OPEN MECHANISTIC PROBLEMS OF QUASILIVING CARBOCATIONIC POLYMERIZATIONOF OLEFINS MEDIATED BY NUCLEOPHILIC ADDITIVES

This study is a comprehensive overview of the open problems and the ex isting views on the mechanism of quasiliving carbocationic polymerizat ions (QLCP) of olefins mediated by nucleophilic additives. The fundame ntal and general aspects of ideal living and quasiliving polymerizatio ns involving other mechanisms, such as free radical, group transfer, r ing-opening metathesis, ring-opening cationic and anionic processes, h ave been also analyzed and summarized. Quasiliving carbocationic polym erization of olefins in the presence of nucleophiles, which form compl exes with the Lewis acid coinitiators, occur by reversible termination . Four different mechanisms have been discussed in this study: (1) rea ctivity leveling by nucleophiles (''electron donors''), (2) propagatio n by species with decreased ionicity (''stretched polarized bonds'') m ediated by Lewis acid-nucleophile complexes (LA.Nu); (3) propagation b y classical ion pair and free ion species; (4) proton scavenging by nu cleophiles and 2,6-di-tert-butylpyridine proton trap. It is shown that mechanisms No. 1, 3 and 4 cannot explain all the existing findings, a nd although the experimental results can be interpreted with mechanism No. 2, the existence of ''stretched polarized bonds'' can be question able. It is also concluded that compared to nonliving carbocationic po lymerization, 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.