Radical Cations in Versatile High Performance Initiating Systems for

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Radical Cations in Versatile High Performance Initiating Systems for Thermal, Redox, and Photopolymerizations Patxi Garra,†,‡ Alexandre Baralle,†,‡ Bernadette Graff,†,‡ Gautier Schrodj,†,‡ Fabrice Morlet-Savary,†,‡ Ceĺ ine Dietlin,†,‡ Jean-Pierre Fouassier,†,‡ and Jacques Laleveé *,†,‡ †

Université de Haute-Alsace, CNRS, IS2M UMR 7361, F-68100 Mulhouse, France Université de Strasbourg, Strasbourg, France

‡

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ABSTRACT: Highly versatile initiating systems for thermal, redox, and photopolymerization processes are proposed. The photopolymerization using the multifunctional amine tris[4-(diethylamino)phenyl]amine (t4epa) and iodonium salt (Iod) as photoinitiating system (PIS) is presented. Methacrylate function conversion up to 84% was reached under LED@850 nm using t4epa/Iod/phosphine PIS when only 60% was possible for the same resin using a commercial camphorquinone/amine/ phosphines blue light (470 nm) PIS showing that longer wavelengths can be used with high final performances. Estimation of the balance between photothermal vs photoinduced electron transfer processes to initiate polymerization was performed exhaustively thanks to thermal imaging, Raman confocal microscopy, FTIR, cyclic voltammetry, UV−vis−NIR spectroscopy, ESR, ESR-ST photolysis, DSC, photo-DSC, and molecular modeling. This method can be used in further works interested in photochemical/thermal processes as it allowed to highlight two unusual reactivity features: (i) the in situ creation of a bicomponent thermal initiator (potentially occurring in several other systems) and (ii) the estimation of light-induced heating rates. Remarkably, a NIR light-absorbing radical cation is responsible for the photoreaction and the high photoinitiating performance. Interestingly, in parallel and without light, the first pure organic peroxide-free redox radical initiating system based on the proposed t4epa/Iod combination will be presented; that is, performances similar to or better than harmful/unstable peroxide-based redox (or thermal) initiating systems are obtained.

1. INTRODUCTION Today, free radical polymerization (FRP) is of high academic/ industrial interest (roughly 45% of the total polymer production).1 For polymerization in eco-friendly conditions (at room temperature (RT) and under air; without purification of monomers), two strategies are generally relevant: redox polymerization2−4 and photopolymerization.5−13 Many exhaustive reviews begin with a clear distinction between these techniques and thermal polymerization (heating of the reaction media). First, redox polymerization is occurring when an oxidizing agent (e.g., bearing a weak O−O or S−S bond14) is mixed with a reducing agent (or system),15,16 leading to the polymerization of the surrounding resin through the generation of initiating radicals in a redox process. This type of process is very convenient for filled sample polymerization (e.g., for the access to composites), but commercial systems © XXXX American Chemical Society

still show some huge drawbacks due to the current toxicity/ instability of oxidizing agents (e.g., peroxides17 ). One important challenge for redox polymerization is to use less harmful/toxic metal-free oxidizing agents (e.g., iodonium salts that are safe as used in dental applications18). In parallel, free radical photopolymerization is occurring when actinic light is absorbed by photoinitiating systems (PIS), leading to the formation of radicals that initiate FRP. One of the most important challenge is the shift of actinic wavelengths toward visible to near-infrared (NIR) wavelengths, particularly for the access to composites19,20 (these latter NIR wavelengths showing much better penetrations in Received: September 13, 2018 Revised: October 8, 2018

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DOI: 10.1021/acs.macromol.8b01975 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. Summary of the previous and proposed (this study) charge transfer complex (CTC) interactions and their UV−vis−NIR illustrations in DCM: (A) (1) 28 mM Iod1; (2) 58 mM 4-N,N TMA; (3) 1:1 mixture of (1) and (2) leading to 29 mM 4-N,N TMA + 14 mM Iod1. (B) (1) 28 mM Iod1; (2) 58 mM t4epa; (3) 1:1 mixture of (1) and (2) leading to 29 mM t4epa + 14 mM Iod1.

