Ziegler-Natta Catalysis: Structural Variations and Tacticity Control

Models for the likely active catalysts in homogeneous Ziegler-Natta systems have been studied using ab initio quantum chemical methods. We investigated the geometries of the isoelectronic model complexes, X2M-R where X = C1 or Cp = (q5-CsHs); where M = Sc and Ti+ (and also Ti); and where R = H, CH3, or SiH3. The general trend is that the M = Sc compounds strongly prefer a planar configuration, whereas the M = Ti+ cases generally prefer pyramidal geometries. This difference in geometry can be related to the differing ground-state electronic configurations for the metals: Sc is (4~)~(3d) l , whereas Ti+ is (4~)1(3d)~. The nonplanar geometry for [CpzTi-R]+ suggests an explanation for the origin of stereospecificity in the syndiotactic polymerization by unsymmetric metallocene catalysts. These results suggest that (($-C5H4)CMe2($-fluorenyl)jSoR would not catalyze syndiotactic polymerization under these conditions.