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Polymer Synthesis This article was originally published in Comprehensive Biomaterials published by Elsevier, and the attached copy is provided by Elsevier for the author's benefit and for the benefit of the author's institution, for non-commercial research and educational use including without limitation use in instruction at your institution, sending it to specific colleagues who you know, and providing a copy to your institutions administrator. All other uses, reproduction and distribution, including without limitation commercial reprints, selling or licensing copies or access, or posting on open internet sites, your personal or institutions website or repository, are prohibited. For exceptions, permission may be sought for such use through Elsevier's permissions site at: http://www.elsevier.com/locate/permissionusematerial Hasirci V., Yilgor P., Endogan T., Eke G., and Hasirci N. (2011) Polymer Fundamentals: Polymer Synthesis. In: P. Ducheyne, K.E. Healy, D.W. Hutmacher, D.W. Grainger, C.J. Kirkpatrick (eds.) Comprehensive Biomaterials, vol. 1, pp. 349-371 Elsevier. © 2011 Elsevier Ltd. All rights reserved. Author's personal copy 1.121. Polymer Fundamentals: Polymer Synthesis V Hasirci, P Yilgor, T Endogan, G Eke, and N Hasirci, Middle East Technical University, Ankara, Turkey ã 2011 Elsevier Ltd. All rights reserved. 1.121.1. Introduction to Polymer Science 350 1.121.1.1. Classification of Polymers 351 1.121.1.2. Polymerization Systems 352 1.121.2. Polycondensation 353 1.121.2.1. Characteristics of Condensation Polymerization 353 1.121.2.2. Kinetics of Linear Polycondensation 354 1.121.2.2.1. Molecular weight control in linear polycondensation 355 1.121.2.3. Nonlinear Polycondensation and Its Kinetics 356 1.121.2.3.1. Prediction of the gel point 356 1.121.2.4. Mechanisms of Polycondensation 356 1.121.2.4.1. Carbonyl addition–elimination mechanism 356 1.121.2.4.2. Other mechanisms 356 1.121.2.5. Typical Condensation Polymers and Their Biomedical Applications 357 1.121.3. Addition Polymerization 357 1.121.3.1. Free Radical Polymerization 358 1.121.3.1.1. Initiation 358 1.121.3.1.2. Propagation 358 1.121.3.1.3. Termination 359 1.121.3.1.4. Kinetics of radical polymerization 359 1.121.3.1.5. Degree of polymerization 359 1.121.3.1.6. Thermodynamics of polymerization 360 1.121.3.2. Ionic Polymerization 360 1.121.3.2.1. Cationic polymerization 360 1.121.3.2.2. Anionic polymerization 360 1.121.3.3. Coordination Polymerization 360 1.121.3.4. Typical Addition Polymers and Their Biomedical Applications 361 1.121.3.5. Comparison of Addition and Condensation Polymerization 361 1.121.3.6. New Polymerization Mechanisms 361 1.121.3.6.1. Atom transfer radical polymerization 361 1.121.3.6.2. Nitroxide-mediated polymerization 362 1.121.3.6.3. Reversible addition–fragmentation chain transfer polymerization 362 1.121.4. Polymer Reactions 363 1.121.4.1. Copolymerization 363 1.121.4.1.1. Types of copolymerization 364 1.121.4.1.2. Effects of copolymerization on properties 365 1.121.4.1.3. Kinetics of copolymerization 365 1.121.4.2. Cross-Linking Reactions 367 1.121.4.2.1. Effect of cross-linking on properties 367 1.121.4.2.2. Cross-linking of biological polymers 367 1.121.4.2.3. Cross-linking agents 368 1.121.5. Conclusion 369 References 370 Glossary Cationic polymerization Polymerization initiated by Addition polymerization Rapid polymerization based cation and propagated by a carbonium ion. on initiation, propagation, and termination of double Condensation polymerization Polymerization in which bonded monomers and no small molecules are polyfunctional reactants produce larger units in a continuous, stepwise manner. eliminated. Anionic polymerization Polymerization initiated by Coordination polymers Polymers based on coordination a anion. complexes. 349 Comprehensive Biomaterials (2011), vol. 1, pp. 349-371 Author's personal copy 350 Polymers Copolymer Polymers composed of chains containing more Kinetic chain length Average length of the polymer than one monomer unit. chain initiated by one free radical. Degree of polymerization Average number of repeating Propagation Continuous successive chain extension in a units in main chains. chain reaction. Gelpoint Point at which cross-linking begins to Repeating unit Basic molecular unit that can represent a produce polymer insolubility. polymer backbone chain. Glass transition temperature (Tg) Temperature at Tacticity Arrangement of the pendant groups in space; that which a polymer gains local or segmental is, isotactic, syndotactic, atactic. mobility. Termination Destruction of active growing chains in a Initiation Start of polymerization. chain reaction. 