Transformation of Living Cationic Polymerization of Vinyl Ethers to RAFT Polymerization Mediated by a Carboxylic RAFT Agent

Novel block copolymers were synthesized by the transformation of living cationic polymerization of vinyl ethers into reversible addition-fragmentation chain transfer (RAFT) polymerization using two types of RAFT agents. Examples include cationically polymerizable monomer of either isobutyl vinyl ether or 2-methoxyethyl vinyl ether (MOVE) and radically polymerizable monomer such as ethyl acrylate, styrene, tert-butoxy styrene, and N-isopropylacrylamide (NIPAM). The key to success in the block copolymerization is to utilize a RAFT agent of the carboxylic trithiocarbonate/SnBr4 initiation system in the presence of an additive such as ethyl acetate and dioxane for living cationic polymerization. The living cationic polymerization was initiated from a proton derived from the carboxylic RAFT agent. After a certain period, the polymerization was quenched and the RAFT group as a counteranion was concurrently recovered, followed by the RAFT process of radical polymerization using azobis(isobutyronitrile) at 70 degrees C. The first segments of the poly(vinyl ether) showed high number-average end functionality for the RAFT terminal group, 0.79 or more, and which were available as a macro-chain transfer agent for radical polymerization. Furthermore, using the transformation process, novel diblock copolymers were also prepared, containing two thermoresponsive segments possessing different lower critical solution temperatures (LCSTs), poly(2-methoxyethyl vinyl ether)-poly(N-isopropylacrylamide) (PMOVE-PNIPAM) block copolymers. The resulting block copolymers reversibly formed or deformed micellar assemblies during two LCSTs. The mean micellar sizes were controlled by the composition of PMOVE and PNIPAM, indicating the successful formation of the block copolymers.