New enantiomeric polylactide-block-poly(butylene succinate)-block-polylactides:: Syntheses, characterization and in situ self-assembly

In situ self-assemblies of new biodegradable triblock PLLA-b-PBS-b-PLLA and PDLA-b-PBS-b-PDLA have been investigated in acetonitrile solution. At first, two series of PLLA-b-PBS-b-PLLA and PDLA-b-PBS-b-PDLA, respectively denoted as the P and Q triblock copolyester series, were prepared with fixed PBS block ((M) over barn,NMR = 6.9 kDa) and diverse enantiomeric PLLA/PDLA blocks. Further, their chemical structures and thermal properties were characterized by means of titration, nuclear magnetic resonance spectroscopy (NMR), gel permeation chromatography (GPC), polarimeter, wide-angle X-ray diffraction (WAXD) and thermal analytical instruments. When mixing the synthesized enantiomeric copolyester pairs denoted as P-1/Q(1) - P-8/Q(8) in acetonitrile solution at 60 degrees C, in situ self-assemblies were found to happen for the P-4/Q(4) to P-8/Q(8) pairs, bearing longer enantiomeric PLA block lengths. DSC and WAXD analysis of the self-assembled microparticles demon-strated that PLLA/PDLA racemic crystals were formed for the P-5/Q(5) - P-8/Q(8) systems, as evidenced by their melting points over 200 degrees C, and a new X-ray diffraction peak detected at 2 theta = 11.8 degrees. Moreover, morphological studies by scanning electron microscopy (SEM) indicated the formation of disk or platelet-like microparticles. It was noted that the diameters of the microparticles self-assembled in situ decreased from 1.28-1.50 mu m down to 480-660 nm, through tailoring the enantiomeric PLA block length. Other factors, such as a central PBS block, the enantiomeric block length and the preparation conditions were suggested to play important roles in the in situ self-assembly of these enantiomeric triblock copolyesters. These results provide a facile way to self-assemble hydrophobic, biodegradable microparticles, through tuning the important van der Waals stereocomplexation interactions between two enantiomeric blocks in solution.