Ethylene Polymerization by Dinuclear Xanthene-Bridged Imino- and Aminopyridyl Nickel Complexes

A series of xanthene-bridged dinucleating ligands bearing imino- and aminopyridyl moieties and their nickel complexes were synthesized and characterized. The properties of these dinuclear complexes in ethylene polymerization were studied in comparison with the corresponding mononuclear nickel complexes. The iminopyridyl dinuclear nickel complexes activated by methylaluminoxane (MAO) showed higher catalytic activities (up to 2.2 × 106 g of PE (mol Ni)−1 h–1), higher molecular weights, and produced polyethylene with much lower branching density (27/1000C) than their mononuclear analogues. Similar trends were observed for the aminopyridyl dinuclear complexes. A metal–metal cooperativity effect was proposed to be able to slow down the β-hydride elimination and the corresponding chain-walking process. These results clearly demonstrated the great potentials of dinuclear nickel catalysts with the xanthene-bridged coordination modes in controlling the ethylene polymerization process as well as the microstructures of the resulting polyethylene products.