Summary: Coumarin containing poly(ethylene glycol) monols and diols were prepared in a telechelic fashion using quantitative end group esterification. The coumarin modified poly(ethylene glycol) monols and diols were solution cast into films (1 to 15 μm) and chain extended via the dimerization of the coumarin derivatives with UVA (>300 nm) light irradiation. The coumarin‐modified PEG diol doubled in molecular weight, and the molecular weight distribution increased from 1.17 to 2.75 upon exposure to 110 J · cm?2 of UVA light. The chain extended poly(ethylene glycol)s were subsequently converted to their original molecular weight via the photocleavage reaction of the coumarin dimer at 254 nm with less than 2 J · cm?2. The photoreversible chain length modulation was monitored utilizing GPC, DSC and UV‐Vis spectroscopy.
