Copper-Based (Photo)redox Initiating Systems as Highly Efficient

Article Cite This: Macromolecules 2018, 51, 679−688

Copper-Based (Photo)redox Initiating Systems as Highly Efficient Systems for Interpenetrating Polymer Network Preparation Patxi Garra,† Méline Carré,† Frédéric Dumur,‡ Fabrice Morlet-Savary,† Céline Dietlin,† Didier Gigmes,‡ Jean-Pierre Fouassier,† and Jacques Lalevée*,† †

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

ABSTRACT: Simultaneous redox free radical polymerization (FRP) and redox cationic polymerization (CP) are combined for the synthesis of methacrylate/diepoxide interpenetrating polymer networks (IPN). At first, the Cu(acac)2/phosphine/iodonium salt operates according to the principles of free radical promoted cationic polymerization (FRPCP). A photoactivation of the reaction for that system was necessary to enhance the mild methacrylate and diepoxide conversions. Second and at least, two complementary copper catalytic cycles are used simultaneously: the recently developed Cu(II)/reducing agent/peroxide FRP system combined with the older Cu(II)/reducing agent/ iodonium salt redox CP system. For this latter hybrid system, outstanding efficiency was observed with more than 90% of epoxy functions conversion for the cationic difunctional monomers and 78% conversion for the vinylic functions conversion for difunctional monomers. The radical and cation generations are discussed in order to fill the interrogations raised by the experimental results. The relevance of dual FRP/CP in IPN synthesis is fully demonstrated. The performance of the hybrid copper catalytic system is remarkable to overcome the oxygen inhibition; i.e., almost no oxygen inhibited layers are observed compared to the >60 μm inhibited layer obtained with a reference redox FRP such as amine/BPO. promoted cationic polymerization (FRPCP)16,17 allowed significant developments of highly active photoinitiating systems for CP.18,19 This FRPCP process is based on the oxidation of free radicals by onium salts which generates cations able to initiate a CP. Indeed, FRPCP approach is interesting as classical and versatile radical photoinitiators can be used in combination with oxidation agents (e.g., onium salts) to initiate CP processes. Such a principle could also be applied to redox processes as redox systems generate free radicals that could be oxidized to cations. Nevertheless, reducing agents used in redox FRP are commonly nucleophilic species (e.g., in amines) that can inhibit the cation propagation in redox CP which explains why redox CP remains quite confidential. Using other chemical mechanisms between 1980 and 2000, Crivello and co-workers contributed significantly to the development of the redox cationic polymerization.20 They proposed many systems based on redox dissociations of onium salts20 such as e.g. metal catalyst/reducing agent/onium salt. Particularly, copper/vitamin C (VitC)/iodonium salt21 and copper/Tin(II)/iodonium salt22 systems allowed extremely high CP conversions at room temperature and in solvent media.

1. INTRODUCTION Nowadays, free radical polymerization (FRP) processes account for about 45% of the total polymer production.1 Among the FRP initiating strategies, redox FRP offers many advantages as it allows excellent in-depth curing even in filled samples for the access to composites.2,3 Redox FRP is a twocomponent mixing approach occurring when an oxidizing agent (or system) is mixed with a reducing agent (or system). For example, mixing dibenzoyl peroxide (BPO) with 4-N,Ntrimethylaniline (4-N,N TMA)system discovered in the 1950s4,5leads to excellent redox FRP and is still in use in many actual high-tech applications requiring in-depth polymerization.6−13 Methacrylate resins (for example, the blend presented in Scheme 1) are used in many cases of redox FRP as they are less toxic/irritating than acrylates, and the final polymer is less sensitive toward hydrolysis. Nevertheless, a huge limitation of redox FRP is its high sensitivity toward the oxygen inhibition (particularly at the surface).14,15 On the other hand, cationic polymerization (CP) is (generally) slower than FRP but not inhibited by the oxygen. Its use remains limited in practical applications but is more and more present as a result of the growing availability of activated monomers in the family of cyclohexene epoxide (for cationic ring-opening polymerization) such as (3,4-epoxycyclohexane)methyl 3,4-epoxycyclohexylcarboxylate (EPOX, Scheme 1). Interestingly, in the photopolymerization area (polymerization induced by light and not by redox mixing), free radical © 2018 American Chemical Society

