The mechanism of the photoinitiation of methyl methacrylate polymerization by the neutral red/triethylamine system

The photoinitiation of the polymerization of methyl methacrylate by the dye neutral red in the presence of triethylamine was studied in order to determine the mechanism that leads to the formation of the initiating radicals. An expression for the yield of free radicals was deduced from that mechanism, and the experimental values for the equilibrium and reaction constants of the ground, singlet and triplet states of the dye in the presence of the other components of the formulation, were used to calculat the behaviour of the system when varying the concentration of the co-initiator triethylamine. The observed dependence fits closely the global polymerization rates obtained directly from polymerization kinetics studied by dilatometry. On the other hand, inhibition times showed a behaviour which follows inversely the polymerization rates, confirming that most of the inhibition is originated from the consumption of residual oxygen.