Copolymerization of carbon dioxide and epoxides by metal coordination complexes

Abstract

The catalytic copolymerization of CO2 with epoxides to give polycarbonates was discovered nearly 50 years ago. The last 10 years have been particularly exciting for the development on highly active homogenous catalyst systems, insights into mechanisms and the design of polymers with promising physical properties. Recent work has identified new copolymerization pathways with increasing diversity in the metals and ligands employed. Cooperative mechanisms with binary catalyst systems using exogenous nucleophiles continue to be studied. Elegant ligand design, however, has been used to generate new, highly active bimetallic or bifunctional catalyst systems. The bimetallic systems allow for “shuttling” of growing polymer chains leading to excellent activity under low CO2 pressures. Bifunctional systems use covalently linked groups that facilitate the role of the nucleophilic co-catalyst. These groups can be neutral Lewis basic sites, such as N-donors, or cationic sites, such as ammonium groups, that improve the association of the anionic nucleophilic co-catalyst components. The cationic groups also improve association of any metal-dissociated anionic polymer chain ends and allow more efficient separation of the catalyst from the polymer products via chromatography. This review gives an overview of the developments of homogeneous catalysts for CO2/epoxide copolymerization grouped by metal site. New mechanistic studies and strategies for future catalyst developments are also discussed.