A collection of ten related 1,2-bis(imino)acenaphthene-nickel(II) halide complexes, [1-[2,6-{(C₆H₅)₂CH}₂-4-{C(CH₃)₃}-C₆H₂N]-2-(ArN)C₂C₁₀H₆]NiX₂ (X = Br: Ar = 2,6-Me₂C₆H₃Ni1, 2,6-Et₂C₆H₃Ni2, 2,6-ⁱPr₂C₆H₃Ni3, 2,4,6-Me₃C₆H₂Ni4, 2,6-Et₂-4-MeC₆H₂Ni5) and (X = Cl: Ar = 2,6-Me₂C₆H₃Ni6, 2,6-Et₂C₆H₃Ni7, 2,6-ⁱPr₂C₆H₃Ni8, 2,4,6-Me₃C₆H₂Ni9, 2,6-Et₂-4-MeC₆H₂Ni10), each bearing one sterically and electronically enhanced N-2,6-dibenzhydryl-4-t-butylphenyl group, have been prepared and fully characterized. The unsymmetrical nature of the chelating bis(imino)acenaphthene is confirmed in the paramagnetic ¹H NMR spectra for Ni1–Ni10, while the molecular structures of Ni1, Ni2 and Ni6 highlight the unequal steric protection of the nickel center imposed by their respective N,N-ligands. On activation with either Et₂AlCl or MMAO, all the nickel complexes were highly active catalysts in ethylene polymerization [as high as 1.26 × 10⁷ g of PE per mol of Ni per h] affording exceptionally high molecular weight (up to 3.1 × 10⁶ g mol⁻¹) hyper-branched polyethylene. Analysis of the mechanical properties reveals the ultra-high molecular weight polymers possess high tensile strength, excellent shape fixity and elastic recovery (up to 69%) as well as high elongation at break (ε_b = 843.9%); such materials offer a promising alternative to current thermoplastic elastomers (TPEs).