Received: November 25, 2017 Revised: January 15, 2018 Published: January 24, 2018 679

DOI: 10.1021/acs.macromol.7b02491 Macromolecules 2018, 51, 679−688

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Macromolecules Scheme 1. Composition of the Methacrylate Resin (Radical Polymerization) and of the Epoxy Resin for Cationic Polymerization ((3,4-Epoxycyclohexane)methyl 3,4-Epoxycyclohexylcarboxylate or EPOX)

Scheme 2. Chemical Compounds Studied for MABLI-Initiated IPN Generation

In recent photopolymerization works, the combination of CP and FRP for interpenetrating polymer networks (IPN) synthesis was proposed in order to combine the advantages of both processes without their respective drawbacks.23,24 Nevertheless, to the best of our knowledge, the only system capable of producing interpenetrating polymer networks (cationic/radical polymerizations with acrylates) from redox mixing was developed by Tehfe et al.25 in a silane/carbazole/ silver salt system (DPSi/NVK/AgSbF6) that was an improvement of their system developed in CP.26,27 However, (i) the efficiencies of cationic and radical (acrylates) polymerization systems remained weak and (ii) the thermal vs redox polymerization process remains unclear. Therefore, in the present paper, we will explore the formation of IPN thanks to systems very recently developed for redox FRP. Particularly, our group proposed two copperbased highly efficient redox FRP initiating systems: (i) first, the metal acetylacetonate bidentate ligand interaction (MABLI) system based on copper acetylacetonate (Cu(acac)2)/2diphenylphosphinobenzoic acid (2dppba) which showed great efficiencies with a mechanism based on simultaneous ligand exchange and release of an acetylacetonate radical (acac•) that can be photoactivated;15,28 (ii) second, copper(I)/reducing agents/peroxide systems showed extremely high efficiencies in redox FRP competitive with the reference 4-N,N TMA/BPO system.29,30 As a result, in the first part of the present study, the Cu(acac)2/2dppba system will be proposed for simultaneous FRP/CP process for the access to IPN using the principles of FRPCP (oxidizing some acac• with onium salts in order to form cations). Critical assessment of such a Cu(acac)2/2dppba/ iodonium salt initiating system (for dual FRP/CP) will be proposed. Also, as a significant light-induced improvement was previously reported in pure FRP, a photoactivation of the IPN

generating system will be proposed. In the second part of the current study, we will propose to mix the Crivello’s systems copper/VitC/iodonium salt (developed in solvent)21 and the copper/Tin(II)/iodonium salt (also developed in solvent)22 with our copper catalytic cycles developed for redox FRP (copper(I)/reducing agents/peroxide systems). The effect of IPN generation on surface (oxygen inhibited) curing will be assessed thanks to Raman confocal microscopy.

2. EXPERIMENTAL SECTION 2.1. Chemical Compounds. All the reactants were used as received and are gathered in Schemes 2 and 4. N-tert-Butyl-α-phenylnitrone (PBN), 4-hydroxy-TEMPO (tempol), ascorbic acid/vitamin C (VitC), dibenzoyl peroxide (BPO), 2diphenylphosphinobenzoic acid (2dppba), 4-N,N-trimethylaniline (4N,N TMA), triphenylphosphine (tpp), copper acetate (Cu(OAc)2), copper 2-ethylhexanoate (Cu(Oct)2), N-vinylcarbazole (NVK), 2-[4(diethylamino)phenylazo]benzoic acid, 4-(diethylamino)azobenzene2′-carboxylic acid (Ethyl red), and diphenyliodonium hexafluorophosphate (Ar2I+) were purchased from Sigma-Aldrich. Tin(II) 2ethylhexanoate (Tin(II)) and copper acetylacetonate (Cu(acac)2) were purchased from Tokyo Chemical Industry (TCI-Europe). Absolute ethanol, toluene, and dichloromethane (DCM) were purchased from Carlo Erba as EPR grade. Highly efficient “G1” copper complex was synthesized according to a procedure already available in the literature.23,31 The efficiency of the different redox radical initiating systems was checked in a reference methacrylate mixture (Scheme 1) with a low viscosity of 0.053 Pa·s (Scheme 1) containing 33.3 wt % (w/w) 1,4butanediol dimethacrylate (1,4 BDMA), 33.3 wt % (w/w) hydroxypropyl methacrylate (HPMA), and 33.3 wt % (w/w) urethane dimethacrylate oligomer; the monomers were obtained from SigmaAldrich. The efficiency of the different redox cationic initiating systems was checked in (3,4-epoxycyclohexane)methyl 3,4-epoxycyclohexylcarboxylate (EPOX) which was obtained from Allnex. 680


