Enhancing the thermal stability of α-Diimine Ni(II) catalysts for ethylene polymerization via symmetrical axial shielding strategy

Targeting practical complex catalyst for ethylene polymerization, the catalytic system requires a good thermo-stability along with high activity as well as reasonable molecular weights of resultant polyethylene. Herein, a series of symmetrical α-diimine-nickel complexes bearing functionalized 2,6-dibenzhydryl-4-t-butylphenyl N-aryl were prepared and examined in detail for ethylene polymerization, and compared the catalytic performance with their unsymmetrical analogues. The symmetrical catalysts displayed high activities in the level of 106 g mol-1 h−1, producing high molecular weight branched polyethylene (Mw: > 1.79 × 105 g mol−1) at all the reaction temperatures ranging from 30 °C to 100 °C, in comparison, both classical and unsymmetrical α-diimine-nickel analogues gave low activities and became completely inactive at 100 °C under similar conditions. It was proposed that π-π interactions between the acenaphthylene and phenyl of benzahydral group suppress the N-aryl rotation and thereby generate a blockage around the metal center by interlocking the axial positions, which in result significantly improve the thermal stability of the nickel complex catalysts.