The first 10 addition products in the free radical polymerization of acrylonitrile initiated by AIBN have been trapped and identified; the resulting yields have allowed individual values of the first eight propagation rate constants to be estimated. These are the first realistic experimentally based estimates of a reasonable sequence of early propagation rate constants in any free radical polymerization. In this experiment, the concentration of the nitroxide trapping agent was maintained at a level sufficiently high to prevent the formation of high polymer, yet low enough to allow a competitive monomer addition to form the lower members of the propagation series before being trapped to form the oligomeric addition products. These oligomers were identified and quantified by electrospray ionization mass spectrometry operated in the selected ion recording mode in series with HPLC−UV. The results have allowed the first eight individual propagation rate constants, kAn(n = 1−8), to be estimated.