Origin of variation in polymerization activity for Ziegler-Natta catalyst with chloride and alkoxy ligands: a density functional study

Abstract

The origin of variation in the activity of Ziegler-Natta (ZN) catalysts having chloride and alkoxy ligands for ethylene and propylene polymerization has been investigated using density functional calculations at B3LYP/LANL2DZ level. The barriers for olefin insertion into [TiCl₂CH₃]⁺ for ethylene and propylene are comparable whereas for catalysts with alkoxy ligands, the insertion barriers for propylene are about 5 kcal mol^-1 higher than the corresponding ethylene insertion barriers. This supports the experimental observation that the propylene polymerization with ZN catalyst having alkoxy ligands has very low catalytic activity as compared to that for ethylene polymerization whereas [TiCl₂CH₃]⁺ polymerizes both ethylene and propylene. It is further revealed that the electronic factors of ligands are dominant over the steric factors in olefin polymerization using such catalysts. The bonding aspects of various stationary structures on the potential energy surface of olefin insertion reaction are investigated using topological properties of electron density.