Trifunctional Photoinitiators Based on a Triazine Skeleton for Visible

Article pubs.acs.org/Macromolecules

Trifunctional Photoinitiators Based on a Triazine Skeleton for Visible Light Source and UV LED Induced Polymerizations Mohamad-Ali Tehfe,† Frédéric Dumur,‡ Bernadette Graff,† Fabrice Morlet-Savary,† Jean-Pierre Fouassier,§ Didier Gigmes,‡ and Jacques Lalevée†,* †

Institut de Science des Matériaux de Mulhouse IS2M, LRC CNRS 7228, ENSCMu-UHA, 15 rue Jean Starcky, 68057 Mulhouse Cedex, France ‡ Aix-Marseille Université, CNRS, Institut de Chimie Radicalaire, UMR 7273, F-13397 Marseille, France § UHA-ENSCMu, 3 rue Alfred Werner 68093 Mulhouse S Supporting Information *

ABSTRACT: Trifunctional photoinitiators (tPIs) based on benzophenone, anthracene, and pyrene chromophores linked to a triazine moiety are proposed as new initiating systems. In combination with an iodonium salt and a silane, these structures are able to initiate the radical polymerization RP of acrylates and the cationic polymerization CP of epoxides and vinylethers under Xe−Hg lamp, LED and very soft irradiation (i.e., halogen lamp). Upon addition of an amine, these new photoinitiators were also able to start the radical polymerization of acrylates. Excellent RP and CP polymerization profiles are obtained i.e. better than those recorded using the reference compounds (benzophenone, anthracence and pyrene). In CP, some of these compounds combined with thianthrenium salts can also be used. The mechanisms involved in the different multicomponent initiating systems were analyzed by ESR, fluorescence, steady state photolysis, and laser flash photolysis experiments.

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INTRODUCTION

compared to those of the individual starting PI units. These known mPIs absorb in the UV or near UV/visible range. Decreasing the length of the spacer or using appropriate spacers in dPIs should allow an efficient coupling of the molecular orbitals MO of the two PI units and could result in enhanced light absorption properties (in term of molar extinction coefficients ε and red-shifted wavelengths). This was first achieved several years ago2k,l in dPIs such as HAP-SpHAP where Sp = −O−, −CH2−. The π−π* transitions are redshifted by 25−35 nm (and centered at ∼260 and 280 nm for Sp = −O− and −CH2−, respectively compared to 245 nm for HAP itself) and the ε are increased by a 2-fold factor. The delocalization of the π orbitals of the HAP moieties takes place through a conjugative interaction with the 2p orbital of the central oxygen atom in HAP−O−HAP or a through-space hyperconjugation in the slightly twisted HAP−CH2−HAP.2s The search for new photoinitiators PI and/or new photoinitiating systems for radical and/or cationic polymerization reactions upon visible lights and/or LED irradiations (365, 395 nm) remains a great challenge to avoid the use of Hg or Xe− Hg lamps.1 In this aim, search of new architectures involving dPIs or even trifunctional photoinitiator arrangements tPIs potentially exhibiting a strong coupling of the molecular

Development of multifunctional photoinitiators mPIs (that can be defined as molecular or macromolecular arrangements where several low molecular weight photoinitiator PI units are incorporated) have received a constant and particular attention of researchers (see, e.g., a book in ref 1 and original papers2). Reasons justifying the interest for such mPIs have been mentioned for many years3 and a recent paper4 pointed out the possible advantages: reduced migratability, reduced release of photolysis products, less odor, better compatibility into the resin... Examples of mPIs (see, e.g., in refs1 and 2) include (i) polymeric PI (where PI = benzophenone, thioxanthone, camphorquinone are generally pendant groups of polymer chains, e.g.;2a,f,o,p,r a dendritic structure has also been proposed2e), (ii) oligomeric PI (with PI = hydroxyacetophenone (HAP) as pendent units),2b and (iii) difunctional photoinitiators dPIs4 where two PI units (such as phenylglyoxylate, benzophenone, thioxanthone; e.g., ref 2m) are usually linked using a rather long ethylene glycol chain spacer Sp. As far as the absorption is concerned in all these systems, the PI units obviously behave as independent single moieties. Bifunctional photoinitiators bPIs (e.g., refs 2h, j, n, and q) where two chemically different PI units (e.g., a benzophenone and a sulfonyl ketone) are linked also refer to dPIs: in that case, a more or less important delocalization of the molecular orbitals occurs leading to improved light absorption properties © 2012 American Chemical Society

