Reentrant phase transition <i>via</i> light–heat antagonism in a single polymer system

Inspired by nature's reliance on antagonistic interactions to orchestrate complex dynamics, synthetic systems often replicate this by integrating multiple competing components-a strategy frequently hampered by synthetic complexity and kinetic mismatch. Here, we report a single-component spiropyran-functionalized polymer system that exhibits programmable reentrant phase transitions mediated by light-heat antagonism in reversible spiropyran isomerization. Within this system, light and heat competitively drive the interconversion of spiropyran between its ring-closed SP^- and ring-opened MCH forms, allowing precise modulation of intermolecular electrostatic interactions and thereby enabling real-time control over polymer conformation and phase transitions. Following this principle, we demonstrate versatile reversible switching of a single polymer system among nonthermoresponsive, monothermoresponsive (UCST-type), and reentrant thermoresponsive states-the latter displaying LCST behavior at low temperatures and UCST behavior at high temperatures. This light-heat regulatory mechanism is further extended to hydrogels, where it enables programmable reentrant volumetric transitions and autonomous oscillatory deformation. By employing noninvasive light to flexibly tailor multimode responsiveness in a single system, our work establishes a robust and generalizable platform for dynamically programmable matter with prospects in soft robotics and biomedicine.