Benzoyl Peroxide Redox

Article pubs.acs.org/Macromolecules

On-Demand Visible Light Activated Amine/Benzoyl Peroxide Redox Initiating Systems: A Unique Tool To Overcome the Shadow Areas in Photopolymerization Processes P. Garra,† F. Morlet-Savary,† C. Dietlin,† J. P. Fouassier, and J. Lalevée*,† †

Institut de Science des Matériaux de Mulhouse IS2M, UMR CNRS 7361, UHA, 15, rue Jean Starcky, 68057 Mulhouse, Cedex, France S Supporting Information *

ABSTRACT: A novel way to counterbalance the drawbacks and advantages of photopolymerization and redox polymerization is proposed. Photoinitiating systems based on charge transfer complexes (CTC) between a phosphine and an iodonium salt as well as an amine and iodonium salt are checked for the free radical polymerization of methacrylates. Polymer materials as thick as 8.5 cm can be produced under air upon exposure to a LED at 405 nm using an amine/phosphine/iodonium three-component photoinitiating system. When combined with benzoyl peroxide (BPO), these systems are active in photoinduced redox polymerization; i.e., the CTC is used to absorb the light and generates initiating radicals and the amine/BPO couple allows a redox polymerization. Therefore, a fast/slow time control of the radical reaction is feasible under air, and an excellent monomer conversion is obtained both at the top layer and in the core of the sample. The efficiency of this photoactivated redox system is assessed in terms of monomer conversion (followed by FTIR and RAMAN analysis) and time control. The chemical mechanisms are studied using UV−vis absorption spectroscopy, steady-state photolysis, 31P NMR, and ESR-spin trapping experiments.

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INTRODUCTION Redox and UV curing are two different techniques widely used in material science and industry to initiate free radical polymerization reactions. The main advantages of UV curing are a high surface polymerization efficiency coupled with a temporal and spatial control. However, the main issue for the curing of thick samples is the light penetration through the entire polymerizable media1 (Scheme 1, top left). In redox curing, when reducing and oxidizing agents are mixed (two-component systems), no time control is possible to accelerate the gel time. Moreover, redox curing only ensures a low top surface conversion because of the oxygen inhibition effect (Scheme 1, top right). The present paper will introduce a hybrid strategy that is based on redox systems that can also be photoactivated under air in order to shorten and control the gel time and to allow both a surface cure and a body cure (Scheme 1, bottom). In this hybrid approach, the combined advantages of both processes (redox and light) can be obtained and polymerization can also occur in the shadow areas or for samples with a low light penetration (filled, dispersed, or pigmented media).2 Since its early discovery in the 1950s,3,4 the amine/benzoyl peroxide (BPO) redox system has been widely used to initiate the free radical polymerization (FRP) of many monomers5−10 and has found applications in many fields such as biomaterials,11,12 orthopedic medicine,13 bone cements,14 and also selfhealing applications.15 Some examples combining light and redox curing mode capabilities were also reported. This results most of the time in better flexural strength of the produced polymers16,17 which make this combination quite interesting. © XXXX American Chemical Society

This can be illustrated by the camphorquinone/amine photoinitiating system mixed with an amine/BPO redox system.11 In a pure photoactivated approach, particular charge transfer complexes (CTC) photoinitiating systems (PIS) related to morpholine/bromine,18 morpholine/sulfur dioxide,18 amine/ Iod couples,18,19 or dye/amine/Iod systems19 were reported in the literature. CTCs have been also invoked in the twocomponent systems, the situation being different in dye-based three-component systems.20 For example, in the amine/alkyl iodide couple, a CTC is responsible for the light absorption properties. Radicals are generated, and a photoinduced reversible complexation mediated polymerization is possible.18 However, for all these systems, the efficiency of the photopolymerization was very limited (e.g., 16% monomer conversion after 80 min of irradiation of MMA (2.8 M) in a deoxygenated CH3CN/H2O solution using triethylamine/iodonium salt under a high pressure mercury lamp).20 Here, in order to combine a photochemical process and a redox process in polymerization reactions (see Scheme 1), we propose new efficient (photo)initiating systems containing an amine/iodonium salt, a phosphine/iodonium salt, or a amine/ phosphine/iodonium system (responsible for the photoinitiation) mixed with benzoyl peroxide (BPO) (for the redox process). These systems will operate according to Scheme 2. Received: October 4, 2016 Revised: November 21, 2016

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DOI: 10.1021/acs.macromol.6b02167 Macromolecules XXXX, XXX, XXX−XXX

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analyzed through FTIR or Raman spectroscopy. The presence of charge transfer complexes (CTC) will be investigated using steady-state photolysis, ESR spin trap, 31P NMR, and UV−vis spectroscopy. The chemical mechanisms of the new (photo/ redox) initiating systems will be discussed in detail.

Scheme 1. Main Features of Photopolymerization (top left), Photoactivated REDOX Polymerization (bottom), and Redox Polymerization under Air (top right)

