Zirconium enolatoimine complexes in olefin polymerization

Abstract

Zirconium complexes with enolatoimine ligands bearing an electron-withdrawing trifluoromethyl group on the alkoxy moiety [k2-N,O-{2,6-R2N=C(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 x 10 5 TO h-1 for 2a and 3.3 x 10 5 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 x 10 2 TO h-1 for 2c and 5.7 x 10 2 TO h-1 for 2d at 25 °C). This goes along with an increase in polymer molecular weight (Mw = 8.1 x 103, Mw/Mn = 2.9 for 2a, Mw = 3.8 x 105, Mw/Mn = 2.0 for 2b, Mw = 1.0 x 106, Mw/Mn = 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 (Mw = 1.4 x 10 4 g mol-1,Mw/Mn = 2.1) with a small portion of regioirregular structures. The reaction of [Zr(CH2Ph)4] with the ketoenamine 2,6-F2C6H3N(H)-CMe=CHC(O)CF3 (1b) yielded the dibenzyl complex [(o-F2C6H3N=CMeCH=C(CF3)O)2Zr-(CH2Ph)2] (3b) which was investigated as a catalyst precursor for ethylene homopolymerization in combination with different activators.