On‐Demand Photodegradable and Thermo‐Reversible, Soft, Transparent Dithioacetal Hydrogels

Stimuli-reversible, chemically cross-linked polymers capable of altering their physicochemical and mechanical properties on demand, upon application of external stimuli (e.g., light, temperature), are highly desirable for the development of multifunctional materials. Herein, we report a facile chemical platform for the synthesis of photodegradable and thermo-reversible, model hydrogels consisting of poly(ethylene glycol) (PEG) as the elastic strands and dithioacetal moieties at the cross-link points. The gels were synthesized via an acid-catalyzed step-growth reaction of a difunctional PEG-thiol macromer with a wisely selected aromatic dialdehyde cross-linker. The formation of the photosensitive dithioacetal bonds at the cross-links rendered the hydrogels photodegradable, whereas the production of the initial comonomers as the main photoproducts after irradiation endowed the material with thermoreversible properties. The linear viscoelastic behavior of water-swollen gels, their photodegradation under UV (λ = 254 nm) irradiation at very low intensity (0.063 mW cm^-1), and the reversible reformation of the hydrogel upon heating were investigated by dynamic shear rheology. Mechanistic insights for the photodegradation mechanism of the system were gained by ¹H NMR spectroscopy and kinetic studies on a model dithioacetal compound.