Zirconium enolatoimine complexes in olefinpolymerization

Abstract

Zirconium complexes with enolatoimine ligands bearing an electron-withdrawing trifluoromethyl group on the alkoxy moiety [κ²-N,O-{2,6-R₂NC(CH₃)C(H)C(O)CF₃]₂ZrCl₂ (2a, R = H; 2b, R = F; 2c, R = CH₃; 2d, R = ⁱPr) were prepared. The isopropyl substituents hinder rotation in solution for 2d, and result in a trans-arrangement of the N-donors in the solid state. Catalyst activities are similar for 2a/MAO and 2b/MAO in ethylene polymerization (1.8 × 10⁵ TO h⁻¹ for 2a and 3.3 × 10⁵ TO h⁻¹ for 2b at 25 °C under 1 bar monomer pressure), increasingly bulky alkyl substituents result in strongly decreased polymerization activities (7.1 × 10² TO h⁻¹ for 2c and 5.7 × 10² TO h⁻¹ for 2d at 25 °C). This goes along with an increase in polymer molecular weight (M_w = 8.1 × 10³, M_w/M_n = 2.9 for 2a, M_w = 3.8 × 10⁵, M_w/M_n = 2.0 for 2b, M_w = 1.0 × 10⁶, M_w/M_n = 2.8 for 2c), apparently bulky substituents retard chain transfer even more strongly than activation of the precursors and/or chain growth. 2b/MAO affords atactic polypropylene (M_w = 1.4 × 10⁴ g mol⁻¹, M_w/M_n = 2.1) with a small portion of regioirregular structures. The reaction of [Zr(CH₂Ph)₄] with the ketoenamine 2,6-F₂C₆H₃N(H)CMeCHC(O)CF₃ (1b) yielded the dibenzyl complex [(o-F₂C₆H₃NCMeCHC(CF₃)O)₂Zr(CH₂Ph)₂] (3b) which was investigated as a catalyst precursor for ethylene homopolymerization in combination with different activators.