A polystyrene (PS)-b-polylactide (PLA) block copolymer was prepared from the combination of atom transfer radical polymerization and ring-opening polymerization with commercially available 2,2,2-tribromoethanol as a dual initiator in a sequential two-step procedure. Hydroxyl-terminated polystyrene (PS-OH)s with various molecular weights were first prepared with polydispersity indices lower than 1.3; these provided valuable macroinitiators for the polymerization of D,L-lactide. A block copolymer with a composition allowing the formation of hexagonally packed PLA cylinders in a PS matrix was then obtained. The PS-b-PLA thin films revealed, after vapor solvent annealing, a hexagonally packed organization of the PLA cylinders, which was oriented perpendicularly to the surface of the film. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 122:2944-2951, 2011