[Problem]To provide a copolymer which includes hydrophilic and hydrophobic blocks, which can form nanoparticles in which a physiologically active substance can be efficiently packaged therein and which are stable under acidic conditions.[Solution]Provided is a copolymer in which the hydrophilic segment is composed of polyethylene glycol (PEG) and the hydrophobic segment is composed of polystyrene. The hydrophobic segment has a side chain, and the side chain has ends to which a cyclic nitroxide radical having a radical scavenging function and a trialkoxysilane are covalently bonded.
