High molecular weight PE elastomers through 4,4-difluorobenzhydryl substitution in symmetrical α-diimino-nickel ethylene polymerization catalysts

Abstract

The following family of N,N-diaryl-2,3-dimethyl-1,4-diazabutadienes, ArNC(Me)C(Me)NAr (Ar = 2,6-Me₂-4-{CH(4-FC₆H₄)₂}C₆H₂ L1, 2-Me-6-Et-4-{CH(4-FC₆H₄)₂}C₆H₂ L2, 2,4-{CH(4-FC₆H₄)₂}₂-6-MeC₆H₂ L3, 2,4-{CH(4-FC₆H₄)₂}₂-6-EtC₆H₂ L4, 2,4-{CH(4-FC₆H₄)₂}₂-6-iPrC₆H₂ L5), each incorporating para-substituted 4,4-difluorobenzhydryl groups but differing in the ortho-pairing, have been synthesized and used as precursors to their respective nickel(II) bromide complexes, Ni1–Ni5. Compound characterization has been achieved through a combination of FT-IR, multinuclear NMR spectroscopy (¹H, ¹³C, ¹⁹F) and elemental analysis. In addition, L1, Ni1 and Ni5 have been structurally characterized with Ni1 and Ni5 revealing similarly distorted tetrahedral geometries about nickel but with distinct differences in the steric protection offered by the ortho-substituents. All nickel complexes, under suitable activation, showed high activity for ethylene polymerization with a predilection towards forming branched high molecular weight polyethylene with narrow dispersity. Notably the most sterically bulky Ni5, under activation with either EtAlCl₂, Et₂AlCl or EASC, was exceptionally active (0.9–1.0 × 10⁷ g of PE per (mol of Ni) per h) at an operating temperature of 40 °C. Furthermore, the polyethylene generated displayed molecular weights close to one million g mol⁻¹ (M_w range: 829–922 kg mol⁻¹) with high branching densities (86–102/1000 carbons) and a selectivity for short chain branches (% Me = 94.3% (EtAlCl₂), 87.2% (Et₂AlCl), 87.7% (EASC)). Further analysis of the mechanical properties of the polymers produced at 40 °C and 50 °C using Ni5 highlighted the key role played by crystallinity (X_c) and molecular weight (M_w) on tensile strength (σ_b) and elongation at break (ε_b). In addition, stress–strain recovery tests reveal these high molecular weight polymers to exhibit characteristics of thermoplastic elastomers (TPEs).