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EXPERIMENTAL SECTION

Chemical Compounds. All the reactants were used as received and are summed up in Scheme 3. Diphenyliodonium hexafluorophosphate (Iod or Ph2I+), N-tert-butylα-phenylnitrone (PBN), 4-hydroxy-TEMPO (tempol), 2-diphenylphosphinobenzoic acid (2-dppba), 4-diphenylphosphinobenzoic acid (4-dppba), 4,N,N-trimethylaniline (4,N,N-TMA), 2-(diphenylphosphino)benzaldehyde (2-dppbald), and (R,R)-DACH-naphthyl Trost ligand (trost) were purchased from Sigma-Aldrich. Dichloromethane (DCM), tetrahydrofuran (THF), and toluene were obtained from Carlo Erba. The efficiency of the different redox systems was checked in two methacrylate-based formulations. The first methacrylate resin noted resin 1 (Scheme 3) consists of 1/3 (w/w) 1,4-butanediol dimethacrylate, 1/3 (w/w) hydroxypropyl methacrylate (HPMA), and 1/3 (w/w) of a representative urethane dimethacrylate (all obtained from Sigma-Aldrich). The second methacrylate blend, noted resin 2 (Scheme 3), is composed of a BisGMA/TEGDMA (30/70 wt %) blend having a much higher viscosity than resin 1.22 All redox formulations were prepared from the bulk resins in two separate cartridges at room temperature (RT) (22−24 °C): a first component containing the oxidizing agents (BPO and/or Iod)with eventually a chemical controller for the gel time (tempol)and another component containing the phosphine and/or the amine. A 1:1 Sulzer mixpac mixer was used to mix both components together at the beginning of each polymerization experiment. All the used molar concentrations are given in the Supporting Information (Table S1). Polymerization in Bulk Followed by Optical Pyrometry. The use of optical pyrometry to follow photopolymerization reactions was developed by Crivello et al.23,24 A rather similar setup was used here: temperature versus time profiles were also followed using an Omega OS552-V1-6 industrial infrared thermometer (Omega Engineering, Inc., Stamford, CT) having a sensitivity of ±1 °C for 2 g (sample thickness ∼4 mm). This setup was also used to monitor the T (°C) vs time upon irradiation with a LED at 405 nm. RT-FTIR Spectroscopy. A Jasco 6600 real time Fourier transform infrared spectrometer (RT-FTIR) was used to record the CC double bond conversion versus time (sample thickness = 1.4 mm). The evolution of the near-infrared methacrylate CC double bond peak was followed from 6130 to 6200 cm−1. Resin 1 or 2 being based on multifunctional monomers, the conversion that is mentioned in the figures does not correspond to the monomer conversion but to the conversion of the polymerizable functions. For example, converting 50% of these functions corresponds thus already to a higher monomer conversion (order of magnitude: 75% for a difunctional monomer and 87.5% for a trifunctional one). A LED at 405 nm (Thorlabs; light intensity about 110 mW/cm2 at the sample) and a LED at 470 nm (Thorlabs; ∼80 mW/cm2) were used for irradiation. The emission spectra are already available in the literature.25,26 Surface and 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 (1690−1755 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 onto uncured monomer to be used as references for double bond conversion. Two methodologies have been used to analyze the samples: Analysis of the Surface Cure of a Sample through z-Profile (Figure 9E). A 100× objective was used in combination with a confocal hole aperture of 200 μm giving an axial resolution of 2.3 μm. Objective displacement in the air was multiplied by a factor of 1.7 following a

Scheme 2. Strategy of Photoactivated Redox Polymerization for This Studya

a

In purple, the CTC light active systems (amine charge transfer complex with iodonium salt) especially efficient at the top surface; in orange, the classical redox BPO/amine reference robust in deep and light unexposed parts of the curing resin.

Several phosphines will be tested (Scheme 3). CTCs based on phosphines have not yet been considered as photoinitiators for FRP. Phosphines can help at least to overcome the oxygen inhibition effect in aerated media through peroxyl radical dissociation.21 Free radical photopolymerizations and photoactivated redox polymerizations in thick (1.4 mm and 8.5 cm) methacrylate-based formulations will be carried out under air and B


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Macromolecules Scheme 3. Chemical Compounds Studied for Photopolymerization, Photoactivated Redox Polymerization, and Redox Polymerization in the Two Methacrylate Resins (1 and 2)

protocol available in the literature27−30 to access the “real” depth in the photopolymerized sample considering a refraction index close to 1.5. Analysis of the Conversion on a Very Thick Sample (8.5 cm, Figure 8). Sample Preparation. A glass Pasteur pipet having an approximate height of 9 cm was sealed at the bottom in order to introduce about 3 g of amine, trost, and Iod formulation. Irradiation by LED at 405 nm (Thorlabs; light intensity about 110 mW/cm2 at the sample) was performed from the top during an arbitrary time of 20 min. After curing, glass was broken, and the obtained polymer sample (thickness: ∼8.5 cm) was analyzed. 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, 1.5, 4.5, 6, and 7.5 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. Redox Photopolymerization under Light Exposure in the Presence of a Light Obstacle (Shadow Area). The protocol to illustrate redox photoactivated polymerization was directly inspired by the work of Aguirre-Soto et al.31 Dimensions of the polymerization zone were 32 mm × 18 mm with a thickness of 500 μm. About 3 mm of the samples were irradiated by LED at 405 nm; the rest of the sample was not exposed to light (using an obstacle; Schemes 1 and 2). As proposed by Aguirre-Soto et al., camphorquinone (CQ, Aldrich) combined with

amine ethyl 4-(dimethylamino)benzoate (EDB, Lamberti) was used as a reference photoinitiating system for pure photopolymerization process. UV−Vis Absorption Spectroscopy. A Varian Cary 3 spectrophotometer was used for recording the UV−vis absorption spectra in THF or in DCM (low polarity solvent used to specifically highlight the charge transfer complex generated between the amine and the iodonium salt). Nuclear Magnetic Resonance (NMR 31P). Nuclear magnetic resonance (NMR 31P) was recorded in CH2Cl2 solution on a Varian Mercury spectrometer at 121 MHz. Electron Spin Resonance (ESR) Spin Trapping (ESR-ST). Electron spin resonance−spin trapping experiments were carried out using an X-band spectrometer (Bruker EMX-plus Biospin). The radicals were observed under nitrogen saturated media at room temperature. Ntert-butyl nitrone (PBN) was used as a spin trap in a mixture of dichloromethane and toluene (25/75 w/w %), similarly to other studies.32 ESR spectra simulations were carried out using WINSIM software.

