Reversible On-Demand Activation of Acid-Catalyzed Dynamic Polymers for Gradient-Driven Reshaping.

Covalent adaptable networks (CANs) unite recyclability and mechanical stability by leveraging externally triggered bond exchange. However, a significant challenge remains in achieving a sharp reversible transition between dynamic and static network states, which broadly limits their applicability. This work introduces a merocyanine photoacid for precise and reversible spatiotemporal control over dynamic bond exchange in thiol-ene photopolymers. When exposed to visible light, the photoacid's activated spiropyran form enables dynamic network rearrangements through acid-catalyzed transesterification. Switching off the light yields the deactivated merocyanine form and a creep-resistant polymer network. Stress relaxation experiments clearly demonstrate a remarkable difference in mechanical properties resulting from the photoacid's isomerization state. The rapid isomerization kinetics and fatigue resistance of this system are utilized to generate gradients of active photoacid at the micrometer level. Applied in a novel mold-free reshaping approach, bending radii are predictable based on the introduction of an empirical model. Finally, the successful fabrication of diverse microstructures via multiphoton laser writing highlights the future potential of these reversibly switchable CANs in light-controlled micromechanics.