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

A series of group 4 bis(isopropoxide) complexes MN^O, stabilized by guanidine-phenolate N^O ligands, have been prepared and used as catalysts for the polymerization of unpurified -lactide under solvent-free conditions at 130 °C. The resulting polylactic acid (PLA) presented heterotactic bias ( = 0.56-0.62) with molecular weights similar to those obtained in control experiments with Zr(OPr)·PrOH, Ti(OPr), and Sn(Oct). 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 s, comparable to that observed with the free guanidine-phenol ligands. The corresponding bis(guanidine-phenolate) titanium dichloride complexes Ti[N^O]Cl were also prepared and tested in ethylene polymerization. The low activity (up to 1.1 kg mol h) was associated to the strong electron-donating ability of the guanidine moiety and to the ,,, coordination mode of the guanidine-phenolate ligand.