Benzosuberyl installed “sandwich” type unsymmetrical α-diimine nickel precatalysts for synthesizing plastomeric to elastomeric polyethylene

Abstract

Precise control over the chain walking and chain transfer capabilities of α-diimine nickel/palladium complexes can generate branched polyethylene with unprecedented thermal, rheological, and mechanical properties. In the present study, with the incorporation of 2,4,6-tris(5-dibenzosuberyl)aniline into the classical α-diimine ligand, a new family of “sandwich” type unsymmetrical nickel precatalysts was designed and prepared for ethylene polymerization. The nickel precatalysts, featuring sandwich steric structures and activated with ethylaluminium sesquichloride, exhibited rarely achieved catalytic activities (up to 32.46 × 10⁶ g_PE mol_Ni⁻¹ h⁻¹) and were able to tune the molecular weight, branching degree and melt temperature of the resulting polyethylene. A high catalytic activity of 4.31 × 10⁶ g_PE mol_Ni⁻¹ h⁻¹ under industrially relevant conditions demonstrated their characteristic thermal stability. With changes in ligand substituents and polymerization temperature, the microstructure varied from moderately branched (77/1000C) to less branched (33/1000C) polyethylene. The reduced number of branches resulted in higher melt temperatures, approaching the characteristics of semi-crystalline polyethylene. This corresponded to the conversion of elastomeric to plastomeric polyethylene with excellent tensile strength. Notably, the number of branches is significantly lower than those of previously reported “sandwich” (8-p-tolyl naphthyl α-diimine)nickel(II) catalysts, perhaps due to non-covalent interactions between the axial capping aryl group and the acenaphthene unit. Moreover, N-alkyl unit containing catalysts were completely inactive, while their N-aryl unit bearing analogues were highly active in ethylene polymerization, shedding light on the concept of catalyst design for ethylene polymerization.