamal, A.K. Srivastava: Polym. Plast. Technol. Eng. 40, 293 (2001) 116. A.V. Gushchin, V.A. Dodonov, in Fundamental Problems of Polymer Science. Int. Conf., 21–23 January 1997 (Moscow, 1997), pp. 1–21 117. M.J. Moloney, B.M. Foxman, Inorg. Chim. Acta 229, 323 (1995) 118. J. Blenkers, H.J. De Liefde Meijer, J.H. Teuben, J. Organomet. Chem. 218, 383 (1981) 119. E. Klei, J.H. Teuben, H.J. De Liefde Meijer, J. Organomet. Chem. 224, 327 (1982) 120. G. Schubert, I. Papai, J. Am. Chem. Soc. 125, 14847 (2003) 121. I. Papai, G. Schubert, I. Mayer, G. Besenyei, M. Aresta, Organometallics, 23, 5252 (2004) 122. R. Fischer, J. Langer, A. Malassa, D. Walther, H. Gorls, G. Vaughan, Chem. Commun. 2510 (2006) 123. A. Galinda, A. Pastor, P.J. Perez, E. Carmona, Organometallics 12, 4443 (1993) 124. C. Collazo, M. Der Mar Conejo, A. Pastor, A. Galindo, Inorg. Chim. Acta 272, 125 (1998) 125. M. Aresta, C. Pastore, P. Giannoccaro, G. Kovacs, A. Dibenedetto, I. Papai, Chem. Eur. J. 13, 9028 (2007)

References

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126. V.A. Maksakov, V.P. Kirin, S.N. Konchenko, N.M. Bravaya, A.D. Pomogailo, A.V. Virovets, N.V. Podberezskaya, I.G. baranovskaya, S.V. Tkachev, Izv. Akad. Nauk. 1293 (1993) 127. N.M. Bravaya, A.D. Pomogailo, in Metal-Containing Polymeric Materials, ed. by C.U. Pittman Jr., C.E. Carraher Jr., M. Zeldin, B. Culberston (Plenum Publ. Corp., New York, 1996), p.51 128. N.D. Golubeva, O.A. Adamenko, G.N. Boiko, A.D. Pomogailo, Inorg. Mater. 40, 363 (2004) 129. O.A. Adamenko, G.V. Lukova, N.D. Golubeva, V.A. Smirnov, G.N. Boiko, A.D. Pomogailo, I.E. Uflyand, Dokl. Phys. Chem. 381, 360 (2001) 130. N.D. Golubeva, S.I. Pomogailo, G.N. Boiko, L.A. Petrova, Yu.A. Olkhov, A.D. Pomogailo, in Polymers-2004. Proceed. 3 Russ. Kargin Conf. (Moscow State University, Moscow, 2004), p. 138 131. A.D. Pomogailo, A.S. Rozenberg, I.E. Uflyand, Metal Nanoparticles in Polymers (Khimiya, Moscow, 2000) 132. J.H. O’Donnell, R.D. Sothman, Radiat. Phys. Chem. 13, 77 (1979) 133. M.L. Post, J. Trotter, J. Chem. Soc. Dalton Trans. 674 (1974) 134. A. Hempel, S.E. Hull, Raja Ram, M.P. Gupta, Acta Cryst. B 35, 2215 (1979)

Chapter 4

Spectral Characteristics and Molecular Structure of Unsaturated Carboxylic Acid Salts

In most typical cases the carboxylate group RCOO is capable of coordinating with metals such as monodentate (syn- and anti- configuration) (I), bidentatecyclic (chelate) (II), bidentate-bridging (III), tridentate (IV), and tetradentate (V) ligands [1–3]:

M

R

R

R

C

C

C

anti

M I

R

M

M III

O

O

R M

C

M

IV

M O

O M

M

M VI

II

O

O

C

M

syn

C O

CH

O

O

R

C O

O

O

O

O

R CH

M

M V

Oligomeric and polymeric coordination complexes with one, two, three, or four carboxylate bridges between each pair of metal atoms are known among experimentally proven structures. Such a large diversity of possible structures and compositions for metal carboxylates can additionally include in the case of the unsaturated analogs, participation of a multiple bond functionality that is known to be able to participate in metal atom coordination via a formation of a  -bond (VI). Therefore, there is an interest to analyze peculiarities of the geometry and the coordination type of metal carboxylate fragment in a series of unsaturated mono- and dicarboxylic acid anions.

4.1 Metal (Meth)acrylates Various methods are employed for the structural studies of metal carboxylates. Vibrational, UV-, and visible spectroscopy, as well as magnetic- and ”-resonance spectroscopic methods are widely used for getting stereochemical A.D. Pomogailo et al., Macromolecular Metal Carboxylates and Their Nanocomposites, Springer Series in Materials Science 138, DOI 10.1007/978-3-642-10574-6 4, c Springer-Verlag Berlin Heidelberg 2010 

57

58

4 Spectral Characteristics and Molecular Structure of Unsaturated Carboxylic Acid Salts

information regarding the coordination number, shape of the coordination polyhedron, coordination mode, and denticity of a carboxylate ligand. Diffraction methods, such as electronic, neutron, and X-ray diffraction allow full quantitative structural information to be received. Also, quantum chemical calculation methods including MO LCAO and density functional theory (DFT) are involved in the studies. The following are examples of identification of spatial forms of the carboxylates of this type based on the data from experimental methods.

4.1.1 IR Spectroscopy Frequencies as (COO / and s (COO / corresponding to asymmetrical and symmetrical vibrations of the carboxylate ion are the most characteristic in the IR spectra of metal carboxylates. Structures I–IV implies the presence of equivalent and unequivalent oxygen atoms in the carboxyl group, which is observed in the IR spectra. In monodentate complexes the difference .as .COO /  s .COO // is much larger than the one for ionic compounds (164–171 cm1 /, while for bidentate carboxylate complexes, the  values are much smaller [2]. Thus, the binuclear Cu(II) methacrylate Cu2 [CH2 DC(CH3 )COO]4 (H2 O)2 in methanol is converted into a mononuclear complex upon replacement of the coordinated water by a stronger donor ligand such as benzimidazole, and the bidentate coordination of the methacrylate ion becomes monodentate . D 203 cm1 / [4]. Spectroscopic data are in concert with the results of X-ray analysis. As in the majority of cases, the CO bond for the coordinated oxygen atom is longer than the one for the non-coordinated atom (see Table 4.1). The structure of the carboxylate ligand is appreciably affected by steric factors. Thus, trialkoxytitanium(IV) methacrylate complexes (CH2 DC(CH3 )COO) Ti(OR)3 (where R D i -Pro, t-Bu, t-Am, 2-ethylhexyl) exist as equilibrium mixtures of structures with bridging and cyclical bidentate coordination of methacrylate group (Table 4.1) [5]: CH2

H3C C

H2C

C O

O

RO RO

C

Ti

Ti

RO O

O

C

OR

C

OR

O Ti

C

C

OR Ti-II

OR

RO

H3C

C C

Ti

O RO RO Ti Ti RO RO OR RO O O C O

C

OR

RO CH2

Ti-I

CH2

H3C

O

C

H3C

OR

H3C

OR O

O

CH2

H3C Ti-III

CH2

ŒLa2 .CH2 DC.CH3 /COO/6 .phen/2   2H2 O ŒLa2 .CH2 DC.CH3 /COO/6 (phen)2 .CH2 DC.CH3 /COOH/2 

Cu3 ŒCH2 DC.CH3 /COO5 (OH) .imH/2

Ti(O-ethylhexyl)3 .CH2 DC.CH3 / COO/ Cu3 ŒCH2 DCHCOO5 .OH/.imH/2

Ti(O-t -Am)3 .CH2 DC.CH3 /COO/

Metal carboxylate Cu2 ŒCH2 DC.CH3 /COO4 .H2 O/2 CuŒCH2 DC.CH3 /COO2 .C7 H6 N2 /2 Cu2 ŒCH2 DC.CH3 /COO4 .C7 H6 N2 /2 Ti(OBu)3 .CH2 DC.CH3 /COO/ Ti(Oi -Pro)3 .CH2 DC.CH3 /COO/ Ti(O-t -Bu)3 .CH2 DC.CH3 /COO/

1,575, 1,564

1,570, 1,565

as (COO) (cm1 ) 1,572 1,564 1,583 1,556 1,561, 1,516 1,588, 1,550, 1,525 1,586, 1,554, 1,517 1,556, 1,519

Table 4.1 Structural parameters of RCOO

1,418, 1,370 1,413, 1,365

1,423

1,423

s (COO) (cm1 ) 1,415 1,361 1,420 1,424 1,424 1,425

199, 162

195, 152

133, 96

163, 131, 94

 (cm1 ) 157 203 163 132 137, 92 163, 125, 100

II, III

II, III

I, III

I, III

II, III

II, III

III I III III II, III II, III

Coordination mode

˚ Distance (A)

1.2620 (13)

COcoord 1.2452 (13)

COtermin

[6]

[5]

[5]

Ref. [4] [4] [4] [5] [5] [5]

1.9751mono , [6] 2.0064mono , 1.9988mono , 1.9878bi , 2.1476bi , 1.9457bi5 [7] 2.498br; av , 2.606ch; av [8] 2.494br; av , 2.612ch; av (continued)

1.9752 (7) 1.961, 1.994

MO

4.1 Metal (Meth)acrylates 59

1,574

1,582, 1,547

1,572 1,558

ŒNdZn2 .CH2 DC.CH3 /COO/6 .NO3 /.2,20 -bipy/2 

Ca.CH2 DCHCH2 COO/2 H2 O

Li.CH2 DC.CH3 /COO/ Na.CH2 DC.CH3 /COO/ 1,423 1,419

1,408

1,414

1,421

1,567

1,574

1,427

1,422

1,549

1,558

s (COO) (cm1 ) 1,419

as (COO) (cm1 ) 1,550

ŒPrZn2 .CH2 DC.CH3 /COO/6 .NO3 /.2,20 -bipy/2 

ŒCuTb.CH2 DC.CH3 /COO/5 .phen/.H2 O/2 ŒLaZn2 .CH2 DC.CH3 /COO/6 .NO3 /.2,20 -bipy/2 

Metal carboxylate ŒCuLa.CH2 DC.CH3 /COO/5 .phen/.C2 H5 OH/2 ŒCoCe..CH2 DC.CH3 /COO/5 .phen/.C2 H5 OH/2

Table 4.1 (continued)

149 139

166

160

146

131

127

 (cm1 ) 131

II, III

III

III

III

II, III

II, III

II, III

Coordination mode COcoord

˚ Distance (A) COtermin

2.429(3)br – 2.507(3)br (LaO), 2003(3)br –2.083(3)br (ZnO) 2.394(3)br – 2.454(3)br (PrO), 2003(3)br –2.077(3)br (ZnO) 2.382(4)br – 2.438(4)br (NdO), 2008(3)br –2.073(3)br (ZnO) 2.318(15)–2.356(13)br , 2.597913)– 2.562(14)ch

2.570(3)ch , 2.529(3)ch , 2.476(3)br , 2.439(3)br 2.455ch; av , 2.339br;av

MO 2.484br , 2.582ch

[16] [16]

[15]

[14]

[13]

[12]

[11]

[10]

Ref. [9]

60 4 Spectral Characteristics and Molecular Structure of Unsaturated Carboxylic Acid Salts

4.1 Metal (Meth)acrylates

61

Meanwhile, tri(n-butoxy)titanium methacrylate containing less bulky n-butoxy groups forms dimer with titanium atoms linked by carboxyl and butoxy bridges. There are also terminal alkoxy groups in the structure: CH2

H3C C C BuO

O

BuO

OBu Ti

Ti BuO

O

OBu O

OBu O

C C CH2 H3C Ti-IV

Formation of the four-membered rings upon chelate coordination of the carboxylate group requires high strains in valence angles of the metal atom. No such strain is present in the compounds of rare earth elements, and the possibility of bidentate cyclic coordination increases. Apparently, strain relief is facilitated by the higher polarity of the metal–ligand bond and high coordination numbers of the metals. For example, the lanthanoid center in heteronuclear Cu2 La2 [9, 17] or CoCe [10] methacrylate complexes bears both the bridging and chelating groups. Moreover, instances of the tridentate coordination of methacrylate groups are not rare in this type of carboxylate complexes. In these cases one oxygen atom of the carboxyl group simultaneously forms bonds with two metal atoms. Metal atoms in isomorphic binuclear La or Gd trans-2,3 dimethacrylate complexes [M(OOC(CH3 /C(CH3 /CH)3 (phen)]2 are connected by two bridging bidentate and two tridentate carboxylate groups [18]:

C O O

O

C

O

O

O

La O

C

C

O

La O

OO C

O C

Binuclear methacrylate complexes [Gd2 (CH2 DC(CH3 /COO)6 (phen)2 ]  2H2 O [19],[La2 (CH2 DC(CH3 /COO)6 (phen)2]  2H2 O[7],and [La2(CH2 DC(CH3 /COO)6

62

4 Spectral Characteristics and Molecular Structure of Unsaturated Carboxylic Acid Salts

(phen)2CH2 DC(CH3 /COOH)2 ] [8] possess the analogous structure. Similarly the structure above the coordination environment of metal atoms in these complexes has a configuration of a slightly distorted tricapped trigonal prism, while two bridging bidentate and two tridentate carboxylate groups connect the metal centers. Tetranuclear zinc – cerium methacrylate complex [Zn2 Ce2 (CH2 DC(CH3 / COO)10 (bipy)2(H2 O)2 ] also has a resembling structure [20]. Each Ce(III) ion is coordinated by nine oxygen atoms from one chelating and five bridging carboxyl groups and a water molecule forming a tricapped trigonal prism. There are two tridentate bridging cyclical CH2 DC(CH3 /COO groups and three bidentate bridging groups between the metal centers CeCe and CeZn, respectively. On the contrary, there are only bidentate bridging carboxylates in the structures of isomorphic trinuclear [PrZn2 (CH2 DC(CH3 /COO)6 (NO3 /(2,20 -bipy)2] [13], [NdZn2(CH2 DC(CH3 /COO)6 (NO3 /(2,20 -bipy)2] [14], and [LaZn2 (CH2 D C (CH3 /COO)6 (NO3 /(2,20 -bipy)2] [12] complexes. IR spectroscopy data . D 146–166 cm1 / also supports this. Note that the transition from the bridging to the cyclical coordination of the RCOO group results in elongation of the MO bond (Table 4.1). Average bond length for the LnObridging , LnOchelating , and LnOtridentate bonds are 2.473, 2.556, ˚ in the La complex above, while the same bonds equal 2.365, 2.455, and and 2.615 A ˚ in the Gd complex [18]. This tendency is also observed for methacrylate 2.530 A complexes, [7, 8, 19] as well as for others [15]. Comparison of MO distances in the tridentate bridging cyclical group demonstrates that the bond M1O2 is weaker than two others. C O1 M1

O2 M2

˚ in comparison with the For example, this distance equals 2.6385(15) A ˚ M1O1 one (2.5674(18) A) in the molecule of the heteronuclear methacrylate complex ZnCe [20], the same distances in the complex [Gd2 (CH2 DC(CH3 / ˚ respectively. It is noteCOO)6 (phen)2]  2H2O [19] being 2.644(3) and 2.456(3) A, worthy that the M2O2 bond is often shorter than the M1O1 one. That is the tridentate oxygen atom has the strongest bond with the metal, although this atom provides part of its binding electron density to the second metal atom. In turn, the higher charge density on the oxygen atom of the monodentate carboxylate group in comparison with the bridging one, also causes the corresponding shortening of the MO bond. This is evidenced, for example, by the structural data for the trinuclear Cu(II) methacrylate [6], which has both monodentate and bidentate bridging carboxylate groups. Average length of the CuOmonodentate bond equals ˚ that is shorter that the average length of the CuObidentate one (2.0270 A). ˚ 1.9934 A It is notable that even in essentially ionic compounds, for example, in lithium and sodium methacrylates [16], the  values may deviate from those in purely

4.1 Metal (Meth)acrylates

63

ionic salts, and the degree of covalence of the metal ion – ligand bond1 increases [21] (Table 4.1). As part of a general tendency, it may be related to the increase of acceptor properties of the carboxylate ligand with the unsaturated fragment.

4.1.2 Magnetic Properties Antiferromagnetic interactions play an important role in the studies of the nature of the metal–metal bond and the structure of metal carboxylates. The value of effective magnetic moments for a number of copper (meth)acrylates is 1.4 B, which is much lower than the purely spin value and is consistent with the dimeric structure of the corresponding carboxylates (Table 4.2). The temperature dependence of the magnetic susceptibility in these systems attests to strong antiferromagnetic exchange interaction of the unpaired electrons of the metal ions, which may occur either directly or through the OCO bridges. Typically, complex carboxylates of divalent transition metals, such as Mn, Fe, Co, Ni, have the composition ML2 (RCOO)2 (LH2 O, ROH, Py, etc.) or ML4 (RCOO)2 and are monomeric. For (meth)acrylate complexes of these metals, this fact is confirmed by the lack of exchange interactions. According to magnetic measurement data, these compounds are high-spin octahedral complexes, the eff values of which change only slightly with temperature changes (see Table 4.2). The magnetic susceptibility values of heteronuclear CuLa, NiLa [9], and CuTb [11] methacrylate complexes correspond to the presence of two uncoupled spins of copper ions. Over the whole temperature range studied, the magnetic susceptibility

Table 4.2 Magnetic properties of metal (meth)acrylates eff (B) Metal carboxylates Cu2 .CH2 DCHCOO/4 2C2 H5 OH Cu2 .CH2 DC.CH3 /COO/4 2H2 O Cu2 .CH2 DC.CH3 /COO/4 2Py Cu2 .CH2 DC.CH3 /COO/4 2(4-VPy) Cr2 .CH2 DC.CH3 /COO/4 4H2 O ŒCuLa.CH2 DC.CH3 /COO/5 .phen/.C2 H5 OH/2 Co.CH2 DCHCOO/2 H2 O Ni.CH2 DCHCOO/2 H2 O Fe.CH2 DCHCOO/2 2H2 O

295 K 1.40 1.35 1.39 1.45 1.45 1.95 5.10 3.60 4.92

78 K 0.22 – – – 1.22 1.91 4.53 3.47 4.35

Antifferomagnetic exchange Strong exchange – – – Exchange No exchange No exchange No exchange No exchange

Ref. [22] [23] [23] [23] [24] [9] [22] [22] [24]

1 The predominantly covalent nature of the TiO bond was found for the cyclopentadienyl Ti(III) maleate and fumarate derivatives [21]. The molar conductivities of the maleate and the fumarate are 18.3 and 6.0 cm2 mol1 , respectively. For comparison, the value for the Cp2 TiCl equals 86.4 cm2 mol1 .

64

4 Spectral Characteristics and Molecular Structure of Unsaturated Carboxylic Acid Salts 300

1,0

200 0,8

0,7 100

1 / χm, mol emu–1

χmT, emu K mol–1

0,9

0,6

0,5 0

100

0 300

200 T, K

Fig. 4.1 The dependence of m T and 1/ m on temperature for [CuLa(CH2 DC.CH3 /COO/5 (phen)(C2 H5 OH)]2

obeys the Curie–Weiss law with relatively low Weiss constants . D 0:82 K/, i.e., the possibility of exchange interactions between the copper atoms is low (Fig. 4.1). It has been found that basic copper(II) methacrylates [Cu(CH2 CHCOO)2 Cu (OH)2 ] [25] and [Cu(CH2 C(CH3 /COO)2 Cu(OH)2 ] [26] are capable of exhibiting ferromagnetic properties. Effective magnetic moment per one Cu(II) ion is 1.86 M.B. at 293 K and 2.45 M.B. at 78 K for the basic copper(II) acrylate. The authors attribute the magnetic properties of the complexes to their possible tetrameric structure: O

R O R

C

C O O

O

O Cu

Cu

O O O

Cu

R C

C

O Cu

O R

O

Other ferromagnetic binuclear -hydroxo copper(II) complexes have been reported [27].

4.1.3 Electron Spectroscopy Diffusion reflection spectra have been under investigation for the transition metal acrylates [22,28] and the copper(II) methacrylate [4,6]. There are three spin allowed theoretically possible d–d transition in these complexes [29]:

4.1 Metal (Meth)acrylates

65

Table 4.3 The data of electronic spectra for (meth)acrylate complexes Cu(II) [4, 6] max .cm1 / Complexes 1 2 3 CuŒCH2 DC.CH3 /COO2 .bimH/2 15,152 – 39,683, 45,455 13,369, 9,760 26,882 39,683, 46,296 Cu2 ŒCH2 DC.CH3 /COO4 .bimH/2 15,152, 9,886 26,418 47,619, 37,313 Cu3 .CH2 DCHCOO/5 .OH/.imH/3 14,388, 9,668 26,884 46,296, 39,370 Cu3 .CH2 DC.CH3 /COO/5 .OH/.imH/3

4

T1g .F / ! 4 T2g .F /.1 / ! 4 A2g .F / .2 / ! 4 T1g .P / .3 /

Bands 2 and 3 in the electron spectrum of Co acrylate are observed at 18,760 and 20,500 cm1 , which agrees with the octahedral coordination structure confirmed also by the effective magnetic moment value (see Table 4.2). Electron transitions 3 A2g ! 3 T2g .F / (13,500 cm1 / and 3 A2g ! 3 T1g .P / (24920 cm1 / attests the existence of the similar chromophor in the Ni acrylate as well. The band at 15,152 cm1 in the spectrum of monomeric Cu methacrylate corresponds to the dxz , dyz ! dx 2 y 2 (2 B1g ! 2 Eg / transition in the flat square ligand field, while the band at 13,369 cm1 with a shoulder at 9,760 cm1 for the binuclear Cu methacrylate was assigned to the dxz , dyz ! dx 2 y 2 and dz2 ! dx 2 y 2 transitions in the tetragonal ligand field. The appearance of a shoulder in the spectra (like the one observed in the last case) is characteristic of the bridging systems with antiferromagnetic interaction. For the complexes with mixed carboxylate functions electron transitions dxz , dyz ! dx 2 y 2 and dxz , dyz ! dz2 typically overlap. For example, in the spectra for the trinuclear Cu(II) acrylate and methacrylate complexes these transitions yield bands at 15,152, 14,338 cm1 and shoulders at 9,886, 9,668 cm1 , respectively [6] (Table 4.3). Certainly, on the basis of the spectral data alone, it is impossible to conclude the exact bond nature and coordination mode in metal carboxylates. Although there is a clear need for the simultaneous analysis of the X-ray diffraction data, crystallochemical studies of the unsaturated carboxylates are rather rare.

4.1.4 Molecular Structure The structural data available deal mainly with methacrylate complexes. Thus the complexes [Cu2 (CH2 DCHCOO)4 (C2 H5 OH)2 ](C2 H5 OH), [Cu2 (CH2 DCHCOO)4 (CH3 OH)2 ] [30], Cu[CH2 DC(CH3 /COO]2 (C7 H6 N2 /2 , Cu2 [CH2 DC(CH3 /COO]4 (C7 H6 N2 /2 [4], Cu[CH2 DC(CH3 /COO]2 (bipy)2 [31], Cu2 [CH2 DC(CH3 /COO]4 [(NH2 /2 CO]2 [32], Eu(CH2 DC(CH3 /COO)3 [33], [La(CH2 DC(CH3 /COO)3 (phen)(CH2DC(CH3 /COOH)]2 [8], as well as the heteronuclear compounds [CuLa(CH2 DC(CH3 /COO)5 (phen)(C2 H5 OH)]2 [9], [CuNd(CH2 DC(CH3 /COO)5 (phen)(EtOH)]2 [17], [CuTb(CH2 DC(CH3 /COO)5 (phen)(H2O)]2 [11], and [NdZn2(CH2 DC(CH3 /COO)6 (phen)(EtOH)] [34] have been studied.

66

4 Spectral Characteristics and Molecular Structure of Unsaturated Carboxylic Acid Salts

a 2′

6 b 2

6′

9

8 7 3

4 11

10

12

5

1 Cu 0 C

6 13

2′

6′

2

a

b

Cu(2)

OH CH

CH3

Cu(1)

Fig. 4.2 Projection of the structure of [Cu2 .CH2 DCHCOO/4 (CH3 OH)2 ] along c axis (a) and formation of lantern complexes combined with hydrogen bonds (dotes) (b)

A typical feature of the compounds considered, as well as their saturated analogs, is the formation of binuclear lantern type complexes LM(RCOO)4 ML. For example, the structure of copper acrylate is presented in Fig. 4.2. Copper atoms are bound into binuclear complexes by four 2 -O,O0 (meth) acrylate groups. Various donor molecules (ethanol, methanol, benzimidazole, bipyridine (bipy), or phenantroline (phen))2 can act as apical ligands in these compounds. According to the structural data, copper atoms have a square pyramid coordination with 2 Some polydentate ligands, such as phen, are capable of substituting Ocarb in the equatorial plane thus destroying the metal–carboxylate core and nets. For example, in the tetranuclear Cu2 La2 methacrylate complex each copper atom is coordinated with three oxygen atoms of the carboxylate

4.1 Metal (Meth)acrylates

67

˚ [4, 30]. The pyramid base is an a typical elongation of the apical bond by 0.2 A ˚ [22]. The CuO disalmost regular square with OO distances equal 2.72–2.78 A ˚ ˚ tances are between 1.93 and 1.98 A in solvated acrylates [30] and are around 1.97 A in methacrylate complexes [4, 32]. The length of the CO bonds varies insignifi˚ in Cu2 [CH2 DC(CH3 /COO]4 [(NH2 /2 CO]2 [32] cantly between 1.248 and 1.267 A ˚ and between 1.244 and 1.260 A in Cu2 [CH2 DC(CH3 /COO]4 (C7 H6 N2 /2 [4], indicating delocalization of density of the  -electrons of the carbonyl groups and the bidentate bridging mode of the coordination. The CuCu distance in (meth)acrylate ˚ [22], 2.617 A ˚ [22], 2.662 A ˚ [4], or 2.609 A ˚ [32]. Varicomplexes equals 2.609 A ation in the steric conditions of the packing of the complexes and the nature of the axial ligands may cause metal atoms to deviate out of the equatorial plane of oxygen atoms, i.e., to increase the CuCu distance. Thus, the value of such devi˚ in the Cu2 [CH2 DC(CH3 /COO]4 (C7 H6 N2 /2 complex [4], which ation is 0.2178 A corresponds to the maximum CuCu distance in the series of the (meth)acrylate ˚ allow the excomplexes considered. While relatively short distances (2.61–2.66 A) istence of the direct metal–metal bond, it is not always clear whether the exchange interaction between metal atoms takes place directly or through bridges of atoms. Therefore, some authors’ statements [4] regarding direct strong metal–metal interaction in the above-mentioned benzimidazole binuclear copper methacrylate complex without the data on the magnetic and resonance properties seem to be not substantiated enough. Lengths of the double bond in the complexes discussed are within the ˚ i.e., they do not take part in the additional coordination usual limits (1.32–1.36 A), with a metal atom. Binuclear fragments with double carboxylate bridges can provide a basis for oligomeric or polymeric structures. Usually in such complexes, oxygen atoms of neighboring fragments act as axial ligands, thus creating chains. The copper(I) acrylate Cu(CH2 DCHCOO) supposedly has a structure of this type [24]. Carboxylate groups occupy trans positions, one relative to another, while neighboring fragments are bound into ribbons by additional CuO bonds, i.e., the acrylate groups function as tridentate ligands. C

O

O Cu Cu O O

C

bridging groups and two nitrogen atoms. One of the nitrogen atoms occupies an equatorial position and another one sits at the top of the pyramid, thus preventing the formation of a polymeric structure [9].

68

4 Spectral Characteristics and Molecular Structure of Unsaturated Carboxylic Acid Salts

Competition between carboxylate and accompanying ligands for the formation of strong bonds with a metal is known to be an important factor defining the structure of a carboxylate complex. In some cases this can cause a change in the structural function of the carboxylate ligand. In particular, synthesis of the copper(II) (meth)acrylate in the presence of a donor ligand imidazole results in an increase of nucleation of the complex formed and yields the binuclear complex Cu2 [CH2 DC(CH3 /COO]4 (C3 H4 N2 /2 [35], as well as the trinuclear carboxylates Cu3 (CH2 DCHCOO)5 (OH)(imH)3 or Cu3 (CH2 DC(CH3 /COO)5 (OH)(imH)3 [6] along with the mononuclear copper methacrylate Cu[CH2 DC(CH3 /COO]2 (C3 H4 N2 /2 . Molecular unit of the complexes includes two 2 -O,O0 ’-(meth)acrylate ligands and 3 -OH group binding copper atoms. Coordination of each copper atom is augmented by monodentate carboxylate groups and the nitrogen atoms of imidazole ligands: O C

C O

Cu

O

O

O

OH

C

O

Cu

Cu O

O

C O

O C

Consequently, the coordination mode of one of the copper atoms is a distorted trigonal bipyramid, while the two other atoms have the square planar mode. Coordination of a copper atom with two monodentate methacrylate groups and benzimidazole molecules results in the formation of the complex with trans square planar configuration (Fig. 4.3a). Each molecular unit is bound with four ˚ thus neighboring units by a system of hydrogen bonds (bond length 2.721 A), creating a two-dimensional supramolecular structure (Fig. 4.3b). Another donor ligand, tris(2-benzimidazolylmethyl)amine (ntb) forms strong bonds with a metal atom with the participation of all four N atoms. Hence, (meth)acrylate groups in the Zn(II) complexes [Zn(ntb)(CH2 DCHCOO)](NO3 /(H2 O) and [Zn(ntb)(CH2DC(CH3 /COO)](NO3 /(H2 O) act as monodentate ligands [36]. In some cases the above mentioned structural changes are accompanied by the reduction of the metal ion. In particular, this was observed upon the synthesis of the mixed valence complex Cu2 I Cu2 II [CH2 DC(CH3 /COO]6 (PPh3 /4 (MeOH)2 from the corresponding binuclear Cu(II) methacrylate crystal hydrate [37].

4.2 Metal Dicarboxylates

69

b

a C4A

c

02A 01A

C11 N2 C10

N1 Cu1 N1A

C5 C9 C8

b

C8A

C7

02

C1 C3

N2A

01

C6

a

C2 C4

Fig. 4.3 Molecular structure of Cu[CH2 DC.CH3 /COO2 .C7 H6 N2 /2 (a) and diagram of its crystal packing (b)

4.2 Metal Dicarboxylates The presence of two carboxyl groups in dicarboxylic acid molecules expands their functional capabilities as ligands and, therefore, defines the structural diversity of the resulting metal carboxylates. Depending on the nature of the metal and the reaction conditions, monosubstituted acid salts, linear, or three dimensional coordination polymers are formed. Let us consider the most typical groups.

4.2.1 Monomeric Salts Acidic maleates of Co, Fe [38, 39], Zn, Ni [40, 41], Mn [42], and Mg [43, 44] with the general composition M(C4 H3 O4 /2  4H2 O are the typical representatives of this class of compounds. In these centro-symmetrical complexes, the metal atom is linked to two monodentate maleic acid residues (bond length for the CoO equals ˚ and 2.157 A ˚ for the FeO), the octahedral metal coordination being com2.123 A, pleted by water molecules (Fig. 4.4). The planar structure of the maleate ligand in these complexes is stabilized by the formation of an intramolecular hydrogen bond. For example, in the Fe maleate the ˚ while it equals 2.439 A ˚ for the Sb bond length for the H.7/    O.5/ equals 1.87 A, tetraphenylmaleate [45]. Overall, the maleate ligand is characterized by the parameters close to those of the free acid (Table 4.4). Indeed, the O    O distances in acidic maleate molecules usually are within a ˚ range. However, the symmetry of the hydrogen bond can vary sig2.39–2.44 A nificantly from perfectly symmetrical to strongly asymmetrical. For example, the most asymmetrical hydrogen bond was found in the molecule of sodium acidic maleate crystal hydrate, Na(OOCCHDCHCOOH)3H2 O, by the neutron diffraction method [54].

70

4 Spectral Characteristics and Molecular Structure of Unsaturated Carboxylic Acid Salts

Fig. 4.4 Molecular structure of Fe(II) hydrogen maleate

There are two unequivalent fumaric acid residues in the binuclear complex [55] [Cu2 (OOCCHDCHCOO)(phen)4](OOCCHDCHCOO)11H2 O, one being a bridge connecting two copper atoms and the other functioning as a counter ion. The COO groups are coordinated to the metal atoms as monodentate ligands . D 197 cm1 /. The maleate dianion also plays a dual role in the molecule of the Cu(II) complex Cu(L)(H2 O)(OOCCHDCHCOO)2 4H2 O (L D 3,10-bis(2-hydroxyethyl)-1,3,5,8,10,12-hexaazacyclotetradecane) [52]. First, this is an axial ligand binding the metal atom with a monodentate carboxyl group . D 249 cm1 /. On the other hand, the second carboxyl group acts as a counter ion. Presence of a broad net of intramolecular hydrogen bonds cause the CO bonds in the non-coordinated carboxyl group to become almost equivalent due to ˚ However, a similar tendency delocalization of electrons (1.255(3) and 1.262(3) A). ˚ as well as for the is also observed for the coordinated CO bonds (1.255(3) A), ˚ due to the participation of the non-coordinated non-coordinated ones (1.273(3) A), O in the formation of three types of hydrogen bonds. Acyl derivatives of metallocenes appear in diverse structural types, including mononuclear, binuclear, and tetranuclear complexes. As it has been noted above (Sect. 3.3.1), the reaction of Cp2 TiCl2 with acetylenedicarboxylic acid at room temperature in a two phase system yields the binuclear carboxylate [Cp2 Ti(OCOCCOCO)]2 , while in the presence of phase transfer catalyst (tetrabutylammonium bromide) and at lower temperatures the isostructural tetranuclear compound [Cp2 Ti(DOCOCCOCO)]4 5CH2 Cl2 is formed [56]. The TiO bond ˚ range, and the TiOC angles are within 138–144ı lengths are within 1.95–1.98 A range. Carboxyl groups of the acetylenedicarboxylate ligand are not coplanar, the corresponding planes forming an angle between 64.6 and 77ı for the bi- and tetranuclear complexes. Formation of the macrocyclic complex, obviously, is more favorable. The presence of solvent molecules capable to occupy central cavity of the macrocycle facilitates the realization of this packing. The monomeric structures of dicyclopentadienyl Ti(III) maleate and fumarate [21] were confirmed by spectroscopic and magnetic measurements. According to IR spectroscopy, the  values are relatively low (75 cm1 for the maleate and 100 cm1 for the fumarate), which corresponds to symmetrically bound carboxyl groups with chelating coordination. Hydrogenmaleate anions act as monodentate ligands in another compound, Cp2 Ti(OOCCHDCHCOOH)2 that is also

1.336(1)

Trigonal bipiramid

Pseudotrigonal piramid, trigonal-o-planaric

P 21 /c

I2/m

P 21

Cc

P1

LiC4 H3 O4 2H2 O

Cu(HC4 H2 O4 )2 2H2 O

CuH2 C4 O4 H2 O

Cu0:06 Zn0:94 H2 C4 O4 2H2 O

Cu2 (OOCCHDCHCOO)

Square-piramidal

Distorted octahedron

1.933(2) 1.959(2) 2.682(2) 1.97(2) 2.00(2) 2.00(2) 1.97(2) 1.988(3) 2.125(2) 2.022(2) 2.109(2) 1.999(3) 1.987(5) 2.027(5) 1.909(5) 1.887(5) 2.231(5) 2.058(6)

1.99 (cp)

1.488(9) 1.371(14)

1.328(4)

1.33(5)

1.336(3)

1.34(1)

CDC

2.402 (cp)

C2=c

Na2 C4 H2 O4 H2 O Square-piramidal

Table 4.4 Some crystallographic and structural characteristics of metal dicarboxylates ˚ MO Metal carboxylate Space group Geometry Distance (A)

1.28(3) 1.25(3) 1.28(3) 1.23(3) 1.281(4) 1.240(4) 1.247(3) 1.278(3)

1.293(1) 1.281(1) 1.250(1) 1.264(1) 1.23(1) 1.28(1) 1.24(1) 1.30(1) 1.235(3) 1.286(3)

CO

(continued)

[50]

[49]

[48]

[48]

[47]

[46]

Ref.

4.2 Metal Dicarboxylates 71

Trigonal bipiramid

Trigonal bipiramid

P 21 =c

P 21 =n

Cc

C2

(C6 H4 )4 Sb(OOCCHD CHCOOH)

(C6 H4 )4 SbOOCCHD CHCOOSb(C6 H4 )4

CoC4 H2 O4 3H2 O

CoC4 H2 O4 5H2 O

Monoclinic syngony

Trigonal piramid

Pm21n

Cu(C4 H3 O4 )H2 O

Geometry

Space group

Table 4.4 (continued) Metal carboxylate

2.071(4) 2.079(4) 2.083(4) 2.096(4) 2.104(4) 2.144(4) Co1 (2.066(2), 2.078(2), 2.131(2))

2.217(3) 2.207(3)

2.509(3)

˚ MO Distance (A) 2.120(6) 2.303(5) 2.306(5) 1.833(6) 1.866(5) 1.996(6)

1.321(3)

1.327(8)

1.317(6)

1.305(7)

1.405(10)

CDC

1.275(2) 1.245(2) 1.273(2)

1.245(2)

1.262(9) 1.292(2) 1.219(2) 1.258(6) 1.298(6) 1.213(6) 1.286(5) 1.226(5) 1.306(5) 1.211(5) 1.251(6) 1.270(6) 1.245(6) 1.260(7)

CO

(continued)

[38]

[38]

[45]

[45]

[51]

Ref.

72 4 Spectral Characteristics and Molecular Structure of Unsaturated Carboxylic Acid Salts

2.3601(18)

1.973(4)monodentate 1.955(3)br 2.054(4)br 2.089(4)ch 2.367(4)ch

P1

P 2(1)

P 2=n

Cu(L)(H2 O)(OOCCHD CHCOO)2 4H2 O L D 3,10-bis(2hydroxyethyl)1,3,5,8,10,12hexaazacyclotetradecane

ŒZn4 .OH/2 (OOCCHD CHCOO)3 (4; 40 bipy)2 ]

Co2 (2.083(3), 2.106(2), 2.119(2)) 2.149(2) 2.157(2)

˚ MO Distance (A)

Fe(C4 H3 O4 )2 4H2 O

Geometry

Space group

Table 4.4 (continued) Metal carboxylate

1.338(4)

1.338(3)

CDC

1.258(5)ch 1.260(5)ch 1.27295)br 1.247(5)br 1.294(6)monodent

1.252(3)coord 1.273(3)noncoord 1.262(3)free 1.255(3)free

1.253(2) 1.260(3) 1.223(2) 1.294(3)

CO

[53]

[52]

[38]

Ref.

4.2 Metal Dicarboxylates 73

74

4 Spectral Characteristics and Molecular Structure of Unsaturated Carboxylic Acid Salts

monomeric [57]. Effective magnetic moments (1.61 and 1.68 B) suggest presence of one unpaired electron at the Ti(III) atom, and relatively low Weiss constants .4 K/ indicate that exchange interactions in the system are weak. It has been shown [58] that presence of multiple bonds in the fumarate (Cp2 TiOCOCHDCHOCOTiCp2 / molecule does not affect parameters of the antiferromagnetic exchange interaction (J D 1:6 cm1 and g D 1:98), the parameters being similar to those found for the saturated analogue containing a succinate bridge. This means that these intramolecular interactions are mainly accomplished through overlapping of the ¢-orbitals. Overall complexes of dicarboxylic acids with bidentate carboxylate bridges are efficient concentrated magnetic systems. For example, high spin octahedral Fe(III) complexes [59] [fFe(salen)g2L] or [fFe(saloph)g2L] (salenH2 D N; N 0 -bis(salicylidene)ethylenediamine, salophH2 D bis(salicylidene)-o-phenylenediamine, L D fumarate, acetylenedicarboxylate) possess high exchange interaction constants (J D 4:75 to 5:47 cm1 /.

O N

O O

Fe

C O

N

N

O

O

C

C

Fe

C O

N O

Also magnetic properties data for the complex [Mn2 (salen)2 (-fumarate)]4H2O provide an evidence to super exchange interaction between paramagnetic centers and to high spin electron configuration [60].

4.2.2 Coordination Polymers As mentioned above, the polydentate nature of dicarboxylic acids along with the tendency for coordination saturation of the central metal ion are important factors conducive to predominant formation of polymeric structures. Additional stabilization of such systems is provided by a network of intra- and intermolecular hydrogen bonds. There is an interesting structural peculiarity of copper(II) dihydrogenmaleate tetrahydrate [48] Cu(C4 H3 O4 /2 4H2 O. Each of the Cu atoms (coordination number, CN D 4) is coordinated directly with four water molecules (bond angle  90ı ) rather than the carboxylate anions. Interaction of the cation [Cu(H2 O)4 ]2C with four anions [C4 H3 O4 ] takes place via water molecules by bridging hydrogen bonds, which results in a formation of the cation chain [Cu(H2 O)4 ]n 2nC . When maleic acid acts as monoprotic, a salt of different composition and structure may be formed.

4.2 Metal Dicarboxylates

75

A molecule of Zn(II) hydrogenmaleate, Zn(C4 H3 O4 /OHH2 O, serves as an example of the polymeric chain comprised of metal ions connected by single carboxylate bridges [61]. Each zinc ion has two valence bonds – with hydrogenmaleate and with hydroxyl group, and its coordination sphere is completed to six by [C4 H3 O4 ] and OH anions from the neighboring zinc ions as well as two water molecules:

C O

C

OH

C

C

O HO

H O H O

Zn H O H

OH

Zn

Connection of a pair of zinc ions by the hydrogenmaleate ligand results in the formation of a spiral-shaped macromolecule typical for the cis-configuration of the acid. Due to intermolecular bonding via hydroxyl groups connecting zinc atoms of neighboring strands, a secondary structure is formed similarly to a cable twisted from several strings (Fig. 4.5). Dehydration of the compound leads to formation of

Fig. 4.5 Morphology of the Zn(C4 H3 O4 /OHH2 O coordination polymer

76

4 Spectral Characteristics and Molecular Structure of Unsaturated Carboxylic Acid Salts

the anisotropic product [ZnH(OOCCHDCHCOO)(OH)]n with fiber type structure which turns into powder upon heating. The distinctive structure of maleic acid, namely its planar configuration with cis-position of the carboxyl group about the double bond, determines its ability to form metal chelates. Both of the carboxylic groups of maleic acid can participate in valent bonding of a metal, and thus its denticity will be 4 (or higher). In most cases, this leads to formation of seven-membered chelate cycles [38, 48, 62–65]. Thus, in the Cu(II) maleate monohydrate, CuC4 H2 O4 H2 O [48], two oxygen atoms from different carboxyl groups of the maleate ligand are coordinated with the Cu ion resulting in the seven-membered chelate cycle creation (average CuO bond length ˚ Square pyramidal metal coordination is completed by oxygen atoms of is 1.99 A). ˚ Each of the the neighboring maleate ligands and H2 O molecule (CuO 2.26 A). maleate groups, acting as a tetradentate ligand, is bound with three Cu atoms of the polymer framework: Cu

H C H C

C

C

O

O

O

O

O

Cu

O

C O

H H

C O

O Cu

Separate maleate moieties are connected via hydrogen bonds formed by water molecules and oxygen atoms of carboxylic groups. The formation of such a twodimensional structure is also typical for a bimetallic CuZn maleate [49]. The analogous chelation pattern is observed in the case of Co(II) maleate trihydrate [38, 66], which coordination polyhedron is a distorted octahedron. The acid anion bound with the metal ion via two oxygen atoms of the carboxyl groups [the ˚ and O(4)Co 2.071A ˚ (see Table 4.4)], probond lengths are O(2)Co 2.096 A duces a seven-membered chelate cycle (Fig. 4.6), [38] while coordination of the O(3) and O(5) with Co atoms leads to formation of a three-dimensional framework (Fig. 4.7).

H

O.

. ..

. H C C O Fig. 4.6 The fragment of the coordination polyhedron of Co (II) maleate

H

C O

C..O

. ..

H

O

H

O.

C

. . ..

O

Co O

O.. . .. O H C

4.2 Metal Dicarboxylates

77

Fig. 4.7 Structure of three-dimensional coordination polymer of Co(II) maleate

In this case, maleic acid acts as a tetradentate ligand. The polyhedron is completed to octahedron by two H2 O molecules (O(6) and O(7)) that are cis-oriented toward each other. The third H2 O molecule is water of crystallization. Its O(12) atom has three short intermolecular contacts: with carboxyl O(3) atom not included ˚ and two atoms O(7) (2.77 A ˚ and 2.86 A) ˚ of the H2 O in the chelate cycle (2.76 A) molecules coordinated with the neighboring Co atoms. A different chelation type is found for the Sn(II) maleinate monohydrate, SnC4 H2 O4 H2 O [67]. Sn atom coordinates one carboxylic group via oxygen atoms. A seven-membered chelate cycle closure is possible with the formation of an additional donor-acceptor bond with O atoms of another functional group due to the ˚ Coordination is completed lone electron pair on the oxygen atom (SnO 2.817 A). to CN D 6 by two O atoms from the neighboring maleic acid anion and a water molecule: H

C

H C

C

O

O C

O

O

Sn

O

Sn

C O O H H

Availability of the trigonal oxygen atoms increases the denticity of the maleate anion to five. The effect of the synthesis conditions on the metal complex structure may be exemplified by cadmium(II) maleinate dihydrate. When obtained by different methods [68, 69], the salt has the same composition, CdC4 H2 O4 2H2 O, but different

78

4 Spectral Characteristics and Molecular Structure of Unsaturated Carboxylic Acid Salts

Fig. 4.8 The crystal structure of Cd(II) maleate dihydrate

structures. In one of them, two crystallographically dissimilar cadmium atoms, Cd(1) and Cd(2) (Fig. 4.8) and two types of maleate ligands (types A and B) are observed [68]. Thus, Cd(1) has the CN D 6 and it coordinates two oxygen atoms, O(1) and O(3), of different maleate ligands along with four water molecules (coordination polyhedron is a distorted octahedron). For Cd(2), the CN is 8, and it chelates with four carboxyl groups from two types of maleate anions. The type A maleate ligand, O(1–4), includes two trigonal oxygen atoms, O(1) and O(3) (one per carboxyl group), connected with Cd(1). In this way, formation of a three-dimensional coordination polymer is achieved which is stabilized due to hydrogen bonds between water molecules and oxygen atoms of the carboxyl groups. Same-formula Cd(II) maleate dihydrate [69] contains only one type of Cd ions, that has a distorted octahedral coordination. The Cd ion (CN D 4) coordinates two water molecules and four oxygen atoms of three maleate groups. Each maleic acid anion, being a tetradentate ligand, interacts with three Cd ions to give the coordination polymer. As might be expected, formation of chelate structures for the trans-isomer, the fumaric acid, is hindered significantly by spatial remoteness of the functional groups from each other. Thus, in the Co(II) fumarate pentahydrate, [CoC4 H2 O4 4H2 O]n nH2 O, two oxygen atoms from two different carboxyl groups connect neighboring cobalt atoms acting as a bridge [38, 70]. The bond length of O(3)Co(1) and ˚ respectively. Four H2 O molecules add up metal O(4)Co(2) are 2.078 and 2.106 A, coordination to octahedron, and the fifth one is water of crystallization. Two crystallographically independent Co atoms are located in the restricted positions on the C2 axis, which leads to the formation of an infinite chain of coordination polymer with cis-positioned anions of fumaric acid the fragment of which is shown in Fig. 4.9. The water of crystallization molecule is also located in two restricted positions on

4.2 Metal Dicarboxylates

79

Fig. 4.9 The fragment of coordination polymer structure of Co(II) fumarate pentahydrate

˚ hydrogen bonds with polymeric chains the C2 axes and forms strong (2.69–2.73 A) arranging them into a three-dimensional framework. Therefore, it may be assumed that ligand conformation, such as cis- and transisomerism of maleic and fumaric acids may serve as the efficient means for regulation of structure and topology of coordination polymers of the kind under consideration. The miscellaneous ligand complexes of Mn(II) maleate/fumarate and 4,4-bipyridine (bipy), fMn(maleate)(-4,40-bipy)]  0.5H2O)g1 and fMn(fumarate) (-4,40-bipy)(H2O)]  0.5(-4,40-bipy)g1 represent another example of such a structural transformation from a two-dimensional to a three-dimensional coordination polymer [71]. In the case of maleate complex, the Mn atoms are chelated by the ligand carboxyl groups forming zigzag-shaped [Mn(maleate)]1 chain:

Here, one can identify two modes of carboxyl group coordination. By one of them, a carboxyl group provides a single atom bridge between two Mn atoms forming a

80

4 Spectral Characteristics and Molecular Structure of Unsaturated Carboxylic Acid Salts

four-membered cycle. It is worth noting at this point that such kind of bridges are important for super-exchange interactions, as will be discussed below. By the second mode, the maleate ligand, as a bidentate one, binds with two Mn centers. This results in the formation of a chain of consecutive seven-, eight-, seven-, and four-membered rings. The -4,40 -bipy units interlink these chains into a two-dimensional coordination polymer. The key factor in the structure of the isomeric fumarate complex is the fumarate bridges connecting different Mn atoms, so that a two-dimensional chain of 14- and 22-membered cycles is formed. The carboxyl group coordination mode is both monodentate and bidentate. Accordingly, the 2-D [Mn(fumarate)]1 layers and -4,40-bipy units produce a three-dimensional open-frame type network. The analogous structural motif was also observed for the interpenetrating three-dimensional framework of coordination polymer for the Zn(II) fumarate with polynuclear core structure [53]. Formation of the 3D coordination polymers is encouraged by the capability of fumarate ligands for different coordination modes of the COO group with metal ions, even in the same compound. This is particularly characteristic for lanthanoid complexes. Thus, three types of fumarate ligands were discovered in the isomorphous structures of [Sm2 (OOCCHDCHCOO)3 (H2 O)4 ]3H2 O [72] and [Eu2 (OOCCHDCHCOO)3 (H2 O)4 ]3H2O [73] (1) with one chelate and one bridgecyclic anti COO ends; (2) with one bridge-cyclic anti and one bridging syn-anti COO ends; and (3) with one chelate and one bridging syn–syn COO ends.

