Suppression of Chain Walking by Heteroatom in 2,3-Bis(imino)butane–Nickel Precatalysts for Ethylene Polymerization

Control of chain walking coupled with chain transfer in nickel-catalyzed ethylene polymerization offers a versatile approach to tune the polyethylene microstructure. In the present work, we report the design, synthesis, and polymerization performance of heteroatomic hydroanthracene- and dibenzosuberyl-functionalized 2,3-bis(imino)butane-nickel complexes. Single-crystal X-ray analysis revealed the close proximity of axial phenyl caps to the chelate plane. This proximity generates weak interactions with the metal center, influencing chain walking, chain transfer, and monomer insertion rates. Upon activation with ethylaluminum sesquichloride (EASC), oxadibenzosuberyl-containing precatalysts exhibited higher activity (up to 5.02 × 10⁶ g mol^-1 h^-1) than thiodibenzosuberyl analogues, whereas nonheteroatomic dibenzosuberyl precatalysts achieved the highest activity (up to 10.4 × 10⁶ g mol^-1 h^-1). Corresponding molecular weights followed the same trend (111.4 vs 490.9 kg mol^-1), while melting points were inversely affected (117.9 vs 104.5 °C). These trends correspond to the electronic richness of steric substituents (thiodibenzosuberyl > oxadibenzosuberyl > dibenzosuberyl) and their capacity for metal-phenyl interactions, where more electron-rich groups suppress chain walking and enhance chain transfer. Temperature-dependent chain walking and β-H elimination generated vinyl- and vinylene-terminated unsaturated polyethylene (vinyl: 100% at 40 °C; vinyl/vinylene = 36/64 at 90 °C), highlighting controllable branching and unsaturation. These results establish a rational framework for the design of high-performance nickel catalysts for polyethylene synthesis.