Paired interacting orbitals (PIO) study on the formation of the active site in Ziegler catalyst system

Abstract

The mechanism of the active site formation in Ziegler polymerization catalysts has been studied using the paired interacting orbitals (PIO) based on an extended Hückel calculation. Deformation of a titanium chloride molecule and partial dissociation of an organoaluminum dimer have been shown to take place by approaching the two species together. A six membered cyclic intermediate complex has been formed in a reaction plane, involving the Ti atom, the C atom, two Al atoms, the Cl* atom and the bridging atom. An elongation of the CAl bond, Al-bridging atom bond, the TiCl* bond and shrinkage of the CTi bond take place in concert in the transition state region. Key factors of activating the complex are the valency of the titanium and the structure whether or not it has a Cl anion trans to the Cl* atom. An active site is the complex between the alkylated titanium chloride and the aluminum chloride produced in the course of the reaction. The formation of the active site in titanium chlorides/organoaluminum systems has been clarified to become easier in the following order: d 1-Sq (square) pyramidal ⪢ d 0-Sq pyramidal ⩾ d 0-hinge like titanium chloride. d 0-triangle pyramidal titanium chloride is unfavorable for the formation of active sites.