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RESULTS AND DISCUSSION The first part of this study will be focused on the development of light active charge transfer complexes (CTC) that can produce radicals upon visible LED irradiation. In a second part, the formation of these CTCs will find a direct application as photoinitiating systems (PIS) for free radical photopolymerization of methacrylates. In a third part, these systems will be used to C


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Figure 1. (A) UV−vis absorption spectra in DCM of 58 mM 4,N,NTMA (black), 35 mM Iod (blue), and 58 mM 4,N,N-TMA + 35 mM Iod mixed (red). (B) Picture in DCM of 0.10 M Iod (left; a), 0.22 M 4,N,NTMA (right; c), and 0.05 M Iod mixed with 0.11 M 4,N,N-TMA (center; b). (C) UV−vis absorbance at 405 nm in DCM as a function of [IOD]/ [4,N,N-TMA] (mol/mol) for 29 mM 4,N,N-TMA and increasing amount of Iod. Figure 2. 31P NMR in CH2Cl2 of (A) 0.23 mM 2dppba alone (dark) and 0.32 mM Iod alone (red), (B) a mixture of [2dppba] = 0.12 mM and [Iod] = 0.15 mM, and (C) the same mixture after LED at 405 nm irradiation for 1 h.

polymerize very thick samples and to produce photopolymers showing depth such as 8.5 cm. Finally, benzoyl peroxide (BPO) will be introduced to generate redox initiating properties in combination with the amine already used in the CTC. Remarkably, these CTC/BPO systems will be characterized by both redox and photochemical activities. Light Active Charge Transfer Complexes: Amine/Iod, Phosphine/Iod, and Amine/Phosphine/Iod. Charge Transfer Complex (CTC) between 4,N,N-TMA and Iod. Most of the time, amines are seen as hydrogen donors in photoinitiating systems (PIS); i.e., they are called co-initiator. They have to bear a labile hydrogen in α position and can quench the excited state of ketones as for example in the camphorquinone/amine couple.26 A good reactivity of three-component systems such as ketone/amine/onium salt18 was also encountered in the literature. Amines have usually low molar extinction coefficients around 405 nm (see Figure S1). Iod does not absorb at all. Therefore, this cannot explain the photolysis of an amine/Iod couple under soft LED at 405 nm irradiation.

CT complexes between a diaryliodonium salt and an electron donor (e.g., an aromatic nucleophile) is already known in the literature in organic synthesis.33 The amine/alkyl iodide interaction was already highlighted in ref 34 using UV−vis spectroscopy. Here, the formation of a charge transfer complex between 4,N,N-TMA and Iod is easily outlined by UV−vis spectroscopy. Indeed, Figure 1A shows that 4,N,N-TMA or Iod in DCM does not absorb light below 380 nm. On the opposite, when mixed together, the solution turns yellow (Figure 1B), and the formation of a strongly absorbing complex is observed (Figure 1A). The formation of such a complex is strongly related to the concentration of both the electron donor (amine) and the acceptor (Iod) but also to the reaction medium. Figure 1C illustrates the change of absorbance at 405 nm as a function of [Iod]/[4,N,N-TMA]. In this figure, when amine concentration in constant, the absorbance increases strongly with both [Iod] and [Iod]/[4,N,N-TMA] clearly showing the formation of a D


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Figure 3. (A) Steady-state photolysis using LED at 405 nm (magenta) in THF for 46 mM 4,N,N-TMA (amine) with 20 mM 2dppba and 15 mM Iod (time following increasing arrow). (B) Steady-state photolysis using LED at 405 nm (magenta) in THF for 20 mM 4,N,N-TMA (amine) with 4 mM trost and 6 mM Iod (time following increasing arrow).

Figure 4. Black: ESR spectra after 10 s LED at 405 nm irradiation of the amine/trost/Iod system in the presence of PBN (as spin trap agent) in toluene/DCM (75/25 w/w). Red: simulation using PBN adduct (aN = 14.5 G; aH = 2.3 G).

Figure 5. Optical pyrometric measurements (sample T (°C) vs irradiation time) upon LED at 405 nm irradiation of resin 1 for 1.0 wt % 4,N,N-TMA + 1.3 wt % Iod (1, dark yellow star); 1.4 wt % 2dppba + 1.3 wt % Iod (2, right pink triangle); 1.0 wt % 4,N,N-TMA + 1.4 wt % 2dppba (3, blue triangle left); 2.8 wt % 2dppba (4, dark squares); 2.6 wt % Iod (5, green triangle down); 2.0 wt % 4,N,N-TMA (6, red triangle up). The irradiation starts for t = 0 s; 4 mm thick samples.

Scheme 4. Proposed CTC Interactions between Amine/Iod and Phosphines/Iod and Its Cleavage by Visible Light

from this CTC equilibrium as the absorption by the superior layers remains probably low. Charge Transfer Complex (CTC) between Phosphine/Iod: An 31P NMR Investigation. The formation of the CT complex in phosphine/Iod is hardly observed by UV−vis spectroscopy (the spectrum for the blend is very similar to the added spectra of the starting compounds; Figure S1). Thus, phosphorus NMR (31P) was used. The 31P NMR spectrum of 2dppba alone is plotted in Figure 2A. A single peak at −9.0 ppm is found which is the same as the one from the literature.35 In the Iod 31P NMR spectrum (coming from the PF6− ion), some multiplets are noted around −151 ppm. The fluor/phosphorus coupling constant is 1JPF = 708 Hz, close to the values available in the literature.36 Mixing both compounds (Figure 2B) leads to the presence of a doublet centered at 28.0 ppm. In the literature, the 31P NMR peak from 2dppba was also shifted to higher chemical shifts at 15 ppm in the Ag−P complex37 with some doublet related to the Ag−P coupling. In the presence spectrum (Figure 2B), this doublet

