Covalent active species, equilibria with ionic

The existence of a dynamic equilibrium between dormant (covalent) and active (ionic) species in controlled carbocationic polymerizations had been debated for years. It has been argued that under certain conditions, polarized covalent species can directly react with monomer examples are the pseudocationic mechanism proposed for the polymerization of styrene initiated by perchloric acid (123,124) (Fig. 5) or the two-component group transfer polymerization proposed for the polymerization of isobutylene initiated by the dicumylacetate/BCls system (125) (Fig. 6). Recent results and theoretical considerations support the now generally accepted view that the true active species are ions, and the dormant species serve as a reservoir from which the propagating ion pairs are formed (126-131). The existence of a dynamic equilibrium between dormant and active species and the ability to suppress the formation of free ions made possible the synthesis of pol5miers with controlled molecular architecture via carbocationic polymerization. 

The second method involves end-quenching of living polymers with appropriate nucleophiles. Although this approach appears to be more attractive than the first one, in situ end funaionali-zation of the living ends is limited to nucleophiles that do not react with the Lewis add coinitiator. Because the ionization equilibrium is shifted to the covalent spedes, the concentration of the ionic active species is very low. Quantitative functionalization can only be accomplished when ionization takes place continuously in the presence of nudeophile. Quenching the vinyl ether polymerization with the malonate anion,certain silyl enol ethers " and silyl ketene acetals have been successfully used to synthesize end-functionalized poly(vinyl ethers). Alkyl amines, " ring-substituted anilines, " " alcohols, " and water " have also been used to quench the vinyl... 

---