An amphiphilic block copolymer of poly(N-vinyl pyrrolidone)-b-poly(epsilon-caprolactone) (PVP-b-PCL) was synthesized by a combination of cobalt-mediated radical polymerization (CMRP) and ring-opening polymerization (ROP). The micellar characteristics of this copolymer were subsequently investigated. PVP (M-n = 11,400, M-w/M-n = 1.32) was synthesized at 20 degrees C via CMRP using a molar ratio of [VP](0)/[V-70](0)/[Co](0) = 150/8/1. The PVP was then reacted with 2,2'-azobis[2-methyl-N-(2-hydroxyethyl)propionamidel (VA-086) to modify its cobalt complex chain end to a hydroxyl group. The cobalt (Co) content in the resulting PVP-OH was 1.2 ppm, indicating that all of the covalent Co-C bonds were cleaved and reacted with VA-086, and that the separated cobalt complexes were successfully removed. The ROP of CL was subsequently carried out using the produced PVP-OH as a macroinitiator at 110 degrees C. The GPC trace of PVP-b-PCL was monomodal without any tailing caused by the residual PVP-OH, indicating that the initiation efficiency was very high. The critical micelle concentration (CMC) of PVP-b-PCL (M-n = 18,000, M-w/M-n = 1.35) was 0.015 mg/mL. The PVP-b-PCL micelles were spherical in shape with an average diameter of 105 nm. The nanosized PVP-b-PCL micelles show promise as novel drug carriers in biomedical and pharmaceutical applications. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3078-3085, 2009