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 MgCl2. First, the possible activation of adsorbed TiCl4 leading to an adsorbed TiIII 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 TiCl4 by AlEt3, followed by transalkylation promoted by another AlEt3 molecule. Second, we investigated the possible reduction of polymerization active TiIII species leading to polymerization inactive TiII 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 TiII species by organohalides, and it was concluded that reoxidation by Cl rich organohalides is thermodynamically more favored

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