Thermostable α-Diimine Nickel(II) Catalyst for Ethylene Polymerization: Effects of the Substituted Backbone Structure on Catalytic Properties and Branching Structure of Polyethylene

On the basis of the strategy of promoting thermostability of α-diimine nickel catalyst by ligand backbone framework, a series of α-diimine nickel(II) complexes with bulky camphyl or diaryl-substituted backbones, [2,6-(R2)2C6H3−N═C(R1)−C(R1)═N−2,6-(R2)2C6H3]NiBr2 (1a, R1 = Ph, R2 = CH3; 2a, R1 = 4-methylphenyl, R2 = CH3; 3a, R1 = 4-fluorophenyl, R2 = CH3; 4a, R1 = camphyl, R2 = CH3; 4b, R1 = camphyl, R2 = i-Pr), were synthesized and used as catalyst precursors for ethylene polymerization. Crystallographic analysis revealed that the bulky camphyl backbone has a valid steric-effect on the nickel center by blocking the axial site for the metal center and suppressing the potential rotation of the CAr−N bond. Ethylene polymerizations catalyzed by these nickel α-diimine complexes activated by MAO were systematically investigated and the influences of the substituted backbones as well as reaction temperature on the catalytic activity, molecular weight and branching structure of the polymers were evaluated. It was...