Well-defined amphiphilic block copolymers of poly(vinyl acetate) (PVAc) and poly(N-vinylpyrrolidone) (PNVP) were synthesized by cobalt-mediated radical polymerization (CMRP). The NVP polymerization initiated by poly(vinyl acetate) end-capped by the cobalt(II)acetylacetonate complex met the criteria of controlled polymerization, i.e., first-order kinetic in NVP, increase of the molar mass with the NVP conversion, and narrow molar mass distribution. Therefore, the length of the two blocks can be tuned by the [VAc]/[Co(acac)(2)] and the [NVP]/[PVAc] ratios for the synthesis of the macroinitiator and the polymerization of the second monomer, respectively. These amphiphilic PVAc-b-PNVP block copolymers were easily converted into the double hydrophilic PVOH-b-PNVP counterparts by selective methanolysis of the PVAc block. These two types of copolymers were prone to self-association into micelles in appropriate solvents.