Enzyme-mediated radical initiation of acrylamide polymerization: main characteristics of molecular weight control

The free-radical polymerization of acrylamide initiated by a redox system (hydrogen peroxide/β-diketone) catalyzed by horseradish peroxidase is studied with emphasis on the control of the molecular weight of the polymer. The case where 2,4-pentanedione (Acac) is used as the β-diketone is particularly examined. It is shown that the concentration of Acac and that of the enzyme readily control the molecular weight of the polymer. The concentration of hydrogen peroxide does not influence the molecular weight to a significant extent except at extreme values for which enzyme degradation or side reactions interfere with the normal enzymatic cycle. The variations of the molecular weight induced by changes in the chemical structure of the β-diketone are attributed to the reactivity toward the enzymatic cycle. The experimental results are rationalized by the use of classical kinetic equations for free-radical polymerization and including the specific expressions of enzymatic catalysis. The tendencies indicated by the theoretical expressions account for the evolutions given by the experimental results.