CRYSTALLINE OLIGOMERIC STEREOCOMPLEX AS AN INTERMEDIATE COMPOUND IN RACEMIC POLY(DL-LACTIC ACID) DEGRADATION

Hydrolytic degradation of intrinsically amorphous poly(DL-lactic acid) was re-examined in pH 7.4 isoosmolar phosphate buffer at 37 degrees C. Size-exclusion chromatography, X-ray scattering and differential scanning calorimetry were used to monitor molecular weight and morphology changes up to the ultimate degradation stage. It was found that heterogeneous degradation of poly(DL-lactic acid) yielded a crystalline residual material of low molecular weight. Comparison with a stereocomplex made of equimolar poly(L-lactic acid) and poly(D-lactic acid) showed that the poly(DL-lactic acid) degradation residue was actually an oligomeric stereocomplex. The formation of stereoregular oligomeric enantiomers agreed well with the predominantly isotactic structure of poly(DL-lactic acid) obtained by ring-opening polymerisation of DL-lactide and argued in favour of the stereodependence of main chain ester bond cleavage.