Step-growth polymers, definition

While these extremely simplified cases are met in practice, some very important exceptions occur that show the need for caution in making generalizations. For example, nylon 6 would normally be considered as a step-growth polymer because it is a polyamide. However, it can also be produced by a ring-opening polymerization that is a kind of chain-growth polymerization. Thus, these definitions are useful but can sometimes be misleading. 

In step-growth polymerization of bifunctional monomers, each molecule, whether monomer or polymer, carries two functional groups There are always half as many molecules as functional groups. Thus, when the number of unreacted groups has decreased to a fraction 1 —/F of the initial, the number of molecules has decreased to that same fraction of its initial. By virtue of its definition 10.15, the number-average degree of polymerization (monomer included in averaging) is the reciprocal of this fraction ... 

Although these definitions were perfectly adequate at the time, it soon became obvious that notable exceptions existed and that a fundamentally sounder classification should be based on a description of the chain-growth mechanism. It is preferable to replace the term condensation with step-growth or step-reaction. Reclassification as step-growth polymerization now logically includes polymers such as polyurethanes, which grow by a step-reaction mechanism without elimination of a small molecule. 

Condensation polymers by the above definition are usually produced by step-growth polymerizations but not all step-growth syntheses are condensation reactions. Thus there is no elimination product in polyurethane synthesis from a diol and a diisocyanate (cf. reaction (1-12)) ... 

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