Thermoresponsive Helical Poly(phenylacetylene)s

Poly­(phenylacetylene) (PPA) bearing dendritic oligo­(ethylene glycol) (OEG) as pendants was synthesized, and its thermoresponsiveness and helical conformation were investigated. Despite the steric hindrance of the bulky pendants in the homopolymer PPA-OEG, the chirality could be efficiently transferred from pendant alanine moieties to PPA main chain through ester linkage. In order to examine the steric effect of pendants on chiral transformation, a model PPA homopolymer PPA-Boc which carries less bulky moieties was prepared for comparison. The chiroptical properties of these thermoresponsive PPAs were further investigated by varying temperature to examine the effects of their thermoresponsiveness. In addition, PPA copolymers PPA-BDY bearing OEG dendron and fluorescent boradiazaindacene (BDY) chromophore showed excellent thermoresponsive properties and interesting fluorescence enhancement at elevated temperatures. To investigate the rigidity effects of polymer backbone on the thermally induced fluorescence enhancement, a nonchiral polymer carrying the same pendants but with polymethacrylate as the backbone (PMA-OEG) was prepared. It was found that the chiroptical and fluorescence properties of these PPAs are dependent not only on their chemical structures but also on the thermoresponsiveness.