A range of well-defined block copolymers were synthesized using 4-cyano-4-(dodecylsulfanylthiocarbonyl)sulfanylpentanol (CDP) as a dual initiator for reversible addition-fragmentation chain transfer (RAFT) polymerization and ring-opening polymerization (ROP) in a one-step process. Styrene, (meth) acrylate, and acrylamide monomers were polymerized in a controlled manner for one block composed of vinyl monomers, and delta-valerolactone (VL), epsilon-caprolactone (CL), trimethylene carbonate (TMC), and L-lactide (LA) were used for the other block composed of cyclic monomers. Diphenyl phosphate was used as a catalyst for the ROP of VL, CL, and TMC, and 4-dimethyamino pyridine for the ROP of LA. These catalysts did not interfere with RAFT polymerization and the synthesis of various block copolymers proceeded in a controlled manner. CDP was found to be a very useful dual initiator for a one-step synthesis of various block copolymers by a combination of RAFT polymerization and ROP. (C) 2012 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 774-779