DFT Study on the Mechanism of Isoprene Polymerization Catalyzed by Ziegler-Natta Catalyst with Esters and Diethers

The internal electron donor is a crucial component of modern Ziegler-Natta catalysts. In this manuscript, we investigated the adsorption behavior of dimethyl phthalate (DMP), dimethyl 2,3-dimethylsuccinate (DMS), and 2,2-dimethyl-1,3-dimethoxypropane (DMMP) on the multisite titanium active center model named TMC-2, as well as the coordination and insertion processes of isoprene using density functional theory (DFT). Among the models, the DMS chelating adsorption configuration (TMC-2*^d) exhibited the lowest reaction energy barrier (ΔG_re = 31.2 kcal/mol) and the highest stereoselectivity (ΔGstereo = 9.2 kcal/mol) during isoprene polymerization. In contrast, the adsorption of DMP and DMMP exerted only a marginal influence on both the geometric parameters of the active centers and the reaction energy barrier. In summary, the adsorption mode and type of internal electron donors significantly influence the coordination free energy and insertion energy barrier of isoprene. For the two insertion modes of isoprene, named re- and si- spatial configurations, the cis-1,4-re configuration presents the methyl substituents pointing to the right (clockwise when viewed down the C1-C4 axis), whereas the cis-1,4-si configuration presents them to the left (counterclockwise); this local chirality dictates which face can readily access the titanium center after donor adsorption. The reconfigured isoprene can spontaneously coordinate across all internal electron donor adsorption models, whereas the coordination process for the si configuration is strongly hindered.