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

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 ...