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 TiCl 4 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 AlEt 3, followed by transalkylation promoted by another AlEt 3 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.