Scheme 1. Main Compounds Used for This Study

filled samples). The challenge is quite important: a UV photon at 300 nm is 3 times more energetic than a NIR one at 900 nm; it explains why photopolymerization was historically developed for UV light.21 A few NIR photopolymerization studies include (i) upconversion particles irradiated by high irradiances NIR lasers (9−60 W/cm2),22−28 (ii) electron transfer between a cyanine dye and an iodonium salt29−32 to generate initiating radicals, or (iii) Cu(II)/phosphines redox photoactivated polymerizations.33,34 Because of (i) the high

irradiances leading to high energy consumptions and (ii) the highly colored final photopolymers in many cases, there is still a huge search and need for highly efficient NIR photoinitiating systems. Very recently, Garra et al. proposed charge transfer complexes (CTC) between N-aromatic amines and iodonium salts as visible light (405−470 nm) photoinitiating systems (PISs).35 The outstanding performances coupled with low optical densities enabled to produce extremely deep samples B


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Macromolecules Scheme 2. Composition of the Methacrylate Resin (Resin 1) Used

an intensity of 1 W was used for the tmpd/Iod2 photoinitiating system. The emission spectra are already available in the literature.42 2.3. Surface Polymer Analysis (z-Profile) through Raman Confocal Microscopy (Figure 4B). Raman spectra of cured polymer surfaces were recorded with a Labram (Horiba) spectrometer mounted on an Olympus BX40 confocal microscope, operating at 632.8 nm with a 1800 lines/mm grating. The carbonyl peak area (1690−1755 cm−1) was measured as an internal standard while the alkene peak area (1630−1660 cm−1) was used to determine the CC conversion. Spectra were recorded on the uncured monomer to be used as reference for double bond conversion. A 100× objective was used in combination with a confocal hole aperture of 200 μm, giving an axial resolution of 2.3 μm. The objective displacement in the air was multiplied by a factor of 1.7 following a protocol available in the literature43−46 to access the real depth in the polymerized sample considering a refraction index close to 1.5. This protocol was already applied in our previous studies.47,48 2.4. Monitoring Photopolymerization Reaction with Thermal Imaging Camera (Figure 4A). An infrared thermal imaging camera (Fluke TiX500) was used to monitor the photopolymerization of the 1.4 mm samples. Fluke SmartView4.1 software was used to present the images. Thermal and spatial resolutions are ±1 °C and about 250 μm, respectively, at a 40 cm distance. A scriptrunning under the Spyder environment (Python language)was used to recover temperature versus time (at the center of the sample) from raw Fluke data files. A complete description of thermal imaging features for photopolymerization monitoring was recently proposed by our group.49 Here, it allowed us to determine gel time from the time required to reach maximum exothermicity (Table 3). 2.5. UV−Vis−NIR Absorption Spectroscopy. UV−vis−NIR absorption spectra in DCM (quartz cell) were recorded (i) using a Varian Cary 3 spectrophotometer (for Figure 1 and Figure S1, working wavelength range 4-N,N TMA. In Table 1, in agreement with the experimental findings (Figure 2), the calculated CTCs (t4epa/Iod1 and tmpd/Iod1) are clearly characterized by a longer absorption wavelength than the donor (t4epa or tmpd) or acceptor (Iod1) alone.

the case of tmpd (Figure 2B), both tmpd and Iod1 are not absorbing visible wavelengths. On the contrary, their mixture leads to a strongly enhanced visible light absorption with two maxima of similar intensities at 540 and 605 nm (Figure 2B). The general light absorption changes from the mixing of t4epa with Iod1 or from the mixing of tmpd with Iod1 will be discussed below (CTC or radical cation formation, Figure 3). Similar changes in the optical properties (triggered by the amine/iodonium mixing) are also observed qualitatively in methacrylate resins (see Figure S1) for tmpd/Iod2 and t4epa/ Iod2 mixtures. As shown previously,35 Iod1 and Iod2 have similar reactivities even in CTC interactions though it was preferred in the present article to use Iod2 in polymerization experiments (to avoid the potential benzene release) and Iod1 in light absorbing/ESR/molecular modeling studies (simpler structure for molecular modeling without tert-butyl moieties). In the case of PSIod3, solubility in DCM was very poor, and it was not possible to study its light absorption properties. Finally, the kinetics of the increasing NIR light absorption was assessed in Figure 2C. Unexpectedly, the light absorbing specie is more and more produced throughout time (even after 14 h) which is very different from previous reports on NE


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Table 1. Frontier Molecular Orbital Properties (Calculated at the UB3LYP/LANL2DZ Level) for Iod1 and the (Multi)functional Donors (t4epa, tmpd) Separately, the CTC Optimized Structures ([Donor-Iod1]CTC), t4epa•+ Radical Cation, and Their Respective Calculated UV−Vis Spectra (Single Point in DCM)a

