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

- The effect of Mg-containing compounds in ethylene polymerization is discussed.
- The reactions of LTiCl2 with R2Mg/R2AlCl (R = Me, Et) are studied.
- The complexes LTiCl2·nRMgCl·(2 − n)R2AlCl (n = 1,2) are the primary products.
- The primary products isomerize into the active centers via the intermediate complexes.
- The energy barriers of the active center formation are small in the Mg-containing systems.

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.

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