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Macromolecules 2.2. Definition of the Two Components Used for Redox Experiments. All redox formulations were prepared from the bulk resins in two separate cartridges at room temperature (RT) (21−23 °C) under air: a first cartridge with the oxidizing agentCu(acac)2 in the first part of the study, copper(II) and/or peroxide and/or iodonium salt in the second part of the studyand the other one containing the reducing agent (and the iodonium salt in the first part of the study). A 1:1 Sulzer mixpac mixer was used to mix both components together at the beginning of each polymerization experiment. The contents of reduction or oxidation agents will be given in weight with respect to the resin in the final mixture (wt %). 2.3. Photopolymerization Monitoring through RT-FTIR Spectroscopy (Figure 6). A Jasco 6600 real time Fourier transformed infrared spectroscopy (RT-FTIR) was used to record both the CC double bond conversion and the epoxide conversion versus time. The evolution of the methacrylate CC double bond peak was followed at about 6150 cm−1. The evolution of the epoxide peak was followed at about 3730 cm−1. A LED at 405 nm (Thorlabs; light intensity about 110 mW/cm2 at the sample surface) was used for redox photoactivation of the Cu(acac)2/2dppba/Iod (Figure 3) initiating system (first part). The emission spectra are already available in the literature.32,33 2.4. Redox Polymerization in Bulk Followed by Optical Pyrometry. The use of optical pyrometry to follow photopolymerization reactions was developed by Crivello et al.21,34 Optical pyrometry is an interesting technique to monitor polymerization:34 qualitative assessment of the polymerization is provided (through exothermicity) with simultaneous measurement of the gel time. Gel time will be defined here as the time to reach the maximum temperature. Temperature versus time profiles were followed here using an Omega OS552-V1-6 industrial infrared thermometer (Omega Engineering, Inc., Stamford, CT) having a sensitivity of ±1 °C for 2 g samples (thickness ∼4 mm). The same setup was used for cationic photopolymerization experiment initiated by a G1/Ar2I+/NVK initiating system (Figure 2) with a smaller sample thickness (∼1.4 mm). A LED at 405 nm (Thorlabs; light intensity about 200 mW/cm2 at the sample surface) was used for that experiment. 2.5. Surface Curing 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 line/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 onto uncured monomer to be used as reference for double bond conversion. For Raman surface analysis (Figures 4D and 5), 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 protocol available in the literature35−38 to access the “real” depth in the photopolymerized sample considering a refraction index close to 1.5. This procedure was already applied in our previous work15,28,29,39 characterizing surface curing. 2.6. UV−Vis Absorption Spectroscopy. A Varian Cary 3 spectrophotometer was used for the determination of the UV−vis absorption spectra in absolute ethanol. The initial solution of Cu(II)/ Ethyl red/Ar2I+ was used for the first spectrum (Figure 7A), and then a 1:1 Sulzer mixpac mixer was used to mix together this solution with a Tin(II) solution (spectra recorded after 2 min mixing). 2.7. Electron Spin Resonance−Spin Trapping (ESR−ST). ESR−ST experiments were carried out using an X-band spectrometer (Bruker EMX-plus Biospin). They were realized using phenyl-N-tertbutylnitrone (PBN) as a spin-trap agent in a similar way as described in other works.31 The radicals were observed at room temperature under air in DCM/toluene (15/85 in volume) solutions.

