N-heterocyclic carbene complexes in ring opening polymerization

Abstract

Metal-N-heterocyclic carbene complexes (M−NHCs) have established themselves as one of the most promising catalytic systems for a variety of transformations. In particular, various well-defined biodegradable polymers have been synthesized by M−NHCs via ring opening polymerization, the field has made impressive progress during the past decades. We present a concise overview of ring opening polymerization of cyclic monomers catalyzed by group 4 to 13 metal-NHC complexes. This review covers major achievements and developments of the field from 2004 to 2020 with a focus on the catalyst/initiator structure–activity relationship of M−NHC complexes. We begin by introducing the general synthetic routes to M−NHCs. The content is organized according to metals and types of N-heterocyclic carbenes (NHCs) i.e. classical and nitrogen-, oxygen-, phosphorus-, and sulfur-functionalized NHCs, latter have additional donor functionality/functionalities in covalent tethered of the ligand. A wide range of cyclic monomers is covered, which includes cyclic esters, carbonates, and epoxides. Furthermore, M−NHCs catalyzed ring opening copolymerization and metal-NHC adducts catalyzed polymerization reactions are also described. In the last section limitations and challenges of the field are discussed.