The first 15 addition products in the free radical polymerization of styrene initiated by AIBN have been trapped using a nitroxide trapping technique. In this experiment, the concentration of trap is maintained at a level sufficiently high to prevent the formation of high polymer yet low enough to allow competitive monomer addition to form the lower members of the propagation series before being trapped to form the oligomeric addition products. These oligomers have been identified and quantified by electrospray ionization mass spectrometry (ESI-MS) in series with HPLC-UV. The observation of single chromatographic peaks for each oligomer indicates that the rate of formation of branched products is negligible. The results have allowed the first 14 individual propagation rate coefficients, kSn (n = 1-14), to be estimated based on the assumption that the successive trapping rate coefficients are constant. RSn堫 S RSn+1宠The results show that at 75 àkSn increases significantly from n = 1 to n = 3 and then stays constant to n = 7. The value of kSn for n = 3-7 is 6.5 ᠲ נ104 M-1 s-1. In the range n = 7-14, the value decreases steadily by a factor of about 2. The value at n = 14 is significantly higher than the IUPAC recommended value for high polymer of 565 M-1 s-1 at 75 î The rate coefficient for the addition of cyanoisopropyl radicals to styrene (n = 0) is 1.0 ᠰ.4 נ104 M-1 s-1 at 75 î