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The non-enzymatic arid enzymatic degradation behaviors of the monomethoxy-poly(ethylene glycol)-b-poly(epsilon-caprolactone) diblock copolymers (MPEG-PCL) micelles in aqueous solution were investigated by DLS, H-1 NMR, SEC and HPLC. It is found that the degradation mechanism of MPEG-PCL micelles in aqueous solution in non-enzymatic case is quite different from that in the presence of enzyme. In nonenzymatic case, the degradation induced by acidic catalysis was not found in low pH aqueous solution but the degradation of the micelles occurred under neutral and basic conditions. The degradation of MPEG-PCL micelles first happens near the interface region of the MPEG shell and PCL core, leading to the part detachment of PEG chains. With increasing degradation time, the degradation inside the PCL core with a random scission on PCL chains occurred. Compared with non-enzymatic degradation, the enzymatic degradation of MPEG-PCL micelles is much fast and the degradation rate of MPEG-PCL micelles is proportional to either the micelles or the enzyme concentration in a certain range. Based on the micelle degradation behaviors that we observed, a possible mechanism for the enzymatic degradation of the MPEG-b-PCL micelles including PCL core erosion which results in cavitization of micellar core and micellar dissociation is proposed. (C) 2008 Elsevier Ltd. All rights reserved.