MABLI relies on the reaction between a metal acetylacetonate and specific bidentate ligands such as 2dppba (Schemes 2 and 3, reactions r1 and r2).15 This system is very interesting as Scheme 3. Free Radical Promoted Cationic Polymerization Reactions Proposed for MABLI-Initiated IPN Generations (Radical/Cationic Polymerizations), with M Being a Vinylic (CC) or Cationic (EPOX) Monomer

neither peroxide (unstable, toxic) nor amines (toxic, carcinogenic) were used for redox FRP initiation at room temperature under air. Moreover, copper acetylacetonate (Scheme 2) shows excellent photoactivation under visible wavelengths and is nowaydays a relatively cheap reagent. Diphenyliodonium hexafluorophosphate (Ar2I+, Scheme 2) will be used to oxidize the acac• (see reaction r3) in order to generate a cation (acac+). As a result, when both cationic and free radical polymerizing resins are present, interpenetrating polymer networks (IPN) should be generated through reactions r5 and r6. 3.1.1. Free Radical Promoted Cationic Polymerization (IPN Generation) Using MABLI Initiating Systems. Experimentally, in Figure 1, the Cu(acac)2/2dppba/Ar2I+ dual FRP/CP initiating system was tested. In Figure 1A, various compositions of the methacrylate/EPOX blends were used from 80/20 (w/ w) to 50/50 (w/w). Exothermicitiesindicating polymerization reactions34are present for all of the blends; nevertheless, the more the EPOX, the lower the exothermicity of the reaction: ∼77 °C for 80/20 (w/w) blend (curve 1) and ∼50 °C for the 50/50 (w/w) blend (curve 4). The latter blend was then analyzed in real-time Fourier transformed infrared spectroscopy (RT-FTIR) which allows a simultaneous study of the methacrylate and of the epoxide conversions as a function of time (Figure 1B). The obtained curves are consistent with the low exothermicities observed by means of optical pyrometry: the conversion reached for CC was low with about 50% obtained after 125 s and only about 15% conversion for the epoxide functions. The best methacrylate/EPOX blend (80/20 (w/w)) was also analyzed in RT-FTIR (Figure 1C). Logically, the noise for the epoxide function follow-up was quite high as the amount of EPOX (20 wt %) in the blend is quite low. The performance of the FRPCP system was quite good with about 60% CC conversion and about 60% epoxide conversion after 75 s. The initial and final FTIR spectra (Figure 1D) confirm the real-time follow-ups with a significant decrease of both peaks areas. In order to explain the low efficiencies obtained particularly when the 50/50 (w/w) blend was used, the influence of phosphines on classical cationic photopolymerization was studied (Figure 2). A reference G1/NVK/Ar2I+ system23,31 was chosen, and two different phosphines were added to the reaction resin (still EPOX): classical triphenylphosphine (tpp) and the phosphine used in the dual FRP/CP initiation shown above (2dppba). Clearly, the initial high performance system (maximum temperature of 75 °C, curve 1) is inhibited by the

3. RESULTS AND DISCUSSION 3.1. Copper-Based MABLI for IPN Generation? We first investigated the MABLI redox FRP systems for FRPCP. 681


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Figure 1. Redox initiating system: 2dppba (1.4 wt %) mixed with Ar2I+ (2 wt %) and Cu(acac)2 (0.8 wt %). (A) Optical pyrometric measurements (temperature vs irradiation time, 4 mm thick sample) under air in the various methacrylate resin/EPOX ratios. 1: 80/20 (w/w); 2: 70/30 (w/w); 3: 60/40 (w/w); 4: 50/50 (w/w). (B, C) Simultaneous radical/cationic polymerizations RT-FTIR follow-ups (CC conversion vs time and epoxide conversion vs time; 1.4 mm sample) under air. (B) In 50/50 (w/w) methacrylate/EPOX blend. (C) In 80/20 (w/w) methacrylate/EPOX blend. (D) Spectra just after mixing and after 102 s for experiment C.

