Periodic nanostructures of poly(glycidyl methacrylate) (pGMA) were grafted onto poly(ethylenealt-tetrafluorethylene) (ETFE) films by reversible addition-fragmentation chain transfer (RAFT) polymerization. ETFE samples were first irradiated in an interference lithographic setup using EUV (extreme ultraviolet) light followed by exposure to air to introduce surface peroxide groups serving as thermal initiators for the graft polymerization. The dependence of height and morphology of the grafted pGMA structures on the exposure dose and on the grafting parameters such as time and concentration of RAFT chain transfer agent was studied with atomic force microscopy (AFM) and scanning electron microscopy (SEM). RAFT mediation reduces the grafted layer thickness by a factor of 5-10 compared to uncontrolled free radical polymerization and significantly improves the spatial resolution into the 50 nm range. A mushroom to brush transition was observed for low absorbed doses. This transition was found to occur at similar to(1.5 +/- 0.7) x 10(-3) and similar to(3 +/- 1.3) x 10(-3) chains/nm(2) for the free radical and RAFT-mediated polymerization, respectively. The grafting density increased with increasing absorbed dose up to a maximum of 0.1-0.2 chains/nm(2). The data interpretation is supported by simulations based on photon statistics.