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A known photoinitiator for a novel technology: 2-(4-methoxystyryl)-4,6-bis(trichloromethyl)1,3,5-triazine for near UV or visible LED† Jing Zhang, Pu Xiao,* Fabrice Morlet-Savary, Bernadette Graff, Jean Pierre Fouassier‡ and Jacques Lalevée* 2-(4-Methoxystyryl)-4,6-bis(trichloromethyl)-1,3,5-triazine (R–Cl) appears as a versatile high-performance photoinitiator (PI) under LED exposure at 385, 395 or 405 nm (intensities in the range ∼9–140 mW cm−2). It has been used as an efficient Type I cleavable PI for the free radical photopolymerization (FRP) of (meth)acrylates under near UV or visible LED irradiation. When combined with various additives (i.e. amine, iodonium salt, or N-vinylcarbazole), the R–Cl based photoinitiating systems can exhibit an even higher efficiency than R–Cl alone. Remarkably, R–Cl alone as well as the R–Cl/additive systems lead to a photoinitiation ability for the FRP of methacrylate under air at 405 nm that is better than that of well-known commercial photoinitiators (e.g. bisacylphosphine oxide (BAPO), 2,4,6-trimethylbenzoyldiphenyl-phosphineoxide (TPO), or 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1

Received 1st June 2014, Accepted 27th June 2014

(BDMB)). In addition, the R–Cl/iodonium salt/N-vinylcarbazole combination can also initiate the cationic

DOI: 10.1039/c4py00770k

polymerization of epoxides in the 385–405 nm range. Moreover, the photochemistry of these systems has been investigated by steady state photolysis, molecular orbital (MO) calculations, and electron spin

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resonance spin trapping techniques.

1.

Introduction

Nowadays, the design of novel high-performance photoinitiators (PIs) well adapted to light-emitting diode (LED)-based irradiation devices (especially near UV and visible LEDs) is still a challenge and attracts increasingly large attention.1,2 Indeed, near UV or visible LEDs possess enormous potential for polymer synthesis under soft conditions as substitutes for existing UV lamps due to their advantages, including better light output, safer usage, higher operating efficiency and lower cost.3 In free radical polymerization (FRP) under UV light, Type I cleavable PIs such as 2-hydroxy-2-methyl-1-phenyl-propan1-one (HAP; Irgacure 1173) or 1-hydroxy-cyclohexyl-phenylketone (HCAP; Irgacure 184) have been proved to be excellent

Institut de Science des Matériaux de Mulhouse IS2M, UMR CNRS 7361, ENSCMu-UHA, 15, rue Jean Starcky, 68057 Mulhouse Cedex, France. E-mail: [email protected], [email protected] † Electronic supplementary information (ESI) available: Emission spectra of different irradiation devices (Fig. S1–S3) and photopolymerization profiles of TMPTA in laminate in the presence of R–Cl, CQ, or CQ/R–Cl upon the LED@470 nm exposure (Fig. S4). This information is available free of charge via the Internet. See DOI: 10.1039/c4py00770k ‡ Formerly, ENSCMu-UHA, 3 rue Alfred Werner, 68093 Mulhouse Cedex, France.

This journal is © The Royal Society of Chemistry 2014

choices.1,4–6 Some commercial Type I PIs (e.g. bisacylphosphine oxide (BAPO), 2,4,6-trimethylbenzoyl-diphenyl-phosphineoxide (TPO), and 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)butanone-1 (BDMB; Irgacure 369)) exhibit visible light absorption (even up to 440 nm for BAPO) but they are mainly used under UV light in industrial applications as well.4–6 Only a few efforts have been recently made to develop novel visible light sensitive Type I PIs: examples include acylgermane-based PIs7–10 or cleavable thioxanthones.11 In contrast, strong efforts have been devoted to the design of various new multi-component photoinitiating systems (PISs) that could allow excitation up to 635 nm.1,12–18 Chlorotriazine derivatives have been widely used as coinitiators in Type II PIs or incorporated into three-component PISs for FRP reactions under exposure to visible light.2,13,15–17,19–26 They have also been reported as Type I PIs in patents to initiate free radical polymerization27–32 or as radical generators in pressure-sensitive adhesive materials under UV light.33–35 Interestingly, the chlorotriazine moiety has also been linked with visible light sensitive dyes, and free radicals usable in FRP are thus generated via an intramolecular electron-transfer process.14,36–40 Even though the capability of the particular vinyl-halomethyl-s-triazines to generate free radicals was mentioned in the patent literature (see above) and some