conversion and ∼60% epoxide conversion after 75 s; without light; Figure 3). Upon soft LED at 405 nm irradiation, it is possible to greatly enhance both (i) the polymerization rate and (ii) the final conversion obtained with about 80% CC conversion and about 90% epoxide conversion during the IPN generation. The FRPCP system can therefore be used in the frame of redox photoactivated polymerization for a highly efficient IPN generation: a slow redox process (mildly efficient) can be boosted by soft LED irradiation. Nevertheless, it should be noted that light penetration in redox photoactivated polymerization is not guaranteed for thicker samples (>1.4 mm).40 3.2. Copper/Reducing Agent/Oxidizing Agents Catalytic Systems for IPN Generation? Though highly efficient initiating systems were proposed in the first part thanks to redox photoactivation of dual FRP/CP, photoactivation is not possible in all applications (e.g., issues of light penetration in shadow areas or for highly filled samples40), and there is still a need to develop pure (without light) highly efficient redox IPN generating systems. Therefore, we proposed using a different strategy in the second part of the current study (compounds used in Scheme 4): the redox FRP system will be based on a copper/reducing agent/peroxide system (Scheme 5A), and the redox CP system will be a copper/reducing agent/iodonium salt system (Scheme 5B). In detail, we propose here for the first time to start the redox FRP catalytic cycle from Cu(II) (previously, Cu(I)/reducing agent/peroxide systems were proposed29,30). This fact is a significant advance as Cu(II)like copper acetate (Scheme 4) is much nowadays cheaper and easier to obtain than the Cu(I) unique complexes previously reported for redox FRP. Two efficient reducing agents are proposed (VitC and Tin(II)). Please note that as in the first

Figure 2. Cationic photopolymerization experiment. Optical pyrometric measurements (temperature vs irradiation time) under air (1.4 mm sample) in EPOX. For 0.15 wt % G1/2 wt % Ar2I+/3 wt % NVK and (1) no phosphine, (2) 0.9 wt % 2dppba, and (3) 1.1 wt % tpp. Irradiated by a LED at 405 nm (200 mW/cm2).

addition of phosphines with only 55 °C for 2dppba (curve 2) and only about 37 °C when tpp (curve 3) is introduced. As a result, it is very likely that the FRPCP system proposed in Figure 1 is inhibited by the presence of 2dppba in the blend: the nucleophilicity of the phosphines ascribed to its lone pair is trapping propagating cationic species, thus inhibiting CP in reaction r7. 3.1.2. Photoactivation of the Free Radical Promoted Cationic Polymerization. Finally, the Cu(acac)2/2dppba reaction can be photoactivated15,39 which should greatly enhance the mild efficiency observed in Figure 1. We selected the 80/20 (w/w) methacrylate/EPOX blend (∼60% CC 682


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Figure 3. Simultaneous radical/cationic polymerizations RT-FTIR follow-ups (A: CC conversion vs irradiation time; B: epoxide conversion vs irradiation time; 1.4 mm sample) under air for the redox initiating system 2dppba (1.4 wt %) mixed with Ar2I+ (2%) and Cu(acac)2 (0.8%). Without and with photoactivation by a LED at 405 nm (110 mW/cm2).

Scheme 4. Chemical Compounds Studied for Copper Catalytic Cycle’s Initiation of IPN

3.2.1. Highly Efficient Copper/Reducing Agents/Onium Salts/Peroxide System for IPN Generations. First, for redox FRP in Figure 4A, the Cu(II)/BPO system is obviously not efficient, so as the VitC/BPO system (see previous reports29). It is necessary to have a catalytic Cu(II)/VitC/BPO system for an efficient redox FRP system with about 58 °C exothermicity (Scheme 5A, FRP). Nevertheless, the system is not as competitive as Cu(I)/VitC/BPO systems (previously reported29,30) because the 4-N,N TMA/BPO redox FRP reference shows a significantly better exothermicity (100 °C). Noteworthy and as previously shown,29 a change of copper ligands structures (and of the concentrations) can change redox polymerization efficiencies. Next, very interestingly, when 5 wt % water is added for the solubilization of VitC in Figure 4B, the Cu(II)/VitC/Ar2I+ system is outstandingly efficient in redox CP with 150 °C exothermicity after 360 s. Water is commonly presented as a CP inhibitor, but here, the highly efficient redox CP is overcoming any water inhibition (also see Figure S1). The system is still quite slow (360 s) but could be even upgraded if present in a dual approach for the IPN generation. Indeed, a dual FRP/CP approach was conducted for the first time in a 50/50 (w/w) methacrylate/EPOX blend (Figure 4C). Unfortunately, RT-FTIR experiments were not possible due to the presence of water in the resin. The hybrid Cu(II)/VitC/ BPO/Ar2I+ system appears to be outstandingly efficient and fast

