Photoinduced Reversible Semicrystalline-to-Amorphous State Transitions of Stereoregular Azopolyesters.

We report the synthesis of isotactic azobenzene-based polyesters (azopolyesters) with main-chain chirality via highly enantioselective resolution copolymerization of racemic azobenzene-containing epoxides with cyclic anhydrides. All polyesters with trans-azobenzene moieties were found to be semicrystalline materials with melting temperatures of 153-231 °C, while the corresponding isotactic cis-azopolyesters were amorphous. The azobenzene groups in the copolymers exhibited reversible trans-to-cis and cis-to-trans photoisomerization upon irradiation with light. This demonstrates that the crystallinity of isotactic azopolyesters can be manipulated via photoinduced reversible isomerization. In addition, mixing isotactic trans-polyesters with different enantiomeric configurations in a 1:1 mass ratio afforded crystalline stereocomplexes for which the crystalline behavior differed significantly from those of the component enantiomer. Also, photoinduced reversible transitions between semicrystalline and amorphous states were observed in various stereocomplexes of isotactic trans-azopolyesters, similar to the isotactic azopolyesters themselves.