A combined ab initio MO-MM study on isotacticity control in propylene polymerization with silylene-bridged group 4 metallocenes. C-2 symmetrical and asymmetrical catalysts

The isotactic control capabilities in propylene polymerization of various substituted ansa-metallocenes are evaluated with a combined ab initio MO-MM method. Steric energy controlling the isotacticity at the transition state is divided into the direct term (steric energy due to the catalyst-olefin interaction) and the indirect term (steric energy due to the catalyst-polymer and polymer-olefin interaction). Among group 4 bis(indenyl) and bis(tetrahydroindenyl) complexes, titanium complexes in general have a substantially better capability of producing an isotactic sequence over zirconium and hafnium analogs, due to the small atomic size of the central metal. To attain good stereoregulation, the substituents at the 2- and 4-positions of the indenyl-based metallocene are very important. However, analysis among congested indenyl-based catalysts suggests that, though the crowded transition structures in general provide better stereocontrol, overcrowding could reduce flexibility and lower stereoregulatory discrimination. An asymmetric catalyst allowing completely one-blocked insertion shows a high isotactic control. The present method reveals qualitatively the control factors of stereoregulation that are consistent with the experiment and may be used for prediction for unknown catalysts.