As was mentioned above, employment of a neutral ligand as an additional chelating agent affects the structural function of the carboxylate ligand and stability of its bond with metal ion. In particular, competition of the ligands for the space in the inner coordination sphere of the complexes is increasing, which generates the tendency for diminishing of the carboxylate ligand denticity. Sometimes, this leads to relocation of the carboxylate anion to the axial positions,3 [74] as it is observed in poly[[[pyrazino[2,3-f][1,10]-phenanthroline]zinc(II)]-4-fumarato-2fumarate] [75] and di--fumarato-bis[o-phenanthroline)-dicobalt(II) [76]:

3

In particular, this is common for neutral ligands of unsaturated amines family, which almost always displace a part of carboxylic group O atoms from the equatorial positions. In the molecule of [Cu(N;N 0 -dimethylethane-1,2-diamine)(-fumarate)(-H2 O)]n [74], each of the copper atoms is located at the C2 axis and has an octahedral configuration which includes equatorial N atoms of the diamine ligand and O atoms of the carboxylate ligand, and also two water molecules as the axial ligands.

4.2 Metal Dicarboxylates

81 O C HC N

O HC

O

C

HC O

O O

C

CH

CH

C

C

O Co

O N

O

C O

Co

N

O O

Co

O C HC

HC

O

n

HC HC O

C

O

N

O

HC

C

O

O

C O

In the latter structure, three crystallographically different fumarate ions link cobalt cations into the two-dimensional framework of an unusual geometry, which consists of consecutively alternating 8- and 28-membered rings. Due to the similar structural pattern, the dianionic ligands in the 1,10phenantroline [77] and benzimidazole [78] fumarate complexes of Ni(II) link the two neighboring metal atoms in a monodentate fashion into a dimeric unit or a molecular polymeric hetero-chain: NH N

O O

O

Ni

H2O

O H2O N NH n

Fumarate bridges act as monodentate ligands in the 1D–3D polymeric Cu(II) ˚ in complexes as well [74, 79, 80]. The dimeric units (CuCu D 3.268 A) f[Cu(OOCCHDCClCOO)(H2 O)2 H2 O]gn are connected by the bifunctional chlorofumarate anions into one-dimensional bands that are cross linked by hydrogen bonds, including those with participation of the third water molecule (Fig. 4.10a). Analogously, the fumarate dianions form a zigzag chain between Cu(II)-diamine centers in the complex [Cu(N;N 0 -dimethylethane-1,2-diamine) (-fumarate)(-H2O)]n [74] (Fig. 4.10b; while formation of 2D structures in [Cu(-fumarate)(piperazine)(H2O)2 ]n [80] takes place with involvement of both

82

4 Spectral Characteristics and Molecular Structure of Unsaturated Carboxylic Acid Salts

Fig. 4.10 Polymer structures of Cu(II) fumarate complexes f[Cu(OOCCHDCClCOO) (H2 O)2 H2 O]gn (a), [Cu(N,N0 -dimethylethane-1,2-diamine)(-fumarate)(-H2 O)]n (b) and [Cu(- fumarate)(piperazine)(H2 O)2 ]n (c)

fumarate bridges and piperazine molecules. Accordingly, the interpenetrating equivalent 2D layers create a three-dimensional interlocked network (Fig. 4.10c). As the limiting case of structural function transformation of carboxyl groups, one may consider the examples of complexes with all of the coordination sites around the metal atom occupied by neutral ligands, in which the carboxylate anion plays only the role of a counter ion [81, 82]: 2+

N

N N M

N OH2

OH2

O

O

4H2O O

O

M = Zn, Cd

4.2 Metal Dicarboxylates

83

The characteristic peculiarity of coordination polymers of itaconates [Cd(C5 H4 O4 / (H2 O)2 ] [83] and [Ba(C5 H5 O4 /2 (H2 O)] [84] is the presence in ˚ between the double bonds of the neighbortheir structure of contacts (CDC< (COO)as (COO)s MO M3O Ref. [Fe3 O(CH2 D CHCOO)6  3H2 O]OH ŒCr3 O.CH2 DCHCOO/6  3H2 OOH ŒV3 O.CH2 DCHCOO/6 ] (CH2 DCHCOO) ŒFe3 O(CH3 CHD CHCOO)6 3H2 O] NO3 H2 O ŒCo3 O(CH3 CHD CHCOO)6 2H2 O] ŒFe2 CoO(CH3 CHD CHCOO)6 3H2 O] 2H2 O ŒFe3 O(CH2 D CHCOO)6 3H2 O] NO3 4H2 O ŒCo3 O(CH2 D CHCOO)6 2H2 O]

1,635

1,575, 1,515

1,435, 1,370

525

[114]

1,635

1,575, 1,525

1,440, 1,370

540

[115]

1,635

1,590, 1,527

1,444, 1,375

1,657

1,562

1,412

1,657

1,569, 1,537

1,409

1,659

1,559

1,413

575, 700

[118]

1,639

1,578

1,445

616

[118]

1,642

1,564, 1,530

1,429

[116] 628

[118]

[118]

[118]

being interlinked by bidentate carboxylate bridges along with hydroxyl groups and in addition, containing methacrylate groups bound with metal atoms in a monodentate fashion [120]:

C

H OH H2O

O Al O

H2 O O O

O

O

H C

OH2

H

O O

Al O

O C

Al O

Al

H

O

O

O Al

H

Al O

O OH

O H

OH2 O

4.4 Unsaturated -Oxo Multinuclear Metal Carboxylates

91

4.4.2 Mass-Spectrometry For the confirmation of multinuclear structure of the studied compounds in the absence of X-ray data, mass-spectrometry analysis coupled with solvent extraction of ions is often employed. In Fig. 4.14, the positive-ion mass spectrum for the Cr(III) acrylate in aqueous alcohol solution is shown, with expanded fragment in the insert. The principal peak in the mass spectrum .m=z D 598/ corresponds to the calculated mass for the cation ŒCr3 O.CH2 DCHCOO/6 C . The presence of peaks with m=z D 596, 599, 600, and 601 is determined by chromium and carbon isotopes. Increase in the electric field potential enables ion fragmentation by dissociation upon collision. In particular, it was reported [114] that in the mass-spectrum of Fe(III) acrylate, ions with m=z D 539, 468, and 397 are attributed to the loss of one, two, and three acrylate anions, respectively, and the ion with m=z D 341 corresponds to the loss of Fe(CH2 CHCOO)3 molecule from the molecular ion ŒFe3 O.CH2 DCHCOO/6 C . The EXAFS spectral data indicate the cluster structure ˚ as well. For example, for the Fe(III) maleate [121], the  length FeFe is 3.29 A,  bond while the distances to the bridging oxygen atom R10 and oxygen atoms of the ˚ and 2.03 A, ˚ respectively. This is in agreement with ligand setting (R1 / are 1.94 A the X-ray analysis data, for example, for ŒFe3 O.2 -bethain/6 .H2 O/3 .ClO4 /  7H2 O ˚ R1 D 2:009 and 2.034 A) ˚ [122]. (R10 D 1:917(2) and 1.917(3) A, It should be noted that the tendency for formation of coordination polymers due to the wide ligating capabilities of unsaturated dicarboxylic acids apparently exists in the case of Cr(III) and Fe(III) unsaturated oxodicarboxylates as well. This is supported by the fact of cluster cations being observed only at a relatively high electric field potential [123]. In addition, in the mass-spectrum of the Cr(III) itaconate (Table 4.8), along with the peak corresponding to unicharged [Cr3 (O(OCOC(COOH)DCH2 /6 ]C ion, peaks for double- and even triple-charged

598

Intensity

598

599 596

600 601

558 m/z m/z

Fig. 4.14 Mass-spectra of the positive ions extracted from an aqueous alcoholic solution of cluster-type Cr(III) acrylate recorded at U D 200 V

92

4 Spectral Characteristics and Molecular Structure of Unsaturated Carboxylic Acid Salts Table 4.8 The data of mass-spectrometry analysis for Cr(III) maleate and itaconate at U D 110 V[124] Cation m=z, Found/calculated Intensity (%) ŒCr3 O(OCOCHD 925.91/926 6:24 CHCOOH)6 ]C 2CH3 OH 893.93/894 8:78 ŒCr3 O(OCOCHD CHCOOH)6 ]C CH3 OH ŒCr3 O(OCOCHD 861.89/862 31:03 CHCOOH)6 ]C ŒCr3 O(OCOCHD 745.93/746 3:31 CHCOOH)4 (OCOCHDCHCOO)]C 629.82/630 0:50 ŒCr3 O(OCOCHD CHCOOH)2 (OCOCHDCHCOO)2 ]C ŒCr3 O(OCOCH2 C(COOH)D 1009.97/1010 4:87 CH2 /6 ]C 2CH3 OH ŒCr3 O(OCOCH2 C(COOH)D 977.95/978 5:65 CH2 /6 ]C CH3 OH 880.90/881 2:55 Œ(Cr3 O)2 (OCOCH2 C(COOH)DCH2 /10  (OCOCH2 C(COO)DCH2 /]2C 815.85/816 8:90 Œ(Cr3 O)2 (OCOCH2 C(COOH)DCH2 /8  (OCOCH2 C(COO)DCH2 /2 ]2C Œ(Cr3 O)2 (OCOCH2 C(COOH)DCH2 /6  750.85/751 11:57 (OCOCH2 C(COO)DCH2 /3 ]2C Œ(Cr3 O)2 (OCOCH2 C(COOH)DCH2 /4  685.83/686 9:98 (OCOCH2 C(COO)DCH2 /4 ]2C 729.48/729 1:14 Œ(Cr3 O)3 (OCOCH2 C(COOH)DCH2 /8  (OCOCH2 C(COO)DCH2 /5 ]3C 642.47/642 1:32 Œ(Cr3 O)3 (OCOCH2 C(COOH)DCH2 /4  (OCOCH2 C(COO)DCH2 /7 ]3C

cluster cations are present in which Cr3 O7 “cores” are interconnected by bridges of tetradentate itaconate ligands. The completion of ligand arrangement for the “cores” is achieved by means of itaconic acid anions in which only one of the carboxyl groups is involved in coordination.

4.4.3 Molecular Structure As it was pointed out above, the structural data for the unsaturated -oxocarboxylates are quite limited, and those available are mostly for multinuclear alkoxyderivatives of Zr and Ti. For example, the structure of Zr6 .OH/4 O4 .CH2 DC.CH3 /COO/12 [125] contains an octahedral Zr6 O4 (OH)4 core in which the triangular faces of Zr6 octahedron are “capped” with 3 -O and 3 -OH groups. The rest of the coordination sites of Zr is occupied by chelating or bridging methacrylate ligands. Therefore, the coordination sphere of each of Zr atoms consists of two 3 -O, two 3 -OH, and four O atoms of the methacrylic acid moiety. Interestingly, the structural characteristics

4.4 Unsaturated -Oxo Multinuclear Metal Carboxylates

Ti(3A)

Zr(2A)

93

Zr(1) Ti(4)

Ti(4A)

Zr(2)

Ti(3)

Fig. 4.15 The structure of a cluster core of Ti4 Zr4 O6 (OBu)4 (CH2 (CH3 )DCHCOO)16

of this complex determined from the ZrK-end of the EXAFS-spectra appeared to ˚ (CN D 2) and 2.24 A ˚ (CN D be in agreement with the X-ray data (ZrO D 2:09 A ˚ 6); ZrZr D 3.52 A (CN D 2) [126]. In contrast to the Zr6 cluster under discussion, the structure of which fits well a spherical model, metal atoms in heteronuclear oxocarboxylates [127] produce extended zigzag-shaped chains of dodecahedral [ZrO8 ] and octahedral (TiO6 ] units (Fig. 4.15).The Zr atom arrangement in this ˚ 0.5 Ti cluster, on average, includes 7.5 O atoms positioned at distance of 2.17 A, ˚ ˚ atoms (3.07 A), and 1.5 Zr (3.45 A). A single Ti atom arrangement comprises of a ˚ (2 atoms) and 2.04 A ˚ (4 atoms), system of O atoms located at distances of 1.83 A 0.5 Zr atoms, and one Ti atom. The structure was determined for various size titanium oxo-clusters, Ti6 O4 (OEt)8 (OR)8 [128, 129], Ti4 O2 (OPrn /8 (OR)8 [129], Ti4 O2 (OPri /6 (OR)6 [128, 129], and Ti9 O8 (OPrn /4 (OR)16 [130] (OR signifies methacrylate or acrylate groups). [Ti9 O8 ((OPrn /4 (OOCC(CH3 /DCH2 /16 [131] is a cluster containing the largest number of carboxylate ligands per Ti atom (1.78). Due to this, the molecule possesses quite an open structure represented by a cycle comprised of six octahedrons attached via the points and two octahedrons joined by the edges. Hence, only two of the oxo-bridges are 3 -oxo, while six others are 2 -oxo. Distribution of bridging methacrylate and OR groups yields a non-symmetric macrocycle. The tendency of dicarboxylic acid carboxylates for the formation of coordination polymers is also valid for their oxo-complexes. Another illustration of this is the crystal structure of Zn(II) fumarate bipyridine complex [53]. According to X-ray analysis, the three-dimensional framework comprises of the tetranuclear hydroxo units, [Zn4 (OH)2 ], that produce two-dimensional polymeric layers by means of bis-bidentate and chelate monodentate fumarate bridges (Fig. 4.16). A threedimensional structure of the coordination polymer is formed by bridges of stacked 4,40 -bipy ligands between the [Zn4 (OH)2 (fumarate)3]1 layers. Also, 12-nuclear Mn(II) oxo-complexes with acrylate [132,133] and methacrylate [134] ligands have been synthesized and characterized by structural studies. These complexes possess molecular magnetic properties.

94

4 Spectral Characteristics and Molecular Structure of Unsaturated Carboxylic Acid Salts

Fig. 4.16 The view of two-dimensional framework of the [Zn4 (OH)2 (fumarate)3 ]1 coordination polymer

4.5 Cluster-Containing Unsaturated Carboxylates Cluster-containing monomers are molecular compounds with a framework com˚ distances conducive prised of metal atoms located at short (no longer than 3.5 A) to direct MM interactions, surrounded by ligands capable of participation in polymerization reactions. These compounds show promise for the development of materials based on individual clusters or ensembles of several atoms with size of 1.5–5.0 nm and well determined structures. One of the approaches to obtaining such systems consists of assembling of multinuclear complexes from the mononuclear ones on a polymer [135]. This approach is often utilized for non-functionalized polymers. Perhaps the most convenient method could be the one based on polymeranalogous transformations of polymers with involvement of particular clusters of mono- or heterometallic type, including polymerization and co polymerization of cluster-containing monomers. The research in this area is presently emerging. The attempts of employment in such syntheses of the carbonyl compounds Co2 (CO)8 and Fe2 (CO)9 and obtaining of cluster-containing monomers derived from methyl ether of p-vinylbenzoic acid have been reported [136]. Carboxylate clusters of the considered type were obtained from the trinuclear clusters M3 (CO)12 (M D Os and Ru) or Os3 (CO)12 (CH3 CN), Os3 (CO)10 (CH3 CN)2 , and (-H)Os3 (CO)10 .-OR) (R D H and Ph) and acrylic acid [137, 138]: Os(CO)4 H Os(CO)3 (CO)3Os O O C CH=CH2

4.5 Cluster-Containing Unsaturated Carboxylates

95

Convenient approaches to structure elucidation for such substances include the comparison of IR and NMR spectra of the reagents, model compounds (cluster analogs of known structure), and the products obtained (in particular, of carbonyl type). This is especially applicable to trinuclear Ru and Os clusters, which are spectroscopically informative due to the absence of bridging ligands and high symmetry. In the above context, complexes of the cluster-of-clusters type seem to be quite interesting. In recent years, they attracted wide attention due to the high coordination ability of organometallic cluster carboxylate ligands, such as [(CO)9 Co3 CCOOH] [90, 139–141]. Interaction of metal acetates, [M2 (OOCCH3 /4 ] where M D Zn, Co [142,143], Cr, Mo, and W [144,145] with cluster-containing acid, [(CO)9Co3C COOH], yields highly organized structures with cluster cores of different geometry and MM bond identity, which, in its turn, is surrounded by Co3 cluster units: Co

Co C

Co O

H

O

O M

O

Co

O O

Co

O Co

Co

Co

Co Co

O

O M O

Co Co

Co

Co

O

H

O Co

C Co

Co

Co

Obviously, there are no principal limitations to functionalization of such cluster molecules with unsaturated ligands according to the method described for other cluster-containing monomers [146, 147]. The carboxylates discussed so far were narrowed to carbonyl-type clusters, although for the solution of many problems, cluster-containing monomers and polymers of other kinds, in particular, halogen-containing ones, are of interest. In this connection, especially promising are derivatives of Mo(II) halogenides, multinuclear complexes of Mo6 Cl12 type that are readily obtained due to a strong tendency of molybdenum(II) for association [148], among which the most propitious are compounds containing a stable [Mo6 Cl8 ]4C group called the staphylonuclear group (Fig. 4.17). This faction embodies the configuration including a central

Fig. 4.17 The staphylonuclearic structure of [Mo6 Cl8 ]4C

96

4 Spectral Characteristics and Molecular Structure of Unsaturated Carboxylic Acid Salts

octahedron comprised of six molybdenum atoms, which is surrounded by eight chlorine atoms located in the points of a somewhat distorted cube. Importantly, the octahedron is incorporated in the cube in such a way that the Mo(II) atoms are placed in the centers of the cube faces, thus providing the equivalence of six directions (normals) along which the shielding of molybdenum ions by chloride ions is minimal. The estimated diameter of such a cluster is about 1 nm (i.e., the cube diagonal length, ClCl, of 0.6 nm plus two times the radius of chloride ion of 0.18 nm). The stable staphylonuclear group is central in complexation reactions [149] and is able of adding up to six axial ligands such as negatively charged ions or polar molecules, including those containing multiple bonds capable of polymerization, for example, acrylate anions in ŒMo6 Cl8 .CF3 COO/6n .CH2 DCHCOO/n 2 [150, 151].

4.6 Metal Carboxylates with Unsaturated Ligands of Acetylene Type These compounds are of interest primarily due to their ability for solid phase polymerization under ionizing irradiation. This is determined by the corresponding distances between the reactive acetylene centers, including the required availability of infinite chain of short acetylene–acetylene contacts, as well as the crystal lattice energy and cross-section for X-ray or ”-ray absorption. Most of the heavy metal salts of propiolic acid meet these structural criteria [152–154]. In the molecules of lanthanoid propynoate complexes [152] acetylene-acetylene contacts are of 3.5– ˚ such distances for thallium dimethylpropynoate [154] and scandium(III) 3.94 A; ˚ respectively. It is important to note propynoate [153] are 3.454 and 3.79–4.02 A, that in most of the cases, an infinite chain of such contacts is formed, as in the structure of La2 O(OOCCCH)6 (H2 O)4 2H2 O (Fig. 4.18a). However, introduction in the molecule of a bulkier ligand, such as 2,20 -bipy, results in shortening of the acetylene–acetylene contacts chain (for instance, in the complex La2 O(OOCCCH)6 (2,20-bipy)(H2O)2 2(2,20-bipy)4H2O, it is limited to 5–6 contacts [152] (Fig. 4.18b), which affects the reactivity of these compounds in polymerization transformations. Another interesting feature of organometallic derivatives of propiolic acid is worth mentioning. The interaction of the latter with cobalt carbonyls yields cluster complexes of VIa type, (CO)6 (Co2 HCCCOOH) and (CO)10 (Co4 HCCCOOH) [90], i.e., complexes of the cluster-containing carboxylic acid, analogs of [(CO)9 Co3 CCOOH] which is widely employed as a carboxylate ligand in organometallic syntheses. It was demonstrated by the quantum chemistry calculations that electron density is transferred from the Co(CO)3 to the HCCCOOH moiety thus decreasing acidity of the carboxyl group. Among the derivatives of dicarboxylic acids containing acetylene bonds, salts of acetylenedicarboxylic acid attract the largest attention. The structure and properties of the acid were discussed in Chap. 2. As is common for metal dicarboxylates,

4.6 Metal Carboxylates with Unsaturated Ligands of Acetylene Type

a C12

97

b C13

C13 C32 C33

C33 C32 C13

C33 C12

C23 C22

C22 C23

C12 C33

C13 C32

C33 C32 C13 C12

C13 C33 C33

C13

C23 C22

C22 C23

C33

C13

Fig. 4.18 The view of dimer structures packing for La2 O(OOCCCH)6 (H2 O)4 2H2 O (a) and La2 O(OOCCCH)6 (2,20 -bipy)(H2 O)2 2(2,20 -bipy)4H2 O (b) complexes with formation of the short acetylene-acetylene contacts

Fig. 4.19 Diamond-like crystal structures for Sr(II) (a) and Zn(II) (b) acetylene dicarboxylates

these compounds feature a wide variety of structures, including monomeric salts (see Sect. 4.2.1) and linear and three-dimensional coordination polymers. A general structural motif may be described as a chain structure comprised of polyhedral metal centers (for example, tetrahedral as in [Be(C4 O4 /(H2 O)4 ]n [155] and [Zn(C4 O4 /2 (HTEA)2 ]n where HTEA is triethylamine [156], square pyramidal in f[Cu(C4 O4 /(H2 O)3 H2 Ogn [157] and [Cu(C4 O4 /(Py)2 (H2 O)]n [158] trigonal prism in [Cd(C4 O4 /(Phen)]n [159], and octahedral in [M(C4 O4 /(Phen)(H2 O)2 ]n (where M D Co(II) [160] or Mn(II) [161]), [M(C4 O4 /(Py)2 (H2 O)2 ]n (M D Fe, Co, and Ni) [158], and [Co(C4 O4 /(H2 O)4 2H2 O]n [162] interconnected by acetylenedicarboxylate dianions that most often are mono-coordinated. The presence of a system of hydrogen bonds and    interactions as, for instance, in the complexes with phenanthroline ligands, normally results in formation of three-dimensional coordination polymers. Of interest are a diamond-like crystal structure for anhydrous acetylenedicarboxylate [Sr(C4 O4 /] [163] (Fig. 4.19a) and that of the above-mentioned complex Zn(C4 O4 /2 (HTEA)2 ]n .[156]. In the latter, each of the zinc ions is bound in a monodentate fashion with four different carboxylate bridges thus producing two interpenetrating diamond-like frameworks (Fig. 4.19b). It is suggested that four carboxylate C atoms in each of the tetrahedral Zn(CO2 /4 units, which occupy C points in the diamond structure, may increase and expand the

III

III

I I I I I I

Sc(OOCCCH)3

f[Cu(C4 O4 /(H2 O)3 ]H2 Ogn ŒFe(C4 O4 /(Py)2 (H2 O)2 ]n ŒCo(C4 O4 /(Py)2 (H2 O)2 ]n ŒNi(C4 O4 /(Py)2 (H2 O)2 ]n ŒCu(C4 O4 /(Py)2 (H2 O)2 ]n ŒCo(C4 O4 /(H2 O)4 ]2H2 O]

I, II, III

Œ(CH3 /2 Tl(OOCCCH)]

La2 O(OOCCCH)6 (2,20 -bipy)(H2 O)2 2(2,20 -bipy)4H2 O

Square-pyramidal Octahedron Octahedron Octahedron Square-pyramidal Octahedron

Octahedron

Square-pyramidal

Table 4.9 Structural characteristics of metal carboxylates with acetylene ligands Coordination mode of the Metal carboxylate COO Geometry La2 O(OOCCCH)6 (H2 O)4 2H2 O II, III

P 21 =c C 2=c C 2=c C 2=c P 21 21 21 P 21 =a

Pa 3

Pnma

P1

Space group P 21 =c

1.273(2) 1.271(1) 1.270(2) 1.268(3) 1.275(8) 1.266(2)

1.248(3)

COcoor

˚ Distance (A)

1.231(3) 1.233(2) 1.231(2) 1.224(4) 1.233(8) 1.237(2)

1.237(4)

COconc

MO 2.599(2)trident 2.523(2)trident 2.658(2)trident 2.547(2)br 2.514(2)br 2.583(2)ch 2.557(2)ch 2.712(1)trident 2.521(2)trident 2.648(2)trident 2.511(2)br 2.484(2)br 2.480(2)mono 2.76(2)br 2.39(2)trident 2.65(2)trident 2.091(2)br 2.081(2)br 1.9555(15) 2.1458(8) 2.111(1) 2.086(2) 1.953(3) 2.105(1)

[157] [158] [158] [158] [158] [162]

[153]

[154]

[152]

Ref. [152]

98 4 Spectral Characteristics and Molecular Structure of Unsaturated Carboxylic Acid Salts

4.6 Metal Carboxylates with Unsaturated Ligands of Acetylene Type

99

Table 4.10 The coordination mode of the COO in unsaturated metal carboxylates Metal carboxylate Structure

Ref.

Pd(O2 CCHDCHCO2 H)2 (dppf)

[165]

ŒEu(O2 CC(Me)DCH2 /3 ]n

[33]

Cu3 [CH2 DC(Me)CO2 ]5 (OH] (imidazole)3

[6]

ŒCu(O2 CCHDCHCO2 / (C10 H8 N2 /]n 2H2 O

[166]

ŒNi(H2 O)6 ][Ni(H2 O)2 (O2 CCHDCHCO2 /]4H2 O

[64]

ŒMg(O2 CCHDCHCO2 /(H2 O)4 H2 O

[167]

ŒMn(O2 CCHDCHCO2 /(phen)]n , ŒMn(O2 CCHDCHCO2 /(phen)]n  nH2 O

[168] syn-anti

syn-anti

ŒMn(O2 CCHDCHCO2 /(bpy)]

[168] anti-anti

syn-syn

100

4 Spectral Characteristics and Molecular Structure of Unsaturated Carboxylic Acid Salts

framework, in other words, to serve as a molecular building block for creation of new elaborated structures. Representative metal carboxylates of the discussed type and their structural characteristics are listed in the Table 4.9. Therefore, the presented data demonstrates that unsaturated carboxylates, similar to their saturated analogs, reveal a variety of structures, from mono- and binuclear to cluster and polymeric complexes. The binding mode of carboxyl group with metal atom is also represented by different types (purely ionic and different degree covalent bond). The most common coordination modes are bidentate bridging, bidentate cyclic, and chelating, a more rare one is monodentate. The cases of their combination in the same molecule are quite frequent. A special manifest of unsaturated function in the carboxylate ligands is  -complexes represented primarily by Cu(II), Ag(I), and Pd(II) carboxylates. Basically all of the mentioned coordination modes have been confirmed by crystallography. It is illustrated schematically by several characteristic examples (Table 4.10). Despite the quite common structure and properties with those of saturated metal carboxylates [164], metal carboxylates of the kind under consideration reveal many specific features which separate them into an independent area and provide them with new properties. It is especially relevant to their polymerization transformations that actually convert these materials into metal-polymeric nanocomposites.

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Chapter 5

Polymerization and Copolymerization of Salts of Unsaturated Carboxylic Acids

Polymerization of unsaturated metal carboxylates is a unique method of synthesis of metallopolymers with a metal atom included in each monomeric unit. In utmost cases, metallopolymers of the type under consideration are prepared by radical polymerization, which comprises the same elementary steps as for conventional monomers. The rate of radical polymerization .w/ is described by the known equation: w D dŒM=dt D ki1=2 .kp =kt1=2 /ŒMŒI1=2

(5.1)

where ki , kp , and kt are the rate constants for chain initiation, propagation, and termination; [M] and [I] are the concentrations of the monomer and the initiator. However deviations from this main equation occur quite often due to the nature of the monomers used. An equation for the kinetic parameters and the average rate of polymerization, P , is as follows: 1 kM .wi kt1=2 / ks ŒS C D D kp ŒM kp kp ŒM PN

(5.2)

where wi , is the initiation rate, kM and ks are the constants for chain transfer to a monomer and to a solvent or to a transfer agent specially introduced into the system chains; [S] is its concentration. It is noteworthy that the determination of the molecular masses of metallopolymers is faced with certain difficulties. Typically, there are no direct methods due to their insolubility in traditional solvents. Indirect methods for molecular mass determination involve metal removal from the final product (for example, by treatment with, or dialysis against HCl, by ion exchange, displacement by zinc amalgam, by treatment with HCl methanol solution, with sodium ethylenediaminetetraacetate, and so on) that is followed by the analysis of the “metal-free” polymers by usual techniques (gel permeation chromatography and ebullioscopy). The molecular masses of the polymers thus formed are most often relatively low. Thus polymerization of maleic acid salts (initiated by tert-butyl hydroperoxide) at 80–180 ıC for

A.D. Pomogailo et al., Macromolecular Metal Carboxylates and Their Nanocomposites, Springer Series in Materials Science 138, DOI 10.1007/978-3-642-10574-6 5, c Springer-Verlag Berlin Heidelberg 2010 

105

106

5 Polymerization and Copolymerization of Salts of Unsaturated Carboxylic Acids

4–10 h results in the formation of a polymer with Mn D 300–5;000 [1]. Probably, chain transfer reactions play a significant role in these systems. For example, in the polymerization of tributyltin acrylate [2], it was found that the initiator, azobis(isobutyronitrile) (AIBN), is a more potent chain transfer agent than lauryl peroxide (the relative constants for chain transfer to initiator, Kn.I/ /Kp , are 0.087 and 0.015, respectively). It is worth noting that examples of analogous reactions for vinylic monomers are characteristic primarily for the initiators that are capable of inducted decay, i.e., for peroxides and hydroperoxides.

5.1 Types of Initiation In principle, radical polymerization of the salts of unsaturated carboxylic acids can be induced by any initiator or initiating radiation. However, AIBN, benzoyl peroxide (BP), potassium or ammonium persulfates, H2 O2 , tert-butyl hydroperoxide, various redox systems are used most often. The styrene–arsenic sulfide complex [3] is an example of nontraditional initiators utilized for the polymerization of metal acrylates. The complex presumably decomposes in a polar medium according to the donor-acceptor mechanism to give the H radical as depicted in the following scheme: CH=CH2

CH=CH2 .... As2S3

+ As2S3

˙ =CH . . . . As S C 2 2 3

˙ H .... As S CH=C 2 3

+ H˙ A

B

The formation of more stable tertiary radical A is probably the preferred route. The suggested scheme agrees with the data from EPR spectra of the reaction polymerizing mixture .g D 1:9572/. Kinetic parameters (see below) are in accordance with the classical equation of radical polymerization that is also attested by the linear correlation of viscosity of the polymer upon the concentration of an initiator,  D f .ŒI0:5 /.

5.1 Types of Initiation

107

Chelated complexes of alkylcobalt with tridentate Schiff bases appeared to be efficient initiators of the low temperature radical polymerization of some metal acrylates [4]: + N

R CoIII

O H2N

NH2 NH2

Br–

[RCo(7-Me-salen)(en)]Br R = Me, Et, i-Pr

Complexes of this type are known [5] to generate free alkyl radicals under mild conditions upon acid treatment. Polymerization of magnesium, zinc, barium, and lead acrylates (MAcr2 ) induced by these organocobalt initiators proceeds at 5–10 ı C even in the absence of acidic additives. Apparently, the function of acidic reagents in these reactions is played by the monomers themselves for which the degree of dissociation .105 Kd / in methanol is rather high: at 20 ı C it is equal to 3.2261075, 3.1561075, and 2.6061075 for the Zn, Pb, and Ba salts, respectively [6]. The MAC cations formed can be coordinated by ethylenediamine giving rise to the active form of the initiator: [RCo(7-Me-salen)(en)]+ + MA+ [RCo(7-Me-salen)(MeOH)2]+

MeOH k1 k–1

[RCo(7-Me-salen)(MeOH)2]+ + M(A)(en)+

(5.3)

. R + [Co(7-Me-salen)(MeOH)2]+

(5.4)

The observed dependence of the rate of polymerization upon the nature of a metal in the acrylates changes in the series Zn > Mg > Ba, which is in line with the suggested mechanism of free radicals formation. That is it correlates with the acidity of the M2C cation. As expected, the obtained metal polyacrylates had higher molecular masses and high syndiotacticity (see below). Polymerization of metal carboxylates in the solid state [7–10] and under matrix devitrification conditions [11] is often initiated by ”-radiation. Owing to relatively low activation energies for the formation of free radicals, the radiation initiation is effective over a broad temperature range, especially at low temperatures. For example, the overall activation energy for emulsion polymerization of sodium acrylate in the presence of K2 S2 O8 is equal to 94.8 kJ/mol, whereas for 60 Co ”-induced polymerization this value is equal to 16.7 kJ/mol [12]. Examples of photo-induced polymerization are known as well. Tetraethoxytitanium(IV) methacrylate derivatives are successfully polymerized in thin layers or on the surface of a metallic substrate upon UV irradiation [13]. However, photochemical initiation in solution for alkoxy derivatives of Ti(IV) with methacrylic,p-vinylbenzoic, phenylacetic acids or itaconic anhydride appeared to

108

5 Polymerization and Copolymerization of Salts of Unsaturated Carboxylic Acids

be less effective due to the high absorbance of Ti(IV) compounds in UV area [14]. Note that the metallopolymers obtained using UV irradiation have a more regular molecular packing compared to the thermally initiated reaction products, as it was observed in the Langmuir–Blodgett films based on cadmium octadecylfumarate or maleate [15]. The polymer formed had the same orientation of the aliphatic part of the chain irrespective of the double bond configuration in the starting monomer: Cd2+ O

O

Cd2+

HC

HC

HC C O

UV

(CH2)17

O

HC C O

C

(CH2)17 H3C

O

O

O (CH2)17

H3C

HC

HC HC

C

O C

C

O

O

O

O

O

C

HC

H3C

O

O

C

(CH2)17

H3C

Photo-induced polymerization ( D 365 nm, photo initiator – ’; ’-dimethoxydeoxybenzoin) of liquid-crystalline metallomonomers with terminal acrylate groups, gave a good yield of highly oriented anisotropic polymers (up to 80%) with quantitative content of the metal [16]. Controlled radical polymerization in the presence of catalytic amounts of transition metal halides by the atom transfer radical polymerization (ATRP) mechanism has been actively developed in recent years [17–19]. These controlling additives are capable of reversible interaction with reactive radicals in the reaction system to give labile adducts, thus creating conditions for “living-chain” radical polymerization. Initiation of all chains takes place almost simultaneously due to the high speed of the process. This gives rise to polymers with polydispersity being close to unity. The first example of ATRP for the metal-containing monomers has been reported for sodium methacrylate. The process occurs in an aqueous solution at 90 ı C in the presence of a macroinitiator based on poly(ethylene oxide), copper(I) bromide catalyst and 2,2-bipyridine in a 2:2:5 molar ratio [20]. At pH < 6, the reaction was ineffective, which may be due to protonation of bipyridine and the lack of solubility of the catalyst under these conditions. The resulting poly(ethylene oxide-block-sodium methacrylate) copolymer had a relatively low molecular mass and narrow polydispersity (1.2–1.3). Recently a similar method has been used to carry out polymerization of sodium methacrylate on the surface of various substrates modified by an initiator of ATRP polymerization [21, 22]. This procedure gives polyelectrolyte layers of controlled composition, thickness, and density. There are virtually no available data on the initiation of anionic or cationic polymerization of the considered monomers. To our knowledge, only one example of anionic polymerization of sodium methacrylate upon the action of phenylmagnesium bromide at 5 to 2 ı C has been reported [23].

5.2 Kinetic and Stereochemical Effects

109

5.2 Kinetic and Stereochemical Effects The presence of a metal in molecules of the salts of unsaturated acids results in carrying various coordination reactions and redistribution of electron density on the growing center thus defining all elemental steps and specifics of the polymerization process as a whole.

5.2.1 Radical Polymerization of Alkali and Alkaline Earth Metal Salts of Unsaturated Carboxylic Acids The main data on the polymerization of these monomers are comprehensively presented in a monograph [24]. Let us consider only a few most characteristic examples. For the discussion of their polymerization distinctions a hypothesis of the kinetic role of ionic pairs in the radical polymerization of the ionizing monomers could be applied. The hypothesis was formulated and developed in a monograph [25]. According to this hypothesis, at the pH > 7 the chain propagation rate is determined only by the rate of the reaction of macroradicals with the terminal ion pair. The observed kinetic effects are interpreted from the standpoint of change in the effective reactivity of the macroradicals: the growing ionized macroradicals are either separated ion pairs or ionic associates. Within the framework of these views, in the polymerization of metal-containing monomers the metal cation apparently acts as a counter ion, the nature of which (the charge, the electrostatic and crystallographic radii, solvation ability) affects the stability of ion pairs and the chain propagation rate: CH3 ~CH2

CH3

.

C

COO –

CH2 M+

C COO –

One of the first quantitative studies of polymerization of magnesium, strontium, barium, and calcium acrylates was performed in 1955 [26]. The effects of concentrations of the monomer, and the K2 S2 O8 Na2 S2 O3 initiating system as well as temperature on the yield of the polymerization product of Ca.O2 CCHDCH2 /2 were studied (Fig. 5.1). The rate constants .2kd f / and the activation energy for initiation of radical polymerization of lithium methacrylate at 333, 338, and 343 K were estimated using inhibition by a stable radical, N; N -diphenyl-N 0-picrylhydrazyl, and were found to be 3.30 ˙ 0.02, 6.19 ˙ 0.21, and 14.31 ˙ 0.60  105 s1 and 134.2 kJ/mol, respectively [27]. Polymerization of magnesium, calcium, and strontium acrylates initiated by ammonium persulfate was studied [28] and the maximum polymerization rates .Wp  106 mol l1 s1 / were found to be 160, 433, and 400, while the molecular masses of the resulting polymers .MM  103 / equaled 92.5, 848, and 990, respectively (Fig. 5.2).

110

5 Polymerization and Copolymerization of Salts of Unsaturated Carboxylic Acids

Fig. 5.1 Yield of calcium polyacrylate vs. monomer (1) and initiator concentration (3) and vs. polymerization temperature (2)

P, % 1

96

2

3

92 88 84

2

22

2

4 10

a

T, °C

42

10 CI, %

6 20

30

CM, %

b P (%)

P (%) 100

3

100

80

2

80

60

3 2

1

60

40

40 1

20

20

1

2

3

t/h

1

2

3

t/h

c P (%) 2

100 3

1

80 60 40 20

0

1

2

3

t/h

Fig. 5.2 Yields of calcium (a), strontium (b) and magnesium (c) polyacrylates vs. time for initial monomer concentrations of 0.2 (1), 0.5 (2) and 0.8 mol L1 (3). Polymerization temperature, 80 ı C, initiator concentration 0.25 mol%

5.2 Kinetic and Stereochemical Effects

111

These differences were attributed to different charge density on the macroradical anion determining the interactions in the growing macroradical–monomeric anion system. The polymerization rate of sodium methacrylate in concentrated aqueous solutions (3.15–4.67 mol/l) in the presence of K2 S2 O8 was found to obey zero order kinetics, that is the rate does not depend on the initial monomer concentration. The rate order with respect to the initiator is 0.51 ˙ 0.20 and the apparent activation energy is 81.5 kJ/mol [29]. The presence of a metal in the monomer molecule does not prevent the emulsion (latex) polymerization, while in the usual emulsion, polymerization salts cause coagulation of the latex. Emulsion polymerization of water soluble sodium acrylate in reverse micelles in a nonaqueous phase follows a nuclear “monomer-drop” mechanism, i.e., the reaction is initiated in the monomer drops because the initiator is dissolved in the internal aqueous phase [30]. The particle size of the monomer emulsion and the resulting polymer latex are virtually the same and are equal to 1 m. The mechanism is also supported by S-shaped kinetic graphs conversion vs. time. The maximum polymerization rate and the molecular mass of sodium polyacrylate formed are described by the following equations: Wmax D ŒK2 S2 O8 0:78 ŒM1:5 ŒSpan 800:1

and M D ŒK2 S2 O8 0:37 ŒM2:9 ŒSpan 800:2 :

It is noteworthy that the polymerization rate and the molecular mass of the polymer formed in this system depend little on the emulsifier concentration (Span 80), in contrast to the case of usual emulsion polymerization. A similar dependence of the reaction rate, Wr , upon the monomer concentration, was observed in the photoinduced emulsion polymerization of sodium acrylate [31]. However, an increase in the concentration of the photoinitiator first causes a gradual increase in the Wr and then a sharp decrease, while the molecular mass follows an opposite dependence upon the photoinitiator concentration (Fig. 5.3). This behavior of the Wr is attributed, on the one hand, to the adsorption effects of the photoinitiator and, on the other hand, to recombination of primary radicals at high concentrations.

a

b 10

2.2

M h (×106)

8 InR p

2.0 1.8

6 4 2

1.6 –16

–15

–14 –13 In[In] DMPA

–12

–11

0

0

2

4 6 [In] DMPA (×10–6)

8

10

Fig. 5.3 Rate of emulsion polymerization of sodium acrylate (a) and viscosity average molecular mass of the polymer (b) vs. DMPA concentration

112

5 Polymerization and Copolymerization of Salts of Unsaturated Carboxylic Acids

5.2.2 Radical Polymerization of Transition Metal (Meth)acrylates Usually polymerization of transition metal (meth)acrylates is carried out in nonaqueous media, i.e., under conditions that rule out dissociation. According to electrical conductivity measurements [6], in ethanol and DMF transition, metal acrylates are weak electrolytes .Kd D .1:97  2:25/  105/ and under the experimental conditions, where the monomer concentration is 103  101 mol/l, their dissociation can be neglected. Thus, polymerization of chromium(III) acrylate in DMF in the presence of styrene–As2 S3 complex follows a radical mechanism and has the orders of 1.0 and 0.5 with respect to the monomer and the initiator, respectively (Table 5.1) [3].

Table 5.1 Kinetic parameters of polymerization of metal (meth)acrylates Order of reaction Polymerization Monomer conditions Chromium (III) DMF, 90 ı C, acrylate initiator – Sterol-As2 S3 Cobalt(II) DMF, AIBN, acrylate 65–75 ı C

kp 2 =kt 103 (L/mol s) 31.0

With respect With respect to monomer to initiator Ea (kJ/mol) Ref. 1 0.5 67.0 [3]

0.84–1.74a

1.27 ˙ 0.06

0.54 ˙ 0.04

0.9

0.6

74.0 ˙ 2.3

[33]

Cobalt(II) acrylate

EtOH, AIBN, 78 ı C

Nickel(II) acrylate

EtOH, AIBN, 78 ı C

0.56–1.40a

1.21 ˙ 0.09

0.53 ˙ 0.05

89.3 ˙ 3.2

[33]

Zinc(II) acrylate

EtOH, AIBN, 78 ı C

4.95–10.28a 1.49 ˙ 0.10

0.86 ˙ 0.06

71.6 ˙ 3.5

[34]

1.5

0.78

94.8

[12]

0

0.5

91.5

[29]

1.05

0.49

[2]

1.10

0.45

[2]

H2 O:kerosene D 1.5:1 (vol.), 50 ı C, 9.8 M K2 S2 O8 , pH 7.02, 13.3 wt% Span 80 Sodium H2 O, 3,15– methacrylate 4.67 mol/L, K2 S2 O8 Tributyltin Decane:benzene acrylate D 90:10 wt%, lauryl peroxide Tributyltin Decane:benzene acrylate D 90:10 wt%, AIBN

Sodium acrylate

a

K  103 ; K D .2kd f =kt /0:5 kp , L1=2 =mole1=2 s

[36]

5.2 Kinetic and Stereochemical Effects Fig. 5.4 The effect of type of transition metal on the rate of metal acrylates: Zn2C (1), Co2C (2), Ni2C (3), and Cu2C (4)

113 P, % 1 2

80

3 60

40

20 4 0

6

12

18

time, h

Table 5.2 The rate constants and activation energy of the initiation reaction of transition metal acrylates polymerization [35] 2kd f  105 .c1 / Metal acrylates Zn(II) Co(II) Ni(II)

343 K 2:93 2:93 2:62

353 K 10.6 7.90 9.52

Ea; 2kd f (kJ/mol) 128.2 ˙ 1.1 116 ˙ 1.5 129.8 ˙ 0.3

Zinc(II), cobalt(II), nickel(II), and copper(II) acrylates show different reactivities in methanol in the presence of AIBN [32–34]. The polymerization rate of zinc acrylate is the highest, the long induction period being followed by fast exothermic reaction (Fig. 5.4). For other salts, no induction period was observed. Copper acrylate polymerizes in a low yield. Studies of the initiation rate of polymerization have shown that the value for the rate constant, 2kd f , vary in the series of acrylate salts in the order Zn2C > Ni2C > Co2C . Hence, the activation energy of initiation varies in the same way (Table 5.2) [35]. For the cobalt(II) acrylate, the value of Ea; 2kd f (116 ˙ 1.5 kJ/mol) is significantly lower, which is in line with the variation of the overall activation energy of polymerization in the series of these monomers (Table 5.1). This decrease in the activation energy for initiation reaction for cobalt(II) acrylate is attributed to the possible formation of the complex Co2C : : : AIBN, though no experimental evidences supporting this view were presented by authors. Under the comparable conditions (AIBN, ethanol), the rate of radical polymerization of transition metal acrylates is

114

5 Polymerization and Copolymerization of Salts of Unsaturated Carboxylic Acids

Fig. 5.5 The dependence of reaction conversion on the time of polymerization for acrylic acid (1) and metal acrylates: Co2C (2), Ni2C (3), Fe3C (4) and Cu2C (5) (CM D 0:9 mol=L; CAIBN D 2:5  102 9 mol/L, ethanol, 78 ı C)

q, % 100 1 2 60

3

20

4 5 0

20

60

100 MИH

lower than the rate of homopolymerization of acrylic acid (AA) (Fig. 5.5), decreasing in the series of cations Co2C > Ni2C > Fe3C > Cu2C [36, 37]. Analogous behavior is characteristic for hydrogenation of these carboxylates [38] that can be caused by decreasing of electro density on the double bond, with increasing of electronegativity of metal. An increase in the initial monomer and initiator concentrations results in an increase in the reaction rate, which is in good agreement with general rules of radical polymerization. Polymerization of metal acrylates comprises the same elementary steps as polymerization of conventional monomers but it is affected by the nature of the transition metal. In view of the fact that the initial and current concentrations of monomers (M0 and M, respectively) are related to the degree of conversion (˛) in the following way: [M] D [M0 ] .1  ˛/, in the quasi-stationary approximation with respect to macroradicals, the polymerization rate can be represented by the following equation: d˛=d D kp .ki =kt /1=2 I1=2 .1  ˛/:

(5.5)

Solution of this equation provided that [I] D [I0 ] exp(ki t) gives the dependence lnfln.1  ˛/ C 2kp ŒI0 =.ki kt /1=2 g D ln2kp ŒI0 =.ki kt /1=2  1=2kit;

(5.6)

that satisfactorily describes the polymer accumulation kinetics in the liquid phase radical polymerization of cobalt(II) acrylate [36] (Fig. 5.6). The equation for the limiting conversion vs. the initial initiator concentration is in good agreement with the experimental data (Fig. 5.7): ln.1  ˛1 / D 2kp ŒI0 =.ki kt /1=2 :

(5.7)

5.2 Kinetic and Stereochemical Effects Fig. 5.6 The graphical solution"of the (5.6)



A D ln.1  ˛/ C 2kp

I0 ki kt

115

1=2 #!

