Recent progress on nickel-based complex pre-catalysts is reviewed herein.The ethylene oligo-/polymerization behaviour is disscussed in terms of the variation of the complex models bearing different kinds of ligand sets. These discussions focus mainly on the influence that the different substituents present have on the observed catalytic activity, the results of which can guide the design of new target structures possessing high ethylene activity.
A series of 2-[1-(4,6-dimethyl-2-benzhydrylphenylimino)ethyl]-6-[1-(arylimino)ethyl]pyridines was synthesized and used to prepare the iron(II) and cobalt(II) chloride complexes thereof. All organic compounds were fully characterized by elemental analysis, IR and NMR ((1)H/(13)C) spectroscopy, whilst the metal complexes were characterized by elemental analysis and IR spectroscopy as well as by single-crystal X-ray diffraction studies (for two representative cobalt complexes), which revealed that the geometry at the metal was either pseudo-square-pyramidal or trigonal bipyramidal. Upon activation with either methylaluminoxane (MAO) or modified methylaluminoxane (MMAO), all metal complex pre-catalysts exhibited high activities for ethylene polymerization. The iron pre-catalysts show much higher activity than did their cobalt analogues; however, the iron catalytic systems generally produced polyethylene of wide molecular weight polydispersity. At elevated reaction temperature, the polyethylene was of lower molecular weight, but revealed narrow polydispersity.
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