Synthesis and Properties of High-Molecular-Weight Stereo Di-Block Polylactides with Nonequivalent D/L Ratios

Di-stereoblock polylactides (di-sb-PLA: PLLA-b-PDLA) having high molecular weight (M-n > 100 kDa) were successfully synthesized by two-step ring-opening polymerization (ROP) of L- and D-lactides using tin(2-ethylhexanoate) as a catalyst. By optimizing the polymerization conditions, the block sequences were well regulated at non-equivalent feed ratios of PLLA and PDLA. This synthetic method consisted of three stages: (1) polymerization of either L- or D-lactide to obtain a PLLA or PDLA prepolymer with a molecular weight less than 50 kDa, (2) purification of the obtained prepolymer to remove residual lactide, and (3) polymerization of the enantiomeric lactide in the presence of the purified prepolymer. Their C-13 and P-31 NMR spectra of the resultant di-sb-PLAs strongly supported their di-stereo block structure. These di-sb-PLAs, having weight-average molecular weights higher than 150 kDa, were fabricated into polymer films by solution casting and showed exclusive stereocomplexation. The thermomechanical analysis of the films revealed that their heat deformation temperature was limited probably because of their low crystallinity owing to the non-equivalent PLLA/PDLA ratio. The blend systems of the di-sb-PLAs having complementary stereo-sequences (the one with a long PLLA block and the other with long PDLA block) were also prepared and characterized to enhance the sc crystallinity. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 794-801, 2010