The free-radical copolymerization of styrene and ethyl acrylate in benzene-d(6) as the solvent in the presence of benzoyl 1 peroxide as an initiator at 70 degrees C was studied by online H-1-NMR spectroscopy. The chemical composition of the copolymer at different reaction times was calculated from the conversion of the monomers to the copolymer, and then the reactivity ratios of styrene and ethyl acrylate were determined at both low and high conversions. Data for the overall monomer conversion versus the time were used to estimate the ratio k(p)k(t)(-0.5) for different compositions of the initial feed (k(p) is the propagation rate constant, and k(t) is the termination rate constant). k(p)k(t)(-0.5) increased with an increasing molar fraction of ethyl acrylate in the initial feed. The monomer mixture and copolymer compositions versus the overall monomer conversion were calculated with the data of H-1-NMR spectra. The incorporation of the styrene monomer into the copolymer structure was more favored than that of the ethyl acrylate monomer. Reducing the molar fraction of styrene in the initial feed intensified this. Drawing the molar fraction of styrene (or ethyl acrylate) in the copolymer chains versus that in the initial feed showed a tendency of the system toward random copolymerization. (c) 2007 Wiley Periodicals, Inc.