CTC. This could also indicate that the stoichiometry of the acceptor−donor charge transfer complex is rather complex. A treatment for this CTC is proposed in the Supporting Information in which a constant competition between the formation of the CTC and its solvation by the solvent (DCM) is taken into account. The equilibrium constant (Kctc) is found to be 6.1, and εCTC (at 405 nm, in DCM) = 280 M−1 cm−1 (Figure S2). Kctc is not very high, which is interesting for photopolymerization reactivity. If some CTC is photolyzed, the equilibrium pushes some amine still free to generate a new CTC without having a high initial epsilon (the free amine does not absorb at 405 nm; see above). High light absorption by the superior layers provokes a decrease of the light penetration. Here one should expect an extended depth of photopolymerization E


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Figure 6. (A) Polymerization profiles (methacrylate functions conversion vs irradiation time) of resin 1 upon LED at 405 nm irradiation in the presence of 1.0 wt % Iod with 0.7 wt % 4,N,N-TMA + 1.4 wt % trost (1, green triangle down); 0.7 wt % 4,N,N-TMA + 1.2 wt % 2dppba (2, red triangle up); 1.2 wt % 2dppba (3, blue triangle left); 1.4 wt % trost (4, sky blue triangle right); and 0.7 wt % 4,N,N-TMA (5, dark square). (B) Polymerization profiles of resin 1 upon LED at 405 nm irradiation in the presence of 1.0 wt % Iod with 0.6 wt % 4,N,N-TMA + 0.9 wt % 2dppbald (3 dark squares); 0.6 wt % 4,N,N-TMA + 0.9 wt % 4dppba (2 green triangle down); and 0.7 wt % 4,N,N-TMA + 1.4 wt % trost (1 sky blue triangle left). Polymerization profiles upon LED at 470 nm irradiation of 1 wt % Iod and 0.6 wt % 4,N,N-TMA + 0.9 wt % 2dppbald (4 red triangle up) and 0.6 wt % 4,N,N-TMA + 0.9 wt % 4dppba (5 blue triangle right). (C, D) Photopolymerization profiles in the presence of 1.1% 4,N,N-TMA with 2.1 wt % Iod of resin 1 (C) upon LED at 405 nm irradiation (dark squares) and LED at 470 nm (red triangles) irradiation; resin 2 (D) upon LED at 405 nm irradiation (dark squares) and LED at 470 nm irradiation (red triangles). 1.4 mm thick samples.

Figure 7. (A) Polymerization profiles (methacrylate functions conversions vs irradiation time) of resin 2 upon LED at 405 nm irradiation in the presence of 2.9 wt % Iod with 0.9 wt % 4,N,N-TMA and 3.0 wt % trost (dark squares) and after 35 days storage at 5 °C (red triangle). (B) Polymerization profiles of resin 1 upon LED at 405 nm irradiation in the presence of 1.3 wt % Iod with 0.4 wt % 4,N,N-TMA and 1.5 wt % 2dppba (dark square) and after 17 days storage at 5 °C (red triangle). 1.4 mm thick samples.

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Figure 8. (A) Experimental setup used for the photopolymerization of the resin 1 (thickness = 8.5 cm) (top: LED at 405 nm; down: Pasteur pipet with polymerizable medium containing 0.25 wt % of 4,N,N-TMA/0.95 wt % Iod/2.0 wt % trost). (B) Pasteur pipet after 20 min of irradiation. (C) 8.5 cm cured polymer after breaking the glass surrounding the polymer. (D) Raman conversion as a function of the distance from the top surface (irradiated zone).

(1JPI = 1123 Hz) may be owed to a coupling between iodide (127I) and phosphorus (31P) and can be ascribed to the presence of a phosphine/Iod CTC. Some remaining lone pair 2dppba 31P can be observed. The height of the new doublet formed did not change after 20 min or 20 h. This mixture was then photolyzed by the LED at 405 nm for 1 h (Figure 2C). Photolysis induced a significant decrease of the 2dppba signal in agreement with the photosensitivity of the 2dppba/Iod system. Therefore, it can be expected that the interaction between 2dppba and Iod results from an equilibrium through a charge transfer complex (CTC) between the phosphine and the iodonium salt. Steady-State Photolysis of the Amine/Phosphine/Iod CTC. The interaction in the three-component amine/phosphine/Iod system upon a visible light irradiation (405 nm) has not been reported yet. Steady-state photolysis (Figures 3A and 3B) in THF was carried out with the LED at 405 nm. The absorption is mainly due to the amine/Iod CTC (see above). Interestingly, a significant photolysis of 4,N,N-TMA/2dppba/Iod occurs. The bleaching is still faster in the 4,N,N-TMA/trost/Iod system (Figure 3). This shows that the three-component system remains photosensitive to irradiation at 405 nm. Free Radical Generation in the Amine/Iod or Phosphine/ Iod Couples: An ESR−Spin Trapping Investigation. ESR−spin trapping experiments in an irradiated amine/trost/Iod system (Figure 4; LED at 405 nm) show that phenyl radicals are produced (hyperfine coupling constants, hfc, of the phenyl/PBN adduct are aN = 14.5 G and aH = 2.3 G, in line with reference values38,39). Therefore, the mechanism (Scheme 4) involved a cleavage of the diphenyliodonium salt after electron transfer and the formation of a diphenyl iodide radical followed by the release of a phenyl radical. In this three-component system, the amine/ Iod and phosphine/Iod couples absorb the light at 405 nm (r1, r2) albeit the light absorption for the phosphine/Iod CTC remains lower than for the amine/Iod CTC. Photoinitiating Systems (PIS) for Free Radical Photopolymerization upon Visible Light Using Amine/Iod, Phosphine/Iod, and Amine/Phosphine/Iod Systems under Air. The polymerization profiles of the methacrylate

