Copper (Photo)redox Catalyst for Radical Photopolymerization in

May 9, 2017 - Third, almost tack-free thick samples (1.4 mm) under air are produced upon sunlight exposure (65% of conversion for the 1.4 mm thick sam...
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Copper (Photo)redox Catalyst for Radical Photopolymerization in Shadowed Areas and Access to Thick and Filled Samples P. Garra,† F. Dumur,‡ D. Gigmes,‡ A. Al Mousawi,† F. Morlet-Savary,† C. Dietlin,† J. P. Fouassier, and J. Lalevée*,† †

IS2M, UMR CNRS 7361, UHA, Institut de Science des Matériaux de Mulhouse, 15, rue Jean Starcky, 68057 Mulhouse Cedex, France ‡ Aix Marseille Univ., CNRS, ICR UMR 7273, F-13397 Marseille, France S Supporting Information *

ABSTRACT: The free radical polymerization of low viscosity methacrylate blends upon a LED irradiation at 405 nm under air is carried out using Cu(I)/iodonium salt/tin(II) organic derivative as photoinitiating systems. The system exhibits a high reactivity; where tin derivative plays a crucial role. It operates through a catalytic cycle in which Cu(I) is regenerated and can be used at low concentrations (0.1−0.3 wt %). Remarkable performances are achieved. At first, a final methacrylate conversion of 82% after 40 s in 1.4 mm thick samples is obtained for an irradiance of 35 mW/ cm2 whereas such a conversion is only reached only when using a Cu(I)/iodonium salt system under a 200 mW/cm2 light exposure. Second, a 55% conversion is still obtained after 150 s under a very low irradiance (2.5 mW/cm2). Third, almost tack-free thick samples (1.4 mm) under air are produced upon sunlight exposure (65% of conversion for the 1.4 mm thick sample after 90 s of irradiation). Fourth, the photocuring of clear samples as thick as 9 cm (and presumably even more) with an impressive homogeneity through the entire polymerizable medium is feasible; the photopolymerization of 8.5 cm thick filled samples is also realized. Fifth and last, a lateral polymerization beyond the irradiated area is demonstrated with unprecedented extensions of 8 mm (tin(II) = 1.3%) and 28 mm (tin(II) = 8%), which allows polymerization reactions to occur in shadowed areas. The chemical mechanisms are studied by steady state photolysis and ESRspin trapping experiments. The subsequent role of the hydroperoxides (ROOH) formed during the polymerization reaction is a key point i.e. for the polymerization in shadowed areas (thick and filled samples), these latent species (ROOH) will be generated from the oxygen inhibition and can diffuse for a full curing of the samples through a ROOH/Cu(I) redox initiation.

1. INTRODUCTION UV curing is a technique widely used in material science to initiate free radical polymerization (FRP) reactions. The main advantages of UV curing are a high polymerization efficiency coupled with both temporal and spatial controls. However, one of the main issues for the photopolymerization of thick samples is the light penetration through the entire polymerizable medium;1−4 this is particularly true for filled, dispersed or pigmented samples. In parallel, numerous reports indicate a growing interest in bulk photopolymerization of thicker millimetric films (e.g., in ref 1). Some studies show different strategies to tackle the problems of in-depth polymerizations: a lot of them are based on the modification of the photonic parameters of the photopolymerization reaction like using higher irradiance2 (and still recently reported in5). This new research5 involved high irradiance and high energy consumption lasers to produce photopolymers of about 10 cm but using NIR activation (9.4 W/cm2). Upconversion particles (0.3 wt % particles in the resin) re-emitted blue light (allowing photoinitiation by © XXXX American Chemical Society

Irgacure 784) upon 980 nm high laser intensity. Another photonic approach is to choose photoinitiators (PI) with low molar extinction coefficients and a capability to decompose without generating absorbing photolysis products6 absorbing the exciting (or actinic) light (in this last case, ∼10 cm thick polymers can be obtained from clear acrylate formulations). In addition, in the case of pigmented matrices, the PI must operate in a well-defined spectral window as observed in the photocuring of paints (few hundred micrometer thick films can only be photopolymerized7). The problem of the light penetration is still crucial. It is well admitted that the conventional strategies based only on the absorption of light by a PI will not lead to the photocuring of thick (centimeter range) highly filled, dispersed or pigmented formulations, and a fortiori to a polymerization in shadowed areas. One possible way to overcome these drawbacks consists Received: March 23, 2017 Revised: April 25, 2017

