The N,N-diaryliminoacenaphthenes, 1,2-[2,4-{(4-FC₆H₄)₂CH}₂-6-MeC₆H₄N]₂-C₂C₁₀H₆ (L1) and 1-[2,4-{(4-FC₆H₄)₂CH}₂-6-MeC₆H₄N]-2-(ArN)C₂C₁₀H₆ (Ar = 2,6-Me₂C₆H₃ L2, 2,6-Et₂C₆H₃ L3, 2,6-i-Pr₂C₆H₃ L4, 2,4,6-Me₃C₆H₂ L5, 2,6-Et₂-4-MeC₆H₂ L6), incorporating at least one N−2,4-bis(difluoro benzhydryl)-6-methylphenyl group, have been synthesized and fully characterized. Interaction of L1–L6 with (DME)NiBr₂ (DME = 1,2-dimethoxyethane) generates the corresponding nickel(II) bromide N,N-chelates, LNiBr₂ (1–6), in high yield. The molecular structures of 3 and 6 reveal distorted tetrahedral geometries at nickel with the ortho-substituted difluorobenzhydryl group providing enhanced steric protection to only one side of the metal center. On activation with various aluminum alkyl co-catalysts, such as methylaluminoxane (MAO) or Et₂AlCl, 1–6 displayed outstanding activity toward ethylene polymerization (up to 1.02 × 10⁷ g of PE (mol of Ni)⁻¹ h⁻¹). Notably 1, bearing equivalent fluorobenzhydryl-substituted N-aryl groups, was able in the presence of Et₂AlCl to couple high activity with exceptional thermal stability generating high molecular weight branched polyethylenes at temperatures as high as 100 °C.