resin upon a LED at 405 nm irradiation are displayed in Figure 5 (for resin 1). Optical pyrometryic measurements were chosen as they offer a fast and reliable assessment of polymerization efficiency (through its exothermicity). No curing of the resin was obtained upon irradiation of 4,N,N-TMA/2dppba (Figure 5, curve 3), Iod alone (Figure 5, curve 5), 4,N,N-TMA alone (Figure 5, curve 6), and 2dppba alone (Figure 5, curve 4). Interestingly, combination of 4,N,N-TMA and Iod leads to a 85 °C exothermicity (Figure 5, curve 1). A higher gel time is observed with 2dppba/Iod, but the process is very efficient (Figure 5, curve 2; maximum temperature of 90 °C). RT-FTIR experiments were also carried out to follow the methacrylate function conversion during the polymerization of resin 1 or resin 2 (1.4 mm thick samples) (LED at 405 nm; Figure 6). In Figure 6A (curve 5), 4,N,N-TMA/Iod leads to a final methacrylate function conversion (FC) = 60% with a rather slow polymerization rate. The use of phosphines (Figure 6A, 2dppba in curve 3, trost in curve 4) and Iod results in longer gel times (about 100 s for 2dppba and trost), but quite good final methacrylate function conversions were reached (70%). Interestingly, a better reactivity is still obtained with threecomponent systems (i.e., when mixing an amine, a phosphine (trost in curve 1 or 2dppba in curve 2), and Iod) with FC = 85%. This positive effect of the phosphine when added to amine/Iod is explained by its dual behavior: (i) to overcome the oxygen inhibition and (ii) to form an additional CTC light active complex with Iod. Photopolymerization was also possible upon a 470 nm light exposure (Figure 6B). The photopolymerization of the two different resins in the presence of the 4,N,N-TMA/Iod couple was also studied using two different LEDs (Figures 6C and 6D). An excellent efficiency at 405 nm is noted in both resins (FC = 75%). The irradiation with LED at 470 nm leads to higher inhibition times and slower polymerization rates than at 405 nm. This may be ascribed to the lower absorption of the involved CTC at 470 nm vs 405 nm (Figure 1A). The efficiency of the amine/trost/Iod system (Figure 7A) is also very high in the BisGMA/TEGDMA (resin 2) (conversion of about 70%), and the time to reach the conversion plateau was G


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Figure 9. Polymerization profiles (methacrylate function conversion vs irradiation time) measured in RT-FTIR for resin 1, 1.4 mm thick samples, in air. Upon LED at 405 nm irradiation (red square) and without light sensitization (black triangle). Initiating systems: (A) 0.7 wt % 4,N,N-TMA (amine) mixed with 1.0 wt % BPO in the presence of 0.14 wt % tempol; (B) 1.4 wt % 4,N,N-TMA mixed with 1.1 wt % BPO and 2.6 wt % Iod in the presence of 0.05 wt % tempol; (C) 0.4 wt % 4,N,N-TMA (amine) and 1.8 wt % trost mixed with 1.0 wt % BPO and 2.3 wt % Iod in the presence of 0.10 wt % tempol; (D) 0.4 wt % 4,N,N-TMA (amine) and 1.3 wt % 2dppba mixed with 1.0 wt % BPO and 1.8 wt % Iod in the presence of 0.07 wt % tempol. (E) Raman surface analysis of the sample polymerized in (D).

as even after 35 days, the performance is the same. The amine/ 2dppba/Iod PIS exhibit a slow degradation after 17 days with a slight loss of the polymerization efficiency (Figure 7B).

lower than in resin 1 (Figure 6B). BisGMA/TEGDMA is much more viscous than resin 1 (see above) and hence less subject to oxygen inhibition. Interestingly, this system is particularly stable H


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Hydroxy-TEMPO (tempol) was added as stabilizer in these experiments to increase the gel time to better exemplify the effect of light. In Figure 9A, the very low absorption of BPO is enough upon light irradiation to shorten the gel time from 170 to 105 s (LED at 405 nm irradiation), and the final conversions are close (about 78%). A LED at 405 nm has a heating of 2 ± 1 °C/10 min (Figure S4) for the resin 1, which increases the decomposition of BPO by 4,N,N-TMA in a MMA medium by a factor of 0.007% according to Zoller et al.6 Therefore, the thermal heating will be considered as negligible. Amine/Iod/BPO Redox and Photosensitive System. Remarkably, when using the amine/Iod/BPO system (Figure 9B), the polymerization starts earlier upon light irradiation, i.e., the gel time being 40 s upon LED at 405 nm vs 100 s without light. Iod has a retarding effect on amine/BPO redox system as the amine is now partly involved in CTC with Iod and not free for the reaction with BPO (Figure S3 and Scheme 4). Amine/Phosphine/Iod/BPO Redox and Photosensitive System with True Fast/Slow Photoactivated Redox Properties. In Figure 9C, a first component containing 0.4 wt % 4,N,N-TMA and 1.8 wt % trost was mixed with a second component containing 1.0 wt % BPO and 2.3 wt % Iod. Upon irradiation, a considerable shortening of the gel time (10 s with light (fast mode) vs 305 s without light (slow mode)) and a better final conversion (about 89% vs 84%) are noted. For this system, the light was a true fast/slow button used to activate the curing. Finally (Figure 9D), the same holds true when changing trost for 2dppba (gel time: 39 s upon LED at 405 nm vs 130 s without light; final conversion: 88% vs 77%). The conversion reaches its maximum within only a few seconds, and therefore the photopolymerization can be considered as the most important process for this fast curing. As shown by the Raman analysis of the surface (Figure 9E), the inhibition layer was quite small, e.g., ∼55% methacrylate conversion with light activation vs 15% conversion at 40 μm in the absence of light. This strong improvement of curing of the surface in contact with air is ascribed to the light activation of the CTCs that generates additional initiating radicals that can overcome the oxygen inhibition. It was also possible to fully illustrate the concept of the redox photoactivated polymerization (Schemes 1 and 2) in Figure 10 for the polymerization even in the presence of a light obstacle. The experimental setup was directly inspired by Aguirre-Soto et al.31 A limited part of the sample was irradiated, and the rest of the sample was not exposed to light (shadow area). As explained in Scheme 1 and shown in Figure 10B, it is not possible to have any photopolymerization for classical PIS (CQ/EDB) in shadow areas. Therefore, the concept of redox photoactivated polymerization (Figure 10C) appears as an elegant way to have highly efficient curing in light irradiated parts (light on) but also efficient curing in shadow areas (light off) due to the redox activation.

