The analysis of photo-initiated poly(methyl methacrylate) via electrospray ionization-mass spectrometry (ESI-MS) (synthesized by pulsed laser polymerization (PLP, at lambda = 351 nm) of methyl methacrylate (MMA) and benzoin as photoinitiator at 6 mJ/pulse laser energy) evidences the presence of unidentified species. The determination of the origin of these species requires a detailed investigation via size exclusion chromatography-electrospray ionization-mass spectrometry (SEC/ESI-MS) and chemically induced dynamic nuclear polarization-nuclear magnetic resonance spectroscopy (CIDNP-NMR). It was found that post-irradiation of benzoin-initiated poly(methyl methacrylate) leads to a-cleavage of the benzoyl fragment leading to a sequence of cascade reactions, including the formation of an additional double bond within the polymer chain as evidenced via ESI-MS. Furthermore, the reaction products of the benzoyl radical post a-cleavage (e.g., benzaldehyde, phenyl methyl ketone, methyl formate, or methane) as well as the formed macroradical can be followed by CIDNP-NMR, which allows establishing a reaction mechanism for the UV-induced cleavage process. The study thus evidence that-if the integrity of UV initiated polymers is to be kept intact during their synthesis-very low irradiation energies need to be employed.