If we use acetonitrile as solvent then how we dissolve a monomer named as itaconic acid in it?

Since the monomer is soluble in water, acetone, ethanol or methanol add any of these co solvents, according to the suitability, in appropriate proportions to acetonitrile and dissolve the monomer.

It seems to be highly polar,  so just add a few drops of  ethanol or Methanol to dissolve and make up with acetonitrile further. Check whether soluble substance is precipitate out or not, if it's not precipitate out, then proceed the analysis.