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.
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