Received: September 14, 2012 Revised: October 16, 2012 Published: October 30, 2012 8639

dx.doi.org/10.1021/ma301931p | Macromolecules 2012, 45, 8639−8647

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Scheme 1

Scheme 2

Avance 400 spectrometer of the Spectropole: 1H (400 MHz) and 13C (100 MHz). The 1H chemical shifts were referenced to the solvent peak DMSO-d6 (2.49 ppm) and the 13C chemical shifts were referenced to the solvent peak DMSO d6 (39.5 ppm). 2,4,6-Tri(1pyrenylamino)-1,3,5-triazine Tz-Py was synthesized according to a procedure previously reported.5 The initiators are yellow powders for Tz_An, Tz_BP and Tz_Py. All the synthesized compounds were prepared with analytical purity up to accepted standards for new organic compounds (>98%) which was checked by high field NMR analysis. Synthesis of 2,4,6-Tri(2-anthracenylamino)-1,3,5-triazine, Tz_An. The reactions were carried out in a 100 mL Parr autoclave under autogenous pressure. The reactor was charged with acetone (50 mL), cyanuric chloride (0.17 g, 0.92 mmol), 2-aminoanthracene (0.87 g, 4.6 mmol) and NaHCO3 (0.3 g, 3.6 mmol). The reaction mixture was heated at 100 °C for 10 h, resulting in a pressure of approximately 6 bar in the reactor. After cooling to room temperature, the greenish precipitate was collected by filtration and washed with acetic acid. The yield was 0.49 g (82%). 1H NMR (DMSO-d6), δ (ppm): 7.31−7.48 (m, 8H), 7.83−7.89 (brs, 4H), 8.07−8.09 (m, 8H), 8.51−8.58 (m, 4H), 8.65−8.80 (m, 3H), 9.77 (brs, 3H). HRMS: m/z calcd for C45H31N6 ([M + H]+ detected), 655.2605; found, 655.2608. Synthesis of 2,4,6-Tri(4-benzoylphenylamino)-1,3,5-triazine, Tz_BP. The reactions were carried out in a 100 mL Parr autoclave under autogenous pressure. The reactor was charged with acetone (50 mL), cyanuric chloride (0.17 g, 0.92 mmol), 4-aminobenzophenone

orbitals of the PI units with those of the spacer thus resulting in better light absorption properties is of crucial interest. In the present paper, we propose, for the first time, the design of tPIs based on a triazine scaffold with benzophenone, pyrene or anthracene moieties as the peripheral photoabsorbing units (Tz_BP, Tz_Py, and Tz_An in Scheme 1). Their ability in photoinitiating systems as photoinitiators/photosensitizers for the cationic polymerization CP of epoxides and vinylethers and the radical polymerization RP of acrylates under very soft irradiation conditions (halogen lamp, LED bulbs, ...) as well as their excited state processes are investigated.

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EXPERIMENTAL PART

i. Triazine-Based Photoinitiators (Tz_BP, Tz_Py, and Tz_An). The investigated triazine derivatives (Tz_BP, Tz_Py, and Tz_An) shown in Scheme 1 were readily prepared by reacting five equivalents of the corresponding amines with cyanuric chloride. All reagents and solvents were purchased from Aldrich or Alfa Aesar and used as received without further purification. Mass spectroscopy was performed by the Spectropole of Aix-Marseille University. ESI mass spectral analyses were recorded with a 3200 QTRAP (Applied Biosystems SCIEX) mass spectrometer. The HRMS mass spectral analysis was performed with a QStar Elite (Applied Biosystems SCIEX) mass spectrometer. 1H and 13C NMR spectra were determined at room temperature in 5 mm o.d. tubes on a Bruker 8640