However, as noted above, polymerization of the carboxylates under consideration can be accompanied by a number of transformations. For example, the coordination of the monomer to the primary radicals .RC : / at the initiation step results in deactivation of the radicals and decreases the initiation efficiency: . CH

Rc CH2 CH2

CH X

+ Mn

.

(5.8)

X Mn

Rc

CH2

CH2

CH . X Mn . Rc

CH

+

X Mn – 1

Rc+

(5.9)

Obviously, the competitive binding of polymeric radicals also accompanies chain propagation and the coordinated radicals thus formed can also undergo intramolecular deactivation: .

~ CH2 CH

(5.10) n

. R + CH2

X M CH X Mn

CH2

CH

.

X Mn . R .

~ CH2 CH X Mn (M – metal, n – its valency, X – functional group)

CH2

+ R+

CH X

Mn – 1

(5.11)

+

~ CH2 CH X Mn – 1

(5.12)

The observed deviations of the reaction orders with respect to the monomer and the initiator in these systems (Table 5.1) compared to the classical radical polymerization may be due to these side reactions, that is to the more complicated initiation mechanism. For example, their increased values attest to a dependence

116

5 Polymerization and Copolymerization of Salts of Unsaturated Carboxylic Acids In(1– α)

Fig. 5.7 The degree of limiting conversion of the monomer vs. Initial initiator concentration during radical polymerization of Co(II) acrylate

1.5

1.0

0.5

0

0.05

0.10 [I0]0.5 / mol0.5

litre– 0.5

Fig. 5.8 Cu2p3=2 and C1s XPS of Cu(II) acrylate (1) and the product polymerization (2); the spectrum of Cu2 O (doted line)

of the initiation rate on the monomer concentration and to an increase in the contribution of the monomolecular termination to the overall kinetic chain termination. Thus, the above mentioned low polymerization rate of the copper(II) acrylate can be attributed to the following reaction: ⋅ ~ CH2 CH X CuII

+

~ CH2 CH

(5.13)

X CuI

This is probably facilitated by the relatively low values of the standard reduction potentials for copper ions. Thus, Eo Cu.II/!Cu.I/ D 0:15 V, while for the comparison Eo Cr.III/!Cr.II/ D 0:41 V and Eo Ti.IV/!Ti.III/ D 0:41 V. By means of special spectroscopic and magnetochemical studies [39], it has been demonstrated that the reduction of some portion of copper(II) ions indeed takes place during the copper(II) acrylate polymerization. The XPS Cu2p3=2 and C1s spectra for the CH2 DCHOCO/2 Cu and the product of its polymerization are presented in the Fig. 5.8. The 1 eV shift of the main Cu2p3=2 peak toward lower bond energy is

5.2 Kinetic and Stereochemical Effects

117

observed in the result of the polymerization. Also, the relative intensity of the satellite located at the high energy side from the main signal decreases from 0.38 to 0.20. There are 2 peaks in the C1s spectrum. One, at 285.0 eV, is due to CH3 , CH2  and CHD groups present in the investigated compounds and vapors of the oil absorbed. The other peak, at 288.5 eV, is due to the carbon atom of the carboxylate group, COO . The half width of the main C1s signal equals 3.0 and 3.6 eV for the CuAcr2 and the copper polyacrylate (PAACu), respectively.

5.2.3 Regulation of Stereochemistry of Radical Polymerization of Metal Carboxylates It is known that the problem of the stereospecific synthesis under the radical polymerization relates to the slight differences between the activation energy for the reaction rates for the growth of isotactic .ki / and syndiotactic .ks / sequences. The difference between the corresponding activation energies equals about 1 kcal/mol. In accordance with the expression ki =ks D e.F ¤

¤

¤ /=kT

;

(5.14)

where F ¤ D Fi  Fs is the difference between the free activation energies for iso- and syndiotactic additions. The microstructure of the polymeric chains in the radical polymerization should be affected by factors that can change the ratio of the constants for the iso- and syndiotactic additions for the growth reaction. First of all, this is low polymerization temperature, because the predominant formation of the syndiotactic conformation with respect to the isotactic one is mainly due to the enthalpy factor. Steric hindrance and polarity also promote the directed growth of the polymer chain. According to the ion pair mechanism and the corresponding calculations, it was shown that the formation of the growing radical – counter ion pairs is accompanied by preferred syndiotactic addition. Polymerization of the salts of unsaturated carboxylic acids creates certain prerequisites for the formation of regular polymers. Apparently, owing to the presence of the polarized metal carboxylate group, the growing center changes its stereochemical configuration to the opposite one in every chain elongation step. As a result, configurations of the carboxylate units in the polymer alternate. The particular role is played by electrostatic interactions between the ionized growing radical and the polar metal containing group. Also, the coordination bonds of the metal cation can have a directing effect. In the earliest publications it was shown [40], that the radical polymerization of alkali metal methacrylates carried out in water gives mainly syndiotactic polymers (the content of the syndiotactic fraction ranges from 90 to 95%). The anionic polymerization of sodium methacrylate affords the polymer containing 78.2–97.8% of the isotactic fraction [23]. Meanwhile, the radical polymerization of this monomer at 70 ı C in benzene yields a polymer with the ratio of the syndio- to hetero- forms of 72.1–26.2. Moreover, even a relatively low content of (meth)acrylate salts in

118

5 Polymerization and Copolymerization of Salts of Unsaturated Carboxylic Acids

their copolymers allows effective control over the microstructure of the polymers formed. Thus, in the triple copolymers MMA–MAK–sodium methacrylate [41], the salt content in a 0.1–0.5 mol% range results in an increase in the number of alternating dyads (Table 5.3) and gives more sterically ordered products. This is made evident by the appearance of isotactic configurations and higher concentration of syndiotactic triads as their fraction increases with the increase of the concentration of salt groups. It is worth noting that the content of syndiotactic units in the growing polymeric chain may be determined by the nature of the counter ion, its sorption ability, the polarity of the medium and so on. For example, bulk radical polymerization of tributyltin methacrylate (TBTM) gives a polymer structure similar to that of the macro complex resulting from the reaction of [(BuO)3 Sn]2 O with atactic polymethacrylic acid (PMAA) [42] (Table 5.4). Apparently, this is related to polymerization conditions: in bulk polymerization, the effect of ion pairs is less pronounced than in polymerization in polar solvents. This was confirmed by polymerization of cobalt and nickel acrylates in ethanol at 60 ı C and their radiation induced low temperature polymerization on defrosting of glassy matrices [11, 36]. After hydrolysis of the resulting metallopolymers, up to 60–65% of syndiotactic polyacrylic acid (PAA) was isolated (the fraction soluble in a dioxane–water (80:20) mixture). Note that the microstructure of the metallopolymers formed upon polymerization of (meth)acrylates of divalent metals (bifunctional monomers) is determined by both the structure of the active site, the reaction temperature, and the nature of the solvent as well as by the steric factors of the spatial cross-linked

Table 5.3 Microstructure of copolymers of MMA with methacrylic acid and sodium methacrylate [41] Concentration of triada (%) Concentration of sodium The number of the methacrylate (mol%)

MMA-MAA diad (%)

i

0 0.10 0.25 0.50

37 50 52 54

0 4 9 11

a

h 45 36 28 24

s 55 60 63 65

i is isotactic, h is heterotactic, and s is syndiotactic configurations Table 5.4 The stereo regular composition of tin-containing polymers [42] Tacticity (%) Macrocomplex Iso-PMAAC[(BuO)3 Sn]2 O Syndio-PMAAC[(BuO)3 Sn]2 O Atactic PMAAC[(BuO)3 Sn]2 O Atactic PMAAa Product of TBTM polymerizationb a b

Iso100 0 15 6:5 18

Polymerization in toluene, BP initiator Polymerization in bulk, AIBN initiator, at 60ı C

Syndio0 78 44 56:5 50

Hetero0 22 41 37:0 32

5.2 Kinetic and Stereochemical Effects

119

Table 5.5 The stereoregular composition of PAA isolated from metal polyacrylates The yield of fraction (%) The starting polymer

Soluble in dioxan (atactic)

Soluble in the mixture of dioxan-water (syndiotactic)

Zna polyacrylate Znb polyacrylate Polyacrylic acidb Baa polyacrylate

20 58 59 26

80 42 41 74

Polymerization at 9 ı C Polymerization at 70 ı C

a b

metallopolymer structure. Indeed, low temperature radical polymerization of zinc, barium, and lead acrylates affords a higher content of the regular fraction, as indicated by the fractionation data for PAA isolated from these metallopolymers (Table 5.5) [4]. According to a known scheme [43], the process of polymerization of bifunctional monomers can be conventionally divided into two stages. The first stage yields a comb shaped linear polymer. Apparently, stereo regular fractions are formed during this period. The second stage includes the formation of a three-dimensional network structure, as chain propagation reactions involve mainly the CDC bonds of the side chains of the macroradical. At this stage chain growth takes place under high steric strain and is accompanied by an increase in the internal stress, giving rise to an atactic structure of polymer chain.

M

M

.

R

M

M M

M

M M

M

M M

M

M M

M

M

Thus, the data of the low frequency IR spectra of metal polyacrylates [4] show that in the region of OMO vibrations the spectrum of the polymer exhibits one broad band with a maximum at 340 cm1 instead of two narrow bands (at 300 and 400 cm1 ) typical for the metal-containing monomer. This is a consequence of distortion of the geometry of bridging groups caused by internal stress in the network structure. Moreover, in some cases, these two stages can be separated kinetically, as has been demonstrated recently in a study of thermal transformations of Co(II) acrylate by dielectric spectroscopy in situ [44]. Figure 5.9 presents the dependence of the relaxation time, m .T /, vs. Arhenius units. It can be seen that till region 3, the spectral pattern is typical for relaxation processes, i.e., the relaxation times decrease with an increase in temperature. The occurrence of polymerization in the region 1 is confirmed by the deviation of the experimental m .T / values from the theoretical ones.

120

5 Polymerization and Copolymerization of Salts of Unsaturated Carboxylic Acids

Fig. 5.9 Relaxation time m in the polymerization of cobalt (II) acrylate vs. temperature (The points are the experiment and the curve shows the results of calculation). (1) polymerization region, (2) ’-relaxation region, (3) cross-linking region

On further increase in temperature, the experimental and theoretically calculated data approach each other. At higher temperatures, polymerization involving the residual double bonds apparently takes place, i.e., a cross-linked polymer is formed. The topochemical factors are also crucial in the formation of isotactic oligomers in the solid phase polymerization of zinc 3-butenoate [45]. Unusual stereo- and regiospecific effects were discovered for other alkenoates as well. The ”-induced stereospecific trimerization of the sodium trans-2-butenoate results in the formation of one of the eight possible diastereomers, namely, the trisodium 2,4-dimethyl-6heptene-1,3,5-tricarboxylate [46]:

CO2Na

γ-rays *

5

4

*

CO2Na

3

*

2

*

CO2Na

CO2Na

Under similar conditions, Ca(II) trans-2-butenoate is subjected to cyclodimerization yielding only one diastereomeric form [47]. CH3 O O

2

Ca

γ -rays

60 ºC,

24 h

*

3 2 * * 1

CO2Ca CO2Ca

When the kinetic effect of the polarized metal carboxylate group is clearly observed, it is possible to obtain crystalline metallopolymers. Thus, the degree of crystallinity of iron(III) polymethacrylate synthesized by ”-induced polymerization depends on the radiation dose. The highest degree of crystallinity (39%) is obtained in the range of 10–25 kGy, although no clear trend for increasing or decreasing of this value with the dose increase was found [48]. The photopolymerization of liquid-crystalline metal carboxylates containing terminal acrylate groups [16] produces anisotropic polymers. The resulting

5.3 Solid Phase Polymerization of Unsaturated Metal Carboxylates

121

Fig. 5.10 Maximum W max (% s–) photopolymerization rate (1) and conversion (2) of the 0.8 liquid crystal monomer Zn(O2 CC6 H3 (O(CH2 /11 OCOCHDCH2 /2 /2 vs. temperature

α (% ) 30 1

2

20

0.6

10

0.4

0.2 10

20

30

40

50

0 60 T / °C

metallopolymers have a hexagonal columnar structure that was confirmed by X-ray diffraction. The mesormorphic structure of the monomer is not changed significantly during polymerization, although the hexagonal packing is compressed for ˚ instead of the polymer. The column-to-column distance in the polymer is 33.7 A ˚ observed in the monomer. Note that the maximum reaction rate and the 39.2 A degree of conversion increase with temperature in the region of existence of the folded mesophase of the monomer .45–55 ıC/ and decrease in the isotropic phase region at 65 ı C (Fig. 5.10). The observed kinetic effects are likely related to ordering of the structure and self-assembling of the polymerizing metal carboxylate in the mesophase, on the one hand, and loss of orientation of the monomer in the isotropic phase, on the other hand. Similar effects were also observed for the traditional monomers [49–51].

5.3 Solid Phase Polymerization of Unsaturated Metal Carboxylates As has been noted, polymerization of this type of monomers in solutions can be accompanied by dissociation of salts, particularly salts of s-elements. Another disadvantage is a limited number of solvents that allow preparation of concentrated enough solutions to carry polymerization. Yet the majority of metal carboxylates are solids (crystalline or amorphous) at room temperature, thus allowing to utilize methods of solid phase polymerization. Metal carboxylates are also most often convenient objects for solid phase polymerization from the structural chemical aspect, since the orientation of their molecules is optimal for formation of chemical bonds between them. Chain propagation takes place in the plane of stacks of tightly packed monomer molecules parallel one to another. Such a process does not take place in either liquid or vitreous state. Therefore, there is no need for significant change in the location of carboxylate molecules in crystals for the solid

122

5 Polymerization and Copolymerization of Salts of Unsaturated Carboxylic Acids

phase polymerization. Regardless of the method of initiation of the solid phase polymerization, the following premises are put [52] into the base of its kinetic scheme: space movements of the growing macroradicals and their collisions with monomer molecules occur only as a result of chain propagation acts (because of almost full absence of onward diffusion of the reacting particles); irregularities of a crystalline lattice (dislocations, cracks, vacancies, and so on) are the breaking points of the growing chains; anisotropy in reactivity of macromolecules growing in a crystalline lattice defines their growth predominantly along one of crystallographic axes. The process of initiation of the free radical (sometimes of the ionic), polymerization of unsaturated metal carboxylates in the solid state, can be started by different types of initiation. The most often used types are thermal, photochemical, and radiation, also mechanochemical one is rarely used.

5.3.1 Thermal Polymerization of Unsaturated Metal Carboxylates Examples of thermal generation of free radicals upon solid phase polymerization of metal carboxylates are rare and mainly involve acrylates of transition metals. Free radicals initiating the polymerization are formed in a result of either the disassociation of CC or CH bonds or the opening of a double bond (formation of biradicals). The rate of solid phase polymerization increases with temperature, since there is a need for higher amplitude of thermal vibrations, so the reactive centers can get closer. Thus, thermal polymerization of sodium acrylate occurs at 145–175 ıC in vacuum [53]. Activation energy for the initiation (determined from EPR data) equals 121 kJ/mol. The process takes place with an induction period that decreases with an increase of temperature (Fig. 5.11) (Ea equals 115.5 kJ/mol, that is in agreement with the value found from the EPR data), and the chain propagation stage has Ea D 68:9 kJ=mol. Barium methacrylate, Ba.OCOC.CH3 /DCH2 /2 H2 O, is one of the first metal carboxylates, with which solid phase polymerization was investigated [54].

P, % 4

3

2

30

1

20

10

Fig. 5.11 Yield of sodium polyacrylate vs. time at temperatures 145 (1), 157 (2), 166 (3) and 175 ı C (4)

0

40

80

120

160

Time, min

5.3 Solid Phase Polymerization of Unsaturated Metal Carboxylates

123

Dehydration of this monomer .65–140 ıC/ is accompanied by the formation of various particles of radical nature [55] that can initiate polymerization. Decomposition that starts at 210 ıC becomes significant at 400 ıC. At this temperature range barium and calcium .300–350 ıC/ methacrylates form di-, tri-, and tetramers [56]. The mechanism of their formation has clearly been identified and the correlation between the structural aspects and the ability to polymerize was found for those monomers [55]. Decomposition of methacrylates and maleates of d -elements is preceded by their thermal polymerization [57–66]. Investigations of the transformation of the monomers under either TA conditions, or in the self generated atmosphere revealed that in the temperature range 200–300 ıC small gas formation occurs along with insubstantial mass loss of the sample (10 mass%). In the case of metal acrylates and maleates major contribution is made by CO2 and the vapors of CH2 DCHCOOH and HOOCCHDCHCOOH, respectively, that condense on reactor walls at room temperature. This is supported by IR and mass spectrometry data. According to the TA data the characteristic temperature polymerization areas are: 270 ıC (Co(II) acrylate), 290 ıC (Ni(II) acrylate), 237 ıC (Cu(II) acrylate), 310 ıC (Fe(III) oxoacrylate), 215–245 ı C (Co(II) maleate), 245 ıC (Fe(III) maleate). During the polymerization changes in the IR absorption spectra take place. These are related to the intensity decrease of the absorption band for the CDC bond valent oscillation and closing absorption bands for the CDO bond valent oscillation, resulting in emerging one broaden absorption band at 1,540–1,560 cm1 . Similar peculiarities are notable for polymerization both in solution (see Sect. 4.2) and in solid state: high reactivity is observed for Zn(II) acrylate, as well as for Co(II) and Ni(II) acrylates and low reactivity is noted for Cu(II) acrylate.

5.3.2 Solid State UV and Radiation Initiated Polymerization Photochemical initiation of metal carboxylates polymerization in solid state can be realized rather rarely. One of a few examples is efficient photopolymerization of potassium acrylate [67] (UV irradiation with wave length 250–300 nm): yield of the polymer is 40% at 73 ı C, however, its molecular mass decreases with the increase of conversion (from 4.7  105 to 2.2  105 ). Photo initiated polymerization of calcium acrylate [68] proceeds with constant rate till high degrees of conversion. Radiation initiation at low temperature is a universal method for initiation of solid state polymerization of unsaturated metal carboxylates. In essence this is the irradiation of the salts at low temperature (typically at the temperature of liquid nitrogen or at 78 ı C) with ”-irradiation of 60 Co, fast electrons, or X-rays (seldom with low temperature plasma). Since under these conditions chains do not grow, there is an accumulation of radicals (or other active centers) in the samples for the following postpolymerization at ambient (or higher) temperature. Relatively early studies reported [69] an easily proceeding polymerization of acrylate salts of Rb and K in vacuum at room temperature (irradiation at 78 ı C),

124

5 Polymerization and Copolymerization of Salts of Unsaturated Carboxylic Acids

while Na and Li salts required heating to 150 ıC. The fastest polymerizing MSM is potassium acrylate: it polymerizes faster at 0 ı C (Ea D 70 kJ=mol) than the sodium salt does at 120 ı C. Chain length of potassium acrylate is higher by an order of magnitude than upon polymerization of the Na or Li salts. Interestingly, the reverse reactivity order was observed for methacrylate salts. Sodium methacrylate is more active than the potassium salt, while lithium methacrylate is not active at all. These differences are attributed to the geometry of crystal lattices of the corresponding salts, which in turn is defined by the nature of a metal ion. Correlation of the mobility of acrylate ions with the rate of their radiation postpolymerization has already been pointed out above [70]. The minimal values for the rate constant for radicals death are observed for potassium acrylate ((2.92 ˙ 0.89)  103 s1 ) and rubidium acrylate ((2.28 ˙ 0.14)  103 s1 ) at temperatures close to those of phase transfers (at 61 ı C and 47 ı C, respectively). The initial rate of polymerization of Ca2C , KC , and Ba2C acrylates is 18.1; 56.8 and 75.1%/h under comparable conditions. Polymerization of the Ca.OCOCHDCH2 /2 2H2 O is rather peculiar as the polymer yield depends on the degree of hydration of the salt [71]. The water from dihydrate was removed at 60 ı C in vacuum and conditions for the solid state polymerization were as follows: I D 0:97 J=.kg s/; D D 8:6 kJ=kg; 78 ı C, postpolymerization at 25 ı C during 9 days. Utilization of the half hydrated form gave rise to the highest yield of the polymer. In the case of barium methacrylate the maximum rate of polymerization was when the substance contained 0.25 mol of water per 1 mol of the salt [72]. The structure of that salt is thought to be crumbly with a number of dislocations that facilitates the solid state polymerization. Hydrogen atoms forming from the hydrated water upon the radiolysis of the salts hydrates contribute significantly to the genesis of free radicals initiating solid phase polymerization. The EPR spectrum of the ”-irradiated barium methacrylate dihydrate is attributed to the radicals generated according to the following scheme [73, 74]: CH3 H + CH2 C

COO −

CH3

CH3 C. COO −

Those initiating radicals compose up to 90% of the forming radicals, the rest 10% are growing radicals of the RCH2 C: (CH3 )COO type. Experiments with D2 O confirmed that up to 75% of the initiating radicals of the monohydrate are formed via addition to double bond of hydrogen atoms generated from the hydrated water. The overall yield of the radicals increases linearly with the irradiation dose, while the polymerization rate is proportional to [I]1=2 , Ea D 132 kJ=mol [75]. Importantly, that the crystal anhydrous salt yields only less than 2% of polymer even after high irradiation doses and long heating (Figs. 5.12 and 5.13). Additionally, the growing radicals on polymerization of dihydrate have a conformation that is different from the one of monohydrate. According to X-ray analysis data [76] the crystal structure of Ba.OCOC.CH3 /DCH2 /2 2H2 O significantly transforms during polymerization. The observed induction period for the polymerization is due to physical capture of short chain growing radicals and its duration substantially decreases upon the

5.3 Solid Phase Polymerization of Unsaturated Metal Carboxylates Fig. 5.12 Product yield of Ba.OCOC.CH3 /DCH2 /2H2 O polymerization vs. time at 78 ı C; the doses of radiation (kJ/kg) 40 (1), 20 (2), 10 (3) and 5 (4)

125

P, %

1

75 2 50 3 25 4 0

Fig. 5.13 The influence of water excess on the yield of polymer at 50 ı C: (1) barium monohydrate; (2) humid monohydrate; (3) monohydrate C5% H2 O

2

4

t, cyn

P, % 3

75 50

2 1

25

0

2

4

t, cym

increase of temperature or power of the dose. Simultaneously, according to spectroscopy data [77] the oxypolynuclear structure of Al(III) oxo methacrylate does not incur any significant changes upon 60 Co ”-induced polymerization at room temperature and various doses (10–50 kGy): [Al(OH) x (OH2) y (OOCC(CH3)=CH2)z]

γ-radiation

{Al(OH) x (OH2) y [OCC(−C(CH3)CH2)−)z ]}n

(5.15) In contrast to the features discussed above, Zn.OCOC.CH3 /DCH2 /2 exhibits a particular capability toward radiation thermal polymerization (the crystals were ground and irradiated at 78 ı C) at 89:5 ı C, and its deceleration occurs at well below the Tm of the monomer. Among a few reports on the ”-initiated solid phase polymerization of acrylates of d -elements the high tendency of Fe(III) methacrylate [48,78,79] to polymerize .I D 0:83 J=.kgs/; D D 45 kJ=kg/ is worth noting.

5.3.3 Reactivity of Unsaturated Metal Carboxylates in Solid Phase As we mentioned above, it was demonstrated in the earliest works [55, 69, 73, 76] that reactivity of various salts containing the same monomeric anion is controlled by the geometry of the crystal lattice. Thus, the parameters of elemental orthorhombic

126

5 Polymerization and Copolymerization of Salts of Unsaturated Carboxylic Acids

˚ presume an inlattice of potassium acrylate (a D 20:5, b D 4:15, and c D 5:73 A) ˚ definite chain of reactive centers within a distance of 4.15 A. This correlates with the main topochemical postulate that reactive groups capable of undergoing solid ˚ phase dimerization or polymerization must be located within distances of 4.2 A from one another [80–82]. Interconnection between crystal structure and reactivity in solid phase is well documented for a wide range of metal carboxylates including propionates, trans-2- and 3-butenoates, trans-2-pentenoates of metals. The presence of a metal mostly plays a definitive role in the activity of ’; “-unsaturated carboxylic acids in solid phase polymerization reactions, by taking into account other factors, such as relative distances between active centers, energy of the crystal lattice, cross section of absorption uptake and so on. Thus, under an influence of an ionizing irradiation crystal metal (Na, K, Rb, Mg, Zn, Cd, Sr, Ba, La, Sc) propionates [83] yielded 23–97% of dark colored polymer products. In contrast, metal free organic acetylenes under analogous conditions display low reactivity in solid phase transformations despite advantageous special orientation of acetylenic centers [84–86]. Heavy metal salts exhibit especially high sensitivity toward ”-irradiation. The pres˚ in the molecule of ence of a chain of short contacts of acetylenic groups (3.454 A) .CH3 /2 Tl.OOCCCH/ facilitates its efficient polymerization under 60 Co ”-rays. This is evident by the correspondent loss of intensity of CC valent oscillation at 2,080 cm1, and the yield for the product linearly increases with an increase of irradiation dose in a range of 7–21 Mrad [87]. Analogous peculiarities were observed during solid phase polymerization of scandium(III) propionate [88]. Chains ˚ and C(2)– of indefinite CC    CC contacts C(2)–C(3) [z, x, y], 3.79 A ˚ are in orthogonal positions, i.e., in this case, C(3) [1=2  z, x  1=2, y], 4.02 A the criterion for parallelism of short contacts of active centers is not a required condition for a topochemical reaction, as it was for postulated alkene derivatives [82]. This structural difference is also characteristic for other propynoates [89]. In addition to the mentioned factors, the nature of accompanied ligands plays an important role in the reaction behavior of carboxylates of the type considered. This can be illustrated by an example of two complexes, La2 .OOCCCH/6 .H2 O/4  2H2 O and La2 (OOCCCH)6 (2,2-bipy)2(H2 O)2 4H2 O2(2,2-bipy) [90], having similar molecular structure, but differing in spatial packing. There is an indefinite chain ˚ in the former compound, while there of acetylene contacts with a distance of 3.95 A are C’  C“ contacts consisting from only five CC    CC sequences in the latter one. In accordance with the observed differences in crystal structure of the complexes, the La2 (OOCCCH)6 (H2 O)4 2H2 O polymerizes efficiently on 60 Co ”-irradiation yielding 59% of polymeric product at 65 Mrad, while the complex with 2,2-bipy ligands appeared to be stable toward ionizing irradiation and did not polymerize in these conditions. Two-layer motif in structural organization is a characteristic feature for many metal alkenoates. First of all this is typical for the monomers that contain metal atoms with small radii. The two-layer arrangement of organic groups facilitates appearance of parallel unsaturated groups, with short distances between reactive centers. Inorganic and organic domains can be identified in the structure of lithium salt of trans, trans-2,4-hexadieneoic acid, CH2 DCHCHDCHCH2 COOLi,

5.3 Solid Phase Polymerization of Unsaturated Metal Carboxylates

127

Fig. 5.14 Two-layered motif of crystal structure of Li sorbate (a) and inorganic LiO tetrahedron layers of the molecule (b)

[91] (Fig. 5.14). The inorganic part of the structure consists of an indefinite two-dimensional chain of LiO tetrahedrons connected by caps and edges. Sorbate groups form organic layers in planes that are almost parallel and arranged in a zigzag fashion. These planes are not fully planar, the torsion angle between carboxylate and the first vinylic group being approximately 10ı . The shortest distances between potential reactive centers found in the structure, i.e., between unsaturated ˚ belonging to ˇ  ı and ˛   contacts or 4.12 A ˚ carbon atoms, are 3.66 and 3.80 A, (˛  ı contacts). Thermal initiation of the lithium sorbate in the temperature range 220–285 ıC (24 h, vacuum) gives rise to an amorphous polymer with 100% yield. In the case of X-ray induced reaction (Cr-irradiation, 40 kV) completely polymerized product is also amorphous though the (111) reflexes on a X-ray spectrum remain rather sharp, i.e., distraction of a crystal structure during the polymerization is not isotropic. Apparently the polymerization takes place primarily along the ˛  ı direction since these contacts are localized in the (111) layer. The topochemical polymerization of benzylammonium muconate is another example illustrating that the crystal structure of a precursor monomer can be retained during a solid state reaction [92, 93]. −

+

CO2NH3C6H5

−

hν

+

CO2NH3C6H5 n

CO2NH3C6H5

CO2NH3C6H5

A layered structure of the polymer crystal obtained reproduces the structure of benzylammonium monomeric crystal consisting from alternating packed layers of the diene carboxylate anion and the benzylammonium cation supported by two-dimensional network of hydrogen and CH  bonds. This is evidenced by the ˚ characteristic diffraction of the polymer product at 2 D 5:2ı .d D 17:0 A/. Salts of less reactive “,”-unsaturated carboxylic acids, such as Zn(II) [45] and Ca(II) [8] bis(3-buteneoates) also reveal a capability to solid state polymerization due to parallel orientation of unsaturated groups. Interestingly, that in the structure

128

5 Polymerization and Copolymerization of Salts of Unsaturated Carboxylic Acids

of the Zn(II) salt one set of buteneoate groups is arranged almost parallel with a dihedral angle 9:1ı and the distance between the unsaturated centers C(3)–C(4) ˚ while the groups in another set are separated by 4.42 A ˚ .x; 2  y; z  1=2/ 4.21 A, ı 1 (C(7)–C(8) .x; 2  y; z  =2/ with a dihedral angle 119:0 . The latter groups appear to be inactive upon the ionizing irradiation, and the maximum yield of polymer product is less than 50%. In contrast, calcium 3-buteneoate that has a structure with a network of parallel contacts CDC    CDC with distances 3.73 and ˚ forms poly(3-buteneoate) with MN w D 400; 000 and 97% yield upon 60 Co 3.90 A ”-irradiation with 305 kGy dose. Metal trans-2-penteneoates display a wide range of reaction ability in solid phase transformations. Some of them (M D Li, Mg, Zn, Cd, Pb) are unable to polymerize upon ”-irradiation, while others form dimers (M D K) or mixtures of oligomers (M D Na, Ca, Sr, Ba) [94]. Metal trans-2-penteneoates undergo solid phase oxidation reaction leading to formation of metal acetylacrylate upon irradiation in air: CO2

n

M

γ-rays

O CO2 M

(5.16)

n

Apparently, there are no principal limitations for the solid state topochemical reaction of Cd(II) itaconate, the structure of which was found to contain contacts ˚ [95]. between CDC bonds with distances less than 4.2 A

5.4 Copolymerization and Terpolymerization Copolymerization with traditional monomers is widely employed for obtaining of metallopolymers based on metal carboxylates. This method allows involving in polymerization processes even those carboxylates, which are incapable of homopolymerization, but copolymerize readily with other monomers. There is another important aspect: since composition of the formed copolymer depends on a variety of causes, copolymerization provides additional possibilities for investigation of factors that affect reactivity of the multiple bond in the metal carboxylate molecule. Like in the case of classical copolymerization, composition of formed metallocopolymers depends on the composition of the initial monomer mixture as well as relative activities of the monomers and their radicals, consistent with Mayo-Lewis equation [96]: Œm1  ŒM1  r1 ŒM1  C ŒM2  D ; Œm2  ŒM2  ŒM1  C r2 ŒM2 

(5.17)

where r2 D k22 =k21 is copolymerization constant which describes the relative activity of metal carboxylate monomer in addition to the “own” and “strange” radicals, m2 and M2 is the content of metal carboxylate in the copolymer and monomer mixture, respectively, and r1 D k11 =k12 , m1 , and M1 are similar characteristics of the “metal-free” analog.

5.4 Copolymerization and Terpolymerization

129

It is known that the ability for polymerization enhances with increasing difference in the resonance stabilization between the adding monomer and the radical formed. In the Q–e scheme, Q is the characteristic of resonance stabilization of a monomer during copolymerization and e is the factor reflecting the magnitude of the polarity effect of a substituent at the multiple bond. These parameters are associated with the relative reactivity constants by the following empirical equations: r1 D .Q1 =Q2 / expŒe1 .e1  e2 /I r2 D .Q2 =Q1 / expŒe2 .e2  e1 /

(5.18)

Copolymerization constants are found by the Mayo-Lewis method [96], with the application of various linearization techniques (such as Fineman–Ross [97], Kelen– Tudos [98] and others). Most commonly used is the Fineman-Ross linear form of copolymerization equation, F .1  f / D r2  r1 F 2 f;

(5.19)

here F D ŒM1 ]/[M2 ] and f D Œm1 ]/[m2 ], which allows for quite simple graphical determination of r1 and r2 ?

5.4.1 The Main Principles of Copolymerization of Alkali and Alkaline Earth Metal Salts The majority of studies point out a significant effect of the reaction medium, primarily ionic strength and solvent polarity, on the kinetic and copolymerization parameters as well as properties of copolymers obtained, such as composition and molecular mass. Change in the nature of interactions in the systems macroradical– counter ion–monomer anion with variation of composition of the medium is suggested to play the key role. In particular, it was shown for copolymerization of acrylic acid salts with acrylamide [99] that at pH D 7.1–7.2, when the salts are dissociated, the rate of copolymerization diminishes with decreasing degree of metal ion binding by the polyacrylamide moiety of the macroradical. The degree of cation binding with acrylate groups depends on the cation size and diminishes in the raw LiC > NaC > KC . Kinetic characteristics of MMA and alkali metal methacrylates copolymerization in methanol were explained by electrostatic repulsion between the salt functional groups and MMA radical [100]. The constants of relative reactivity for monomers (Table 5.6) indicate that a radical with terminal metal methacrylate unit prefers binding with MMA monomer rather than with its own one. This ability, defined as 1/r2 , decreases with increasing ionic radius of alkali metal cation (Fig. 5.15). This tendency may be correlated with the order of variation of homopolymerization rates for these salts, assuming that k22 increases, while k21 remains practically unchanged. Such a behavior is also typical for other salts of this kind, and normally, the copolymers formed are enriched in M1 .r1 > 1; r2 < 1/ over the whole range of

Styrene

Methylmethacrylate

Styrene

M1 Methylmethacrylate

DMSO, AIBN, 0,5%, 60 ı C Methanol, AIBN 0.01%, 60 ı C Methanol, AIBN 0.01%, 60 ı C DMSO, AIBN 0.5%, 70 ı C DMSO, AIBN 0.5%, 70 ı C DMSO, AIBN 0.5%, 70 ı C DMSO, AIBN 0.5%, 70 ı C

Na(OOCC(CH3 )DCH2 /

K(OOCC(CH3 )DCH2 /

Mg(OOCCHDCH2 /2

Ca(OOCCHDCH2 /2

Sr(OOCCHDCH2 /2

Ba(OOCCHDCH2 /2

DMSO, AIBN, 0,5%, 60 ı C

Methanol, AIBN 0.01%, 60 ı C

Li(OOCCHDCH2 /

M2 Li(OOCC(CH3 )DCH2 /

0.173 0.18 ˙ 0.14 0.12 ˙ 0.02 0.14 ˙ 0.05 0.11 ˙ 0.02

5.31 ˙ 0.07 6.10 ˙ 0.22 4.12 ˙ 0.02 3.95 ˙ 0.15

0.126

5.65

3.97

0.07

0.72

1.30 7.29

0.073

r2

0.59

r1

Table 5.6 The parameters of copolymerization of alkaline and earth-alkaline metal unsaturated carboxylates Comonomers Copolymerization conditions

0:12

0:13

0:11

0:11

0:06

0:24

0:59

0:01

0:54 0:16

[100]

0:18 1:36

[102]

[102]

[102]

[102]

[100]

[101]

0:02

0:07

[101]

0:54 0:62

Ref. [100]

0:3

e2

0:64

Q2

130 5 Polymerization and Copolymerization of Salts of Unsaturated Carboxylic Acids

5.4 Copolymerization and Terpolymerization

131

1/r2 14 Li 12

10

Na 8 K

0 0.6

0.8

1.0 Ion radius / Å

1.2

Fig. 5.15 Ratio 1=r2 vs. cation radius in the copolymerization of MMA with alkali metal methacrylates

monomer mixture compositions (Table 5.6). Only in the case of lithium salts, in such systems as Li methacrylate-MMA [100] and Li acrylate-styrene [101], the expressed tendency for a regular alternation of monomer units is observed. At the same time, the product of multiplication of copolymerization constants for the pair styrene-Li methacrylate .r1 ; r2 D 0:94/ is close to 1, which is indicative of nearly ideal copolymerization. Differences in reactivity of acrylate and methacrylate ligands are reflected in the parameters Q and e. Thus, the value of Q is 0.64 and 0.07 for Li methacrylate and Li acrylate monomers, respectively, which is in agreement with general tendency of 1,1-disubstituted ethylenes for having larger Q values than the monosubstituted ones. Substitution of a hydrogen atom with a CH3 group in methacrylate causes change both in the magnitude and the sign of the double bond polarity (eLi acrylate D C0:02 and eLi methacrylate D 0:54). The negative value of e indicates an elevated electron density on the vinyl group due to electron donating nature of methyl group. Polarity effects influence noticeably copolymerization of Mg, Ca, Sr, and Ba acrylates with styrene in DMSO [102]. For the monomers studied, the magnitude of e increases in the order: Mg < Ca < Sr < Ba. Escalation of the unit alternation, i.e., gradual decrease of the product r1 , r2 follows the same order.

5.4.2 Reactivity of Tin-Containing Carboxylates The key features of copolymerization of trialkyltin (meth)acrylates and maleates with vinyl monomers [103–105] are that the comonomers are randomly distributed

132

5 Polymerization and Copolymerization of Salts of Unsaturated Carboxylic Acids

Table 5.7 The parameters of copolymerization of Sn-containing unsaturated carboxylates Comonomers M1 MMA

r1

r2

Q2

E2

Ref.

15.40 ˙ 0.98

0.01 ˙ 0.02

0:05

1:40

[103]

Styrene

6.70 ˙ 0.39

0.05 ˙ 0.10

[103]

Butylacrylate

9.39 ˙ 0.21

0.11 ˙ 0.08

[103]

Acrylamide

122.44 ˙ 6.04 0.06 ˙ 0.20

[103]

Acrylamide

M2 (n-Bu)3 Sn(OOCCH DCHCOOH)

(n-Bu)3 Sn(OOCCH DCH2 /

0.11 ˙ 0.02

0.82 ˙ 0.06

0:38

0:74

[103]

Itaconic acid

0.011 ˙ 0.109 1.088 ˙ 0.044 0:313 0:774 [105]

Dimethylitaconate

0.767 ˙ 0.181 0.932 ˙ 0.040

Acrylamide

(n-Bu)3 Sn(OOCC(CH3 ) DCH2 /

1.460 ˙ 0.40

0.85 ˙ 0.10

[105] 0:62

0:57

[103]

Itaconic acid

0.073 ˙ 0.090 2.272 ˙ 0.080 0:575 1:300 [105]

Dimethylitaconate

0.829 ˙ 0.101 1.223 ˙ 0.036

[105]

(n-Bu)3 0.643 ˙ 0.039 0.139 ˙ 0.058 Sn(OOCCH2 C(COOSn (n-Bu)3 /]DCH2 / 1.729 ˙ 0.129 0.316 ˙ 0.100

[106]

Styrene

MMA

[106]

over the chain, the trend for alternation increasing with an increase in the length of the alkyl chain in the (meth)acrylate monomer. The type of variation of the copolymerization parameters on going from acrylate to methacrylate (the r1 , r2 value increases from 0.09 to 1.24) for copolymerization of AAm with tributyltin (meth)acrylate derivatives [103] (Table 5.7) is similar to that discussed previously for lithium salts. For tributyltin maleate, the copolymers are enriched with the comonomer units, also the effective constants are abnormally high, especially for acrylamide .r1 D122:44; r2 D0:06/. Hence, tributyltin maleate copolymers with styrene, MMA, butyl acrylate or AAm are composed of large blocks of the comonomer separated by single maleate units. Moreover, in all cases, the r2 value is close to zero (see Table 5.7), which indicates the inability of the maleate monomer to homopolymerization. Good agreement was found between the theoretically calculated and experimental values of triads in the chain sequences of di(tributyltin) itaconate and MMA copolymer [106] (Table 5.8). This fact confirms the preferred addition of methyl methacrylate units to the growing macroradical and, hence, the formation of longer polymer chains from them.

5.4 Copolymerization and Terpolymerization

133

Table 5.8 Theorethical and experimental triad distributions in copolymers of di(tributyltin)itaconates .M2 / and MMA .M1 / [106] f111 C f112 f212 f1

Experimental

Theorethical

Experimental

Theoretical

0:50 0:55 0:65 0:75 0:90

0:8333 0:8750 0:9280 0:9688 1:0000

0:8636 0:8861 0:9411 0:9713 0:9951

0:1667 0:1250 0:0720 0:0312 0:0000

0:1364 0:1140 0:0589 0:0287 0:0040

The remote position of a tin carboxylate center in respect to double bond in molecules of tributyltin 4-(p-styryl)-butaneoate [107] and tributyltin 4-(p-styryl)propaneoate [108] ensures relatively high yields of polymeric products and the reaction rates. x x

+

1-x

1-x

AIBN

OSnBu3

OSnBu3

O

O x = 0.2-1

5.4.3 Copolymerization of Transition Metal Salts Since metal di(meth)acrylates and dicarboxylates are nonconjugated divinyl monomers, the equation of copolymerization in these systems has the form: Œm1  ŒM1  r1 ŒM1  C ŒM2  D ; Œm2  2ŒM2  ŒM1  C 2r2 ŒM2 

(5.20)

It is assumed that intramolecular cyclization or intermolecular ion cross-linking can be neglected, at least, at low degrees of conversion: ~M1 ~M·1 + C C

C C

M

C

. M1

(5.21)

Mn+

C C n+

C

~M1

C

. C C

C Mn+

M1

(5.22)

134

5 Polymerization and Copolymerization of Salts of Unsaturated Carboxylic Acids

It was shown by special studies [109, 110] that the products formed in initial copolymerization stages are soluble in organic solvents; therefore, no intermolecular cross-linking by acrylate groups takes place. The number of the non-consumed double bonds in metal acrylates increases in the series Zn2C (35%) < Co2C (39%) < Ni2C (49%). Despite the general rule on relatively lower activity of the analyzed carboxylates, compared to traditional monomers, as observed in homopolymerization reactions, copolymers on their base can differ significantly by microstructure of polymer chains, depending on the nature of the comonomer and the reaction medium as a whole. Copolymers based on carboxylic acid salts may differ considerably in the microstructure of polymeric chains, which depends on the comonomer nature and the reaction medium. In the copolymerization of transition metal acrylates with styrene in methanol, it was found that r1 > 1 and r2 < 1 (Table 5.9) [111]. It is obvious, that with these copolymerization constants, the resulting copolymer will be enriched in styrene irrespective of the composition of the reaction mixture. Conversely, when the reaction is carried out in DMF, alternation of the monomer units is observed in the polymer formed from the same monomer pairs. In the nickel acrylate–styrene copolymer, 46% of the acrylate units are incorporated in regularly alternating structures [110]. This behavior may be due to the change in the double bond polarity and, hence, parameters of the reactivity of acrylates in polar solvents, as is the case, for example, of styrene copolymerization with acrylamide in DMSO [112]. However, in the case of copolymerization of copper acrylate with styrene in methanol or acetonitrile the copolymerization parameters are not changed much [111]. The effective values of relative activity of the monomers (Table 5.9) attest to a statistical structure of the copolymer of MMA with chromium [113], copper [114, 115], and nickel [116] acrylates obtained by bulk polymerization. In the metal acrylate– acrylonitrile system, the opposite signs of double bond polarity (parameters e and Q for acrylonitrile are 1.2 and 0.6, respectively) account for S-shaped composition curves according to the classical copolymerization theory and for the clear-cut trend for alternation of copolymer units in the product [117–119] (Fig. 5.16). The trend for comonomer alternation is also found in the copper maleate–styrene copolymer: the product r1 ; r2 < 1, and the regularly alternating structures account for more than 50% of elementary units) [120]. However, in the cobalt hydrogen maleate– styrene system, the trend for alternation is slight, as the styrene concentration in the monomer mixture increases, long .n > 10/ polystyrene chains separated by maleate units are formed in the copolymer.

