Free radical solution copolymerization of acrylonitrile (AN) and itaconic acid (IA) was performed with DMSO-d(6) as the solvent and 2,2'-azobisisobutyronitrile (AIBN) as the initiator. Weight ratio of the monomers to solvent and molar ratio of initiator to monomers were constant in all experiments. The initial comonomer composition was the only variable in this study. On-line H-1 NMR spectroscopy was applied to follow individual monomer conversion. Mole fraction of AN and TA in the reaction mixture 0 and in the copolymer chain (F) were measured with progress of the copolymerization reaction. Overall monomer conversion versus time and also compositions of monomer mixture and copolymer as a function of overall monomer conversion were calculated from the data of individual monomer conversion versus time. Total rate constant for the copolymerization reaction was calculated by using the overall monomer conversion versus time data and then k(p)/k(t)(0.5) was estimated. The dependency of k(p)/k(t)(0.5) on IA concentration was studied and it was found that this ratio decreases by increasing the mole fraction of IA in the initial feed. The variation of comonomer and copolymer compositions as a function of overall monomer conversion was calculated theoretically by the terminal model equations and compared with the experimental data. Instantaneous copolymer composition curve showed the formation of alternating copolymer chain during copolymerization reaction. (c) 2006 Wiley Periodicals, Inc.