(Salen)Cr^IIIX Catalysts for the Copolymerization of Carbon Dioxide and Epoxides:  Role of the Initiator and Cocatalyst

The copolymerization of CO(2) and cyclohexene or propylene oxide has been examined employing (salen)Cr(III)Nu complexes (Nu = Cl or N(3)) as catalysts. The addition of various cocatalysts, including phosphines and PPN+ or Bu4N+ Cl- salts serves to greatly enhance the rate of copolymer production. In these instances, the mechanism of the initiation step appears to be unimolecular in catalyst concentration, unlike the bimolecular process cocatalyzed by N-methylimidazole. The copolymers were produced with >95% carbonate linkages with TOFs in the range 39-494 mol epoxide consumed/mol Cr.h. In the presence of phosphine cocatalysts, no cyclic carbonate was produced as a byproduct.