Photoreactive Cholesteric Polyesters Derived from 4-Carboxycinnamic Acid and Novel Chiral Spacers

Abstract

A homopolyester was prepared from 4-carboxycinnamic acid and a diol derived from 4-mercaptophenol and (S)-3-bromo-2-methyl-1-propanol. With this spacer and a 1 :1 combination of 4-carboxycinnamic acid and naphthalene-2,6-dicarboxylic acid, copolyesters with random and with alternating sequences of the dicarboxylic acids were synthesized. The homopolyester and the random copolyester are noncrystalline materials with a broad cholesteric phase, whereas the alternating copolyester is semicrystalline. In contrast to the homopolyester, the copolyesters form a blue Grandjean texture over a broad temperature range. Upon irradiation in solution the cinnamoyl moiety undergoes trans-cis photoisomerization and (2 + 2) photocycloaddition. The cinnamic, benzoic, and naphthalene-2,6-dicarboxylic phenyl esters undergo photo-Fries rearrangements. The dominating reaction in spin-coated films is the photocycloaddition causing photocrosslinking. The efficiency of cycloaddition and their competition with the Fries reaction are dependent on the composition of the polymers and the conversion. The rearrangement is completely suppressed in films of the homopolyester 4, and the photocycloaddition is more efficient than in the copolymers.