Synthesis of meso-lactide by thermal configurational inversion and depolymerization of poly(l-lactide) and thermal configurational inversion of lactides

Meso-lactide (MLA) was synthesized by thermal degradation of poly(l-lactide) [i.e., poly(l-lactic acid) (PLLA)] at relatively high temperatures of 250 and 300 °C for thermal degradation periods up to 750 and 360 min, respectively. Maximum MLA yields as high as 30 and 40 mol% were successfully attained at 250 and 300 °C, respectively, when thermal degradation was continued for 750 and 180 min, respectively. This indicates that the intentional thermal configurational inversion and depolymerization of PLLA and configurational inversion of lactides (LAs) at the higher temperature are favorable for the synthesis of MLA in a short period. The molecular weight and optical purity and tacticity of poly(lactide) [i.e., poly(lactic acid) (PLA) decreased and the d-lactyl unit content of PLA increased with an increase in reaction time. The higher MLA fraction for 250 °C compared to that for 300 °C at the reaction times when PLA samples had similar number-average molecular weights can be ascribed to the lower tacticity or higher racemization at chiral carbons in PLA chains.

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