A theoretical study of Ziegler-Natta olefin polymerization on the TiCl 3 crystalline surface

Abstract

Olefin insertion into Ti-methyl bonds of methyltitanium chloride clusters on a TiCl 3 crystalline surface has been studied by using paired interacting orbitals (PIO). We have shown that at least two types of active sites can be assumed on the edge of the basal face of violet TiCl 3 crystallites: a edge type active site which possesses a dangling bonded Cl atom, four bridged Cl atoms and a Cl vacancy and a corner type active site which possesses a dangling bonded Cl atom, three bridged Cl atoms and two Cl vacancies. The size effects on these active site have been estimated by changing the number of titanium atoms of the TiCl 3 model clusters. The most important interaction in the olefin coordinated state is electron delocalization from the occupied Ti d xz orbital of the methyltitanium chlorides to the π∗ orbital of the coordinated olefin molecule, while in the transition state is electron delocalization from the occupied C p and Ti d xz orbital of the methyltitanium chloride species to the π∗ orbital of the olefin. These interactions are compactly shown in PIO's. The coordination energy and the activation energy for the olefin insertion are almost independent of the cluster size when the number of Ti atoms is greater than three. The corner type active site has been shown to be stabilized at the transition state by the absence of one of the Cl atoms which are located orthogonal to the insertion plane. Thus, the olefin insertion on the corner type active site is more favorable than that on the edge type active site. In the case of propylene insertion, syn-orientation of propylene is favored. This regioselectivity should be lower on the corner type active site than that on the edge type active site.