Mechanism and Implementation of Oxygen Inhibition Suppression in Photopolymerizations by Competitive Photoactivation of a Singlet Oxygen Sensitizer

Here, we report a novel method of suppressing oxygen inhibition while simultaneously performing photopolymerizations with a visible light photoinitiator. A singlet oxygen sensitizer, Zinc tetrakis(tert-butyl) phthalocyanine (Zn(ttp)), is used to excite oxygen at a rate greatly exceeding the initiation rate to promote the reaction of ground state oxygen preferentially with Zn(ttp). The inhibition times of such polymerizations decreased from 280 s for polymerization without Zn(ttp) to 40 s for Zn(ttp)-mediated polymerizations in air. Thermal polymerization studies shows that the suppression mechanism happens only when the formulation is exposed to visible light that is absorbed by Zn(ttp). The excited state or zn(ttp) resulting from light absorption transfers its energy to oxygen, consequently exciting it to the singlet state. Because of the presence of deactivating mechanisms, the singlet oxygen decays back to its ground state and a quasi-steady state is established between the ground state and singlet state oxygen species. Because of the high rate of light absorption by Zn(ttp) as compared to the photoinitiator, the singlet state oxygen becomes the predominant oxygen species, thereby resulting in significant suppression of radical scavenging by ground state oxygen. Experimental results are corroborated with a mathematical model to describe the variation in inhibition times with changing Zn(ttp) concentration. The value of the characteristic lifetime of singlet oxygen calculated from the model is 1 ms, consistent with that which is reported in the literature for several organic monomers. Finally, optimal initiating conditions are designed to suppress oxygen inhibition while achieving high conversions in the most challenging condition of a sample equilibrated with a pure oxygen atmosphere.