Trifluoromethoxy-substituted nickel catalysts for producing highly branched polyethylenes: impact of solvent, activator and N,N′-ligand on polymer properties

Abstract

The para-trifluoromethoxy-substituted 1,2-bis(imino)acenaphthenes, 1-[2,6-{(C₆H₅)₂CH}₂-4-F₃COC₆H₂N]-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 utilized to generate their respective nickel(II) halide complexes, LNiBr₂ (Ni1–Ni5) and LNiCl₂ (Ni6–Ni10). Characterization of the complexes has been achieved via elemental analysis, ¹⁹F NMR, ¹H NMR and FT-IR spectroscopy. In addition, representative molecular structures have been determined with bromide-containing Ni1, Ni3 and Ni4 adopting discrete mononuclear species while Ni9 assumes a chloride-bridged dinuclear arrangement. The polymerization of ethylene promoted by Ni1–Ni10 in combination with either MAO (methylaluminoxane), MMAO (modified methylaluminoxane) or EASC (Et₃Al₂Cl₃) as activator, furnished highly branched polyethylenes; evidence for hyperbranched materials is also presented. As a notable observation, higher activity was seen in hexane than in toluene with levels reaching up to 21.0 × 10⁶ g of polyethylene (PE) (mol of Ni)⁻¹ h⁻¹ at 30 °C for Ni5/MAO. The resulting polyethylenes exhibited very high molecular weights (up to 16.0 × 10⁵ g mol⁻¹ for Ni8/MMAO) with branching densities reaching up to 186 branches per 1000 carbon atoms. Moreover, the run temperature, solvent and the type of aluminum activator employed have been shown to strongly influence the branching density and type of branch. Furthermore, good mechanical properties (ε_b up to 984%) and elastic recovery (up to 71%) were a feature of these polymeric materials, properties that are distinctive of thermoplastic elastomers (TPEs).