Iod1 and CTC structures calculated in the cationic form; HOMO/LUMO visualization: isovalue = 0.02; λmax and F stand for the calculated maximum absorption wavelength and oscillator strength, respectively. a

However, computation of the UV−vis−NIR absorbance for t4epa/Iod1 CTC complex was not consistent with exper-

imental UV−vis absorption spectra, particularly for the maximum at 1181 nm compared to 973 nm experimentally. F


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2.235 Å and the other one at 2.175 Å (very tight angle of 93.5°). This last [t4epa-Iod1]CTC CTC suggests a very strong t4epa/Iod1 donor−acceptor interaction leading to a redox process between t4epa and Iod; that is, the C−I bond length increases in agreement with a reduction of the iodonium salt and an oxidation of the amine leading to t4epa•+ (r1). This process can lead to the dissociation of the iodonium salt with one aryl radical (Ar•) produced, the byproduct being Ar−I. The reactivity of these systems will be applied to FRP below.

This discrepancy can be ascribed to the actual light absorbing species that is not a CTC but a byproduct of t4epa/Iod1 redox reaction (see below for the ESR experiments in which the light absorption properties can be clearly ascribed to the radical cation t4epa and not to the CTC). Indeed, reduction of the iodonium salt by t4epa would lead to the production of t4epa•+ radical cation; this latter shows a strong bathochromic shift compared to t4epa (i.e., computed maximum NIR absorption at 878 nm which is rather close to experimental value: 973 nm; Table 1). This will be confirmed below in ESR experiments. For the tmpd/Iod1 complex, the computed light absorption is better correlated to the experimental one though still not fully accurate (756 nm computed compared to 605 nm experimentally observed). In that case, the effect of solvent on the computed CTC light absorption properties could account for the computational inaccuracy. A tmpd/Iod1 CTC is clearly likely to be the light absorbing species as no tmpd/Iod1 redox reaction (leading to a byproduct) was observed in twocomponent redox FRP (see Figure 5). It should also be mentioned that increasing the number of iodonium (one, three, and five moieties) synthesized onto a polystyrene backbone leads to a bathochromic effect on the absorption wavelength (Table S1) even though no π resonance is present in that structure (resonance through space). It led us to consider PSIod3 for this study. 3.1.3. Iodonium Salt Geometry as an Indicator of the Amine/Iodonium Interaction. Interestingly, the changes in iodonium’s geometry are highlighted in Table 2. The stronger

t4epa + Iod1 ⇆ [t4epa−Iod1]CTC → t4epa•+ + Ar• + Ar−I (r1)

In the case of tmpd, the changes of iodonium geometry where less important which suggests that the tmpd/Iod1 interaction is remaining at the state of a CTC: [tmpd-Iod1]CTC. This is also consistent with the absence of redox radical reaction between tmpd and an iodonium salt (see below). tmpd + Iod1 ⇆ [tmpd−Iod1]CTC

3.1.4. Electron Spin Resonance (ESR). Next, we propose to complete the study of the amine/iodonium salt interactions by means of electron spin resonance (ESR) and ESR spin trapping (ESR-ST). In Figure 3A and without any light exposure, the tmpd−Iod1 interaction leads to an unresolved radical observed. Direct observation of CTC through ESR was possible in some cases of the literature.55,56 More interestingly, in Figure 3B, ESR-spin trapping (ESR-ST) monitoring the photolysis of [tmpd−Iod1]CTC under LED@530 nm leads to (i) photolysis of the unresolved signal (attributed to the CTC) and (ii) production of Ph• radicals (PBN/Ph• radical adducts detected; see the simulation of the experimental spectrum in Figure S3E). In the case of t4epa/Iod1, a well-resolved stable radical was observed directly at room temperature in solution (Figure 3C, g = 2.003 at the center, Figure S3C) without light exposure. Interestingly, using light or electrochemical means,57,58 stable triphenylamine substituted radical cations were also observed in ESR. In our case (no light or additional energy for radical cation production), we propose to attribute the very original spectrum (particularly for metal-free solutions) to the radical cation (t4epa•+) of Scheme 3. Indeed, it was possible to

Table 2. Selected Geometrical Features of the Iodonium Salt Isolated and When Involved in a CTC Interaction with the Donors of This Study structure optimized Iod1 [4-N,N TMA-Iod1]CTC [tmpd-Iod1]CTC [t4epa-Iod1]CTC

