Role of the Counteranion in the Reaction Mechanism of Propylene Polymerization Catalyzed by a (Pyridylamido)hafnium(IV) Complex.

We studied the stereoselective propylene polymerization catalyzed by the ion pair (IP) active species of (pyridylamido)hafnium(IV) complex, the catalyst. In particular, we focused on the role of the counteranion (CA) for the reaction mechanism of propylene insertion because it had never been investigated from the molecular point of view. First, we searched for a variety of IP configurations by the molecular dynamics (MD) method and quantum chemical calculation, and located the transition states (TSs) of propylene insertion. By comparing the CA-bound catalysts with the isolated ones, it was revealed that the manner of energetically favorable insertion is interestingly altered due to the location of the CA. Next, we showed that the CA is essential to determine the coordination structures of the Hf atom at transition state. Therefore, it was concluded that the role of the CA is critical to accurately control the polymerization mechanism. Finally, we proposed a synthesis experiment which would be advantageous to increase the stereoselectivity of the catalyst on the basis of the present polymerization mechanism. [ABSTRACT FROM AUTHOR]