The copolymerization of zinc methacrylate (ZMA) and 2-(N-ethylperfluorooctanesulfonamide)ethyl acrylate (RFA) in benzene was studied as a model for the copolymerization of ZMA and RFA in synthetic rubber during the peroxide curing process (in situ copolymerization). To confirm copolymer formation from combination of the specified monomers bearing functional groups with different characteristics, experimental procedures to quantify unchanged monomers and two types of monomer units in the copolymer were established. As a result, it was ascertained that the copolymer composition was the same with that of the comonomer composition for an equimolar mixture of the monomers regardless of conversion. We can conclude that the composition of the copolymer resulting from the in situ copolymerization can be determined and the in situ copolymerization could be a novel method to modify the properties of rubber.