a hydrophilic material having a hydrophilic nature and being biocompatible and processable into nanoparticles wherein a hydrophilic monomer such as threitol or a hydrophilic polymer such as polyethylene glycol is attached to the βposition of an α , β - unsaturated carboxyl group of a polymer comprising an aromatic compound such as 4 - hydroxycinnamic acid in which an aromatic ring is substituted by the α , β - unsaturated carboxyl group and one or more hydroxyl groups . in particular , a hydrophilic material having a nanoparticle size which is highly dispersible in water , can well migrate into the blood and is useful as a drug carrier , a medical material and a sustained drug release material .