External Catalysis of Polymerization

The slow increase in molecular weight was mistakenly thought originally to be due to the low reactivity of functional groups attached to large molecules. It is, however, simply a 

Equations 2-27 and 2-33 and Fig. 2-2 describe the much greater difficulty of performing a successful polymerization compared to the analogous small-molecule reaction (such as the synthesis of ethyl acetate from acetic acid and ethanol). Consider the case where one needs to produce a polymer with a degree of polymerization of 100, which is achieved only at 99% reaction. Running the polymerization to a lower conversion such as 98%, an excellent conversion for a small-molecule synthesis, results in no polymer of the desired molecular weight. Further, one must almost double the reaction time (from 450 min to 850 min in Fig. 2-2) to achieve 99% reaction and the desired polymer molecular weight. For the small molecule reaction one would not expend that additional time to achieve only an additional 1 % conversion. For the polymerization one has no choice other than to go to 99% conversion. 

The kinetics of step polymerizations other than polyesterification follow easily from those considered for the latter. The number of different general kinetic schemes encountered in actual polymerization situations is rather small. Polymerizations by reactions between the 

A and B functional groups of appropriate monomers proceed by one of the following situations [Saunders and Dobinson, 1976]  

Some polymerizations, such as the formation of polyamides, proceed at reasonable 

Other polymerizations such as those of urea, melamine, or phenol with formaldehyde (see Table 1-1) require an externally added acid or base catalyst to achieve the desired rates of reaction. 

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