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Kinetics of Different Methods of Polymerization

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Kinetics of Different Methods of Polymerization Technische Universität Berlin Institut für Chemie Polymerization Technology Karl-Heinz Reichert Reinhard Schomäcker Third Edition SS 2017 Preface This teaching booklet has been written for students attending the Master Program of Polymer Science, established as a joint program by four universities in the cities of Berlin and Potsdam. This text book focuses on fundamental aspects of polymerization reaction engineering. In the development of a polymerization process the type of reactor and its mode of operation are key factors, which not only affect reactor performance and safety, but also to a large extend the quality of the polymeric product. This is due to the fact that polymers are non uniform materials and the degree of non uniformity is affected by chemistry and reaction engineering conditions as well. I hope that the contents of this text book will be of help to those students who will be envolved in large scale synthesis of polymers in times to come. I would like to thank my secretary Veronika Schott for writing the manuscript of this booklet and especially for her patience with respect to numerous changes of the text, which I have made all the time. Thanks also go out to Monika Klein, who drew all the figures presented in this book and Scott Kibride who improved the English language. Finally we would like to thank all my former PhD students and many of our colleagues for some of their scientific results, which we have used in this text book. Karl Heinz Reichert Berlin in October 2002 Reinhard Schomäcker Berlin in April 2017 TABLE OF CONTENTS 1. Introduction 1 1.1 Classification of Polymers 1 1.2 Types of Polymerization Reactions 2 1.3 Methods of Polymerization 3 1.4 Types of Polymerization Reactors 4 1.5 General References 4 1.6 Tables and Figures 6 2. Kinetics of Polymerization and Molecular Weight of Polymers 9 2.1 Free Radical Polymerization in Solution 9 2.1 Free Radical Polymerization in Emulsion 16 2.3 Free Radical Copolymerization in Solution 23 2.4 Coordination Polymerization in Gas Phase 24 2.5 Coordination Polymerization in Liquid Phase 32 2.6 List of Symbols 34 2.7 References 35 2.8 Tables and Figures 37 3. Viscosity of Reaction Mixture 55 3.1 Introduction 55 3.2 Viscosity of Homogeneous Systems 55 3.3 Viscosity of Heterogeneous Systems 58 3.4 List of Symbols 59 3.5 References 60 3.6 Figures 61 4. Data Acquisition of Polymerization Reactions 66 4.1 Introduction 66 4.2 Reaction Calorimetry/Kinetic and Caloric Data 66 4.3 Reaction Viscosimetry/Rheological Data 70 4.4 Solubility and Diffusivity of Monomer in Polymer 71 4.5 List of Symbols 73 4.6 References 74 4.7 Figures 75 5. Polymerization in Stirred Tank Reactors 83 5.1 Mode of Operation 83 5.2 Mixing of Reaction Mixture 84 5.3 Heat Removal and Safety Aspects 92 5.4 Residence Time Distribution 98 5.5 Reactor Performance 101 5.6 Reactor Selectivity 105 5.7 Reactor Scale-up 109 5.8 List of Symbols 111 5.9 References 113 5.10 Tables and Figures 114 6. Polymerization Processes 139 6.1 General Aspects 139 6.2 Processes for Chain-Growth Polymerization 140 Solution Polymerization/High Density Polyethylene Suspension Polymerization/Poly(vinyl chloride) Emulsion Polymerization/Styrene-Butadiene-Copolymer Slurry Polymerization/High Density Polyethylene Gas Phase Polymerization/High Density Polyethylene 6.3 Processes for Step-Growth Polymerization 146 Condensation Polymerization in Solution/Phenolic Resins Condensation Polymerization in Melt and Solid State/Poly- (ethylene terephthalate) Addition Polymerization in Liquid Phase/Polyurethanes 6.4 References 149 6.5 Tables and Figures 150 1. INTRODUCTION 1.1 Classification of Synthetic Polymers Synthetic polymers can be classified according to their specific properties into thermoplastics, thermosets and elastomers. Examples of major polymers of each kind are listed in Tab. 1.1. Thermoplastic polymers are organic materials, which consist of linear or branched macromolecules having molecular weights on the order of 100 000 gram per mole. On heating above melting point thermoplastic polymers melt and form highly viscous liquids with a typical flow pattern. On cooling the melt solidifies again. In this way thermoplastic polymers can easily be processed into materials of different shapes. According to the physical structure and chemical composition of the polymers they can be partially crystalline or amorphous materials in the solid state. Amorphous polymers like polyvinyl chloride, poly- styrene, and polyesters are transparent materials. Partially crystalline polymers like high density polyethylene and polypropylene are not transparent in the solid state due to their heterophasic structure. Thermosets are organic materials, which are formed by higly crosslinked macro- molecules with extremely high molecular weights. On heating they can not be molten but they do decompose and lose their original properties. Therefore thermosets have to be processed in such a way that synthesis and processing of the polymer material is done at the same time in a given