Tuning 2,3-Bis(arylimino)butane-nickel Precatalysts for High-Molecular-Weight Polyethylene Elastomers

The catalytic performance of α-diiminonickel complexes is highly sensitive to structural modifications in their ligand frameworks. In this study, a series of unsymmetrical 2,3-bis(arylimino)butane-nickel complexes featuring ortho-2,6-dibenzhydryl groups as sterically demanding motifs and para-methyl groups as electron-donating enhancers were proposed and synthesized. These nickel complexes were thoroughly characterized using FTIR, elemental analysis, and single-crystal X-ray diffraction (for Ni4 and Ni5), revealing deviations from ideal tetrahedral geometry. Upon activation with Et₂AlCl, these complexes demonstrated exceptional ethylene polymerization activity, achieving a remarkable value of 13.67 × 10⁶ g PE mol^-1 (Ni) h^-1 at 20 °C. Notably, even at 80 °C, the nickel complexes maintained a high activity of 1.97 × 10⁶ g PE mol^-1 (Ni) h^-1, showcasing superiority compared to previously reported unsymmetrical 2,3-bis(arylimino)butane-nickel complexes. The resulting polyethylenes exhibited ultra-high molecular weights (M_w: 3.33-19.47 × 10⁵ g mol^-1) and tunable branching densities (84-217/1000C), which were effectively controlled by polymerization temperature. Moreover, the mechanical properties of the polyethylenes, including tensile strength (σ_b = 0.74-16.83 MPa), elongation at break (ε_b = 271-475%), and elastic recovery (SR = 42-74%), were finely tailored by optimizing molecular weight, crystallinity, and branching degree. The prepared polyethylenes displayed outstanding elastic recovery, a hallmark of high-performance thermoplastic elastomers, making them promising candidates for advanced material applications.