Crystallization and melting behavior of polylactides

Six polylactides, polymerized with Salen-Al-OCH3 initiator and having optical purities between 43% and 100%, were analyzed by differential scanning calorimetry, X-ray diffraction, and optical microscopy, following various crystallization conditions. It was found that each of those polylactides can crystallize, even those with low optical purities; their crystallization rate is, however, slower than those for high optical purity polyesters. Moreover, the low optical purity polymers tend to form stereocomplexes between the L and D sequences of the same polylactide, which behavior is ascribed to their multiblock microstructure. A correlation was found between the measured melting temperature of optically active polylactides and their average sequence length.