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chemical mechanisms were proposed,41,42 there is no report, to the best of our knowledge, of a thorough investigation of the photoinitiation ability of this kind of compound under near UV/visible LEDs. In addition, most of the reported triazine derivatives were concerned with FRP, their applications in cationic polymerization (CP) being limited; a new efficient system for CP upon near UV or visible LED is reported here. Herein, we will study the photochemical mechanisms involved in 2-(4-methoxystyryl)-4,6-bis(trichloromethyl)-1,3,5triazine (R–Cl) for the initiation of polymerization using new steady state photolysis, electron spin resonance (ESR) experiments, and molecular orbital calculations. Furthermore, the photoinitiation ability of R–Cl and R–Cl based systems in the FRP of (meth)acrylates and the CP of epoxides under LEDs (385 nm, 395 nm or 405 nm: intensities in the range ∼9–140 mW cm−2) will be investigated using real-time Fourier transform infrared spectroscopy (RT-FTIR).

theory at the B3LYP/6-31G* level on the relaxed geometries calculated at the UB3LYP/6-31G* level; the molecular orbitals involved in these transitions can be extracted.43,44 The geometries were frequency checked. 2.3.

Different lights were used for the irradiation of photocurable samples: UV LED centered at 385 nm (M385L2 – ThorLabs; ∼9 mW cm−2), LED centered at 395 nm (Taoyuan Electron Limited; ∼140 mW cm−2), LED centered at 405 nm (M405L2 – ThorLabs; ∼110 mW cm−2), and polychromatic light from a halogen lamp (Fiber-Lite, DC-950; incident light intensity: ∼12 mW cm−2 in the 370–800 nm range). The emission spectra of the irradiation sources are given in the ESI (Fig. S1–S3†). 2.4.

2. Experimental 2.1.

Materials

The investigated 2-(4-methoxystyryl)-4,6-bis(trichloromethyl)1,3,5-triazine (R–Cl), methyl diethanolamine (MDEA), ethyl 4-(dimethylamino)benzoate (EDB), diphenyliodonium hexafluorophosphate (Iod), and N-vinylcarbazole (NVK) were purchased from Sigma-Aldrich or Alfa Aesar, and their chemical structures are shown in Scheme 1. Bisacylphosphine oxide (BAPO), 2,4,6-trimethylbenzoyl-diphenyl-phosphineoxide (TPO), and 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1 (BDMD) were obtained from BASF. Trimethylolpropane triacrylate (TMPTA) and (3,4-epoxycyclohexane)methyl 3,4epoxycyclohexylcarboxylate (EPOX) from Allnex were used as benchmark monomers for radical and cationic photopolymerization. Bisphenol A-glycidyl methacrylate (Bis-GMA) and triethyleneglycol dimethacrylate (TEGDMA) were obtained from Aldrich and used with the highest purity available. 2.2.

Computational procedure

Molecular orbital calculations were carried out using the Gaussian 03 package. The electronic absorption spectrum for R–Cl was calculated with the time-dependent density functional

Scheme 1

Irradiation sources

Steady state photolysis experiments

The triazine-based photoinitiator R–Cl (and optionally with MDEA) in acetonitrile was irradiated with the LED@405 nm, and the UV-vis spectra were recorded using a JASCO V-530 UV/ Vis spectrophotometer at different irradiation times. 2.5.

