Free-radical copolymerization of vinyl acetate (VAc) and dibutyl maleate (DBM) in deuterated chloroform (CDCl₃) initiated with 2,2′-azobis(isobutyronitrile) at 60 °C was monitored via online ¹H-NMR kinetic experiment. The reactivity ratios of the VAc (r_VAc) and DBM (r_DBM) were calculated using data obtained from two online experiments. Two, i.e. classic and new, approaches were used to collect conversion and copolymer composition data. Results showed that for a copolymerization with a relatively large difference between the reactivity ratios (r_VAc to r_DBM ratio of about 5) the reactivity ratios can be calculated by data collected only from one online ¹H-NMR experiment. In the case of copolymerization with the chloroform where copolymers with a number-average molecular weight (M_n) equal to or below 10⁴ g mol⁻¹ were synthesized, r_VAc and r_DBM were found to be 0.1135 and 0.0562 (r₁r₂ = 0.0064), respectively, [J. Polym. Res., 2014, 21, 582]. In the present work where copolymers with an M_n value of about 2 × 10⁴ g mol⁻¹ were synthesized, r_VAc and r_DBM were found to be 0.1911 and 0.0381 (r₁r₂ = 0.0073), respectively. The chemospecificity of the chloroform and corresponding trichloromethyl (CCl₃) radical in the reaction with the propagating radical and comonomers, respectively, was deduced from instantaneous copolymer composition curves. The results revealed that DBM adds preferentially onto the CCl₃ radicals derived from AIBN/CHCl₃ or AIBN/CDCl₃, indicating that copolymer composition and hence apparent reactivity ratios can be affected by the copolymer's molecular weight. Results may also be attributed to the preferential addition of the propagating radicals with a terminal DBM unit to CHCl₃ or CDCl₃.