Specific examples of unsaturated carboxylic acids for the photo polymerization are as follows: ##STR1## wherein R1 is a hydrogen atom or methyl group; R2 is a hydrocarbon group which may contain a heterocyclic ring; and n is an integer of 1 or more. ##STR2## wherein R3 is a hydrogen atom, or methyl group, and R4 is a group containing a heterocyclic ring.