Synthesis and stereocomplex formation of enantiomeric alternating copolymers with two types of chiral centers, poly(lactic acid--2-hydroxybutanoic acid)s.

Stereocomplex (SC) formation was reported for the first time for enantiomeric alternating copolymers consisting of repeating units with two types of chiral centers, poly(lactic acid--2-hydroxybutanoic acid)s [P(LA--2HB)s]. l,l-Configured poly(l-lactic acid--l-2-hydroxybutanoic acid) [P(LLA--l-2HB)] and d,d-configured poly(d-lactic acid--d-2-hydroxybutanoic acid) [P(DLA--d-2HB)] were amorphous. Blends of P(LLA--l-2HB) and P(DLA--d-2HB) were crystallizable and showed typical SC-type wide-angle X-ray diffraction profiles similar to those reported for stereocomplexed blends of poly(l-lactic acid) and poly(d-lactic acid) homopolymers and of poly(l-2-hydroxybutanoic acid) and poly(d-2-hydroxybutanoic acid) homopolymers, and of l,l-configured poly(l-lactic acid--l-2-hydroxybutanoic acid) [P(LLA--l-2HB)] and d,d-configured poly(d-lactic acid--d-2-hydroxybutanoic acid) [P(DLA--d-2HB)] random copolymers. The melting temperature values and melting enthalpy values at 100% crystallinity for stereocomplexed solvent-evaporated and precipitated P(LLA--l-2HB)/P(DLA--d-2HB) blends were correspondingly 187.5 and 187.9 °C, and 98.1 and 91.8 J g. Enantiomeric polymer blending of P(LLA--l-2HB) and P(DLA--d-2HB) can confer crystallizability by stereocomplexation and the biodegradable materials with a wide variety of physical properties and biodegradability are highly expected to be prepared by synthesis of alternating copolymers of various combinations of two types of chiral α-substituted 2-hydroxyalkanoic acid monomers and their SC crystallization.