Photodegradable physically cross-linked polymer networks are prepared from self-assembly of photolabile triblock copolymers. Linear triblock copolymers composed of poly (o-nitrobenzyl methacrylate) and poly(ethylene glycol) (PEG) segments of variable molecular weights were synthesized using atom transfer radical polymerization. Triblock polymers with low-molecular-weight PEG segments form solid films upon hydration with robust mechanical properties including a Young's modulus of 76 ± 12 MPa and a toughness of 108 ± 31 kJ m−3. Triblock polymers with high-molecular-weight PEG segments form physically cross-linked hydrogels at room temperature with a dynamic storage modulus of 13 ± 0.6 kPa and long-term stability in hydrated environments. Both networks undergo photodegradation upon irradiation with long wave UV light.