RAFT polymerization of 4-acryloxyphenyldimethylsulfonium triflate (SR-AEM) using pentafluorophenyl-(4-phenylthiocarbonyl-thio-4-cyanovalerate) as the chain transfer agent and AIBN as a radical source in acetonitrile at 90 degrees C yielded poly(4-acryloxyphenyldimethylsulfonium triflate) (poly(SR-AEM))s in high yields with well-controlled molecular weights and polydispersity indices. The reactive polymer poly(SR-AEM) was converted using an excess amount of amines, e.g., isopropylamine yielding predominantly poly(N-isopropylacrylamide) with defects of poly(4-acryloxyphenyl methylsulfide) in a minor ration of less than 16%. Furthermore, poly(SR-AEM) could be converted by thermo-triggered release of methyl group at dimethylsulfonium ion group to afford poly(4-acryloxyphenyl methylsulfide) (poly(APM)) as proved by H-1 NMR measurements, which did not react with amines anymore. To the best of our knowledge, this represents the first temperature-induced "on-off switching" of the reactivity of an activated ester by using 4-dimethylsulfonim phenol as the stimuli-responsive leaving group.