Ultra‐Fast and Reversible [2 + 2] Photocycloaddition Enabling Green Light‐Mediated Switching of Polymer Materials

Light‐activated covalent adaptable networks (CANs) can reversibly rearrange their structures under a photonic field, enabling adaptability and customization for various materials applications. However, most light‐based CAN rely on high‐energy short‐wavelength UV light for activation, which may cause unwanted side reactions. In this report, we introduce a fully reversible [2 + 2] photocycloaddition system‐based on an ortho‐palladated oxazolone motif that can be activated by wavelength at 525–590 nm, achieving quantitative photo‐conversion in less than 5 min. The cyclobutane adduct can be completely reversed to the starting molecule at 60℃ in 12 h. Critically, the photo‐ and thermal‐switching can be repeated for more than 20 cycles without any loss of chemical integrity and reactivity. Employing time‐dependent density functional theory calculations and extensive experimental validation, we elucidate the mechanism of the photoreaction to occur via singlet biradical formation, followed by an electron transfer process to yield a zwitterionic intermediate that forms the cycloadduct. We further develop a synthesis method to incorporate the ortho‐palladated oxazolones into methacrylate polymer structures, forming green light‐triggered adaptive polymer networks. The fast response to green light enables temporal control of polymer elasticity and shape manipulation, despite the lack of optical transparency.