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(0.91 g, 4.6 mmol), and NaHCO3 (0.3 g, 3.6 mmol). The reaction mixture was heated at 160 °C for 4 h, resulting in a pressure of approximately 13 bar in the reactor. After cooling to room temperature, the brown precipitate was collected by filtration and washed with acetic acid. The yield was 0.53 g (86%). HRMS: m/z calcd for C42H31N6O3 ([M + H]+ detected), 667.2452; found, 667.2455. Anal. Calcd for C42H30N6O3: C, 75.7; H, 4.5; N, 12.6. Found: C, 75.5; H, 4.4; N, 12.4. ii. Compounds. Tris(trimethylsilyl)silane ((TMS)3Si−H), diphenyliodonium hexafluorophosphate (Iod1), N-Methyldiethanolamine (MDEA), triethylene glycol divinyl ether (DVE), anthracene (An), pyrene (Py), and benzophenone (BP) were obtained from Aldrich and used with the best purity available (Scheme 2). [Methyl-4-phenyl(methyl-1-ethyl)-4-phenyl]iodonium tetrakis(pentafluorophenyl) borate (Iod2) 6 was obtained from Bluestar Silicones. (4Hydroxyethoxyphenyl)thianthrenium hexafluorophosphate (TH) was obtained from Lamberti Spa (recrystallized form of Esacure 1187). (3,4-Epoxycyclohexane)methyl 3,4-epoxycyclohexylcarboxylate (EPOX; Uvacure 1500) and trimethylol−propane triacrylate (TMPTA) were obtained from Cytec and selected as representative monomers for cationic and radical polymerization, respectively (Scheme 2). iii. Irradiation Sources. Several lights were used: (i) polychromatic light from a halogen lamp (Fiber-Lite, DC-950; incident light intensity, I0 ≈ 12 mW cm−2 in the 380−800 nm range); (ii) polychromatic light delivered by a Xe−Hg lamp (Hamamatsu, L8252, 150 W, λ > 300 nm) or Hg lamp (Omnicure S1000); (iii) 365 nm (LED from Hamamatsu: LC-L1, Δλ ∼ 20 nm; I0 ∼ 50 mW cm−2). The emission spectrum for halogen lamp was already given in ref 7a. iv. Radical Polymerization (RP). For film photopolymerization experiments, TMPTA (Scheme 2) was used as the monomer. The experiments were carried out in laminate (polypropylene films). The films (25 μm thick) deposited on a BaF2 pellet were irradiated (see the irradiation sources). The evolution of the double bond content was continuously followed by real time FTIR spectroscopy (JASCO FTIR 4100) at about 1630 cm−1 as in.7 v. Cationic Polymerization CP. The epoxy films (25 μm thick) were irradiated on a BaF2 pellet under air. The evolution of the epoxy group content was continuously followed by real time FTIR spectroscopy (JASCO FTIR 4100) as in.7a,c,d The absorbance of the epoxy group was monitored at about 790 cm−1. The conversion of the Si−H group of (TMS)3Si−H is followed at about 2050 cm−1. The formation of the polyether was well characterized at 1080 cm−1. vi. ESR Spin Trapping (ESR-ST) Experiments. ESR-ST experiments were carried out using an X-Band spectrometer (MS 400 Magnettech). The radicals were produced at room temperature under a halogen lamp exposure and trapped by phenyl-N-tertbutylnitrone (PBN) according to a procedure described in detail in refs 8 and 9. vii. Fluorescence Experiments. The fluorescence properties of the different triazine based photoinitiators were studied in toluene using a JASCO FP-750 spectrometer and the fluorescence lifetimes were determined with a Fluoromax 4 spectrofluorimeter (Jobin-Yvon). The rate constants were extracted from a Stern−Volmer treatment as presented in ref 7a. viii. Laser flash photolysis. Nanosecond laser flash photolysis (LFP) experiments were carried out using a Q-switched nanosecond Nd/YAG laser (λexc = 355 nm, 9 ns pulses; energy reduced down to 10 mJ) from Continuum (Minilite) and an analyzing system consisting of a ceramic xenon lamp, a monochromator, a fast photomultiplier and a transient digitizer (Luzchem LFP 212).7

Figure 1. (A) UV−visible absorption spectra of the different triazine derivatives (Tz_Py in toluene; Tz_An and Tz_BP in toluene/ acetonitrile). Inset: zoom for Tz_BP. (B) HOMO and LUMO of a model Tz_Py derivative (UB3LYP/6-31G* level).

