Impact of cyclic backbone and thioether functionality on thermoresponsive behaviors of multi-responsive Y-junction-bearing polyacrylamides

Alteration of chemical composition and chain topology of polymers allows for the rational regulation of their thermoresponsive behaviors. Compared with oxyether group, thioether moiety with higher hydrophobicity and oxidation sensitivity is proposed to present some differences in phase transition. To elucidate cyclization and thioether functionality dependent thermoresponsive behaviors, two pairs of thermo/pH/oxidation-responsive linear and cyclic Y-junction-bearing polymers (YJPs) involving dual thioether groups or coexistent thioether and oxyether moieties are designed. Multi-tunable LCST behavior can be achieved upon variations of hydrogen bonding, electrostatic and hydrophobic interactions. Owing to distinct stability of supramolecular interactions, copolymer solutions prepared by direct dispersion or self-assembly are liable to present a reverse order of phase transition temperature (T_c,l) relying on the thioether functionality. Cyclization of the backbone can result in an elevated T_c,l up to about 14 °C, revealing Y-junction-induced amplification of topology effect. T_c,l is prone to decreasing upon solvent switch from water to heavy water or pH increment, while the distinct oxidation sensitivity of monothioether and dithioether groups affords oxidation-triggered LCST “on/off” behaviors. In addition, intriguing sphere-to-vesicle-to-lamella-to-vesicle, vesicle-to-lamella-to-sphere and lamella-to-nanoribbon-to-lamella transitions occur in thermo-induced self-assembly. The success of this study paves the way for exploring thioether functionality dependent physicochemical properties and multipurpose applications of complex architectural polymers.