Open mechanistic problems of quasiliving carbocationic polymerization of olefins mediated by nucleophilic additives

摘要

This study is a comprehensive overview of the open problems and the existing views on the mechanism of quasiliving carbocationic polymerizations (QLCP) of olefins mediated by nucleophilic additives. The fundamental and general aspects of ideal living and quasiliving polymerizations involving other mechanisms, such as free radical, group transfer, ring-opening metathesis, ring-opening cationic and anionic processes, have been also analyzed and summarized. Quasiliving carbocationic polymerization of olefins in the presence of nucleophiles, which form complexes with the Lewis acid coinitiators, occur by reversible termination. Four different mechanisms have been discussed in this study: (1) reactivity leveling by nucleophiles ("electron donors"), (2) propagation by species with decreased ionicity ("stretched polarized bonds") mediated by Lewis acid-nucleophile complexes (LA.Nu); (3) propagation by classical ion pair and free ion species; (4) proton scavenging by nucleophiles and 2,6-di-tert-butylpyridine proton trap. It is shown that mechanisms No. 1, 3 and 4 cannot explain all the existing findings, and although the experimental results can be interpreted with mechanism No. 2, the existence of "stretched polarized bonds" can be questionable. It is also concluded that compared to nonliving carbocationic polymerization, kinetic analysis indicates that the propagating species cannot be the same in quasiliving carbocationic polymerizations and in chain transfer dominated classical carbocationic polymerizations with ion pairs and free ions. [References: 33]