Ti and Zr complexes bearing guanidine-phenolate ligands: coordination chemistry and polymerization studies

Abstract

A series of group 4 bis(isopropoxide) complexes M[N^O]₂(OⁱPr)₂, stabilized by guanidine-phenolate N^O ligands, have been prepared and used as catalysts for the polymerization of unpurified rac-lactide under solvent-free conditions at 130 °C. The resulting polylactic acid (PLA) presented heterotactic bias (P_r = 0.56–0.62) with molecular weights similar to those obtained in control experiments with Zr(OⁱPr)₄·ⁱPrOH, Ti(OⁱPr)₄, and Sn(Oct)₂. The molecular weights were lower than expected for living polymerization due to chain transfer and/or transesterification. Zr complexes were more active than the Ti homologues, with rate constants ranging from 1.17–3.21 × 10⁻⁴ s⁻¹, comparable to that observed with the free guanidine-phenol ligands. The corresponding bis(guanidine-phenolate) titanium dichloride complexes Ti[N^O]₂Cl₂ were also prepared and tested in ethylene polymerization. The low activity (up to 1.1 kg_PE mol⁻¹ h⁻¹) was associated to the strong electron-donating ability of the guanidine moiety and to the trans-N,N-cis-O,O-cis-Cl,Cl coordination mode of the guanidine-phenolate ligand.