different light source emission spectra: (i) the halogen lamp that mostly delivers visible light (e.g., for both Tz_Py and Tz_An having ε > 2000 M−1 cm−1 at 400 nm), (ii) the Xe−Hg lamp that delivers a UV−visible light (λ > 300 nm), and (iii) the LED at 365 nm. Remarkably, when going from Py to Tz_Py or An to Tz_An, a noticeable bathochromic shift of the UV absorption spectra is observed (Figure S1, Supporting Information: λmax ∼ 334 nm; ε ∼ 33 000 M−1 cm−1 for Py vs λmax ∼ 348 nm; ε ∼ 38 000 M−1 cm−1 for Tz_Py and λmax ∼ 356 nm; ε ∼ 9000 M−1 cm−1 for An vs λmax ∼ 404 nm; ε ∼ 8500 M−1 cm−1 for Tz_An). For Tz_BP, the absorption properties are moderately improved compared to the parent compound BP (λmax ∼ 330 nm; ε ∼ 120 M−1 cm−1 for BP vs λmax ∼ 330 nm; ε ∼ 330 M−1 cm−1 for Tz_BP; Figure 1). Molecular orbital MO calculations (using the time-dependent density functional theory at B3LYP/6-31G* level on the relaxed geometries calculated at UB3LYP/6-31G* level) show that the MOs involved in the π → π* transition are strongly delocalized for Tz-Py i.e. the two different pyrene units are involved and a participation of the triazine linker is also noted (Figure 1B). This is in line with the improved light absorption properties of Tz_Py compared to that of Py; a rather similar behavior is observed for Tz_An. For Tz_BP, the MOs are less affected by the presence of the triazine skeleton and accordingly, the absorption properties are less modified

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RESULTS AND DISCUSSION 1. Photochemical Properties of Tz_BP, Tz_An, and Tz_Py. Because of their absorption spectra (Figure 1A) and their high molar extinction coefficients (about 38 000, 8500, and 330 M−1 cm−1 for Tz_Py, Tz_An, and Tz_BP at ∼348, ∼404 and ∼330 nm, respectively), the absorption of the selected triazine derivatives allows a good matching with the 8641


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The excitation of Tz_Py in laser flash photolysis reveals the presence of a transient specie characterized by an absorption maximum at about 420 nm (Figure 3). This is close to the 3Py

compared to that of the parent compound. Tz_Py exhibiting the best absorption properties of the new photoinitiators, the mechanistic investigation will be reported only for this compound in the rest of the discussion (moreover, the reduced solubility of Tz-An and the fast degradation of Tz_BP in laser flash photolysis experiments prevent a thorough investigation of their excited state processes). a. The Tz_Py/Iod1 and Tz_Py/Silane/Iod1 Initiating Systems. In fluorescence experiments (Figure S2A, Supporting Information), a strong quenching of the 1Tz_Py by Iod1 is found (i.e., k = 1.2 × 1010 M−1 s−1); the fluorescence lifetime under air is 16 ns. For 1Py/Iod1, a diffusion controlled reaction rate constant is also found7a indicating that a Py moiety of Tz_Py is involved. This 1Tz_Py/Iod1 interaction corresponds to an efficient electron transfer process which promotes (as in the known thermodynamically favorable process calculated in 1 Py/Iod1)10 the decomposition of the iodonium salt r2.

TzPy → 1TzPy(hν) 1

(r1)

TzPy + Ph 2I+ → (TzPy)•+ + Ph 2I• → (TzPy)•+ + Ph• + Ph − I

(r2)

Interestingly, upon a halogen lamp exposure, a very fast bleaching of the Tz_Py/Iod1 solution occurs (e.g., in less than 2 min for Tz_Py/Iod1; Figure S2B, Supporting Information). This bleaching is ascribed to the oxidation of Tz_Py by Iod1 r2. Reaction r2 is also strongly supported by ESR-ST experiments where the formation of a phenyl radical (Ph•) during the irradiation of Tz_Py/iod1 is clearly observed (Figure 2): the Figure 3. Time-resolved absorption spectra after laser excitation (355 nm) of Tz_Py in toluene. Inset: transient decay at 420 nm under air.

