Rare Earth Metal‐Containing Ionic Liquid Catalysts for Synthesis of Epoxide/Cyclic Anhydride Copolymers

The perfectly alternating ring‐opening copolymerization of epoxides and cyclic anhydrides has been an emerging route to diverse polyesters. Simple catalysts have been targeted for this polymerization to decrease the cost and air‐sensitivity of the catalysis. This report improves upon the recently reported YCl3 ⋅ 6H2O/[PPN]Cl (bis(triphenylphosphine)iminium chloride) binary catalyst system, which shows fast rates of polymerization but suffers from low molecular weights due to the presence of many water chain transfer agents. In this study, use of a phosphonium chloride ionic liquid as both the cocatalyst and solubilizing agent for the metal salt to form a metal‐containing ionic liquid (MIL) leads to polymerization of high molecular weight polyesters at significantly faster rates than the original system. The identity of the ionic liquid, dryness of MIL, and synthetic prep of the MIL were all found to have a greater impact on the polymerization rate than the metal choice. Metal‐containing ionic liquids (MILs), involving the combination of rare earth metal chlorides with a phosphonium chloride ionic liquid, were found to be highly active and selective for the ring‐opening copolymerization of epoxides and cyclic anhydrides. These MILs were able to produce high molecular weight polymers >30 kDa faster than other catalysts in the literature.