Robust chiral zirconium alkoxide initiators for the room-temperature stereoselective ring-opening polymerisation of rac-lactide

Chiral Schiff bases (1H to 4H) and a series of their Group 4 metal alkoxide complexes [(R-1)(2)Ti((OPr)-Pr-i)(2), (R-2)(2)Ti((OPr)-Pr-i)(2), (R-1)(2)Zr((OPr)-Pr-i)(2), (R-2)(2)Zr((OPr)-Pr-i)(2), (R-3)(2)Zr((OPr)-Pr-i)(2), (R-4)(2)Zr((OPr)-Pr-i)(2), (S-1)(2)Zr((OPr)-Pr-i)(2) and (rac-1)(2)Zr(OiPr)(2)] have been prepared and characterised by H-1, and C-13 NMR spectroscopy. In solution, both K and D isomers were observed, suggesting a low degree of chiral induction from the ligand. One ligand (R-4H) and three complexes [Delta-(R,R-2)(2)Ti((OPr)-Pr-i)(2), Lambda-(R,R-1)(2)Zr((OPr)-Pr-i)(2) and Delta-(R,R-3)(2)Zr((OPr)-Pr-i)(2)] have also been characterised by single crystal X-ray diffraction. All complexes were found to have a pseudo-octahedral alpha-cis geometry. The complexes were tested as initiators for the ring-opening polymerisation of rac-lactide in solution and in the melt. The titanium complexes are inactive in solution and afford atactic polylactide in the melt. Zr(IV) complexes afford heterotactically enriched polylactide both in toluene solution (at 20C and 80C) and in the melt. Polymerisations were generally found to be well-controlled, giving predictable molecular weights and low molecular weight distributions. Ligand variation (substituents and/or chirality) has little effect on either the activity or selectivity of initiators. Zirconium initiators were found to be unusually robust as they were able to maintain well-controlled polymerisation following addition of water to reactions in solution and when using unpurified monomer for reactions in the melt.