The polymerization of acrylonitrile in aqueous solution. Part II. The reaction photosensitized by Fe3+, Fe3+OH−, Fe2+, and I− ions

The polymerization of acrylonitrile in aqueous solution photosensitized by Fe2+, Fe3+OH−, and I− has been determined dilatometrically at 25°C. After an acceleratory phase caused partly by increasing intensity of absorbed light due to light‐scattering by the insoluble polymer particles, a steady rate Rp is established and continues up to ∼10% conversion. Rp is proportional to Iabs[m1]1.0, where x is slightly greater than 0.5 unless ferric ions are present which may oxidize and terminate polyacrylonitrile radicals and cause x to be greater than 0.5. In accordance with earlier measurements, ϕ for the reaction {article}{empty}{document}Fe3 + OH- 3130\ A. --4pt--4pt--4pt--4pt--4pt--4pt--5pt Fe2 + + OH{document} is found to be 0.15. Propagation occurs by addition (a) of dissolved monomer molecules to polymer radicals S* (kp = 1.45 × 104 l. mole−1 sec.−1) and (b) of adsorbed monomer molecules to the radical in suspended polymer particles P* which contain several polymer molecules and one growing radical. S* radicals are subject to mutual termination (kt = 2 × 109 l. mole−1 sec.−1) and oxidative termination by ferric ion (k4 = 1.3 × 104 l. mole−1 sec.−1). As soon as the steady state is established [P*] is constant, but after stopping irradiation [P*] decays slowly and bimolecularly and therefore gives rise to a small aftereffect. Agitation causing the polymer suspension to coagulate (a) reduces Rp due to changes in Iabs and a decrease in the number of particles and (b) decreases the half‐life of the aftereffect by increasing the rate of mutual destruction of P* particles.