Stereocomplex crystallization of chiral hydroxyalkanoic acid-based biodegradable alternating copolyesters with two types of chiral centers and opposite monomer configurational combinations of L-D and D-L (S-R and R–S)

Abstract

Stereocomplex (SC) crystallization of chiral copolyesters was normally reported for those having the two types of chiral centers and identical monomer configurational combinations (MCCs) of L-L and D-D (or S–S and R-R). Herein, we report for the first time the SC crystallization of chiral hydroxyalkanoic acid-based biodegradable alternating copolyesters with the two types of chiral centers and the opposite MCCs of L-D and D-L (or S-R and R–S). Poly(l-lactic acid-alt-d-2-hydroxybutanoic acid) [P(LLA-alt-D-2HB)] with MCC of L-D and poly(d-lactic acid-alt-l-2-hydroxybutanoic acid) [P(DLA-alt-L-2HB)] with MCC of D-L were synthesized and their SC crystallization was confirmed by wide-angle X-ray diffractometry. The crystallizability was not enhanced by SC crystallization in enantiomeric polymer blending compared to homocrystallization in unblended specimens. The melting temperature was not elevated by SC crystallization as in the normal case of SC crystallization reported previously for chiral homopolyesters and copolyesters. However, the fact that only SC crystallization occurred without homo-crystallization in the enantiomeric blend indicates that interaction between the enantiomeric polymer chains were stronger than that between the identical polymer chains. It is expected that the newly proposed novel combinations of chiral hydroxyalkanoic acid-based alternating copolyesters with the two types of chiral centers and the opposite MCCs of L-D and D-L (S-R and R–S) can facilitate the preparation of biodegradable materials having a variety of physical properties and biodegradability.