cavity corresponding to the shape of the material which is to be produced. In general, thermosets are filled with glass fibre to improve the mechanical strengh of the materials. Elastomers are linear or branched macromolecules which are very flexible. The molecules contain double bonds, which can easily react with added crosslinker at elevated temperatures, forming a crosslinked material with rubber-like properties. Typical properties of organic polymers are low specific weight, low heat and electrical conductivity, and good resistant to corrosion. Feedstocks for major polymers are crude oil, natural gas, salt, air, and water. Organic polymers are produced by a relatively small number of large chemical companies. Approxi- mately seventy percent of all polymers produced are thermoplastics, twenty percent are thermosets, and ten percent are elastomers. 1 1.2 Types of Polymerization Reactions Polymerization reactions can be very complex chemical reactions with many different side reactions. One way of classification of polymerization reactions is to look at the polymer growth reaction, which is essential for polymer formation. By looking at the polymer growth reaction, chainwise and stepwise poly- merization reactions can be distinguished. See Tab. 1.2. In chainwise polymerization reactions the propagation of a molecule happens by the consecutive addition of bifunctional monomer molecules (M) to an active * site ( Pn ) of chain length n. Once the active sites are formed they start a chain of monomer addition reactions until the chain is terminated by a termination reaction. The active sites can be free radicals, organo metallic complexes or anionic or cationic species of very different kinds. Depending on the nature of active sites polymerization reactions can be classified into free radical polymerization, coordination polymerization, and ionic polymerization. If these polymerization reactions do not have any chain termination or chain transfer reaction they are called living polymerization. In case of a living polymerization the life time of active sites are long (at least on the order of total reaction time). The life time of free radicals is in general on the order of seconds. Active sites of organo metallic catalysts can have very different life times. In general they are on the order of seconds or minutes. The concentration of active sites of chainwise polymerization reactions is in general very low and it can be constant or non-constant with conversion of monomer in batchwise reaction. As mentioned before, chainwise polymerization reactions are complex reactions consisting of initiation, propagation, termination and transfer reactions. All of the reactions are running simultaneously. The molecular weight of polymers formed during chainwise polymerization can remain constant or decrease or increse with conversion of monomer. This depends on the contribution of each single reaction. In case of a free radical polymerization run in a batch reactor at constant temperature the molecular weight remains constant with conversion if chain transfer reactions play a dominant role. If not, it will fall with conversion due to decreasing concentration of monomer. The same is true for coordination polymerization. In case of living polymerization the molecular weight of polymer formed is increasing with conversion in any case since no termination and transfer reactions are present in the reacting system. The molar concentration of polymer molecules of chainwise polymerization reactions also depends on the kind of polymerization. It remains constant with conversion for a living polymerization and is increasing for free radical and coordination polymerization since at any time new polymer molecules are formed. The situation can be quite different in the case of stepwise polymerization reactions. Here the polymer growth reaction takes place by stepwise reactions of bifunctional molecules (Pn and Pm in Tab 1.2). The molecules can be monomers, 2 oligomers, or polymers depending on the degree of conversion. At the beginning of reaction only monomer molecules are present in the reaction mixture. With increasing conversion monomer concentration is rapidly falling and oligomers are formed. High molecular weight polymers are only formed at very high conversion of functional groups (above 99 %). The polymer growth reaction is a typical condensation reaction like the reaction of carboxylic groups with hydroxylic groups; forming ester groups and water. This kind of polycon- densation reactions are in general reversible reactions, which have to be shifted to the right side of the equilibrium for high conversions. The active sites are the functional groups of the reacting molecules, with an infinite life time on its own. The concentration of functional groups is decreasing with increasing conversion. In an ideal case there are no other side reactions in stepwise polymerization reactions beside growth reaction. The avarage molecular weight of the condensation products increases with conversion of functional groups. First there is a very slow increase, then at high conversion there is a very strong increase in molecular weight.
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