C−I distances (Å)

C−I−C angle (deg)

2.177 2.177 2.173 2.185 2.190 2.193 2.175 2.235

100.9

(r2)

95.2 93.6 93.5

Scheme 3. Proposed Structure for the t4epa•+ Radical Cation Observed in ESR (Figure 3C)

the donor, the more the iodonium salt seems to undergo dissociation. In detail, the structure of the model iodonium salt (Iod1) is symmetrical with two carbons located equidistant to the iodine at 2.177 Å. The CIC angle is 100.9°. On the contrary, when a strong donor like 4-N,N TMA is creating a CTC,35,47 the iodonium salt geometry is getting more tense in terms of CIC angle (95.2°), and even more interestingly one of the distances between iodine and aryl groups is now asymmetrical with one carbon placed at 2.173 Å and the other one at 2.185 Å. The geometry was also modified for [tmpd-Iod1]CTC with carbons located at 2.190 and 2.193 Å from iodine (tighten angle of 93.6°) though the geometry of that CTC is symmetrical (iodine close to the central aromatic ring of the N-aromatic donor) when asymmetrical (iodine closer to nitrogen of the N-aromatic donor) for [4-N,N TMAIod1]CTC and [t4epa-Iod1]CTC. That latter CTC is still proposed in line with the previous literature;35 its UV−vis absorption could be masked by the extremely high absorption of t4epa•+ product. The changes of geometry are even stronger for the strongest donor (t4epa) with one carbon located at

accurately simulate the spectrum using the following coupling constants: aN = 6.94 G (nitrogen bearing the radical), aH = 3.39 G (accounted for the first six hydrogen in beta position on the aryl group), and aH = 1.25 G (accounted for the second six hydrogen in gamma position on the aryl group). Also, the same spectrum was present with a very weak intensity (see Figure S3A,B) for pure t4epa (possible traces of oxidized amine t4epa•+). This peak cannot be observed when two oxidations G


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Macromolecules are performed (with BPO, Figure S3B, generation of t4epa2+). The intensity of this ESR signal was much stronger in the presence of Iod1, which leads us to propose reaction r1 to explain the formation of t4epa•+. Also, Figure 3D shows that it is possible to photolyze this structure under soft LED@850 nm irradiation to form the Ph•/PBN radical adducts (Figure S3C). This led us to propose reactions r3 and r5 (see Scheme 4).

component systems are necessary for light-induced polymerizations. t4epa/Iod2 PIS is mildly efficient but rapid with 36% conversion obtained (1.4 mm, under air, in resin 1 and upon LED@850 nm irradiation). The addition of TPP outstandingly enhances conversion to 84% in slightly more than 2 min (see effects of phosphines on oxygen inhibition in the literature41,59,60). The same holds true when using a low migration phosphine such as 4-diphenylphosphinostyrene (4dpps, 75% conversion obtained). tmpd/Iod2 is mildly efficient with 25% conversion, but the addition of phosphine also greatly enhances conversions to about 69% under green light. On the contrary, PSIod3 shows very low photoinitiating ability with only 30% conversion reached after 800 s under purple (LED@405 nm) light in a three-component systems (Figure S6). The poor behavior of PSIod3 is ascribed to its very poor solubility in methacrylate resins. The influence of the PSIod3 counteranion (BF4− against PF6− for Iod2) in free radical photopolymerization is expected to be much lower than what is known in cationic photopolymerization.61 When having a look at CC bond conversions for multifunctional monomers, it is of outmost importance to compare the values obtained with state of the art radical initiating systems. Consequently, the value of conversion for a standard CQ/amine/phosphine three component PIS in resin 1 was added to Table 3. Despite a longer polymerization time, the newly developed green light (69% conversion) and NIR light (84% conversion) PIS outrank the commercial blue light CQ/amine/phosphine system (only 60% conversion, same setup and resin) (Table 3). In line with the literature, we account the higher conversion obtained to the increased curing temperature highlighted in Figure 4.62 NIR light is in many cases more interesting (and challenging) compared to blue light as it has a much better light penetration in composites.20 Also, photochemical bleaching is outstanding (Figure S4A) which could extend the applications for the final photopolymer. 3.2.2. Pure Thermal Effects on Polymerization Initiation. As the light-induced temperature increase was very fast for the t4epa/Iod2 system (see Figure 4A), we propose to perform a deconvolution of the photothermal vs photoinduced electron transfer effects leading to the curing of that resin. To do so, we studied that t4epa/Iod2 formulation without NIR photons but with temperature increase, that is, thermal polymerization in Figure 5A. It has to be noted that the pot life of this formulation was not optimum: after 7 days (storage at RT), a solid (polymer) could be observed at the bottom of the formulation (suggesting a slow radical production per unit of time). Logically, t4epa in resin (curve 2), Iod2 in resin (curve 3), or the resin alone (curve 4) showed no curing at all when between 0 and 150 °C. More interestingly, the t4epa/Iod2 acts as thermal initiator with a decomposition temperature (starting at 75 °C) close to dibenzoyl peroxide (BPO, see Figure S7). Both thermolysis and photolysis of CTC complexes are here worthwhile; it has to be mentioned that in the field of radical organic synthesis, some literature examples64 exist of both thermal and photonic responsive CTCs (with completely different donors/acceptors chemistries). 3.2.3. Redox Two-Component Polymerization. Finally, due to the redox reaction between t4epa and iodonium salt, we tried to use the t4epa/Iod2 combination as a pure twocomponent redox system (Figure 5B). This is fundamentally different compared to the monocomponent resin (t4epa + Iod2 in resin) studied above (Figure 4 and Table 3). Indeed,

