Thermo/Light Dual-Responsive Helical Dendronized Poly(phenylacetylene)s

Stimuli-responsive helical polymers form an intriguing class of materials that may find broad applications in chiroptical materials or switchable chiral devices. Here we report the synthesis of a class of helical dendronized copolymers with poly(phenylacetylene) (PPA) backbones. These copolymers carry methoxy-terminated dendritic oligoethylene glycol (OEG) pendants and spiropyran moieties. Both block copolymers and random copolymers were prepared to investigate the effect of structure on their stimuli-responsiveness and helicities, as well as their supramolecular assemblies in aqueous conditions. The copolymers exhibited typical thermoresponsive behavior through thermal dehydration of the dendritic OEGs with tunable cloud point temperatures (Tcps). Polarity differences between the hydrophilic dendritic OEG pendants and the lipophilic PPA backbones afforded all copolymers radial amphiphilicity. In addition, for the block copolymers, polarity differences between the hydrophilic dendronized block and the lipophilic SP block further provided the copolymers with axial amphiphilicity. Their unprecedented amphiphilic nature drove the copolymers to preferentially aggregate in water even below their Tcps. On heating across their Tcps, the thermally induced phase transitions of the copolymers resulted in switches in their helicity. Photoisomerization of spiropyran moieties between the closed spirocyclic (SP) form and the open merocyanine (MC) form within the copolymer matrices was dependent on the balance of interactions between spiropyran moieties and dendritic OEGs, which did not exhibit an obvious influence on their helicities. Supramolecular assembly of the block copolymers tend to form extremely long fibers with thicknesses of about 25 nm, while anisotropic nanocubes with dimensions around 400–600 nm were observed from the random copolymers, indicating that topology of the copolymers plays an important role in modulating their assemblies.