Activation in anionic polymerization: Why phosphazene bases are very exciting promoters

Recently, nitrogen–phosphorous hybrid organobases such as phosphazene bases (PBs), which possess a remarkably high basicity, have been extensively studied in organic synthesis. Their applications in the domain of anionic polymerization are reviewed. Those non-ionic superbases generate highly reactive anionic species according to two different pathways: firstly by deprotonation of weak acids in which the protonated phosphazene base forms the cation, and secondly by complexation of the lithium cation by the phosphazene base when organolithium compounds are used as initiators. They have been successfully used for the anionic ring-opening polymerization (AROP) of epoxides, cyclosiloxanes, cyclic esters, caprolactam, and very recently cyclopropane-1,1-dicarboxylates, as well as for the anionic polymerization of vinyl monomers such as methacrylates, acrylates, butadiene, and isoprene. Polymerizations with metal-free non-protonated phosphazenium counterions are also reviewed. In all cases, the rates of polymerization are much higher than those observed with metal cations, and similar to the values obtained with cryptated counterions. The use of protonated and non-protonated phosphazenium counterions leads generally to polymers with narrow molecular weight distributions, and well-controlled end groups. Advantages of PBs are discussed, and perspectives in the revisited domain of anionic activation applied to polymer chemistry are presented.