Controlled free-radical copolymerization of styrene (S) and butyl acrylate (BA) was achieved by using a second-generation nitroxide, N-tert-butyl-N-[1-diethylphosphono-(2,2-dimethylpropyl)I nitroxide (DEPN), and 2,2-azobisisobutyronitrile (AIBN) at 120 degreesC. The time-conversion first-order plot was linear, and the number-average molecular weight increased in direct proportion to the ratio of monomer conversion to the initial concentration, providing copolymers with low polydispersity. The monomer reactivity ratios obtained were r(S) = 0.74 and r(BA) = 0.29, respectively. To analyze the convenience of applying the Mayo-Lewis terminal model, the cumulative copolymer composition against conversion and the individual conversion of each monomer as a function of copolymerization time were studied. The theoretical values of the propagating radical concentration ratio were also examined to investigate the copolymerization rate behavior. (C) 2004 Wiley Periodicals, Inc.