A quantitative analysis of peroxy-mediated cyclic regeneration of eosin under oxygen-rich photopolymerization conditions

Abstract

Eosin, a photoreducible xanthene, reacts with tertiary amines and initiates the free radical photopolymerization of aqueous solutions of acrylate monomers. This reaction proceeds even in the presence of a large excess (∼1000×) of inhibiting oxygen via a mechanism that has not been established conclusively. This chemistry has proven useful in the area of biosensing, where the formation of a hydrogel on the time scale of seconds serves as a macroscopic, amplified signal that can be connected to molecular recognition events. In this work, we built a kinetic model to quantitatively explore a mechanism in which eosin is regenerated through the reaction of eosin-based radicals with peroxy-radicals formed from oxygen-inhibition reactions. To determine whether the predictions of this model are consistent with conversion profiles measured using real-time FTIR, we refrained from fitting rate constants or other unknown parameters associated with individual steps in the mechanism to the conversion profile. Rather, we considered physical upper bounds and performed sensitivity analyses spanning several orders of magnitude to predict the reactivity of the system. We explored the effects of the peroxy-mediated regeneration rate constant, kregen, and the initial eosin concentration on the irradiation time that is required to reach a CC bond conversion of 0.2 (t0.2). At this CC bond conversion, the aqueous monomer solutions studied herein have become hydrogels. The predictions of the model capture several trends that we have observed experimentally. However, even when the rate constants associated with eosin regeneration via reaction with peroxy-species are set at the physical upper bounds, the values of t0.2 predicted by the model are much larger than those that we observed experimentally. The results presented herein motivate and provide a framework for future work to more fully elucidate the mechanism of this interesting and useful photopolymerization reaction.