Abstract Radical copolymerization of 3-methylstyrene with methacrylic acid and methyl methacrylate in toluene, acetone, and isopropyl alcohol is studied. The calculated copolymerization reactivity ratios are smaller than unity. The established, well-defined alternating tendency in this case can be reasonably explained by the polar effect of comonomersolvent H-bond formation on comonomer reactivity. The significant effect of methacrylic acid dimerization in toluene (again by H-bond formation) and dimer participation in chain propagation on the copolymerization of this monomer with 3-methylstyrene and methyl methacrylate is also shown.