ESR spin trapping (ESR-ST) experiments

ESR-ST experiments were carried out using a Bruker EMX-plus spectrometer (X-band). The radicals were generated at room temperature upon ambient light exposure under N2 and trapped by phenyl-N-tert-butylnitrone (PBN) according to a procedure45 described elsewhere in detail. The ESR spectra simulations were carried out using the WINSIM software. 2.6.

Photopolymerization experiments

For photopolymerization experiments, the conditions are given in the figure captions. The photocurable formulations were deposited on a BaF2 pellet under air or in laminate (25 μm thick films) for irradiation with different lights. The evolution of the double bond content of TMPTA (or a blend of Bis-GMA/TEGDMA) and the epoxy group content of EPOX were continuously followed by the real time FTIR spectroscopy (JASCO FTIR 4100)46,47 at about 1630 cm−1 and 790 cm−1, respectively.

Chemical structures of the investigated triazine-based photoinitiator, additives, and monomers.

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3. Results and discussion


3.1. Light absorption property of the studied triazine-based photoinitiator R–Cl The ground state absorption spectrum of R–Cl in acetonitrile (Fig. 1) exhibits an absorption maximum centered at 373 nm (with a corresponding molar extinction coefficient ε373 nm ∼ 44 000 M−1 cm−1). Moreover, the absorptions at the LED wavelengths (ε385 nm ∼ 39 000 M−1 cm−1; ε395 nm ∼ 29 000 M−1 cm−1; ε405 nm ∼ 17 000 M−1 cm−1) are excellent, making it an excellent potential PI. Additionally, it also exhibits satisfactory overlap with the emission spectrum of a halogen lamp. From the molecular orbital (MO) calculations, the near-UV/ visible absorption band is associated with a HOMO → LUMO transition and exhibits a partial charge transfer from the methoxystyryl to the triazine moieties (Fig. 2). Remarkably, the LUMO exhibits an anti-bonding character (σ*) for a C–Cl bond

Fig. 1

UV-vis absorption spectrum of R–Cl in acetonitrile.

Fig. 2 Highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of R–Cl at the UB3LYP/6-31G* level (isovalue = 0.04). The arrow indicates the anti-bonding character for a C–Cl bond (see text).


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(Fig. 2). This can be in agreement with a cleavage process of this bond in the excited state. 3.2.

Photochemistry of R–Cl

Steady state photolysis experiments of R–Cl alone or R–Cl/ MDEA in acetonitrile upon LED@405 nm irradiation are given in Fig. 3. A very fast decrease of the ground state absorption band of R–Cl occurred (Fig. 3(a)). This is in agreement with a cleavage process of a C–Cl bond (see below for the ESR data). The R–Cl/MDEA two-component system exhibited a slightly slower photolysis than R–Cl alone (Fig. 3(b) and 3(c)): this indicates a relatively low R–Cl/MDEA interaction compared to the primary process arising in R–Cl alone. In ESR spin trapping experiments (Fig. 4(A)), chlorine radicals Cl• (PBN/Cl• adduct: aN = 12.3 G, aH = 0.71 G; aCl = 4.7 G and 6.2 G for Cl37 and Cl35 in full agreement with Cl• generated by other approaches48) and chloro–carbon radicals Cl–C• (PBN/Cl–C• adduct: aN = 13.4 G, aH = 1.75 G) were clearly observed. The Cl–C• and Cl• radicals result from the cleavage of C–Cl bonds in R–Cl (Scheme 2). This cleavage process can be examined by molecular orbital calculations. The bond dissociation energy BDE (C–Cl) is relatively weak (51.5 kcal mol−1 at the UB3LYP/6-31G* level). This value is quite similar to that reported for a carbon–carbon bond BDE(C–C) ∼ 51 kcal mol−1 for a very efficient Type I photoinitiator (2,2′-dimethoxyl-2-phenyl acetophenone).1 This weak BDE(C–Cl) is in agreement with a Type I behavior for R–Cl and is also in agreement with the previously proposed mechanisms for chloro-triazine derivatives.41,42 Interestingly, the triplet excited state energy level is calculated at the UB3LYP/6-31G* level as ET[R–Cl]) = 47.5 kcal mol−1. This can suggest that the dissociation from the triplet state is endergonic and not favorable. From fluorescence experiments (Φfluo ∼ 2 × 10−4; this work), the singlet state energy level has been evaluated as ES[R–Cl] = 71.2 kcal mol−1; this is much higher than the BDE(C–Cl) showing a highly exothermic cleavage process from the first excited singlet state. Previous phosphorescence, laser flash photolysis and sensitization experiments on 4,6-bis(trichloromethyl)-1,3,5-triazines41,42 demonstrated that the intersystem crossing ISC quantum yield is low and, accordingly, a major singlet state pathway for the C–Cl cleavage process under direct excitation was postulated. For 9-chlorofluorene, a singlet state cleavage was fully shown by picosecond time resolved spectroscopies.49 By analogy and according to the ESR experiments and MO calculations, we could expect that the photodissociation of R–Cl occurs in the singlet state. The low yield of radiative deactivation suggests a fast process. This is confirmed here in R–Cl by the following experiment. The irradiation of camphorquinone CQ at 470 nm (intersystem crossing quantum yield = 1.0,1 3CQ lifetime ∼20 μs,50,51 triplet state energy: ET[CQ] = 52 kcal mol−1 50) in the presence of R–Cl (which is quasi transparent at this wavelength and very weakly excited) obviously forms 3R–Cl through triplet–triplet energy transfer: indeed, in reaction 1, ET[CQ] > ET[R–Cl] and 3CQ cannot lead to electron transfer reaction with RCl. The FRP of

