Summary: The free‐radical addition of ω‐functional mercaptans to the vinyl double bonds of 1,2‐polybutadiene‐block‐poly(ethylene oxide) copolymers was used for modular synthesis of well‐defined functional block copolymers. The modification reaction proceeds smoothly and yields quantitatively functionalized block copolymers (1H NMR and FT‐IR spectroscopy) without disturbing the molecular‐weight distribution of the parent copolymer (PDI < 1.09, size exclusion chromatography).