Two types of photoreactive water-soluble oligo(ethylene glycol)s (OEGs; M-n = ca. 6500) were prepared by derivatization of OEG with photodimerizable groups such as cinnamates or coumarinates at both ends. Upon U`V light irradiation of biscinnamated OEG in an aqueous solution, almost all trans-cinnamate groups were isomerized to cis form in preference to dimerization even in an associated state at high concentrations of cinnamate groups, whereas dimerization via intermolecular photocycloaddition was the dominant process in methanol solution and in the film state. Coumarinate groups attached to OEG were photodimerized by UV irradiation, and this process was enhanced in an aqueous solution and in film cast from a polar solvent by hydrophobic association between coumarinate groups, resulting in the production of macromolecules. Kinetic analysis revealed that for low-concentration aqueous solutions or a film cast from a nonpolar solvent, predominantly circular trimers to tetramers were produced on average, whereas from high-concentration aqueous solutions or film cast from a polar solvent, mostly cyclic heptamers were produced. These results suggest that the molecular design of nanostructured, cyclic polymers of various sizes is possible by changing the concentration of aqueous solutions and cast solvents in this photoaddition system. (c) 2005 Wiley Periodicals, Inc.