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DOI: 10.1021/acs.macromol.7b00622 Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. (A) Proposed Cu Redox Catalytic Cycles for the Propagation of Polymerization beyond the Irradiated Area and Access to Very Thick Samples and (B) the Proposed Strategy for the Photopolymerization Propagation in Shadowed Areas

Copper is a cheap transition metal (5.78 $/kg12 by the beginning of March 2017) quite abundant in the Earth’s crust13 which is usually not considered as a highly toxic element. It was used in some polymerization reactions for example the azide− alkyne cycloaddition (click reaction14). The reversibility of Cu(I) to Cu(II) oxidation made it a candidate of choice for atom transfer radical polymerization (ATRP).15,16 One report indicates a possible use of Cu(acac)2 in combination with a specific phosphines in order to initiate redox polymerization.17 Cu(I) complexes can also be readily oxidized upon LEDs18 in the visible and initiate free radical and/or cationic photopolymerizations. Cu(I) complexes combined with a (hydro)peroxide, have already been proposed for redox initiated polymerization and likely work according to reactions r1 and r2.19 The r2 reaction produces ROO• radicals much less effective than RO• to initiate FRP.20 It would therefore be preferable to lower the Cu(II) content in the media in order to favor the r1 reaction and have more RO• generated; simultaneously r2 will be much more limited. The idea of introducing a reducing agent to efficiently convert Cu(II) in Cu(I) is therefore emerging as an attractive way to favor r1 vs r2. Moreover, this ensures a recovery of the starting photoinitiator and makes possible a decrease of the concentrations used. The current metal-free reference for redox polymerization (without light) is the amine/benzoylperoxide system21−23 which involves (i) toxic carcogenic amines (like 4,N,N-trimethylaniline) and not in catalytic concentrations and (ii) unstable (explosive) benzoyl peroxide. We believe that the proposed approach to generate in situ hydroperoxides is less hazardous in terms of chemicals toxicity.19

in the generation of latent species that can further start and propagate reactions in the dark. This approach was described e.g. in the use of a four-component photoinitiating system where three first components (Rose Bengal/ferrocenium salt/ amine) initiates the photopolymerization and the presence of a fourth component (hydroperoxide), interacting with suitable formed species, leads to subsequent dark reactions involving hydroxyl radicals and allows the body cure of a 300−400 μm thick pigmented film.7,8 This strategy was also exploited by Ermoshkinet al.9 (referred as photopolymerization without light or remote curing) and illustrated by the oxalate ester/hydrogen peroxide system which produces hydroxyl initiating radicals in the absence of light (the polymerizations were carried out with acrylatesthat exhibit higher propagating rate constant than methacrylates but a higher toxicityand under a nitrogen flow saturated with H2O2, which strongly reduces the practical uses of this concept). Somehow, remote curing in the case of Ermoshkinet al.9 is very close to redox initiated polymerization10 where an oxidizing agent reacts with a reducing agent to generate the initiating radicals. Another approach was recently defined as lateral photopolymerization in a very elegant example developed by AguirreSoto et al.11 Upon light exposure, a Methylene Blue/amine system leads to a metastable leuco form of the dye as latent species which slowly reacts in shadowed areas with an iodonium salt to generate polymerization initiating aryl radicals (Ar•). About 3.5 mm of lateral extension of the polymerization was possible. The photopolymerization of thick samples was also achieved. In the present paper, we are looking for a system being able to polymerize on demand thick samples or/and parts of samples in shadowed areas (no access to light irradiation). This paper is devoted to the polymerization of multifunctional monomers for the synthesis of polymer networks. The access to the polymerization in shadowed areas will be very useful in many high-tech applications. We will here propose a new approach lying on catalytic redox cycles based on copper complexes. These cycles will use very low concentrations to ensure a deep light penetration, photochemically start (triggering of the reaction) and be followed by the diffusion of reactive species in nonilluminated areas.

