Novel materials through stereocomplexation

Both (-)- and (+)-poly(lactide) (PLA) crystallize into a stereocomplex with a melting point that is 50°C higher than that of crystals of the same-handed enantiomers. According to Boyer et al. [Polym. Prepr., 36 (1995) 87] and Jiang et al. [J. Am. Chem. Soc., 117 (1995) 7037], alternating isotactic propylene-CO-copolymers (P(P-alt-CO)) also form a stereocomplex with a higher melting point (60°C). Force-Field-simulated structures for both polymer systems were found to agree well with X-ray data, irrespective of whether they had a chiral or racemic packing. The almost similar results for both stereocomplexes indicate that they might form a mixed stereocomplex of (-)-PLA and (+)-P(P-alt-CO). In acetonitrile, both enantiomers of an AB block copolymer derivative, poly(lactide)-poly(ethyleneglycol) (PLA-PEG), were found to crystallize exclusively into a racemic lattice. The influence of racemic packing on self-assembly of the two-block copolymer was analyzed by atomic-force microscopy. An equimolar mixture of (-)- and (+)-PLA-PEG formed spherical particles through stereocomplexation, in contrast to chiral block copolymers that formed large crystal needles and long rods.