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Fig. 3 Steady state photolysis of (a) R–Cl and (b) R–Cl/MDEA ([MDEA] = 0.1 M): UV-vis spectra recorded at different irradiation times; (c) decrease of R–Cl absorption at 373 nm in the absence/presence of MDEA. In acetonitrile. LED@405 nm irradiation.

Fig. 4 ESR spectra of the radicals generated in (A) R–Cl and (B) R–Cl/EDB upon an ambient light exposure and trapped by PBN in tert-butylbenzene: (a) experimental and (b) simulated spectra (EDB but not MDEA was used here to avoid a high polarity of the sample preventing an ESR analysis).

Scheme 2

Radicals generation from R–Cl under light.

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TMPTA using CQ alone or the CQ/R–Cl system upon LED@470 nm exposure (Fig. S4 in the ESI†) exhibits, however, the same polymerization profiles (these profiles are relatively slow; indeed, CQ itself initiates the FRP of TMPTA through the hydrogen abstraction from the monomer but this process is not very efficient;51 dramatically better profiles will be reported below under the direct excitation of R–Cl at 405 nm). As the


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CQ excitation should produce 3R–Cl in 1, this FRP result demonstrates that no efficient C–Cl bond cleavage occurs in 3 R–Cl and can confirm the unfavorable endergonic character of the triplet state dissociation. Therefore, the R–Cl photodissociation obeys a singlet route mechanism. An alternative path which cannot be completely excluded is one involving the population of an upper dissociative triplet state after direct excitation and intersystem crossing (this state being not populated by the triplet sensitization reaction 1) prior to relaxation to the nondissociative T1 state. Experiments to test mechanistic alternatives can be interesting by ultrafast spectroscopies but are beyond the scope of the present paper. CQ ! 1 CQ ðhν at 470 nmÞ and 1 CQ ! 3 CQ 3

CQ þ RCl ! CQ þ 3 RCl

ð1Þ

Upon addition of the amine (EDB) (the same likely holds true for MDEA), the ESR signal of Cl• and Cl–C• is strongly reduced and a new signal appeared (Fig. 4(B)) [aN = 15.1 G, aH = 2.85 G assigned to a α-aminoalkyl radical EDB(–H)• (the hyperfine coupling constants for the PBN radical adduct of EDB(–H)• can be affected by the presence of acid and the protonation of the amine)]: it can be assigned to the newly produced radicals arising from the hydrogen abstraction of Cl• or Cl–C• with EDB (2), but the R–Cl/EDB electron/proton transfer route (3) cannot be absolutely ruled out (3 is probably a minor pathway due to the expected short lifetime of the excited state of RCl (see above)).