spectrum reported in the literature12 and can be safely ascribed to 3Tz_Py. Considering the concentration of the iodonium salt used in the polymerization experiments (e.g., [iod1] = 0.048 M; see below), the resulting strong singlet state quenching (see above) will prevent, in these conditions, a significant triplet state pathway in Tz_Py/Iod1. In the presence of a silane, reactions r4-r5 occur as observed in the other related systems:1,7a the formation of the silyliums (TMS)3Si+ (known as efficient structures for the epoxide ringopening reaction) explains the excellent polymerization initiating ability of Tz_Py/(TMS)3SiH/Iod1 (see below). Figure 2. ESR spectra obtained after a halogen lamp irradiation of Tz_Py/Iod1 (in tert-butylbenzene), [Iod1] = 0.01 M: experimental (a) and simulated (b) spectra. Phenyl-N-tert-butylnitrone (PBN) is used as spin-trap.

(TMS)3 Si• + (TzPy)•+ → (TMS)3 Si+ + TzPy •

+

•

(TMS)3 Si + Ph 2I → (TMS)3 Si + Ph + Ph − I

(r4) (r5)

In the absence of a possible experimental support (see above), we could assume, as the behavior of Tz_Py is rather close to that of Py, that Tz_BP and Tz_An should behave as BP (triplet state reactivity) and An (singlet state reactivity), respectively. b. Tz_Py/Amine Initiating System. For this part, triethylamine will be used as a representative amine to avoid a high polarity of the sample ensuring the ESR investigation. An aminoalkyl radical is generated upon a halogen lamp irradiation of Tz_Py/amine as observed in ESR-ST experiments, i.e., the aminoalkyl radical adduct onto PBN is characterized by hyperfine coupling constants hfc (aN = 14.5 G; aH = 2.5 G)9e in agreement with the reported data. This highlights a α(C−H) hydrogen abstraction between Tz_Py and the amine according

hyperfine coupling constants hfc (aN = 14.2 G; aH = 2.2 G) agree with the known data for the PBN adduct of this radical.9 For the irradiation of a Tz_Py/(TMS)3Si−H/Iod1 solution, these Ph• radicals are easily converted into silyl radicals (TMS) 3Si • by a hydrogen abstraction reaction r3 on (TMS)3Si−H7,11 as revealed in ESR-ST experiments by the formation of silyl radicals (aN = 15.0 G; aH = 5.7 G in agreement with ref 9 for the associated PBN radical adduct). In the irradiation of Tz_Py/(TMS)3Si−H, no free radicals were observed indicating that the presence of Iod1 is required in agreement with r3. Ph• + (TMS)3 Si− H → Ph− H + (TMS)3 Si•

+

(r3) 8642


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Figure 4. (A) Photopolymerization profiles of EPOX under air. Upon a Xe−Hg lamp exposure (λ > 300 nm) in the presence of (1) Tz_BP/Iod1 (1%/2% w/w); (2) (TMS)3Si−H/Iod1 (3%/2% w/w); (3) Tz_BP/(TMS)3Si−H/Iod1 (1%/3%/2% w/w); (4) Tz_Py/(TMS)3Si−H/Iod1 (1%/ 3%/2% w/w); (5) Tz_An/(TMS)3Si−H/Iod1 (1%/3%/2% w/w); (6) Py/(TMS)3Si−H/Iod1 (1%/3%/2% w/w); insert: Si−H conversion during the photopolymerization of curve 4. (B) Photopolymerization profiles of EPOX under air. Upon a Xe−Hg lamp exposure (λ > 300 nm) in the presence of (1) Tz_BP/(TMS)3Si−H/Iod1 (1%/3%/2% w/w) and (2) Tz_BP/(TMS)3Si−H/Iod1 (1%/3%/2% w/w) after 7 days of storage.

