The free-radical copolymerization of itaconic acid (IA) and styrene in solutions of dimethylformamide and d(6)-dimethyl sulfoxide (50 wt %) has been studied by H-1 NMR kinetic experiments. Monomer conversion versus time data were used to estimate the ratio k(p).k(t)(-0.5) for various comonomer mixture compositions. The ratio K-p.k(t)(-0.5) varies from 5.2.10(-2) for pure styrene to 2.0.10(-2)mol(0.5)L(-0.5)s(-0.5) for pure IA, indicating a significant decrease in the rate of polymerization. Individual monomer conversion versus time traces were used to map out the comonomer mixture-composition drift up to overall monomer conversions of 60%. Within this conversion range, a slight but significant depletion of styrene in the monomer feed can be observed. This depletion becomes more pronounced at higher levels of IA in the initial comonomer mixture. The kinetic information is supplemented by molecular weight data for IA/ styrene copolymers obtained by variation of the comonomer mixture composition. A significant decrease in molecular weight of a factor of 2 can be observed when increasing the mole fraction of IA in the initial reaction mixture from 0 to 0.5. (C) 2001 John Wiley & Sons, Inc.