Revisiting the Stereochemistry of Propylene Isotactic Polymerization Reaction Mechanism on C-2 Symmetric [SiH2(Ind)(2)ZrCH3](+) and [SiH2(Ind)(2)ZrCH3](+)[CH3B(C6F5)(3)](-)

摘要

The [SiH2(Ind)(2)ZrCH3](+) (Ind = indenyl) catalyzed stereoregularity of propylene polymerization mechanism has been investigated at M06 level of theory. Four different approaches of propylene to the reactive catalyst lead to four isomeric products due to the C-2 symmetry of bridged Ind ligand of the catalyst. Consequently, various possibilities of propylene attack as well as orientation of polymer chain yield numerous stereoisomers. The calculations of the first and second insertions with various conformers clarified the most favorable reaction pathway and showed that isotactic propagation is more favorable (3.5 kcal mol(-1)) than syndiotactic propagation. The structures of resting state catalysts displayed various agostic interactions of the CH bond with the Zr center which stabilize the catalytic systems and play important roles in determining the favorable reaction pathway. The influence of counter anion [CH3B(C6F5)(3)](-) on the reactivity of the catalyst was also studied. The results also confirm that the trans orientation of the counter anion with respect to propylene is more favorable than its cis orientation and clarify the most favorable reaction pathway in the first and second insertion. Because agostic interactions are involved in various aspects, AIM analysis has been used to find the bonding nature of agostic interactions as well as ion-pair bonds. The overall results suggest that rigidity of ansa- zirconocene, unique structure of C-2 symmetric ansa ligand, influence of [CH3B(C6F5)(3)](-) and beta agostic interaction may restrict the attack of propylene only to isotactic polymerization and not to syndiotactic polymerization.