Scheme 4. Proposed Chemical Mechanisms Involved in the Free Radical Generation Initiated by a t4epa/Iod2 System: without Light (r1, r1′, and r6) and in the Presence of Light (r1, r3, r4, r5, and r6)

3.2. Application to Polymer Synthesis. 3.2.1. Photopolymerization. In this part, the donor/iodonium salt reactivities were applied to polymer synthesis. First, the ability of tmpd/iodonium salt and t4epa/iodonium salt to generate aryl radicals (Ar•) under visible and NIR light irradiations will be applied to photopolymerization. In Figure 4A, t4epa/Iod2 light-induced polymerization is monitored by thermal imaging. Using small amounts of reactants, t4epa/Iod2 formulation during irradiation is rapidly reaching 100 °C (in 150 s) where a second heat generation is recorded (≈130 °C) (Figure 4A, curve 2). In a control radical inhibited experiment, due to the 4-hydroxy-TEMPO (tempol, Figure 4A, curve 1), that second peak was absent. We could therefore attribute this second heat peak to the exothermicity of the polymerization, that is, fast CC conversion. Interestingly, the completely inhibited formulation (Figure 4A, curve 1, 1 wt % tempol) still reaches about 110 °C after 4 min.30 No polymerization and bleaching were observed for that formulation. Here, we can therefore suggest that the NIR light absorbing species convert significant light into heat under soft LED@850 nm irradiation (see our full study of thermal imaging monitoring49). This data implies that thermal stability of the PIS up to 110 °C will have to be studied to balance photothermal vs photoinduced electron transfer effects. For the monomer alone, no exothermicity is observed showing that the presence of t4epa/Iod2 is required for the heat generation. The addition of phosphine to overcome oxygen inhibition (here triphenylphosphine TPP) is leading to an even faster reaction with 160 °C reached after about 2 min (Figure 4A, curve 3). The analysis of the final polymer through RAMAN confocal microscopy (Figure 4B) suggests that no oxygen inhibited layer is remaining (already 60% CC conversion at the surface) which reflects extremely high radical generating rates competitive with the one of UV sensitive photoinitiating systems.41 A summary of the photopolymerization experiments performed in this study is given in Table 3 (see FTIR absorbances in Figures S4 and S5). Control experiments (Iod2, t4epa, or tmpd alone as photoinitiator) clearly show that twoH


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Figure 4. Photopolymerization experiments followed by thermal imaging (temperature vs irradiation time) measured for the resin 1; 1.4 mm thick samples, under air, LED@850 nm irradiation (the irradiation starts for t = 5 s, 1.1 W/cm2) for (if mentioned) 0.5 wt % t4epa, 1 wt % Iod2, and 2 wt % TPP. (A) 1: t4epa/Iod2 + 1 wt % tempol; 2: t4epa/Iod2; 3: t4epa/Iod2/TPP. Inset: corresponding thermal imaging acquisitions at the highest temperature peak. (B) Raman confocal microscopy analysis (CC conversion vs distance from the surface) of the polymer generated in curve 3.

