Living polytetrahydrofuran was terminated with pyridinium or isoquinolinium N-oxide to yield polymers with the corresponding end groups. Direct and sensitized irradiation of these photoactive polytetrahydro-furans produced alkoxyl radicals at both chain ends capable of initiating the radical polymerization of methyl methacrylate. In this way, triblock copolymers were formed. The block copolymer composition was determined with the aid of g.p.c. and optical and H-1 n.m.r. spectroscopy.