A special kind of reversible addition-fragmentation chain transfer (RAFT) agent with two ending hydroxy groups, S, S'-bis(2-hydroxyethyl-2'-butyrate) trithiocarbonate (BHBT), was successfully synthesized by using polymeric supports. BHBT can act not only as initiating centers in polymerization of lactide with tin(II) 2-ethylhexanoate catalyst but also as an efficient chain transfer agent in RAFT polymerization. The thermally sensitive and biodegradable block copolymers, poly(lactide-b-N-isopropyl-acrylamide-b-lactide) (PLA-b-PNIPAAM-b-PLA), were successfully synthesized by ring-opening polymerization (ROP) of lactide initiated from two hydroxy groups of BHBT and then RAFT polymerization of N-isopropylacrylamide (NIPAAM) using PLA with a centered trithiocarbonate unit as RAFT agent. The results of H-1 NMR and GPC analyses show that the triblock copolymer of PLA-b-PNIPAAM-b-PLA with well-defined structure and controlled molecular weight has been prepared by combination of ROP and RAFT methods. The aqueous solution of the micelles prepared from block copolymers using a dialysis method showed reversible changes in optical properties: transparent below a lower critical solution temperature (LOST) and opaque above the LOST.