Triazine‐Based Type‐II Photoinitiating System for Free Radical Photopolymerization: Mechanism, Efficiency, and Modeling

Abstract

Isopropylthioxanthone, a versatile photoinitiator (PI) for free radical photopolymerization (FRP), is combined with a triazine derivative (Tz) in a Type-II photoinitiating system (PIS). Initiation ability of this system for acrylate photopolymerization is assessed using a diacrylate monomer. Involvement of a photoinduced electron transfer mechanism is demonstrated by time-resolved spectro scopic measurements. Further insights in this mechanism are obtained through the use of a photopolymerization kinetic model taking into account the main reaction steps from the absorption of photons to the formation of the polymer. Prediction ability of the model is also tested with different initial concentrations of PI and co-initiator, as well as a different Tz. This last experiment reveals the noticeable role of back electron transfer in the FRP mechanism of Type-II PIS. by the use of a photoinitiating system (PIS). It converts photons into chemical energy through the production of radicals capable of reacting with acrylic monomers to initiate macromolecular chains. Three main families of PIS have been developed, each with their own advantages and limitations. [2,[4][5][6][7][8][9] Type-I PIS rely on the photoinduced dissociation of the initiator to produce primary radicals. Despite a high quantum yield of radical production, typical bond dissociation energies require the use of UV lamps that are known to be harmful and to release ozone in the atmosphere. Therefore, Type-II PIS have been developed, which combine a photoinitiator (PI) able to absorb light and a co-initiator capable of reacting with the excited PI through hydrogen abstraction or electron transfer. Nevertheless, radical production is limited by the diffusion of the species into the viscous monomer medium and competition of the bimolecular reaction with PI deactivation pathways. In the case of hydrogen abstraction, hydrogenated PI radicals (PIH • , such as ketyl radicals) could also act as terminating agents and reduce final conversion. [10,11] Photocyclic initiating systems (PCIS) have then been developed in order to combine advantages of previous PIS, i.e.,

Experimental and theoretical investigations of free radical photopolymerization: Inhibition and termination reactions

“…It has been notably used to determine the effect of oxygen inhibition on the propagation and termination rate constants [82], as well as model layer-by-layer photopolymerization [83] by Taki et al. Such a kinetic model has been recently adapted for the photoinitiation involving a type-II PIS combining isopropylthioxanthone and triazine derivatives, evidencing the crucial role of back electron transfer in the kinetics of three-dimensional polymer network formation [84]. However, there is still a lack of quantitative data concerning the amount and the competition between the termination reactions for such a simple system as a type-I photoinitator.…”

Photoreduction of triplet thioxanthone derivative by azolium tetraphenylborate: a way to photogenerate N-heterocyclic carbenes

“…Laser flash exposure was performed at 355 nm in deaerated and anhydrous acetonitrile, where only the sensitizer (ITX) absorbs the light. Given the short lifetime of ITX singlet state as well as its high intersystem crossing quantum yield, 30,31 ITX is assumed to interact mainly via its triplet state. As can be seen in Figure 3a, the transient absorption spectra of ITX alone shows an absorption maximum at 630 nm, fully consistent with the ITX triplet ( 3 ITX*) in acetonitrile reported in the literature.…”

“…As can be seen in Figure 3a, the transient absorption spectra of ITX alone shows an absorption maximum at 630 nm, fully consistent with the ITX triplet ( 3 ITX*) in acetonitrile reported in the literature. 29,30,[32][33][34] exhibits a fairly endothermic character ( Fig. S1 in Supporting Information).…”

Photochemical Study of a Three‐Component Photocyclic Initiating System for Free Radical Photopolymerization: Implementing a Model for Digital Light Processing 3D Printing

“…Consequently, commonly used photosensitive 3D printing resins are mostly composed of (meth)acrylates monomer/oligomer mix with TPO or BAPO type I photoinitiator (PI) . Recently, more complex systems were proposed, but only few examples of two and three component PIS can be found in literature, and none of them was deeply investigated in terms of 3D printing performance despite the well‐known reactivity of such systems …”