Effects of molecular weight and catalyst on stereoblock formation via solid state polycondensation of poly(lactic acid)

• Stereoblock PLA (sb-PLA) was prepared via solid state polymerization (SSP).
• A catalyst of the PLA oligomers affects the final structure and yield of sb-PLA.
• Methanesulfonic acid catalyst generates sb-PLA with higher MW at a higher yield.
• Tin(II) catalyst generates sb-PLA with lower MW at a lower yield.
• Lactide formation occurs vigorously by the tin catalyst under the conditions of SSP.

This work reports the synthesis of stereoblock poly(lactic acid) (sb-PLA) by solid state polymerization (SSP) using a combination of PLA enantiomers with low molecular weight, focusing on effects of catalyst and molecular weight of the enantiomers. It was revealed for the first time that the PLA enantiomers synthesized with a novel methanesulfonic acid (SO) catalyst form sb-PLA with higher molecular weight at a higher yield, whereas the PLA enantiomers synthesized with a widely used tin(II) (Sn) catalyst causes a significant side reaction resulting in a low yield. These phenomena would be characteristic of the sb-PLA synthesized via SSP, since the side reaction of the lactide formation is promoted by vigorous sublimation of the lactide under the conditions of high temperature and a low pressure used for the SSP process. Reaction mechanisms were proposed for the SSP using the two different catalysts, which successfully explained the present results. It was also demonstrated that the molecular weight of the starting PLA enantiomers significantly affects the final molecular weight and sc crystallinity of sb-PLA.

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