Since the discovery of Ziegler-Natta catalyst, great breakthrough has been made in the design of catalyst in olefin polymerization field, but the development of catalyst is still the fresh source in this field. It will always be a frontier scientific problem in this field to influence the olefin polymerization process through the structural design of catalysts. Due to the low cost of nickel and its high abundance, nickel-based catalysts have shown great prospects in the application of olefin polymerization. In this work, three kinds of unsymmetrical α-diimine nickel catalysts with different steric effects were designed, and their catalytic performances in ethylene polymerization were explored. It is found that the variety of catalysts and the change of polymerization conditions have important effects on the catalytic activity of ethylene polymerization, the molecular weight, branching density, thermodynamic parameters and mechanical properties of the prepared polyethylene. Among them, the polyolefin prepared by Ni3 catalyst with large steric hindrance is ultra-high molecular weight polyethylene, and the above materials maintain excellent mechanical properties and elastic properties at the same time. Specifically, tert-butyl substituted ketoimine intermediate B was condensed with three different anilines to synthesize ligands L1~L3 with different steric effects. The catalysts Ni1~Ni3 were successfully synthesized by efficient coordination reaction between the obtained ligands and DMENiBr₂, and the yields were over 90%. The single crystal structures of catalysts Ni1 and Ni3 were also prepared, and the steric hindrance around the nickel center was quantified utilizing the steric maps [V_bur(Ni1)=46.1%, V_bur(Ni3)=48.7%]. Under 0.8 MPa pressure of ethylene and AlEt₂Cl as cocatalyst, the ethylene polymerization performance was explored using three nickel catalysts at different temperatures. Among them, Ni2 exhibited the best catalytic activity in ethylene polymerization at 50 ℃ (up to 1.7×10⁷ g•mol^-1•h^-1), generating polyethylene with higher branching density. The polyethylene prepared by Ni3 with large steric hindrance is ultra-high molecular weight polyethylene with a molecular weight of 172×10⁴ g•mol^-1. Moreover, polyethylene materials prepared by Ni3 also showed excellent mechanical properties (breaking stress, 6~8 MPa; elongation at break, ≈600%) and elastic properties (strain recovery value up to 68%).

Key words: ethylene polymerization, catalyst design, unsymmetrical ligand, α-diimine nickel, materials property