Bimetallic Complexes as Catalysts for the Copolymerisation of Epoxides and CO2

Abstract

A series of novel macrocyclic ligands bearing amine and phenol donor atoms was synthesised, with differing substituents on the aromatic ring. These ligands were designed to allow incorporation of two metal centres in proximity within a flexible macrocyclic cavity. From these ligands, three new bimetallic zinc acetate complexes were produced. These complexes were the most active catalysts for the copolymerisation of CO2 and cyclohexene oxide at 1 atm CO2 pressure, which could significantly reduce the energy cost of plastic production. The catalysts produced poly(cyclohexene carbonate) with near-perfect CO2 incorporation. Variation of reaction conditions showed the catalysts were tolerant of water/oxygen, and active under low loadings. Trimetallic complexes were also synthesised, showing an externally bound zinc site to exhibit reduced activity. The copolymers produced had low molecular weights (<20,000 g/mol), probably due to chain transfer reactions. Replacement of zinc with cobalt(II) increased the activity tenfold, probably due to the more nucleophilic cobalt-carbonate bonds. Oxidation of the Co(II)/Co(II) complex produced a mixed valence Co(II)/Co(III) complex; characterised by X-ray crystallography, UV-Vis spectroscopy and magnetic measurements. The complexes showed extremely high activity under 1 atm CO2, furthermore, at 10 atm, an activity similar to the best catalysts was observed. Both ionic and neutral cobalt(II) halide complexes were synthesised. These complexes showed slightly higher molecular weights, but lower activities. By coordinating strong nucleophilic donors (such as 4-dimethylaminopyridine) to the metal centre, the catalytic activity was significantly reduced, a result which led to the proposal of a bimetallic mechanism. The use of other epoxides (e.g. propylene oxide and styrene oxide) in the copolymerisation was attempted. However, the catalysts were inactive for copolymerisation with these epoxides. The terpolymerisation of cyclohexene oxide, CO2 and propylene oxide showed poor control and activity. The block copolymerisation of PCHC with lactide was also investigated producing, tri-block copolymers.