The role of ketyl radicals in free radical photopolymerization: new experimental and theoretical insights

In this work, the role of ketyl radicals produced by the abstraction of a hydrogen from an amine by a thioxanthone-based photoinitiator is considered within the scope of the free radical photopolymerization process. Transient absorption experiments are combined with kinetic modeling of the polymerization reaction. It reveals that thioxanthone-based ketyl radicals are able to react with oxygen, hence decreasing the extent of oxygen inhibition during the polymerization. Moreover, the introduction of a termination reaction involving the ketyl radicals in the kinetic model clearly demonstrates the already suspected ability of this species to act as a terminating agent of the polymerization reaction, thus reducing the maximal rate of photopolymerization. The role of aminoalkyl radicals resulting from the photoinduced hydrogen abstraction in the reduction of oxygen inhibition is also discussed in light of kinetic modeling results. The robustness of the model is finally assessed by modifying the incident light intensity as well as the initial amounts of photoinitiator and amine.