Classes of Step-Growth Polymers

The dependence of polymer properties on chemical compositions is reviewed in basic polymer texts.9,10 The backbone structure of a polymer defines to a large extent the flexibility and stability of a polymer molecule. Consequently, a great range of polymer properties can be achieved within each class of step-growth polymers by varying the backbone structure using different monomers. 

The following chapters will provide detailed discussions of the structure-property relations with various classes of step-growth polymers. 

Epoxy resins represent a class of step-growth polymer familiar to anyone who has used epoxy to glue together a broken object. An epoxy resin consists of two components a fluid prepolymer composed of short polymer chains with reactive epoxides on each end, and a hardener, usually a diamine or triamine that ring opens the epoxides and cross-links the chains together. 

Condensation polymers are an important class of step-growth polymers formed by the common condensation reactions of organic chemistry. These involve the elimination of a small molecule, often water, when two molecules join, as in amidation ... 

The chemical nature of the main-chain linkages of step-growth polymers makes this class of polymers particularly reactive to a wide variety of chemical species. Solvolysis reactions break the C-X bond at the polymer linkage bonds. These types of reactions are often pH-dependent, so the stability of the polymer is highly dependent on the acidity or basicity of the prodegradant. 

All of the addition reactions seen earlier in this chapter (radical, cationic, and anionic polymerization) are examples of chain-growth processes. Polymers created by any of these addition processes are chain-growth polymers. In contrast, most condensation polymers are not chain-growth polymers, but instead belong to a class called step-growth polymers. 

In the next group of chapters we shall discuss condensation or step-growth polymers and polymerizations in Chap. 5, addition or chain-growth polymers and polymerizations in Chap. 6, and copolymers and stereoregular polymers in Chap. 7. It should not be inferred from this that these are the only classes of polymers and polymerization reactions. Topics such as ring-opening polymeri-... 

Step-growth polymers, the second major class of polymers, are prepared by reactions between difunctional molecules, with the individual bonds in the polymer formed independently of one another. Polycarbonates are formed from a diester and a diol, and polyurethanes are formed from a diisocyanate and a diol. 

Condensation polymers, which are also known as step growth polymers, are historically the oldest class of common synthetic polymers. Although superseded in terms of gross output by addition polymers, condensation polymers are still commonly used in a wide variety of applications examples include polyamides (nylons), polycarbonates, polyurethanes, and epoxy adhesives. Figure 1.9 outlines the basic reaction scheme for condensation polymerization. One or more different monomers can be incorporated into a condensation polymer. 

Condensation or step growth polymers are formed by the incremental growth of monomer(s) or condensed product(s) of the reactant molecules through covalent bonds after the elimination of by-products such as H2O, NH3, HCl, HCHO, phenol, and so on. The molecular mass of a repeating unit is less than the molecular mass of a monomer(s) or reactant(s). Examples of this class of polymer are nylons, polyesters, polyimides and polycarbonates. 

There are two main classes of synthetic polymers chain-growth polymers and step-growth polymers. Polyethylene and other alkene and diene polymers like those we saw in Sections 8.10 and 14.6 are chain-growth polymers because... 

It is the third of these criteria that offers the most powerful insight into the nature of the polymerization process for this important class of materials. We shall frequently use the terms step-growth and condensation polymers as synonyms, although by the end of the chapter it will be apparent that step-growth polymerization encompasses a wider range of reactions and products than either criteria (1) or (2) above would indicate. 

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