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

The mechanism of copolymerization of CO 2 with cyclohexene oxide catalyzed by the Zn(II) organometallic compound (BDI)ZnOCH 3 (BDI = N(2,6- i Pr 2 C 6 H 3 )C(Me)CHC(Me)N(2,6- i Pr 2 C 6 H 3 ) 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 CO 2 and cyclohexene oxide/ethylene oxide into zinc−alkoxyl and zinc−carbonate bonds have been investigated in detail. The insertion of CO 2 into either a zinc−alkoxyl (epoxide + CO 2 alternating insertion) or zinc−carbonate (consecutive CO 2 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 CO 2 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.