Well-defined poly(vinyl acetate) macroinitiators, with the chains thus end-capped by a cobalt complex, were synthesized by cobalt-mediated radical polymerization and used to initiate styrene polymerization at 30 degrees C. Although the polymerization of the second block was not controlled, poly(vinyl acetate)-b-polystyrene copolymers were successfully prepared and converted into amphiphilic poly(vinyl alcohol)-b-polystyrene copolymers by the methanolysis of the ester functions of the poly(vinyl acetate) block. These poly(vinyl alcohol)-b-polystyrene copolymers self-associated in water with the formation of nanocups, at least when the poly(vinyl alcohol) content was low enough. (c) 2006 Wiley Periodicals, Inc.