Theoretical investigation of the mechanism of ethylene polymerization with salicylaldiminato vanadium(III) complexes

The use of vanadium-based catalysts allows the preparation of high molecular mass polymers with uniform molecular mass distributions, polypropylene and ethylene/alpha-olefin copolymers with high a-olefin incorporation. However, the design of ligand systems with vanadium catalysts would face difficulties, because it is difficult to experimentally determine the structures of the active species of vanadium catalysts. In this paper, possible structural candidates for the active species in ethylene polymerization catalyzed by the salicylaldiminato vanadium complex combined with AlEt2Cl were investigated using density functional theory. By comparing theoretical simulation results with previous experimental investigations, especially regarding the crucial role of the diethyaluminum chloride (AlEt2Cl) cocatalyst, it was concluded that a neutral bimetallic species containing two Al-Cl-V bridging bonds is the most favorable structure model for the active vanadium species. A notable effect of Al co-catalysts was clarified in the theoretical investigation. During the formation of the active species, AlEt2Cl act as an assistant for the alkylation and alkyl abstract processes of precursors. More importantly, AlEt2Cl is necessary for the formation of the bis(chlorine-bridged) structure in the active species, which showed a notable effect on the structural stability of the active species and its catalytic activity. Additionally, we investigated the chain termination mechanism in this system. (C) 2015, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.