Radical copolymerizability of 6-methyl-2-vinyl-3-pyridazinone (I) has been reinvestigated in acetic acid and also in the presence of polyacrylic acid (PAA). Copolymerization of styrene with I was carried out in a few solvents. A positive e2value was obtained in acetic acid while negative ez values were obtained in benzene and DMF. I was allowed to copolymerize with a few monomers such as styrene, methyl methacrylate, and acrolein in the presence of PAA to give block-type copolymers. All these observations indicate that polymerizability of I is controlled by the hydrogen bonding interaction between the carbonyl group of I and the hydroxyl group of the additive.