Classifications of Polymers and Polymerization Reactions

Because a quantitative understanding of the polymerization process used to prepare a sample is essential for the specification of its molecular structure, this chapter presents a summary of some common polymerization reactions. Much more comprehensive treatments can be found in books devoted to polymer chemistry. A few examples are referenced [5-7]. 

Wallace Carothers, duPont s famous polymer chemist, proposed classifying polymers by reference to the stoichiometry of the polymerization reaction. If the entire monomer molecule ends up in the polymer, he called it an addition polymer, whereas if there is a byproduct, often water, the primary product is called a condensation polymer. He thus considered vinyl polymers to be addition polymers and polyesters to be condensation polymers. However, it was later learned that it is possible to make some addition polymers by reactions in which there is a byproduct and to make some condensation polymers by reactions in which there is no byproduct. Paul Flory, who started his career as the theoretician in Carother s duPont research group, later proposed that the reaction mechanism be used as the basis for classifying polymers. In this scheme, in a step polymerization, any two reactive molecules can combine, so polymerization occurs uniformly throughout the reaction mixture. In chain growth polymerization, on the other hand, monomer units are added only to species containing an active center or initiator, which can be a free radical, an ion, or an active catalyst site. Condensation polymers are usually produced by step reactions, and addition polymers are usually made by 

The most important chain-growth polymers are polyolefins and other vinyl polymers. Examples of the former are polyethylene, and polypropylene, and examples of the latter are poly(vinyl chloride), polystyrene, poly(vinyl alcohol), polyacrylonitrile, and poly(methyl acrylates). The most common stepwise reactions are condensation polymerizations. Polyamides, such as nylon 6-6, which is poly(hexamethylene adipamide), and polyesters, such as poly(ethylene terephthalate), are the most important commercial condensation polymers. These polymers were originally developed for use in fiber manufacture because of their high melting points but are now used also as thermoplastics. Polycarbonate is an engineering plastic that is made from bisphenol A and phosgene by a stepwise reaction. 

Addition polymers are made by use of three processes free radical, ionic, and complex coordination catalyst. Ionic polymerization can be either anionic or cationic. Except for some elastomers that are made by ionic polymerization, commercial polymers are made by free radical polymerization or by using complex coordination catalysts. So-called living polymers are widely used in polymer research, because they can be made with very narrow molecular weight distributions. These were traditionally made using anionic polymerization, but it has been shown that living polymers can also be made using a free-radical or cationic process. 

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