Polylactones: 32. High-molecular-weight polylactides by ring-opening polymerization with dibutylmagnesium or butylmagnesium chloride

Abstract

The dibutylmagnesium (Bu 2Mg)-initiated polymerizations of l-lactide at 120°C in bulk yielded poly( l-lactide) of low molecular weights ( η inh ⩽ 0.4 dlg −1 in CH 2Cl 2), regardless of reaction time and initiator concentration. Higher molecular weights were obtained in solution when carried out in the presence of ethers. The addition of crown ethers to polymerizations conducted in toluene at 0°C gave the best results, with η inh values up to 3.0 dlg −1 (in Ch 2Cl 2), corresponding to number-average molecular weights ( M ns) the order of 3 × 10 5g mol −1. These high-molecular-weight poly( l-lactide)s possess almost complete optical purity and show melting temperatures of ∼170°C. Relatively high-molecular-weight poly( d,l-lactide)s were also obtained by the Bu 2Mg-initiated polymerizations of racemic- or meso- d,d-lactides, with η inh values up to 1.4 dl g −1 (in CH 2Cl 2), corresponding to M n = 10 5, being found. The Grignard reagent, BuMgCl, gave somewhat better results than Bu 2Mg when used as an initiator for l-lactide in bulk. However, BuMgCl is not reactive enough to initiate satisfactory polymerizations in solution at temperatures ⩽25°C.