Solution copolymerization of styrene (Sty) and acrylonitrile (AN) was carried out in dioxane at 60 +/- 6 1 degrees C for 90 min using diphenylselenonium 2,3,4,5-tetraphenylcyclopentadienylide (selenonium ylide) as radical initiator. The kinetic expression is as follows: R(p) similar to [Ylide](0.5) [Sty](1.0) [AN](1.0). The overall activation energy is 28.72 kJ mol(-1). The composition of copolymer calculated from (1)H-NMR and elemental analysis was used to evaluate reactivity ratio as r(1) (Sty) = 0.351 and r2 (AN) = 0.0185, using kelen-Tudos method. It confirmed the alternating nature of the copolymer. The copolymer was characterized using Fourier transform infrared (FTIR) spectroscopy, (1)H-NMR, (13)C-NMR, differential scanning calorimetry, and thermal gravimetric analysis. Electron spin resonance spectroscopy confirmed the presence of the phenyl radical responsible for initiation. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 1052-1058, 2011