Phosphazene Bases as Organocatalysts for Ring-Opening Polymerization of Cyclic Esters

Over the past several years, organocatalyzed polymerization reactions have attracted considerable attention, and these efforts have led to major advances. A large number of organic compounds have been proven active for the polymerization of a large variety of monomers. In particular, phosphazene bases (PBs) are a family of extremely strong, non-nucleophilic, and uncharged auxiliary bases, and have shown their remarkable potential as organocatalysts for the ring-opening polymerization (ROP) of cyclic monomers. By deprotonation of weak acids or in combination with lithium cation, PBs significantly enhance the nucleophilicity of the initiator/chain-end, thus allowing fast and usually controlled anionic polymerization. In this feature article, the recent advances in phosphazene-catalyzed ROP of cyclic esters are summarized. This review is divided into three sections, including general features, design and synthesis, and catalytic applications. It aims to provide a critical analysis of PB-mediated ROP systems and a useful guide for the further design of organocatalysts applied to polymer synthesis. An outlook is given at the end.