A series of α‐diimine nickel(II) catalysts bearing different para‐substituted groups, {[(4‐R‐2,6‐Me2‐C6H2NC)2Nap]NiBr2, Nap: 1,8‐naphthdiyl; R = CHMePh (C1); R = CHPh2 (C2); R = tBu (C3); R = Br (C4); R = Me (C5)}, were synthesized and characterized. The molecular structures of C1 and C3 were determined by single crystal X‐ray analysis and show distorted tetrahedron geometry with C2v symmetry. The catalytic behaviors of these catalysts toward ethylene polymerization were investigated in the presence of diethylaluminum chloride (Et2AlCl) as a cocatalyst. The fast chain‐walking process in N,N‐chelated nickel system can be efficiently modulated via the introduction of para‐sec‐phenethyl substituents with improved thermal stability and catalytic performance. C1‐Et2AlCl exhibited high thermal stabilities and high activities reaching 4.61 × 106 g PE (mol Ni h)−1, producing highly branched amorphous polyethylenes (up to 143 branches/1000C) with high molecular weight and narrow polydispersity. This catalyst system conducted successfully conducted 2‐octene polymerization in comparison with the corresponding 1‐octene polymerization. The result demonstrated 2‐octene polymerized slower than 1‐octene and produced the highly‐branched poly(2‐octene)s with approximately 120 branches/1000C, including methyl, ethyl, amyl, and hexyl branches and a small amount of the long methylene sequence due to monomer isomerization.

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