Cationic Polymerization with Lewis Acids as Initiators

The initiator concentrations required for cationic polymerizations are smaller than those for radical polymerizations frequently 10 to 10 mol of initiator per mol monomer is sufficient to achieve a high rate of reaction. The effect of initiator concentration on the rate and average degree of polymerization depends on the monomer and a variety of other factors and does not follow a consistent pattern. 

The type and amount of cocatalyst required for optimum polymerization conditions must be determined for each case generally the amount of cocatalyst required (in mol) is much less than that of the initiator. 

In polymerizations of unsaturated compounds with Lewis acids the required reaction temperatures are below room temperature, down to -100 °C or even lower (see Example 3-16). On the other hand, cyclic monomers (see Sect. 3.2.3) frequently require higher temperatures. 

In solution polymerizations catalyzed by Lewis acids, the polymerization frequently does not begin immediately after addition of the initiator, and there is an induction period that cannot be completely eliminated even by careful purification of the starting materials. In contrast, some cationic polymerizations proceed so quickly (flash polymerization), even after dilution, that conversion is already complete after a few minutes (e.g., isobutylene. Example 3-16). 

Finally, it should be noted that cationic polymerizations are very sensitive to impurities. These can act as cocatalysts, accelerating the polymerization, or as inhibitors (e.g., tertiary amines) they can also give rise to chain transfer or chain termination and so cause a lowering of the degree of polymerization. Since these effects can be caused by very small amounts of impurities (10 mol% or less), careful purification and drying of all materials and equipment is imperative. 

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Cationic Polymerization with Lewis Acids as Initiators... 

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