Exploring fluoride effects in sterically enhanced cobalt ethylene polymerisation catalysts; a combined experimental and DFT study

Abstract

The fluoro-substituted 2,6-bis(arylimino)pyridine dichlorocobalt complexes, [2-{CMeN(2,6-(Ph₂CH)₂-3,4-F₂C₆H)}-6-(CMeNAr)C₅H₃N]CoCl₂ (Ar = 2,6-Me₂C₆H₃ Co1, 2,6-Et₂C₆H₃ Co2, 2,6-iPr₂C₆H₃ Co3, 2,4,6-Me₃C₆H₂ Co4, 2,6-Et-4-MeC₆H₂ Co5), were synthesized in good yield from the corresponding unsymmetrical N,N,N′-ligands, L1–L5. Besides characterization of Co1–Co5 by FT-IR spectroscopy, ¹⁹F NMR spectroscopy and elemental analysis, the molecular structures of Co2 and Co5 were also determined highlighting the unsymmetrical nature of the terdentate ligand and the pseudo-square pyramidal geometry about the metal center. When either MAO or MMAO were employed as activators, Co1–Co5 were able to achieve a wide range of catalytic activities for ethylene polymerisation. Co5/MAO exhibited the highest activity of the study at 60 °C (7.6 × 10⁶ g PE mol⁻¹ (Co) h⁻¹) which decreased to 3.3 × 10⁶ g PE mol⁻¹ (Co) h⁻¹ at 80 °C. In addition, it was found that the polymerisation activity increased as the steric hindrance imparted by the ortho groups was enhanced (for MMAO: Co3 > Co5 > Co2 > Co1 > Co4), a finding that was supported by DFT calculations. Furthermore, it was shown that particularly high molecular weight polyethylene could be generated (up to 483.8 kg mol⁻¹) when using Co5/MMAO at 30 °C, while narrow dispersities (M_w/M_n range: 1.8–4.7) and high linearity (T_m > 131.4 °C) were a feature of all polymers produced. By comparison of Co3 with its non-fluorinated analogue using experimental data and DFT calculations, the substitution of fluorides at the meta- and para-positions was demonstrated to boost catalytic activity and improve thermal stability.