5.4.4 Kinetic Features The rate of radical copolymerization of styrene or acrylonitrile with zinc, cobalt, and nickel acrylates increases with an increase in the salt content in the monomer mixture [109–111]. This is especially pronounced in the case of zinc acrylate

Co(OOCCHDCH2 /2

Styrene

Co(HOOCCHDCHCOO)2

Co(OOCCHDCHCOO)

Styrene

Cr(OOCCHDCH2 /3

Cu(OOCCHDCH2 /2

Styrene

Acrylonitrile

MMA

MMA

Acrylonitrile

Styrene

Styrene

Acrylinitrile

Styrene

Ni(OOCCHDCH2 /2

Zn(OOCCHDCH2 /2

Acrylonitrile

Acrylonitrile

M2 Zn(OOCCHDCH2 /2

M1

Ethanol, 70 C, 0.01 mol/L AIBN Ethanol, 70 ı C, 0.01 mol/L AIBN

ı

DMF, AIBN 9  10 mol/L, 60 ı C In bulk, 2:688  103 mol/L BP, 60 ı C In bulk, 2:688  103 mol/L BP, 60 ı C DMF, As2 S3 -styrene, 85 ı C

0.34 ˙ 0.001 0.07 ˙ 0.004

0.45 ˙ 0.004

0.03

0.18 1.45 ˙ 0.001

0.95

0.8

0.08 ˙ 0.02

0.12 ˙ 0.08

0.11

1.08

1.05

0.21 ˙ 0.01

5.94 ˙ 0.05

3

6.34

Acetonitrile, AIBN 2%, 80 ı C

0.53 ˙ 0.06 0.17 ˙ 0.02

1.83 ˙ 0.02 0.09 ˙ 0.02

0.15 ˙ 0.01

0.56 ˙ 0.09

1.74 ˙ 0.03 0.14 ˙ 0.02

0.24 ˙ 0.03

r2 0.90 ˙ 0.07

0.41 ˙ 0.02

r1 1.10 ˙ 0.02

DMF, AIBN 9  103 mol/L, 60 ı C Methanol, AIBN 2%, 80 ı C

DMF, AIBN 9  10 mol/L, 60 ı C Methanol, AIBN 0.5%

3

DMF, AIBN 9  103 mol/L, 60 ı C Methanol, AIBN 0.5%

Methanol, AIBN 0.5%

Table 5.9 The parameters of copolymerization of transition metal unsaturated carboxylates Comonomers Copolymerization conditions

0.50

0.35

0.26

0.11

0.59

0.48

0.42

0.51

0.24

Q2 0.84

1:04

0:04

0:82

0:22

0:85

0:63

0:75

0:64

0:30

e2 0:70

[120]

[120]

[119]

[113]

[114]

[117]

[111]

[111]

[117]

[111]

[117]

[111]

[118]

Ref. [111]

5.4 Copolymerization and Terpolymerization 135

136

5 Polymerization and Copolymerization of Salts of Unsaturated Carboxylic Acids

a

b [m2]

0.8

0.8

0.4

0.4

0

c

[m2]

0.2

0.6

[M2]

0

d

[m2]

0.8

0.4

0.4

0.2

0.6

[M2]

0.6

[M2]

0.2

0.6

[M2]

[m2]

0.8

0

0.2

0

Fig. 5.16 Copolymerization diagrams of the systems acrylonitrile (M1 / – zinc(II) (a), cobalt(II) (b), nickel(II) (c) and copper(II) (d) acrylates

(Fig. 5.17), because the highest electron delocalization is expected due to the lowest electronegativity of zinc ion in this series of metals. Copper acrylate sharply inhibits the process, probably, by a mechanism similar to the mechanism of its homopolymerization. When dicarboxylic acid salts are used, for example, in pair cobalt hydrogen maleate–styrene, the rate of copolymerization decreases monotonically with an increase of the dicarboxylate monomer fraction in the initial mixture. However, in the case of neutral cobalt maleate–styrene system, this dependence passes through a maximum at the equimolar reactant ratio (Fig. 5.18) [120]. This type of behavior is often attributed to the donor-acceptor interactions between the comonomers, as in the copolymerization of sodium 2-acrylamido2-propanesulfonate with N -vinylpyrrolidone in water and DMSO [121].

5.4 Copolymerization and Terpolymerization

137

P (%)

Fig. 5.17 Yield of the product of styrene copolymerization with metal acrylates vs. time. Content of zinc acrylate in the comonomer mixture: 2 (1), 6 (2), 10 wt% (3); content of copper acrylate: 2 (4), 6 wt% (5) for Cu2C ; For comparison, the curve for styrene homopolymerization is given

3

15

2 12

1

9

6

6

3

4

0

2

4

5

6

8

time, h

Wo / cal mol–1 s–1 1.4

1.2

1.0

0.8 0.2

0.3

0.4

0.5

0.6

[M2] / mole fraction

Fig. 5.18 Initial rate of styrene copolymerization with Co(cis-HOOCCHDCHCOO)2 (1) and Co(cis-OOCCHDCHCOO) (2) vs. carboxylate content

Mathematical simulation of the experiment gave equations that describe adequately the dependence of the rate of copolymerization of Zn2C , Co2C , Ni2C , and Cu2C acrylates with acrylonitrile (AN) [122, 123]: Wp D K.ŒM1  C ŒM2 /n1 ŒIn2 ;

(5.23)

Wp D A0 .ŒM1  C ŒM2 /n1 ŒIn2 exp.Ea =RT/;

(5.24)

138

5 Polymerization and Copolymerization of Salts of Unsaturated Carboxylic Acids

Table 5.10 The copolymerization kinetic parameters of transition metal acrylates with acrylonitrile Metal Parameter L0:5

A0 , mol0:5 s0:5 n1 n2 Ea , kJ mol1 k 2 p /kt , L mol1 s1 K 348 , L0:5 mol0:5 s0:5

Zn2C [122]

Co2C [123]

Ni2C [123]

Cu2C [122]

(4.67 ˙

(4.4 ˙

(3.3 ˙

(6.8 ˙

0.06)107

0.3)1010

0.3)109

Cr3C [119]

1.1)109

1.67 ˙ 0.10 0.61 ˙ 0.07 72.5 ˙ 4.9

1.59 ˙ 0.10 0.57 ˙ 0.14 92.6 ˙ 6.8

1.65˙0.05 0.55 ˙ 0.07 86.0 ˙ 3.6

1.12 ˙ 0.10 0.78 ˙ 0.07 88.2 ˙ 5.0

6.16  104

5.55  104

4.15  104

3.98 ˙ 104

1 0.5 96.2 ˙ 0.2 1:0  105

where [M1 ] and [M2 ] are concentrations of the precursor monomers, [I] is concentration of the initiator, mol/L, n1 and n2 are the orders of reaction with respect to the total concentration of monomers and the initiator, respectively, K is the overall reaction rate constant, A0 is the preexponential factor, Ea is the overall activation energy. The kinetic parameters of copolymerization for the monomer pairs considered suggest a complicated influence of the nature of a metal on the elementary steps of the polymerization process (Table 5.10). As in the homopolymerization, the general trend of variation of the reaction rate constant in the series Zn2C > Co2C > Ni2C > Cu2C is retained. However, copolymerization of Cr(III) acrylate with acrylonitrile follows the ideal radical polymerization kinetics (see Table 5.10) [119]. It is noteworthy that when a great excess of AN is present in the system AN–zinc acrylate, the rate constant .2kd f .1:6 ˙ 0:26/  105 s1 ) and the activation energy (126:8 ˙ 3:7 kJ mol1) for initiation of the copolymerization do not differ much from those found for the homopolymerization of AN [124].

5.4.5 Terpolymerization Terpolymerization is important for practical purposes as it provides even more possibilities of varying the properties of the final product. In most cases, two termonomers are present in the terpolymer in greater amounts and are responsible for its key properties, while the third one is added only for modification of the required property of the polymeric material. From this point, the use of salts of unsaturated acids as the third monomer allows control of the reactivity of vinyl monomers, and determination of the spatial configuration and morphology of the products. Indeed, even minor additives (0.5–2.0 mol%) of alkali and alkaline earth metal methacrylates introduced in the MMA–MAA system (90:10) in bulk copolymerization have a noticeable effect on the parameters r1 and r2 and the radical copolymerization

5.4 Copolymerization and Terpolymerization

139

kinetics, thus determining the physicochemical properties of the copolymers having the following structure [41, 125, 126]: CH3 CH2 C O n C OCH3

CH3 CH2 C O m C OH

CH3 CH2 C C

O

p

OM

The introduction of metal methacrylate increases the yield and molecular mass of copolymerization products. These unusual facts were explained by inclusion of the salt into complexation with the system components. This affects the intermolecular interactions, giving rise to specific areas (intermediates) with a definite orientation of the monomer molecules. As a result, the reaction is accelerated and more regular and longer macrochains are formed (i.e., similar phenomena, that accelerate the polymerization of complexed monomers [127]) (Fig. 5.19). In addition, in the presence of a salt, the product r1 , r2 decreases, which corresponds to an increase in the degree of alternation of monomer units, as is also indicated by the change in the Harwoord parameter (Table 5.11). The formation of terpolymers, for example, in the systems tributyltin methacrylate–butyl methacrylate–acrylonitrile [128], di(tributyltin) itaconate (TBOI)–methyl (MA) or ethyl acrylate (EA)–acrylonitrile (AN) [129], follows copolymerization kinetics constants for binary systems of monomer pairs. This fact was also noted for terpolymerization of zinc [130], chromium [131], and copper [132] acrylates with styrene and acrylonitrile. Ternary azeotropic compositions, for example, for the systems TBOI–MA–AN and TBOI–MA–EA are equal to 37:48:15 and 9:80:11 mol%, respectively [129], and are in good agreement with the theoretically calculated values (Fig. 5.20), that is the terpolymerization in such systems obeys the ideal copolymerization laws. The kinetic parameters of terpolymerization for the monomers under consideration are also in agreement with the classical views on the effects of initiator and monomer concentrations on the reaction rate. Thus, for example, terpolymerization

P, % 1

2

3

4

80 60 40

Fig. 5.19 Copolymerization conversion vs. time for the system of MMA and MAA in the presence of 1% metal methacrylate: (1) Bi, (2) Ba, (3) Na, (4) without salts

20

0

30

60

90 time, min

140

5 Polymerization and Copolymerization of Salts of Unsaturated Carboxylic Acids

0

10

0

Table 5.11 The change of the copolymerization constants of MMA (M1 / and MAA(M2 / at introducing metal methacrylate into copolymerization system [126] The system r1 r2 r1 , r2 The Harwoord parameter MMA–MAA 0.37 ˙ 0.06 0.85 ˙ 0.03 0:315 36.90 MMA–MAA– 0.12 ˙ 0.02 0.16 ˙ 0.02 0:019 66.45 LiMAA MMA–MAA– 0.19 ˙ 0.03 0.11 ˙ 0.03 0:021 53.76 NaMAA MMA–MAA– 0.27 ˙ 0.02 0.08 ˙ 0.04 0:022 45.05 KMAA MMA–MAA– 0.36 ˙ 0.02 0.34 ˙ 0.02 0:122 41.7 CuMAA MMA–MAA– 0.59 ˙ 0.01 0 0 30.7 CoMAA

20

80 MA

AN

40

60

60

40

10

0

0

80

20

0

20

40

60

80

100

TBTI

Fig. 5.20 Composition of the bis(tributyltin) itaconate–methyl acrylate–acrylonitrile terpolymer vs. composition of the initial mixture. The ends of arrows point to the molar composition of the copolymer obtained from monomer mixtures with contents corresponding to the coordinates of points in the arrow; the azeotrope boundary is shown by the dashed line

of copper acrylate, styrene, and acrylonitrile is described by the equation: W D ŒI0:5 [St][AN] (1/[CuAA]) (where I stands for p-acetylbenzylidenetriphenylarsenic ylide), while constants of the relative reactivity, r1 .St/ D 5 ˙ 2 and r2 (AN C CuAA) D 0.4 ˙ 0.02, point to a random distribution of monomer units in the polymer chain [132]. Like in numerous cases mentioned above, and assumingly, by the analogous mechanism, an increase in copper acrylate concentration results in the decreased polymerization rate (Table 5.12).

References

141 Table 5.12 The influence of the concentration of copper (II) acrylate on the rate of terpolymerization [132] [CuAA] (mol/L) Conversion (%) Wp  105 (mol/L s) 0.062 12.3 – 0.093 10.3 2.1 0.124 7.9 1.7 0.155 6.6 1.3

At the same time, for copper methacrylate copolymerization with hydroxyethyl methacrylate in the presence of 1,1,1-tris(hydroxymethyl)propanetrimethacrylate, the terpolymer yield was quite high (70–80%) and essentially invariant with salt concentration changing from 2.7 to 3.82 mol% [133]. Therefore, homo- and copolymerization of unsaturated metal carboxylates are extensively studied areas of polymer science. On one hand, this is one of the most important methods for obtaining of metallopolymers. On the other hand, the presence of metal in a comonomer provides additional opportunities for the study of the finest mechanisms of copolymerization processes. In addition, a huge amount of information has been accumulated which still needs to be systematized. This requires development of the theory of copolymerization kinetics, in particular, a revision of the Q  e scheme, as well as detailed account for accompanying complexation, redox, and other processes. Copolymerization of comonomers with different metals needs to be developed. Intense development of this area of science allows for solving of these problems in the nearest future.

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5 Polymerization and Copolymerization of Salts of Unsaturated Carboxylic Acids

100. A. Hamoudi, I.C. McNeil, Eur. Polym. J. 14, 177 (1978) 101. A. Gronowski, Z. Wojtczak, Eur. Polym. J. 25, 241 (1989) 102. A. Gronowski, Z. Wojtczak, Macromol. Chem. 190, 2063 (1989) 103. N.E. Ikladious, A.F. Shaaban, Polymer. 24, 1635 (1983) 104. N.A. Ghamen, N.N. Messiha, N.E. Ikladious, A.F. Shaaban, Eur. Polym. J. 15, 823 (1979); 16, 339 (1980) 105. A.F. Shaaban, M.A. Salem, M.M. Azab, N.N. Messiha, Acta Polymerica 39, 654 (1988) 106. A.F. Shaaban, N.M.H. Arief, A.A. Mahmoud, J. Appl. Polym. Sci. 33, 1735 (1987) 107. L. Angiolini, D. Caretti, E. Salatelli, L. Mazzocchetti, R. Willem, M. Biesemans, J. Inorg. Organomet. Polym. 18, 236 (2008) 108. L. Angiolini, M. Biesemans, D. Caretti, E. Salatelli, R. Willem, Polymer 41, 3913 (2000) 109. A. Gronowski, Z. Wojtczak, Acta Polymerica 36, 59 (1985) 110. G.I. Dzhardimalieva, A.D. Pomogailo, Izv. Akad. Nauk SSSR, Ser. Khim. 352 (1991) 111. Z. Wojtczak, A. Gronowski, Macromol. Chem. 186, 139 (1985) 112. Yu. A. Begantseva, A.S. Malyshev, S.D. Zaitsev, Yu. D. Semchikov, Vysokomol. Soedin. A 44, 560 (2002) 113. S.M. Sayyah, A.A. Bahgat, A.I. Sabby, F.I.A. Said, S.H. El-Hamouly, Acta Polymerica 39, 399 (1988) 114. E.S.M. Higgy, S.M. Sayyah, I.H. Rashed, E. El-Mamoun, A.M. Hussein, Acta Polymerica 37, 606 (1984) 115. A.A. Razik, H. Talaat, S. Fayek, Polym. Degrad. Stab. 46, 41 (1994) 116. S.M. Sayyah, M.A. Khaled, A.I. Sabry, I.A. Sabbah, Acta Polymerica 40, 293 (1989) 117. T. Cherniawski, Z. Wojtczak, Acta Polymerica 35, 443 (1984) 118. Z. Wojtczak, T. Cherniawski, B. Rozwadowska, Acta Polymerica 34, 125 (1983) 119. B. Chaturvedi, A.K. Srivastava, Polymer 35, 642 (1994) 120. G.I. Dzhardimalieva, A.D. Pomogailo, Kinetika i Kataliz. 39, 893 (1998) 121. V.F. Kurenkov, T.A. Zhelonkina, Zh. Prikl. Khim. 77, 310 (2004) 122. T. Czerniawski, Z. Wojtczak, Acta Polymerica 41, 201 (1990) 123. T. Czerniawski, Z. Wojtczak, Acta Polymerica 42, 277 (1991) 124. T. Czerniawski, Z. Wojtczak, Acta Polymerica 43, 219 (1992) 125. V.P. Prokop’ev, B.I. Utei, L. Kh. Khazryatova, E.V. Kuznetsov, Vysokomol. Soedin. B 19, 222 (1977) 126. E.A. Gonyukh, E.V. Kuznetsov, L. Kh. Khazryatova, V.P. Prokop’ev, N.A. Akhmerov, Izv. Vyssh. Ucheb. Zaved. Khim. Khim. Tekhnol. 27, 1070 (1984) 127. V.A. Kabanov, V.P. Zubov, Yu.D. Semchikov, Complex-Radical Polymerization (Khimiya, Moscow, 1987) 128. N.A. Ghamen, N.N. Messiha, N.E. Ikladious, A.F. Shaaban, J. Appl. Polym. Sci. 26, 97 (1981) 129. A.F. Shaaban, A.A. Mahmoud, J. Appl. Polym. Sci. 36, 1191 (1988) 130. B.P. Agrawal, A.K. Srivastava, Polym. Eng. Sci. 34, 528 (1994) 131. P. Shukla, A.K. Srivastava, Polymer 35, 4665 (1996) 132. P. Shukla, A.K. Srivastava, Polym. Int. 41, 407 (1996) 133. A. Baccante, R. Quaresima, S. Lora, G. Palma, R. Volpe, B. Corain, J. Appl. Polym. Sci. 67, 11 (1998)

Chapter 6

Polymer-Analog Transformations in Reactions of Synthesis of Metal Macrocarboxylates

The application of polymers in modern technology, of concentration and isolation of metal ions from solution, water preparation, water purification, processes of removal, concentration, and separation of metal ions (including heavy), and catalysis, is determined to a great extent by the ability of metal ions to form stable contacts with functional groups of macromolecules as sorbents. They appear as a result of the creation of a system of electrovalent and coordination bonds between metal ions and certain groups of polymers, with formation of new polymer systems – macromolecular metal complexes (MMC). All organic polyacids are capable of forming in alkaline, neutral, and low acidic media quite strong metal complexes. Thus, polyacrylic acid (PAA) binds Cu(II) much stronger than its low molecular weight analogs. This process can proceed with ionized as well as with nonionized carboxylic groups. In the former case, contribution from ionic component into the total coordination bond energy becomes the definitive [1, 2]. Carboxyl groups containing polymers can be utilized in complexation processes in combination with micro- and ultrafiltration [3]. For example, PAA (mol. mass 30,000) is used for the removal of Zn2C and Ni2C ions from water on a polysulfonic membrane with retention coefficient of 97–99% [4]. This process is competitive with osmosis, nanofiltration, electrodialysis, liquid membranes, and so on. The binding of metal with complexing agents of natural systems is of particular importance, since functioning of transitional metal ions in biological systems, transport, and assimilation of metal ions in living organisms are based on the binding of the ions by functional groups of biopolymers. Polycarboxylates are widely used in interdisciplinary areas, such as ecotoxicology, water chemistry, plant nutrition; they are included into protein formations. Thus, interaction of Ca2C ions with COO groups of a protein results in a change of biopolymer swelling and is a critical stage of blood coagulation, irritation, and contraction of nerves and muscles, cell movement [5, 6]. These macroligands have received broad utilization for the preparation of new types of detoxicants, immobilized ferments, pharmaceuticals, and so on. Chemical reactions in systems polyacids–metal salt (metal ion) are called polymer-analog transformations if during the reactions the nature of carboxyl

A.D. Pomogailo et al., Macromolecular Metal Carboxylates and Their Nanocomposites, Springer Series in Materials Science 138, DOI 10.1007/978-3-642-10574-6 6, c Springer-Verlag Berlin Heidelberg 2010 

145

146

6 Polymer-Analog Transformations in Reactions of Synthesis of Metal Macrocarboxylates

groups bound with the main chain changes, while the length and the structure of the main chain skeleton of a polyacids remain the same. In other words, these reactions do not involve the polymer chain, just its side groups [7].

6.1 Complexation of Metal Ions with Macromolecular Ligands The efficiency of formation of macro complexes and stability of the system ion–polymer depend on the number and the energy of electrovalent and coordination bonds, hence, from the charge, coordination number and the nature of the ion, as well as from the number, nature, and the arrangement of charged groups and electronegative atoms in the macromolecule, i.e., from the chemical structure of the polymer. Analysis of interactions in such systems can be approached differently, depending on what is considered the central particle: a macroligand or a metal ion. Thus, two approaches for the calculation of equilibrium constants in the systems have been developed. A method based on an approximation of an independent interaction of each unit of macromolecule with low molecular weight compound is called the Scatchard method, and an approach counting mutual effects of separate chain units (cooperativity) is called the Hill method [110]. Subsequent addition of metal ions can be described by a general scheme: LMi 1 C M

Ki

! LMi

(6.1)

where LMi is a chain containing i of attached M. Therefore Ki D

ŒLMi  ŒLMi 1 ŒM

(6.2)

Thus, the total complexation constant displayed through current concentrations of chains, [L], and a metal, [M], is equal to K D ŒLMi =ŒLŒMi D

j Di

Y

Kj :

(6.3)

j D1

Material balance equations for concentrations of the starting components [L]0 and [M]0 can be presented as follows: ŒL0 D ŒL C ŒL

iX DN

Ki ŒMi ;

(6.4)

i D1

ŒM0 D ŒM C ŒL

iX DN i D1

Ki ŒMi ;

(6.5)

6.1 Complexation of Metal Ions with Macromolecular Ligands

147

where [L] and [M] are current concentrations of polymer and metal ions; N is a number of monomeric units in the chain. Then the formation function (˜n) that is a number of ligand groups per one metal ion is determined as: iP DN

nQ D

iKi ŒMi

i D1 iP DN

1C

i D1

: Ki

(6.6)

ŒMi

Then it is convenient to use the following equation for the calculation of Keff on the formation of similar type macro complexes: Keff D 

ŒMLn   ˚ ŒL0 ”  ŒMLn  ŒM0   ŒMLn  n

(6.7)

where  is the factor counting the maximum number of units L of the polymer participating in the formation of MLn complexes. An approach based on the consideration of a metal ion as a central particle on the formation of macro complexes utilizes the Flori principle for indefinitely long chains. It presumes that reactivity of binding centers do not depend on their location in a polymer chain or in a low molecular weight analog when components of the model reaction are chosen right. Assuming that there is only one type of reaction center present, the formation constant is determined as following: K D ŒMb =.ŒL0  ŒL/ŒM;

(6.8)

where [L]0 – [L] is the concentration of unreacted units in the chain, [M]b is the concentration of the complex. Since the aforementioned scheme accounts for only the number of bound metal ions, the equilibrium constant does not depend upon the molecular weight of a polyacid. For the calculation of a polyacid formation constant, a variation of the Beurrum method is most often utilized. In particular, for potentiometric titration a modified Gregor method is used. For this aim, two main parameters, the concentration of free carboxyl groups (L) and the formation function (˜n), are calculated. Some of the step formation constants, Kj , calculated by this method for polycarboxylates and their low molecular weight analogs are presented in the Table 6.1. However, due to some special features of the complexation by polyacids (high local concentration of reactive groups in the bunch, change of the charge and the conformation of macromolecules during the reaction, participation of reactive centers from different chains in metal binding, unavailability of some groups for the binding and so on) the Beurrum method is often not applicable for the description of these processes.

148

6 Polymer-Analog Transformations in Reactions of Synthesis of Metal Macrocarboxylates Table 6.1 The successive formation constants of the macrocomplexes and their low molecular mass analogs (cit. on [8]) M lg K1 lg K2 lg K3 lg K4 lg ˇ Sm(III) 5.7 5.4 – – 11:1 Eu(III) 5.7 5.2 – – 10:9 Pr(III) 5.6 5.2 – – 10:8 Cu(II) 4.8 4.2 – – 9:0 Ni(II) 3.9 3.4 – – 7:2 Co(II) 3.7 3.1 – – 6:8 Sm(III) 6.2 5.9 – – 12:1 Tb(III) 3.86 – – – – Cu(II) 5.9 5.2 – – 11:1 Sm(III) 3.9 3.2 2.9 10:0 Eu(III) 3.8 3.2 2.9 – 9:9 Pr(III) 3.7 3.1 2.9 – 9:7 Cu(II) 3.4 2.9 – – 6:3

The most important characteristic feature of reactions of chain molecules (including polyacids) is their ability to form numerous complexes having the same chemical composition but different arrangement of the reacted metal ions. This leads to compositional inhomogeneity, namely to distribution by length of the reacted and unreacted blocks. This should be taken into account on calculations of the formation constants. If upon metal binding unreacted blocks with the number of functional groups less than the number required for the binding of a metal ion were formed, then these blocks will not participate in the reaction. In other words, the concentration of unreacted centers is not equal to the active concentration of reacting particles. There are different ways possible for binding q metal ions with a chain containing p functional groups. The number of complexes formed that have a different structure is equal to Cpq (number of combinations of q molecules by p centers). In order to take these factors into account on the calculations of complex formation constants, the real concentration of units reacted with metal compounds is accounted for [9]. Usually metal ions are bound to a macroligand by a few bonds from either one (intramolecular) or several (intermolecular complexes) chains. In diluted solutions the intramolecular complexes are preferably formed [7, 10], while in concentrated solutions and in the matrix the intermolecular ones are preferred. During the formation of intramolecular complexes the first binding can be considered as the second order reaction (first on each component) and all the following reactions (building of intramolecular bonds) are the first order ones. Overall reaction of M with one chain is a second order reaction. Rates of complexation reactions usually are high and it is difficult to isolate experimentally stages of the process. In those relatively rare cases when this was done, the constants Ki were found to increase with the increase of the binding rate of a metal, that is in contrast to low molecular weight ligands. This result is a manifestation of another cooperative “chain effect”, when the shape

6.1 Complexation of Metal Ions with Macromolecular Ligands

149

of a macromolecule changes upon complexation. The addition of a metal ion to the chain is accompanied not by just this chemical act but also by a change in the “local hardness” of the polymeric chain in the point of its addition, thus resulting in the increase of reactivity of the polymer, for example, according to this type:

L L L

+ Mn+ L

K1

L

L Mn+

L L

L

L

K2

L

Mn+

L

L

L

L

K3

L

Mn+

L

L L

L

The formation constant for the complex can be presented in the form KD

iY DN

Ki D Kj

(6.9)

i D1

where  D Ki =Ki 1 , the cooperativity parameter that shows how big the formation constant is for the current addition in comparison with the previous one. It is postulated that all subsequent constants except the first one are identical, since practically all the entropy is lost in the first act of addition; the subsequent steps are all intramolecular cyclizations. The value K2 =K1 D  D 104  108 : In other words, the coordination causes reshaping of the polymer chain and makes its conformation more appropriate for further reactivity. As a consequence of this effect, there is an uneven distribution of metal ions between macromolecules during the reaction. Since the first addition act is accompanied by the largest entropy loss in comparison with the minimal losses in the subsequent addition acts, the interaction with the chain lasts till the saturation of all potential reactive centers. This results in the coexistence in the reaction volume of both, the macro complexes with the maximum number of bound functional groups and the unreacted macroligands, i.e., the principle “all or nothing” is realized. The cooperative character of interaction in the systems polymer–metal ion is made evident by not just the shape change of a macromolecule in solution, but as was mentioned earlier, by the change of chain charge, by dependence of thermodynamic parameters from the molecular mass, molecular mass distribution, degree of conversion for functional groups, and flexibility of the chain. Significant influence on metal ion binding can be produced by hydrophobic, hydrophilic, or electrostatic interactions between the components. A macromolecule is elongated during the formation of charged chains in the result of interaction with MnC with the growth of the conversion degree [11]. A macroligand itself can contain positively or negatively charged atoms that facilitate or prevent binding of metal ions. Therefore, electrolyte solutions are specially introduced for the prevention of electrostatic repulsion. Thermodynamic characteristics for the interactions of metal ions with polymer reagent are estimated from temperature dependencies K D f .1=T /. As a rule, the

150

6 Polymer-Analog Transformations in Reactions of Synthesis of Metal Macrocarboxylates

largest contribution into the total change of free energy is made by the entropy part, while the enthalpy part has only slight changes. For this analysis three levels of spatial arrangement of macro complexes have to be taken [12] into account: – Local level, that reflects chemical structure of a single binding unit of a metal ion with a chain molecule (nature of the metal and carboxyl groups, reaction conditions and so on) – Molecular level, defined by the chemical structure of a polymer chain (its length, composition of elementary units, shape and conformation of the chain, and so on) – Supramolecular level, reflecting features of supramolecular interactions of macromolecules and the degree of their mutual arrangements. This is especially relevant for studies of metallopolymers in the solid phase, for example, those obtained by lyophilic drying of the corresponding solutions, when the effects of supramolecular organization are manifested to higher degree in comparison with solution By taking into account these three levels of spatial arrangement, the difference in the free energy on the formation of macro complexes can be presented as follows (assuming the additivity of its components): G D G1 C G2 C G3 ;

(6.10)

where G1 , G2 , and G3 are the free energy differences for local, molecular, and supramolecular levels, correspondently. Under certain conditions, the free energy difference for one or another level can be neglected, allowing analyzing enthalpy (H ) and entropy (TS ) contributions into the G value for each level in more detail: G D H C TS (6.11)

6.2 Metal Ion Binding by Polyacids Polymeric acids as macroligands attracted attention of a number of researchers rather early (see, for example, [13–16]). Let us consider the main features of MXn binding by polyacids with taking into account the structural organization (topology) of a polyligand. Polymers and copolymers based on acrylic and methacrylic or sulfonic acids are used most often, usage of maleic, itaconic, or others is rare. The necessary cation can be bound to polymer by either the treatment of its acidic form with the corresponding metal hydroxide, or substitution of a weakly bound cation. For the polyacids the affinity toward metal cations changes in the following series: HC >> AgC > NaC . However, in the vast majority of cases preliminary ionization of the polymeric acids is carried out.

6.2 Metal Ion Binding by Polyacids

151

Binding of alkali metal ions by PAA macromolecules increases in the series KC < NaC < LiC [17], although from the position of ion atmosphere binding the reverse order should be expected. The normal order follows the law of electrostatic interactions: a solvated ion with the smallest radius interacts stronger than the one with the largest radius. The reverse dependence in the case of polycarboxylic acids attests for their specific binding, according to which the viscosity of aqueous solutions of polycarboxylic acid salts increases in the row LiC > CsC . Binding of small ions with polyions to large extent determines the solubility of polyelectrolytes in solutions of salts. Typically, introduction of low molecular weight electrolytes into aqueous solutions of polyelectrolytes decreases the solubility of the latter. The stronger the binding of ions with the chain, the stronger is the effect. Accordingly, solubility of polyelectrolytes decreases most significantly on the introduction of multicharged counter ions. Interaction of PAA with divalent cations has been investigated rather thoroughly, since exactly this reaction was put into a base of classical studies of complexation with participation of macroligands (see, for example, papers [18–21]). Interaction of metal salts with polymer reagents differs significantly from reactions with low molecular weight analogs, since they feature the presence of a number of reaction centers and also are accompanied by a change of conformation and shape of macromolecules in solution. Successive attachment of metal ions to functional groups of a polymer (presumed to be a central particle) is considered most often. Strong decrease in the pH of the aqueous solution of a polyacid occurs upon addition of a metal salt, indicating ionization of carboxyl groups. K1

C C * RCOOH C M2C )  RCOOM C H

(6.12)

K2

C * RCOOMC C RCOOH )  .RCOO/2 M C H

(6.13)

This is manifested by two jumps of the pH on potentiometric titration curves, for example, for Cu(II), the first jump takes place between the pH values 6 and 8, while the second one does at about 10.5. However, these processes typically proceed along more complicated routes, including disassociation of a polyacid and hydrolytic equilibria with participation of the hydroxo complexes, M.OH/n : RCOOH • RCOO C HC

(6.14)

.2n/

M.OH/n C nHC • M2C C nH2 O RCOO C M2C • .RCOO/2 M

(6.15)

The structure of the binding center can have a different character: C

O e M+ O

C

O O→ M

C

O O

M

152

6 Polymer-Analog Transformations in Reactions of Synthesis of Metal Macrocarboxylates

The high concentration of HC in the solution facilitates a shift in the two latter equilibria to the right. Hydrogen ions are less likely to undergo the exchange for the transition metal ions than the alkali metal cations. Therefore, the preliminary ionization of polyacids is carried out to facilitate the exchange. Let us consider this process in more detail on an example of Cu(II) binding with PMAA [22]. Each Cu(II) ion interacts with one, two, or four carboxyl groups depending on the ionic strength and the pH. Adducts with 2:1 composition are formed predominantly at low concentration of copper ions, while at high concentration the 1:1 adducts are formed. The stability constant for the 1:1 Cu2C complexes (25ı C; NaNO3 0.1 mol/L, pH 6) with PAA mol. mass 3  106 is estimated [23] to be lg K1 D 5:2 ˙ 0:2. There are two variants possible upon this: the intramolecular interaction with two carboxyl groups from the same chain and the intermolecular one with participation of carboxyl groups from different polymer chains. Doubling of the molecular mass of PMAA upon the interaction with Cu(II) means that binding of divalent cations by ionized PMAA is conducted through bridges between two polymer chains [24]. In other words, dimerization of a polyacid is a consequence of covalent binding in such systems. The same phenomenon is observed in the PMAA–Zn(II) system when the molecular mass of PMAA increases from 0:47  106 to 1:025  106 in the product [25]. The composition of the compounds formed depends on the concentration ratios of the reacting components. Products with 1:1 composition are formed at low concentration of Zn (degree of neutralization is 0.3–0.75), while at high concentration the 2:1 products are formed. Complexes with 2:1 composition are more stable than the 1:1 complexes. The question of binding of counter ions is important. The binding is carried out by formation of ion pairs or complexes with participation of the counter ions and the charged portions of the polyion. The corresponding bound portion is thus discharged. Typically, it is difficult to separate experimentally the two types of binding. The presence of a specific binding was proven by the studies of interaction of different counter ions with the same polyion. Binding of cations by polyanions is determined by the size of the charged group of the polyion, radius of the hydrated and nonhydrated forms of a counter ion as well as their solvation energy. The pH value of the medium is also important. At low pH values, the PAA chain has an elongated shape due to repulsion between the negatively charged COO groups and metal ions bind to one or two neighboring groups. At the pH < 4.5 the macromolecular tangle compresses, and a metal ion can coordinate to 2–4 carboxyl groups. PMAA forms with Cu2C ions three types of complexes, depending on the concentration of copper and the degree of neutralization of carboxyl groups when exchanged interactions are realized [26]. These are mononuclear, tetragonal, and polynuclear associates (clusters that are described below in the grafted fragments PE-gr-PAA) and even Cu2C nitrate adsorbed on the surface of PMAA at lyophilic drying. Upon neutralization of PMAA solutions with NaOH, polynuclear clusters are destroyed already at ˛ D 0:1 and dimeric complexes are formed. Interestingly, PAA that has a similar structure behaves differently. Concentration of polynuclear

6.2 Metal Ion Binding by Polyacids

153

complexes increases and mononuclear ones decreases with the increase of the ˛ value. Cu(II) forms with the ionized PMAA and PAA complexes of the D2h and D4h symmetry (dimers) depending on the pH [27]: C

C

O

O

O

Cu O

C O

O

O

O

O C

O

Cu

Cu

C

O

O

C O Cu

O

O

C

O C

D2h

D4h

Thus, two main structures of the coordination polymers are formed. The first one is the square planar structure of Cu2C with four oxygen atoms of two carboxyl groups, the coordination number (CN) of the copper is 4 and the Cu2C –O distance is 0.196 nm. The second one is the binuclear coordination structure Cu2C –Cu2C , the ionic pair with 8 oxygen atoms of four carboxyl groups. In this structure the Cu2C –O distance is equal 0.196 nm and the Cu2C –Cu2C one is 0.264 nm [28]. These coordination linked network structures are more stable than their low molecular weight analogs or the precursor ammonia complexes. O

O

C

Cu

C O

O

O

O

Cu

C C O O C O

O O

Cu O

C C C

It is necessary to take into account that typically structures of this type are dynamic formations and may change in solution with time, transforming into more stable ones. There are a number of examples of this. For example [29], Cp2 TiCl2 when in slight excess forms with the copolymer, poly(styrene-co-methacrylic acid), Mw 36,000, Mw =Mn D 1:7, in THF a soluble product (Mw 41,000, Mw =Mn D 1:9). All attempts to remove the excess of Cp2 TiCl2 by reprecipitation of metallopolymer led to cross-linking of the product. It was demonstrated by different physico-chemical methods (including spectral ones coupled with the dialysis) [30], that interaction of Ca2C ions with PAA occurs through a series of intermediate stages, such as: the starting NaC ionic complex binds to carboxyl group in bidentate fashion (a), then intermediate monodentate Ca2C complex is formed (b), pseudo bridge complex with H2 O (c), pseudo bridge complex with NaC (d), and, finally, bidentate chelate complex (e).

154

6 Polymer-Analog Transformations in Reactions of Synthesis of Metal Macrocarboxylates

b

+ O Ca

Ca++

O

a

O

+ Ca++ Na

–

O

c

+ O Ca O O

H

e

O

H

+ Ca

O

d

Ca++

+ O Ca O

+ Na

The interaction of organometallic compounds with polyacids, for example, the interaction of diethyl zinc with copolymer ethylene-co-methacrylic acid (HA) [111] takes place through the series of equilibrium transformations, that include formation of acidic dimers, formation of tetracoordinated zinc carboxylates, then (with the excess of acidic groups) hexacoordinated zinc carboxylates, and, finally, formation of the salt of hexacoordinated zinc(II): K1

* 2AH )  ŒAH    AH acid dimer K2

ZnEt2 C 2AH ! ZnA2 C 2EtH tetracoordinated zinc carboxylate K3

 C * ZnA2 C AH  )  ŒZnA3  H hexacoordinated zinc carboxylate K4

* ZnA2 C 4AH )  Œ.AH/2    AZnA    .HA/2  Zinc acid salt

(6.16) (6.17) (6.18) (6.19)

Since the stability of complexes depends on many factors, some formal (empirical) equations were suggested that connect molecular mass of PAA (1:4  lgN  2:4, where N is a number of monomeric units) with the complexation constant for calcium and magnesium ions and the solution ionic strength (0  I  1), and the degree of protonation (˛). The formation constant for Ca2C complexes is higher than that one for Mg2C : at I D 0:1 mol L1 (NaCl), log N D 1:8 and ˛ D 0:5 the Mg2C lg K2Ca2C D 4:43 and the lg K2 D 4:24 [31]. For the pair Cu2C and Ni2C the formation constants for ML2 complexes (ˇ) are equal 6.3 and 5.4, correspondingly (for the calcium at these conditions ˇ D 4:6). Mg2C ions form less stable bonds with PAA than Ba2C , Sr2C , and Ca2C ions do. Changes in the viscosity and the specific conductivity of the polyion charge observed upon the neutralization of MgO are smaller for the PAA than those for the PMAA. Upon titration of 0.02 M solution of PAA with barium hydroxide [32] the transition from loose to compact tangle was noted in the region 0:3 < ˛ < 0:55 and the difference in the free energy for these states was calculated: Gı =N D 156–173 cal/mol. The series of the stability constants (lg K2 ) for the metal ion complexes with PAA or PMAA obtained under the same conditions is presented [33] as

6.2 Metal Ion Binding by Polyacids

155

the following: Cu2C  Ca2C  Mg2C . Since calcium ions bind two neighboring carboxyl groups, their lg K2 has the same value as in the case of dicarboxylic ligands. There were attempts (in particular, on an example of Ca2C ions binding [34]) for searching for equations connecting the protonation constant and the disassociation constants for macrocomplexes as a function of the ionic strength (I ) and the temperature (T , K) according to the type: pKH D 4:856  0:984 I 1=2 C 0:253 I  198:7=T for protonic dissociation (6.20) and

pKCa D 3:968  2:671 I 1=2 C 0:750 I  1102:3=T:

(6.21)

3C

Complexation of Al ions with a statistical copolymer of acrylic and maleic acids (composition 0.7:0.3, molecular mass 92,000) was investigated in detail by titration method as well as by applying the stopped-flow technique [35]. Low molecular mass analogs, such as glutaric and tricarballylic acids, were used as models for the comparison analysis. COOH

COOH

CH2

CH2 CH COOH

The main conclusions were, that the dominating part of the Al3C complexes are intrasphered and monodentate, they include neighboring COO groups and the deprotonation constant of the macroligand, pKd D 3:0, (the maximum degree of deprotonation is achieved at the pH equal 3.6), is significantly lower than those for the low molecular weight analogs (Table 6.2). Substitution of a water molecule inside the hydration shell of the aluminum ion by a carboxyl group of the macroligand is the rate-determining step. The rate constant for this reaction (k1 D 3:1 s1 ) is also lower than those for the model ligands. A positive value for the change in activation entropy indicates the low hydration degree in the intermediate complex. Formation of triple charged cations, similar to the ion pair type that includes polyacrylate ion, occurs upon the neutralization in the Co3C – PAA system. Their formation is suppressed upon introduction of an electrolyte. Q – described of the outer sphere association. Q depends only on electrostative interaction. In addition to the potentiometric titration method, the stopped-flow technique, different variations of spectral and electro-chemical methods, there are also other ways for the quantitative evaluation of the efficiency of the formation of macromolecular metal complexes. One of them is based on the effect of quenching of the luminescence of macromolecules with luminescence labels (0.1–0.3 mol% of groups of the 9-alkylanthracene structure) by transition metal ions in dilute solutions under the conditions of ionization of all carboxyl groups [36]. Under these

156

6 Polymer-Analog Transformations in Reactions of Synthesis of Metal Macrocarboxylates Table 6.2 Thermodinamic and kinetic data for the complexation of Al3C with carboxylates at 298 K [35] Glutaric acid Tricarballylic acid Polycarboxylic acid pKa1 4.37 3.72 4.0 5.50 5.05 5.9 pKa2 pKa3 – 6.6 7.8 5.4 3.5 3.0 pKd 1.15 1.15 0.54 lg Ki 1.3 3.3 lg .Q0 dm3 mol1 / 1.3 k1 Œs1  37 19 3 2.7 1.4 1.0 k1 Œs1  7:0  102 3:9  102 8:0  104 k2 [mol dm3 s1 ] 3 1 3 3 k2 [mol dm s ] 5:0  10 2:8  10 2:6  104 1 0 Hi [kJ mol ] 20 28 19 90 115 72 Si0 ŒJK1 mol1  0 0 0 Hd0 [kJ mol1 ] 100 67 57 Sd0 ŒJK1 mol1  H1C [kJ mol1 ] 70 92 78 S1C ŒJK1 mol1 ] 30 70 20 H2C [kJ mol1 ] 120 70 63 90 30 100 S2C ŒJK1 mol1 ]

conditions for the complexes of AgC , Cu2C , and Ni2C with carboxyl containing polymers and copolymers in aqueous saline solutions at 25ı C the stability constant, Kst , is higher than 108 [37]. A portion of Cu2C ions forms electrovalent contacts with one ionized carboxyl group in the ratio “ > 0:5 (ratio of molar concentrations of metal ions and carboxyl groups at different fixed concentrations of the polymer in solution). Then, the Kst values decline to 105 –103 . Since electrovalent interactions play a definitive role in the formation of these macrocomplexes, the complexes can be destroyed upon the increase of ionic strength of the solution. Thus, the Kst is diminished upon addition to the solution of a neutral salt, NaCl, (Kst is equal 6  107 and 1  107 at i D 0:1 and 0.5, correspondingly). The structure of carboxyl containing copolymers (isomeric structure of carboxyl containing units) also affects the stability of MMC. Compacting of macromolecules occurs due to enhancement of intramolecular contacts (upon mutual compensation of carboxyl groups and metal ions). Infrared spectra provide significant information regarding the structure of carboxylate units. The frequency COO D 1;580 cm1 is observed in the IR spectra for the products that do not contain bridging COO groups, while for those with bridging carboxyl groups the frequency  D 1;520–1;560 cm1 is observed. The frequencies as and s equal to 1,390–1,440 and 1,300–1; 340 cm1 , correspondingly (Table 6.3). Characteristic adsorption bands for the valent oscillations of COO in the IR spectra for the solution mixture (1:1) of metal nitrates (1 mol/L) and 25% PAA in the case of Al.NO3 /3 appear at 1;616 cm1 .as /, 1;449 cm1 .s /,  D 167 cm1 ;

6.2 Metal Ion Binding by Polyacids

157

Table 6.3 The stretching frequencies (cm1 ) of COO – groups in the IR spectra of MXn adducts with PAA [13, 38] M CDO as COO s COO 1,700 1,437 1,400 1,336 Fe(III) 1,560 1,440 1,400 1,313 Cr(III) 1,546 1,440 1,394 1,324 Co(II) 1,546 1,438 1,390 1,307 Ni(II) 1,536 1,436 1,390 1,300 Pd(II) 1,520 1,440 1,393 1,340 1,640 Al(III) 1,616 1,449 Fe(III) 1,632, 1,526

and for the Fe.NO3 /3 in the same system they appear at 1,632, 1526 cm1 .as /, 1; 450 cm1 .s /,  D 182 and 76 cm1 . The interaction mechanism and the structure of ionic bound carboxylates of rare earth elements (REE) are investigated in detail (see, for example, [39, 40]). Formation of intrachain complexes that have a fragment of 10–13 monomeric units per one metal ion takes place upon addition of trivalent REE ions [Tb, Ce, La, Eu, Nd, and others] to aqueous solutions of PAA, PMAA, or polyglutamic acid. The state of Eu(III) ions in their polycarboxylates, in particular the number of water molecules in the first coordination sphere of the metal ion, was studied by the laser induced fluorescence [41]. A cage from carboxylate ions is formed around the metal ion when the ratio ŒCOO =ŒEu3C  is 20. Each metal ion is surrounded by between 5.5 and 8 water molecules (pH D 5.5) and the pH dependence indicates the change of conformation of PMAA depending on the number of water molecules around the metal ion. Complexes of Ce(III) with PAA, especially with the low molecular weight one, were utilized [42] for the preparation of CeO2 . Only COO groups take part in the binding of Tb(III) by the hydrolyzed polyacrylamide; amide groups do not participate in the reaction [8]. Platinum group metals easily interact with polymeric acids, the type of the bond formed being determined to a substantial extent by their nature [8, 43, 44]. Thus, the bridging binding is more typical for ruthenium compounds: polymer bound ruthenium acetate has a structure of 3 -oxo-metal acetate complex, while rhodium (I) gives nonbridging complexes (Scheme 6.1). Poly(carboxylato)hydrocarbonyltriphenylphosphineruthenium (II) and poly(carboxylato)triphenylphosphineruthenium (II) chloride were synthesized by the reaction of benzene solutions of RuH2 .CO/ .PPh3 /3 / or RuCl2 .PPh3 / with copolymers of acrylic acid and ethylene, maleic acid and alkyl- or arylvinyl ethers [45]. In the latter, the carboxyl groups of maleic acid fragments act as either mono- or bidentate binding ligands. Chemical and physical properties of these macromolecules are determined by hydrophobicity, electronegativity, and volume of the ether fragments. Compounds of Rh and Pd are efficiently bound to copolymers of styrene and maleic acid [46].

