Kinetics of acrylamide photopolymerization as investigated by capillary zone electrophoresis

Riboflavin-mediated photopolymerization of acrylamide had been investigated earlier and found to give a poor conversion of the monomer into the polymeric phase (barely 60% with standard 1-h light exposure at room temperature with a 12-W neon source). The kinetics of such process were re-examined by using capillary zone electrophoresis and it was found that the efficiency could be augmented to > 95% (even higher than in peroxodisulphate polymerization) under the following conditions: by conducting the photopolymerization process at 70°C 1 h, instead of at room temperature, alterantively, by using a 105-W UV-A lamp, having a radiation spectrum extending also in the UV region, instead of the standard 12-W neon bulbs, and by increasing the amount of catalyst (riboflavin 5′-phosphate) from 6 to 12 ppm. In all instances, it was found that atmospheric oxygen would act as a retarder, the lag time being ca. 17 min at an oxygen partial pressure of 35 mmHg and increasing progressively to > 70min at > 200 mmHg. However, even at a partial pressure as high as 900 mmHg (as obtained by oxygen insufflation from a tank), once the retarder had been consumed a conversion of monomers into the polymer chains of > 92% was still obtained. In contrast, oxygen acts as an inhibitor in peroxidisulphate polymerization: from an incorporation efficiency of ca. 92% at 25 mmHg oxygen partial O₂ pressure, the conversion is lowered to only 40% at 900 mmHg.