3.3.

EDB þ Cl• ðCl–C• Þ ! H–Cl ðCl–CHÞ þ EDBð–HÞ •

ð2Þ

*R–Cl þ EDB ! H–Cl þ R• þ EDBð–HÞ •

ð3Þ

Photoinitiation ability of R–Cl upon LEDs

3.3.1. Free radical photopolymerization (FRP) of (meth)acrylates. On the basis of the above results, the radicals generated from R–Cl alone or R–Cl/amine (Scheme 2, or reactions 1 and 2) are expected to initiate FRP reactions of TMPTA upon LED exposure at 405 nm, 395 nm or 385 nm. As seen in Fig. 5 and Table 1, high polymerization rates Rp and final conversions FC (58%) were obtained with R–Cl alone upon exposure to the

Table 1 TMPTA final conversions (FCs) obtained in laminate or under air upon exposure to the LEDs at 385 nm or 405 nm for 400 s in the presence of R–Cl based PISs (R–Cl: 0.5 wt%; MDEA or EDB: 2 wt% in the formulations)

PISs

LED 385 nm

LED 405 nm

R–Cl R–Cl/MDEA R–Cl/EDB

59%b

25%a|58%b 42%a|61%b 35%a|61%b

a

Under air. b In laminate.

LED@405 nm (110 mW cm−2) in the laminate. When the photopolymerization was carried out under air, FC dramatically dropped (25%) due to the usual oxygen inhibition effect. Interestingly, the addition of an amine led to enhanced conversions under air (42 and 35% with MDEA and EDB, respectively); almost no change is noted in the laminate. Remarkably, the R–Cl/MDEA system efficiently initiates the polymerization of TMPTA in laminate (FC = 59%) even under the low-intensity UV LED@385 nm (∼9 mW cm−2). The FRP of methacrylates (i.e. Bis-GMA/TEGDMA blend (70%/30%, w/w)) at 405 and 385 nm is also feasible (Fig. 6(a) and Table 2). Using R–Cl alone, FC of 48% and 64% were obtained under air and in laminate, respectively (LED@405 nm exposure): this is likely due to the higher viscosity of the methacrylate (∼2000 mPa s for Bis-GMA/TEGDMA blend (70%/30%, w/w) vs. 80–135 mPa s for TMPTA at 25 °C) which would reduce the oxygen inhibition effect. As before, the addition of MDEA improves the polymerization profiles (see in Fig. 6(a) and Table 2), FC increasing up to 57 and 69% under air and in the laminate, respectively. Using NVK instead of MDEA also improved polymerization profiles (Table 2; by analogy with the behavior of other PI/NVK systems,52 excellent polymerization initiating NVK derived radicals (resulting from the addition of Cl–C• and Cl• to the double bond) are expected to be formed (4)). No improvement is noted when employing PI/Iod probably due to the lack of an efficient electron transfer. At 405 nm, due to their lower absorption, the well-known commercial UV-photoinitiators (BAPO, TPO, or BDMB) are less efficient (44, 37, 38% vs. 48% for R–Cl and 57% for

Fig. 5 Photopolymerization profiles of TMPTA (a) in laminate or (b) under air in the presence of R–Cl based PISs (R–Cl: 0.5 wt%; MDEA or EDB: 2 wt%). LEDs exposure at 385 nm or 405 nm.


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Fig. 6 Photopolymerization profiles of Bis-GMA/TEGDMA blend (70%/30%, w/w) in laminate or under air in the presence of R–Cl based PISs (and BAPO, TPO and BDMB used as references). R–Cl, BAPO, TPO and BDMB: 0.5 wt%; Iod: 1 wt%; MDEA or NVK: 2 wt%. Upon LED exposure at 385 nm or 405 nm.

