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

Poly(E,L-lactide) has been obtained by thermal elimination of alkali chlori de from alkali 2-chloropropionates. Preparative studies and differential sc anning calorimetry (DSC) have shown that while the sodium and potassium com pounds undergo a clean polymerisation, the lithium analogues decompose on h eating without polymerisation. The energetics of the polymerisation reactio n have also been investigated by determining the enthalpies of formation of sodium and lithium (S)-2-chloropropionates (from reaction-solution calorim etry), and the enthalpies of formation of poly(L-lactide) and poly(D,L-lact ide) (from micro combustion calorimetry). The results suggest that the poly merisation 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 aci d (polylactide) have been prepared by thermal reaction of crystal mixtures of sodium or potassium chloroacetates and sodium or potassium 2-chloropropi onates, which were obtained by co-precipitation of the precursor compounds from methanol solutions. The crystal mixtures and reaction products have be en characterised by a number of techniques, including solid state NMR spect roscopy and X-ray powder diffraction. For high contents of glycolide, the r eaction occurred entirely in the solid state, whereas for cases with more t han 10 mol% lactide, a melt is formed which solidi es on cooling to room te mperature. The products are lactide-terminated block copolymers with typica l chain lengths of 40 monomer units. Due to the solvent-free nature of the synthesis, all polymers can be obtained with microporous morphology.