A water soluble, biodegradable AB type diblock copolymer which comprises 61 to 85% by weight of a biodegradable, hydrophobic A block comprising a biodegradable polyester, and 15 to 39% by weight of a biocompatible, hydrophilic B block comprising a monofunctional polyethylene glycol(PEG) having a number average molecular weight less than 5000, and wherein said diblock copolymer has a number average molecular weight less than 15000 and possesses reverse thermal gelation properties.