158

CH2

CH~

Rh(PPh3)x (x = 1÷3) CH

C O

L

L ~CH2

,N

PPh3

CH~ O C

OC H

CH

O

O n MCI3–n

CH2

CH C

m

O

O M

n–m

CH2

CH

n

CH2

O O

U

O

CH2

m–I

O

O H2O

C

C

CH

CH

CH2

CH

n n

CH2 CH2

CH2

m

I

O

O O H2O H2O O U O O H2O O O

U

C

O

C O

CH CH

O CH2

m

CH2

CH C

O RuCI(CO)3

CH

O

C O

I

O

C

C

CH2

CH

O O H2O O H2O O U O O U O H2O O O HO 2 O O N N O O

O

O O

CH C

C ) 3] 2 (CO CI 2 OH CH 3

Ru O

PPh3

CH2

C O

C

C3H7OH / C6H6

H

O C

CH2

COO– RuH2L(PPh3)3

Ca

CH~

PPh3 (L = CO, PPh3)

O

O

UO2(NO3)2

CH~

[Ru

~CH2

Ru

d)

PPh3

~CH2

O

a,

O

CH

CH2

n

O

MOCOCH3

CH~

CH2

C

=L

O

I3 ( M

~CH2

3 )2 2H 5 OH

O 5-x

CH C

C

O

M( OC

CH~ C

O H2 )4 •3 3 CI 3 OH h Ru H 3 (P P H – C hH O R H7 C3

~CH2

ΦA μM

(n = 1÷5 L = μMΦA)

(M =N i, C –H o,C 2O u)

+ 6-nRu3(OCOCH3)nL3]

COO

MC

~CH2

6 Polymer-Analog Transformations in Reactions of Synthesis of Metal Macrocarboxylates

O C

CH

I

m

O

O Ru(CO)3

O

Scheme 6.1 Formation of macrocomplexes via carboxylic group of polymer acids

In principle, a similar mechanism is realized also upon binding of complexes of f -elements, for example UO2 .NO3 /2  6H2 O [8, 47–49]. Interaction of uranyl ions with carboxyl groups leads to a change of chemical and physical nature of neighboring groups, so that it facilitates involvement in the process of subsequent groups. Different variations of immobilization are observed: UO2 (II) ion can be attached to two carboxyl groups that are monodentate or give chelates with formation of linked or not linked bridging structures (Scheme 6.1). About 85% of carboxyl groups are chelated, which is caused by steric factors allowing only limited number of carboxyl groups to participate in this interaction. Uranyl complexes with PAA have hexagonal (bipyramid) structure that includes two intrasphere H2 O molecules per each uranium atom. There are numerous macrocomplexes with derivatives of carboxylic acids. The most abundant are, in particular, macrocomplexes of Co(II), Ni(II), Cu(II), Fe(III) with copolymers of acrylonitrile (or methacrylonitrile) and methacrylic acid [50] and Cu(II) with copolymers of maleic acid and ethylene, styrene, or n-butylvinyl ether [51]. Both carboxyl groups of maleic acid fragments participate in the interaction that is accompanied by local changes around the chain and its conformation affects strongly neighboring groups upon the complexation. Copolymers of acrylamide and acrylic acid (prepared by polymer analog transformations upon hydrolysis of polyacrylamide) efficiently bind Fe(III), Cu(II), Ni(II), Cr(III), and other cations. Macrosalts of Cr(III), Fe(III), Ni(II), and Co(II) with polyethylene gly-

6.3 Metal Ion Binding by Stereoregular Polyacids

159

col methacrylate phtalate are of interest, as well as are the reaction products of polyfunctional esters with amphoteric metal oxides [52]: HO

ROCOR'COO HO

n

H + MO

ROCOR'COO

n

M

OH

HO ROCOR'COO n H

HO ROCOR'COO

n 2

M

Formation of metal hydroxocarboxylates is the first act of these interactions, while the next one is formation of metal dicarboxylates. More deep processes include coordination of terminal hydroxyl or carbonyl groups by a metal, leading to the increase of molecular mass of the products formed. Involvement in the reaction of reactive groups of these multifunctional macroligands takes place in the following order [53]: –COO > diol OH groups > polyether terminal groups > polyether carbonyl groups.

6.3 Metal Ion Binding by Stereoregular Polyacids Investigation of specifics of MXn binding by stereoregular polyacids is of significant interest, although these data are extremely limited. Out of general consideration, it can be suggested that distinctive configuration of a polymer chain can affect the structure of a macrocomplex, and the lower the flexibility of the ligand the stronger the effect of chain macrotacticity will be. A big difference between syndio- and isotactic PMAA in selective sorption of univalent metal ions was noticed a while ago [8]. Activity of sodium ion changes in the series iso- > syndio- > atactic for all degrees of neutralization and regardless of the molecular mass and the concentration of a polyacid. Formation of macrocomplexes with bivalent metal salts also points to the big importance of microstructure of polyacids, for example PMAA. The feature of metal ions interaction with the stereoregular PMAA, is that neighboring carboxyl groups of the chain are involved in the macro complex formation. Each Cu (II) ion reacts with two neighboring carboxyl groups of one chain of the isotactic PMAA. It was established by methods of potentiometric titration, dialysis, and viscosimetry that interaction of Cu(II), Mg(II), and Zn(II) ions with the isotactic PMAA of varying degree of neutralization (˛ D 0:3  0:9) takes place according to two-step scheme [112]: At high concentration of Cu(II) the products of 1:1 composition are formed, while at low concentration those of 2:1 composition are formed. The characteristic viscosity, [], increases from 2.0 to 9.9 for the isotactic PMAA, depending on the ˛ value, and for the macrocomplexes of PMAA with Cu(II) from 0.4 to 3.0 (see Fig. 6.1). These facts are explained similarly to the cases discussed above by compression of PMAA macromolecules that increases in the series Mg(II) < Zn(II) < Cu(II).

6 Polymer-Analog Transformations in Reactions of Synthesis of Metal Macrocarboxylates

Fig. 6.1 The characteristic viscosity of the isotactic PMAA vs. neutralization degree (˛) undo neutralization without salts (1) and in presence of Mg(II) (2), Zn(II) (3), and Cu(II) (4) ions

[h], L / g 1 8.0

6.0

2

4.0

3

2.0

4

0 0.2

0.6

6

a

0.8

60 3

ΔH, Kcal / mol

Fig. 6.2 Thermodynamic parameters of metal ions interactions with syndio(1, 3) and isotactic (2) PMAA. (1,2) the change of enthalpy, (3) enthropy

0.4

4

50 2

2

ΔS, Kcal / molK

160

40 1

0

Mn Fe Co

Ni

Cu Zn Cd

30

The nature of reacting cations also plays an important role upon their binding with the iso- and syndiotactic polymethacrylic acids (PMAA). Thus, the isotactic PMAA is 1.5 times more reactive toward binding of Cu(II) than the syndiotactic one [2, 54], whereas in the case of Mg and Na, the opposite is observed. Upon precipitation of PMAA from solution by Cu(II) ions, the isotactic polymer precipitates first at the concentration of Cu(II) three times lower than that one required for precipitating the syndiotactic polymer. Such an influence of stereo regularity on the precipitation is not observed for the more flexible PAA macromolecule. The interaction of Mn, Co, Ni, Cu, Zn, Cd, and Mg ions with isomeric polyacids is an endothermic process, and therefore, complexes are stabilized due to relatively large entropy changes. It is obvious from the Fig. 6.2, that the H value for the complexes of isotactic PMAA is always higher than the one for the syndiotactic PMAA (with the exception of the Cu(II) complexes) and the isotactic complexes are more strained.

6.4 Peculiarities of MXn Binding by Cross-Linked Polyacids Table 6.4 The thermodynamic data of Cu(II) and Mg(II) ions complexation isotactic PMAA K  107 G H Macrocomplex (L/mol) (kcal/mol) (kcal/mol) Cu(II)-syndiotactic PMAA 400 13.2 5.1 Mg(II)-syndiotactic PMAA 1.0 9,6 0.14 Cu(II)-isotactic PMAA 1,200 13,9 3,8 Mg(II)- isotactic PMAA 0.4 9.1 0.8

161 with syndio- and S (kcal/(mol K)) 61 32.5 59 33

The differences between the magnesium and the copper complexes (Table 6.4) are entirely caused by their nature: Cu(II) forms covalent type compounds, while Mn(II) forms ionic ones with lower steric constrains and the lower H values. The high values of H and S for the Cu(II) complexes are due to the release of H2 O molecules upon their formation. Two water–metal bonds are destroyed during this process, leading to the entropy increase. According to UV spectroscopy data Co(II) binds with three carboxyl groups of one polymer chain. This is explained by the stronger charge redistribution on the polymer anion (contribution of a covalent component in the formation of Co(II) carboxylates is lower than in the case of Cu(II)). The data of equilibrium dialysis indicate that the iso-PMAA binds Cu2C 3 times stronger than the syndiotactic one, while the binding of Mg2C by the iso-PMAA is weaker (especially at high ionization degrees). Cu2C forms covalent complexes of strict geometry, while Mg2C forms ionic in which it is less determined. That can lead to different dependence of the degree of association of these ions with PMAA upon the stereoregularity of the polymer. Many other cations are more efficiently bound by the syndio-PMAA than by the iso- isomer, the atactic PAA holding the intermediate position. Investigations of complexation processes of PMAA often help evaluate its conformational state. Further studies are needed for establishing of general peculiarities of metal ions binding by isomeric polyacids, in particular analogs of PAA and PMAA.

6.4 Peculiarities of MXn Binding by Cross-Linked Polyacids Significant attention is paid to this subject in literature (see, for example, a monograph [55]), in particular, due to a wide spread and systematic investigation of carboxylic cationites and ampholites. The following main features are observed upon binding of MXn by these macro ligands. The composition of coordination centers depends on the preliminary treatment of the polymer, the degree of ionization (˛) of carboxyl groups. At the ˛ value close to 0, structures with the maximum number of nonionized carboxyl groups dominate. At high ˛ values as a result of significant electric field the ionic type of M(II)–PAA bond is dominant. A coordination center can include both, protonated and deprotonated groups. For example, there are four carboxyl groups in the plane of a square and two water molecules on top of an octahedron in the Cu.RCOO /2 .RCOOH/2  .H2 O/2 [56]. The degree of binding

162

6 Polymer-Analog Transformations in Reactions of Synthesis of Metal Macrocarboxylates

of functional groups by a metal, i.e., concentration of the bound metal (polymer “loading”) also substantially affects the composition of the products formed. The Kst value decreases with an increase of the degree of binding of functional groups by a metal. This is caused by energetic heterogeneity of the surface, stereochemical, and geometrical defectiveness of the centers formed (including the lower number of available ligand groups). Therefore, an important role is given to the change of mobility of a polymer matrix by controlled cross-linking, introduction of other functional groups into polymer, and so on. Structural features of linked ligands can introduce certain changes into the composition, structure, and stability of the products formed, leading to the inversion of stability series. Thus, stability constants of complexes with the PMAA without cross-linking decrease in the series Cu(II) > Zn(II) > Ni(II) > Co(II), whereas for the cross-linked PMAA the stability of complexes (probably due to formation of Zn(II) complexes of tetrahedron structure) changes in another sequence: Cu(II) >> Ni(II) > Zn(II) [2]. Typically, mostly mixed diffusion mechanism of binding is realized. Contribution of external or internal diffusion into kinetics of the process can change depending on the degree of cross-linking of these carriers, concentration, and the nature of MXn in the solution. Perhaps, the same mechanism takes place also upon attachment of MXn to carboxyl containing fragments of “mosaic” gels [57]. Thus, the diversity of binding forms of transition metals by carboxyl containing polymers, provides significant possibilities for the fine tuning of composition and structure of coordination centers and the bond types realized there. The desired results can be achieved by varying the conditions of binding (preliminary treatment of the polymer, the pH and ionic strength, the nature of a media, concentration ratios, and so on).

6.5 Formation of Macrocomplexes with Grafted Polycarboxylic Fragments Grafted polymers is a type of copolymers. Their distinction is that practically all reactive groups are located on the surface, and are accessible to reagents including metal salts in the suspension binding method. Ion exchange membranes are obtained by grafting acrylic and methacrylic acids (see, for example, [58]). It has been demonstrated [59] on the example of Rh3C binding, that strong binding of even small amounts of a metal (0.005–0.08%) improves thermal, electrical, and optical properties of copolymers. The general scheme of binding of M.OCOCH3 /2 in aqueous alcohol suspension onto polyethylene with grafted polyacrylic acid (PE-gr-PAA) can be presented as the following [60, 61]: CH2 CH

m

+ M(OCOCH3)2

COOH M = Cu, Ni, Co

CH2 CH m–(l+n) CH2 CH l CH2 CH CH2 CH COOH

COOH

O

C

O

O M

C

O

n

6.5 Formation of Macrocomplexes with Grafted Polycarboxylic Fragments

163

According to the scheme, part of the grafted carboxyl groups do not take part in the reaction and the M(II) attachment is carried out by one carboxyl group with the formation of mixed ligand products (monosubstituted form C1 ), as well as by two (disubstituted form C2 – “anhydride” cycles) carboxyl groups of the grafted fragments. Equilibrium for this reaction is established almost immediately after mixing the components. With the increase of temperature (283–363 K) the amount of bound M(II) increases almost linearly (Table 6.5). An increase of the molar ratio, ŒM.II/0 =ŒCOOH0 , enhances the rate of carboxyl groups participation in the reaction. However, a significant portion of them is not involved in the process even with the excess of M.OCOCH3 /2 . The concentration dependence (Fig. 6.3) of the M(II) binding upon the ratio ŒM.II/0 =ŒCOOH0 has the Langmuir type. Its analysis by applying the transformed Langmuir equation for the isotherm of localized adsorption lead to K D 300 L/mol and k D 0:35. Table 6.5 The characteristics of Cu(II) ions binding with PE-gr-PAA [60] [Cu(II)]CB Concentration, mol. part T,K wt% mmol/g [COOH] C1 2:1 333 2.48 0.39 0.28 0.06 1:1 333 2.07 0.325 0.44 0.08 0.5:1 333 1.51 0.24 0.53 0.01 0.3:1 333 1.37 0.22 0.56 0.01 0.2:1 333 0.94 0.15 0.70 0.002 0.1:1 333 0.55 0.087 0.83 0.002 0.05:1 333 0.29 0.045 0.91 0.003 0.02:1 333 0.20 0.032 0.93 0.001 1:1 293 1.96 0.31 0.43 0.05 1:1 313 2.03 0.32 0.43 0.07 1:1 353 2.35 0.37 0.37 0.11 1:1 363 2.69 0.42 0.25 0.09

C2 0.66 0.48 0.46 0.43 0.295 0.17 0.09 0.064 0.52 0.50 0.52 0.66

Note: The content of grafted fragments is 1  103 mol/g

b

0.4

[Cu] / [Cu]b

[Cu]b ·102, mmol / g

a

0.2

0

4 [Cu]·102, mol / L

8

0.2

0.1

0

4 [Cu]·102, mol / L

8

Fig. 6.3 The concentration dependence of Cu(II0 binding upon the ratio ŒM.II/0 =ŒCOOH0 . (a) the dependence of ŒCub D f ŒCu; (b) ŒCu=ŒCub D f ŒCu (Langmuir isotherm)

164

6 Polymer-Analog Transformations in Reactions of Synthesis of Metal Macrocarboxylates

ŒCu.II/=ŒCu.II/CE D 1=K C .1=k/ŒCu.II/

(6.22)

where [Cu(II)] is the concentration of Cu(II) in the solution, k is a constant corresponding to the saturated adsorption of Cu(II) by carboxyl groups. These values attest that the grafted fragments during the interaction with MXn behave similarly to homopolymers by the degree of functional groups availability (at least under these conditions). The observed peculiarities are also retained upon the increase in the amount of grafted PAA from 1.5 till 11.0 mass% (0.2–1.5 mmol/g, average thickness of a grafted layer is 4–28 nm). It is important to establish the correlation between the forms C1 (one point binding of M(II)) and C2 (cyclic binding) as a function of reaction conditions. This was done by using tritium labeled salts, M.OCOC3 H3 /2 , during the immobilization [61]. The radioactivity of the final product will be determined only by the content of the form C1 . The ratio between the immobilized transition metal and the radioactivity of the product characterizes the content of the products C1 and C2 . It is seen from the Table 6.3 that with the increase of Cu.OCOCH3 /2 concentration in solution, the portion of C1 increases. However, in the case of ionized PAA in the systems PE-gr-PAA-M(II), the contribution of these structures does not exceed 8.2 mol% for the Cu(II), which is in contrast to the preferential formation of 1:1 products noticed in many papers. That is, this reaction is strongly shifted toward formation of cyclic products of the C2 type. Most likely that similar to the isotactic PAA, the cyclization involves neighboring units of the same grafted chain. Upon temperature increase, the ratio of the C1 and C2 forms increases slightly due to the increase of the portion of carboxyl groups involved in the reaction. Note, that up to 70% of carboxyl groups participate in the reaction, due to the solubility of the grafted layer under the reaction conditions. The situation of insoluble polymer support – soluble grafted layer is modeled in these systems. In other words, in the systems based on PE-gr-PAA advantages of both soluble (relative flexibility of the grafted chains, availability of the functional groups, high their concentration in the polymer domain and so on), and tridimensional linked (easiness of product isolation from the reaction volume, predomination of structurally homogenic complexes and so on) polymers are realized. Other peculiarities take place on the interaction of MXn with PE-gr-PAA in nonaqueous media [62], such as thickness of the grafted layer induces significant influence on the effectiveness of the binding. Since outer fragments are more available for the reaction than the inner ones, the effectiveness of the reaction decreases with an increase of the degree of grafting. In particular, almost each carboxyl group reacts with the VO.OC2 H5 /3 [or with the Ti.OC4 H9 /4 ] at the grafting degree of 0.5 mass%, while grafting of 13.5 mass% of PAA leads to a binding ratio of one vanadium atom per a chain of 12 PAA units (Fig. 6.4.). Most likely that immobilization of transition metal acetylacetonates and carboxylates in nonaqueous media takes place according to ligand exchange mechanism, though the degree of participation of the grafted groups in those reactions is low. Similar to the cases discussed above, for the MXn with bulky substituents it is more difficult to enter ligand exchange reactions (Table 6.6). Note for the comparison, that solubility of the carboxylates Cu.Cn H2nC1 COO/2 (n D 3, 7, 11, 17, and 29)

6.5 Formation of Macrocomplexes with Grafted Polycarboxylic Fragments [V]b.104, g-at./g 1.5

165

[COOH] / [V]b, mol / mol 20 1 15

1.0 2

10

0.5 5

0

3 6 9 12 Grafting degree of AA, wt.%

Fig. 6.4 The interactions in the system of PE-gr-PAA-VO.OC 2 H3 /3 at 333 K in heptane. (1) the amount of the bounded vanadium vs. grafting degree of acrylic acid; (2) the average number of AA units per atom of the bounded vanadium Table 6.6 The covalent binding of MXn with PE-gr-PAA solutions [62] The content of bounded MXn transition metal, mmol/g TiC14 0.06 VC14 0.14 0.138 Ni.CH3 COO/2 0.045 Ni.C17 H35 COO/2 0.024 NiL2 (L – the residue of naphtenic acid Co.CH3 COO/ 0.067 0.22 Co.C17 H35 COO/2 Cr.CH3 COO/3 0.15 0.25 Cu.CH3 COO/2 0.10 Ni.acac/2 0.32 Co.acac/2 VO.acac/2 0.08 0.053 Pd.acac/2

in nonaqueous f , mol/mol of [–COOH]0 0.07 0.18 0.17 0.056 0.030 0.085 0.03 0.19 0.31 0.13 0.40 0.10 0.066

Note: It is grafted of 5.6 wt.% of PAA (0.8 mmol/g)

in low density PE at 363 K can increase by a few orders of magnitude if the polymer is preliminary oxidized, since the salt binding by ligand exchange occurs in this case [8]. It is well known that in selective solvents, block-copolymers of the PS-PAA type (block-ionomers) exist as reversed micelles. In organic solvents blockcopolymers are segregated into micro phases with spherical, cylindrical, and lamellar morphology. Metal ions are bound with carboxyl groups of the micelle core, due to formation of covalent or ionic bonds. Recently more complex blockcopolymers have been developed for these aims, for example, diblock copolymer

166

6 Polymer-Analog Transformations in Reactions of Synthesis of Metal Macrocarboxylates Block-copolymers with isolated microphases

Metal salts

~COOH ~COOH

~COO

M2+

~COO

Nanoreactor

Fig. 6.5 A principal scheme of the block-copolymer and its metallocomplex formation

1200

C, mg / g

% (Au) % (Ag) % (Pd) % (Cu) % (Fe)

1000 800 600 400 200 0

0

2

4

6 t, days

8

10

Fig. 6.6 The change of the degree of metal ions loading by block copolymer of (methyltetracyclododecene)400 (2-norbornene-5,6-dicarboxylic acid)50 vs. time. (C is loading capacity, mg of metal/g of polymer)

(methyltetracyclododecene)400(2-norbornene-5,6-dicarboxylic acid)50 [63]. Ions of Ag, Au, Cu, Ni, Pb, Pd, Pt, and others form macrocomplex with units of the micelle core (Fig. 6.5). The loading of a metal can reach quite significant values, over 1 g/g of PAA block under the optimal conditions (Fig. 6.6).

6.6 Bimetallic Polycomplexes Binding of different metal ions by polyacids is required for the solving of numerous problems. Two variations of the preparation of these heterometallic polycomplexes are utilized. The first one is a simultaneous binding of different metals with polyacids, and the second is the subsequent one. In the latter case, the macrocomplex obtained (with vacant carboxyl groups) is a specific ligand for the second (M0 ) metal binding. Often such products are called “complexes of complexes”. Thus,

6.6 Bimetallic Polycomplexes

167

for the polymers with grafted functional cover, it is relatively easy to choose conditions for an introduction of M0 X0m , which do not significantly affect the state of already immobilized MXn [64]. Methods of subsequent covalent or donor-acceptor interaction of metal complexes, with the similar type functional groups of a polymer, are the most convenient for this. Such reactions were realized for polymers of different types, including carboxyl groups. For example, according to the following scheme [65]: CH2 CH

kMXn

CH2 CH

l

COOH

CH2 CH

l-k

COOH

CH2 CH

l-k-p

COOH

CH2 CH

k

pM'X'm k

COO MXn–1 CH2 CH

p

COO MXn–1 COO M'X'm–1

M = Ni(II), Co(II), Cu(II); M' = Ti(IV), V(IV), V(V), Zr(IV)

It was established by special studies that there was no substitution of immobilized MXn upon the introduction of M0 X0m . The degree of binding of M and M0 , concentration ratios, the bond nature of a transition metal with polymer, and the structure of immobilized metal center obey the same peculiarities as in the case of immobilization of separate compounds, for example Ni.OCOCH3 /2 or TiCl4 . Although these methods do not allow conducting an efficient control of distribution of transition metals on a polymer carrier, the desired quantitative ratios between them can be achieved by variation of the reaction conditions. It can be supposed that in such polymer complexes M and M0 are spatially separated and act as weakly interacting (or even disconnected) centers, although they are bound one to another by polymer chain. However, magnetic behavior studies of Ni(II) [or Co(II)]–V(IV) systems immobilized on PE-gr-PAA allowed to reveal [66], that even with the statistical distribution of transition metals, the system is not completely disconnected (regarding the electron localization). Thus, the dependence 1/ M D f .T / for the immobilized Ni(II) ( eff D 3:77 B at 298 K) has a slight curve at 110 K (Fig. 6.7) revealing a presence of weak spin–spin antiferromagnetic interactions between the immobilized nickel ions. Layering the VCl4 on this sample leads to a change in the magnetic behavior of the system: the dependence 1/ M D f .T / for the polymer bimetallic complexes strictly obeys the Curie–Waiss law (T D 5 K), evidencing the disappearance of the interaction between Ni(II) ions. Formation of ionic bridging complexes of the following type is possible in the case of bimetallic complexes. ~C

O→M ← O O→M′← O

C~

The preparation of the LaMnO3 with utilization of PAA has been reported [67]. Condensation of metal carboxylate with a metal salt or a mixture of metal salts in the presence of a polymer can be used to this aim [68].

168

6 Polymer-Analog Transformations in Reactions of Synthesis of Metal Macrocarboxylates

Fig. 6.7 The magnetic properties of metallocomplexes immobilized into PE-gr-PAA: (1) PE-gr-PAA CNiCl2 C VCl4 ; (2) PE-gr-PAA C NiCl2 ; (3) PE-gr-PAA C VCl4

(1 / χm)⋅10–2, g–1 100

2 1

80

60 (1 / χm)⋅10–2, g–1 40

3

400 300

20

200 100 100

200

300 T, K

For comparison, note that even in solutions containing cations of varied types, for example, Mo(V) and M(II) (M D Cu, Co, Fe, and Ni), in the case when one of them or both have an asymmetric structure (i.e., when the enhanced electron density is localized on the periphery of an ion) associates are formed [69] by mostly involving bridging ligands or (rarely) by charge transfer. The associates formed are polynuclear heterometallic complexes. These reactions apparently also take place in macrocomplexes, although they are complicated by an influence of a macrochain. Therefore, the immobilized systems are not electronically disconnected even upon binding of heterometallic complexes with statistical distribution of MXn and M0 X0n . Cooperative type interactions are observed between paramagnetic metals.

6.7 Formation of Organic–Inorganic Composites It has been demonstrated that such compounds as PdCl2 2H2O and H2 PtCl6 6H2 O, are capable of binding with miscellaneous type ligands comprised of organic– inorganic SiO2 hybrid with grafted copolymers of acrylic acid, and m- or p-divinylbenzene obtained by radical copolymerization in the presence of SiO2 [70]. Immobilization of AA on the surface of silicon plates is performed in a similar fashion, triggered by its free-radical polymerization, involving self-assembling thin layers of azo-initiator, leading to formation of “brushes” on the surface (Scheme 6.2) [71]: Layered ultra-thin films, based on the ionized polyacids and self-assembling polycations, are new materials obtained by stepwise adsorption of polymers on the solid surfaces [72–74]. An interesting way to their formation consists of oxidative

6.7 Formation of Organic–Inorganic Composites Si–OH

H3C Cl

Si O H3C

CN

CH3

CH3 CN

CH3

N

CH3

N

O

CH3

N

O

O Si

CH3

O Si

169 immobilization CH3 NEt toluene 3 CN

polymerization

CH3 CN

N

O

H2O

OH

Si O H3C

CH3 CH3

O Si CH3

O

CN O

n

OH

Scheme 6.2 Formation of polymer brushes onto the surface of silicon

O + M

C

–O

O

+ M

–O

HO

M

O

M

O

M

– O

O

M

O

M

O

M

– O

OH OH – O

C O C O

HO C O – C O

C O

Fig. 6.8 A schematic view of crystal region of “inorganic core” and organic part of polymer metal carboxylate complexes

dissolution of metals [75]. In such a case, metals (Cr, Mn, Fe, Co, Ni, Cu, Mn– Co) or their oxides are treated with a mixture of C3 –C40 acids (including aromatic ones) in the presence of polypropylene glycol and water. The reaction mixture is stirred at 75ı C and then water is removed at 150ı C. The high molecular weight metal carboxylates formed are soluble in octane, cyclohexane, CCl4 , benzene, THF, etc., and they very slowly (over several days) precipitate from these solvents. They constitute “reverse micelles” (Fig. 6.8) with inorganic inner “core”, size of 3–8 nm, and an outer organic shell (carboxylates). The “molecular mass” of such micelles,

170

6 Polymer-Analog Transformations in Reactions of Synthesis of Metal Macrocarboxylates

determined from their sedimentation rate, is estimated at 5  104 –1:5  106 . In their turn, these particles may aggregate (especially in polar solvent that remove the “organic” shell) forming 10–30 nm size crystallites. Such carboxylates may be heterometallic as well. The products obtained are efficient catalysts of different processes. The catalysis takes place at the interface of a soluble layer and the “core”, and it resembles a homogeneous one. Most likely, this is a new kind of interfacial catalysis. This type of complexes may be heterogenized from an organic solvent on a surface of another polymer. Of special importance, is the problem of behavior of dispersions and solutions of acrylic polymers containing carboxyl groups, present in dye composites used for protection of metals and glues, and also for regulation of properties of different dispersion systems, in particular, metal oxide dispergators employed in ceramics manufacturing. Metal polycarboxylates produce high reactivity ceramic oxide powders, with large specific surface area [76]. For this purpose, PAA complexes of divalent, trivalent, and tetravalent metals are used [77]. This requires a detailed investigation of adsorption and desorption processes, taking place on the surface of particles of dispersions upon their contact with carboxyl containing polymers [78]. Thus, a kinetic study of copper(II) oxide [79] or ZnO [80] dissolution in polyacrylic acid (in the presence of hydrogen peroxide) showed that the rates of these reactions depend on the amount of adsorbed PAA. In its turn, PAA adsorption on copper increases with its molecular mass. A decrease in the molecular mass results in increasing rate of copper dissolution, which is determined by the rate of PAA salts desorption [81]. A lot of attention is being paid to organic–inorganic hybrid materials, so-called polyelectrolyte-based cements, which were first synthesized in the late 1960s [82]. The main area of their application is ceramics manufacturing, which includes their employment in dentistry and in the biomedical field, due to the good biocompatibility and adhesion of these compounds. They are most often prepared by the reaction of PAA with metal oxides (mainly ZnO). The homogeneous material obtained contains up to 31.5% of zinc. A technological scheme for its manufacturing includes the following steps (Fig. 6.9)[83]. Among other organic–inorganic hybrids, zinc polycarboxylates with calcium fluoroaluminosilicates forming high quality dental cements are worth mentioning [84–86]. Introduction of even small amounts of ionic additives based on trivalent metals, such as Al.NO3 /3 and Fe.NO3 /3 , to zinc polycarboxylate accelerates the cement setting reaction [38, 87]. Emulsion coatings (for example, see [88]) have received a relatively wide dissemination. They are based on highly concentrated (about 35% by weight of solid residue) polyacrylate hydrosoles of acrylic acid-derived copolymers with M.NH3 /2C 4 linking reagents (in particular, M D Cu or Zn [89]) which react readily with carboxylate ions at relatively low temperatures (120–150ı C). Metallopolymer coatings arising from fine emulsions (particle diameter 0.01–0:1 m) possess enhanced physico-mechanic properties, they are water-resistant and their films are glossier. There are many more examples to list.

6.8 Binding of MXn by Natural Carboxyl Group Containing Polymers Fig. 6.9 A schematic representation of the precipitate method for preparing the zinc-acetate–PAA complex

171

PAA

Zinc Salt

Aqueous solution 0.1 N

Aqueous solution 0.1 N

Sodic Salt preparation PAA + NaOH(0.5N)

Mixing Filtering Washing Drying Grinding Characterization

6.8 Binding of MXn by Natural Carboxyl Group Containing Polymers Distribution of metal ions in different physico-chemical phases causes definite influence on their mobility and bioaccumulation. In this regard, multi-charged macromolecular ligands like humic acids or polysaccharides play a key role in localization and accumulation of metal ions in natural objects. Carboxymethyl cellulose (CMC) is the most often used among other natural polymers for the synthesis of metallopolymers. It is a homogeneous, fine, powderlike polymer that does not contain any groups but carboxyl, (up to 5  103 mol=g) capable of participating in ionic binding at moderate pH values (up to 10). Thus, 60–80% of all deprotonated carboxyl groups of CMC reacts with Cu(II) [90], the simultaneous coordination of one Cu ion with two carboxyl groups being impossible because of purely steric reasons. Therefore, one of them forms a coordination bond, while the other participates in the binding by electrostatic interaction. Complexes of Cu(II), Ni(II), and Fe(II) with CMC-based membranes are less stable [91].

172

6 Polymer-Analog Transformations in Reactions of Synthesis of Metal Macrocarboxylates

Unlike the Cu(II), molybdenum (VI) is adsorbed by CMC without excretion of protons with the formation of immobilized complex acid [90]: COOH + H2MoO4 + H2O

C

O O HO

OH Mo H2O

O OH

The coordination number six, that is typical for protonated forms of Mo(IV), is achieved by water molecules or by the nearest carboxyl group of the polymer. Formation of coordination tridimentional networks takes place upon chemical interaction of functional groups of sodium methyl cellulose with a Cr3C salt [92]. The linking is considered as a first rate reaction by the Cr3C concentration, the effective rate constant of the chromium binding reaction being k D 0:025 h1. The critical concentration of the gel formation is 0.3–0.5 mass%, depending on a molecular mass of the polymer, while the minimum concentration of the linking agent is 0.012–0.014 mol/L. The efficiency of binding of Zn(II), Pb(II), Cu(II), and Cd(II) by pectin (polygalacturonic acid, that has a similarity to cellulose chain structure, and carboxyl groups of which are partially esterified with methanol) has been studied. It was shown [93] that each Zn(II) atom binds with two free carboxyl groups. Interestingly, the stability constant of the macrocomplex decreases with the increase of the esterification degree (between 0 and 90%) of the pectin. This reaction is considered to be used for prevention of poisoning by toxic metal cations and their removal from an organism, while the dependence of the amounts of the bound Zn(II), upon the esterification degree is suggested to regulate Zn(II) content in an organism. Similar features are also observed upon binding of Ca2C ions [94]. It is also worth mentioning the binding of humic compounds by aluminum hydroxocation nanoclusters on the surface of kaolin [95]. Carboxymethyl dextran ether possess good binding properties toward Cu(II), Ni(II), and Co(II) [96], however, the degree of binding in this case is smaller than for PAA or PMAA. In recent years metal binding properties of humic and fulvic acids are studied intensively [97–99]. These acids are necessary and are the main links in soil forming processes, and create a specific “depot” of bioelements that regulates plant nutrition regime, depending on the environment conditions. The problem of heavy metals complexation with these macroligands is important from the point of binding of their mobile forms. Finally, complexation of metal ions with humic acids plays an important role in processes of migration and delivery of biogenic metals into biological systems, in ore formation processes, for solving ecological problems. Macromolecules of humic acids contain functional groups that are different by acidity (Chap. 1); each of those is a potential center for metal binding. Many kinetic features for complexation by humic acids are similar to those of their synthetic analogs: centers formed upon ionization of weaker

6.8 Binding of MXn by Natural Carboxyl Group Containing Polymers

173

Table 6.7 The formation constants of the coordination centers at the interaction of metal ions with humic acids [98] Ionic strength ( ) 0.01 0.1 1.0 Me2C Cu2C Ni2C Zn2C Cu2C Cd2C Mn2C

lg K1 4.91 4.82 4.68 4.45 4.35 4.07

lg ˇ2 9.45 9.05 8.89 8.45 7.32 7.15

lg K1 4.51 4.23 3.96 3.75 3.58 3.35

lg ˇ2 7.95 7.45 7.27 6.85 5.84 5.79

lg K1 4.13 3.70 3.44 3.34 3.22 –

lg ˇ2 6.54 6.44 6.28 – – –

acidic groups participate in the reaction with the increase of the pH, an increase of the ionic strength causes enhancement of their acidic properties. However, this results in an opposite effect on the stability of the metal complexes formed. Stability of the coordination centers increases with the decrease of ionic strength that is due to polyelectrolyte properties of humic acids (Table 6.7). Upon an increase of molecular mass of these acids, the stability of the complexes formed decreases slightly due to an increase in this case, of the portion of strong acidic groups in the whole pool of protonogenics. Additionally, at the pH > 4, when the macroligand is soluble, coordination centers of the ML2 type are formed, the cause of that being the optimal conformation required by stereochemistry of the complexes. In these conditions, macromolecules of humic acids exist as macroions, become flexible and adopt conformations that are energetically favorable for the formation of coordination units of the ML2 composition. Note, that well characterized synthetic PAA is often used as a model for complex formation by humic acids [100–104]. Interaction of metal ions with natural polymers significantly affects redistribution of microelements in geological deposits and soils. Formation of actinide (III) complexes with natural polyelectrolytes such as humic compounds, is considered in the context of migration processes of actinides in natural waters and stability of their complexes formed [105, 106]. Broad thermodynamic studies of processes and mechanisms of complexation of their synthetic polyelectrolyte analogs, such as PAA, polymaleic acid, –CH(COOH)n , PMAA, poly(’-hydroxyacrylic) acid, – .C.OH/.COOH/–CH2 /n – and so on, are likely to be connected with just that. A carboxyl group connected with a polymer chain is a unique and widespread ligand capable of efficiently binding practically any metal salts. During their interaction with polyacids, formation of a whole series of structures is possible. This depends upon experimental conditions, as well as structure and composition of a polyligand. Polyacids are convenient objects for analysis of polymer effects, and finding peculiarities of complexation in macromolecular systems. Many aspects of these problems, such as carboxyl containing polymers of condensation type, heteropolyacids (polyaminoacids, carboran containing, and so on), metallopolymer chelates (of the type [107]) and others, were not analyzed here. Consideration of biological activity of metal polyacrylates, for example, hemostatic properties

174

6 Polymer-Analog Transformations in Reactions of Synthesis of Metal Macrocarboxylates

of the water soluble polymer feracryl (an iron containing salt of PAA), which hemostatic action is already efficient in a form of 1% solution or dried gauze dressings [113], is worth a separate discussion. Another one out of numerous examples is an application of PAA and its Co(II), Zn(II) macrocomplexes as novel immunological additives [114]. Although PAA itself increases formation of antibodies [108], its high toxicity (LD50 D 70 mg=kg) limits application of the substance. Application of macrosalts of these metals based on the copolymers of acrylic acid and N vinylpyrrolidone confirmed [109] their immunomodulatory properties, they have low toxicity and low influence on critically important functions of animal organisms. Triple metal salt of PAA of the composition, .CH2 CH.COONa/n .CH2 CH.COO/2:3 Fe/m .CH2 CH/.COO/2 Hg/p (n D 97–99 mol%, m D 0:04–0.06 mol% and p D 0:08– 2.85 mol%), exhibits high antimicrobial activity (test cultures were strains of E. coli, Prot. vulgaris, Ps. aeruginosa, Staphylococus) [115]. This triple complex possesses a strong bacteriostatic effect toward inhibition of those strains and a low toxicity value.

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6 Polymer-Analog Transformations in Reactions of Synthesis of Metal Macrocarboxylates

68. M.M. Milanova, M. Kakihana, M. Arima, M. Yashima, M. Yoshimura, J. Alloys Compd. 242, 6 (1996) 69. Z.A. Saprykova, N.D. Chichirova, Izv.Vuz’ov. Khimiya i khim. tekhnol. 25, 1039 (1982) 70. X.-Y. Guo, H.-J. Zong, Y.-J. Li, Y.-Y. Jiang: Makromol. Chem. Rapid Commun. 5, 507 (1984) 71. R. Konradi, J. Ruhe, Macromolecules. 37, 6954 (2004) 72. P. Bertrand, A. Jonas, A. Laschewsky, R. Legras, Macromol. Rapid Commun. 21, 319 (2000) 73. G. Decher, Science 277, 1232 (1997) 74. G. Decher, J.B. Schlenoff, Multilayer Thin Films (Wiley-VCH, New York, 2003) 75. C.U. Pittman Jr., E.H. Lewis, M. Habib, J. Macromol. Sci. A 15, 897, 915 (1981) 76. M. Balastre, J.F. Argillier, C. Allain, A. Foissy, Colloids Surf. A 211, 145 (2002) 77. R. Roma, L. Sarraf, M. Morcellet, Eur. Polym. J. 37, 1741 (2001) 78. J. Cesarano, I.A. Askay, J. Am. Ceram. Soc. 71, 250, 1062 (1988) 79. Y. Cohen, A.B. Metzner, Macromolecules 15, 1425 (1982) 80. Y. Cohen: Macromolecules 21, 494 (1988) 81. V.N. Kislenko, R.M. Verlinskaya, Kolloid. Zh. 63, 558, 613 (2001); 64, 447 (2002); Zh. Prikl. Khim. 77, 1374 (2004) 82. J.H. Adair, J.A. Casey, S. Venigalla (eds.), Characterization Techniques for the Solid-Solution Interface (American Ceramic Society, 1994) 83. M.E. Nicho, J.M Saniger, M.A. Ponce, A. Huanosta, V.M. CastaLno, J. Appl. Polym. Sci. 66, 861 (1997) 84. S. Matsuya, T. Maeda, M. Ogata, J. Dent. Res. 75, 1920 (1996) 85. E.A. Wasson, J.W. Nicholson, J. Dent. Res. 72, 481 (1993) 86. J.W. Nicholson, J. Mater. Sci. Mater. Med. 4, 404 (1999) 87. Y. Haga, S. Inone, M. Nakajima, R. Yosomiya, Mater. Chem. Phys. 19, 381 (1988) 88. P.J. Moles, Polym. Paint Color J. 178, 154 (1988) 89. L.Q. Yang, Z.M. Xie, Z.M. Li, J. Appl. Polym. Sci. 66, 2457 (1997) 90. A.P. Fillipov, Teoret. Eksperim. Khimiya. 19, 463 (1983) 91. C. Kamizawa, J. Appl. Polym. Sci. 22, 2867 (1978) 92. V.V. Medvedeva, L.I. Myasnikova, Yu.D. Semchikov, L.Z. Rogovina, Vysokomol. Soedin. B 40, 492 (1998) 93. A. Malovikova, R. Kohn, Collect. Czech. Chem. Commun. 48, 3154 (1983) 94. D. Durand, C. Bertrand, A.H. Clark, A. Lips, Int. J. Biol. Macromol. 12, 14 (1990) 95. Yu. Tarasevich, V.V. Lukyanova, G.M. Telbiz, Teoret. I Eksperim. Khimiya. 41, 45 (2005) 96. L.I. Shevchenko, Z.A. Lugovaya, V.N. Tolmachev, Vysokomol. Soedin. A 27, 1993 (1985) 97. J. Butfle, Complexation Reactions in Aquatic Systems (Ellis Horwood Ltd, Chichester, 1989) 98. Sh. Jorobekova, Macro-Ligand Properties of Humic Acids (Ilim, Frunze, 1986) 99. K. Kydralieva, Sh. Jorobekova, Metal Ions in Enzyme-Inhibitory Systems (Ilim, Bishkek, 2002) 100. J. Buffle, Complexation Reactions in Aquatic Systems. An Analitical Approach (Ellis Horwood, Chichester, 1988) 101. R.F. Cleven, H.P. Van Leeuwen, Int. J. Environ. Anal. Chem. 27, 11 (1986) 102. M. Esterban, H.G. De Jong, H.P. Van Leeuwen, Int. J. Environ. Anal. Chem. 38, 75 (1990) 103. M.A. van Hoop, J.C. Benegas, Coll. Surf. A 170, 151 (2000) 104. S.B. Clark, G.R. Choppin, in A Comparison of the Dissociation Kinetics of Rare Earth Element Complexes with Synthetic Polyelectrolytes and Humic Acids in Humic and Fulvic Acids: Isolation, Structure and Environmental Role. ASC Symposium Series, vol. 651, ed. by. J.C. Gaffney, N.A. Marley, S.B. Clark (ASC, Washington, DC, 1996), p. 207 105. G.R. Choppin, Radiochim. Acta 44/45, 23 (1988) 106. J.I. Kim, P. Zeh, B. Delakowitz, Radiochim. Acta 58/59, 147 (1992) 107. A.D. Pomogailo, I.E. Uflyand, Macromolecular Metal Chelates (Khimiya, Moscow, 1991) 108. R.V. Petrov, R.M. Khaitov, R.I. Ataulkhanov, Immonogenetics and Artificial Antigents (Moscow, 1983) 109. F.N. Muratkhodzhaev, AS.A. Batyrbekov, A.P. Sirota, R.Z. Rafikov, Khim. Farm. Kh. 40 (1990) 110. G.M. Barrow, Physical Chemistry for the Life Sciences (McGraw-Hill, New York, 1974)

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111. M.M. Coleman, J.Y. Lee, P.C. Painter, Macromolecules 23, 2339 (1990) 112. E.C. Kolawole, M.A. Bello, Eur. Polym. J. 16, 325 (1980) 113. V.Z. Annenkov, A.E. Platonova, G.M. Kolonchuk, N.G. Dianova, V.B. Kazimirskaya, V.M. Annenkova, G.S. Ugrumova, M.G. Voronkov, Khim. Farmaz. kh. 3, 322 (1982) 114. P.A. Podkuiko, L.Ya. Tsarik, N.V. Zaitsev, Khim. Promyshl. 80, 30 (2003) 115. E.L. Zhdanovich, O.A. Trifonova, T.I. Nikiforova, T.Ya. Pushechkina, V.M. Annenkova, V.Z. Annenkova, M.G. Voronkov, Khim. Farm. Kh. 50 (1990)

Chapter 7

Molecular and Structural Organization of Metal-Containing (Co)Polymers

It is well known that properties of the polymeric materials based on (co)polymers depend strongly on the sequence in which units of different nature are distributed throughout a polymeric chain. Distinctions in the units’ distribution are displayed in the character of intermolecular interactions that finally causes the variety of supramolecular structures affecting the properties of the materials.

7.1 Ionic Aggregations and Multiplets Many properties of the considered type metal-containing polymers are determined by aggregation of ions, especially in the case of alkaline and alkaline earth metals, that allows to consider metal-containing polymers as typical representatives of a known class of polymeric compounds – ionomers. Ionomers are polymers usually containing carbon atoms in the main chain and including small amounts (up to 15 mol%) of acidic groups (carboxyl, sulfo-, phospho-, etc.) in the composition [1–3]. These groups can be the side units or can be included in the main polymeric chain. Carboxylated ionomers are the most interesting from the practical point of view. As it was shown [4], ionic associations, for example, in the sulfoionogenic polymers, are much stronger than in their carboxylated analogues. It results in very high viscosity of the melts of sulfoionogenic polymers. Besides, residual sulfo-groups, are exposed, as a rule, to thermal destruction at their incomplete neutralization under the treatment at elevated temperatures. It is necessary to note, that most part of the commercially available ionomers is also obtained on the basis of copolymers of acrylic or methacrylic acids, for example, sodium or zinc salts of copolymers of ethylene with methacrylic acid (EMAA), etc.

7.1.1 Ionomers Synthesis Basic obtaining methods of macromolecular metal carboxylates have been considered in detail in the previous chapters. So, in this chapter we will stress attention only on those approaches which allow us to receive polymers of the required composition and with the characteristics necessary for the development of specific properties of A.D. Pomogailo et al., Macromolecular Metal Carboxylates and Their Nanocomposites, Springer Series in Materials Science 138, DOI 10.1007/978-3-642-10574-6 7, c Springer-Verlag Berlin Heidelberg 2010 

179

180

7 Molecular and Structural Organization of Metal-Containing (Co)Polymers

ionomers. It is known that conventional methods of the ionomers synthesis are in two-stages. In the first stage, the component, containing an ionogenic functional group, is entered into a nonionic framework, after that these groups are subjected to full or partial neutralization. In most cases, elastomers with relatively high molecular weights are used as ionomeric precursors in the polymer-analogous reactions. It results in the obtaining of ionomers with high viscosity characteristics of their melts [5–7]. At the same time, use of the maleate-modified ethylene–propylene copolymers with molecular weights Mn 11;00040;000 as precursors allows to obtain ion-containing polymers with properties acceptable for processing [8, 9]. Acetate salts or metals (hydro)oxides are usually used for neutralization of carboxyl groups [10–12]. An interesting method was offered for the synthesis of maleate-modified ionomers. It includes ring opening reaction of the epoxidated three block-copolymer (styrene-butadiene-styrene (SBS)) using acidic potassium maleate [13]: CH

CH2

m

CH CH2

CH O

CH

CH2

m

CH

n

CH2

CH

CH CH2

OH

O C

n

HOOC

CH

CH

COOK

m

CH

CH2

m

O

CH HC

COOK

Dimethylaniline (5 mass%) was used as a catalyst in this reaction. It is necessary to note that disubstituted potassium maleate was applied for the regulation of medium pH. It resulted in an increase of epoxidation degree up to the 40% potassium dimaleate/monomaleate ratio. Conversion of epoxy groups was more than 90% under optimal conditions. It was discussed above that conventional approaches for obtaining the ioncontaining polymers are multistage and laborious. So, copolymerization of unsaturated acid salts (see Sect. 5.4) is the unique possibility of a single-stage synthesis of such polymers. Stable ionomeric emulsions on the basis of sodium or zinc acrylates were received under their copolymerization with methyl methacrylate (MMA) and butylacrylate (BA) inpresence of dodecyl sulfonate (NaSO3 .CH2 /11 CH3 / and K2 S2 O8 as the initiator at 60ı C [14, 15].

