Mutual Impact of Benzhydryl and Benzhydryloxy Groups on Enhancing Thermal Stability of α‐Diiminonickel Precatalysts for Plastomeric Polyethylene

ABSTRACT Diimine‐nickel catalysts usually show low activity and poor polyethylene properties during ethylene polymerization at high temperatures, which can be addressed by precise tuning of the ligand structure. In this study, a series of unsymmetrical 1,2‐bis(imino)acenaphthene‐nickel(II) complexes bearing a fixed, sterically demanding 2,6‐dibenzhydryl‐4‐(benzhydryloxy)phenylimine and systematically varied N‐aryl substituents were synthesized and evaluated for ethylene polymerization. These complexes are well characterized by FTIR, elemental analysis and single crystal x‐ray diffraction ( Ni2 Et and Ni5 Et ). Polymerization behavior was strongly influenced by both cocatalyst choice and reaction conditions, with EASC delivering the highest activity (up to 1.34 × 10 7 g·mol −1 ·h −1 ) and producing high molecular weight (2.0–11.6 × 10 5 g mol −1 ), semicrystalline polyethylene, while MMAO produced more amorphous materials with lower melting points. Of significant note, high‐temperature polymerization demonstrated excellent catalyst thermal stability, with maintained activity of 1.8 × 10 6 g·mol −1 ·h −1 and molecular weight of 2.1 × 10 5 g mol −1 at 100°C. Sterically less hindered N‐aryl groups favored higher activity, whereas bulkier substituents promoted chain propagation for higher polymer molecular weight polyethylene. Change of steric substituent resulted in precise control over crystallinity ( X c : 5.7 to 80.8%), which showed a strong relationship with the mechanical properties of resulting polyethylene, displaying high tensile strength (up to 11.6 MPa) and elongation at break (up to 586%). Compared to previous unsymmetrical nickel catalysts, these polyethylenes offer enhanced tensile performance, emphasizing the structural control exerted by the catalyst structure on material properties.