[2+2] Photocycloaddition-Mediated Intra- And Intermolecular Cross-Linking Of Thermoresponsive Dendronized Polymethacrylates

Precise control on cross-linking of polymers is highly valuable to tune their structures and properties as well as for material applications. In this work, we present a robust methodology to mediate intra- and intermolecular cross-linking of thermoresponsive dendronized polymers through [2 + 2] photocycloaddition reactions. These polymers are constituted of polymethacrylates carrying a cinnamate-cored dendritic oligo(ethylene glycol) (OEG) pendant in each repeating unit. In dilute solutions, adjacent cinnamate moieties were efficiently dimerized by UV irradiation to afford intramolecularly cross-linked dendronized polymers. Their photocycloaddition behavior was followed by UV-vis and H-1 NMR spectroscopy, and the crowding effect from dendron dimerization on chain conformations was studied by SEC-LS and AFM. Interestingly, dendronized polymers retained single-chain cylinder topologies without branched and loop microstructures after photodimerization, and their hydrodynamic sizes in solutions kept nearly invariable, which is very different from the single-chain collapse of linear cinnamate polymers without OEG dendron pendants. These polymers exhibited characteristic thermoresponsive behavior in water, and their phase transition temperatures increase continuously with dimerization degrees. However, thermally induced interchain aggregations were surprisingly retarded during the phase transition, especially for polymers with high dimerization degrees. Furthermore, cross-linking of dendronized polymers was switched from intramolecular onto intermolecular fashion by photodimerization of the polymers above their phase transition temperatures in aqueous phases, which can be utilized to prepare thermoresponsive dendronized nanoparticles with uniform shapes and controllable sizes.