STEP-GROWTH CONDENSATION POLYMERIZATION

The reaction proceeds via step growth polymerization (condensation). The degree of polymerization depends on conversion and is sensitive to impurities and molar (stoichiometric) imbalance. Water should be excluded from the reaction medium since it reacts with the diisocyanate and converts it to CO plus an amine, which reacts with unreacted diisocyanate to give polyurea. This prevents the formation of high molecular weight and therefore leads to... 

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. 

Discussion of ladder polymers also enables us to introduce a step-growth polymerization that deviates from the simple condensation reactions which we have described almost exclusively in this chapter. The Diels-Alder reaction is widely used in the synthesis of both ladder and semiladder polymers. In general, the Diels-Alder reaction occurs between a diene [XVI] and a dienophile [XVll] and yields an adduct with a ring structure [XVlll] ... 

This situation seems highly probable for step-growth polymerization because of the high activation energy of many condensation reactions. The constants for the diffusion-dependent steps, which might be functions of molecular size or the extent of the reaction, cancel out. 

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