Solvent-free isothermal tuning of viscoelasticity of polymer materials is important for an emerging photochemical molding technology and photoreversible adhesives. In this study we designed a four-armed star-shaped poly(butyl acrylate, BA) oligomer having four coumarin end groups. The irradiation of UV at the wavelength of 365 nm (UV365) to the viscous poly(BA) oligomer under a solvent-free condition produced a solid network material along with the progress of dimerization reaction with coumarin end groups. The subsequent irradiation of UV at the wavelength of 254 nm (UV254) caused dimer dissociation reaction to attain change in the mixing degree of star and network architectures in the material. Moreover, viscoelasticity of the network material was tunable by repetitive UV365 and UV254 irradiations. (c) 2017 Wiley Periodicals, Inc.