DFT and QSAR Studies of Ethylene Polymerization by Zirconocene Catalysts

A computational study of olefin polymerization has been performed on 51 zirconocene catalysts. The catalysts can be categorized into three classes according to the ligand framework: class I, Cp2ZrCl2 (10 catalysts), class II, CpIndZrCl2 (38 catalysts), and class III, Ind2ZrCl2 (3 catalysts), Ind = η5-indenyl. Detailed reaction pathways, including chain propagation and chain termination steps, are modeled for ethylene polymerization using zirconocene catalysts. Optimized structures for reaction coordinates indicated the presence of α-agostic interactions in the transition states (TSs) for both the first and second ethylene insertions, as well as in the ethylene π-complex of the ZrnPr cation. However, β-agostic interactions predominate in the cationic n-propyl and n-pentyl intermediates. The calculated activation energy barrier energies show that the TS for the insertion of ethylene into the Zr–CH3+ bond is the highest point on the computed reaction coordinates. Quantitative structure–activity relationship studies were also performed for 38-mixed zirconocene dichlorides. This study, in concert with the previous work, suggests that the type of ring attached to Zr (Cp vs Ind) affects the reaction kinetics and thermodynamics less significantly than the type of substituents attached to the Cp and indenyl rings and that substituent effects are even greater than those arising from changing the metal (Zr vs Hf).