Synthesis of bimodal molecular weight distribution polyethylene with α-diimine nickel(II) complexes containing unsym-substituted aryl groups in the presence of methylaluminoxane

A series of unsymmetrical complexes of 2,3-bis(2-phenylphenyl)-butanediimine nickel(II) dibromide (complex 1), 1,4-bis(2-isopropyl-6-methylphenyl)-acenaphthenediimine nickel(II) dibromide (complex 2) and meso- and rac-1,4-bis (2,4-di-tert-butyl-6-methylphenyl)-acenaphthenediimine nickel(II) dibromide (meso-3 and rac-3) were synthesized and activated by methylaluminoxane (MAO) for ethylene polymerization. By 13C NMR characterization, meso- and rac-stereo-isomers were detected in the condensation products resulting from the reaction of unsym-substituted anilines with diketones. It was notable that meso- and rac-isomers in ligand 1 or ligand 2 could not be separated owing to their interconversion, however, meso- and rac-isomers in ligand 3 could be isolated and identified by X-ray diffraction and NMR analysis. At low polymerization temperatures, complex 1/MAO afforded polyethylene with bimodal molecular weight distribution, while complex 2/MAO prepared polyethylene with single-modal distribution. Moreover, by raising polymerization temperature or extending time of catalyst aging, bimodal molecular weight distribution polyethylene was also produced by complex 2/MAO. The hypothesis of bimodal molecular weight distribution polyethylene synthesized by unsymmetrical α-diimine nickel(II) complexes was supported that the molecular weight of polyethylene produced by rac-3/MAO was significantly higher than that produced by meso-3/MAO under identical polymerization conditions. A unique methodology to prepare polyethylene with bimodal molecular weight distribution was demonstrated.