Mono- and dinuclear olefin reactions at aluminium

As an alternative to the standard Cossee mechanism, in which olefin insertion involves two sites at a single metal centre, we have investigated a true dinuclear alternative where the chain switches between two different metal centres at each insertion. The corresponding dinuclear variations of  hydrogen elimination (BHE) and  hydrogen transfer to monomer (BHT) were also investigated. Surprisingly, calculations indicate that the barriers for both insertion and BHT at two different metal centres are rather similar to those for the more usual mononuclear mechanisms. Dinuclear BHE is more competitive as a chain transfer mechanism, although it always has a higher barrier than BHT. In any system where polymerization at an unknown aluminium active species is believed to occur, dinuclear insertion should be considered as a real alternative to the "standard" mononuclear mechanism. However, from the systems we have studied the prospects for designing highly active, high-MW Al polymerization catalysts (i.e. significantly better than trialkylaluminium) appear just as dim for dinuclear as for mononuclear species.