7.1.2 Morphology and Structure of Ionomers Various models including such structures as multiplet-cluster [16, 17], core-shell [18, 19], cylinder [20], hard sphere [19], and others have been developed for the

7.1 Ionic Aggregations and Multiplets

181

description of the ionomers morphology. Despite their distinctions, it is considered that the main factor determining the peculiarities of structure and properties of the ionomers is the formation of stable aggregates of ion pairs [16], which can compose multiplets or clusters in dependence on their concentration. The small compact aggregates of ion pairs are called multiplets, and aggregates formed from the separate multiplets are called clusters. It is supposed [17] that there is an area with the reduced mobility of polymeric chains around each multiplet. In case of high concentrations of the charged groups in a polymeric matrix (>5–7%) these areas start to overlap giving extended formations enriched with ions. The formations are called domains or clusters and they frequently show properties of an individual phase. Contribution of the separate aggregations is estimated by various physical and chemical methods: small angle X-ray scattering (SAXS) [19, 21–23], studying of dynamic, mechanical, rheological, or dielectric properties of metal-containing polymers, Raman and IR-spectroscopy in the near range, and also EXAFS [24, 25] and rare-earth probe methods [26, 27]. Dominating formation of Cu(II)–Cu(II) pair in the copolymer of ethylene with methacrylic acid (5.4 mol%) was revealed by electron paramagnetic resonance (EPR) method [28]. Isolated Cu(II) ions had only weak absorption at 300 mT (Fig. 7.1). Absorption band at 254 cm1 in the Raman-spectra of sodium polymethacrylate [29] was assigned to the vibrations of ions in the multiplets, and absorption band at 166 cm1 was assigned to the vibrations of ions in the clusters. It is explained by the fact that electrostatic interactions in big aggregates became more shielded and frequency of ionic vibrations is lower in comparison with the basic band of a separate cation or a multiplet. Intensity of the band correlates well with the concentration of ions in the clusters founded by the dielectric method (Fig. 7.2). Similar regularities were also observed in case of copolymers of alkali metals acrylates with styrene [30]. Ionic bands corresponding to the clusters appeared at

Fig. 7.1 EPR spectrum of copolymer of ethylene/methacrylic acid neutralized with 60% of Cu(II)

182

7 Molecular and Structural Organization of Metal-Containing (Co)Polymers

Fig. 7.2 The relative intensity of adsorption band at 254 cm1 (1) and 166 cm1 (2) in the Raman spectrum vs. the content of methacrylate sodium in its copolymer with styrene

155 cm1 for KC and NaC ions and at 95 cm1 for CsC . The principal scheme of similar aggregations can be represented as follows [31]: +

+ + + +

+

+

+

+

+ +

+

+ +

+

+

+ + + +

+

It is necessary to note that state of the ionic aggregation depends on many factors, for example, on the cation nature, on the polymeric chain microstructure, on ways of obtaining the ionomer, etc. For example, metal-containing polymers obtained by solid phase polymerization under the action of high pressures in combination with shearing deformations (HP C SD) show strong antiferromagnetic exchange between paramagnetic centers, both in homopolymers and in heterometallic copolymers on the basis of Ni(II), Cu(II), and Ti(IV) acrylates (Table 7.1) [32, 33]. Antiferromagnetic exchange is connected, most probably, with the interchain interactions of the paramagnetic centers developed as a result of conformational changes in macrochains under the action of HP C SD, i.e. the favorable conditions for the formation of multiplet and cluster domain structures are created. It is typical that polymers and copolymers obtained by liquid-phase polymerization do not show

7.1 Ionic Aggregations and Multiplets Table 7.1 Magnetic properties of heterometal-containing copolymers ef (B.M.) Method The content of synthesis of M1 (mol%) Copolymer 295 K 80 K (NiAcr2 ) .M1 /– Radical, in 54 3:40 3:36 Cp2 Ti(MAA)2 solution Radical, in 84 3:38 3:30 (NiAcr2 ) .M1 /– Cp2 Ti(MAA)2 solution Radical, in 92 3:27 3:18 (NiAcr2 ) .M1 /– Cp2 Ti(MAA)2 solution Homopolymer Radical, in 100 3:29 3:28 of (NiAcr2 ) solution Solid-phase 42 4:30 3:75 (NiAcr2 ) .M1 /– Cp2 Ti(MAA)2 (HP C SD) (NiAcr2 ) .M1 /– Solid-phase 76 4:05 3:38 Cp2 Ti(MAA)2 (HP C SD) (NiAcr2 ) .M1 /– Solid-phase 91 3:73 3:36 Cp2 Ti(MAA)2 (HP C SD) Solid-phase 100 4:73 3:78 Homopolymer (HP C SD) of (NiAcr2 ) (CuAcr2 ) .M1 /– Solid-phase 39 1:58 1:05 Cp2 Ti(MAA)2 (HP C SD) Solid-phase 62 1:57 1:03 (CuAcr2 ) .M1 /– Cp2 Ti(MAA)2 (HP C SD) Solid-phase 73 2:53 1:56 (CuAcr2 ) .M1 /– Cp2 Ti(MAA)2 (HP C SD) (CuAcr2 ) .M1 /– Solid-phase 84 1:48 1:07 Cp2 Ti(MAA)2 (HP C SD) Homopolymer Solid-phase 100 1:42 1:15 of (CuAcr2 ) (HP C SD)

183

Antiferromagnetic exchange No exchange No exchange No exchange No exchange Exchange Exchange Exchange Exchange Strong exchange Strong exchange Strong exchange Strong exchange Strong exchange

antiferromagnetic exchange after HP C SD treatment. It confirms that structure of the complexes, combined into clusters with antiferromagnetic interaction, is formed at the stage of copolymers formation in afterflow conditions. It was shown by WAXS and SAXS methods [22] that small ionic aggregates are mainly formed in the copolymer of zinc acrylate with styrene,1 [34, 35]. The ionic peak 2‚ D 5:5ı appears at concentration of Zn acrylate equal to 7.02 mol% on a spectrum of wide angle X-ray scattering. Intensity of the ionic peak increases with an increase in metal acrylate content (Fig. 7.3). Acrylates of alkali metals in their copolymers with styrene [30] form cluster ag˚ [23, 36], along with ion pairs and multiplets gregates with the sizes 70–100 A already at concentrations 3.85 and 5.16 mol%. As a rule, temperature increase at

1

It is characteristic also for other Zn-containing ionomers. For example, the size of ionic aggregates is equal to 0.45 nm in all range of metal ion concentration in copolymers of ethylene with methacrylic acid with neutralization degree 0.32–0.83. Ethylenic ionomers of copper and iron have the same features.

184

7 Molecular and Structural Organization of Metal-Containing (Co)Polymers Intensity

6 5

4

3 2

1 2

10

18

26

2 q / deg

Fig. 7.3 WAXS spectra of zinc acrylate – styrene copolymer with a salt content of 3.67 (1), 5.51 (2), 7.02 (3), 9.83(4), 17.59 (5), 19.48 mol% (6)

Table 7.2 SAXS dataa for the ionomers based on zinc acrylate and styrene at various contents of zinc acrylate [22]

ZnAA (mol%) 7.02 9.83 17.59 19.48

Q.mol=cm3 /2 0:109  103 0:147  103 0:211  102 0:534  102

˚ d (A) 17.3 17.7 15.8 19.8

˚ R (A) 3.7 3.8 3.7 4.0

Q is the scattering invariant, d is the average distance between the domains, R is the radius of the ionic domain

a

SAXS studying, especially at higher than glass-transition temperature (Tg ) of a polymeric matrix temperature, results in an increase in the sizes and quantity of cluster particles because of aggregation of free ion pairs and multiplets [22, 23]. But in the case of the copolymer of zinc acrylate, perceptible change of the sizes of ionic aggregates and distance between them was not observed; only volume fraction of ionic aggregates was increased. It was testified by change of Q value (Table 7.2).

7.1 Ionic Aggregations and Multiplets

185

The character of the ionic associations is also determined by the nature of a surrounding polymeric matrix. Removal of the charged groups from the main chain can favor the formation of ionic multiplets owing to the reduction of steric hindrances. In its turn, influence of the formed multiplets on phase behavior of a macromolecule in such systems become weaker as it was shown in the case of the functionalizated liquid-crystalline ionomers containing carboxyl groups of acrylic acid, 3-acryloyloxypropionic acids, etc. [37]. If induction of a smectic phase in the copolymers with acrylic acid occurs already at low metal ion concentrations (2mol%), then an increase in distance between ionogenic groups and main polymeric chain is revealed in the inability of an ionomer to form SA -phase. Tg of a polymeric matrix is one more tool for the effective control of the properties of ionomers. [38]. Poly(ethyl acrylate-co-itaconate) containing two ionic groups in one unit COO-Na+ CH2

CH

x

COOC2H5

CH2

C

y

CH2 COO-Na+

shows a high degree of cluster formation in comparison with a similar ionomer based on the polystyrene characterized by the formation of several multiplets only. It is confirmed by the values of the relaxation module and the tangent of the angle of mechanical and dielectric losses of the copolymers given [39]. Such behavior is connected with the noticeably low value of Tg of the polyethylacrylate matrix against the polystyrene system (125ıC) [40]. According to the model [17], with an increase in ions content in a system, if the cluster’s size exceeds some fixed minimum size, then ionomeric polymer shows the second glass-transition temperature connected with the combined effect of relaxation of a polymeric chain in a cluster area and transfer of ionic groups in multiplets. According to the dynamics-mechanical thermal analysis data [41], the ionomeric copolymers of polyethylacrylate and acrylic acid [poly(ethylacrylate)-co-acrylic acid (3.6–15.2 mol%)], neutralized by various cations, are characterized by two glass-transition temperatures. Lower glass points correspond to the Tg of the polyethylacrylate matrix while temperature transfers revealed in the high-temperature area are caused by cluster aggregates [42–44]. From the functional dependence of the glass-transition temperature upon ion content (Fig. 7.4) it is seen that Tg of the matrix linearly increases relatively slowly with an increase in ion concentration, and the cation type does not influence on Tg . At the same time, Tg of the clusters are differed among themselves depending on the cation nature and force of the ionic interactions, in particular on the q/a parameter, where q is an ion charge, a is a distance between cation and anion. The ionomeric polymers feature (as a consequence of the ionic aggregations) is an increase in glass transition temperature that testifies the presence of an ionic

186

7 Molecular and Structural Organization of Metal-Containing (Co)Polymers

Fig. 7.4 The glass temperatures of poly(ethylacrylate) ionomers vs. the content of ions (a) and the parameter of cq/a (b)

Table 7.3 Glass-transition temperatures .Tg / of copolymers of styrene (St) and acrylic acid (AA) and their Na-salts [48] Tg (K) Temperature region of Tg for a Na-containing ionomers, K Copolymer Copolymer Ionomer PS St–AA(3.9) St–AA(5.2) St–AA(6.4) St–AA(11.7) St–AA(14.1) a

373 381 384 388 395 399

– 389 394 400 410 441

– 11 15 20 32 42

In bracket the content of acrylic acid or sodium acrylate (mol%) is given.

cross-links in a polymeric matrix [45–48]. It is necessary to note, that Tg of the neutralized copolymers is appreciably higher than Tg of the acidic form of ionomers (Table 7.3). It is seen that with an increase in content of carboxylated groups temperature transition range extends too. The character of the ions distribution in a polymeric chain and their configuration influence the glass-transition temperature of the obtained ionomers. It was shown, for example, that Tg of the ionomeric copolymers St–AA synthesized by emulsion method (for these copolymers mainly block structure is characteristic [49]) is lower than Tg of the products of bulk polymerization [48]. Emulsion type copolymers have more long sequences of AA units due to better solubility of AA in water, while carboxyl groups of the copolymers obtained in bulk are distributed quite homogeneously in a polymeric chain. It results in the same sequence of NaC ions and, accordingly, is homogenously distributed along all chain ionic interactions, that is revealed in the chain mobility reduction and Tg increase. The important characteristic of the ionomers obtained is the degree of neutralization of acid groups of an ionomeric precursor. The neutralization degree influences

7.1 Ionic Aggregations and Multiplets 100 K ionomers

75 DNexp (%)

Fig. 7.5 The experimental neutralization degree vs. the given neutralization degree for K- and Zn-ionomers. The doted line indicates the full neutralization

187

Zn ionomers

50

25

0 0

25

50 DNt (%)

75

100

both the microstructure and the final properties of the polymer formed. Thus, potassium acetate can neutralize only one of two carboxyl groups in the copolymer of maleic anhydride-gr-ethylene-co-propylene (MalAn-gr-EP) that results in the appearance of the plateau on the graph of dependence of the experimental neutralization degree (DNexp ) vs. the specified neutralization degree (DNt ) at 50% level (Fig. 7.5) [50]. DNexp value was calculated from the integral intensity of the asymmetric vibration of a carbonyl group of the anhydride (CDO) at 1,785 cm1 (A1785 ) using intensity of rocking vibration of methylene group of ethylene/propylene chain at 723 cm1 (A723 / as the internal standard: DNexp D 1 

.A1785 =A723 /ionomer .A1785 =A723 /precursor

!!  100%

(7.1)

Apparently, formation of the K-carboxylated group reduces considerably the activity of the second carboxyl group of maleate unit against K acetate as a weak base. At the same time, practically full neutralization is observed in case of Zn2C cation. During neutralization, microphase division in an initial ionomer system caused by a polarity distinction between anhydride groups and non-polar ethylene-propylene chains is conserved, but its level depends greatly on the counterion nature. SAXS profile for the K-ionomer is characterized by a sharp peak of dispersion while for the Zn-containing ionomer this peak is appeared as a shoulder (Fig. 7.6). Probably this distinction is connected with the features of coordination behavior of Zn2C cations against dicarboxylated ionomer units. Spectroscopic investigations [51–54] testify the presence of the specific local ionic structures in the considered type polymers. Proceeding from the known coordination tendencies for various cations and from the analysis of symmetry of probable structures, various types of

188

7 Molecular and Structural Organization of Metal-Containing (Co)Polymers

a

b Absolute intensity (Å–3)

Absolute intensity (Å–3)

50 40 30 20

K-100 K-50 K-25 MAn-g-EPM

10 0 0.00

0.05

0.10

0.15

0.20

0.25

60 50 40 30

Zn-100 Zn-25

Zn-50

20 10

MAn-g-EPM

0 0.00

0.05

0.10

q (Å–1)

0.15

0.20

0.25

q (Å–1)

Fig. 7.6 SAXS profiles for maleic anhydride-gr-copolymer of ethylene/propylene and corresponding K- (a) and Zn-ionomers (b) with different neutralization degree

the local structures for alkaline, alkaline earth and zinc salts of copolymer of ethylene with methacrylic acid (4 mol%) were suggested [51]: O

O

O

C O

C

O

O O

O (3)

C

(2)

O

O

O

O

O (1)

C

O

C

C

C

C

O

O

O

O O

O

C O

O

C C

(4)

In particular, multiplets of Li-and Na-ionomers of EMAA form octahedral structures (3) and asymmetric stretching vibrations of COO appears as a doublet 1,568/1,547 cm1 (Na-ionomer) and 1,573/1,548 cm1 (Li-ionomer) in the IRspectra according to the D3 symmetry group. On the contrary, analysis of the K- and Cs-salts symmetry supposes the coordination numbers 8 (structures (4)) and only one IR-active frequency of asymmetric vibrations observed in the experimental spectra (1,550 and 1;548 cm1 for K- and Cs-ionomers, respectively). Change of a microstructure was observed even at the obtaining of binary mixtures of ionomers, for example, on the basis of Na and Zn salts of EMAA of copolymer [55]. Appearance of the new band of asymmetric vibration of COO group at 1;569 cm1 was observed in the IR-spectrum of the polymeric mixture (Fig. 7.7). This band was

1540 cm–1

189

1569 cm–1

1585 cm–1

7.1 Ionic Aggregations and Multiplets

EMAA-0.3Na

EMAA-0.3Na /EMAA-0.6Zn (75 / 25 w/w)

EMAA-0.3Na /EMAA-0.6Zn (50/50 w/w)

EMAA-0.6Zn

1650

1600

1550

1500

n /cm–1

Fig. 7.7 IR spectra of binary mixture of the EMAA-0.3 Na and EMAA-0.6 Zn

referred to the bridging carboxylated group between sodium and zinc cations. Contribution of this bridging structure is especially observable at temperature increase up to the area corresponding to the order–disorder transition in the ionic aggregations. This transition yields the peak on the DSC curve in the field of 325 K, while melting of the crystal phase of PE occurs at 360 K. It is necessary to note, that the nature of the cluster transition is, as a whole, an important problem. It is a subject of much research [56–59], as such properties of ionomers as viscoelasticity and rigidity undergo changes in this temperature area. It was shown that this transition includes two relaxation processes – the fast process (reversible) and the more slow process (irreversible) [60–62]. The more slow process means that formation of the ionic crystallites during cooling of a melt occurs not at the melting point of a cluster, but it occurs gradually in time by keeping them at room temperature. As it was noted above, increase of Tg of a polymeric matrix in ionomeric systems can be caused by the aggregation of ions into small dense multiplets [63]. Tg is the linear function of the ion content in this area. However, when Tg value is higher than some Tg value, fast increase of Tg is observed which is connected with the beginning of cluster formation in ionomers [64]. Formation of the ionic clusters or domains in the Zn-containing ionomer PE-c-AA was observed at C50ı C [65] similar to the E-MAA copolymer [66]. Such

190

7 Molecular and Structural Organization of Metal-Containing (Co)Polymers

a transition was interpreted as “an ionic transition”, accompanying with the water molecules elimination from the ionic clusters [67]. It results in loss of crystallinity of a polymeric matrix and in amorphization of the matrix, and it was confirmed by DSC-thermograms. At the same time, a new peak appears in this temperature area. This peak is connected with the vibrations of the ionic clusters and multiplets having various coordination structures [68] and testifying formation of a separate ionic phase. Sizes of the forming domains can vary in a wide range depending on many factors, including neutralization degree of carboxyl groups, coordination of a metal, a level of microphase division and also nature and molecular weight of an ionomeric molecule. So, radii of domains for the maleate-modified copolymers of ethylene/propylene (Table 7.2) exceed considerably corresponding sizes for the other types of ionomers, for example, for the sodium or zinc neutralized copolymers of ethylene/methacrylic acid [34, 69]; or for the copolymers of acrylate of zinc and styrene [22] (see Table 7.1). It is necessary to note, that SAXS parameters for the systems under consideration (Table 7.4) are in good agreement with the modified Yarusso-Cooper model [19]. According to this specified model, there are small dispersive structures in a matrix along with the large domains in the MAn-g-EP ionomers. It can be connected with the big distances between domains because of higher molecular weights, that increases probability of the fact that functional groups remain isolated in a matrix. In some cases, such ion-containing polymers can be considered as nanocomposites on a molecular level. For example, according to the transmission electron microscopy data for the K-maleate ionomeric three block-copolymer styrene– butadiene–styrene, diameter of ionic domains is 3–8 nm (Fig. 7.8) [70]. Thus, the analysis shows that the dominating effect in the ionomers is the microphase division into polar and non-polar domains due to the presence of ions. It is revealed not only in bulk or in the swelling state of the ion-containing polymers, but also in a solution. Formation of the ion pairs and multiplets was founded in gels of sodium polymethacrylate in methanol [71]. Collapse of PAA neutralized by the monovalent cations in methanol was detected by various methods [72]. AccordTable 7.4 SAXSa data for maleate-modified ethylene/propylene copolymers [50] ˚ ˚ ˚ 3) The sample R (A) R1 (A) Vp (A MAn-g-EP 17.3 44.9 3:9  106 K-ionomer (25) 26.1 63.3 5:3  106 K-ionomer (50) 23.4 56.1 3:3  106 K-ionomer (100) 24.8 58.2 3:4  106 Zn-ionomer (25) 18.0 47.9 2:7  106 Zn-ionomer (50) 18.8 50.3 3:8  106 Zn-ionomer (100) 16.9 49.6 3:3  106 a

R is the radius of domains; R1 is the radius of the polymer layer with a restricted mobility; Vp is the average volume containing one scattering particle; in brackets the neutralization degrees are given.

7.2 Morphology and Topological Structure of Metal-Containing Polymers

191

Fig. 7.8 TEM microphotograph of lead-containing maleate ionomers of styrene/butadiene/styrene triblock copolymer

20nm

ing to the model [73, 74], ion pairs suffer collapse in a polymeric chain in solvents with low permittivity and associate then into multiplets, passing in “supercollapsed state” or in an ionic mode. Ion pairs dissociate in polar solvents, and polyelectrolitic mode predominates. The character of the interactions responsible for the particular state, depends not only on the solvent, but also on the nature of counterions, on the type of ionic groups in a polymer, and on the pH of medium. So, for example, in case of such divalent complexing cations as Cu2C or VO2C , counterions can be associated into ionic groups in a polymeric chain even in such polar mediums as water [75]. Investigations of the “metal cation-polymeric molecule” interactions by the methods of molecular dynamics modeling [76, 77], show an essential role of the entropic factors in such systems. Linkage of the short chains of polyacrylic acid with Ca2C -ions in an aqueous solution results in the formation of the stable conformation of a coil with a free surface energy equal to 18 kcal/mole at low concentration of a polymer. At the same time local structures arise, in which Ca2C ions ˚ and the most part of oxygen form clusters with Ca2C  Ca2C distance equal to 4 A, atoms are shared between calcium ions (Fig. 7.9). On the contrary, probability of a coil formation at high concentrations of PAA and Ca2C -ions is low because of the local rigidity caused by coordination of plenty of Ca2C by initially extended short polymeric chains, and interchain interactions became more preferable. Various kinds of the “clusters of clusters” type aggregations are revealed also for the organo-inorganic hybrid polymers containing true oxometallic cluster units (see Chap. 3), though the nature of their formation has not been understood yet [78].

7.2 Morphology and Topological Structure of Metal-Containing Polymers The structural organization of the considered type metal-containing (co)polymers is determined by a unique combination of properties of a metal-containing coordination polyhedron, by polyfunctional character of an initial monomeric salt and

192

7 Molecular and Structural Organization of Metal-Containing (Co)Polymers

Fig. 7.9 Model of PAA chain conformation in the presence of Ca2C ions

by a variety of the nature of inter- and intramolecular bonds of polymeric chains. A majority of the forming metal-containing copolymers has amorphous structure in spite of the regular, as it was discussed above, mainly syndiotactic structure and a developed system of intermolecular interactions, that, in the aggregate, creates favorable conditions for the chains stacking and crystallizations of polymers. It is connected, probably, with the large number of cross-linking bonds and ionic groups, that increases rigidity of the polymer’s chains and, consequently, complicates their packing.

7.2.1 Three-Dimensional Network Polymers Even a minor change of the chemical composition or nature of the coordination bonds of an initial monomeric salt can result in a change of a structure and properties of the polymeric products. Visually it can be shown by the example of methacrylate derivatives of Ti(IV) alkoxides [79, 80]. As it was shown earlier, Ti.OR/3 .OOCC.CH3 / D CH2 / structure can be described by the equilibrium structures I and II (R D i -Pro, 2-ethylhexyl), I, II, and III (t-Bu, t-Am), and IV (R D Bu) [79]. The presence of a bidentate carboxylated bridge in a monomer molecule results in the formation of three-dimensional network polymer during its (co)polymerization:

7.2 Morphology and Topological Structure of Metal-Containing Polymers

193

CH2

H3C C C RO RO

C

O

O

Ti

Ti

RO O

O

OR

RO RO

OR

O

O

Ti

Ti

RO O

OR

O

OR OR OR

C

C C H3C

CH2

Special research of the etherification reaction of titanium alkoxides with tetra(copolymer of methacrylic acid, MAA, ethyl acrylate and ethyl methacrylate) and ter-(copolymer of methacrylic acid, MAA, and butyl methacrylate)polymers having acidic functions were conducted [80]. It was shown that it is possible to control effectively the formation of the cross-linked structures by such factors as molar Ti/COOH ratio, nature of alkoxide groups, concentration, and composition of an initial polymer. Instantaneous formation of a three-dimensional gel was observed at the stoichiometric molar ratio of tetraalkoxide and acid function (Ti/COOH D 1) or at the molar deficiency of titanium alkoxide (Ti/COOH < 1). However, the character of a mixture changed at gradual increase in a molar fraction of titanium alkoxide (Ti/COOH > 1) – the gel became less dense and transformed into a stable solution at fixed Ti/COOH value, which was accepted as threshold value. IR-spectra of the polymeric film obtained from this solution revealed bands as (COO) and s (COO) at 1,550 and 1;450 cm1 , that indicated a bidentate-chelate coordination of carboxylated group with titanium atom ( D 100 SM1 ). The role of the ligand environment of the metal atom in the interchain space is also important. So, the more lengthy Ris in an alkyl chain, the below threshold value of Ti/COOH ratio is (Table 7.5) and the more easy the formation of chelated structures is. A similar approach was successfully used for the prevention of a threedimensional network formation during copolymerization of alkoxides of Ti(IV) methacrylate with MMA: H3C CH2

H3C

CH2 C C

C

C C O RO

Ti

Ti

RO O

O

OR OR OR

Ti(OR)4

OR

RO OR

Ti/COOH > –1

MMA

RO

OR

Initiator

OR OR

RO

Ti O

C

O

O

O C

C H3C

OR Ti

C

CH2

OR

RO

OR

C

H3C

Ti

Ti O

RO

O

O

O

O

CH2

194

7 Molecular and Structural Organization of Metal-Containing (Co)Polymers

Table 7.5 Change of Ti/COOH threshold value in dependence on the nature of the alkoxide ligand Ti(OR)4 [80]

Fig. 7.10 The temperature dependencies of ©0 for cross-linked sodium polymethacrylate for an initial concentration of methacrylic acid of 20 (1), 30 (2) and 40% (3) (f D 1 kHz)

Ti(OR)4 Ti(OEt)4 Ti(OPr)4 Ti.Oi Pr/4 Ti(OBu)4 Ti(OEH)4

Threshold Ti/COOH 13/1 12/1 17/1 7/1 3/1

ε′

ε′ 80

2 1

600

60 400 3

20

30

40

50

40

T, °C

Efficiency of cross-linking of the forming gel increased at rising of the initial sodium methacrylate concentration in the presence of the cross-linking agent, N ,N -methylenebisacrylamide [81]. Dielectric properties of the polymeric system are especially sensitive to the rearrangements of the mutual chains order. It was confirmed by a sharp increase of the real part of the dielectric constant ("0 ) of dry cross-linked metal-containing polymers with an increase in an initial salt concentration at the obtaining of the corresponding hydrogels (Fig. 7.10). Such change of "0 occurs because of the residual water; concentration and state of the water are determined by the structure of a polymer network.

7.2.2 Interpenetrating Polymer Networks The ability of the analyzed metal-containing copolymers to form spatially crosslinked structures is of crucial importance in the obtaining of the polymeric mixtures and alloys on their basis and gives vide opportunities for the modification of the properties of polymers. As it is known, interpenetrating polymer networks (IPPN) are the combination of two cross-linked polymers, and synthesis and cross-linking,

7.2 Morphology and Topological Structure of Metal-Containing Polymers

195

at least, one of them is carried out in the presence of the second polymer. Metal polyacrylates (for example, Zn [82, 83], Cr and Cu [83] or their monomeric salts) on the one hand, and vinyl monomers and (co)polymers on the other hand can be the basic components of such systems. As a rule, an additional cross-linking agent as divinyl benzene (till 20–25 mol%) is used. Interpenetrating polymer networks can be obtained on the basis of metal polyacrylates only, certainly, differed by the nature [84–87]. It is worth noting the following fact. If metal polyacrylates are used as initial precursors in the polymer network formation, then in most cases they are considered as linear polymers soluble in benzene, DMFA, DMSO, dioxane [82, 83, 85, 86]:

n

n O

O

O

O

O

Cu

Cr O

n

O

O

O

Zn

O

O

O

2

But there are no evidences of the proposed structures in the analyzed works except the fact of the polymers solubility, as it was discussed earlier, not all double bonds can be involved in the polymerization process. The number of the residual unsaturated bonds depends on the monomeric salt nature, type, and conversion of the reaction and it is equal to 4–49% for the bifunctional carboxylates according to the different estimations [88–90]. It is seen that the degree of unsaturation practically in all cases is less than 0.5. On the other hand, decrease in the residual unsaturated bonds quantity can be explained, for example, by a formation of the intramolecular cycle structures of the following type: ~CH2

CH

CH2

CH~

Mn+

However, it is also impossible to exclude acts of interchain cyclization which will inevitably result in the cross-linked chains. So, in case of Co2C polyacrylate heated during 0.5 h at 100, 150, 200, and 250ıC significant decrease in intensity of the bands of the residual unsaturated bonds .CDC/ (1,640 cm1 ) in the IR-spectrum is observed, and this band practically disappears at 250ı C (Fig. 7.11) [91]. Such processes can take place in the analyzed systems but their role during cross-linking, seemingly, is insignificant. It seems, for example, surprising that the swelling degree

196

7 Molecular and Structural Organization of Metal-Containing (Co)Polymers

Fig. 7.11 The fragments of IR spectra of Co2C acrylate (1), its polymer (2), and polymers after heat treatments during 0.5 h at 100 (3), 150 (4), 200 (5) and 250ı C (6)

I

6 5 4

3 2 1 1400

1600

1800

n, cm–1

Table 7.6 The effect of concentration of Zn(II) polyacrylate on the properties of the IPPN [82] N c (g/mol) [ZnPAA] 102 (mol/L) Yield (%) Swelling (%) M 14.49 6.3 41.4 322 9.66 16.7 39.3 294 4.78 20.8 37.0 241 2.42 24.0 36.1 198 [Initiator] D 5:19  103 mol/L, [Styrene] D 1.38 mol/L, [MMA] D 1.44 mol/L, [DVB] D 0.83 mol/L

of a polymer network in DMFA and the average molecular weight of chains between cross-links (MN c ) (Table 7.6) increase with an increase in concentration of Zn2C polyacrylate at the obtaining of IPPN on the basis of Zn2C polyacrylate [82]. At the same time, converse effects are observed when a monomeric salt is used as an initial basic component of the IPPN. The cross-linking degree increases and, accordingly, MN c decreases with an increase in the monomeric salt concentration [92, 93]. A wide spectrum of physicochemical properties of polymeric mixtures is determined, first of all, by a compatibility of their components. It is possible to evaluate a degree of compatibility of two polymers by the glass-transition temperatures (Tg /. Thus, as polyacrylate/polyacrylonitrile system (1:1) [85] has two close Tg (146 and 152ı C), that confirms sufficiently homogeneous distribution of two phases, each of which is enriched, seemingly, with one of the polymers – PAC-As or PAN, accordingly. DSC-investigations confirm formation of a polymeric alloy for the Bi polyacrylate/PAN mixture, which shows one, and higher, glass-transition

7.2 Morphology and Topological Structure of Metal-Containing Polymers

197

temperatures (281ıC). Intermolecular Van der Waals interactions and formation of complexes at participation of CN-groups of PAN and metal atoms of polyacrylates promote, probably, effective compatibility of the polymeric components. It is confirmed by a shift of valence vibrations .CN/ into the long-wave region in the IR-spectra of polymeric alloys (from 2,260 till 2,241 and 2;243 cm1 for Bi polyacrylate/PAN and As polyacrylate/PAN, accordingly). Degree of bond strength of the polymeric complexes can be estimated by the viscosity parameter kAB [94]: .A /B D

.A / Œ1 C 2kAB .B / CB C     .r /B

(7.2)

where .r /B is relative viscosity of the polymer B at concentration CB , .A /, and (B / are internal viscosities of polymers A and B. Similarly to the Huggins constants kA and kB , the kAB parameter depends on several types of the interactions, and hydrodynamical and thermodynamic interactions are determining. Thermodynamic contribution includes effects of the excluded volume both intramolecular, resulting in an increase of a coil, and intermolecular, resulting in the compression of a coil. Thus, for the As polyacrylate (A) – Sb polyacrylate (B) system, kAB values in such solvents as DMSO, DMFA, and dioxane are 0.60, 0.42, and 0.26 accordingly, i.e., the strongest complex is formed in DMSO and the least strong complex is formed in dioxane in dependence on a permittivity of a solvent [93]. And, as a whole, these values indicate that interaction of As and Sb polyacrylates between themselves is weak and has Van der Waals character. It is known that salts of unsaturated carboxylic acids are frequently used as activating and reinforcing agents at vulcanization of rubbers. Cross-linking processes arising in such systems, result in the formation of the interpenetrating polymeric networks with the improved physical-mechanical properties [95–97]. An especially effective method is the method in which synthesis of a metal carboxylate and its polymerization are carried out during formation of the IPPN in situ. One of the obtaining methods of the IPPN on the basis of nitrile-butadiene rubber (NBR) and Zn(II), Al(III), Zr(IV) methacrylates [98] is the following. NBR and metal oxide are mixed in a Brabender mixer, then methacrylic acid and dicumyl peroxide are added and the obtained composition is consolidated at 150ı C and 12 MPa during 30 min. According to another method, cross-linkage of NBR in the presence of metal oxide and peroxide (150ıC, 12 MPa, 30 min) is conducted at first. Then, obtained vulcanizate was swelled in the MAA solution containing 0.5 mass% of AIBN with the subsequent heat treatment in a furnace at 150ı C during 8–10 h. Noticeable increase in tensile strength (up to 17–19 MPa in comparison with NBR filled by metals oxides) and decrease of elongation (%) indicate formation of the IPPN in these systems. The typical morphologic pattern of the analyzed polymer networks looks as a continuous phase of the one component with characteristic inclusions of the second component (Fig. 7.12) of spheric [85], cellular [92] and other forms.

198

7 Molecular and Structural Organization of Metal-Containing (Co)Polymers

Fig. 7.12 Electron microscopy image of the polymer network on the base of Zn polyacrylate and copolymer of styrene with MMA (magnification 790)

7.2.3 Hybrid Supramolecular Structures It is known that binuclear metal carboxylates of the general formula M2 .O2 CR/4 L2 have a characteristic structure of the lantern type (Fig. 7.13a). Formation of the high-symmetric two-dimensional structures (Fig. 7.13b) is observed in case of linear dicarboxylated bridges, and an indefinite chain of homogeneous micropores (Fig. 7.13c) is created due to the interplane interactions in such systems. Typical representatives of this class of compounds among unsaturated carboxylates are fumarates, trans–trans-muconates containing binuclear Mo2 4C [99–101], Rh2 4C [102] and Ru2 2C;3C [103] units. Synthesis of the considered type metals dicarboxylates in the presence of polymers with one-dimensional chains results in a formation of supramolecular inclusion complexes [99, 100]:

Mo2(O2CCH3)4

dicarboxylic acid, polymer CH3OH, Ar

Mo2(O2C-X-CO2)2 polymer

X=CH=CH; CH=CH–CH=CH polymer = HO

CH2–CH2–O

n

H; HO

CH2–CH–O

n

H

CH3

The formed products are the noncovalently interacting ensembles of the guest– host type supramolecules (Fig. 7.14). In the complexes of Mo(II) fumarate with polyesters [99], number of the included molecules of the polymer-guest depends on its molecular weight and reaches the saturation at the molecular weight more than

7.2 Morphology and Topological Structure of Metal-Containing Polymers

199

b

a

Mo

Mo

Mo

Mo

Mo

Mo

R C L

M

R C L M

Mo

Mo Mo

Mo

Mo Mo

C R

C Mo

Mo

Mo

Mo

R

Mo

Mo

8

c Mo Mo

Mo Mo

Mo

Mo Mo

Mo

Fig. 7.13 Microporous structure of coordination polymers of metal dicarboxylates: (a) lantern structure; (b) two-dimensional infinite chain; (c) the formation of linear pores

Fig. 7.14 A schematic view of the structure of Mo(II) dicarboxylate containing of poly (ethyleneglycole) in micropores

Polymer

200

7 Molecular and Structural Organization of Metal-Containing (Co)Polymers

N

N

M

M

N

N

self assembly

N

N

π – π stacking

M N

M

= dicarboxylic ligand

N

N

N

M

M N

N

linear chain complex two-dimensional layer and micropores three-dimensional network

[Zn(O2C

CO2)(

N ) 3] ·

N·(H2O)0·5

[Ni (O2C

CO2)(

N ) 3] ·

N·(H2O)2

Fig. 7.15 A scheme of the formation of a microporous structure for mononuclear metal dicarboxylates

3,000, and it is equal to 4 ethylene glycol units per Mo2 unit in case of polyethylene glycol. This value is twice less for polypropylene glycol, apparently, because of the greater size of a monomer unit. Mononuclear metal dicarboxylates show similar tendency in the formation of microporous systems. In particular, peculiar stacks are formed because of  – interactions in the molecule of fumarate-pyridine complexes of Cu(II), Zn(II), Ni(II) due to the processes of self-assembly from one-dimensional regular linear chains including metal atoms, connected by dicarboxylated ligands. It results in succession in two- and three-dimensional structures with the great number of micropores (Fig. 7.15) [104]. This type of complexes are capable of absorbing significant amounts of reversible absorbing gases, for example, N2 , Ar, O2 , CH4 , X. Thus, Zn(II) complex absorbs up to 10.6 moles of N2 per mole of a metal atom. Intercalation of acrylate ions with the subsequent polymerization in situ in molecules of layered double hydroxides (LDHs) as an inorganic host can be also noted among perspective synthetic strategies [105, 106]. Acrylate and polyacrylate nanocomposites on the basis of the replaced nickel hydroxide LDH(Ni0:7 L0:3 (poly)acrylate) (LDFe, Co, Mn) were obtained according to this scheme [107, 108]. Monomeric and polymeric systems can be isolated in consistent stages of the synthesis in case of Fe-containing intercalites, while for Co- and Mn-containing nanocomposites intercalation and polymerizations occur in one stage with the formation of polyacrylate composites immediately. Interlayer distances are equal to ˚ (Fig. 7.16) in dependence on the synthesis conditions. 7.8–12.5 A Liquid and organic crystals, micelles, two-layer lipids, etc. are often used as highly organized mediums along with micro- and mesoporous compounds for

7.2 Morphology and Topological Structure of Metal-Containing Polymers

a

201

b –

O

–

O

–

O

O

COO– COO– COO– COO– COO–

O

O

~12–12.5 Å

~13.4 Å O

O–

O

O–

O

c

–

COO– COO– COO– COO– COO–

O

d COO–

COO–

COO–

~12 Å

COO–

~7.8 Å COO–

COO–

COO–

–

OOC

–

OOC

–

OOC

Fig. 7.16 Schematic view of intercalated nanocomposites: LDH(Ni0:7 Fe0:3 -acrylate) (a), two layer LDH(Ni0:7 M0:3 -polyacrylate) (M D Fe, Co (b)), monolayer LDH(Ni0:7 M0:3 -polyacrylates) (M D Fe or Co (c) and Mn or Co (d)), The layered double hydroxides were obtained by solidphase synthesis (b, c) and coprecipitation (d)

obtaining the supramolecular systems [109]. Thus, at mixture of the water-soluble poly(p-phenylenevinylene)dimethyl sulfide (PPPV) with amphiphilic acrylate monomers [110, 111] of the following type O(CH2)11OOCCH=CH2 Na+ –OOC

O(CH2)11OOCCH=CH2 O(CH2)11OOCCH=CH2

the inverted lyotropic liquid-crystal phase, in which PPPV pierces a hexagonal column and orients parallel with its c-axis (Fig. 7.17), is formed. Photopolymerization of an acrylate salt and subsequent heating of a cross-linked matrix results in the formation of hybrid PPPV with more intensive fluorescent properties in comparison with the volumetric poly(p-phenylenevinylene). Moreover, a new intensive band in the emission spectrum at 670 nm appears in case of Eu(III) [111] salt. It testifies interaction of metal cation with PPPV chains with possible energy transport between components. Sizes of the inverted hexagonal phase depend on the metal nature and type of the metal–caroboxylate interaction [111, 112] and are also determined by length and structure of the aliphatic part of the amphiphilic mesogenic metal-containing monomer [113] p-styryl-(LC-1) or p-styryloxy-(LC-2) octadecanoates:

202

7 Molecular and Structural Organization of Metal-Containing (Co)Polymers

hν

Δ

+ Cl– SMe2 H

n

Δ -nSMe2 -nHCl

n

Fig. 7.17 Scheme of stabilization of lyothropic liquid-crystalline phase by polymerization of an acrylate monomer

or + – ( )m ( )n CO2 Na

m + n + 3 = 18

LC-1

O + – ( )m ( )n CO2 Na

LC-2

Interplanar distances of the hexagonal mesophase are the function of charge and size of the metal ion (Table 7.7). Salts on the basis of trivalent lanthanide ions have minimal sizes of unit cells in comparison with corresponding analogues of divalent ions of transition metals that agrees also with IR-spectroscopy data on the nature of the carboxylated ion coordination. Stronger chelate coordination “carboxyl grouplanthanide ion” ( D 96–97 cm1 ) results in a decrease in general size of a head group of an amphiphilic molecule and provides more dense packing. Metal ions of the greater size at identical charge value are more inclined to form lamellar or regular hexagonal structure. For example, potassium salts form a lamel˚ while a well-defined inverted hexagolae with interchannel distance (ICD) 38.9 A, ˚ nal phase with ICD 41.9 A [112] is characteristic for Na-p-styryloctadecanoate. In contrast to other salts, size of the mesophase formed by Cu-p-styryloctadecanoate is much less (Table 7.7) and X-ray diffraction data indicate formation of the thermotropic columnar-hexagonal phase. It is probably connected with the peculiarities of binuclear structure which is characteristic, as it is known, for copper carboxylates. It is important to note that microstructure of the liquid-crystalline phase does not undergo essential changes during polymerization and cross-linking of the chains (Table 7.7). A similar approach to the synthesis of highly organized systems was realized at the obtaining of polymeric micelles. The rod-like form of the surphactante vinyl benzoate monomer

7.2 Morphology and Topological Structure of Metal-Containing Polymers

203

Table 7.7 Powder X-ray diffraction data for nanostructured polymers based on liquid-crystalline salts of mesogenic metallomonomers [111, 112] Compound LC-1a Compound LC-2b After polymerization Monomer mixture After polymerization

Metal ion NaC KC Ca2C Mg2C Ni2C Co2C Cd2C Cu2C Eu3C Ce3C

d100 d110 d200

d100 d110 d200 ICDc

˚ (A) 34.5 20.2 17.5

˚ (A) 36.7 38.2 30.9 28.9

35.8 20.8 18.2 35.6 20.3 17.7 35.0 20.6 18.0

21.1 19.5 18.1 16.9

18.3 13.0 15.7 14.6

d100 d110 d200 ICD Phase RH L RH RH

42.1 38.8 36.1 33.6

˚ (A) 35.7 38.2 30.4 28.1

d

27.7 15.9 13.8 31.9 RH 30.0 17.4 15.3 34.9 RH 23.0 13.3 11.5 26.7 CH

20.5 19.2 17.7 16.5

17.7 13.0 15.4 14.2

41.0 38.6 35.4 32.9

Phase RH L RH RH

28.2 15.6 13.6 31.5 RH 29.5 17.2 15.1 34.5 RH 22.6 13.1 11.4 26.2 CH

30.2 17.6 15.7 30.9 17.8 15.5

a

Monomer LC-1: H2 O: 2-hydroxy-2-methylpropiophenone (20 wt% in xylene) – 85:10:5 (wt%) Monomer LC-2: DVB: H2 O: 2-hydroxy-2-methylpropiophenone (20 wt% in xylene) – 87:7:5:1 (wt%) c ICD is interchanell distance d Phase designations: RH is reverse hexagonal, L is lamellar, CH is columnar hexagonal b

CH3 + COO H3C N (CH2)11CH3 CH3

is stabilized during its radical polymerization, and the formed product has high thermal stability and does not show long dissociates in diluted solutions [114]. Presence of the hydrophobic aliphatic groups in the copolymers of acrylic acid with N -dodecylacrylamide [115] or in the alternate copolymers of sodium maleate with alkylvinyl esters [116–121] of the following chemical constitution CH2

CH O

O

C12H25

CH

CH

C

C

ONa

ONa

O n

promotes the formation of micelle-like aggregates from macromolecules of copolymers in aqueous mediums at critical concentration of micelle formation 2  103 g=L. Quantities of the polymeric chains .m/ participating in the formation of micelle-like aggregates were estimated from average molecular weights (Mw ) of copolymers of dodecylvinyl ester and sodium maleate (pC12 M) differed in molecular weights and from average molecular weights (Mwm / of micelle-like aggregates which were determined according to the data of sedimentation equilibrium measurement) [122]. The number of such polymeric chains is 16, 8, 2, and 1 as molecular

204

7 Molecular and Structural Organization of Metal-Containing (Co)Polymers Table 7.8 Molecular-weight characteristics of copolymers of sodium maleate with dodecylvinyl ether [122] Polymer Mn  103 Mw =Mn Mwm  104 m pC12 M-1 3.3 1.4 7.6 16 6.0 1.5 8.0 8 pC12 M-2 pC12 M-3 18 1.9 6.7 2 89 3.0 19 0.7 pC12 M-4

masses of polymeric products increase that testifies significant tendency of pC12 M copolymers to the intramolecular association (Table 7.8). The ability of ion-containing polymers to self-organization increases in case of their complexes, for example, with various types of surphactantes and amines that can result in the formation of complex types of hierarchical structures. It was established that interaction of diblock-copolymer of poly(ethyleneoxide)poly(methacrylate of sodium) (PEO176 -b-PMANa186 ) with surphactantes is attended by the formation of micelle-like aggregates – vesicles, in which insoluble polyion–surphactante fragments are stabilized by polyethylene glycol chains (PEG) in an aqueous medium [123]. The diameter of the particles of such stoichiometric complexes changes within the limits from 85 up to 120 nm. In case of inclusion complexes of, for example, ’-cyclodextrin (’-CD) with the above mentioned pC12 M copolymers, reaction of cooperative linkage induces dissociation of self-organizing micelle aggregates [121, 122]. Just competitive processes of self-association in the considered type ion-containing polymers explain, seemingly, selective linkage of ’-CD since any interaction, for example, with ”- or “-CD for pC12 M copolymers were not observed [122]. Self-organizing template complexes of “polyacrylic acid-cetyl trimethyl ammonium bromide” play a key role in morphogenesis of mesoporous silica gel surface in the presence of ions of alkali-earth metals [124, 125]. Complexation of metal ions with carboxyl groups of a polyelectrolitic chain causes, probably, bending of the polymeric chain and, finally, results in the formation of the curved micelle and generation of discoid or gyroid particles in dependence on pH (Fig. 7.18). The orienting influence of a “parent” polymeric chain and nonvalent interactions promote the ordered polyelectrolitic complexes formation during matrix polymerization of Na methacrylate [126] or acrylate [127] in the presence of poly(allyl amine) of hydrochloride as a template polymeric molecule. Kinetic effects of the reaction (high initial rate of polymerization and its dependence on a molar ratio of template units to a monomer) are in agreement with the growth of “daughter” chains on the so-called zip-mechanism. According to this mechanism, molecules of the monomer are initially adsorbed on a template macromolecule and then their propagation occurs. In its turn, deviations from the ideal zip-mechanism can result in the formation of a network-type structure (Fig. 7.19), for example, if the growing radical reacts with a monomer of another template chain, or chain growth occurs with involving of not adsorbed monomer molecules with the subsequent addition of another template chain. Reaction at this becomes diffusion- controlled and its rate accordingly decreases.

7.3 Basic Types of Units Variability in Metal-Containing (Co)Polymers

205

Fig. 7.18 Scheme of the formation of micelle (a) and microphotographs of the hyroid particles of template complexes PAA-STAB-Mg (b) and PAA-STAB-Sr (c) Scale bar at 1 m

Thus, morphology and topological structure of the metal-containing polymers are much more complex than structures of traditional polymers, the structure is enriched by the presence of metal ions in chains, their clusters and associates of higher organization. Now the structure is more or less studied for alkaline, alkaline-earth, and transition metals, and its study has only been started for polyvalent metals.

