Thermoresponsive Dendronized Poly(phenylacetylene)s Showing Tunable Helicity

Abstract

Switching the macromolecular helicity of poly(phenylacetylene)s (PPAs) through external achiral stimuli provides convenient possibilities in designing smart chiral materials. Herein, a series of PPAs carrying dendritic oligoethylene glycol (OEG) pendants were synthesized with different linkages between the backbone and dendron pendants, including benzamide (PPA-AC), inverted acylamide (PPA-AM), diamide (PPA-dAM), benzoate (PPA-ES), and benzyl ester (PPA-OEG). The periphery of the OEG dendrons is capped with either an ethoxy (PPA-ACe) or a methoxy group (PPA-ACm) to afford the PPAs with different overall hydrophilicity. These dendronized PPAs inherit unprecedented thermoresponsiveness from the dendritic OEGs and show characteristic thermally induced phase transitions with cloud points (Tcps) in the range 30–54 °C. Hydrogen bonding interactions between neighboring repeating units were found to play a key role in transferring chirality from pendants to polymer backbone. Dependent on the linkages, these PPAs behave differently about helix–helix transitions driven by the thermally induced collapse of the pendanted OEG dendrons. Interestingly, dendronized homopolymer with methoxy terminals (PPA-ACm) shows radial amphiphilicity in aqueous solutions, which renders the polymer a different chiroptical property. For comparison, chiroptical properties of these polymers were also investigated in organic solvents. Furthermore, coordination with Ba2+ was found to mediate the chiroptical properties of these dendronized PPAs in aqueous solutions, which can be surprisingly shielded by OEG dendrons below their Tcps.