final conversions of EPOX as exemplified by curves 3, 4, or 5 in Figure 4A (presence of the silane) compared to curve 1 (absence of the silane). Final conversions of about 90%, 80% and 70% for Tz_BP, Tz_Py, and Tz_An, respectively, are reached after 5 min of irradiation and tack free coatings are obtained. The Si−H conversions are also high (i.e., ∼ 80% for Tz_Py): this demonstrates that these Si−H conversions are directly related to the efficiency of the initiation step in agreement with reactions r3−r5. Remarkably, a quite slow conversion is obtained when using (TMS)3Si−H/Iod1 (Figure 4A, curve 2) in line with the very low absorption of the iodonium salt (Iod1) under the Xe−Hg lamp (λ > 300 nm). Compared to the well-known parent compounds Py and An,10 a higher efficiency of the triazine derivatives is clearly observed (e.g., Figure 4A, curve 4 vs curve 6; Xe−Hg lamp exposure); the same holds true upon a 365 nm LED irradiation (Figure 5 curve 1 vs curve 2). The much better absorption properties of Tz_Py (compared to Py) at 365 nm (Figure S1, Supporting Information) are presumably the driving factor for its enhanced initiating ability. The storage of all the triazine-containing formulations (e.g., Tz_BP/(TMS)3SiH/Iod1) is quite good as similar polymerization profiles are obtained either immediately after preparation (Figure 4B, curve 1) or after 7 days of storage at room temperature without the presence of inert atmosphere (Figure 4B, curve 2). 2.b. Photopolymerization of EPOX upon Visible Light. While comparing the different triazine derivatives for the EPOX photopolymerization under the halogen lamp irradiation (Figure 6), a decrease of the polymerization rates in the series Tz_An > Tz_Py > Tz_BP could be evidenced. A very fast polymerization is however noted with the multicomponent initiating system Tz_An/(TMS)3Si−H/Iod1. This trend of reactivity has to be related to the light absorption properties of the different initiators at λ > 380 nm: Tz_An > Tz_Py > Tz_BPsee Figure 1A. No polymerization is observed when using the Tz_BP/(TMS)3Si−H/Iod1 initiating system in

to an electron/proton transfer r6. The rate constant for this process r6 has been determined from fluorescence quenching experiments: 2 × 108 M−1 s−1 (Figure S3, Supporting Information). The same reactions should likely arise in Tz_BP and Tz_An. 1

TzPy + MDEA → (TzPy)•− + MDEA•+ → (TzPyH)• + MDEA•(−H)

(r6)

2. Cationic Photopolymerization. 2.a. Photopolymerization of EPOX upon UV Light. The best conversion−time profiles of EPOX using Tz_Py (or Tz_BP, Tz_An)/ (TMS)3Si−H/Iod1 and Tz_Py (or Tz_BP, Tz_An)/Iod1 are depicted in Figures 4 and 5. All the experiments were carried out under air. Interestingly, when using the Xe−Hg lamp (λ > 300 nm), the addition of the silane to triazine derivatives/Iod1 drastically improves both the polymerization rates Rp and the

Figure 5. Photopolymerization profiles of EPOX under air upon a 365 nm LED exposure in the presence of (1) Py/(TMS)3Si−H/Iod1 (1%/ 3%/2% w/w) and (2) Tz_Py/(TMS)3Si−H/Iod1 (1%/3%/2% w/w). 8643


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Scheme 3

∼9000 M−1 cm−1 at 404 nm for Tz_An and TX-An, respectively), Tz_An exhibits the highest efficiency (Figure S5, Supporting Information) again highlighting its excellent reactivity. 2.c. Sensitization of Thianthrenium Salts. Sulfonium (and also thianthrenium) salts and iodonium salts correspond to well-known cationic photoinitiators.1,3 Different anthracene derivatives being efficient structures to sensitize the sulfonium salts decomposition,10 the effect of the Tz_An addition to 4(hydroxyethoxyphenyl) thianthrenium hexaflorophosphate (TH) has also to be investigated. In the EPOX photopolymerization, the Tz_An/TH couple is better than TH alone (both the polymerization rates and the final conversions are increased) (Figure 7, curve 1 vs curve 2). This highlights the

Figure 6. Photopolymerization profiles of EPOX under air. Upon a halogen lamp irradiation in the presence of (1) Tz_Py/Iod1 (1%/2% w/w), (2) Tz_BP/(TMS)3Si−H/Iod1 (1%/3%/2% w/w), (3) Tz_Py/(TMS) 3 Si−H/Iod1 (1%/3%/2% w/w), (4) Tz_An/ (TMS)3Si−H/Iod1 (1%/3%/2% w/w), (5) An/(TMS)3Si−H/Iod1 (1%/3%/2% w/w), and (6) Py/(TMS)3Si−H/Iod1 (1%/3%/2% w/ w). Inset: Photobleaching of the polymer film under air of curve 3; UV−vis absorption spectra before (a) and after (b).

