The novel photosensitive chain extender (ICM) was prepared by condensation of 5-hydroxyisophthalic acid (1) and 7-chlorocarbonylmethoxycoumarin (CM). ICM acid chloride was polycondensed with poly(ethylene glycol)s (PEG)s of different molecular weights (M-n = 1000, 3000, 8300 g/mol). The resulting high-molecular-weight PEGs having a coumarin moiety in the pendent group (ICM) underwent rapid reversible photo-cross-linking upon exposure to alternating wavelength of irradiation (>280/254 nm) in the absence of photoinitiators. The melt-pressed PEG films were exposed to the wavelength of >280 nm to produce a hydrogel. Gel contents attained over 90% for ICME3000 and 80% for ICME8300, respectively, within 5 min, These gels were swollen in water and showed characteristic properties of a hydrogel. The degree of swelling could be controlled by the irradiation time as well as MWs of PEG. (C) 2008 Elsevier Ltd. All rights reserved.