h). Therefore, t4epa reduction of Iod2 is likely to occur. Remarkably, the t4epa/Iod2 redox system developed is a peroxide-free system but already characterized by a performance similar to the well-known peroxide-based 4-N,N TMA/ BPO commercial system (see 100 °C exothermicity in a similar resin and same setup).15,16 To the best of our knowledge, this is the first example of iodonium-based redox FRP initiating system (no light). It is of high interest as iodonium salts are much more stable than peroxides and can be used in highly selective biorelated applications.18 3.3. Mechanistic Proposition. Our investigations led us to consider the chemical mechanisms gathered in Scheme 4: (i) Without light, the t4epa/Ar2I+ interaction led to the slow (see Figure 2) generation of Ar2I• radicals; these latter have a lifetime of a few hundred picoseconds65 and are fast

during preparation of a monocomponent (0.5 wt % t4epa/1 wt % Iod2 formulation in resin 1), solubilization of both components from mixing is slow (potentially leading to a slow radical production per unit of time); if some radicals are generated from the partial redox interaction between both components, they are rapidly quenched by the renewed oxygen in the stirred media.41 For that one component, 24 h stability (at RT) was observed, but after 7 days a polymer was present at the bottom of the formulation. In Figure 5B, on the contrary, redox mixing of two perfectly solubilized components (t4epa one and Iod2 one) is presented (with potentially higher radical production per unit of time). Unexpectedly, an outstanding exothermicity was recorded after about 250 s with 95 °C reached. For comparison, no polymerization at all was obtained for tmpd/Iod2 redox system (after more than 72 I


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Macromolecules ΔGet = Eox (t 4epa•+) − Ered(Iod2) − E*

Table 3. Summary of the Main Photopolymerization Experiments under LED@850 nm (1.1 W/cm2) and Laser Diode@530 nm (1 W) Irradiations, under Air, in Resin 1, at Room Temperature (RT, 23 ± 2 °C)a PIS Iod2 t4epa [t4epa-Iod2] [t4epa-Iod2] + TPP [t4epa-Iod2] + 4dpps tmpd [tmpdIod2]CTC [tmpdIod2]CTC + tpp CQ/amine + 4dpps

irradiation source

FTIR CC conversion (%) n.p.

= 0.39 − (− 0.20) − 1.13 = − 0.54 eV

It has to be mentioned that a theoretical study recently proposed a lower reduction potential for diphenyliodonium hexafluorophosphate (−0.7 V vs SCE)68 in opposition to the early work by Crivello and co-workers.9 Our own CV measurements, as another one performed few years ago,69 show two peaks for Iod2 in ACN (Figure S2E); one close enough to −0.2 V (vs SCE) and the other one closer to the new value. We choose to use the value proposed by Strehmel et co-workers for Iod2 (same tert-butyl substitution) derivative (−0.59 V vs SCE, p 2288) which would give a second ΔGet = −0.15 eV. Therefore, the electron transfer is favorable in all the cases, and therefore (r5) is proposed to occur as both t4epa•+ photolysis and Ph• radicals are observed (Figure 3D). Lightinduced nonradiative deactivation of t4epa•+ leading to the conversion of light into heat will be discussed below. (iii) Finally, heating the reaction media enhances radical production rates through reaction r1 (i.e., r1′, t4epa/Iod2 formulation acts as a thermal initiator). Heating can be provided by external source (DSC, thermal polymerization) or by nonradiative deactivation of absorbing species, that is, t4epa•+ in (r3) and (r4). Indeed,the heating rate is much more important for the t4epa/Iod2 formulation under NIR light compared to t4epa formulation alone (see photo-DSC experiment in Table S2 and Figure 5A). This last point could account for the outstanding performances of the NIR PIS: indeed, recent works tend to demonstrate that higher curing temperatures lead to higher photopolymerization conversions.62 For the tmpd/Iod2 interaction under 532 nm light, a classical N-aromatic amines/Iod2 CTC interaction is proposed.35 3.4. Methodology for Photothermal vs Photoinduced Electron Transfer Deconvolution. The following standard methodology for deconvolution of photothermal versus photoinduced electron transfer effects in photopolymerization (or other light-induced reactions) can be proposed: First, the reaction under full NIR irradiation should be studied by

polymerization timeb

LED@850 nm or LD@532 nm LED@850 nm LED@850 nm LED@850 nm

n.p.

n.p. 36 84

n.p. 3 min 9 s 2 min 8 s

LED@850 nm

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Radical Cations in Versatile High Performance Initiating Systems for

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