Quantitative comparison of selectivities in the polymerization of cyclic esters

Chain transfer processes (k_tr) taking place in the polymerization (anionic and pseudoanionic) of cyclic esters (lactones) are reviewed. These reactions are mostly responsible for the departure of these systems from the fully controlled (living) polymerizations. The ratios of k_p/k_tr have been determined for a number of initiating systems and the structures of the growing species are related to their selectivity, expressed by k_p/k_tr. It has been shown that the less reactive and more sterically crowded active species polymerize more selectively.