agreement with the lack of absorption of Tz_BP at λ > 380 nm (Figure 1A). The photobleaching of the polymer film in these conditions is also quite remarkable (Figure 6). This can be useful for applications requiring colorless coatings. This bleaching clearly indicates that these initiators are strongly inserted in the polymer network as initiating structures. Accordingly, the extractability of the initiator evaluated as presented by us in [2d] is found to be very low. The threecomponent systems based on the parent compounds (benzophenone, pyrene, anthracene) does not lead to an efficient polymerization (i.e., final conversion 400 nm (ε ∼ 8500 and

Figure 7. Photopolymerization profiles of EPOX under air, upon Hg lamp irradiation in the presence of (1) TH (3% w/w) and (2) Tz_An/ TH (1.2%/3% w/w).

high versatility of these triazines derivatives that can be used in combination of iodonium or thianthrenium salts in CP initiating systems. 2.d. Photopolymerization of DVE/EPOX Blends. Interestingly, when using Tz_An/Iod2 to polymerize a DVE/EPOX blend (20%/80% w/w) upon a halogen lamp irradiation in laminate, a copolymer is readily formed and a tack free coating is obtained after only 10 min. At this time, the EPOX and divinyl ether double bond (CC) conversions are about 50% and 80%, respectively (Figure S6, Supporting Information). The conversion of the divinyl ether DVE is faster than that of EPOX (Figure S6, Supporting Information) as resulting from the usual higher reactivity of vinylethers in cationic polymerization. The polymerization initiation can be easily explained by eq r7, where the VE+ cation is the initiating structure produced by the oxidation of the VE• radical (obtained by addition of the (TMS)3Si• or Ph• radicals to the vinylether double bond) by the iodonium salt. (TMS)3 Si•(or Ph•) + VE → VE• 8644

(r7a)


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Figure 8. (A) Photopolymerization profiles of TMPTA upon a Xe−Hg lamp irradiation (λ > 300 nm) in laminate in the presence of (1) Tz_BP/ Iod1 (1%/2% w/w), (2) Tz_BP/(TMS)3Si−H/Iod1 (1%/3%/2% w/w), (3) Tz_Py/(TMS)3Si−H/Iod1 (1%/3%/2% w/w), and (4) Tz_An/ (TMS)3Si−H/Iod1 (1%/3%/2% w/w). (B) Photopolymerization profiles of TMPTA upon a halogen lamp irradiation in laminate in the presence of (1) Tz_Py (1% w/w), (2) Tz_Py/Iod1 (1%/2% w/w), and (3) Tz_Py/(TMS)3Si−H/Iod1 (1%/3%/2% w/w). (C) Evolution of the IR band of (TMS)3Si−H recorded during the photopolymerization of Tz_Py/(TMS)3Si−H/Iod1 (1%/3%/2% w/w) under air and upon a halogen lamp irradiation. (D) Photopolymerization profiles of TMPTA upon a Xe−Hg lamp irradiation (λ > 300 nm) under air in the presence of (1) Tz_Py/ Iod1 (1%/2% w/w) and (2) Tz_Py/(TMS)3Si−H/Iod1 (1%/3%/2% w/w).

VE• + Ph 2I+ → VE+ + Ph• + Ph − I

(Figure 8A). The high reactivity of the new three-component system is ascribed to the formation of free radicals (Ph• and R3Si•) through r1-r3 that can initiate the polymerization of TMPTA (Figure 8). Indeed, Ph• and (TMS)3Si• are excellent polymerization initiating radicals exhibiting high rate constants for their addition to the acrylate double bond (k > 106 M−1 s−1).7b,c,14,15 The same holds true when using a halogen lamp irradiation with Tz_Py/(TMS)3Si−H/Iod1 (Figure 8B). A final conversion higher than 50% is reached within 5 min of irradiation and a tack-free coating is obtained. A high Si−H consumption is still found in agreement with eq r3 (Figure 8C). Remarkably, no polymerization is observed when using Tz_Py/Iod1 upon a Xe-lamp exposure under air. The addition of (TMS)3 Si−H improves the photopolymerization of TMPTA. Rather high polymerization rates and conversions (∼35%) are obtained (Figure 8D). This rather interesting behavior of the proposed silane containing photoinitiating systems under air is in line (as already shown elsewhere, see e.g. in7b,c,15) with the ability of the silanes to convert the stable peroxyls into new polymerization initiating silyls. 3.b. Triazine Derivative/Amine Photoinitiating Systems. Interestingly, when using N-methyl diethanolamine (MDEA) as a co-initiator (triazine derivative/MDEA initiating systems), good photopolymerization profiles are recorded (Figure 9). With Tz_Py/MDEA, a final conversion >70% is reached within 5 min of irradiation, forming a tack-free coating. A final conversion >70% is excellent for a trifunctional acrylate monomer.[1] The aminoalkyl radicals generated from the Tz_Py/MDEA initiating system (see above in reaction r6) are known as very efficient structures for the addition to acrylates.16