Cu(I) + ROOH → Cu(II) + RO• + OH−

(r1)

Cu(II) + ROOH → Cu(I) + ROO• + H+

(r2)

Therefore, our idea is (i) to combine a Cu(I)/ROOH redox system with the recently proposed18 Cu(I)/Iodonium salt photoinitiating system that has been developed recently18 and (ii) to add a reducing agent being able to favor reaction r1. The expected mechanism is described in Scheme 1. The Cu(I) complex is excited and produces phenyl radicals in the presence B

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Macromolecules Scheme 2. Chemical Compounds Used for the Catalytic Cycles

of diphenyl iodonium hexafluorophosphate (Iod) reactions r5 and r6. The Cu(II) form is generated and its interaction with the reducing agent leads to the regeneration of Cu(I). Here, Cu(neo) (DPEphos)]BF4 is used as the Cu(I) source, tin(II) or Bi(III) derivatives as reducing agents and Iod as photoredox co-initiator. Hydroperoxides (R′OOH) are formed in the medium during the polymerization under air and act as oxidizing agents (Scheme 1A where R• stands for the radicals being able to initiate FRP: R• = Ar• or R′O• when using Iod or a hydroperoxide, respectively). R′O• radicals are generated in the irradiated area according to the following reactions: (i) in the excited state, the Cu(I)/Iod system forms Ph•, (ii) macroradicals are produced during the polymerization propagation, (iii) the interaction of all radicals produced (R′•) with oxygen yields peroxyl radicals reaction r3) and then R′OOH (reaction r4. The diffusion of these latent species from the irradiated areas to the shadowed areas allows a redox initiation (without light) from the ROOH/Cu(I) interaction. Therefore, this system can use low Cu(I) concentrations, allowing light to deeply penetrate into the resin (no or limited inner filter effect) leading to the formation of hydroperoxides that can diffuse and decompose in the shadowed areas. Therefore, the polymerization of thick materials or the polymerization in shadowed areas can be envisioned. R′• + O2 → R′OO•

(r3)

R′OO• + R − H → R′OOH + R•

(r4)

Scheme 3. Composition of the Model Methacrylate Model Resin

Diphenyliodonium hexafluorophosphate (Iod), N-tert-butyl-α-phenylnitrone (PBN), and cumene hydroperoxide (R″OOH), were purchased from Sigma-Aldrich. Toluene and dichloromethane (DCM) were purchased from Carlo Erba. Tin(II) 2-ethylhexanoate (Tin(II)) and bismuth(III) 2-ethylhexanoate (Bi(III)) were purchased from Alfa Aesar. Barium glass fillers (average diameter of 400 nm) were used for the filled samples in the preparation of composites. A Cu(I) bearing one diphosphine ligand (bis[2(diphenylphosphino)phenyl] ether or DPEphos) and one diamine ligand (neocuproine) was synthesized as already reported in the literature.18,24 It will be referred as Cu(I) or [Cu(neo)(DPEphos)]BF4 in the paper. This particular Cu(I) was used as the Cu(I)*/Iod photoinitating performances were already very good for thin samples photopolymerization.18 However, for this latter system, no polymerization in shadowed areas was found. Interestingly, 6 month stability of the Cu(I)/iodonium salt system was previously demonstrated. The efficiency of the different redox systems was checked in a methacrylate mixture with a low viscosity of 0.053 Pa.s containing 33 wt % of 1,4-butanediol-dimethacrylate (1,4-BDMA), 33 wt % of hydroxypropyl methacrylate (HPMA), 33 wt % of urethanedimethacrylate oligomer. All the monomers were obtained from Sigma-Aldrich. This reference resin will be referred as model resin (Scheme 3). All formulations were prepared from the bulk resin and all photopolymerizations were carried out at room temperature (21−23 °C) under air. 2.2. RT-FTIR spectroscopy. A Jasco 6600 real-time Fourier transformed infrared spectrometer (RT-FTIR) was used to follow the CC double bond conversion versus time for polymerizations of 1.4 mm thick samples. The evolution of the near-infrared methacrylate CC double bond peak was followed from 6130 to 6200 cm−1. A LED@405 nm (Thorlabs) having an adjustable intensity range of 0− 200 mW/cm2 at the sample position was used for the photopolymerization experiments. The emission spectrum is already available in the literature.25

