Properties of stereo multi-block polylactides obtained by chain-extension of stereo tri-block polylactides consisting of poly(L-lactide) and poly(D-lactide)

Stereo multi-block polylactides (multi-sb-PLA) having controlled block sequences of poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) were synthesized by chain-extension of stereo tri-block polylacitdes (PLLA-PDLA-PLLA) that were readily synthesized by two-step ring-opening polymerization (ROP) of D- and L-lactides. The weight-average molecular weight of the resultant multi-sb-PLAs reached about 14.3 × 10⁴ Da. These multi-sb-PLAs were found to exclusively form stereocomplex (sc) crystals without homo-chiral (hc) crystallization occurring because the quickly formed sc crystals effectively suppressed the hc crystallization as cross-linkers. Their thermal properties could be tuned by controlling the block lengths of PLLA-PDLA-PLLA in the two-step ROP. Particularly, the multi-sb-PLAs consisting of longer PLLA and PDLA blocks in 1:1 ratio exhibited higher sc crystallinity and thermo-mechanical properties after thermal annealing.