Vanadium-based imido-alkoxide pro-catalysts bearing bisphenolate ligands for ethylene and ε-caprolactonepolymerisation

Abstract

The pro-catalysts [V(NAr)(L)(OR)] (Ar = p-tolyl, p-ClC₆H₄, p-(OMe)C₆H₄, p-(CF₃)C₆H₄; R = t-Bu, i-Pr, n-Pr, Et, C(CH₃)(CF₃)₂) have been prepared in good yields from the reaction of [V(NAr)(OR)₃] and the bisphenol 2,2′-CH₃CH[4,6-(t-Bu)₂C₆H₂OH]₂ (LH₂). X-Ray crystal structure determinations for the Ar = p-tolyl, R = t-Bu (1), R = C(CH₃)(CF₃)₂ (2) and Ar = p-ClC₆H₄, R = t-Bu (3) derivatives revealed monomeric complexes, whereas use of R = i-Pr, n-Pr or Et led to alkoxide-bridged dimeric structures of the form [V(NAr)(L)(μ-OR)]₂ (R = i-Pr, Ar = p-tolyl (4), p-ClC₆H₄ (5), p-(CF₃)C₆H₄ (6), p-(OMe)C₆H₄ (7); R = n-Pr, Ar = p-tolyl (8), p-(CF₃)C₆H₄ (9); R = Et, Ar = p-ClC₆H₄ (10), p-tolyl (11)). Complexes 1–11 yield highly active ethylene polymerisation catalysts when treated with DMAC (dimethylaluminium chloride) in the presence of ETA (ethyltrichloroacetate), with activities in the range 38 800 to 75 200 g mmol⁻¹ h⁻¹ bar⁻¹. The molecular weights of the resultant polymers were in the range 37 000 to 411 000 g mol⁻¹, with molecular weight distribution 2.2 to 4.7. The effect of the nature of the para-arylimido substituent and the alkoxide group OR upon the catalytic activity has been investigated. For ε-caprolactone polymerisation, mononuclear 1–3 exhibit low conversion (≤25%; 0% for 2), whereas use of the dimeric species 4–11 led to higher conversions (41–78%).