Preparation of Polymer Gel with Homogeneous Network Structure Composed of Photoreactive Star-Shaped <i>N</i> -Isopropylacrylamide Polymers

The construction of polymer gels with well-defined and ordered network structures remains a significant challenge due to inherent heterogeneities arising from conventional cross-linking methods. In this study, we report the synthesis and photoinduced gelation of a star-shaped poly(N-isopropylacrylamide) (PNIPA) derivative, terminal anthracene (AN) 4-armed star PNIPA, functionalized with photodimerizable anthracene groups at each chain terminus. This precursor was prepared via reversible-deactivation radical polymerization using a tetrafunctional initiator, followed by postpolymerization modification through click chemistry. Semidilute solutions of the modified polymer in dimethylformamide (DMF), a good solvent, were irradiated with 395 nm light to initiate anthracene dimerization-driven cross-linking without the addition of chemical cross-linkers. Dynamic light scattering (DLS) measurements during the cross-linking reaction revealed that gelation occurred without creating noticeable spatial heterogeneities. Swelling experiments, 1H NMR spectroscopy, and methanolysis followed by molecular weight analysis confirmed efficient covalent bond formation through photodimerization. Small-angle X-ray scattering (SAXS) profiles of swollen gels further revealed a highly ordered internal network structure. Although the formation of loop defects cannot be entirely excluded, the network more closely reflects the molecular symmetry and architecture of the star-shaped precursors compared to conventional gels. Moreover, the photo-cross-linked gels were fully depolymerizable back to their building blocks upon 254 nm light exposure, enabled by the absence of chain entanglements. This reversible network formation strategy, based solely on photochemistry and structurally defined polymers, offers a promising approach toward recyclable, structurally regular soft materials, contributing to sustainable materials design.