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.