A THF-insoluble polymeric product was obtained by the reaction between acetaldehyde azine (AcAz) and maleic anhydride (MAnh) without any initiator. The reduced viscosity values for this product indicate polymer formation from AcAz and MAnh. Results of the reactions between AcAz and MAnh at varying feed ratios of AcAz indicate that the polymer is formed mainly by reaction between equimolar amounts of AcAz and MAnh. (2Z)-4-{1-(5-Methyl-2-pyrazolinyl)}-4-oxo-2-butenoic acid (compound 1), a one-to-one adduct of AcAz with MAnh, was isolated as a crystalline compound from the THF-soluble fraction of the reaction mixture. Comparing the spectroscopic results for the polymer with those for compound 1 and 4-{1-(5-methyl-2-pyrazolinyl)}-4-oxobutanoic acid (compound 2), a one-to-one adduct of AcAz with SAnh, the polymer from AcAz and MAnh was found to be formed mainly by polymerization of compound I through the C=C bond. Results of the polymerizations of compound 1 using typical initiators show compound 1 can be polymerized by radical initiators, indicating that the polymer from AcAz and MAnh is formed by a radical mechanism. On the basis of the electron-donating property of 5-methyl-2-pyrazoline (compound 3) formed by isomerization of AcAz in the presence of compound 1, the spontaneous polymer formation from AcAz and MAnh was inferred to be initiated by electron transfer from compound 3 to compound 1.