7.3 Basic Types of Units Variability in Metal-Containing (Co)Polymers Analyzing polymerization of the considered type metal-containing monomers, it is reasonable to start with the prerequisite that composition of the forming polymer corresponds to the composition of the initial monomer, and many experimental data

206

7 Molecular and Structural Organization of Metal-Containing (Co)Polymers

a Template chain Adsorbed monomer unit

1

2 Growing radical

3

b

Fig. 7.19 Mechanism of template polymerization of sodium methacrylate in the presence of poly(allylamine hydrochloride) (a) and schematic view of the network structure of the resulting polymer (b). (1) Formation of network nodes, (2) attachment of non-adsorbed monomer, (3) linear chain formation

confirm the given thesis as it is followed from the foregoing examples. However, units in which structure and geometry differ from the basic type units are also formed during formation of such polymers. It can result in breaking of chemical and structural homogeneity (“defectiveness”) of a macromolecular chain, and real macromolecules, in principle, cannot be represented as monotonously repeating identical units, i.e., every possible type of anomalous additions (units variability) in them should be taken into account. These problems are very important for the determination of a connection between composition, structure, and properties of the products received by (co)polymerization of unsaturated metal carboxylates, and also frequently cause or limit various areas of products application.

7.3 Basic Types of Units Variability in Metal-Containing (Co)Polymers

207

Metal-containing polymers have two types of units variability. The first is characteristic for traditional high-molecular compounds (stereoregularity, presence of the residual multiple bonds, structuring, and cyclization during polymerization, etc.) and the second – for metal-containing polymers [128, 129]. Types of units variability of metal-containing polymers have principal importance for many areas of their use. They are partial elimination of metallogrouping and breaking of electronic structure in the units (changes of the valence of a metal ion, its nuclearity and ligand environment, extracoordination and changes of the form of the polyhedron of metal-containing complexes especially in case of polyvalent d -elements, character of distribution of metallogrouping in a polymeric chain, etc. during formation of metal-containing polymers). We shall analyze some of them.

7.3.1 Units Variability, Caused by Elimination of Metallogrouping During Polymerization It is one of the most important types of structural defectiveness in metallopolymeric chains: unsuccessful attempts of polymerization of many MCM are connected with elimination [130] of a metal hydride and formation of “free-metal” polymers. (y +z ) CH2

initiation

CH

CH2

C O

CH

y

CH

CH

z

+ z MHXn –1 + zCO2

C O

MXn / 2

O

O MXn / 2

Chemical transformations of unsaturated metal carboxylates proceeding in aqueous and polar solvents are accompanied by ionization and dissociation of carboxylates, that also can result in the formation of “free-metal” products. Especially it concerns carboxylates of d -metals, which are strong electrolytes in water. Salts of unsaturated carboxylic acids are practically completely dissociated in aqueous or aqueous-organic mediums at pH > 7 (value of molar electric conductivity at infinite dilution is 0 D 146 154 cm2 Ohm1 mol1 ) and other particles already act as monomers – acrylate- and methacrylate-ions in case of salts of acrylic and methacrylic acids [131]. Aqueous solutions of Cu(II), Co(II), and Ni(II) acrylates are also average force electrolytes (dissociation degree is 0.52–0.53), concentration dependence of molar electric conductivity is described well by the Kohlrausch equation [132]. Therefore, as it was shown in Sect. 5.2.2, polymerization of the investigated metal-containing monomers, is carried out, as a rule, in the conditions excluding their dissociation, and metal content in the formed metallopolymers corresponds to the calculated value. Thermal polymerization of unsaturated metal carboxylates is also frequently accompanied by elimination of metallogrouping. So, in case of copper(II) acrylate [133], relatively weak bond Cu–O breaks with the formation of CH2 DCHCOO radical which, interacting with a matrix, can be the channel, resulting in the

208

7 Molecular and Structural Organization of Metal-Containing (Co)Polymers

formation of “free-metal” units. Combination of thermal decomposition process with polymerization in solid phase occurs by a lot of transformations (190240ı C). The final empirical composition of the polymers containing metallic fragments both carboxylated and not carboxylated (products of the elimination of CO2 , Cu(COO)2 groups with the formation of CHDCHCHDCH, CH2 CHDCHCH2 , and others fragments) types can be expressed as ŒCu.C6 H4 O4 /px .C4 H4 /x .CuC6 H6 O4 /qy .C4 H6 /y .C6 H8 O4 /j z .C4 H8 /z n .

7.3.2 Units Variability, Caused by Various Oxidation Rate of d-Metals It is also one of the most widespread types of units variability. By analogy with macromolecular complexes, it was possible to expect that homo- and copolymerization of metal-containing monomers will prevent or will slow down oxidative or reducing processes with participation of metal ions. There are numerous experimental data confirming that a polymeric matrix stabilizes low-valent metals complexes (for example [134], Pd(I)). Moreover, stability of Cu(I) condition during polymerization of copper(I) acrylate (including thermal polymerization) allows to use [135] this method for the obtaining of coordination Cu(I) compounds. However during polymerization of the monomers including ions of high-valent metals, reduction of the ions frequently occurs, for example: V5C ! V4C ! V3C , Fe3C ! Fe2C , Mo5C ! Mo4C , etc. The reasons for it can be very different. So, polymerization of Cu(II) acrylate as it was discussed earlier, is accompanied by the reactions of intramolecular chain termination [136], it is promoted, probably, by the relatively low values of standard potentials of reduction of copper ions (E0Cu.II/!Cu.I/ D 0:15 V). Electron transport in such systems is realized by a complex way and accompanied by reduction of Cu2C up to CuC . Character of the electronic spectrum of the polymeric product testifies that the electronic structure of copper ions has essentially changed during polymerization. The absorption band corresponding to the excitation of d–d-transfer in electron shell of Cu2C ion is observed at 14;800 cm1 in the electronic spectrum of the initial monomer of Cu(II) acrylate (Fig. 7.20). In the spectrum of the polymerization product, this band completely disappears but band at 28;000 cm1 instead appears. At least, for those ions which have kept the valence, such change of a spectrum (hypsochromic shift) can be described (see [137]) as change of the symmetry in the structure of their nearest environment which is a square pyramid from oxygen atoms in an initial complex. Oxygen atoms of carboxyl groups are at the base of the pyramid, and oxygen atom of the coordinated alcohol molecule are at the top of the pyramid. Set of the received data including IR spectroscopy ones (Fig. 7.21) allows to assume that units of the polymeric chain are dimers of one- and divalent copper with the preservation of the bridge structure:

7.3 Basic Types of Units Variability in Metal-Containing (Co)Polymers 200 D 2.0

250

300

350 400

500

209

700 900 ν, nm

1

1.6 2 1.2

0.8

0.4

0 50

42

34

26

18

ν . 103, cm–1

Fig. 7.20 Absorption electron spectra of copper(II) acrylate (1) and the product of its polymerization (2) CH2=CH O

C

Cu

O

O C

CH CH2 CH H2C HC H2C HC O

O

2+

Cu

O O

C C

O Cu1+

1+

CH2

O O

Cu2+ O C O C CH CH2 CH CH2 CH=CH2

7.3.3 Anomalies in Metal-Containing Polymers Chains Caused by a Variety of Chemical Linkage of a Metal with a Polymerized Ligand This type of units variability can be most visually shown by the example of metals salts with unsaturated carboxylic acids. As was discussed in Chap. 4, structural functions of a carboxylated group RCOO are very varied. It can act as mono-(I),

210

7 Molecular and Structural Organization of Metal-Containing (Co)Polymers 2

100

Transmission, %

80

1

60

40

20

0

20

18

16

14

12

10

8

4 6 ν . 10–2, cm–1

Fig. 7.21 IR spectra of copper(II) acrylate (1) and of the polymer product (2)

bi-(II, III), three- (IV) or tetradentate (V) ligand at interaction with a metal ion even on the condition that the multiple bond does not participate in complexing. The spectroscopic data of metals acrylates testify mainly ionic character of a M–O bond with a degree of covalence 0.13 0.16, and, as it was noted above, some reduction of the orbital contribution to the metal–ligand ¢-bond in the macromolecule composition were observed. It is interesting, that an increase in the length of metal–ligand bonds and reduction of the degree of their covalence [138] occur at transferring from acrylate complexes to their saturated analogues (acetates). It was shown by IR- and dielectric spectroscopy [139–142] that distortion of bridge groups geometry occurs during polymerization of metal acrylates under the action of internal stresses in the structure of the forming network. Sometimes it can resultin destruction of the bridge bonds. For example, the new absorption band at 1;700 cm1 appears in the IR-spectrum of the polymeric product at polymerization of polynuclear Cr(III) oxoacrylate [143]. It is connected, most likely, with the formation of monodentate carboxylated groups (Fig. 7.22). Overlapping of the frequencies corresponded to the bridge and nonbridge bidentate carboxylated groups, as it is typical for the polymeric Fe(III) oxoacrylate, were not observed [144].

7.3.4 Extracoordination as One of the Types of Anomalies (Spatial and Electronic Structure of a Polyhedron) Coordination unsaturation of the central atom, promoting comparatively easy passing of a lot of the side processes, is one of the reasons of relatively low stability of some metal-containing monomers and polymers on their basis. Both specially entered substances and molecules of a solvent, most often – water, can act as an additional ligand. Not all chemically bonded with a metal atom ligands are removed from

7.3 Basic Types of Units Variability in Metal-Containing (Co)Polymers

211

100 1 Transmission, %

80 2 60 40 20 0 22

18

14

10

ν.10–2, cm–1

Fig. 7.22 IR spectra of chromium(III) acrylate (1) and of the polymer product (2)

the product after polymerization, it also is a source of units variability in polymers. As a whole, as it was discussed above, geometry of the metal atom does not undergo essential changes during polymerization, but reorganizations in the structure of the nearest environment of a metal atom can sometimes take place. For example, at thermal solid-phase polymerization in the Fe3C maleate molecule with the local symmetry of Fe–O bonds close to cubic, reorganization of the structure towards more asymmetric one occurs [145]. Higher asymmetry of the nearest environment and spin–lattice relaxation are characteristic also for Fe(III) ions in the polymeric oxoacrylate [144]. Bathochromic shift of one of the bands (up to 21;500 cm1 ) in the electronic spectrum of the Fe(III) polyacrylate corresponding to the forbidden d –d -transitions in the electronic shell of Fe(III) (Fig. 7.23) is observed.

7.3.5 Unsaturation of Metal-Containing Polymers and Their Structurization These phenomena can be caused by various reasons: involving not all multiple bonds into polymerization, peculiarities of the restriction reactions and chain transfer reactions, etc. Ability of the metal-containing copolymers [146] based on Co or Ni acrylates with styrene to dissolve in polar solvents up to the fixed contents of acrylate units testifies passing of the process mainly by one of the multiple bonds; it relates also to their graft polymerization [147]. Part of the unreacted double bonds in (co)polymers of transition metal diacrylates [88, 90, 146] with traditional vinyl monomers increases in the series Zn2C .35%/ < Co2C (39%) < Ni2C (49%), which correlates with the ability of these acrylates to homopolymerization (unsaturation is equal to 50% in those hypothetical cases when only one acrylate group reacts). It is necessary to note that presence of the residual double bonds and opportunity of their add-polymerization can be used for the obtaining of the network polymers with

212

7 Molecular and Structural Organization of Metal-Containing (Co)Polymers 200

250

300 350

500

λ, nm

2 1.0

24000 21500

36000

1.5

28000

D

1 0.5

50

40

30

20

ν.10–3, cm–1

Fig. 7.23 Absorption electron spectra of iron(III) oxoacrylate (1) and of the polymer product (2)

high strength and plasticity at high softening points. Such properties of the metalcontaining polymers obtained will be analyzed in the subsequent chapters. Thus, various types of units variability in metal-containing polymers chains can arise during polymerization of MCM. Some of them make the essential contribution to their structure and serve as specific levers of control of composition and properties of the metal-containing polymers formed, others have only hypothetical character. The analysis of units variability carried out in metal-containing polymer chains can cause the impression of multiplicity of the side processes concomitant polymerization of MCM. It can seemproblematic to achieve uniqueness of the polymerization products of such monomers connected just with structural homogeneity of the macrocomplexes formed, which were noted in the introduction. Actually it is not so. Many of the considered side reactions can be prevented or their role can be essentially reduced. There are many approaches for this purpose, most convenient of them – low-temperature polymerization processes including polymerization with use of the irradiation. This approach allows to carry out polymerization in a wide temperature range (including and post-radiation variant) and at any phase states of a monomer. Even if it is impossible to exclude units variability, it is almost always possible to take it into account (sometimes quantitatively). According to the IUPAC [148] nomenclature, metal-containing polymeric chains can be referred to the regular macromolecules at low contents of the units with anomalous structure. Structures of these macromolecules include mainly recurrence of the identical constituent units connected with each other in the same way. Irregular macromolecules are formed at the considerable contribution of units variability. Structure of the irregular macromolecules included the constituent units connected with each other along the chain not in the same way.

References

213

Thus, the given data testify that the range of the molecular organization of the considered type metal-containing polymers is rather wide: from linear polymers up to di- and three-dimensional network and supramolecular structures. The structurechemical control at all levels of the metal-containing copolymers organizations (molecular, topological, and the supramolecular) allows to obtain polymers with a complex of valuable properties, that is especially important from the standpoint of the modern tendencies of creation of the new generation materials.

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120. W. Binana-Limbele, R. Zana, Macromolecules 23, 2731 (1990) 121. D. Taura, A. Hashidzume, A. Harada, Macromol. Rapid Commun. 28, 2306 (2007) 122. D. Taura, A. Hashidzume, Y. Okumura, A. Harada, Macromolecules 41, 3640 (2008) 123. A.V. Kabanov, T.K. Bronich, V.A. Kabanov, K. Yu, A. Eisenberg, J. Am. Chem. Soc. 120, 9941 (1998) 124. C.C. Pantazis, A.P. Katsoulidis, P.J. Pomonis, Chem. Mater. 18, 149 (2006) 125. C.C. Pantazin, P.N. Trikalitis, P.J. Pomonis, M.J. Hudson, J. Microporous Mesoporous Mater. 66, 37 (2003) 126. G. Polacco, M.G. Cascone, L. Petarca, G. Maltinti, C. Cristallini, N. Barbani, L. Lazzeri, Polymer Int. 41, 443 (1996) 127. P. Cerrai, G.D. Guerra, S. Maltinti, M. Tricoli, P. Giusti, L. Petarca, G. Polacco, Macromol. Rapid Commun. 15, 983 (1994) 128. G.I. Dzhardimalieva, A.D. Pomogailo, Variability of Mixed-Unit Chains in Metal-Containing Polymers. Chapter in Book: Metal-Containing Polymeric Materials, ed. by C.U. Pittman Jr., et al. (Plenum Press, New York, 1996), pp. 63–79 129. A.D. Pomogailo, G.I. Dzhardimalieva, Izv. Akad. Nauk. SSSR Ser. Khim., 2403 (1998) 130. C.U. Pittman Jr., in Organometallic Reactions, vol. 6, ed. by E.I. Becker, M. Tsutsui (New York, 1977), p. 1 131. V.A. Kabanov, D.A. Topchiev, Polymerization of Ionizing Monomers (Nauka, Moscow, 1975) 132. B.S. Selenova, G.I. Dzhardimalieva, E..B. Baishiganov, O.N. Efimov, A.D. Pomogailo, Izv. Akad. Nauk. SSSR Ser. Khim., 1025 (1989) 133. E.I. Aleksandrova, G.I. Dzhardimalieva, A.S. Rozenberg, A.D. Pomogailo, Izv. Akad. Nauk. SSSR Ser. Khim., 308 (1993) 134. L.B. Krentsel, A.D. Litmanovich, N.A. Plate, E.I. Karpeiskaya, L.F. Godunova, E.I. Klabunovskii, Vysokomol. Soedin. A. 34, 10 (1992) 135. G.I. Dzhardimalieva, I.N. Ivleva, E.N. Frolov, A.D. Pomogailo, Izv. Akad. Nauk. SSSR Ser. Khim., 1145 (1998) 136. G.I. Dzhardimalieva, A.D. Pomogailo, S.P. Davtyan, V.I. Ponomarev, Izv. Akad. Nauk. SSSR Ser. Khim., 1531 (1988) 137. F. Cotton, J. Wilkinson, Modern Inorganic Chemistry (Mir, Moscow, 1969) 138. R.C. Mehrotra, R. Bohra, Metal Carboxylates (Academic Press, London, New York, 1983) 139. B.S. Selenova, G.I. Dzhardimalieva, M.V. Tsykalova, S.V. Kurmaz, V.P. Roshchupkin, I. Ya. Levitin, A.D. Pomogailo, M.E. Vol’pin, Izv. Akad. Nauk. SSSR Ser. Khim., 500 (1993) 140. I.A. Chernov, G.I. Dzhardimalieva, G.F. Novikov, A.S. Rozenberg, A.D. Pomogailo, in Structure and Dynamics of Molecular Systems, vol. 2 (Yalchik, 2002), pp. 258–261 141. G.F. Novikov, I.A. Chernov, G.I. Dzhardimalieva, A.D. Pomogailo, Condensation Medium and Interphases 7, 239 (2005) 142. I.A. Chernov, G.F. Novikov, G.I. Dzhardimalieva, A.D. Pomogailo, Vysokomol. Soedin. A 49, 428 (2007) 143. M. Shulga, I.V. Chernushevich, G.I. Dzhardimalieva, O.S. Roshchupkin, A.F. Dodonov, A.D. Pomogaylo, Math. AN. Ser. Chem 6, 1047–1051 (1994) 144. Yu.M. Shulga, O.S. Roshchupkina, G.I. Dzhardimalieva, I.V. Chernushevich, A.F. Dodonov, Y.V. Baldokhin, P.Ya. Kolotyrkin, A.S. Rozenberg, A.D. Pomogailo, Izv. Akad. Nauk SSSR, Ser. Khim., 1739 (1993) 145. A.T. Shuvaev, A.S. Rozenberg, G.I. Dzhardimalieva, N.P. Ivleva, V.G. Vlasenko, T.I. Nedoseikina, T.A. Lubeznova, I.E. Uflyand, A.D. Pomogailo, Izv. Akad. Nauk SSSR, Ser. Khim., 1505 (1998) 146. G.I. Dzhardimalieva, A.D. Pomogailo, Izv. Akad. Nauk SSSR, Ser. Khim., 352 (1991) 147. V.S. Savostyanov, A.D. Pomogailo, D.A. Krytskaya, A.N. Ponomarev, Izv. Akad. Nauk SSSR, Ser. Khim. 45; 353 (1986) 148. A.D. Jenkins, P. Kratochvil, R.F.T. Stepo, U.W. Suter, Pure Appl. Chem. 68, 2287 (1996)

Chapter 8

Properties and Basic Fields of Application of Metal-Containing Polymers

The properties of metal-containing (co)polymers and also of traditional polymers modified by them are determined in many respects by the potential ability of metal ions to form ionic and coordination cross-links, to realize electron transitions in metal atoms under both electric field effect and high-energy radiations, to show cohesive and adhesion interactions.

8.1 Improvement of the Polymeric Materials Properties Based on Cross-Linking Action of Monomeric and Polymeric Salts As was shown above, various cross-linking mechanisms with participation of the considered compounds are possible: (co)polymerization of monomeric salts; additional interchain coordination interaction of a metal ion with an electron-saturated heteroatom; add-polymerization of residual double bonds, and, lastly, by means of the formation of aggregates and multiplets in ionomers molecules as nodes of a physical network. Cross-linking at photopolymerization of metal acrylates in a gelatine matrix (R, R0 – gelatine macromolecules) occurs by the addition mechanism to a double bond [1, 2]: ~R–NH2 + CH2=CH–CO–O–M–O–CO–CH=CH2 + H2N–R~ ~R–NH–CH2–CH2–CO–O–M–O–CO–CH2–CH2–NH–R ~ ~R'–OH + CH2=CH–CO–O–M–O–CO–CH=CH2 +HO–R' ~ ~R'–O–CH2–CH2–CO–O–M–O–CO–CH2–CH2–O–R' ~

Substantial increase in thermo- and heat-resistance of metal-containing carboxylated (co)polymers is observed at their cross-linking in comparison with “freemetal” analogues. The value of the destruction temperature .Td / of metal-containing macromolecules frequently is 300–400ıC and higher. So, Td for Li and Na [3] polymethacrylates are equal to 457 and 491ıC accordingly. Decomposition of Co(II), Ni(II) and Zn(II) polyacrylates occurs at the same high temperatures (Table 8.1) [4]. Mass losses up to 210ıC are caused by the liberation of methanol (solvent at polymerization), occluded in polymers. The second endothermic peak refers already A.D. Pomogailo et al., Macromolecular Metal Carboxylates and Their Nanocomposites, Springer Series in Materials Science 138, DOI 10.1007/978-3-642-10574-6 8, c Springer-Verlag Berlin Heidelberg 2010 

217

Residue of solvent (MeOH) release Decomposition

Co(II) polyacrylate

Residue of solvent (MeOH) release Decomposition a The differential thermal analysis

Residue of solvent (MeOH) release Decomposition

Zn(II) polyacrylate

Ni(II) polyacrylate

Process

Polymer

Cu2C and the initial sorption rate was equal to 33.06, 12.92 and 12.53 mg/g min accordingly [122]. But at that, presence of the IA residues in the copolymer results in an increase in the initial rate of removal of Cu2C in comparison with the homopolymer AMPS (5.27 mg/g min). Copolymers of cellulose-gr-acrylic acid [123, 124] and also hydroxyethylcellulose-gr-poly(acrylic) acid [125] have selective character of sorption to Pb2C ions in a mixture of the above mentioned cations. It is interesting to note, that double increase in the Cu2C ions concentration in the initial mixture of metal ions (at the equal molar ratio of the remaining metal ions) results in the reduction of sorptive capacity to Pb2C and Cd2C ions (0.49 mmol Cu2C =g, 0.43 mmol Pb2C =g and 0.26 mmol Cd2C =g), that can means that sorbent becomes selective with respect to Cu2C ions. Studying of the mechanism and kinetics of sorption of metal ions by polymeric hydrogels testifies that removal of metal is the very fast process, adsorption equilibrium is reached quickly enough, and linkage of metal occurs on the adsorptive, ion-exchange or chelated mechanisms. So, sorption isotherms of Ni2C [126] or Cr6C [127] ions by the hydrogels of poly(acrylamide--acrylate Na) or polymethacrylate Fe(III) [128] are well described in linearized Langmuir coordinates: Ce Ce 1 C D ; qe Q0 b Q0

(8.3)

where Ce is the equilibrium concentration of metal ions in a solution (mg/L), qe is the content of a metal in a sorbent (mg/g), Q0 is the maximum sorption (mg/g), b is the Langmuir constant (L/mg). As can be seen from Table 8.7, received graphically values of constants of sorption isotherms for the Ni2C -poly(acrylamide-co-acrylate Na) system are in accordance with the data of regression analysis. It is typical that sorptive capacity of the gel grows with an increase in a mole fraction of Na acrylate in a copolymer; at 44 mol% content of Na acrylate 84% of Ni2C ions is linked for the solutions with the initial concentration equal to 20 mg/L. However absorption degree of Ni2C ions reduces for higher concentration. The ability to form a ternary polymer–metal ion complex was revealed for the copolymer of sodium acrylate with maleic anhydride and diethylenetriamine during adsorption and division of trace quantities of Au3C , Ru3C , Bi3C and Hg2C

240

8 Properties and Basic Fields of Application of Metal-Containing Polymers Table 8.7 Isotherm constants Ni2C -ions sorption with poly(acrylamide-cosodium acrylate) hydrogel at different temperatures Temperature (ı C) Q0 (mg/ga ) b (L/mga ) 30 4.52 (4.03) 0.646 (0.599) 40 4.30 (4.31) 0.283 (0.281) 50 3.41 (3.42) 0.258 (0.253) a

In brackets the data of regression analysis are given

ions from aqueous solutions that was expressed in sufficiently high sorptive capacity with respect to the above mentioned metal ions (220, 105, 155 and 176 mg/g, accordingly) [129]. The technique of molecular recognition is especially suitable for the obtaining of such type sorbents [130–133]. It is known, that such systems reveal, for example, high substrate selectivity, at the same time they can have low sorptive capacity. In this connection, metal-containing monomers give supplementary functionalities for template effect at linkage of metal ions or organic molecules [134–137]. (Co)polymerization of metal (meth)acrylates in the presence of a cross-linking agent with the subsequent removal of a metal by a suitable eluent results in the formation of a cross-linked copolymer with preservation of the favorable for complexation with the given ions conformation of a macromolecule of an initial metal-containing copolymer. Cross-linking M1

M1

M1 + M2 + M3 ...

M1

M1

M1

"template" polymer

M1 M1

Removing M1

M1

M1

According to this schema, the cross-linked macroporous copolymers of Cu(II) methacrylate and their macromolecular templates, having selective sorption properties with respect to Cu(II) ions, were obtained (Table 8.8) [135]. Bonded “own” ions make 60–70% from initial content of ions in a copolymer and their selective adsorption is realized from sufficiently diluted solutions (103 –105 M). It is important for concentration and extraction of metal ions at their content, for example, in the polluted waters at the level lower than the detecting level. The original method of the precipitation polymerization [138] was offered for the obtaining of the metal-template polymer at copolymerization of Cu(II) methacrylate with dimethacrylate of ethylen glycol (DMEG); the (DMEG:Cu(MAA)2 ) molar ratios were varied from 2 up to 14. Polymerization was carried out in isopropanol with the use of a rotary evaporator for the formation of homogeneous microspheres with the sizes from 1 up to 4 mm in dependence on the polymerization

8.4 Sorption Properties of Metal-Containing (co)Polymers

241

Table 8.8 Effective sorption capacity (in ( mol/g) of “copper tuned” and analogous “untuned” polymers cross linked by ethylene glycol dimethacrylate with respect to the metal cationsa Copolymers Zn2C Cd2C Pb2C Cu2C Copolymer of [Cu(OCOC(CH3 /DCH2 / 2 H2 O (1) 10:3 7:4 15:5 45 Untuned polymer 1 4:6 4:6 16:5 12.5 Copolymer [Cu(OCOC(CH 3 /DCH2 / 2 Py (2) 9:7 7:8 15:2 49 12:4 5:7 6:7 52 Copolymer [Cu(OCOC(CH3 /DCH2 / 2 VPy (3) Untuned polymer 3 3:2 4:1 12:6 30 a Sorption capacity: salt concentration, 4:08  103 mol=L, 25ı C, 2.5 h, pH 4.7

Fig. 8.17 SEM image of Cu(II)-template polymer microgranules prepared at different crosslinking agent concentrations: (a) 2, (b) 6, (c) 10 and (d) 14 molar ratio DMEG:Cu(MAA)2 . Initiator concentration: 4 wt% AIBN, monomer concentration: 14 wt/vol% of solvent

conditions (Fig. 8.17). Sorptive capacity of the template polymer and selectivity were determined after removal of Cu(II) ions from the cross-linked copolymer. Adsorption equilibrium was reached in 10 mines and absorption of a sorbed ion was 90% from the initial contents. Maximum sorptive capacity with respect to the Cu(II) ions was 0.331 mmol/g, that was 40–200 times more than linkage of other ions and exceeded by order capacity of a non-arranged cross-linked hydrogel. The opportunity to use the considered type metal-containing copolymers for creation on their basis of selective sorbents for radionuclides seems to be suitable. Using the above described approach and proceeding from the corresponding monomeric salts, it is possible to obtain the cross-linked copolymers, prearranged, for example, to the Sr2C [136] or U4C [139] ions:

242

8 Properties and Basic Fields of Application of Metal-Containing Polymers O

Cl

O

OH2

U

O

O H2O O

Copolymerization with crosslinking

O

O Cl

Cl

Removing UO22+

OH Cl

O

O H2O

O

OH2

U

O

O

O

Cl

HO O

Cl

Repeated linkage of uranyl ions in the presence of a strong complexing ions reveals high selectivity of a template copolymer with respect to UO2 2C : the factor of selectivity (ratio of the quantity of a sorbed “own” ion to the quantity of a “strange” ion) is equal, for example, in case of competing ions, to Cu2C 8.8, VO2C 3.8, Al3C 8.6, Fe3C 8.1, Th4C 2.7 [139]. Sorptive capacity decreases, as a rule, with an increase in the cross-linking degree. So, there is an optimal area of compositions when sorption properties are revealed most efficiently for the copolymers of Sr2C acrylate with ethyleneglycol dimethacrylate [136]: the factor of selectivity is 20–27 at the content of a crosslinking agent 42–53 mol% (Table 8.9). Probably, at higher M2 content in a copolymer, decrease of selectivity occurs because of change of the mechanism of ions sorption which is carried out not only by the “prearranged” centers, but also by others, in particular, ester groups by the coordination mechanism. It is interesting, that triple copolymerization137 (M3 – styrene) with participation of strontium diacrylate and DMEG is accompanied with a decrease in a copolymer yield with an increase in M3 part in a comonomeric mixture, and a decrease in Table 8.9 Sorption properties of the copolymers of Sr.OCOCHDCH2 /2 (M1 ) with dimethacrylate ethyleneglycol (M2 ) Metal concentration after Copolymer Amount [Sr] in the sorption (mg-equiv/g) composition (%) initial sorbent M1 61 58 49 46 18 7

M2 39 42 51 54 82 93

(mg-equiv/g) 6.03 5.73 4.84 4.5 1.78 0.68

[Sr] – 2.74 3.07 1.23 0.54 0.80

[Ba] – 0.10 0.14 0.06 0.78 0.96

Selectivity factor – 27.4 21.9 20.5 0.69 0.83

8.4 Sorption Properties of Metal-Containing (co)Polymers

243

Table 8.10 Copolymerization of Sr(CH2 DCHCOO)2 (M1 ) with dimethacrylate ethyleneglycol (M2 / and styrene (M3 ) (70ı C, ethanol, 2 mol% AIBN) Metal concentration Composition of after sorption monomer Amount [Sr] in (mg-equiv/g) mixture (mol%) Copolymer copolymer M1 9 27 52

M2 16 19 22

M3 75 54 26

yield (%) 32 64 92

(mg-equiv/g) 0.91 3.75 4.96

[Sr] 0.50 0.83 0.96

[Ba] 0.32 0.55 0.70

Selectivity factor 1.5 1.5 1.3

sorption properties of a copolymer and reduction of its selectivity (Table 8.10). It is possible to suppose, that corresponding optimization (by cross-linking degree, the nature of a cross-linking agent and the third comonomer, and also by conditions of sorption, medium pH) of this method will allow to obtain effective sorbents for the linkage of strontium, including radionuclide, at presence of significant excess of accompanying ions (Ca, Ba, Na, To, Mg, etc.). With the purpose of creation of polymeric sorbents, proof against effect of climatic factors and corrosive mediums, the method allowing to obtain the “prearranged” sorbents on the basis of inert bearers such as polyethylene was developed [136]. This method is based on graft polymerization and copolymerization of strontium diacrylate to a surface of a polyethylene-powder. This powder was exposed to  -irradiation 60 Co (a radiation dose is equal to 10 Mrad) on air for creation of the radical centers initiating graft copolymerization of Sr.CH2 DCHCOO/2 with DMEG (333–353 R, methanol, 25 mol.% DMEG). Under these conditions, the content of grafted Sr2C was equal to 1:2  104 g-equiv/g PE, that corresponds to the ˚ A sorbent in which the grafted layer is “prearthickness of a grafted layer 30–50 A. ranged” to the strontium ions is formed after removal of the initial Sr2C . However, sorptive capacity of such polymer is low (30–40% from the content of the grafted strontium diacrylate) because of a small specific surface of the PE-powder. Temperature dependence of linkage of Sr2C by such polymer is shown on Fig. 8.18. It can be seen that sorptive capacity of the polymer grows and reaches the optimal value with rise in temperature. Apparently, use of the polymers with the developed surface as substrates, and also optimization of processes of graft homo- and copolymerization’s for the formation of the graft layer can improve sorption properties of such graft polymers. In this connection, polymeric hydrogels, sorption properties of which are easily controlled, in dependence on composition of copolymers, swelling degree, thicknesses of a cross-link, etc are especially attractive for utilization of radioactive nuclides. Adsorption of uranyl ions on poly(acrylamide-co-acrylic acid) increases with an increase in the content of acrylic acid in the hydrogel and concentration of uranyl ions [140]. The adsorption isotherms have S shaped character (Fig. 8.19) and sorption values are 70–320 mg of UO2 2C =g and 70–400 mg of UO2 2C =g from

244

8 Properties and Basic Fields of Application of Metal-Containing Polymers

Fig. 8.18 The dependence of the amount of bounded Sr2C -ions on temperature (sorbent PE-grPAA)

[Sr]fx × 105, mol / g 1.6

1.4

1.2

1.0

0.8 10

30

50 T, °C

70

90

800 700

qe(mgUO2+g–1)

600 500

3

400 2 300 1 200 100 0 0

500

1000

1500

C°(mgUO2+L–1)

Fig. 8.19 The adsorption isotherms of uranyl ions from aqueous solutions of uranyl nitrate onto poly(AAm-co-AA) hydrogels at pH 7.0 and 25ı C. Initial molar ratios of AAm/AA 30/70 (1), 20/80 (2), and 15/85 (3)

the solutions of uranyl-nitrate and uranyl-acetate accordingly, in dependence on the content of acrylic acid in the hydrogel. As a whole, sorption properties of the polyacrylamide hydrogels on the basis of copolymers of unsaturated carboxylic acids with respect to the ions of heavy metals are in correlation with their ability to swell in water [140–142].

8.5 Catalysis by Macromolecular Metal Carboxylates

245

8.5 Catalysis by Macromolecular Metal Carboxylates Polymeric metal-containing complexes are widely used as immobilized catalysts of various processes [143,144]. Immobilization of metal-containing complex catalysts on the polymeric bearers allowed to raise their stability and selectivity, to simplify stages of division of a product and a catalyst in many cases. Recent tendencies of development of catalysis by polymer-linked metal-containing complexes and also a specific role of macroligands (including macroligands with carboxylated functions) in the catalyzed processes, have been analyzed in detail in the recent reviews [145, 146]. Data concerning catalytic properties of macrocomplexes on the basis of polymeric acids and macroligands with carboxyl groups in various reactions are covered sufficiently fully in numerous monographs and reviews [144, 147, 148]. Therefore, the basic attention will be focused on some catalytic reactions with participation of metal-containing polymers obtaining by homo-copolymerization of unsaturated carboxylic acids. As has been noted above, traditional methods of immobilization of metalcontaining complexes are multi-stage; the processes accompanying them can be complicated by a lot of conversions that results in composition heterogeneity of the formed products. It is possible to overcome these restrictions by use of the heterogeneous catalysts obtained by (co)polymerization of metal-containing monomers [149]. The spectrum of the reactions catalyzed by such catalysts is very wide – hydrogenation of alkenes and functionalized olefins, oxidation of various substrates, polymerization of alkenes and alkynes, etc. Products of polymerizations of the unsaturated carboxylates of d -elements are especially suitable in this class of macromolecular complexes. As a rule, they are effective in the same reactions which are catalyzed by usual metal-containing complexes, though the presence of such unusual ligands in a coordination sphere causes specificity of their behavior. Moreover, as it was noted in Chap. 4, many of metal carboxylates, first of all on the basis of dicarboxylic acids, are supramolecules with a structure from unidimensional chains, two-dimensional layered and three-dimensional coordination polymers:

O

M O O OO O M M O OO O

O M

O

M O

O

O M O

O

O

M

M OO

OO O M

O M

O O M O

O O

M O O M

M M OO O

OO O M

O O M M O

246

8 Properties and Basic Fields of Application of Metal-Containing Polymers

As a rule, the products formed are insoluble in the usual reaction mediums, and thus, can potentially be used as heterogeneous catalysts. In the last few years, such compounds have been considered as an independent class of fixed metal-containing complexes, so-called self-supported catalysts [150, 151]. They are highly organized functional organometallic ensembles in which metal centers are connected among themselves by means of polydentate ligands with the help of coordinate bonds. So, the fumarate complexes of rhodium ŒRh2 .trans-O2 C–CHDCH–CO2 /2 n are very active in catalytic hydrogenation of olefines [152, 153]. In principle, bimetallic complexes of this type can also be obtained. For example, such complexes as Rh-containing carboxylated polymers with metal-porphyrinic units [154, 155], which are catalysts of hydrogenation of propylene, ethylene, 1-butene. Synergetic effect of a macrocomplex action is revealed in these reactions. It lies in the fact that both metal centers work. Activation of hydrogen atoms occurs on binuclear Rh centers, while an increase in local concentration of olefin in micropores occurs due to coordination of a metal atom of a porphyrin ring with olefin. Thus, the opportunity of designing active centers on a molecular level at the obtaining of coordination polymers is an effective way for the synthesis of multimetal heterogeneous catalysts. The increased thermal stability and stability to corrosive medium of the considered metal-containing polymers allow us to use catalytic systems on their basis in hard regime conditions (for example, at elevated temperatures, in oxidizing atmosphere). We shall consider several examples of reactions with participation of metal-containing polymers.

8.5.1 Catalytic Reactions of Oxidation of Hydrocarbons On the one hand, these reactions reveal features of the oxidizing catalysis under the action of polymer-immobilized complexes, and on the other hand – they have many common features with enzyme catalysis since they proceed at low temperatures, demand low quantity of a catalyst and have high selectivity [156]. For example, oxygen oxidation of cyclohexene is the model reaction for research of the mechanism of catalytic and not-catalytic oxidation of olefines. Oxidation of cyclohexene proceeds on two parallel routes: with participation of >CDC< or DC–H bonds:

(8.4)

The main products of liquid-phase oxidation are 2,3-epoxycyclohexane, cyclohexene-1-ol-2 and hydroperoxide of cyclohexyl (HPCH). The mechanism of this reaction was investigated in detail by the example of Co2C complexes, connected with various types of carboxyl-containing polymers [157, 158]. At use of Co.AcAc/2 as the catalyst (Fig. 8.20), the process is characterized by

8.5 Catalysis by Macromolecular Metal Carboxylates Fig. 8.20 Oxidation of cyclohexene in the presence of cobalt-containing catalysts: (1) Co(AcAc)2 , (2) PE-gr-Co2C -PAA, (3) copolymer of cobalt acrylate with styrene

247

Adsorption of O2, mol / L 0.4 1

2

3

0.3

0.2

0.1

0

Fig. 8.21 Inhibition of cyclohexene oxidation with dimer Ph–Ph. Œcyclohexene

D 5 mol=L, catalyst PE-grCo2C -PAA, 328 K, [Ph–Ph], mol/L: 1:7  104 (1, 3, 4), 3:4  104 (2), 1:7  105 (5). Arrows indicate the moment of inhibitor adding

20

40

60 80 time, min

Adsorption of O2, mol / L 5 4 5

4 3

3 1

2

2

1

0

20

40

60 time, min

the induction period with the subsequent fast attainment of the maximum rate and its further decrease. The induction period is absent at catalysis by macrocomplexes, oxidation rate remains constant, catalysts are active up to deep oxidation rates and can be used repeatedly after their separation from the reaction medium. The composition of the formed products is almost identical, the main products are HPCH, cyclohexenone, cyclohexenole and cyclohexene oxide. Oxidation of cyclohexene by such systems is the heterogenic-homogeneous chain process accompanied by release of radicals into volume. It was confirmed by a method of inhibitors: by introduction of an acceptor of free radicals – dimmer of 1,2-bis(4,40dimethylaminophenyl-1,2-diphthaloylethane (Ph–Ph) – into a system. Addition of an inhibitor on the part of stationary development of the reaction (Fig. 8.21) results in occurrence of the braking periods independent of depth of oxidation.

248

8 Properties and Basic Fields of Application of Metal-Containing Polymers

Calculations show that the initiation rate at the initial moment of time is 30 times less than on the stationary part. Hence, superficial processes determine rate of generation of chains; free radicals are not formed in volume. In the beginning of the reaction, when ROOH are absent in the system, free radicals are formed by the reaction of the polymer-linked cobalt with oxygen: ] – [Co3+ ...O•–O–]2+ RH

]–Co2+ + O2

]– Co2+ + R• + HO•2.

Decomposition of the co-coordinated ROOH introduces the main contribution to the initiation of chains in the developed process: ]– Co2+ + n ROOH

k

]–Co2+...n ROOH

kj

RO• + ]– [Co(OH)]2+ + (n –1) ROOH

K is the equilibrium constant Initiation rate is determined by the equation wj D

kj KŒROOH n0 ŒCo2C 0 : 1 C KŒROOH n0

(8.5)

The main kinetic parameters of oxidation under the action of macrocomplexes are given in Table 8.11. Because of the fact that a small increase in the parameter of oxidability (for non-catalytic oxidation of cyclohexene by oxygen at 303 K k2 k61=2 D 2:3 L1=2 =mol1=2 c1=2 / cannot provide formation of significant amounts of oxidation products, it is supposed that they are formed also as a result of a reaction of linear chain termination with participation of fixed Co2C complexes. The K value (>102 L=mol) and independence of wi on [ROOH] allow us to make the important conclusion that high local concentration of hydroperoxide is created near the surface of the metal-containing polymeric catalyst. Transport of ROOH to the active centers is carried out as a result of its migration on the surface of a

Table 8.11 The kinetic parameters of cyclohexene oxidation in the presence of macromolecular metal carboxylates

Catalyst PE-gr-Co2C PAA Copolymer of cobalt(II) acrylate with styrene

ŒCo2C  103 (gatom/L) 0.88 10.00

1=2

6

wi  10 (mol/L s) 6.5 2.8

4

w  10 (mol/L s) 1.4 0.97

2C

wŒCo  102 (s1 ) 160.0 9.7

k 2 k6 (L1=2 =mol1=2 s1=2 ) 5.4 5.8

8.5 Catalysis by Macromolecular Metal Carboxylates

249

C, mol / L 1.5 COOH

1.0

0.5

C OH, C 0

C O C

O 100

300

500

700 time, min

Fig. 8.22 The kinetics of cyclohexene oxidation with molecular O2 in the presence of metallopolymer catalyst based on copolymer of Ni(II) acrylate (64 mol%) and styrene. CNi D 4 mol=L, PO2 D 1 atm, 333 K

catalyst, and only after decomposition of ROOH the formed radicals leave into a volume where chain radical oxidation of cyclohexene develops [159]. The main problem in such processes is an essential increase in selectivity of a reaction. The main product of oxidation of cyclohexene at presence of the copolymer of Ni2C acrylate and styrene [160] is cyclohexenylhydroperoxide, its content is about 90% of total amount of products (Fig. 8.22). Besides cyclohexenylhydroperoxide, slight amounts of cyclohexenone, cyclohexenole and cyclohexene oxide are formed. High selectivity of effect of Ni(II)-copolymers, is connected probably, with the fact that process of generation of active centers is suppressed at the investigated reaction conditions due to decomposition of hydroperoxide at participation of a transition metal compound, as it was considered above, by the scheme of reactions (8.6). However, the final mechanism of such processes is not clear and, probably, requires further research.

8.5.2 Reactions of Peroxidase Decomposition Undoubtedly the diluted H2 O2 solutions are ecologically profitable oxidizing agents besides atmospheric oxygen for large-scale processes. The traditional approach

250

8 Properties and Basic Fields of Application of Metal-Containing Polymers

to the estimation of their activity is the comparative study of the model reaction of disproportionation of hydrogen peroxide on homogeneous and heterogenizated complexes. The general equation for the initial rate of disproportionation of hydrogen peroxide includes rates of parallel processes of non-catalytic [on walls, in a liquid phase and on a surface of a polymer (Wo )] and catalytic decomposition (k is the constant of its rate) on the metal ions: W D Wo C kŒMnC ŒH2 O2 :

(8.6)

The contribution part of each of these processes into the total reaction rate of disproportionation of hydrogen peroxide at presence of the macromolecular metalcontaining complex of Co2C polyacrylate [160] is presented below: W  103 (mol/L min) In homogeneous medium and on the wall of vessel 0.79 On the surface of polymer 0.41 With Co(II) polyacrylate 3.17 Catalytic activity of metal polyacrylates changed in the series: Co2C .3:45/ > Cu .1:57/ > Mn2C .1:23/ > Fe3C .1:01/ > Cr3C .0:93/ > Ni2C (0.75) (values of the reaction rate are given in brackets, k 102 , min1 ). Comparative analysis of catalytic properties of polymeric metal-containing complexes with their low-molecular analogues has shown that metal polyacrylates have stability of effect without loss of activity in repeated experiments (Fig. 8.23) and can be easily isolated from the reaction medium and used again. 2C

Conversion, %

40

30

1

20

2 3 10

0 0

5

10

15

20

25

time, min

Fig. 8.23 Decomposition of H2 O2 in the presence of Co(II) polyacrylate: first (1), second (2) and third (3) cycles at 313 K, V D 20 ml, ŒH2 O2 D 0:115 mol=L, mcat D 0:002g

8.5 Catalysis by Macromolecular Metal Carboxylates

251

Table 8.12 Decomposition of H2 O2 in the presence of copolymers of Co(II) maleates and styrene (V D 20 mL, mcat D 0:002 g, 25ı C) Co(II) hydromaleate (M1 ) – styrene Co(II) maleate (M1 ) – styrene M1 , mole fraction 0:20 0:38 0:59 0:33 0:44 0:52 1:98 2:90 10:38 2:75 8:64 11:61 k  103 (min1 ) Ea (kcal/mol) 14:62 6:02

Catalytic activity of the copolymers of both maleate and hydromaleate of cobalt in the decomposition reaction of hydrogen peroxide grows with an increase in MCM part in the copolymer (Table 8.12), rate constants of decomposition of hydrogen peroxide for the copolymer of cobalt maleate are higher in comparison with the copolymer of hydromaleate salt. Higher catalytic activity of the copolymers of cobalt maleate in comparison with Co(II) hydromaleate is confirmed also by the values of activation energy which are equal: Ea D 6:02 kcal/mol for the copolymers of styrene and cobalt maleate (molar fraction of in the polymer is equal to 0.44) and Ea D 14:62 kcal/mol for the copolymer of cobalt hydromaleate (molar fraction of MCM in the polymer is equal to 0.38). Such distinction in properties of the complexes under consideration is connected, probably, with considerably distinguished ligand environment of a metal atom. As it was shown in Chap. 4, the coordination polyhedron of cobalt in the molecule of Co(II) maleate is a little bit distorted octahedron in which the acid residue is connected with the metal ion with the help of two oxygen atoms of both carboxyl groups forming a chelated seven-membered cycle.

8.5.3 Other Catalytic Reactions Polymerisation of vinyl monomers occurs efficiently under action of the initiator – copolymer of styrene with Na acrylate or MMA with Na methacrylate in aqueous solutions at 85ı C in absence of usual initiators [161]. It was offered [162] to use Zr methacrylate of general formula Zr4 ŒOCOC.CH3 /D CH2 10 O2 X2  nH2 O.X D OH , CH2 D C.CH3 /COO etc.; n D 2; 4) as the catalyst of block radical polymerization of vinyl monomers. Acyloxy-derivatives of titanium, di- and tributoxytitanium butylmaleates Tin On1 .OCOCHD CHOCOC4 H9 /m .OC4 H9 /2nC2m (n D 3, 5, 8; m D 1:5, 2.5, 3.0, 4.0, 6.0), are nor only epoxy hardeners, but they also improve apparently physical-mechanical characteristics of the formed polymers [163, 164]. Na-salt of fumaric, maleinic and itaconic acids were investigated as the alternative ecological catalysts of the etherification reaction of 1,2,3,4-bytene-tetracabonic acid and cellulose instead of traditionally used sodium hypophosphite at production of fabrics [165].

