Three types of pegylated amphiphilic copolymers of poly(delta-valerolactone) (PVL) were copolymerized with methoxy poly(ethylene glycol) (MePEG) and poly(ethylene glycol) (PEG(4000) and PEG(10,000)), respectively. Pegylation of PVL allowed copolymers possessing amphiphilic property and efficiently self-assembled to form micelles with a low critical micelle concentration (CMC) in the range of 10(-7)-10(-8)M. The average molecular weight of copolymers was in the range of 10,000-20,000 Da, and the polydispersity of copolymers was about 1.7-1.8. Higher mobility of low molecular weight PEG (i.e., MePEG and PEG(4000)) than high molecular weight PEG(10,000) allowed valerolactone ring opening more efficient in terms of PVL/MePEG and PVL/PEG(4000) copolymers possessing longer chain length in hydrophobic domain. Pegylated PVL with low CMC and triblock structure was preferred to encapsulate drug during micelle formation. Although all of these amphiphilic copolymers exhibited controlled release character, the micelles formed by triblock copolymer possessed a more stable core-shell conformation than that by diblock copolymer, and resulted in the release of drug from triblock micelles slower than that from diblock micelles. (c) 2006 Wiley Periodicals, Inc.