An efficient solid-state polycondensation method for synthesizing stereocomplexed poly(lactic acid)s with high molecular weight

Simultaneous solid-state polycondensation (SSP) of the powdery prepolymers of poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA) can produce entire stereocomplexed poly(lactic acid)s (sc-PLA) with high molecular weight and can be an alternative synthetic route to sc-PLA. Ordinary melt polycondensations of L- and D-lactic acids gave the PLLA and PDLA prepolymers having medium molecular weight which were pulverized for blending in 1:1 ratio. The resultant powder blends were then subjected to SSP at 130-160 °C for 30 h under a reduced pressure of 0.5 Torr. Some of the products thus obtained attained a molecular weight (Mw) as high as 200 kDa, consisting of stereoblock copolymer of PLLA and PDLA. A small amount of the stereocomplex should be formed in the boundaries of the partially melted PLLA and PDLA where the hetero-chain connection is induced to generate the blocky components. The resultant SSP products showed predominant stereocomplexation after their melt-processing in the presence of the stereoblock components in spite of containing a small amount of racemic sequences in the homo-chiral PLLA and PDLA chains. ©2008 Wiley Periodicals, Inc.