Photocatalysis reactions using [Ru(II) (bpy)3 ](2+) were studied on the example of visible-light-sensitized reversible addition-fragmentation chain transfer (RAFT) polymerization. Although both photoinduced electron- and energy-transfer mechanisms are able to describe this interaction, no definitive experimental proof has been presented so far. This paper investigates the actual mechanism governing this reaction. A set of RAFT agents was selected, their redox potentials measured by cyclic voltammetry, and relaxed triplet energies calculated by quantum mechanics. Gibbs free-energy values were calculated for both electron- and energy-transfer mechanisms. Quenching rate constants were determined by laser flash photolysis. The results undoubtedly evidence the involvement of a photoinduced energy-transfer reaction. Controlled photopolymerization experiments are discussed in the light of the primary photochemical process and photodissociation ability of RAFT agent triplet states.

In this work, the inhibition and termination reactions occurring throughout a free radical photopolymerization initiated by a type-I photoinitiator are studied by kinetic modeling. The role of the macroradicals as the main oxygen trapping agents during the inhibition time is identified, and the absence of primary radical consumption by oxygen can be related to a high initiation efficiency at early times. The ratio of the termination reactions reveals that bimolecular termination remains the principal pathway for the cessation of macromolecule growth, even at high polymer conversion. Moreover, the evolution of the termination ratio during the polymerization can be correlated to both the diffusional control of the polymerization reactions as the polymer network grows and the photoinitiator consumption. Finally, the effect of the incident light intensity and the initial photoinitiator concentration on the termination reactions is assessed, and the validity of the steady-state assumption applied to the macroradical concentration discussed.

SummaryThe combination of a dye which absorbs the photon, an electron acceptor and an electron donor leading to energy conversion through electron transfer, was the basis of the so called three-component systems. In this paper, an experimental work combining Rose bengal dye with a triazine derivative as electron acceptor and ethyl 4-(dimethylamino)benzoate as electron donor, will underline the benefit of the photocyclic behavior of three-component systems leading to the dye regeneration. A thermodynamic approach of the photocycle is presented, followed by a mechanistic and computational study of ideal photocycles, in order to outline the specific kinetics occuring in so called photocatalytic systems. The simple kinetic model used is enough to outline the benefit of the cyclic system and to give the basic requirements in term of chemical combination needed to be fulfilled in order to obtain a photocatalytic behavior.

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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.,