Cluster size effects in models of the active site for stereospecific heterogeneous Ziegler-Natta polymerization

The extended Huckel molecular orbital method has been used to study models of the active site for Ziegler-Natta catalyzed olefin polymerization on the surface of crystalline TiCl₃ containing from one to twenty titanium atoms, and the coordination of an ethylene monomer by the active site. Significant differences are observed in the coordination of ethylene to the small model systems compared to the larger model systems. These differences are analyzed using the complexation energies, bond orders and orbital populations, and are interpreted in terms of back donation from low-lying d-orbitals of the active site to the π* orbital of the olefin. Conclusions are drawn on the basis of these results regarding the requirements of model active sites suitable for study with more sophisticated theoretical methods.