Light-Controlled Topology Switching Enables Continuous Modulation of Thermally Induced Phase Behavior in Polymer Solutions

Dynamic regulation of polymer solution phase behavior with real-time reversibility remains challenging. Here, we report a light-responsive poly(ethylene glycol) (PEG) system that enables reversible switching between linear and cyclic topologies, allowing continuous modulation of thermally induced phase transitions. Linear PEG functionalized with styrylpyrene end groups undergoes efficient intramolecular [2 + 2] photocycloaddition under visible light to form cyclic polymers, while UV irradiation restores the linear precursor. This reversible topology interconversion allows continuous tuning of the cyclic/linear ratio. As a result, the cloud point temperature of aqueous solutions can be modulated over a wide range from 37 to 65 °C. Aggregation appears to be enhanced in linear polymers through end-group-mediated π-π stacking and intermicellar bridging, whereas it is suppressed in cyclic polymers due to topological constraints and reduced stacking interactions. This work establishes topology switching as a noninvasive strategy for dynamically regulating polymer solution thermodynamics and highlights topology as a tunable parameter for controlling thermoresponsive behavior.