Copper(II) Ethylene Polymerization Catalysts: Do They Really Exist?

The reactions of two types of copper(II) ethylene polymerization catalysts, [(sal)CuCl]2 (sal = 2-{C(H)N(2,6-iPr2-C6H3)}-4,6-tBu2-phenoxide) and (α-diimine)CuCl2 (α-diimine = [(2,6-iPr2-C6H3)NC(Me)]2), with methylaluminoxane (MAO) and trimethylaluminum (TMA) have been investigated. In both examples, facile and irreversible ligand (L) transfer from copper to TMA present in MAO was observed, resulting in formation of the corresponding (sal)AlMe2 and (imino-amido)AlMe2 complexes. The (imino-amido)AlMe2 complex is formed by α-diimine ligand transfer to aluminum followed by alkylation of one imino moiety in the ligand backbone. Both aluminum complexes were active catalysts for ethylene polymerization with activities similar to their Cu(II) precursors. Simple addition of a neutral salicylaldimine or α-diimine ligand to MAO in the absence of any copper species resulted in the formation of the corresponding LAlMe2 complexes, which are again active for ethylene polymerization. These results indicate that ethylene polymerization does not occur by a migratory insertion mechanism at the copper center, but is the result of ligand transfer to aluminum, and it is the resulting LAlMe2/LAlMe+ complexes that are likely the active species.