Figure 10. (A) Experimental setup used to illustrate redox photoactivated experiment. A limited part (∼3 mm) of the sample was irradiated by LED at 405 nm (the obstaclealuminum layersinduces shadow areas). (B) Photopolymerization reference for resin 1; 500 μm thick sample, under air. CQ = 0.4%; EDB = 2.2% (the polymerization is found only in the irradiated area). (C) Redox photoactivated polymerization for resin 1; 500 μm thick sample, under air. 0.4 wt % 4,N,N-TMA (amine) and 1.3 wt % 2dppba mixed with 1.0 wt % BPO and 1.8 wt % Iod in the presence of 0.07 wt % tempol: a polymerization in both light on and light off areas is clearly observed.

In-Depth Photopolymerization of 8.5 cm Thick Samples. The first photopolymerization experiments reported above (Figures 6 and 7) were done using 1.4 mm thick samples which can be already considered as thick samples in the photopolymerization area. The molar extinction coefficients of the starting compounds as well as the associated CTCs being very low at 405 nm (Figure 3 and Figure S1), the polymerization of even thicker media should be possible. Figure 8 shows that the manufacture of a 8.5 cm thick polymer material using 4,N,NTMA/trost/Iod is feasible within 20 min of irradiation without additional redox activity. Confocal Raman microscopy allows following the conversion as a function of the depth. Conversions of 70 and 60% (Figure 8D) were obtained at the top of the sample (1 and 15 mm)the oxygen inhibition is not observed in this figure as the polymerization is very efficient, leading to an inhibited layer of less than 50 μm (e.g., Figure 9E); then the conversion reached an asymptote around 50% (4.5−7.5 cm from the surface; Figure 8D). Some drawbacks still exist: heterogeneity related to shrinkage inside the mold and a nonhomogeneous conversion throughout the entire polymerizable medium. A second experiment was carried out (Figure S5) using the same protocol but with a Pasteur pipet fully wrapped in black tape (to avoid any other light activation). A similar 8.5 cm photopolymer was obtained with the same heterogeneity. Nevertheless, the present result makes these systems particularly suitable (contrary to most of the developed PISs in the literature) for the polymerization of clear thick samples and can be a starting point to develop new types of applications where a deep curing is necessary. However, this photochemical CTC approach can hardly be used for samples with shadow areas or with low light penetration (filled, dispersed, or pigmented media) which is why a combined redox/photochemical approach will now be presented. Addition of BPO To Introduce a Redox Process. The redox interaction between an amine and BPO was already widely used in the literature. Amine/BPO is one of the most robust redox initiating system which explains its many uses in material science.5,7−9 The chemical mechanisms are now clear, and the reaction characteristics can even be predicted through simulation.6 Photosensitization of the Amine/BPO Reference under a LED Exposure. As shown in refs 40 and 41, BPO can be cleaved upon panchromatic Hg lamp irradiation. Figure 9A compares the thermal and the light activated amine/BPO redox system. 4-

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CONCLUSIONS A photoactivated redox polymerization of methacrylates under air is described. The new photoinitiating systems developed here are based on a phosphine/iodonium salt (Iod) CTC or an amine/Iod CTC or a combination of both (three-component amine/phosphine/Iod system). They allow producing very thick materials, up to 8.5 cm, under a LED at 405 nm irradiation. The introduction of benzoyl peroxide in the amine/phosphine/Iod combination allows turning to slow/fast polymerization mode in I


