Plastomeric-like polyethylenes achievable using thermally robust N,N′-nickel catalysts appended with electron withdrawing difluorobenzhydryl and nitro groups

Abstract

A new set of five unsymmetrical N,N′-diiminoacenaphthenes, 1-[2,6-{(4-FC₆H₄)₂CH}₂-4-NO₂C₆H₄N]-2-(ArN)C₂C₁₀H₆ (Ar = 2,6-Me₂C₆H₃L1, 2,6-Et₂C₆H₃L2, 2,6-ⁱPr₂C₆H₃L3, 2,4,6-Me₃C₆H₂L4, 2,6-Et₂-4-MeC₆H₂L5), have been synthesized and used to prepare their corresponding nickel(II) halide complexes, LNiBr₂ (Ni1–Ni5) and LNiCl₂ (Ni6–Ni10). The molecular structures of Ni3(OH₂) and Ni4 reveal distorted square pyramidal and tetrahedral geometries, respectively, while the ¹H NMR spectra of all the nickel(II) (S = 1) complexes show broad paramagnetically shifted peaks. Upon activation with either methylaluminoxane (MAO) or ethylaluminum sesquichloride (Et₃Al₂Cl₂, EASC), Ni1–Ni10 displayed very high activities for ethylene polymerization with the optimal performance being observed using 2,6-dimethyl-containing Ni1 in combination with EASC (1.66 × 10⁷ g PE mol⁻¹ (Ni) h⁻¹ at 50 °C) which produced high molecular weight plastomeric polyethylene (M_w = 3.93 × 10⁵ g mol⁻¹, T_m = 70.6 °C) with narrow dispersity (M_w/M_n = 2.97). Moreover, Ni1/EASC showed good thermal stability by operating effectively at an industrially relevant 80 °C with a level of activity (6.01 × 10⁶ g of PE mol⁻¹ (Ni) h⁻¹) that exceeds previously disclosed N,N′-nickel catalysts under comparable reaction conditions. This improved thermal stability and activity has been ascribed to the combined effects imparted by the para-nitro and fluoride-substituted benzhydryl ortho-substituents.