Ethylene polymerization catalyzed by dinickel complexes with a double-decker structure
Double-decker dinickel complexes with a macrocyclic ligand containing two salicylaldimine groups are employed as catalysts for ethylene polymerization. The ligand, whose two imine nitrogens are connected by a 2,3,6,7,9,9-hexamethylxanthene-4,5-diyl spacer (1-2H) and a 2,7-tert-butyl-9,9-dimethylxanthene-4,5-diyl spacer (2-2H), forms dinickel complexes [Ni2Me2(PMe3)2(1)] and [Ni2Me2(PMe3)2(2)]. The catalytic activity of [Ni2Me2(PMe3)2(1)] is higher than those of [Ni2Me2(PMe3)2(2)] and the mononickel complex [NiMe(PMe3)(1-H)] at r.t. The GPC results of the polyethylene obtained suggest that the reaction catalyzed by the dinickel complexes produced polyethylene with a much wider molecular weight distribution (Mw/Mn > 4.0) than that catalyzed by the mononickel catalyst [NiMe(PMe3)(1-H)] (Mw/Mn = 1.8). The polymerization catalyzed by [Ni2Me2(PMe3)2(2)] showed higher activity at 60 °C than at room temperature. Polyethylenes obtained using [Ni2Me2(PMe3)2(1)] and [Ni2Me2(PMe3)2(2)] catalysts were divided into a fraction with Tm = 76–77 °C and a more branched structure (69 and 95 (/1000 C)) and another with Tm = 118–120 °C and a less branched structure (36 and 39 (/1000 C)). Polymerization by the mononuclear complex does not produce the polyethylene corresponding to the less branched fraction. The cooperative effect between the two nickel centers is responsible for the formation of the less branched crystalline polyethylene with a higher molecular weight.