Photodependent release from poly(vinyl alcohol)/epoxypropoxy coumarin hydrogels

To obtain a photoresponsive hydrogel, poly(vinyl alcohol) (PVA) having coumarin residue as a pending group was crosslinked by photodimerizing the coumarin pendants. PVA having coumarin pendants was prepared by reacting the polymer with epoxypropoxy coumarin (EPC) in a strong alkali condition with EPC/PVA molar ratio of 200/1. According to the 1H NMR spectrum of PVA‐EPC conjugate, the molar ratio of EPC residue to PVA was about 4.3/1, indicating that EPC was attached to PVA every 444 repeating units of vinyl alcohol. An aqueous solution of PVA‐EPC (5%, w/v) became a semi‐solid hydrogel by the irradiation of a light (λ = 365 nm; 400 W) for 1 hr. The dimerization degree of EPC residues of the hydrogel decreased and increased in a periodical manner under the cyclic irradiation between two UV lights (λ = 365 nm and λ = 254 nm). The release of 5(6)‐carboxyfluorescein (CF) from PVA‐EPC hydrogel was significantly enhanced by the 5 min‐irradiation of λ = 254 nm (6 W), possibly due to the photodedimerization of EPC residues of the hydrogel.