Photosensitized polymerization of acrylic monomers. II. Kinetics of polymerization in the presence of oxygen

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Abstract
The kinetics of the photopolymerization of acrylamide and methacrylic acid in aqueous and semi‐organic solutions have been examined; the initiating redox system used in the presence of oxygen were eosin‐thiourea, eosin‐1‐ascorbic acid, or in some cases riboflavine. The rates of photopolymerization were determined by measuring the thermal rise of the reaction cell using a thermistor technique. In aqueous and in semi‐organic solutions, the rate is proportional to the square of the monomer concentration in both cases. Below a critical concentration of sensitizer the rate of polymerization of monomers is proportional to the square root of sensitizer concentration (acrylamide in the presence of riboflavine, methacrylic acid in the presence of eosin‐ascorbic acid); above it, the rate becomes inversely proportional to the square root of the sensitizer concentration. With respect to the reducing agent concentration, the exponent for thiourea is located between 0.5 and 1 in the case of acrylamide and 0.5 for methacrylic acid. Finally with respect to the oxygen concentration the rates are proportional to the square root of its concentration, at least below a given oxygen‐concentration where the induction period becomes predominant. These results are interpreted assuring a participation of the monomer in the initiation step, and a low initiation efficiency of the primary radicals (cage recombination). A kinetic scheme is presented and the experimental results discussed on the basis of it.