Porous poly(D,L-lactide) and poly(D,L-lactide-co-glycolide) produced by thermal salt elimination from halogenocarboxylates

Abstract

Poly(D,L-lactide) has been obtained by thermal elimination of alkali chloride from alkali 2-chloropropionates. Preparative studies and differential scanning calorimetry (DSC) have shown that while the sodium and potassium compounds undergo a clean polymerisation, the lithium analogues decompose on heating without polymerisation. The energetics of the polymerisation reaction have also been investigated by determining the enthalpies of formation of sodium and lithium (S)-2-chloropropionates (from reaction-solution calorimetry), and the enthalpies of formation of poly(L-lactide) and poly(D,L-lactide) (from micro combustion calorimetry). The results suggest that the polymerisation process is thermodynamically favourable for the sodium compounds but not for the lithium compounds, in good agreement with the experimental observations. Copolyesters of glycolic acid (polyglycolide) and lactic acid (polylactide) have been prepared by thermal reaction of crystal mixtures of sodium or potassium chloroacetates and sodium or potassium 2-chloropropionates, which were obtained by co-precipitation of the precursor compounds from methanol solutions. The crystal mixtures and reaction products have been characterised by a number of techniques, including solid state NMR spectroscopy and X-ray powder diffraction. For high contents of glycolide, the reaction occurred entirely in the solid state, whereas for cases with more than 10 mol% lactide, a melt is formed which solidifies on cooling to room temperature. The products are lactide-terminated block copolymers with typical chain lengths of 40 monomer units. Due to the solvent-free nature of the synthesis, all polymers can be obtained with microporous morphology.