Comparative Study of Various Mechanisms for Metallocene-Catalyzed α-Olefin Polymerization

In this paper we try to give answers to some unexplained flaws in the widely accepted Cossée−Arlman mechanism (CAM) for the propagation step of the polymerization of α-olefins. We first take the bare cationic catalyst and make a comparison of the CAM with the so-called Trigger mechanism (TM) proposed by Ystenes, and not only see whether the latter model can predict, qualitatively, some reaction behaviors not explained by the former one but also theoretically calculate both reaction paths and make a quantitative comparison, in terms of reaction free energy. It turns out that, while in terms of electronic energy the TM is favored, inclusion of free energy corrections gives a lower energy profile for the CAM reaction path. We then make use of a model for an experimentally used catalyst counterion and study how it affects the reactivity of the metallocenic cation fragment toward the monomer.