On preference of insertion mechanism in the ethylene polymerization catalyzed by half-titanocene complexes with aryloxy ligands: Static and dynamic theoretical studies

The Ziegler-Rauk bond-energy decomposition analysis was performed for the frontside (FS) and backside (BS) transition states of ethylene insertion in the processes catalyzed by half-titanocenes with phenoxy ligands to rationalize the origin of the energetic preference of the backside insertion observed for the complexes with monosubstituted phenoxide(Type 4 catalysts). The final preference of the backside or frontside transition state comes as a balance between the electronic preference of the former, and the steric preference of the latter. The unique energetic preference of the backside insertion observed for Type 4 catalysts appears to be a result of reduced steric crowding. The openness near the metal center and conformational flexibility leads to enhanced catalytic activity of those systems. In addition, Car-Parinello molecular dynamic simulations were carried out to examine the influence of entropic effects on the preference of the insertion mechanism. For Type 4 catalysts, the spontaneous frontside insertion was observed. Therefore, at the free-energy level, frontside insertion becomes viable due to entropic destabilization of the backside transition state.