Scheme 5. Hybrid Copper Catalytic Cycles for (A) FRP (Cu(II)/Reducing Agent/Peroxide) and (B) Cationic Polymerization (Cu(II)/VitC/Iodonium Salt) in IPN Generations

part, academic diphenyliodonium hexafluorophosphate can be easily replaced by 4-tert-butyldiphenyliodonium hexafluorophosphate as they have similar (photo)redox reactivities.41 This should avoid potential benzene releases. The reference redox FRP system is 4-N,N TMA/BPO. Contrary to Crivello’s systems studied in monomer/solvents mixtures, we performed CP in bulk resins which is necessary for many redox polymerization applications (coatings, paints, adhesives, composites, etc.). 683


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Figure 4. (A, B, C) Optical pyrometric measurements (temperature vs mixing time, 4 mm samples) under air. A: BPO = 1.1 wt %/0.75 wt % Cu(OAc)2 and VitC = 1.1 wt % (+ 4.4 wt % water) or (reference) 4-N,N TMA = 0.8 wt %/BPO = 1.0 wt % in the presence of 0.12 wt % tempol. FRP: Free radical polymerization, in methacrylate resin. B: Ar2I+ = 2.0 wt %/VitC= 2.2 wt % (+ 5 wt % water)/Cu(OAc)2 = 0.75 wt %. CP: cationic polymerization, in EPOX. C: BPO = 1.1 wt %, Ar2I+ = 1.0 wt %, VitC = 1.1 wt % (+ 4.5 wt % water), Cu(OAc)2 = 0.75 wt %. IPN: interpenetrated network, in 50/50 (w/w) methacrylate/EPOX blend. (D) Raman surface analysis of the samples polymerized in (A) and (C).

with about 117 °C exothermicity in less than 60 s. This IPN approach is particularly efficient at the very surface (Figure 4D) with an oxygen inhibition layer (for CC polymerization) of less than 5 μm when all the system presented in redox FRP show a very thick inhibition layer: >50 μm for the Cu(II)/ VitC/BPO system and >70 μm for the 4-N,N TMA/BPO system. The IPN approach is therefore fully relevant to overcome the oxygen inhibition at the top surface: the redox CP network (formed and not sensitive to oxygen inhibition) is preventing the oxygen diffusion inside the IPN, leading to a redox FRP system that is then much less sensitive toward the oxygen inhibition. In order to confirm the IPN generation, a dual FRP/CP approachstill in the 50/50 (w/w) methacrylate/EPOX blendwas followed by RT-FTIR experiment in Figure 5. Tin(II) reducing agent was used instead of vitamin C as no water is required for its solubilization. A slight noise particularly for epoxide conversion vs time curveis observed in Figure 5A probably as a result of small presence of oxygen bubbles (from mixing) or low signal area of the epoxide function (see example FTIR absorbance in Figure 1D). The hybrid Cu(II)/Tin(II)/BPO/Ar2I+ system is clearly creating simultaneously radicals and cations (Figure 5A and Figure S2): kinetics of epoxide and vinyl monomers conversion are synchronized. Also, the performance without light is outstanding with (after 35 s) more than 90% epoxide functions conversion for EPOX and about 75% CC conversion for the methacrylate blend. These results are remarkable because both

cationic and vinylic resins are composed by bifunctional monomers which are always limited in final conversions (one function is trapped in the polymer network and the second one gets less mobile thus less reactive). As a comparison in photopolymerization, one of the best IPN generatedresulting from years of developments/optimizations of IPN initiations42−44resulted in only 80% epoxide conversion (same cationic monomer).23 Lower conversions can lead to dramatic losses of mechanical properties particularly throughout time (aging of the polymer).23 Finally, photoactivation of the hybrid Cu(II)/VitC/BPO/ Ar2I+ redox system for IPN generation was proposed in Figure 5B,C. The system without light is very efficient with more than 160 °C exothermicity after 60 s (slightly higher exothermicity than Figure 4C due to more VitC29). As a result to the already very high reactivity without light, photoactivation had (i) a mere effect on exothermicity (almost the same) and (ii) an accelerating effect on polymerizations (50 s). An effect of the photoactivation on surface curing was also noted as about 60− 65% CC conversion (at 10 μm) was observed using light activation when about 50% CC conversion (at 10 μm) was obtained without light. The polymerization initiation results are summed up in Table 1. 3.2.2. Mechanistic Considerations for the Hybrid Copper Catalytic Redox Initiation. Some chemical mechanistic questions remain, particularly if the Cu(II)/VitC/Ar2I+ is generating free radicals through a Cu(I)/Ar2I+ reaction (r9) like what is present in the photochemical mode for the Cu(I)*/ 684


