Photochemical method to eliminate oxygen inhibition in free-radical photopolymerization

Photopolymerizations based on free radical mechanism have been extensively utilized in industry due to their high reaction rate and a wide selection of commercially available monomers. However, one of the greatest limitations to the application of free radical polymerizations is the inhibitory effect of molecular oxygen on these types of reactions. Oxygen interacts with photopolymerization systems both at the initiation stage and in the propagation stage. The ground triplet state oxygen plays an active role in quenching the excited triplet state of the photoinitiators and scavenging the primary radicals/propagating chains to form non-initiating peroxide radicals. The presence of molecular oxygen in photopolymerization system generally introduces an induction period accompanied by the decreased polymerization rate. The ultimate conversion of the system is lowered and the polymer chain length is significantly reduced due to the premature termination of the propagation by oxygen. For the thin film and coating application, the formation of an inhibition layer at the top surface is often observed because the diffusion rate of oxygen into the sample from the surrounding environment is more rapid than its consumption by the polymerization.; In this study, a novel photochemical method was proposed for consuming molecular oxygen prior to the photopolymenzation, thereby allowing the reaction to proceed in an oxygen-free environment. The method is based on the generation of singlet oxygen by reaction of the ground state oxygen with the excited triplet state of the singlet oxygen generator (a porphyrin, Znttp). The singlet oxygen is then consumed by reaction with a second compound (the singlet oxygen trapper, dimethylanthracene). The possible factors that might affect the efficiency of the singlet oxygen generation/trapping processes were discussed and the criteria for the selection of oxygen consuming components were provided. A technique capable of measuring the dissolved oxygen concentration in organic liquids was developed based on the singlet oxygen production/trapping mechanism. The effectiveness of a dimethylanthracene/Znttp combination for consumption of oxygen was demonstrated in different acrylate systems. The effects of oxygen diffusion on the photochemical method were also investigated and discussed.