Question

Synthesis of poly (d-lactide) with different molecular weight via melt-polymerization

Answer 1

To enhance the reaction rate of solid state polycondensation (SSP) of poly(l-lactic acid) (PLLA) and thus to prepare high molecular weight PLLA by SSP in a short time, the effects of precrystallization as well as SSP temperature (Ts) on the SSP of PLLA were investigated. It was found that precrystallization at low Tc (70 °C) and then SSP at step-raised Ts could greatly favor the growth of the molecular weight of PLLA. After precrystallization at 70 °C for 60 min and then SSP at 150–160 °C for 20 h, the product’s weight average molecular weight (Mw) exceeds 200 000 g·mol–1 while its optical purity remained as high as 90%. The prepolymer precrystallized at 70 °C for 60 min had smaller crystal size and looser packing of lamellae structure than that at Tcmax (110 °C) and moderate crystallinity of 45%. Such crystal properties may result in enhanced concentration and mobility of chain-end groups and improved diffusion of byproduct water in an amorphous area, so the SSP reaction is accelerated. PLLA continues to crystallize during SSP. The crystallinity and melting temperature increase rapidly at the early stage and slowly at the late stage, implying that the effect of precrystallization on the SSP rate is significant at the early stage, and weakens gradually at the late stage.