Table 2 Final conversions (FCs) of Bis-GMA/TEGDMA blend (70%/30%, w/w) obtained under air or in laminate upon exposure to LEDs at 385 nm or 405 nm for 300 s in the presence of R–Cl based PISs (and BAPO, TPO and BDMB used as references). R–Cl, BAPO, TPO and BDMB: 0.5 wt%; MDEA: 2 wt%

PISs

LED 385 nm

R–Cl R–Cl/MDEA R–Cl/Iod R–Cl/NVK BAPO TPO BDMB

32%a 32%a 29%a 37%a

LED 405 nm 48%a|64%b 57%a|50%c|69%b 50%a 57%a 44%a 37%a 38%a

systems52) led to a FC of 58% or 63% under air upon LED@385 nm or LED@405 nm exposures, respectively; tackfree polyether coatings were obtained (Fig. 7) (the too low light intensity available with a halogen lamp allows a FC of only 18%). The R–Cl/Iod/NVK system was pretty stable in the EPOX formulation (only 2% of conversion decrease, from 63% to 61%, after three weeks of storage in the dark at room temperature; LED@405 nm exposure). Cl–NVK• ðor Cl–C–NVK• Þ þ Ph2 Iþ ! Cl–NVKþ ðor Cl–C–NVKþ Þ þ Ph• þ Ph–I

ð5Þ

Under air. b In laminate. c After three weeks of storage in the dark at room temperature, polymerization under air.

a

R–Cl/MDEA under air; see in Fig. 6(b) and Table 2). At 385 nm, however, they exhibit a similar efficiency. Interestingly, the R–Cl/MDEA also worked well for methacrylate under air upon LED@395 nm exposure and a FC of 49% was obtained. Cl• ðor Cl–C• Þ þ NVK ! Cl–NVK• ðor Cl–C–NVK• Þ

ð4Þ

After three weeks of storage in the dark at room temperature, a slight decrease of the photoinitiation ability of R–Cl/ MDEA (FC decreased from 57% to 50%; LED@405 nm) is noted, thereby demonstrating the quite good stability of this system in a methacrylate formulation. The starting color of the formulation was already pale before polymerization and the final coating became lighter after the reaction. Therefore, the photobleaching property of R–Cl endows it with potential applications in the manufacture of colorless coatings. This is in full agreement with the bleaching observed in solution (Fig. 3). 3.3.2. Cationic photopolymerization (CP) of epoxides. The CP of EPOX was also investigated. When using R–Cl (0.5 wt%) alone or R–Cl/Iod (0.5%/2%, w/w), they were not effective (LED@405 nm) which is attributed to the fact that no cation or radical cation was generated. In contrast, the R–Cl/Iod/NVK PIS (where the formation of polymerization initiating NVK based cations (4–5) can be expected as observed in other

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Fig. 7 Photopolymerization profiles of EPOX under air in the presence of R–Cl/Iod/NVK (0.5%/2%/3%, w/w/w) upon the LEDs at 385 nm or 405 nm or the halogen lamp exposure. Insert: NVK consumption for run LED@405 nm.


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4.

Conclusion

This paper reveals that 2-(4-methoxystyryl)-4,6-bis(trichloromethyl)-1,3,5-triazine (R–Cl) can be used as an efficient Type I cleavable photoinitiator (even better than several well-known commercial compounds) for free radical polymerization of (meth)acrylates upon LED (e.g. 405 nm) exposure. Its high FRP photoinitiation ability under air demonstrates its capability to reduce the oxygen inhibition effect. Moreover, the R–Cl/amine (or R–Cl/N-vinylcarbazole) two-component system exhibits a higher photoinitiation ability than R–Cl alone in some cases. Interestingly, R–Cl/Iod/NVK is also efficient for the cationic photopolymerization of epoxides, especially at 405 nm. The whole set of results make R–Cl a high-performance versatile photoinitiator under LEDs.

Conflict of interest The authors declare no competing financial interest.

Acknowledgements JL thanks the Institut Universitaire de France for the financial support.

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