Analytical and kinetic aspects of the photoinitiated polymerization of acrylonitrile in the presence of chromium(VI) salt in aqueous solution are reported. The proposed mechanism includes a photoredox process that takes place in the excited state. A new complex, formed from the excited [HCr04"]* anion and acrylonitrile, was inferred from the UV absorption spectra of irradiated solutions of the components at pH 3.12. The protonated form of this complex is written as [HCr04"-acrylonitrile] without any assumption about the stoichiometry or the site of complexation. This complex may dissociate to yield a chromium(V) complex and an organic radical, and the radical initiates the polymerization of acrylonitrile.The effect of various parameters on the initial quantum yields of chromium(VI) reduction is studied. °is proportional to the monomer concentration and decreases when the pH of the solution is increased, with a pseudoplateau at pH 2.5-3.5. The final products formed in the redox phenomena are chromium(III) and polyacrylonitrile. The CrVI -» Crln photoreduction involves three H+ ions per chromium reduced. The mechanism of the initiation is governed by two acid-base equilibria between HCr04" and Cr041
The photoredox process occurring between HCrO, and acrylamide (AAm) leads to the formation of chromium(I11) and polyacrylamide. The influence of pH, monomer concenlration, absorbed light intensity I , and chromium(V1) concentration on the polymerization rate R , are reported. Two different domains are detected as a function of chromium(V1) concentration: At low concentration, where R , increases with chromium concentration, a process of termination by mutual annihilation of the macroradicals accounts for the experimental dependence R , a [AAmI3l2 .Id'*. At higher concentration, R , decreases when the chromium concentration increases; a process of oxidative termination of the radicals by chromium(V1) is involved corresponding to a dependence R , a [AAm]' . I , . The evolution of the polymer molecular weight as a function of the different parameters controlling the polymerization is described.
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