Long poly(ethyl methacrylate) (M-n = 2,300,000) and polystyrene (M-n = 1,200,000) chains were subjected to ultrasonic scission in the presence of a radical scavenger, 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO). This procedure yielded polymers with lower molecular weights and TEMPO terminal units. Application of these polymers in stable radical mediated polymerization of styrene resulted in chain extension and block copolymers, depending on the precursor polymer. Block copolymer formation was evidenced by NMR measurement, and chain extension was shown by GPC analysis. (C) 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1950-1953, 2000.