Types Of Polymers And Polymerizations
There has been and still is considerable confusion concerning the classification of polymers. This is especially the case for the beginning student who must appreciate that there is no single generally accepted classification that is unambiguous. During the development of polymer science, two types of classifications have come into use. One classification is based on polymer structure and divides polymers into condensation and addition polymers. The other classification is based on polymerization mechanism and divides polymerizations into step and chain polymerizations. Confusion arises because the two classifications are often used interchangeably without careful thought. The terms condensation and step are often used synonymously, as are the terms addition and chain. Although these terms may often be used synonymously because most condensation polymers are produced by step polymerizations and most addition polymers are produced by chain polymerizations, this is not always the case. The condensation-addition classification is based on the composition or structure of polymers. The step-chain classification is based on the mechanisms of the polymerization processes.
Principles of Polymerization, Fourth Edition. By George Odian ISBN 0-471-27400-3 Copyright © 2004 John Wiley & Sons, Inc.
1-1a Polymer Composition and Structure
Polymers were originally classified by Carothers [1929] into condensation and addition polymers on the basis of the compositional difference between the polymer and the monomer(s) from which it was synthesized. Condensation polymers were those polymers that were formed from polyfunctional monomers by the various condensation reactions of organic chemistry with the elimination of some small molecule such as water. An example of such a condensation polymer is the polyamides formed from diamines and diacids with the elimination of water according to nH2N-R-NH2 + nHO2C-R'-CO2H -
where R and R' are aliphatic or aromatic groupings. The unit in parentheses in the polyamide formula repeats itself many times in the polymer chain and its termed the repeating unit. The elemental composition of the repeating unit differs from that of the two monomers by the elements of water. The polyamide synthesized from hexamethylene diamine, R = (CH2)6, and adipic acid, R' = (CH2)4, is the extensively used fiber and plastic known commonly as nylon 6/6 or poly(hexamethylene adipamide). Other examples of condensation polymers are the polyesters formed from diacids and diols with the elimination of water and the nHO-R-OH + nHO2C-R'-CO2H -»-
polycarbonates from the reaction of an aromatic dihydroxy reactant and phosgene with the elimination of hydrogen chloride:
The common condensation polymers and the reactions by which they are formed are shown in Table 1-1. It should be noted from Table 1-1 that for many of the condensation polymers there are different combinations of reactants that can be employed for their synthesis. Thus polyamides can be synthesized by the reactions of diamines with diacids or diacyl chlorides and by the self-condensation of amino acids. Similarly, polyesters can be synthesized from diols by esterification with diacids or ester interchange with diesters.
Some naturally occurring polymers such as cellulose, starch, wool, and silk are classified as condensation polymers, since one can postulate their synthesis from certain hypothetical reactants by the elimination of water. Thus cellulose can be thought of as the polyether formed by the dehydration of glucose. Carothers included such polymers by defining condensation polymers as those in which the formula of the repeating unit lacks certain atoms that are present in the monomer(s) from which it is formed or to which it may be degraded. In this
TABLE 1-1 Typical Condensation Polymers
Type
Characteristic Linkage
Polymerization Reaction
Polyamide
Protein, wool, silk Polyester
Polyurethane Polysiloxane
Phenol-formaldehyde
Urea-formaldehyde
Melamine-formaldehyde
Polysulfide Polyacetal
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