Kinetic model for radical trapping in photopolymerization of multifunctional monomers

An improved kinetic model is presented which accounts for radical trapping during the photopolymerization of multifunctional monomers such as diacrylates and dimethacrylates. Following earlier suggestions, the model assumes that trapping of radicals behaves as a unimolecular first-order reaction. The novel feature is that the trapping rate constant is presumed to increase exponentially with the inverse of the free volume; this treatment is qualitatively consistent with the free volume dependence previously proposed for the other rate constants. This improved model predicts the experimental reaction rate trends as well as previous models developed in the literature; more importantly, though, this improved model newly predicts, as no other model has, the following experimental trends in the trapped and active radical concentrations: (1) that the active radical concentration passes through a maximum while the trapped radical concentration increases monotonically; (2) that a higher light intensity leads to a lower fraction of trapped radicals at a given conversion of functional groups but to a higher trapped radical concentration at the end of the reaction. Moreover, unlike its antecedents, the improved model correctly predicts that the polymerization rate depends more on light intensity the higher the conversion and that higher light intensity can lead to a higher final conversion.