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 alpha-diimine nickel catalyst by ligand backbone framework, a series of alpha-diimine nickel(II) complexes with bulky camphyl or diaryl-substituted backbones, [2,6-(R-2)(2)C6H3-N=C(R-1)-C(R-1)=N-2,6-(R-2)(2)C6H3]NiBr2 (1a, R-1 = Ph, R-2 = CH3; 2a, R-1 = 4-methylphenyl, R-2 = CH3; 3a, R-1 = 4-fluorophenyl, R-2 = CH3; 4a, R-1 = camphyl, R-2 = CH3; 4b, R-1 = camphyl, R-2 = 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 C-Ar-N bond. Ethylene polymerizations catalyzed by these nickel alpha-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 found that the catalysts containing a camphyl backbone have excellent thermal stability and polymer structure control for ethylene polymerizations. Even at 80 degrees C, the 4b/MAO system still kept high activity and relatively stable kinetics and produced high molecular weight polyethylene. Moreover, the branching degrees and branched chain distribution of the polyethylenes obtained by the complex could also be controlled by tuning the reaction temperature.