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Figure 5. Redox polymerization experiments (under air) in 50/50 (w/w) methacrylate/EPOX blend for BPO = 1.1 wt %/Ar2I+ = 1.0 wt %/0.75 wt % Cu(II) and a reducing agent (Tin(II) or VitC). (A) Simultaneous radical/cationic polymerizations RT-FTIR follow-ups (CC conversion vs time and epoxide conversion vs time; 1.4 mm sample). Reducing agent = 3.0 wt % Tin(II); Cu(II) = Cu(OAc)2. (B) Optical pyrometric measurements (temperature vs mixing time; 4 mm samples) under air reducing agent = 1.5 wt % VitC (+ 5.0 wt % water); Cu(II) = Cu(Oct)2; with and without LED at 405 nm photoactivation. (C) Raman surface analysis of the samples polymerized in (B).

completely shut down by the inhibition reactions (with oxygen or stabilizers such as phenols). Next, in Figure 6B and in the following chemical mechanistic part, the Tin(II) reducing agent was used instead of VitC as it is soluble in weakly polar solvents for ESR and UV−vis experiments. Also, no ambiguous release of H+ from the reduction of the reducing agent (which is likely with VitC) can be proposed. Similarly, copper octoate (Cu(Oct)2 that is more soluble in toluene (weakly polar solvent necessary for ESR) was used instead of copper acetate (Cu(OAc)2). The Cu(II)/ Tin(II)/Ar2I+ redox CP system is very active (Figure 6B) with more than 120 °C reached after about 400 s. The Cu(II)/ Tin(II) blank shows no polymerization at all after more than 2 h: Ar2I+ is necessary for cations generation. When adding high amounts of tempol (curve 2) which should trap radicals and shut down the reaction if radicals are present in the mechanism, the cationic system Cu(II)/Tin(II)/ Ar2I+ is still active even if a decrease in exothermicity is observed (about 37 °C). It can be attributed to the amine nature of tempol which can also extinguish cationic polymerization like phosphines (see r7). This experiment tends to demonstrate that radicals are not present in the CP initiation through the Cu(II)/Tin(II)/Ar2I+ system. Afterward, the reaction was studied out of the polymerization resins. In UV−vis spectroscopy, the Cu(II)/Ar2I+ + pH indicator (ethyl red, pH end point for 4.0−5.8) solution is the sum of the respective absorption of each component (curves 1, 2, and 3, Figure 7A). Adding Tin(II) as what is done

Table 1. Summary of the Main Results for the CP, FRP, and Dual FRP/CP (IPN Generation) Initiations polymerization mode FRP IPN

oxidizing agent(s)

IPN photoactivated FRP FRP

Cu(acac)2 Cu(acac)2 + Ar2I+ Cu(acac)2 + Ar2I+ BPO Ar2I+

CP

Ar2I+

IPN

Ar2I+ + BPO

IPN photoactivated

Ar2I+ + BPO

a

reducing agent

metal catalyst

efficiency

2dppba 2dppba

+++15 +

2dppba

+++

VitC VitC or Tin(II) VitC or Tin(II) VitC or Tin(II) VitC or Tin(II)

Cu(OAc)2 Cu(OAc)2 or Cu(Oct)2 Cu(OAc)2 or Cu(Oct)2 Cu(OAc)2 or Cu(Oct)2 Cu(OAc)2 or Cu(Oct)2

+ n.p.a +++ +++ +++

n.p.: no polymerization.

Ar2I+ reaction.45,46 As a result, without any peroxide, a methacrylate/EPOX 20/80 (w/w) blend was used for a combined redox FRP/CP (Figure 6A). The reaction was completely shut down by the methacrylate resin as no polymerization was observed (Figure 6A, curves 1 and 2). It was still true for a 50/50 (w/w) methacrylate/EPOX blend (curve 3). This indicates that if a radical productionthrough a Cu(I)/Ar2I+ reactionis present, its efficiency is very low and 685


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Figure 6. Optical pyrometric measurements (temperature vs time) under air. (A) For 1.3 wt % VitC/5 wt % water/1.2 wt % Cu(OAc)2/2.0 wt % Ar2I+ in (1) pure EPOX (2) 80/20 (w/w) EPOX/methacrylate blend and (3) 50/50 (w/w) EPOX/methacrylate blend. (B) In pure EPOX, for 2.1 wt % Tin(II)/0.6 wt % Cu(Oct)2; (1) 1.9 wt % Ar2I+, (2) 1.9 wt % Ar2I+ and 0.16 wt % tempol, and (3) no additive.

