Communication: Living free-radical butyl acrylate polymerization in miniemulsion was initiated by polystyrene bearing a nitroxyl end group to yield polystyrene-block-poly(butyl acrylate) block copolymers. Polystyrene macroinitiator was obtained using different initiating systems (potassium persulfate or benzoyl peroxide) in the presence of 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) or the more water-soluble 4-hydroxy-2,2,6, 6-tetramethylpiperidin-N-oxyl (OH-TEMPO). The nitroxide water-solubility has an important influence in determining molecular weight distributions and controlling the growth of the second block.