DFT modeling of the post-titanocene catalytic system LTiCl2-Bu2Mg-Et2AlCl for alkene polymerization: The role of alkyl bridge Mg–C–Ti and β-agostic C–H–Ti bonds in the formation of active centers

Abstract

DFT modeling of the active centers formation in the catalytic system LTiCl₂-Bu₂Mg-Et₂AlCl for alkene polymerization, where L is a bidentate ligand of saligenin type, suggests the three-step mechanism of this process. This mechanism includes the addition of the most probable alkylating agent, RMg(μ-Cl)₂AlR₂ or RMg(μ-Cl)₂MgR (R = alkyl), to LTiCl₂ with the formation of the trinuclear heterocomplex with the alkyl bridge bond Mg–C–Ti, followed by its two-step isomerization into the active center via the β-agostic intermediate. The free energy changes at the stages of the addition and isomerization are negative; the maximal energy barrier on the reaction pathway is small. In the Mg-free system LTiCl₂-AlR₃, the isomerization proceeds through the single energy barrier with a significantly higher amplitude. This could be the reason for a relatively high activity of the Mg-containing system and inactivity of the Mg-free system.