252

8 Properties and Basic Fields of Application of Metal-Containing Polymers

The copolymer of styrene and porphyrin acrylate of Fe3C (the cytochrome model P450) reveals higher catalytic activity in the hydroxylation reaction of cyclohexane in comparison with unfixed Fe3C porphyrin [166]: H

H H2C

C

CH2 x

C y

O=C O

N

N Fe

N Cl N

It is supposed, that hydrophobic environment of the metal-containing porphyrin prevents formation of the inactive -oxo dimer owing to the polymeric chain. Thus, it is possible to obtain purposefully metal-containing complex catalysts of various processes, varying a metal nature, ligand environment in a coordination sphere of a metal and character of distribution of monomeric units in a metal-containing copolymeric chain. Polymerization and copolymerization of metal-containing monomers is the effective approach for the obtaining of the heterogenizated metal-containing complex catalysts. The properties of such catalysts can be controlled by changing of a geometrical and configuration structure, of distribution of metal-containing complexes in a chain while the traditional way of immobilized catalysts does not allow to affect on these factors. The tendency to use bimetallic catalysts containing metal atoms differing by catalytic functions was noted. Probably, the number of similar processes will increase, especially as synthetic opportunities allow us to design systems with controllable distances between such centers.

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Chapter 9

Monomeric and Polymeric Metal Carboxylates as Precursors of Nanocomposite Materials

The interest in the metal-containing polymeric nanocomposites is caused by a unique combination of properties of metals nanoparticles, their oxides and chalcogenides, and by mechanical, film-forming and other characteristics of polymers with opportunities for their use as magnetic materials for record and storage of information, as catalysts and sensors, in medicine and biology [1]. Homo- and copolymers of acrylic and methacrylic acids and their salts are widely used for the stabilization of metal-containing dispersions. For example, nanocomposites of the PbS/copolymer of styrene-methacrylic acid [2], PbS/copolymer of ethylene-methacrylic acid [3], CuS/polyvinyl alcohol-polyacrylicacid [4], Cu2C -polyacrylic acid/CdS [5], Co/PAAblock-PS[6] wereobtained by variousmethods.HeterometallicZnS/CdSnanocrystals with luminescent properties were synthesized by the treatment of the triple copolymer of styrene-Zn diacrylate-Cd diacrylate by the general reagent H2 S (Mn D 4:7  104 , atomic ratio Zn=Cd D 3:3 W 1) [7]. Such examples are very numerous. On the one hand, carboxylated compounds of a monomeric and polymeric structure can be molecular precursors of nanocomposite materials. On the other hand, carboxyl groups of macroligands are efficient stabilizers of nanoparticles; these functions are frequently developed together in one system. Amphiphilic character of carboxylated polymers and copolymers allows not only to encapsulate nanoparticles of metals or to combine them with polymeric and inorganic matrixes or biological objects, but also allows to give such properties as solubility in various mediums, ability to self-organization, etc., to nanoparticles.

9.1 Formation and Stabilization of Nanoparticles at Presence of Macroligands with Carboxyl Functional Groups Aggregative stability of particles in a polymeric matrix is defined by the processes of steric stabilization, flocculation, phase division, electrostatic interactions, etc. It was shown by AFM researches [8], that Van der Waals attraction forces act between two uncovered polymer surfaces of the yttrium-stabilized zirconyl (YSZ) nanoparticles at distance up to 200 nm, causing their aggregation. At the same time, the presence of an adsorbed layer of ammonium polyacrylate or polymethacrylate on the surface A.D. Pomogailo et al., Macromolecular Metal Carboxylates and Their Nanocomposites, Springer Series in Materials Science 138, DOI 10.1007/978-3-642-10574-6 9, c Springer-Verlag Berlin Heidelberg 2010 

257

258

9 Monomeric and Polymeric Metal Carboxylates as Precursors

of YSZ nanoparticles results in the occurrence of the repulsive forces between them at 35 nm distance between surfaces. At that, ammonium polymethacrylate provides stronger repulsion (4 nN at a distance of 25 nm), than ammonium polyacrylate, that, probably, is caused by an additional steric barrier due to CH3 -group. Conformation effects of a polymeric chain are especially sensitive to the reaction conditions, for example, pH. So, radiation-chemical reduction of the AgC ions in aqueous weak alkaline solutions is accompanied by their coloration into blue color in the presence of PAA [9–11]. In an alkaline solution, PAA molecules are unfolded into chains due to repulsion of COO-groups, and AgC ions receive an opportunity to interact with COO-groups. The formed “blue silver” has high stability even in air and can be isolated in a pure form at water evaporation. Sizes of the Co nanoparticles during high-temperature reduction of Co2C in the presence of the polymeric surfactant of PAA-block-PS were controlled by the [Co2C ]/[COOH] change or by the length of the PS block unit, by content of the co-surfactant and reaction time [6]. Concentration of PAA influenced essentially on the morphology of the Ag nanoparticles: dendritic Ag particles with the 400–500 nm diameter were formed at 0.1 mass% content of the polymer [12]. An increase in the PAA concentration up to 0.5 mass% resulted in the formation of the Ag particles of mainly spherical form (particle size is 160–200 nm). Spherical Cu nanoparticles were also obtained in the PAA-Cu2C films by the reduction in the H2 atmosphere at temperatures higher than 220ıC [13]. It is interesting that such nanocomposite films were less stable and Cu particles were subjected to the oxidation to Cu2C during several weeks, while reduction at temperatures higher than 230ıC was accompanied by the occurrence of keto-groups due to condensation of carboxyl groups and formation of the cross-links between PAA chains that provided higher stability of the formed Cu nanoparticles. As a whole, interactions of polymeric chains with a nanoparticle are various, they are differed by the nature and intensity and are frequently revealed simultaneously. Polymeric chains can form covalent1 [14–18], ionic or coordination bonds with the atoms of a surface layer of a metal at chemical adsorption. So, stabilization of silver nanoparticles in the presence of liquid-crystalline polymers containing cyanobiphenylic mesogenic groups and units of acrylic acid is carried out due to interaction of a macromolecule with a surface of a nanoparticle with the formation of various types of bonds (Fig. 9.1) [19]. It is interesting that stable dispersions of nanoparticles can also be obtained in monomeric acids or solutions of their salts. Deprotonated carboxylated groups (1,547, 1;437 cm1 ) were revealed in the IR spectrum of ZnS:Mn nanoparticles isolated from their dispersions in acrylic acid and stable till 8 months [20].

1

Recently, researchers more often turn to this type of bond at design of nanomaterials from molecular blocks [14–17]. With this purpose, nanoparticles are additionally functionalized, including functionalization by carboxyl groups, for their subsequent covalent linkage with other components. Alkane-thiolate-protected nanoparticle of gold, supplied with a carboxyl group, was covalently joined by such principle to the polylysine molecule with the formation of the conjugate of polymer-nanoparticles in the form of rings, loops, cycles, etc. [18].

9.1 Formation and Stabilization of Nanoparticles

259

2 1

C≡N

O

4

C ≡ N:-

O Ag

Ag

3

HO

δ+ δ O

−

Ag

Fig. 9.1 Scheme of the interaction of liquid crystalline polymer molecule with the surface of silver nanoparticle: (1) silver nanoparticle, (2) a mesogenic group, (3) polymer chain, (4) a carboxylic group

Interaction of carboxylated ligands with a surface of nanoparticles of titanium (IV) oxide was investigated with the use of the probe molecule of all-trans-retinoic acid [21]. Information about various forms of superficial bonds can be received from the excited triplet state of an acid molecule at photoinduction of recombination of a charge of an adsorbed superficial monomolecular layer [22]. It was shown, that the character of bonds of carboxyl groups with nanoparticles of titanium (IV) oxide depends on their sizes. At the reduction of the nanoparticles sizes from 6 nm to 8:5), when each charged group of a polyelectrolyte is in the neighborhood with a counter-ion, results in the break of hydrogen bonds. It is typical, that pH-induced changes of the nature of specific intermolecular interactions are accompanied by the transition of a colloid system from a transparent form in the strong-acidic medium to a lactic dispersion at pH  3:5 and again to a transparent solution at high pH values. We shall note, that just temperature and

9.1 Formation and Stabilization of Nanoparticles

261

Scheme 9.2 The type of complexes of carboxylic polymers with nanoparticles depending on pH

medium pH are the most attractive for biomedical purposes among many external influences (temperature, a solvent, light, pressure, ionic strength of medium, etc.) causing sensitive response of the so-called smart polymeric hydrogels. Heatand pH-sensitive nanocomposite was synthesized in situ by radical polymerization of N -isopropylacrylamide (NIPA) in the presence of the synthetic hectorite ŒMg5:34 Li0:66 Si8 O20 .OH/4 Na0:66 and linear polyacrylic acid [28]. The obtained hydrogels had a structure of a semi-interpenetrating organic-inorganic network and showed ability to change repeatedly their volume in response to the changing conditions (Fig. 9.3). It is important, that PAAm -(hectorite)n-NIPA (m=n D 2:5–3) show sufficiently high mechanical properties at the optimal composition. New opportunities are opened at the creation of the materials on the basis of hydrophobic-modified polymeric acids [29]. Octylamine [30, 31]- or dodecylamine [32]-modified polyacrylic acid is widely used for the encapsulation of nanocrystalline quantum dots (NQds), giving them high water solubility. Diblock copolymer of poly(styrene)-block-poly(acrylic) acid show the same effect in its adducts with carbon nanotubes; the obtained nanocomposites have good solubility in various mediums [33]. In all these cases, an amphiphilic polymer surrounds a nanoparticle forming a micellar layer around it. Using this methodology, it is possible to enter the formed NQds-polymeric complex into an inorganic matrix, for example, TiO2 [31], or to form layered nanostructures on various surfaces. One of these methods was offered in the work [34]. It is based on the ability of the CdS nanoparticles, capped by the polyacrylate-anion, to self-organization into layered ensembles with cationic poly(diallyldimethylammonium chloride) on a surface of solid substrates of silicon or quartz (Fig. 9.4). It is remarkable that Coulomb repulsions of the surface charges, arising after full compensation of the positive charges of a cationic polyelectrolyte by PAA-CdS

262

9 Monomeric and Polymeric Metal Carboxylates as Precursors

a

20°C 40°C

Wgel(t) / Wgel (eqs)

1.2 0.9 0.6 0.3 0 –12

0

12

24 36 Time (hr)

48

60

72

b

pH7 pH2

Wgel(t) / Wgel (eqs)

1.2 1 0.8 0.6 0.4 –12

0

12

24 36 Time (hr)

48

60

72

Fig. 9.3 Swelling behavior (wgel (t)/wgel (eqs)) of NIPA-hectorite-PAA nanocomposite gel vs. temperature (a) and pH (b) in cyclic experiments

– – – – – – + + – + – – – – + +– + + – + – – –+ + – – + – + + +– + + + – + + – – – – – – – – –

– – – +–

– – –+ + – –

Fig. 9.4 A model of layered assemble of the PAA-CdS nanoparticles embedded into poly(diallyldimethylammonium chloride) cationic polyelectrolyte

anions, promote formation of only monomolecular layer of nanoparticles in each act of deposition. An opportunity of synthesis of the similar type of interpolyelectrolytic complexes including metal particles was shown in a lot of research (see, for example [35]). It is emphasized, that interhelium interactions in such reactions have the electrostatic nature. Questions of the effective stabilization and modification of nanoparticles are closely interdependent with the development of their obtaining methods.

9.2 Basic Obtaining Methods of Metal-Containing Polymeric Nanocomposites

263

9.2 Basic Obtaining Methods of Metal-Containing Polymeric Nanocomposites on the Basis of Monomeric and Polymeric Carboxylates All known obtaining methods of the considered type of nanocomposites are reduced to the so-called bottom up synthesis, since nanoparticles in the nanocomposites are created from separate atoms. The primary majority of physical-chemical methods of “assembly” of nanoparticles give an opportunity to control the size and composition of nanoparticles and to obtain nanoparticles with sufficiently narrow size distribution. It is a complicated problem for, for example, alternative methods (top-down) – grinding and dispersion.

9.2.1 Thermal Conversions of Metal-Containing Carboxylated Precursors One of the perspective obtaining methods of metal-containing nanoparticles and their polymeric composites are thermal conversions of metal-containing monomers. It is possible to combine in situ formation of superfine metal particles and a stabilizing them polymeric matrix during these thermal conversions [36–40]. Metalcontaining polymeric nanocomposites on the basis of acrylates of Cu(II) [36], Co(II) [41, 42], Fe(III) [43], Ni(II) [44], their cocrystallizates [45] and also maleates of Co(II) [46] and Fe(III) [47] were obtained using such an approach. Microstructure of the formed composites is represented by the metal-containing nanoparticles of 5–30 nm diameter and close to spherical form, they are dispersed homogeneously in the polymeric matrix with average distance 10–12 nm [48] (Fig. 9.5). Uniformity

Fig. 9.5 TEM microphotograph (a) and diagram of nanoparticles distribution on the size (b) for the products of thermolysis of Co(II) acrylate at 643 K

264 Fig. 9.6 Distribution of metal-containing nanoparticles on the size. The products of thermolysis of metal carboxylates: (1) Fe(III) acrylate; (2) Fe.HCOO/2  2H2 O; (3) Co(II) maleate

9 Monomeric and Polymeric Metal Carboxylates as Precursors Nd / ∑Nd 0.3 3 2 0.2

1

0.1

0

8

16

24

32 d, nm

40

of distribution of the metal-containing particles in the matrix and their narrow sizes distribution, testifies, apparently, a big degree of homogeneity of processes of decarboxylation and formation of a new phase. It is important to note, that the average size of the particles, formed during thermal conversions of unsaturated metal carboxylates, is lower than for the products of thermal conversions of saturated metal carboxylates (Fig. 9.6) [49]. Systematic research of thermolysis of unsaturated metal carboxylates allowed to reveal community of character of their conversions, consisting in sequences of three basic macrostages [36, 41, 43, 44, 50]: 1. Dehydration of crystalline hydrates of monomers (Tterm < 423 K) with simultaneous reorganization of ligand environment, accompanying by separation of a part of carboxylated ligands. 2. Solid-phase polymerization of the reorganized dehydrated monomer (Tterm  453–493 K). 3. Decarboxylation of the formed (co)polymer at high temperatures (Tterm > 473 K). Main gas evolution and mass loss of a sample at thermolysis are connected with the last process. It is possible to estimate thermal stability of metal carboxylates by the relative strength of interatomic M–O and C–O bonds in their crystal-chemical structure. Lengths of M–O and C–O bonds within the limits of a coordination polyhedron can be essentially differed, that testifies their energy nonequivalence. Dentate ability of the fixed part of unsaturated ligands can be changed during dehydration and, similar to anhydrous carboxylates of saturated acids, [51] they start to carry out simultaneously both the role of a ligand and function of a lacking solvate in a crystalline structure. An increase in dentate ability of ligands results in distortion of oxygen surrounding of a metal with the respective change of M–O and C–O distances in the structure and, hence, in change of their strength. In particular, dependence of the fragment CH2 DCHCOO– ions yield vs. the electric field intensity in mass-spectrometric research at fragmentation of the ŒFe3 O.CH2 DCHCOO/6 C ion

9.2 Basic Obtaining Methods of Metal-Containing Polymeric Nanocomposites

265

Fig. 9.7 Mass spectra of the positive ions from aqua-alcohol solution of Fe(III) oxoacrylate at U D 200 V (a) and 400 V (b)

indicates [52] energy nonequivalence of the M–OOCCHDCH2 bonds in the Fe(III) oxoacrylate. In the mass spectrum at U D 400 V (Fig. 9.7), ions with m=z D 539, 468, 397 correspond to the separation of one, two and three CH2 DCHCOO-groups, and the ion with m=z D 341 corresponds to the separation of Fe.CH2 DCHCOO/3 molecule from the molecular ŒFe3 O.CH2 DCHCOO/6 C ion. During the study of the acidic maleates of the M.C4 H3 O4 /2  4H2 O composition (M D Mn, Fe, Co, Ni), dependence between structural and thermal characteristics of these connections was determined: with reduction of an ionic radius from Mn(II) cation to Ni(II) cation there is a reduction of interatomic distance between a metal cation and an oxygen anion and decrease in decomposition temperature occur in the series of bimaleates Mn, Fe, Co, Ni – 400, 355, 350, and 300ı C, accordingly [53]. It was revealed by thermogravimetry and DTA methods, that thermal conversions of maleate and fumarates of metals (M D Mn, Co, Ni) [54] and chromium(III) acrylates [55] include processes of dehydration and oxidative decomposition of carboxylaled ligands. Intermediate product of the anhydrous M3 [Fe(OOCCH=CHCOO)3 ] complexes (M D Li, Na, K) [56] in the 215–300ıC temperature range is Fe(II) maleate, with the subsequent formation of ”-Fe2 O3 and maleates/oxalates of alkali metals, and at 430–550ı C fine-dispersed particles of the corresponding ferrites are formed.

266

9 Monomeric and Polymeric Metal Carboxylates as Precursors h 1.0

1

2

3

a

4 5

0.8 0.6 0 –2.0

0.4

1.6 1 .103 T

b

–3.0

0.2 0

1.5

lgk 500

1000

1500

time, min

Fig. 9.8 The kinetics of thermal decomposition of Co(II) acrylate: degree of conversion  vs. time (a) at different temperatures: 663 K (1), 653 K (2), 643 K (3), 633 K (4), 623 K (5); and lgk vs.1/T (b)

Generally for acrylates and maleates of metals [57], kinetics of gas evolution vs. degree of conversion .t/ is satisfactorily approximated by the dependence (Fig. 9.8): .t/ D 1f Œ1  exp.k1 / C .1  1f /Œ1  exp.k2 /;

(9.1)

where  D t t0 (t0 is heating time of a sample, ˜1f D ˜.£/ at k2 t ! 0, k1 t ! 1, k1 , k2 – are the effective rate constants. Analysis of possible ways of chemical conversions of dehydrated metal carboxylates in the assumption of energy nonequivalence of M–O bonds and formation of the acrylic CH2 D CH–COO and maleic  OCOCH D CHCOO radicals in the primary decomposition act has shown, that these radicals initiate polymerization of a metal-containing monomer with the subsequent decarboxylation of metal-containing groups. Compositions formed during thermal conversion of metal acrylates and maleates solid products can be expressed as quotas of the C–H–Ofragments; the detailed calculations are given in the works [36, 39, 46, 58] MOz .CH2 CHCOO/px .CHCHCOO/qy .CH2 CH/x .CHCH/H ; .for acrylates/; (9.2) MOz .D CHCOO/2px .D CCOO/2qy .D CH/x .ı C/H ; .for maleates/; (9.3) where x D y D z D 0 (z ¤ 0 in case of acrylate and maleate of iron(III)), p and q are quantity of intrachain and end groups, depleted by hydrogen (p C q D 1), accordingly. The most probable way of the formation of metal oxides is the oxidation reactions.

9.2 Basic Obtaining Methods of Metal-Containing Polymeric Nanocomposites

267

M C 1 CO2 D MOz C .1  z/CO2 C zCO

(9.4)

M C 2 H2 O D MOz C .2  z/H2 O C zH2 :

(9.5)

Escaping CO can be spent in carbonization reactions as it was observed at thermolysis of the product of radical polymerization of nickel acrylate, Ni(II) polyacrylate [59]. 3Ni.s/ C 2CO.g/ D Ni3 C.s/ C CO2.g/ C  122 kJ=mol Œ60

.9:6/2

4Ni.s/ C CO.g/ D Ni3 C.s/ C NiO.s/ C  84 kJ=mol:

(9.7)

2

It can be seen in (Fig. 9.9), that absorption of CO is accompanied by an increase in the rate of CO2 accumulation. Growth of Texp results in the reactions rate increase [(9.6)–(9.7)], the consequence of this effect is the observed reduction of maximum of CO yield, its displacement towards early stages of thermolysis up to its disappearance. Evolutionary conversions of metal carboxylates during thermolysis were considered by the example of oxoclusters of acrylate [48] and maleate of Fe(III) [47, 61]. They are convenient model objects for studying of mechanism of formation of a short range ordering structure near iron atoms at thermal conversions as an initial stage of nanoparticles nucleation in metal-containing polymeric systems.

Fig. 9.9 The yield of gaseous products during thermolysis of Ni(II) polyacrylate at 573 K: (1) ’P;t , (2) ˛Co2 ; t , (3) ’CO;t , (4) ˛CH4 ; t , (5) ˛H2 ; t . Arrays indicate the moment of sampling of the products for mass spectrometry analysis

Hf ŒNi3 C.s/   C50:2 kJ=mol – is an estimation on the basis of Hı f ŒFe3 C.s/  D C25:1 kJ=mol and comparison of the series of formation heats of Ni(II) and Fe(II).

2

268

9 Monomeric and Polymeric Metal Carboxylates as Precursors

In particular, questions about an opportunity of inclusion of a metal-containing cluster group of a monomer into a formed at thermolysis polymer and character of their conversion during decarboxylation process are important. At which stages of thermolysis does the destruction of a ferriferous cluster as an initial stage of heterogeneous nucleation of nanoparticles in metal-containing polymeric systems occur? Is this process accompanied by the formation of metal–metal bonds, etc.? In the polynuclear oxocomplexe of Fe(III) maleate, elimination of three molecules of crystallization water and three molecules of maleic acid, accompanying by the reorganization of ligand environments of Fe atoms, occurs already at early stages of thermolysis. R1

R1 O

R1

c

R1

Fe

O

O o H

c

Fe O O

O

c c

O

o H

Fe

o

O

O

c R1

R1: CH = CHCOOH

O c

O

Fe O

O O

R1

O

O

c

O

c

Fe

O

O O

c

O

OO O

o

c c

c

c

Fe OO

+

c

O O

c

c c

c

Results of a mass-spectrometer research of the Fe(III) maleate (FeMal) also indicate on the opportunity of passing of such elimination. The peak with maximal size m=z 525 in the mass spectrum corresponds to the Fe3 OŒOOCCHDCHCOOC 3 ion. For studying of evolution of a short range ordering structure near Fe atoms during isothermal decomposition of FeMal, EXAFS3 -research of solid-phase intermediate products of thermolysis, isolated at various conversion degrees (Table 9.1, Fig. 9.10), was carried out. According to these data, dehydration and polymerization of a desolvated monomer with retention of a structure fragment of a metal-carboxylated [Fe3 OR6 ]cluster occur during thermal conversions of FeMal to the degrees of conversion corresponding to the FeMal-a and FeMal-b samples. However, changes of the EXAFS spectrum occur already at the initial stage of decarboxylation of the formed

3

The method is based on the phenomenon of diffraction of photoelectrons on the surrounding of the atom, absorbing X-rays. Diffraction is revealed as a long-range fine structure of the X-ray absorption spectrum (EXAFS) of the chosen atom. Separating an oscillating part of the EXAFS and applying Fourier transformation to it, it is possible to receive the Fourier-transformate module (FTM) which is a function of radial distribution of the surrounding of atoms near the absorbing atom accurate within phase corrections. Position (r) of the maximums of FTM, as a rule, corresponds to radiuses R of coordination spheres (CS) (R D r C ˛, where ˛ is the phase correction), their amplitudes ® are proportional to coordination numbers (N). Proportionality coefficient and ˛ value are determined on the basis of the analysis of the EXAFS-data of the suitable compounds with a known structure. Besides, R, N and  2 sizes (thermal dispersion of interatomic distance, the Debye–Waller factor) can be determined on the basis of selection of values of the specified magnitudes providing good conformity of the calculated and experimentally defined functions of the oscillating part of the EXAFS (fitting method).

9.2 Basic Obtaining Methods of Metal-Containing Polymeric Nanocomposites

269

Table 9.1 The structural data and FTM parameters for iron atoms in oxo maleate (III), products of thermolysis and standard compounds [61] FTM parameters Structural data The number of Compound j2  104 'j (rel. coordination (Texp ; m, 2 units). R rj (nm) Q (%) KC wt%) sphere, j j (nm) Nj (nm ) Fe(acac)3 1 0.147 3.6 0.202 6.0 0.27 2 Fe–O (0.200) (6.0) – 0.216 0.6 – – – s.m. 2 0.254 0.6 (0.295) (5.4) – Fe–C 3 0.292 1.2 (0.333) (4.2) – Fe–C FeMal (Troom ) 1 0.149 3.2 0.203 5.0 0.22 1.7 Fe–O 2 – – 0.194 1.0 0.33 Fe–Oa 3 0.293 1.2 0.329 2.0 0.23 Fe–Fe FeMal-a 1 0.155 3.00 0.205 4.0 0.39 0.3 Fe–O (393 K; 6.2) 2 – – 0.186 1.0 0.15 Fe–O 3 0.294 1.1 0.336 2.0 0.92 Fe–Fe FeMal-b 1 0.155 2.8 0.205 4.0 0.37 0.5 Fe–O (438 K; 21.7) 2 – – 0.182 1.0 0.35 Fe–O 3 0.300 1.0 0.342 2.0 0.70 Fe–Fe FeMal-c 1 0.155 2.2 0.207 3.5 0.52 2 Fe–O/C (513 K; 34.5) 3 0.294 0.3 – – Fe–O/C FeMal-d 1 0.152 1.5 0.205 2.5 0.53 1.2 Fe–O/C (643 K; 46.8) – 0.217 0.3 – – – s.m. 2 0.267 0.2 0.297 FeMal-e 1 0.150 1.5 0.204 3.0 0.67 1.7 Fe–O 0.219 0.5 0.246 0.3 0.31 2.2 Fe–Fe (643 K; 48.2) 2b s.m. 2 0.262 0.4 0.292 Fe–Fe FeMal-f 1 0.148 1.6 0.203 3.0 0.70 2.5 Fe–O (643 K; 54.2) – 0.225 0.3 – – – s.m. 2 0.266 0.5 0.296 Fe–Fe FeMal-g 1 0.147 1.9 0.199 4.0 0.74 0.9 Fe–O (643 K; 57.1) – 0.213 0.3 – – – s.m. 2 0.264 1.1 0.294 Fe–Fe ’-Fe2 O3 1 0.149 2.1 (0.196) 3.0 – Fe–O (0.208) 3.0 – Fe–O 2 0.261 2.1 (0.287) 1.0 – Fe–Fe (0.296) 3.0 – Fe–Fe Note: The data of X-ray diffraction studies are given in brackets. Q denotes the values of the objective function characterizing the adjustment accuracy a The distances between Fe and -O (bridging oxygen) b The peak comprises the Fe–Fe coordination sphere and the secondary maximum (s.m.) of the first coordination sphere

270

9 Monomeric and Polymeric Metal Carboxylates as Precursors

Fig. 9.10 Fourier transform module of EXAFS spectra of K-edge absorption for samples: (1) FeMal, (2) FeMal-a, (3) FeMal-b, and (4) FeMal-c

polymer. Amplitude of the FTM peak corresponding to the first CS decreases almost twice in comparison with the value for FeMal already for the FeMal-c sample, and the second basic maximum practically disappears that can be caused by the destruction of a trinuclear bridge and formation of the new phases containing a set of Fe–O and Fe–C distances. Systematic increase in the amplitude of the peak from r  0:264 nm during thermolysis is observed, it testifies oxidation of Fe atoms with an increase in thermolysis time. Thus, thermal conversions of unsaturated metal carboxylates allow to combine processes of synthesis of nanoparticles with their simultaneous stabilization by a formed decarboxylated polymeric matrix. Sufficiently narrow character of size distribution of metal-containing nanoparticles and morphological peculiarities connected with their spherical form are caused, most probably, by comparative homogeneity of processes of thermal conversion of monomeric carboxylates. It is also testified by evolution of topography of a solid phase during thermolysis of metal acrylates indicating on only partial heterogeneity in the region of macrodefects [62]. Oleates and octanoates of metals are the most frequently used as molecular precursors of nanostructured materials among others monomeric carboxylates. Thermolysis of these complexes in combination with surfactants and other reagents is usually carried out in a solution of high-boiling solvents (octadecane, octadecene, docosane, octyl ether, etc.). Doubtless advantages of thermal decomposition of carboxylated compounds in an inert solvent are the opportunity of the controlled synthesis of practically monodisperse nanocrystals with a high yield, narrow size distribution and high crystallinity [63–67].

9.2 Basic Obtaining Methods of Metal-Containing Polymeric Nanocomposites

271

General strategy of the obtaining of nanocrystals of semiconductor metal sulfides by the considered method consists in thermal decomposition of metal-oleates complexes in an alkane-thiol [68,69]. For example, monodisperse crystal Cu2 S nanoparticles with 18 nm (230ıC, 20 min), 15 nm (215ıC, 20 min) and 19 nm (215ı C, 60 min) sizes and various forms from spherical to disk-shaped were obtained at variation of temperature and time of a reaction and a molar ratio of oleylamine and dodecanethiol. Nanocrystals of ZnS, CdS, MnS and PbS were synthesized similarly. Stoichiometry of the initial reagents is the key factor of formation of PbS nanowires at thermolysis (280ıC, 1 h) of the precursor obtained from Pb.NO3 /2 /octanoate Na/ethylenediamine/dodecanethiol reaction mixture, an optimal molar ratio is 1:2:1:1.6 [70]. By special research it was shown [71, 72] that division of processes of nucleation and growth in a time or temperature scale is critical for the formation of monodisperse particles. Nonequivalence of a M–O bond and a structure of a molecular precursor play an important role alongside with such factors influencing on these processes as temperature and reaction time, concentration of reagents, etc. [61, 73]. Detailed studying of the Fe(III) oleate structure [74] by FTIR, element analysis, X-ray photoelectron spectroscopy and DSC methods revealed that not subjected to the special treatment after its obtaining Fe(III) oleate contains oleic acid in the composition which is connected with the monodentate oleinic ligand (Scheme 9.3) in the form of dimer. If this oleate (as prepared) is heated at 70ı C, removal of crystallization water and destruction of the dimer occur, and the Fe(III) complex in this form is thermally stable up to 380ı C. Additional extraction in ethanol or acetone results in full removal of oleic acid (Scheme 9.3), that affects very essentially on thermal behavior of the complex: an increase in nucleation temperature occurs and processes of nucleation and growth start to overlap, and, as a result, polydisperse particles are formed. We shall note, that the process of a nucleus formation is connected with removal of oleic acid or the monodentate oleinic ligand (200–240ı C), and

C

C O

O Fe O H2O O Fe O O O C

O O

C OH

C O

O C OH

C

O

O Fe

extraction

C2H5OH

Scheme 9.3 Removing oleic acid from Fe(III) oleate dimer complex

O Fe O O C

O

HOH5C2

272

9 Monomeric and Polymeric Metal Carboxylates as Precursors

dissociation of the residual ligands (300ıC) is connected already with the formation of ferric oxide nanoparticles [71]. The same laws and the formation mechanism of monodisperse nanoparticles are characteristic for thermal conversions of Co2C Fe2 3C -oleates complexes in 1-octadecene at 300ıC [73,75]. Prenucleation intermediates of CoFe2 O4 are formed in the 250–300ı C temperature range, but growth of nanocrystals is not observed at that; and concentration of the prenucleation centers increases sharply and they start to grow only at temperatures exceeding thermal decomposition of Co2C Fe3C 2 oleates complexes (300–320ı C). Fine control of temperature, rates and reaction time at this stage allows to influence efficiently both on sizes and form of nanocrystals (Fig. 9.11).

Fig. 9.11 TEM microphotographs of the CoFe2 O4 nanocrystalls obtained at temperatures 305ı C (a) and 314ı C (b) and 320ı C at heating of 0 (c), 5 (d), 60 (e), 120 min (f)

9.2 Basic Obtaining Methods of Metal-Containing Polymeric Nanocomposites

273

It is interesting, that it is possible to obtain nanocrystals of Fe(III) oxide of octahedral form at similar temperature conditions, but in the presence of trioctylammoniumbromide [76]. For the obtaining of MFe2 O4 (MDCo, Ni, Mn, Fe) nanocrystals from the corresponding oleates complexes, the determining factor is the fact that they have close decomposition temperatures. Otherwise, as, for example, for copper and indium oleates, thermolysis of a mixed complex results in segregation and formation of the nanocrystalline Cu2 S and In2 S3 heterostructures [77].

9.2.2 Polymer Carboxylate Gels and Block Copolymers as Reactors for Nanoparticles Introduction of ready metal nanoparticles into a polymeric matrix results frequently in change of properties of nanoparticles and in their aggregation and it does not allow to achieve high concentrations of a nanophase, problems of compatibilization appeared inevitably, etc. Sometimes methods of ex situ polymerization are used. According to this method, for example, silver nanoparticles obtained by the reduction in AgNO3 solution by ascorbic acid, were added into a solution of acrylic acid and poly(ethylene glycol)methylacrylate ether at presence of a cross-linking agent and were subjected to photopolymerization (UV-irradiation, 8 min) [78]. Alternative approaches (i.e., template-mediated synthesis) are based on the obtaining of metal nanoparticles in the medium of a polymeric (or monomeric) matrix in situ. Amphiphilic diblock copolymers, for example, polystyrene-block-polyacrylic acid in organic and aqueous solutions are widely used for the encapsulation of semi-conductor nanoparticles of metal sulfides at the stage of their formation [79, 80]. One of such examples is interesting by the fact that di- and three-block copolymers give unique opportunities for fine regulation not only of sizes of nanoparticles, but also regulation of morphology of metal-containing polymeric nanocomposites on their basis. Quantum dots of CdS were obtained in the micelles of the three-block copolymer of poly(ethylene oxide)-block-polystyreneblock-poly(acrylic acid) of various architecture [81]. At mixing of dihydrate of cadmium acetate with a diluted solution of the copolymer in THF spherical micelles are formed. They are so-called primary spherical inverse micelles (PSIMs) (10–50 nm), consisting of cadmium acrylate core surrounded by PS and then by PEO. Three kinds of populations of micelles with the average size equal to 40 nm are formed in an aqueous solution: mononuclear, multinuclear and aggregations of multinuclear micelles. Spherical micelles with a core from Cd-acrylate or CdS (after treatment with H2 S) are transformed into rod-like micelles at an increase in water content. A new multistructure with Cd-acrylate cores located in a PS matrix, surrounded by a PEO crown, is formed if PSIMs are transferred from THF into water before influence of H2 S and this architecture is retained after converting into CdS – a water-soluble supermicelle is formed. Basic parameters for such

274

9 Monomeric and Polymeric Metal Carboxylates as Precursors

micelle of PEO(45)-block-PS(150)-block-PAA(108) composition considering physical characteristics of copolymers (density, molecular weight, chain length) were calculated. It was founded that size of average diameter of nanocrystals and the supermicelle equal to 4.8 nm and 50 nm, accordingly; number of quantum dots in the supermicelle is 75, and number of CdS ion pairs in a quantum dot is 1,150. Another structure is observed at the replacement of THF by water in case of PSIMs with a cores from CdS. It is represented by micelles with a core from PS, and quantum dots of CdS are located in its crown. Quantum dots of CdS, in turn, are surrounded by PAA or PEO blocks. In contrast to other polymeric templates such as dendrimers or considered above block-copolymers, polymeric gels are sufficiently available because of simplicity of their synthesis and opportunities of easy functionalization. Indubitable advantage of many polymeric gels and nanocomposites on their basis is their biocompatibility. Due to this fact they can be used in medicine for creation of carriers of medicinal substances and for their transportation. Network structure of microgels providing probability of nucleation and growth of nanoparticles in each void and high sensitivity of these systems to changes of external factors also have important value. The general scheme of obtaining of metal nanoparticles in polymeric microgels can be represented as follows (Scheme 9.4): Reduction

n+ cool Me cool cool

-

n+ Me cool

Oxidation

cool n+ Me cool

Sulfidation

Scheme 9.4 The general scheme of metal nanoparticles synthesis in polymer gels

Various types of polymeric microgel nanocomposites containing metallic, magnetic, semi-conductor, ceramic and other nanoparticles have been developed at the present time [82, 83]. Properties located in microgels nanoparticles, such as structure, sizes, size distribution, polydispersity, and also morphology of nanoparticles and hybrid gels, level of doping of microspheres by nanoparticles are defined by the reaction conditions and composition of microgels. So, hydrodynamical radius of particles of the microgel of poly(N -isopropyl acrylamide-acrylic acid-2-hydroxyethylacrylate) [84] in the region of 2:3 < pH < 9:2 increases from 230 to 600 nm that results in an increase in the CdS content from 0.04 to 0.12 g/g. The CdS content practically linearly depends also on a mole fraction of acrylic acid in a hydrogel composition. Sizes of the synthesized in situ nanoparticles are small and monodisperse, for example, sizes of the magnetite particles are 8:5 ˙ 1:0 nm;

9.2 Basic Obtaining Methods of Metal-Containing Polymeric Nanocomposites

275

level of the nanoparticles content is also easily controlled. For example, in one reaction the cycle content of the magnetite nanoparticles, obtained by precipitation in situ in pores of the copolymer of styrene and acrylic acid [85], is 3.5–8 mass% depending upon the concentration of the initial reagents; in repeated cycles it can be increased to 20 mass%. Microgels also give a unique opportunity for the controlled design of interphase/superficial structures and manipulation of superficial morphology on nano- or micrometer level. It is achieved by introduction of defined functional groups into composition of microgels and by character of their distribution. Thus, nanocomposite microspheres of ZnS- and CdS-poly(N -polyisopropylacrylamideco-methacrylic acid) reveal very interesting superficial morphology in the form of figured structures that is connected with nonuniform precipitation of metal sulphide because of unhomogeneous distribution of metal ions within the microgel [86]. The in situ method is widely used also for the obtaining of nanocomposites on the basis of mineral clays (montmorillonite, bentonite, and other silicate clay are the most frequently used) and polymeric hydrogels. Such systems reveal improved mechanical characteristics due to high dispersion of mineral clays and ability to exfoliation in a polymeric matrix. Polymeric hydroxyapatite nanocomposite was obtained by a method of precipitation in situ in the microgel of polyacrylic acid [87]. Synthesized similarly intercalates of hydrated magnesium–aluminum silicate with poly(hydroxyethylmethacrylate)-poly(ethylene glycol methacrylate)methacrylic acid [88] showed high strength characteristics and significant thermal stability in comparison with usual hydrogels. An efficient variety of template assisted synthesis of metal-containing polymeric nanocomposites is, in our opinion, homo- and copolymerization of monomeric metal carboxylates with the subsequent formation of nanoparticles in situ. Nanocrystals of the PbS/polymeric gel were obtained by a combination of homopolymerization [89] of Pb(II) dimethacrylate and its copolymerization with styrene and an exchange reaction with H2 S [90]. ZnS nanoparticles were synthesized similarly in a polymeric matrix [91]. Scheme of the obtaining of PbS/polymethacrylic acid nanocomposite is shown on Fig. 9.12. It is interesting that it is possible to obtain a monomeric precursor in the form of nanofibres with diameter 200–300 nm and length from tens to hundreds of microns, selecting corresponding solvent during synthesis of the initial Pb(II) methacrylate. The subsequent ”-initiated polymerization allows to keep morphology of the monomer in the polymeric product and in the formed nanocomposite. In principle, in situ method is also the perspective for obtaining hybrid nanocomposite polymeric gels in the form of thin films. One of few examples [92] consists in polymerization of a monomeric carboxylate with the use of techniques of atom transfer radical polymerization (ATRP). Controlled polymerization in the presence of p-toluenesulfonyl chloride has allowed to carry out growth of the Pb(II) polymethacrylate film on a surface of Si plate and to obtain monodisperse PbS nanoparticles with 4 nm size and high density after exposition of the film by gaseous H2 S. The same methodology can be put in a basis of creation of hybrid nanocomposites with a core-shell morphology. Obtaining of the SiO2

276 Fig. 9.12 Scheme of synthesis of PbS in a polymer matrix in situ

9 Monomeric and Polymeric Metal Carboxylates as Precursors PbO H2O

MA

Pb(MA)2 Sonicate

Layer structure

Ethanol (60°C)

Pb γ-ray

60Co

n –x Pb Pb H2S

= PbS nanoparticles

n –m

= Polymer chains

nanospheres with a shell from the block-copolymer composite containing CdS nanoparticles includes several stages (Scheme 9.5) [93]. Controlled superficiallyinitiated ATRP of the Pb(II) methacrylate (SiO2 @PPbMAA, d D 215 nm) or MMA (SiO2 @PPbMAA, d D 219 nm) is carried out on a surface of silica gel nanospheres (d D 206 nm), and then block copolymerization with the formation of silica gel with a shell from block copolymers (SiO2 @ PPbMAA @ PMMA, d D 226 nm) or (SiO2 @PMMA@PPbMAA, d D 229 nm) is carried out. CdS nanoparticles are formed in situ at treatment with H2 S at 100ıC during 2 h. These and some other methods of obtaining hybrid metal-containing polymeric nanocomposites of the considered type were discussed in detail in the recent review [94]. Finally, we shall note one more interesting method of the obtaining of metalcontaining polymeric nanocomposites based on the combined synthesis both a polymeric matrix and metal nanoparticles in situ. Polyacrylic and methacrylic acids doped by metal-containing clusters were obtained at cocondensation of monomeric acids and metals at 77 K according to the Scheme 9.6 [95]. Seemingly, linkage of the cluster particles with the polymeric matrix is carried out with participation of the superficial atoms differing, as is well known, by high reactivity. Content of metal-containing clusters in final composites was equal to 0.22–113.3 mass% for PAA and 0.17–8.37% for MAA.

9.2 Basic Obtaining Methods of Metal-Containing Polymeric Nanocomposites

277

Scheme 9.5 Synthesis of hybrid nanocomposites with a core-shell morphology R CH2

C

+ Matom

COOH

Cocondensation (77 K) Polymerization

R CH2

C

x

C

O

H O R = H, CH3

Mn

M = Pd, Cu, Ag, Au, Bi, Sn, Cd, Zn

Scheme 9.6 Preparation of polymer acids doped by metal-containing clusters

9.2.3 Sol–Gel Methods in the Obtaining of Oxocluster Hybrid Materials Significant interest in unsaturated oxocarboxylates as structural elements or peculiar structural blocks of hybrid organo-inorganic nanocomposites (Fig. 9.13) has been revealed recently [96–100]. Traditional ways of obtaining such systems are frequently accompanied by phase division because of the difficulty of the control over size, form and distribution of particles of an inorganic component in an organic matrix [101]. One of the methods of elimination of such effects is the presence of covalent bonds or strong intermolecular interactions, for example, hydrogen, between basic components of a system [102]. Such an approach can be realized by the participation of oxocarboxylate clusters functionalizated by groups able to polymerize. Due to these groups covalent linkage of a metal-oxocluster unit with a polymeric chain is achieved. A linkage of an inorganic core with unsaturated groups of a hybrid molecule can be realized

278

9 Monomeric and Polymeric Metal Carboxylates as Precursors

Fig. 9.13 Molecular structure and size of a typical metal oxo cluster

Sn O

alkyl group Fig. 9.14 Fragment of the structure of hybrid nanocomposite obtained by copolymerization of f(BuSn)12 O14 (OH)6 g(OOCC.CH3 /DCH2 /2 and MMA

also by means of electrostatic interactions and hydrogen bonds, as it was shown for the f(BuSn)12 O14 (OH)6 g(OOCC(CH3 /=CH2 /2 oxocluster [103]. Polymethyl methacrylate–methacrylate copolymer, cross-linked by oxo-hydroxy butyltin cluster units, is formed at copolymerization with MMA. In this copolymer structure of the f.BuSn/12 O14 .OH/6 g2C macrocation does not undergo changes during polymerization (Fig. 9.14). Methacrylate-substituted metal-oxocluster Hf4 O2 (OOCC(CH3 /DCH2 /12 and methacryloylpropyltrimethoxysilane were used for the obtaining of hybrid thin films on the basis of silica gel with introduced hafnium oxoclusters [104, 105]. Chemical binding of the components was carried out by photochemical polymerization of methacrylate groups; alkoxy groups of the silane were exposed to hydrolysis and condensation with the formation of an oxide network (Scheme 9.7):

9.2 Basic Obtaining Methods of Metal-Containing Polymeric Nanocomposites Hf4O2(OOC(CH3)=CH2)12

+

279

CH2=C(CH3)COO-(CH2)3-Si(OMe)3 Hydrolysis / condensation Polymerization O O

O O

O Si O

Si O

Si

Si

Scheme 9.7 Synthesis of Hf(IV)/silica hybrid nanocomposite by sol-gel reactions

A similar approach was realized in a three-component system at copolymerization of two oxozirconium and oxohafnium clusters (M4 O2 (OCOC(CH3 /DCH2 / with (methacryloxypropyl)trimethoxysilane [106]. Methacrylate groups of the cluster molecules and silane were exposed to thermal or photoinitiated polymerization, and alkoxyde groups formed an oxide SiO2 network by means of hydrolysis and condensation. Calcination of a hybrid nanocomposite at a temperature of more than 800ıC is accompanied by pyrolysis of an organic part of the nanocomposite and condensation of the oxide network and results in the formation of the nanostructured oxide material (Scheme 9.8). Carboxylated ligands in organo-inorganic composites provide a high degree of cross-links in such systems due to coordination bonds between a polymer and a mineral component. Aggregates of inorganic particles in the hybrid poly(MMA-coBMA-co-MAA)/TiO2 (4.6–30 wt%) composite are distributed evenly in a copolymer matrix and phase division in the system is not observed in contrast to the optically opaque material, poly(MMA-co-BMA)/TiO2 [107].

9.2.4 Metal-Containing Polymeric Langmuir–Blodgett Films Nanoparticles in the Langmuir–Blodgett (LB) films are prospective materials for molecular designs. Various sensory groups or their precursors with nonlinear optical groups, metal-containing complexes and nanopaticles can be introduced into such self-organized layers. The majority of research refers to the self-organized hybrid nanocomposites on the basis of low-dimensional semi-conductor particles in the Langmuir-Blodgett films. So, films containing sulphides of cadmium, zinc or lead with 100 nm thickness (34 layers) were obtained by sulfidizing of layers of behenates of these metals, (C21 H43 COO/2 M [108, 109]. Films are anisotropic and their anisotropy increases at sulfidizing; on the basis of this fact it was concluded about layered arrangement of the formed nanoparticles. Length of an acid molecule

280

9 Monomeric and Polymeric Metal Carboxylates as Precursors silane + THF + HCl 0.5M mix and stir for 8 hrs (hydrolysis-condensation)

weighed amount of Zr and Hf clusters

mix and stir for 15′

add the thermal initiator

add the photoinitiator

stir for 15′

stir for 15′

thermal polymerization in oil bath at 60°C

deposition of the film by spin-coating

drying under vacuum at 70°C

UV exposure for 10′ (polymerisation)

calcination

calcination

gel

film

Scheme 9.8 Diagram of producing of hybrid nanocomposites

is 2.68 nm; thickness of the layer made from clusters is 1.12 nm; nanoparticles are not spherical, diameter is 5–10 nm, thickness is 1.1–1.3 nm. There are data in literature [110], that formation of the CdSe nanoparticles by treatment of the cadmium arachidate films ((C19 H31 COO/2 Cd) by H2 Se vapor occurs in interlamellar space of the films in a solid phase and is accompanied by their essential deformations and even by destruction of a lamellar structure. Multilayer Langmuir–Blodgett films are comparatively often obtained from stearates of cadmium [111], magnesium [112], ’-Fe2 O3 -stearate [113]. Formation of the self-organized structures in hydrophobic layers of stearic acid from a silver stearate film (8–14 layers) was established. The silver stearate film was moved to the electrodes ( D 25 nN=m) and was electrochemically reduced in neutral or acid solution with the formation of twodimensional Ag clusters 20–30 nm in diameter [114]. Multilayer films of nanoparticles of cadmium, lead and copper sulphides in oligomerous monooctadecanol ester of polymaleic acid (PMAO) were obtained by the combination of technique of inverse micelles and Langmuir– Blodgett [115–117].

9.3 Metal-Containing Polymeric Nanocomposite Materials of the Carboxylated Type

281

COOH CH

x

CH y n C O O

C18H37

x : y = 1.5, 7; x > y; n = 14

Inverse CdS-PMAO micelles were obtained at passing of H2 S through a solution of cadmium salt of PMAO in chloroform. Monomolecular layers of CdS-PMAO nanoparticles were transferred onto a solid surface of CaF2 and Si substrates with use of LB technique. It is important to note, that the sizes of the nanoparticles were less than those obtained in stearate films and were