Poly(l-lactide) XI. Lactide formation by thermal depolymerisation of poly(l-lactide) in a closed system

Thermal degradation (depolymerisation) of poly(l-lactide) [i.e., poly(l-lactic acid) (PLLA)] was carried out in a sealed tube as a closed system in the temperature range of 250–290 °C for 15 h without further addition of depolymerisation catalyst, and lactide formation by depolymerisation of PLLA was monitored using X-ray diffractometry, gel permeation chromatography (GPC), and gas chromatography (GC). The highest yields of lactides and l-lactide were as low as 14 and 8%, when thermal degradation was performed for 10 h at 270 and 250 °C, respectively. The fractions of l-lactide and of d- and meso-lactides respectively decreased and increased with increasing degradation temperature and time. The low yield of lactides in this study can be explained by polymerization of the formed lactide and by formation of low molecular weight compounds other than lactides, while the low yield of l-lactide is attributable to the formation of high amounts of meso- and d-lactides, in addition to the low yield of lactides. The probable pathways for the formation of D- and meso-lactides are discussed.