Functional polytetrahydrofurans containing N,N'-diethyl dithiocarbamates were synthesized by the living cationic polymerization of tetrahydrofuran using trifluorosulfonic acid anhydride and N,N'-diethyl dithiocarbamic acid sodium salt as an initiator and terminator, respectively. The molecular weight distribution was narrow and the number average molecular weights of the polymer were very close to the theoretical one, M-n,M-th. N,N'-diethyl dithiocarbamoyl polytetrahydrofuran was used as a macroiniferter in the photopolymerization of methyl methacrylate and block copolymers were obtained.