α-Diamine Nickel Catalysts with Nonplanar Chelate Rings for Ethylene Polymerization

A series of novel alpha-diamine nickel complexes, (ArNH-C(Me)-(Me) C-NHAr) NiBr2, 1: Ar= 2,6-diisopropylphenyl, 2: Ar= 2,6-dimethylphenyl, 3: Ar= phenyl), have been synthesized and characterized. X-ray crystallographic analysis showed that the coordination geometry of the alpha-diamine nickel complexes is markedly different from conventional adiimine nickel complexes, and that the chelate ring (N-C-CN-Ni) of the alpha-diamine nickel complex is significantly distorted. The alpha-diamine nickel catalysts also display different steric effects on ethylene polymerization in comparison to the alpha-diimine nickel catalyst. Increasing the steric hindrance of the alpha-diamine ligand by substitution of the o-methyl groups with o-isopropyl groups leads to decreased polymerization activity and molecular weight; however, catalyst thermal stability is significantly enhanced. Living polymerizations of ethylene can be successfully achieved using 1/Et2AlCl at 35 degrees C or 2/Et2AlCl at 0 degrees C. The bulky alpha-diamine nickel catalyst 1 with isopropyl substituents can additionally be used to control the branching topology of the obtained polyethylene at the same level of branching density by tuning the reaction temperature and ethylene pressure.