Synthesis of highly branched polyethylene using para‐phenyl‐substituted α‐diimine nickel catalysts

A series of para-phenyl-substituted α-diimine nickel complexes, [(2,6-R2-4-PhC6H2N═C(Me))2]NiBr2 (R = iPr (1); R = Et (2); R = Me (3); R = H (4)), were synthesized and characterized. These complexes with systematically varied ligand sterics were used as precatalysts for ethylene polymerization in combination with methylaluminoxane. The results indicated the possibility of catalytic activity, molecular weight and polymer microstructure control through catalyst structures and polymerization temperature. Interestingly, it is possible to tune the catalytic activities ((0.30–2.56) × 106 g (mol Ni·h)−1), polymer molecular weights (Mn = (2.1–28.6) × 104 g mol−1) and branching densities (71–143/1000 C) over a very wide range. The polyethylene branching densities decreased with increasing bulkiness of ligand and decreasing polymerization temperature. Specifically, methyl-substituted complex 3 showed high activities and produced highly branched amorphous polyethylene (up to 143 branches per 1000 C).