We will focus the paper on highlighting the efficiency of the reducing agent in catalytic redox cycles, the photocuring of thick filled samples and the photopolymerization beyond the irradiated area in shadowed areas (lateral polymerization). The redox cleavage of a model hydroperoxide (Cumene hydroperoxide = R″OOH in Scheme 2) in the presence of Cu(I) will be studied to explain the propagation of the reaction. At last, since this photoinitiating system is being able to work under low light intensities, its ability to induce a radical polymerization under sunlight exposure will be checked. Steady state photolysis and ESR spin trapping experiments will help to support the proposed mechanisms.

2. EXPERIMENTAL SECTION 2.1. Chemical Compounds. All the reactants were used as received and are summed-up in Schemes 2 and 3. C

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Figure 1. Photolysis in DCM of 0.44 mM Cu(I) with 10 mM Iod with a LED@405 nm: (A) without tin(II) and (B) in the presence of 66 mM tin(II). (C) Cu(I) consumption (calculated at 380 nm) for the chemical system presented in part B. [Cu(neo) (DPEphos)]BF4/tin(II)/Iod formulation, 45 wt % of fillers was added and 5 min of manual mixing was performed in order to homogenize fillers/resin formulation. This formulation was then inserted into the Pasteur glass pipet. Raman Analysis. A 50× LW objective was used in combination with a confocal hole aperture of 200 μm. The sample was deposited on its side on the microscope stage and the side surface of the sample was analyzed at different distance from the irradiated surface: 0.1 cm; 1.5 cm; 3 cm; 4.5 cm; 6 cm; 7.5 cm; 8.5 cm. Three spectra were recorded for each distance (with a rotation of around 120° of the polymer sample to get a mean value over the circumference) and the standard deviation of the conversion values was considered as the experimental uncertainty. 2.5. Monitoring of the Photopolymerization Propagation of Very Thick Samples through Thermal Imaging (Figure 5). An infrared thermal imaging camera (Fluke TiX560) was used to monitor the propagation of the photopolymerization over 8.5 cm in the filled sample. Fluke SmartView4.1 software was used to present the images. Thermal and spatial resolution are ±1 °C and about 250 μm respectively, at a 40 cm distance. 2.6. Photopolymerization beyond the Irradiated Area. The protocol to illustrate the photopolymerization without light (or beyond the irradiated area) was directly inspired from the work of AguirreSoto et al.11 Dimensions of the uncured sample were 32 mm ×18 mm with a thickness of 500 μm. About 3 mm of the samples were irradiated by a LED@405 nm, the rest of the samples was protected from the light exposure by an aluminum mask. As proposed by Aguirre-Soto et al., camphorquinone (CQ, Aldrich) combined with ethyl 4-(dimethylamino)benzoate (EDB, Lamberti) was used as a reference for a photopolymerization process that does not lead to a polymer in shadowed areas. 2.7. UV−Visible Absorption Spectroscopy. UV−vis absorption spectra in DCM were acquired using A Varian Cary 3 spectrophotometer. Photolysis was performed using LED@405 nm also used in

