Molecular weight increase driven by evolution of crystal structure in the process of solid-state polycondensation of poly(L-lactic acid)

Relationship between the increase in molecular weight and evolution of crystal structure was studied in the solid-state polycondensation (SSP) of poly(L-lactic acid) (PLLA). Here, a PLLA prepolymer having a weight-average molecular weight (M-w) of c.a., 3000 Da and a high optical purity (OP) over 99 %ee was prepared by bulk melt-polycondensation of L-lactic acid with dodecylbenzenesulfonic acid as a catalyst. It was first annealed at 80-110 degrees C and subjected to SSP at 120-140 degrees C to obtain a PLLA polymer having an M-w higher than 100,000 Da. The highest increase in M-w was observed at 140 degrees C, while the growth of crystal structure had been induced in an early stage of SSP at 120 degrees C, reaching 71% in crystallinity. Wide- and small-angle X-ray scatterings of the PLLA polymers revealed that the PLLA polymer retained the alpha-form crystals throughout the annealing and SSP with little change in lamellar morphology. It was therefore suggested that the acid-catalyzed polycondensation is driven by the reaction of the terminal groups of the PLLA prepolymer accumulated on the surface ab planes of the lamellar crystals above 120 degrees C. Therefore, the crystal morphology of the annealed samples was found to strongly influence the final molecular weight of PLLA in the SSP. (C) 2017 Elsevier Ltd. All rights reserved.