a b s t r a c tEosin, 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 (~1000Â) 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 peroxymediated regeneration rate constant, k regen , and the initial eosin concentration on the irradiation time that is required to reach a C]C bond conversion of 0.2 (t 0.2 ). At this C]C 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 t 0.2 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.
Eosin, a photosensitizer dye that can exist in multiple oxidation states, has been shown to initiate visible‐light photopolymerizations of acrylates in open air when used in combination with tertiary amines. The exact mechanism behind this reactivity with micromolar concentrations of eosin in aqueous monomer solutions initially containing millimolar concentrations of oxygen has not been conclusively established, although pathways for regeneration of eosin in the presence of oxygen certainly play a role. In this work, a reaction–diffusion model incorporating a peroxy‐mediated eosin‐regeneration mechanism is built to explore the effects of oxygen diffusing into the system as the light‐activated reaction proceeds. An oxygen concentration‐dependent flux boundary condition is used to model the continuous replenishment of oxygen at the surface open to air as it is consumed by reaction. The model predicts the formation of a free radical concentration front that initially forms closer to the open surface and gradually moves toward the closed surface, with polymer film thickness increasing and the time required for polymerization to begin decreasing as the initial eosin concentration is increased. These results suggest that oxygen's dual role as both a free radical inhibitor and a precursor of the oxidizing species required for regeneration of eosin brings about interesting spatial variations when the reaction is carried out in a thin film geometry that is exposed to open air. In this case, an assumption of a well‐mixed system is not appropriate and the kinetics of the reaction and conversion as a function of position are likely to depend on the geometry of the system.
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