Synthesis and Stereocomplex Formation of Star-Shaped Stereoblock Polylactides Consisting of Poly( l -lactide) and Poly( d -lactide) Arms

The synthesis of star-shaped polylactides (PLAs) having both poly( l -lactide) (PLLA) and poly( d -lactide) (PDLA) arms in one molecule, i.e., stereo-miktoarm star-shaped PLAs, is described. The azido-functionalized PDLAs and ethynyl-functionalized PLLA possessing linear and two- and three-branched structures were prepared by the ring-opening polymerization of d -lactide and l -lactide using azido- or ethynyl-functionalized initiators. The number-average molecular weights ( M n,NMR s) of the PLAs were ca. 5000 g mol –1 with narrow molecular weight distributions ( M w / M n s) of less than 1.18. The click reaction of the azido-functionalized PDLAs and the ethynyl-functionalized PLLAs using the copper(I) bromide/ N, N, N ′, N ″, N ″-pentamethyldiethylenetriamine catalyst in a mixed solvent of dichloromethane/1,1,1,3,3,3-hexafluoro-2-propanol (14/1, v/v) gave a linear stereoblock PLA as well as 3-, 4-, 5-, and 6-armed stereo-miktoarm star-shaped PLAs with M n,NMR s of ca. 10 000 g mol –1 and M w / M n s of less than 1.16. The wide-angle X-ray scattering and differential scanning calorimetry measurements proved that these stereo-miktoarm star-shaped PLAs formed stereocomplex crystals without any trace of homochiral crystallization.