(r7b)

3. Radical Photopolymerization of Acrylates. 3.a. Triazine Derivative/Silane/Iod1 Photoinitiating Systems. The best typical conversion−time profiles of trimethylolpropane triacrylate TMPTA are shown in Figures 8 and 9. The triazine derivatives/(TMS)3Si−H/Iod1 combination is a good initiating system; i.e., high polymerization rates and final conversions are obtained in laminate under a Xe−Hg lamp exposure and tackfree coatings are formed (see Tz_Py or Tz_An in Figure 8A). Using Tz_BP, the conversion reaches a maximum of ∼20%

Figure 9. Photopolymerization profiles of TMPTA upon a Xe−Hg lamp irradiation (λ > 300 nm) in laminate in the presence of (1) Tz_Py (1% w/w) and (2) Tz_Py/MDEA (1%/5% w/w). 8645


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Tz_Py is also able to initiate alone the polymerization (Figure 9 curve 1) but to a lower extent (Figure 9, curve 2 vs curve 1). This can be ascribed to a hydrogen abstraction reaction between *Tz_Py and the monomer M thereby generating an initiating radical M(−H)• , reaction r8. *TzPy + M → (TzPy − H)• + M(−H )•

(r8)

CONCLUSION In the present paper, original trifunctional photoinitiators tPIs comprising anthracenes, pyrenes and benzophenones as peripheral chromophores of a triazine central core are proposed. The strong molecular orbital coupling existing between the peripheral chromophores and the triazine moiety enabled to significantly improve the light absorption properties of the new photoinitiators. The three-component (tPI/silane/ iodonium salt) systems in RP and CP and the two-component (tPI/amine) combination in RP are more efficient than the reference systems. Very soft irradiation conditions as well as visible lights (Xe−Hg lamp, 365 nm LED, halogen lamp) can be successfully employed. Some of these compounds can also sensitize the thianthrenium salt decomposition; this can likely be extended to other sulfonium salts. Development of triazine based organic photocatalysts should likely be proposed in the future. This work should also open the way to highly efficient light harvesting photoinitiators where the PI/spacer coupling is much more important. ASSOCIATED CONTENT

S Supporting Information *

(i) Synthesis of thioxanthone−anthracene, (ii) UV−visible absorption spectra of Py, Tz_Py, An, and Tz_An, (iii) fluorescence quenching of 1Tz_Py by Iod1, (iv) fluorescence quenching of 1Tz_Py by triethylamine, (v) photopolymerization profiles of EPOX in the presence of Tz_Py/Iod2 and Tz_Py/(TMS)3Si−H/Iod2, (vi) photopolymerization profiles of EPOX in the presence of Tz_An/(TMS)3Si−H/Iod1 and thioxanthone−anthracene/(TMS)3Si−H/Iod1, and (vii) photopolymerization profiles of an EPOX/DVE blend in the presence of Tz_An/Iod2.This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: (J.L.) [email protected]; (D.G.) didier.gigmes@ univ-provence.fr. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by the “Agence Nationale de la Recherche” grant ANR 2010-BLAN-0802. JL thanks the Institut Universitaire de France for the financial support. The authors thank G. Norcini (Lamberti Spa) for the gift of recrystallized (4-hydroxyethoxyphenyl) thianthrenium hexaflorophosphate (TH). The authors also thank Dr. Nicolas Blanchard and Cedric Tresse (COB-UHA, Mulhouse, France) for the preparation of the thioxanthone−anthracene. 8646


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Trifunctional Photoinitiators Based on a Triazine Skeleton for Visible

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