Design of Hybrid Steric α-Diimine Nickel Precatalysts with Intramolecular π-π Interactions for the Synthesis of Polyethylene Elastomers

Abstract

A series of hybrid steric α-diimine Ni(II) precatalysts incorporating a rigid diarylmethyl group and a flexible isopropyl substituent were designed and synthesized for ethylene polymerization. Single-crystal X-ray diffraction analyses revealed intramolecular offset π-π stacking between the electron-deficient acenaphthene-based backbone and the electron-rich aryl rings of the diarylmethyl group. This non-covalent interaction rigidified the ligand framework, suppressed N-aryl rotation, and conferred exceptional thermal stability to the precatalysts, particularly at lower temperatures where enhanced π–π interactions further stabilize the structure. Upon activation with MAO, these precatalysts exhibited high activities (up to 2.31 × 107 g mol-1 h-1), which were largely retained even at 110 °C. They produced high-molecular-weight polyethylene elastomers with tunable branching densities (54–96/1000C). The catalytic performance was critically influenced by remote para-substituents on the diarylmethyl moiety, with electron-donating groups (–OCH3) yielding both the highest activity and molecular weight. Notably, the "ABAB"-type hybrid steric precatalysts displayed superior activity and branching density to their unsymmetric "AABB"-type counterparts. The resulting polymers demonstrated excellent elastomeric properties, including high elongation at break (up to 1329%) and outstanding strain recovery (up to 86%), thereby demonstrating the efficacy of this synergistic design for the direct synthesis of high-performance thermoplastic elastomers.