Figure 7. (A) UV−vis absorption spectra in absolute ethanol for (if mentioned) Ethyl red (pH indicator) = 0.02 mM Cu(Oct)2 = 4.2 mM; Ar2I+ = 21 mM and Tin(II) = 31 mM. 1: Cu(Oct)2 + Ar2I+; 2: Ethyl red; 3: Cu(Oct)2/Ar2I+/Ethyl red; 4: 120 s mixing of Cu(Oct)2/Ar2I+/Ethyl red and Tin(II). (B) Black: ESR spectra after 120 s mixing of 0.5 mM Cu(Oct)2 and 5 mM Ar2I+ mixed with 7 mM Tin(II) in the presence of 3 mM PBN (as spin trap agent) in toluene/DCM (85/15 v/v), under air.

4. CONCLUSION This paper describes a simultaneous combination of redox free radical polymerization (FRP) and cationic polymerization (CP) processes thanks to two copper-based approaches recently developed for redox FRP. One approach was based on the principles of free radical promoted cationic polymerization (FRPCP): onium salts oxidized acac• radicals (generated in the Cu(acac)2/2dppba reaction) in order to form acac+ useful for redox CP. The performances in IPN generations for that approach were mild because of the presence of 2dppba phosphine that inhibits the CP; it was necessary to photoactivate the redox system in order to obtain high performances. Then, another approach was based on copper/reducing agent/ peroxide FRP systems which were combined with previously reported Cu(II)/reducing agent/onium salt redox CP system in order to form a hybrid dual FRP/CP. The performance of this hybrid system was outstandingly high for the IPN generation (almost 90% epoxy function conversion for the cationic difunctional monomer and 78% for the methacrylate multifunctional resin). In that system, the dissociation of the iodonium salt thanks to Cu(I) in situ generated was found to occur only through protons generation in the Cu(I)/iodonium reaction (H+ detection and no radical detection in ESR spin trapping mode). Dual FRP/CP in IPN was particularly efficient at the surface (as demonstrated in Raman confocal microscopy)

in resin leads to two unambiguous changes: Cu(II) is transformed in Cu(I) (blue solution to a transparent one with a decrease of the Cu(II) band at 700 nm) and, more importantly, the pH indicator changes toward its acidic form (apparition of a 510 nm band).47 One can therefore clearly conclude that (i) the reaction of Cu(II) with a reducing agent (Tin(II)) forms Cu(I) in reaction r8 and that (ii) H+ are generated from the Cu(I)/Ar2I+ reaction (r10). Also, in ESR spin trapping experiment (Figure 7B), a similar experiment was conducted in the presence of N-tert-butyl-α-phenylnitrone (PBN). No radicals at all were trapped which indicates that the Cu(I)/Ar2I+ reaction is not (significantly) leading to the dissociation of the iodonium salt in order to form aryl radicals (see reaction r9). Scheme 6. Chemical Mechanisms Proposed for the Redox Cationic Initiation of Cu(II)/Reducing Agent/Iodonium Salt Initiating System

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Macromolecules as the inhibition layer was of about 5 μm when redox FRP reference systems (amine/BPO) showed an inhibition layer of more than 60 μm. The performances were so high that photoactivation was not necessary for these copper catalytic cycles IPN generating systems.

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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.7b02491. Figure S1: optical pyrometric measurements (temperature vs mixing time, 4 mm thick samples) under air for various amounts of water; Figure S2: FTIR spectra before and after polymerization for IPN (PDF)

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AUTHOR INFORMATION

Corresponding Author

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

Jacques Lalevée: 0000-0001-9297-0335 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Agence Nationale de la Recherche for the grants “PhotoRedox” and “FastPrinting”. REFERENCES

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Copper-Based (Photo)redox Initiating Systems as Highly Efficient

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