The role of organoaluminum and electron donors in propene insertion on the Ziegler–Natta catalyst

Abstract

Alkyl aluminium plays a primary role in activating Ti within Ziegler–Natta (ZN) catalysts for propylene polymerization. We performed density functional calculations to explore the additional roles of AlEt₃ and AlEt₂Cl, in conjunction with diisobutyl phthalate (DIBP) internal donor and dicyclopentyl dimethoxysilane (DCPDMS) external donor, to enhance the stereoselectivity of propene insertion. Based on our calculated adsorption energies on the (MgCl₂)₁₃/TiCl₂ⁱBu cluster model for the ZN catalyst, the presence of DIBP on the cluster essentially facilitated AlEt₂Cl adsorption while AlEt₂Cl also promoted the adsorption of DIBP. The reaction between AlEt₃ and DIBP on the cluster led to the extraction of DIBP, creating an available site for DCPDMS adsorption. While the stereoselectivity, represented by the difference in the activation energies between 1,2-re and 1,2-si insertions of propene, was negligible on the cluster containing only DIBP, it became significant on the clusters containing both AlEt₂Cl and DIBP (and DCPDMS). AlEt₂Cl plays a pivotal role in imposing steric effects near the Ti active site, thereby increasing stereoselectivity. Our findings suggest the importance of including AlEt₂Cl alongside DIBP (and DCPDMS) in the ZN cluster model to investigate stereoselective propene insertion. Considering AlEt₂Cl adsorption and AlEt₃ reaction with internal donors is essential in developing Ziegler–Natta catalysts.