The role of 1,1-diphenylethylene (DPE) in radical emulsion polymerization of methyl methacrylate is investigated. The presence of DPE causes a strong decrease in both the rate of polymerization and the molecular weight. According to the results of structure analysis by means of MALDI-TOF mass spectrometry, UV-vis spectroscopy, and H-1-NMR spectroscopy, DPE is incorporated in the copolymer chain exclusively as a reactive recombinant alpha,p-dimer, underlining the peculiar role of DPE in radical polymerizations.