Moving up and down the titanium oxidation state in Ziegler-Natta catalysis

DFT molecular modeling studies were undertaken to shed light on possible activation and deactivation mechanisms of Ziegler-Natta catalytic systems, as well as on the possible mechanisms for their reactivation by organohalides. We focused our efforts on Ti species attached to the (110) lateral cut of MgCl ₂. First, the possible activation of adsorbed TiCl₄ leading to an adsorbed Ti^III species bearing a Ti-alkyl bond and a coordination vacancy, which is a species able to undergo chain-growth, was considered. According to our calculations formation of the first active species can be easily rationalized by cleavage of a Ti-Cl bond of coordinated TiCl ₄ by AlEt₃, followed by transalkylation promoted by another AlEt₃ molecule. Second, we investigated the possible reduction of polymerization active Ti^III species leading to polymerization inactive Ti^II species, and we found that a Ti-H bond, possibly formed after chain termination, is weaker than the Ti-Et (polymeryl) bond. Third, we investigated the mechanism of reactivation of Ti^II species by organohalides, and it was concluded that reoxidation by Cl rich organohalides is thermodynamically more favored.