Direct synthesis of hyperbranched oligoethylene using o-phenylene-bridged binuclear pyridine-imine Ni(II) catalysts

Abstract

Recently, nickel and palladium complexes based on pyridine-imine platform have found extensive application in olefin polymerization and copolymerization. In this study, we created two o-phenylene-bridged dinuclear nickel pyridine-imine catalysts for the oligomerization of ethylene. The dinuclear catalyst with an H atom at the 6-position of pyridine exhibited exceptionally high polymerization activity, which is an order of magnitude higher (2.82–4.65 vs 0.57–0.66 × 10⁶ g.mol⁻¹h⁻¹) than that of its corresponding brominated analog. On the other hand, the oligoethylenes produced by this catalyst had a molecular weight that is also an order of magnitude higher (1582–2213 vs 315–382 g/mol) and possesses a lower branching density (88–101 vs. 133–161/1000C) compared to the oligoethylenes generated by the brominated analog. When further compared to the corresponding mononuclear nickel catalyst, the dinuclear catalyst demonstrated a stronger ability to inhibit chain transfer and chain walking, resulting in the production of oligoethylenes with higher molecular weight and lower branching density. This may be attributed to a bimetallic synergistic effect or merely a steric hindrance superposition effect. Additionally, nuclear magnetic resonance (NMR) analysis further confirmed the hyperbranched structure of the oligoethylenes.