One-step synthesis of well-defined poly(alkyl methacrylate)-b-polyester block copolymers was successfully performed at 30 °C using a dual initiator for reversible addition–fragmentation chain transfer (RAFT) polymerization and ring-opening polymerization (ROP). 4-Cyano-1-hydroxypent-4-yl dithiobenzoate, a hydroxyl-functionalized RAFT agent, was used as a dual initiator for the RAFT polymerization of methacrylic monomers and the ROP of lactones. Diphenyl phosphate (DPP) was used as a catalyst for ROP. The two polymerization reactions proceeded in a controlled manner without mutual interference. DPP was found to be a suitable catalyst for a dual initiator route combining RAFT polymerization and the ROP of lactones.