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Macromolecules

(13) Sideridou, I. D.; Achilias, D. S.; Karava, O. Reactivity of Benzoyl Peroxide/amine System as an Initiator for the Free Radical Polymerization of Dental and Orthopedic Dimethacrylate Monomers: Effect of the Amine and Monomer Chemical Structure. Macromolecules 2006, 39 (6), 2072−2080. (14) Achilias, D. S.; Sideridou, I. Study of the Effect of Two BPO/ amine Initiation Systems on the Free-Radical Polymerization of MMA Used in Dental Resins and Bone Cements. J. Macromol. Sci., Part A: Pure Appl.Chem. 2002, 39 (12), 1435−1450. (15) Wilson, G. O.; Henderson, J. W.; Caruso, M. M.; Blaiszik, B. J.; McIntire, P. J.; Sottos, N. R.; White, S. R.; Moore, J. S. Evaluation of Peroxide Initiators for Radical Polymerization-Based Self-Healing Applications. J. Polym. Sci., Part A: Polym. Chem. 2010, 48 (12), 2698−2708. (16) Lu, H.; Mehmood, A.; Chow, A.; Powers, J. M. Influence of Polymerization Mode on Flexural Properties of Esthetic Resin Luting Agents. J. Prosthet. Dent. 2005, 94 (6), 549−554. (17) Feng, L.; Suh, B. I. The Effect of Curing Modes on Polymerization Contraction Stress of a Dual Cured Composite. J. Biomed. Mater. Res., Part B 2006, 76 (1), 196−202. (18) Wiley: Photoinitiators for Polymer Synthesis: Scope, Reactivity, and Efficiency - Jean-Pierre Fouassier, Jacques Lalevée; http://eu.wiley. com/WileyCDA/WileyTitle/productCd-3527332103.html (accessed May 18, 2016). (19) Padon, K. S.; Scranton, A. B. A Mechanistic Investigation of a Three-Component Radical Photoinitiator System Comprising Methylene Blue, N-Methyldiethanolamine, and Diphenyliodonium Chloride. J. Polym. Sci., Part A: Polym. Chem. 2000, 38 (11), 2057−2066. (20) Junhui, H.; Erjian, W. Radical Photopolymerization Using Aryliodonium Salt/tertiary Amine Complex System as the Photoinitiator. Chin. J. Polym. Sci. (Engl. Ed.) January 1990, pp 36−43. (21) Lalevée, J.; Allonas, X.; Fouassier, J. P. New Access to the Peroxyl Radicals Reactivity? Chem. Phys. Lett. 2007, 445 (1−3), 62−67. (22) Gonçalves, F.; Kawano, Y.; Pfeifer, C.; Stansbury, J. W.; Braga, R. R. Influence of BisGMA, TEGDMA, and BisEMA Contents on Viscosity, Conversion, and Flexural Strength of Experimental Resins and Composites. Eur. J. Oral Sci. 2009, 117 (4), 442−446. (23) Falk, B.; Vallinas, S. M.; Crivello, J. V. Monitoring Photopolymerization Reactions with Optical Pyrometry. J. Polym. Sci., Part A: Polym. Chem. 2003, 41 (4), 579−596. (24) Crivello, J. V.; Lam, J. H. W. Redox Cationic Polymerization: The Diaryliodonium Salt/ascorbate Redox Couple. J. Polym. Sci., Polym. Chem. Ed. 1981, 19 (2), 539−548. (25) Dietlin, C.; Schweizer, S.; Xiao, P.; Zhang, J.; Morlet-Savary, F.; Graff, B.; Fouassier, J.-P.; Lalevée, J. Photopolymerization upon LEDs: New Photoinitiating Systems and Strategies. Polym. Chem. 2015, 6 (21), 3895−3912. (26) Morlet-Savary, F.; Klee, J. E.; Pfefferkorn, F.; Fouassier, J. P.; Lalevee, J. The Camphorquinone/Amine and Camphorquinone/ Amine/Phosphine Oxide Derivative Photoinitiating Systems: Overview, Mechanistic Approach, and Role of the Excitation Light Source. Macromol. Chem. Phys. 2015, 216 (22), 2161−2170. (27) Everall, N. J. Modeling and Measuring the Effect of Refraction on the Depth Resolution of Confocal Raman Microscopy. Appl. Spectrosc. 2000, 54 (6), 773−782. (28) Everall, N. J. Confocal Raman Microscopy: Common Errors and Artefacts. Analyst 2010, 135 (10), 2512−2522. (29) Shin, W. S.; Li, X. F.; Schwartz, B.; Wunder, S. L.; Baran, G. R. Determination of the Degree of Cure of Dental Resins Using Raman and FT-Raman Spectroscopy. Dent. Mater. 1993, 9 (5), 317−324. (30) Courtecuisse, F.; Dietlin, C.; Croutxé-Barghorn, C.; Van Der Ven, L. G. J. Depth Characterization of Photopolymerized Films by Confocal Raman Microscopy Using an Immersion Objective. Appl. Spectrosc. 2011, 65 (10), 1126−1132. (31) Aguirre-Soto, A.; Lim, C.-H.; Hwang, A. T.; Musgrave, C. B.; Stansbury, J. W. Visible-Light Organic Photocatalysis for Latent RadicalInitiated Polymerization via 2e−/1H+ Transfers: Initiation with Parallels to Photosynthesis. J. Am. Chem. Soc. 2014, 136 (20), 7418− 7427.

a few seconds with a quite good efficiency and reaching higher monomer conversions and better surface conversions. The development of other redox photosensitive systems is under way.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b02167. Table S1 and Figures S1−S5 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (J.L.). ORCID

J. Lalevée: 0000-0001-9297-0335 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Simon GREE (IS2M) for helpful support during the Raman experiments. REFERENCES

