Initiator-Free Photohydrogelation of Linear Coumarin-Containing Poly(ethylene glycol) Methacrylates and Study of Ultrasound-Triggered Gel Breakdown AITranslate

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DOI：https://doi.org/10.1021/acsapm.1c00752

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We report a serendipitously discovered initiator-free photohydrogelation of linear poly(ethylene glycols) (PEGs) containing coumarin and methacrylate end groups. The mechanistic studies by NMR spectroscopic reaction monitoring with various control experiments and by real-time UV curing rheology reveal that hydrogelation occurs via a 3-reaction, 1-step process, involving two [2 + 2]-cycloadditions between coumarin and coumarin and coumarin and methacrylate groups and an initiator-free radical polymerization of the methacrylate. We also elucidate the important role of PEGs in this initiator-free UV-induced gelation. Coumarin groups are shown to rheologically impact the network strength and flexibility of hydrogels through [2 + 2]-cycloaddition spacing elongation. Using this methodology, coumarin-containing PEG methacrylates of various molecular weights afford a range of hydrogels G1–G8 with good yields after purification and drying. Interestingly, the same chemistry can be applied to react a dicoumarin containing PEG with a PEG dimethacrylate at various weight ratios (G9–G11). The gels could be partially broken down under several types of ultrasound triggers. In polymer solution, coumarin photodimers (CPDs) are shown to return partially to the coumarin state in response to 30 kHz ultrasound. In gel states, the breakdowns triggered by 30 kHz sonication and by 1.1 MHz high-intensity focused ultrasound are observed at PEG chains without any conversion of CPDs into coumarin. Our methodology may be applicable to the development of a controlled delivery system for hydrophobic actives where a conventional photohydrogelation with the use of a photoinitiator and the subsequent presence of its byproduct can cause migration and toxicity concerns to the surrounding environment.