Effect of Different Alkylaluminums on Butadiene–Isoprene Copolymerization Catalyzed by the Ziegler–Natta Catalyst: A DFT Study

Alkylaluminum is used as a cocatalyst in the Ziegler–Natta catalytic system, and its structural characteristics directly affect the activity of the catalyst. In this article, the mechanism of butadiene–isoprene copolymerization in the TiCl4/MgCl2-type Ziegler–Natta catalyst system was investigated using density functional theory (DFT) calculations, and the effects of different cocatalysts on catalytic activity were examined. Active center models (M-1, M-2, and M-3) were constructed using trimethylaluminum (TMA), triethylaluminum (TEA), and triisobutylaluminum (TIBA) as cocatalysts, and the reaction pathway and free energy barrier for butadiene–isoprene copolymerization were calculated. The results show that alkylaluminums reduce Ti4+ to Ti3+ by alkylation to form active centers. Among them, the average free energy barriers of butadiene–isoprene copolymerization on M-1, M-2, and M-3 models are 33.8, 30.7, and 31.5 kcal/mol, respectively, indicating that TEA and TIBA as cocatalysts can enhance the catalyst activity to a greater extent, with TEA showing the most significant improvement.