Polystyrene/poly[styrene-co-(butyl methacrylate)] block copolymers with controlled molecular weights and with polydispersities generally below (M) over bar w/(M) over bar(n) = 1,45 and partially as low as (M) over bar w/(M) over bar(n) = 1,19 were synthesized by a free radical bulk copolymerization using a 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO)-capped polystyrene macroinitiator. The influence of the macroinitiator concentration on the block copolymerization was studied. The polymerization rates are independent of the macroinitiator concentration and are close to that of thermally self-initiated styrene/butyl methacrylate copolymerizations showing the important role of self-initiation for N-oxyl mediated free radical polymerizations.