End-group analysis was achieved on poly(methyl methacrylate) and poly(methyl methacrylate-r-butyl acrylate) by liquid chromatography-electrospray ionization-ion trap mass spectrometry (LC-ESI-IT MS) and electrospray ionization-Fourier transform ion cyclotron resonance tandem mass spectrometry (ESI-FTICR MS2). The two polymers were produced by radical polymerization in butyl acetate at relatively high temperature using the same initiator, tert-butylperoxy-3,5,5-trimethylhexanoate. In both polymers, structures with different end-groups were successfully assigned using gradient elution LC-IT MS, with the aid of exact mass experiments in an FTICR MS instrument. Isobaric materials in PMMA were discriminated using accurate mass FTICR MS2. Isocratic LC-MS reduced the complexity of the data of P(MMA-r-BA), made the attribution of peaks easier, and shortened the experimental time compared to gradient elution LC-MS. The identification of end-groups in both the methacrylic homo- and copolymer demonstrated that beta-scission and radical transfer to solvent play an important role in the polymerization.