DFT Study of Ethylene and Propylene Copolymerization over a Heterogeneous Catalyst with a Coordinating Lewis Base

Abstract

The copolymerization of ethylene and propylene over a heterogeneous Ti(III) catalyst containing tetrahydrofuran (THF) as a Lewis base and MgCl2 as a support has been studied by means of DFT. Two feasible models of active sites have been examined thoroughly, and one of them turned out to be favorable in terms of both catalytic activity and the microstructure of the resulting polymer. The external barriers of olefin insertion for this model range from 3.1 to 16.0 kcal/mol and are influenced by a variety of factors, such as the structure of the growing polymer chain and the nature of the incoming olefin as well as the orientation of the ligands around the titanium atom. Stochastic simulations performed on the basis of insertion and termination barriers provided us with the insight into the composition and microstructure of the copolymer as well as its molecular weight as a function of comonomer partial pressures. It is demonstrated that the reactivity of ethylene in the copolymerization process is significantly higher than that of propylene, which is consistent with known experimental data. Our results also indicate moderate regiospecificity and stereoselectivity toward propylene, depending on the partial pressures of the comonomers.