A novel biodegradable unimolecular reversed micelle consisting of a poly (L-lactide) (PLA) shell and a hyperbranched D-mannan (HBM) core, that is, a chestnut-shaped polymer (PLA-HBM), was synthesized by the polymerization Of L-lactide on HBM with 4-(dimethylamino)pyridine (DMAP) as the catalyst. The obtained polymers were soluble in dimethyl sulfoxide, tetrahydrofuran, and chloroform but insoluble in H2O. The molecular weights of the PLA chain on PIA-HBM tended to increase with increasing polymerization time. The number of PLA chains on PLA-HBM could be controlled by the ratio of DMAP to the sugar unit in HBM. The obtained copolymer, PLAHBM, acted as a unimolecular reversed micelle with an encapsulation ability toward the hydrophilic molecule. In addition, the entrapped hydrophilic molecules were slowly released from the core of PLA-HBM, and the release rate was accelerated by the breaking of the PLA chains of the shell when proteinase K as a hydrolase of PLA was used. (c) 2005 Wiley Periodicals, Inc.