Synthesis of a Linear-Hyperbranched Supramolecular Polymer and Its Light-Responsive Self-Assembly Behavior

Herein we reported the synthesis, self-assembly and light-responsive disassembly of a "linear-hyperbranched" supramolecular polymer. Firstly, a hyperbranched polymer CD-g-HPG composed of hyperbranched polyglycerol with a beta-cyclodextrin group in the center was synthesized by ring-opening multibranching polymerization (ROMBP). Secondly, a linear polymer AZO-PS composed of polystyrene with an azobenzene group at the end was synthesized via atom transfer radical polymerization (ATRP). Then, the linear AZO-PS and hyperbranched CD-g-HPG were conjugated together through the specific CD/AZO host-guest interactions, leading to the formation of the "linear-hyperbranched" supramolecular polymer PS-b-HPG. This supramolecular polymer was amphiphilic and could self-assemble into vesicles in water. The host-guest complexation ability was characterized by UV-Vis titration. In the case of keeping the concentration of AZO-PSs unchanged, the absorption peak at 330 nm increased gradually with the addition of CD-g-HPGs, which supported the occurrence of complexation between beta-CD groups in CD-g-HPGs and AZO groups in AZO-PSs. The host-guest CD/AZO complexation constant of 4.14x10(4) M-1 was calculated by the Benesi-Hildebrand plot. A Job plot was generated, from which it was determined that the binding stoichiometry between AZO-PS and CD-g-HPG is 1:1. The self-assemblies of the amphiphlic linear-hyperbranched supramolecular polymers were characterized by SEM and TEM. The SEM images showed that the self-assemblies were spherical particles, and the holes directly be seen in some particles indicated that they were vesicles or hollow spheres with a very thin wall thickness. The TEM images of self-assemblies stained with ruthenium tetroxide (RuO4) indicated that the spherical particles were vesicles according to a clear contrast difference between the inner pool and the outer thin wall. At last, we showed that the vesicles could disassemble under UV light due to the trans-to-cis isomerisation of the AZO groups. With the continuous UV irradiation on vesicles, the absorption peak of trans-AZO diminished gradually and almost completely disappeared after 900 seconds. Meanwhile, the solution was transformed from turbid to transparent followed with the appearance of yellow precipitates in the bottom of the bottle.