2.3. Sunlight Induced Photopolymerization of Methacrylates. Sunlight induced photopolymerization (Figure 3, parts Band C) was performed in Mulhouse (France, N 47° 43′ 47.319″; E 7° 18′ 33.7536″) on the 18th of January 2017 at 1:30 pm, which can be considered as a sunny day. Sunlight was simply redirected from the horizon using a planar mirror (Figure S1) behind a glass window. A light intensity (from 190 to 25000 nm) of 110 mW/cm2 at the sample position was recorded using a Thorlabs PM100D- S310C digital optical powermeter. It was possible to analyze the sun emission spectrum (behind the glass window) from 200−1021 nm using a Thorlabs Compact Spectrometer CCS200/M (Figure S1D, showing the spectrometer limitation in the IR region). Using this curve, the light intensity between 200 and 450 nm (absorption band of Cu(I)) is less than 6 mW/cm2. The conversion was followed using RT-FTIR as described in b) but without LED. 2.4. Very Thick Polymer Analysis (z-Profile) through Raman Confocal Microscopy. 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. Carbonyl peak area (1690 to 1755 cm−1) was measured and used as an internal standard while the alkene peak area (1630 to 1660 cm−1) was used to determine the CC conversion. Spectra were recorded onto uncured monomer to be used as a reference for double bond conversion. Analysis of the Conversion on a Very Thick Sample (8.5−9 cm, Figures 4 and 5). Sample Preparation. A Pasteur glass pipet having an approximate height of 8.5 to 9 cm was sealed at the bottom in order to introduce about 3 g of [Cu(neo)(DPEphos)]BF4/tin(II)/Iod formulation. Irradiation from the top by a LED@405 nm (Thorlabs; light intensity aroundt 230 mW/cm2 at the surface of the sample) was performed from the top during an arbitrary time of 4 min. After curing, the glass pipet was broken and the obtained polymer sample (thickness: ∼8.5−9 cm) was analyzed. A similar protocol was used for filled photopolymer (Figure 5); after preparation of the containing D

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Macromolecules RT-FTIR experiments with a fixed distance between the irradiation source and the sample. The [Cu(neo) (DPEphos)]BF4 consumption was calculated from the height of the 380 nm absorbance peak. 2.8. Electron Spin Resonance−Spin Trapping (ESR−ST). Electron spin resonance experiments were carried out using a Xband spectrometer (Bruker EMXplus Biospin). The radicals were observed at room temperature under air in DCM/toluene (15/85 in volume) solution. ESR-ST experiments were realized using PBN as a spin trapping agent in a similar way as described in other works.18 ESR spectra simulations were carried out using the WINSIM software.

3. RESULTS AND DISCUSSIONS 3.1. The Cu(I)/Iod and Cu(I)/Iod/Tin(II) Based Catalytic Cycles To Enhance Radical Generation upon Light Exposure. For simplification purposes, the copper complex [Cu(neo) (DPEphos)]BF4 will be noted Cu(I) in the following paper. Figure 1 reports the photolysis of Cu(I) in the presence of Iod. Without reducing agent (Figure 1A), the conversion of Cu(I) to Cu(II) is fast in agreement with r5−r6 as previously reported.18 The effect of the ligand structure on the photoinitating ability of Cu(I) species in (r6) has been investigated for other Cu(I).24 A steady value of the absorption value is reached after about 40 s of photolysis (Figure 1C). In contrast, in the presence of tin(II) as reducing agent, the UV− visible spectrum shows very weakly changes after 100 s (Figure 1B). Indeed, the consumption of Cu(I) in the presence of tin(II) (Figure 1C) is very slow compared to the one of the system without tin(II) during the photolysis suggesting a regeneration of Cu(I) in full agreement with Scheme 1 (see also r7). This is totally different from other depths/conversion studies1 where once the photoinitiator is consumed, it cannot be regenerated. Here, it can be concluded that the introduction of tin(II) allows an extended reactivity even for a limited concentration of Cu(I) or at low light intensity. Reactions r5, r6 and r7 account for the observed results; r5 and r6 were previously published.18 Cu(I) → *Cu(I)

Figure 2. Photopolymerization profiles (methacrylate function conversion vs irradiation time) measured in RT-FTIR for the model resin, 1.4 mm thick samples, in air, upon LED@405 nm irradiation. Initiating system: Cu(I) = 0.3 wt % with 0.85 wt % Iod. Without tin(II): upon 200 mW/cm2 irradiance (green) and upon 35 mW/cm2 irradiance (black curve). In the presence of 6 wt % tin(II) and upon 35 mW/cm2 irradiance (blue).