(1) Lee, J. H.; Prud’Homme, R. K.; Aksay, I. A. Cure Depth in Photopolymerization: Experiments and Theory. J. Mater. Res. 2001, 16 (12), 3536−3544. (2) Garra, P.; Dumur, F.; Morlet-Savary, F.; Dietlin, C.; Fouassier, J. P.; Lalevée, J. A New Highly Efficient Amine-Free and Peroxide-Free Redox System for Free Radical Polymerization under Air with Possible Light Activation. Macromolecules 2016, 49 (17), 6296−6309. (3) Horner, L.; Scherf, K. Ü ber Den Einfluß Der Substituenten Auf Die Chemische Reaktivität I. Der Zerfall von Dibenzoylperoxyd Durch Substituierte Tertiäre Amine. Justus Liebigs Ann. Chem. 1951, 573 (1), 35−55. (4) Lal, J.; Green, R. Effect of Amine Accelerators on the Polymerization of Methyl Methacrylate with Benzoyl Peroxide. J. Polym. Sci. 1955, 17 (85), 403−409. (5) Yang, H.; Zhang, L.; Ma, W.; Pu, H.; Gong, F. Polystyrene/ Attapulgite Nanocomposites Prepared via In Situ Suspension Polymerization with Redox Initiation System. J. Appl. Polym. Sci. 2015, 132 (9), 41567. (6) Zoller, A.; Gigmes, D.; Guillaneuf, Y. Simulation of Radical Polymerization of Methyl Methacrylate at Room Temperature Using a Tertiary amine/BPO Initiating System. Polym. Chem. 2015, 6 (31), 5719−5727. (7) Zhang, Y.; Gao, B.; Gu, L.; Zhao, X. Constructing Aryl Tertiary Amine-Bpo Redox Initiation System to Realize Highly Effective GraftPolymerization of Styrene on Silica Gel Particles. Acta Polym. Sin. 2012, No. No. 3, 264−271. (8) Worzakowska, M. The Influence of Tertiary Aromatic Amines on the BPO Initiated Cure of Unsaturated Epoxy Polyesters with Styrene Studied by Non-Isothermal DSC. J. Therm. Anal. Calorim. 2011, 105 (3), 987−994. (9) Misra, G. S.; Bajpai, U. D. N. Redox Polymerization. Prog. Polym. Sci. 1982, 8 (1−2), 61−131. (10) Sarac, A. S. Redox Polymerization. Prog. Polym. Sci. 1999, 24 (8), 1149−1204. (11) Kwon, T.-Y.; Bagheri, R.; Kim, Y. K.; Kim, K.-H.; Burrow, M. F. Cure Mechanisms in Materials for Use in Esthetic Dentistry. J. Invest. Clin. Dent. 2012, 3 (1), 3−16. (12) Sideridou, I. D.; Achilias, D. S.; Kostidou, N. C. Copolymerization Kinetics of Dental Dimethacrylate Resins Initiated by a Benzoyl Peroxide/amine Redox System. J. Appl. Polym. Sci. 2008, 109 (1), 515− 524. J


Article

Macromolecules (32) Xiao, P.; Dumur, F.; Zhang, J.; Fouassier, J. P.; Gigmes, D.; Lalevée, J. Copper Complexes in Radical Photoinitiating Systems: Applications to Free Radical and Cationic Polymerization upon Visible LEDs. Macromolecules 2014, 47 (12), 3837−3844. (33) Dohi, T.; Ito, M.; Yamaoka, N.; Morimoto, K.; Fujioka, H.; Kita, Y. Unusual Ipso Substitution of Diaryliodonium Bromides Initiated by a Single-Electron-Transfer Oxidizing Process. Angew. Chem., Int. Ed. 2010, 49 (19), 3334−3337. (34) Ohtsuki, A.; Goto, A.; Kaji, H. Visible-Light-Induced Reversible Complexation Mediated Living Radical Polymerization of Methacrylates with Organic Catalysts. Macromolecules 2013, 46 (1), 96−102. (35) Vasam, C.; Modem, S.; Kankala, S.; Kanne, S.; Budige, G.; Vadde, R. Synthesis Characterization and Hydroformylation Activity of New Mononuclear rhodium(I) Compounds Incorporated with Polar-Group Functionalized Phosphines. Open Chem. 2010, 8 (1), 77−86. (36) Honeychuck, R. V.; Hersh, W. H. Coordination Of“ noncoordinating” anions: Synthesis, Characterization, and X-Ray Crystal Structures of Fluorine-Bridged Hexafluoroantimonate (1-), Tetrafluoroborate (1-), and Hexafluorophosphate (1-) Adducts of [R3P (CO) 3 (NO) W]+. An Unconventional Order of Anion Donor Strength. Inorg. Chem. 1989, 28 (14), 2869−2886. (37) Miguel-Coello, A. B.; Bardají, M. Influence of the Substitution Pattern (Ortho versus Para) on the Structure and Luminescence of silver(I) Complexes Ligated by Diphenylphosphinobenzoic Acids. Inorg. Chim. Acta 2014, 423 (Part A), 219−224. (38) Tehfe, M.-A.; Lalevée, J.; Telitel, S.; Contal, E.; Dumur, F.; Gigmes, D.; Bertin, D.; Nechab, M.; Graff, B.; Morlet-Savary, F.; Fouassier, J.-P. Polyaromatic Structures as Organo-Photoinitiator Catalysts for Efficient Visible Light Induced Dual Radical/Cationic Photopolymerization and Interpenetrated Polymer Networks Synthesis. Macromolecules 2012, 45 (11), 4454−4460. (39) Lalevée, J.; Blanchard, N.; Tehfe, M.-A.; Morlet-Savary, F.; Fouassier, J. P. Green Bulb Light Source Induced Epoxy Cationic Polymerization under Air Using Tris(2,2′-bipyridine)ruthenium(II) and Silyl Radicals. Macromolecules 2010, 43 (24), 10191−10195. (40) Wang, T.; Wang, Z. H. Cationic Photopolymerization of Epoxy Systems Initiated by Cyclopentadien-Iron-Biphenyl Hexafluorophosphate ([Cp-Fe-biphenyl]+PF6 -). Polym. Bull. 2005, 53 (5−6), 323− 331. (41) Wang, T.; Chen, J. W.; Li, Z. Q.; Wan, P. Y. Several Ferrocenium Salts as Efficient Photoinitiators and Thermal Initiators for Cationic Epoxy Polymerization. J. Photochem. Photobiol., A 2007, 187 (2−3), 389−394.

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Benzoyl Peroxide Redox

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