Mononuclear and dendritic nickel(II) complexes containing N,N'-iminopyridine chelating ligands:: Generation effects on the catalytic oligomerization and polymerization of ethylene

A series of carbosilane dendritic compounds Gn-ONNMem, containing one (n = 0), four (n = 1), eight (n = 2), or 16 (n = 3) terminal pyridylimine ligands, substituted with m methyl groups (m = 0, 2, 3), and nickel complexes Gn-ONNMemNiBr2, comprising monometallic to metallodendritic structures, have been synthesized. The nickel complexes, in combination with methylaluminoxane (MAO), are active catalysts for the concurrent transformation of ethylene into mixtures of toluene-insoluble polyethylene and oily oligomers. Oligomers consist of mixtures of olefins that follow a Schulz-Flory distribution, and polymers are found to be highly branched low molecular weight polyethylene. The variation of the pyridylimine ligand framework by methyl substituents has a decisive influence on the activities of the nickel compounds. Also, the size (i.e., generation n) of the dendritic precursor acutely affects the catalyst performance and the microstructure of the insertion products. Thus, higher generation catalysts show superior oligomerization activities and produce less branched polyethylene polymers with higher molecular weights.