ance upon low irradiance (for example in deep polymerization samples) will be better and second the use of small amounts of Cu(I) will be possible. This last point is interesting for the photopolymerization of very thick samples: upper layers hardly absorb light as the absorption is proportional to the concentration of Cu(I) (Beer−Lambert’s law) limiting the inner filter effect that is encountered in traditional PIS. It was therefore decided to try to photopolymerize a 9 cm thick sample following a protocol recently published.26 Further investigations of photopolymerizations of methacrylates at low irradiance are proposed in Figure 3. At a relatively low irradiance (35 mW/cm2) the differences between Cu(I)/ Iod (curve 3), Cu(I)/Bi(III)/Iod (curve 2) and Cu(I)/tin(III)/ Iod (curve 1) PIS are illustrated in Figure 3A. Adding Bi(III) as a reducing agent does not significantly improve the Cu(I) regeneration and the photopolymerization efficiency remained at the same level (FC ∼ 70% after 100 s) than the initial Cu(I)/ Iod PIS (FC ∼ 65% after 100 s). On the contrary, the Cu(I)/ tin(III)/Iod system is very fast and strongly efficient (FC ∼ 82% after 28 s). Catalytic cycle is therefore much more efficient using the oxidation reaction tin(II) to tin(IV) than the Bi(III) to Bi(V) one. Also, as the irradiance of the LED@405 is tunable, an attempt of very low irradiance (2.5 mW/cm2) photopolymerization is shown in Figure 3A, curve 4. As a noteworthy point, it is possible to obtain a significant photopolymerization of methacrylates at this very low irradiance (FC ∼ 55% after 150 s). This outstanding performance is very important for a future practical use of this new PIS due to the very low energy consumption of the LED. 3.2.2. Photopolymerization upon Sunlight. To go even further in the way to use costless and sustainable light emission sources, the sun was considered. It delivers a limited irradiance in the absorption band of Cu(I) (less than 6 mW/cm2 from 350 to 450 nm). Indeed, few studies27,28 show efficient PIS under sunlight exposure. The sun has the advantages of being a cheap and inexhaustible emission source and can find use in outdoor applications (e.g., for paint drying). In the literature,

(r5)

*Cu(I) + Ar2I+ → Cu(II) + Ar2I• → Cu(II) + Ar • (r6)

Cu(II) + 1/2tin(II) → Cu(I) + 1/2tin(IV)

(r7)

3.2. Photopolymerization under Low Light Intensities and Access to Very Thick Samples. 3.2.1. Role of the Light Intensity. The Cu(I)/Iod PIS was already compared in the literature to reference PISs such as CQ/amine, CQ/Iod or even BAPO.18 Its performance was almost similar or even better for the photopolymerization of trimethylolpropane triacrylate (TMPTA). In Figure 2, an outstanding performance for the photopolymerization of the model resin under air (1.4 mm thick samples) is obtained for an irradiance of 200 mW/cm2 using the LED@405 nm i.e. a final methacrylate function conversion (FC) of 82% is obtained after 40 s. Nevertheless, when irradiance is lowered to 35 mW/cm2, the performance was significantly reduced (FC of 60% after 240 s). Interestingly, when tin(II) is introduced in order to regenerate Cu(I) in a three-component Cu(I)/Iod/tin(II) system (as demonstrated in Figure 1), it was possible to obtain again a high performance (FC = 83% after 40 s) but now upon a very low irradiance of 35 mW/cm2, i.e., the Cu(I)/Iod/tin(II) PIS with 35 mW/cm2 exhibits the same performance than the Cu(I)/Iod PIS for 200 mW/cm2. This performance opens new possibilities for the Cu(I)/Iod/tin(II) PIS: first, the performE

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Figure 3. Photopolymerization profiles (low irradiances) measured in RT-FTIR for the model resin, 1.4 mm thick samples, under air, room temperature. Initiating system: Cu(I) = 0.2 wt % with 1.0 wt % Iod; tin(II) or Bi(III) (if added) = 5 wt %. (A) LED@405 nm (1−3): irradiance = 35 mW/cm2; (1) Cu(I)/tin(II)/Iod; (2) Cu(I)/Bi(III)/Iod; (3) Cu(I)/ Iod; (4) irradiance= 2.5 mW/cm2, Cu(I)/tin(II)/Iod. (B, C) Sunlight; irradiance