1-Hexene was polymerized with mono- (MC_n, n = 1–3) and dinuclear (BC_n, n = 1–7) α-diimine Ni-based catalysts bearing different backbones, ortho-substituents and spacers between the active centers. Significantly, catalyst BC₂ through an optimum bulkiness in its structure and electronic and steric effects, had the highest activity among all the dinuclear and mononuclear analogues, and made poly(1-hexene) with high M_w (1.7 × 10⁶ g mol⁻¹) and a narrow MWD (2.2). Moreover, a lower level of branching density was observed for the samples obtained by catalysts BC₁ and BC₂ (83–85/1000C) in comparison to MC_1–3 (107–120/1000C) and BC_3–7 (131–150/1000C).