1.121.1. Introduction to Polymer Science solvent, the chains start to separate from each other and for linear and branched polymers this separation leads to com- A polymer is a macromolecule composed of a combination of plete solubility. The cross-linked network polymers, however, many small units that repeat themselves along the long mole- cannot dissolve in a solvent; they swell, forming gels. cule. The small starting molecules are called monomers, and The process of creating macromolecules from monomers is the unit which repeats itself along the chain is called the called polymerization. If only one type of monomer is used in repeating unit. In general, polymer chains have several thou- polymerization, there will be only one type of repeating unit sand repeat units. The length of the polymer chain is specified in the chain. In this case, the macromolecule is a homopoly- by the number of repeating units in the chains and this mer. If the polymer is formed from two different monomers number is called the degree of polymerization. Most of the (have two different repeating units), it is known as a copoly- monomers are composed of carbon, hydrogen, oxygen, and mer. If a chain is formed from only ethylene, the polymer is a nitrogen. Few other elements such as fluorine, chlorine, sul- homopolymer and named as polyethylene. On the other fur, etc. may also exist. Syntheses of polymers are carried out hand, the copolymer of ethylene and vinyl acetate has two in vessels or large reactors, sometimes with application of monomers and, therefore, has two different repeating units. heat and pressure, and the small monomeric units connect If three different monomers are used to produce a polymer, to each other through the chemical reactions. The chemical the product is a terpolymer. Biological polymers, such as process used for the synthesis of polymers is called the poly- enzymes, are formed from many different amino acids, and merization process. therefore, their structures contain a variety of repeating units. Polymers which have the ability to melt and flow are used Since a large number of combinations of these molecules in manufacturing and are generally identified with the com- are available, it becomes possible to design and synthesize mon name, plastics. In general, plastic products contain other polymers with the desired properties ranging from fibers added ingredients such as antioxidants and lubricants to give to films, sponges to elastomers. This versatility makes them the desired properties to the object produced. essential materials to be used in various applications ranging Most of the macrochains obtained in polymerization reac- from macro-sized products used in the households to nano- tions are linear polymers and are formed by the reactions of scale devices used in nanotechnological and biomedical monomers containing either carbon–carbon double bonds applications. or have two active functional groups or difunctionality. Polymers such as cellulose, silk, and chitin can be obtained Many monomers have different active groups on the same from natural sources and polymers such as polyethylene, poly- molecule such as one end of the monomer contains a carbox- styrene, and polyurethanes can be synthesized in the labora- ylic acid and the other end contains an alcohol, and the tories and plants. The macrochains such as DNA, RNA, and reaction of the acid group of one molecule with the alcohol enzymes have biological importance and are crucial for life. groupof the other forms polyesters. Polymerization reactions In general, the backbone of a polymer is formed mainly of also take place when one of the monomers contains two acid carbon atoms. These are called the organic polymers. There are groups and the other contains two alcohol or two amine also a few inorganic polymers, and the atoms in their back- groups. If there are some monomers which have more than bones are different than carbon. An example is silicone, the two functionalities (e.g., 3- or 4-functionality), their presence backbone of which is constituted of silicon and oxygen. in the chain cause the formation of extra chains linked to the One very important property which strongly influences main backbone. In this case, branched polymers are obtained. the mechanical strength of the polymer is its molecular weight. If the extent of branching is very high and all the macrochains Hydrocarbon molecules with increasing number of carbons are connected to each other, then they form a highly cross- are methane, ethane, propane, etc. The ones containing up to linked, three-dimensional structure which is called a network. five carbons are in the gaseous state. As the number of carbons, These networks have infinite molecular weights since all and therefore, the molecular weight increases, they become chains are connected to each other. In a polymer structure, liquids, wax type solids, and eventually hard solids. The ones all chains are tangled around each other forming the bulk called polymers contain more than 100 carbons along the structure. At low temperatures they are solid, but in a good chain. Most polymers which are useful as plastics, rubbers, Comprehensive Biomaterials (2011), vol. 1, pp. 349-371 Author's personal copy Polymer Fundamentals: Polymer Synthesis 351 fibers, etc. have at least 50 repeating units and have molecular 4 6 1 weights between 10 and 10 g molÀ .
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