A 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 diacryla tes and dimethacrylates. Following earlier suggestions, the model assumes t hat trapping of radicals behaves as a unimolecular first-order reaction. Th e novel feature is that the trapping rate constant is presumed to increase exponentially with the inverse of the free volume; this treatment is qualit atively consistent with the free Volume dependence previously proposed for the other rate constants. This improved model predicts the experimental rea ction rate trends as well as previous models developed in the literature; m ore importantly, though, this improved model newly predicts, as no other mo del has, the following experimental trends in the trapped and active radica l 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 radi cals at a given conversion of functional groups but to a higher trapped rad ical concentration at the end of the reaction. Moreover, unlike its anteced ents, the improved model correctly predicts that the polymerization rate de pends more on light intensity the higher the conversion and that higher lig ht intensity can lead to a higher final conversion.