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.