Synthesis of stereo multiblock polylactides by dual terminal couplings of poly-L-lactide and poly-D-lactide prepolymers: A new route to high-performance polylactides

Poly-L-lactide (PLLA) having two anthracene (diene) terminals and poly-D-lactide (PDLA) having two maleimide (dienophile) termainals were prepared by the ordinary ring-opening polymerization (ROP) of L- and D-lactides with the corresponding functional initiators followed by the coupling reaction of the resultant terminal hydroxyls of the PLLA and PDLA prepolymers with hexamethylene diisocyanate (HMDI), respectively. The di-functional PLLA and PDLA obtained were then mixed and subjected to Diels-Alder coupling to spontaneously form PLLA-PDLA stereo multiblock copolymers (m-sb-PLA: PLLA-multi-PDLA) whose block sequences could readily be controlled by changing the molecular weight of the PLLA and PDLA prepolymers first synthesized by the ROP. The finally chain-extended m-sb-PLAs were found to have excellent thermal stability and thermo-mechanical properties because of the easy formation of stereocomplex crystals of PLLA and PDLA by retaining high molecular weight. This synthetic method based on the dual terminal couplings can afford a convenient route to stereo block copolymers of PLLA and PDLA having excellent properties and processability.