Free-radical homo- and copolymerization of vinyl acetate (VAc) and n-butyl acrylate (BA) in benzene-d(6) were performed by using benzoyl peroxide as an initiator at 70 degrees C. Polymerization kinetic was followed by online H-1 NMR kinetic experiments. Significant drift in the comonomer mixture composition with reaction progress was observed. Reactivity ratios of VAc and BA were calculated by terminal unit model (TUM) as well as by simplified penultimate unit model (PUM) with r(VAc) = 0. It was found that copolymer composition can be described well by the TUM. "Lumped" kinetic parameter (k(p).k(t)(-0.5)) was estimated from experimental data. A good fitting between the theoretical and experimental drifts in the comonomer mixture and copolymer compositions with reaction progress was observed. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 123: 543-553, 2012