Free radical polymerization of acrylonitrile: Mass spectrometric identification of the nitroxide-trapped oligomers formed in and estimated rate constants for each of the first eight propagation steps

The first 10 addition products in the free radical polymerization of acrylo nitrile initiated by AIBN have been trapped and identified; the resulting y ields have allowed individual values of the first eight propagation rate co nstants 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 t he nitroxide trapping agent was maintained at a level sufficiently high to prevent the formation of high polymer, yet low enough to allow a competitiv e monomer addition to form the lower members of the propagation series befo re 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 re sults have allowed the first eight individual propagation rate constants, k (An) (n = 1-8), to be estimated: [GRAPHICS] The values of k(An) at 75 degrees C decrease from 5.4 x 10(3) L mol(-1) s(- 1) with n = 1 to a minimum of 2.1 x 10(3) L mol(-1) s(-1) with n = 3 before leveling off with a constant value of 4.8 x 10(3) L mol(-1) s(-1) at n > 5 .