Mechanism of Photoinitiated Free Radical Polymerization by Thioxanthone−Anthracene in the Presence of Air

The mechanism of formation of initiating radicals for free radical polymerization by the thioxanthone−anthracene (TX-A) photoinitiator was investigated by laser flash photolysis, fluorescence and phosphorescence spectroscopy, and polymerization studies. The proposed mechanism of photoinitiation involves photoexcitation of TX-A and quenching of triplet excited states of TX-A by molecular oxygen to generate singlet oxygen. Singlet oxygen reacts with the anthracene moiety of TX-A to form an endoperoxide. The endoperoxide intermediate was isolated and characterized by MS and 1H NMR. The endoperoxide undergoes photochemical or thermal decomposition to generate radicals which are able to initiate free radical polymerization. A significant deuterium effect on the polymerization rates using CDCl3 and CHCl3 as solvents confirmed the involvement of singlet oxygen in the initiation process.