Introduction Polymer materials used in advanced products, including electrical and optical devices, generally consist of copolymers. For reason of its low cost, most industrial copolymers have been produced using free radical polymerization. Improvement of copolymer materials has been accomplished by detailed characterization of copolymers, including copolymer composition and comonomer sequences, as well as end-group structures, molecular weights, and their distributions. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) is commonly used as a technique for polymer characterization. 1) Copolymer characterization has also been attempted using MALDITOFMS. 2–8) However, the structural characterization of copolymers remains a challenging task, since the random nature of comonomer distribution generates very complicated mass spectra. During free radical polymerization of copolymers, a variety of end-group combinations are formed, resulting in extremely intricate mass spectra. A mass of different combinations of comonomer distributions and end-groups increases the likelihood of isobaric interference, in which the peaks of different chemical compositions with the same nominal mass overlap. For these reasons, the characterization of copolymers by MALDI-TOFMS currently appears to be limited to copolymers with a uniform end-group structure.