PURPOSE: To provide a new 2-(4-alkenyl phenyl)-5-oxazolone compound having good polymerization activity capable of producing homopolymer and copolymer easily, and having ring-opening-reactivity of azulactone pendant group with monomer configuration or polymer configuration.

CONSTITUTION: This compound is 2-(4-alkenyl phenyl)-5-oxazolone compound of formula I, for example, 2-(4-vinylphenyl)-5-oxazolone. This compound is produced by allowing the compound of formula II to react with the same compound, then dehydrizing obtained compound of formula III. IN the formula I, R1 is H, an alkyl, aryl, alkaryl, aralkyl, cycloalkyl, R2 and R3 form carbon ring or heterocyclic ring togeter with C to which they are bonded.