Provided is a triblock copolymer represented by General Formula (I):CNR-PEG-CNR (1)or a polycation thereof, whereineach CNR is independently a polymer segment having a repeating unit containing as part of a pendant group a cyclic nitroxide radical that binds to the polymer main chain via a linking group having at least one imino group or ether bond, and PEG is a segment containing poly(ethylene glycol).