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

Abstract

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-alt-2-hydroxybutanoic acid)s [P(LA-alt-2HB)s]. L,L-Configured poly(L-lactic acid-alt-L-2-hydroxybutanoic acid) [P(LLA-alt-L-2HB)] and D,D-configured poly(D-lactic acid-alt-D-2-hydroxybutanoic acid) [P(DLA-alt-D-2HB)] were amorphous. Blends of P(LLA-alt-L-2HB) and P(DLA-alt-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-co-L-2-hydroxybutanoic acid) [P(LLA-co-L-2HB)] and D,D-configured poly(D-lactic acid-co-D-2-hydroxybutanoic acid) [P(DLA-co-D-2HB)] random copolymers. The melting temperature values and melting enthalpy values at 100% crystallinity for stereocomplexed solvent-evaporated and precipitated P(LLA-alt-L-2HB)/P(DLA-alt-D-2HB) blends were correspondingly 187.5 and 187.9 °C, and 98.1 and 91.8 J g⁻¹. Enantiomeric polymer blending of P(LLA-alt-L-2HB) and P(DLA-alt-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.