Molecular Orbital Study of Zinc(II)-Catalyzed Alternating Copolymerization of Carbon Dioxide with Epoxide

Abstract

The mechanism of copolymerization of CO2 with cyclohexene oxide catalyzed by the Zn(II) organometallic compound (BDI)ZnOCH3 (BDI = N(2,6-iPr2C6H3)C(Me)CHC(Me)N(2,6-iPr2C6H3) chelating β-diimine ligand) has been studied with the hybrid molecular orbital (MO) method ONIOM, combining the density functional method B3LYP/LANL2DZ(d) with the semiempirical MO method PM3. In particular, the insertions of CO2 and cyclohexene oxide/ethylene oxide into zinc−alkoxyl and zinc−carbonate bonds have been investigated in detail. The insertion of CO2 into either a zinc−alkoxyl (epoxide + CO2 alternating insertion) or zinc−carbonate (consecutive CO2 insertion) bond has been found to be thermodynamically less favorable but is in general kinetically favored over the insertion of epoxide, due to a high barrier for the latter. This high barrier is associated with a rather asynchronous transition state where the ring opening has taken place and yet the C−O bond is not formed. However, only in the case of insertion of sterically strained cyclohexene oxide into the zinc−carbonate bond is the barrier low enough to compete with CO2 insertion, resulting in alternating copolymerization. This lowering is driven by the release of the extra strain energy in the three- and six-membered-ring bicyclic structure in cyclohexene oxide. The rate-determining step in copolymerization is epoxide insertion, which can be controlled by the catalyst and the epoxide.