High molecular weight polyethylene via the spatial proximity of benzosuberyl in iminopyridine nickel catalysts

Abstract

Iminopyridine nickel catalysts are typically prone to facile chain transfer reactions, resulting in low molecular weight polyethylenes. In this study, a spatial proximity strategy was employed in 5-dibenzosuberyl-modified iminopyridine nickel catalysts to enhance ethylene polymerization. Using a template reaction between acetylpyridine and 5-dibenzosuberyl-functionalized aniline, a series of 2-(1-(2,6-bis(5-dibenzosuberyl)-4-(alkyl)phenylimino)ethyl)pyridine ligands were synthesized and subsequently reacted with (1,2-dimethoxyethane)NiBr₂ to afford the corresponding nickel complexes. Single-crystal X-ray diffraction revealed a sandwich-like arrangement in the resulting nickel complexes, with short centroid-to-plane distances (average: 3.194 Å for Ni1^Me and 3.268 Å for Ni2^iPr), suggesting close spatial proximity between the benzosuberyl phenyl caps and the chelate plane. Compared to DEAC, activation with MAO resulted in a higher activity (up to 2.2 × 10⁶ g mol⁻¹ h⁻¹), significantly increased molecular weights (56–182 kg mol⁻¹), and a narrower dispersity (PDI = 1.5–1.8). Notably, the polymer molecular weights were 10 to 100 times greater than those of most previously reported iminopyridine nickel catalysts lacking benzosuberyl steric substituents, indicating the strong and advantageous impact of the spatial proximity of benzosuberyl toward the chelate plane on polymerization. Moreover, in the case of substituent variations in the catalyst, dibenzosuberyl substituents at all the ortho- and para-positions of aniline demonstrated a positive effect on both the monomer insertion rate and chain propagation, leading to high catalytic activity and polymer molecular weights. The resulting polyethylene predominantly contained methyl branches, with an overall branching density of 53 to 99/1000C, as confirmed by high-temperature NMR measurements.