Thermosensitive, biocompatible poly(ε-caprolactone)-b-poly(N-vinylcaprolactam) (PCL-b-PVCL), poly(δ-valerolactone)-b-PVCL, and poly(trimethylene carbonate)-b-PVCL block copolymers were synthesized at 30 °C using a hydroxyl-functionalized xanthate reversible addition-fragmentation chain transfer (RAFT) agent, 2-hydroxyethyl 2-(ethoxycarbonothioylthio)propanoate (HECP), as a dual initiator for ring-opening polymerization (ROP) and RAFT polymerization in a one-pot procedure. The hydrophobic blocks were first synthesized by the ROP of cyclic monomers using diphenyl phosphate (DPP) as a catalyst and the RAFT polymerization of the PVCL block was followed by adding N-vinylcaprolactam (VCL) and 2,2′-azobis(4-methoxy-2,4-dimethyl valeronitrile) (V-70) as an initiator to the reaction mixture. This novel one-pot process is convenient and powerful method for the synthesis of the PVCL-based biocompatible block copolymers. The lower critical solution temperature (LCST) of the PVCL-based biocompatible block copolymer can be readily tuned by controlling the hydrophobicity of the block copolymers. By copolymerizing a hydrophilic N-vinylpyrrolidone moiety to the PVCL blocks by RAFT copolymerization, the LCST of the copolymer was matched with the body temperature for its future biomedical applications.