Poly(L-glutamic acid)-block-poly(ethylene glycol)-block-poly(L-glutamic acid) sodium salt is a A-B-A type hydrophilic block copolymer that has a poly(L-glutamic acid sodium salt) section on both sides of a non-reactive PEG section. It is a crystalline solid soluble in water. Upon derivatization of the poly(L-glutamic acid sodium salt) parts, it can self-assemble into particles in aqueous solution with a hydrophobic poly(L-glutamic acid) core and a hydrophilic PEG shell.