Four examples ofN,N-bis(aryl)butane-2,3-diimine–nickel(II) bromide complexes, [ArN=C(Me)-C(Me)=NAr]NiBr2(where Ar = 2-(C5H9)-4,6-(CHPh2)2C6H2(Ni1), Ar = 2-(C6H11)-4,6-(CHPh2)2C6H2(Ni2), 2-(C8H15)-4,6-(CHPh2)2C6H2(Ni3) and 2-(C12H23)-4,6-(CHPh2)2C6H2(Ni4)), disparate in the ring size of theortho-cycloalkyl substituents, were prepared using a straightforward one-pot synthetic method. The molecular structures ofNi2andNi4highlight the variation in the steric hindrance of theortho-cyclohexyl and -cyclododecyl rings exerted on the nickel center, respectively. By employing EtAlCl2, Et2AlCl or MAO as activators,Ni1–Ni4displayed moderate to high activity as catalysts for ethylene polymerization, with levels falling in the orderNi2(cyclohexyl) >Ni1(cyclopentyl) >Ni4(cyclododecyl) >Ni3(cyclooctyl). Notably, cyclohexyl-containingNi2/MAO reached a peak level of 13.2 × 106g(PE) of (mol of Ni)−1h−1at 40 °C, yielding high-molecular-weight (ca. 1 million g mol−1) and highly branched polyethylene elastomers with generally narrow dispersity. The analysis of polyethylenes with13C NMR spectroscopy revealed branching density between 73 and 104 per 1000 carbon atoms, with the run temperature and the nature of the aluminum activator being influential; selectivity for short-chain methyl branches (81.8% (EtAlCl2); 81.1% (Et2AlCl); 82.9% (MAO)) was a notable feature. The mechanical properties of these polyethylene samples measured at either 30 °C or 60 °C were also evaluated and confirmed that crystallinity (Xc) and molecular weight (Mw) were the main factors affecting tensile strength and strain at break (εb= 353–861%). In addition, the stress–strain recovery tests indicated that these polyethylenes possessed good elastic recovery (47.4–71.2%), properties that align with thermoplastic elastomers (TPEs).