AbstractThe copolymerizations of 2‐vinylpyridine (2‐VP), 4‐vinylpyridine (4‐VP), and 5‐ethyl‐2‐vinylpridine (5‐Et‐2‐VP) with styrene (St), methyl methacrylate (MMA), and methyl acrylate (MA) have been investigated. The copolymer compositions have been determined by visual titration with crystal violet indicator of the copolymers in the 20:80 glacial acetic acid‐acetonitrile mixture withN/10‐perchloric acid in glacial acetic acid. It has been proved that this titration method is consistently successful. The monomer reactivity ratios,r1andr2, were calculated for the systems St‐2‐VP (0.56 ± 0.02 and 0.9 ± 0.2, respectively), MMA–2‐VP (0.439 ± 0.002 and 0.77 ± 0.02, respectively), MA–2‐VP (0.168 ± 0.003 and 1.58 ± 0.05, respectively), St–4‐VP (0.54 ± 0.03 and 0.7 ± 0.1, respectively), MMA–4‐VP (0.574 ± 0.004 and 0.79 ± 0.05, respectively), MA‐4–VP (0.22 ± 0.01 and 1.7 ± 0.2, respectively), St–5‐Et‐2‐VP (0.79 ± 0.03 and 1.2 ± 0.2, respectively), MMA–5‐Et‐2‐VP (0.395 ± 0.003 and 0.69 ± 0.03, respectively), and MA–5‐Et‐2‐VP (0.179 ± 0.006 and 1.16 ± 0.08, respectively). The alternating tendencies of the pyridines with the comonomers measured by the reactivity ratio products have been discussed in connection with styrene and, further, theQ‐evalues have been calculated. On the basis of these data, it has been concluded that the pyridyl groups of these monomers act as electron donors to the double bond, as does the phenyl group in the styrene molecule; these groups show comparable resonance stabilization to styrene,