Theoretical investigation into the mechanism of ethylene polymerization by a cationic dinuclear aluminum complex: A longstanding unresolved issue

The mechanism for the polymerization of ethylene via a mononuclear aluminum catalyst has been shown previously to lead to inconsistent results. We propose here for the first time a plausible mechanism involving a dinuclear aluminum species which overcomes the problems of the mononuclear catalyst. With the aid of computational quantum chemistry, we have shown that the dinuclear pathway has a much lower activation energy than the mononuclear pathway, a result which can be explained in terms of a greater orbital overlap being maintained in the dinuclear transition structure. We show that the generation of one growing polymer chain is more likely than that of two or three growing polymer chains. Importantly we find that the propagation step is more favorable than termination, which is in contrast to the findings with the mononuclear catalyst.