Multicolor Photoinitiators for Radical and Cationic Polymerization

Article pubs.acs.org/Macromolecules

Multicolor Photoinitiators for Radical and Cationic Polymerization: Monofunctional vs Polyfunctional Thiophene Derivatives Pu Xiao,† Frédéric Dumur,‡ Damien Thirion,§ Sébastien Fagour,§ Antoine Vacher,§ Xavier Sallenave,§ Fabrice Morlet-Savary,† Bernadette Graff,† Jean Pierre Fouassier,∥ Didier Gigmes,*,‡ and Jacques Lalevée*,† †

Institut de Science des Matériaux de Mulhouse IS2M, UMR CNRS 7361, UHA, 15, rue Jean Starcky, 68057 Mulhouse Cedex, France ‡ CNRS, Institut de Chimie Radicalaire, Aix-Marseille Université, UMR 7273, F-13397 Marseille, Cedex 20, France § Laboratoire de Physico-chimie des Polymères et des Interfaces, Université de Cergy-Pontoise, 5 mail Gay Lussac, Neuville sur Oise, 95031 Cergy-Pontoise Cedex, France ∥ Formerly, UHA, 3 rue Alfred Werner, 68093 Mulhouse Cedex, France S Supporting Information *

ABSTRACT: Thiophene and polythiophene derivatives have been prepared and used as photoinitiators upon visible light exposure. Their abilities to initiate, when combined with an iodonium salt (and optionally Nvinylcarbazole), a ring-opening cationic photopolymerization of epoxides and radical photopolymerization of acrylates under various different irradiation sources (i.e., very soft halogen lamp irradiation, laser diode at 405, 457, 473, 532, and 635 nm and blue LED bulb at 462 nm) have been investigated. These systems are characterized by a remarkable performance for purple to red light exposure. They are also particularly efficient for the cationic and radical photopolymerization of an epoxide/acrylate blend in a one-step hybrid cure and lead to the formation of an interpenetrated polymer network IPN (30 s for getting tack-free coatings). Their migration stability is excellent in the cured IPNs. The photochemical mechanisms are studied by steady state photolysis, fluorescence, cyclic voltammetry, electron spin resonance spin trapping, and laser flash photolysis techniques.

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INTRODUCTION In the photopolymerization area, the design of high performance photoinitiating systems PISs upon visible lights and soft irradiation conditions (low intensity) becomes important and is attracting an increasing attention.1−22 Such typical PISs contain a photoinitiator PI that has to absorb the light and one or two additives so that suitable PI/additive interactions can lead to initiating radicals (for free radical polymerization FRP), cations, or radical cations (for cationic polymerization CP or free radical promoted cationic polymerization FRPCP). The absorption of PI in term of red-shifted wavelength maxima and huge molar extinction coefficients is a key point for getting a high reactivity/efficiency of the polymerizable formulation in the above considered experimental conditions. Using substituted UV PIs or a lot of dyes where the classical π-electron delocalization ensures lower transition energies has been already achieved but very often restricted to radical systems (see e.g. refs 1−22). In recent papers, we have particularly looked for PIs of cationic and radical polymerization being able to exhibit a broad and intense absorption in the whole visible range. Examples include the design of light harvesting organic PIs (exhibiting highly coupled molecular orbitals23−26) and, in © 2013 American Chemical Society

the current year, multicolor PIs working over the 400−700 nm such as a known photochrome (2,7-di-tert-butyldimethyldihydropyrene DHP27) or several synthesized new push−pull PIs (e.g., thiobarbituric, indanedione, or Michler’s ketone derivatives28−30) where charge transfer transitions occur. In the present paper, we report on a monofunctional thiophene and a polyfunctional thiophene derivative usable as multicolor PIs (QXTP and PQXTP, Scheme 1), the expected high delocalization in PQXTP being expected to allow an unprecedented purple-to-red light irradiation for both the ringopening polymerization ROP of epoxides and the FRP of acrylates (i.e., household halogen lamp and blue LED bulb at 462 nm, laser diodes at 405, 457, 473, 532, and 635 nm). These particular compounds are particularly well-known for the manufacture of polymers in photovoltaic applications (see refs 31−38 and references herein). Other thiophene derivatives are also known as photosensitizers for the onium salt photoinitiated cationic polymerization39−42 and radical polyReceived: July 3, 2013 Revised: July 28, 2013 Published: August 20, 2013 6786

dx.doi.org/10.1021/ma401389t | Macromolecules 2013, 46, 6786−6793

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intensity: I0 ≈ 12 mW cm−2 in the 370−800 nm range) and a household blue LED bulb at 462 nm (∼10 mW cm−2). Photopolymerization Experiments. For photopolymerization experiments, the conditions are given in the figure captions. The photosensitive formulations were deposited on a BaF2 pellet under air or in laminate (25 μm thick) for irradiation with different lights. The evolution of the epoxy group content of EPOX and the double-bond content of TMPTA were continuously followed by real-time FTIR spectroscopy (JASCO FTIR 4100)52,53 at 790 and 1630 cm−1, respectively. Fluorescence Experiments. The fluorescence properties of QXTP and PQXTP were studied in tetrahydrofuran using a JASCO FP-750 spectrometer. The interaction rate constants kq between QXTP (or PQXTP) and Iod were extracted from classical Stern− Volmer treatments54 (I0/I = 1 + kqτ0[Iod], where I0 and I stand for the fluorescent intensity of QXTP or PQXTP in the absence and the presence of the Iod quencher, respectively; τ0 stands for the lifetime of the excited QXTP or PQXTP in the absence of Iod). Redox Potentials. The oxidation potentials (Eox) of QXTP and PQXTP were measured in acetonitrile/toluene (50%/50%, V/V) by cyclic voltammetry with tetrabutylammonium hexafluorophosphate (0.1 M) as a supporting electrolyte (Voltalab 6 Radiometer). The working electrode was a platinum disk and the reference electrode was a saturated calomel electrode (SCE). Ferrocene was used as a standard, and the potentials determined from the half-peak potential were referred to the reversible formal potential of this compound (+0.44 V/SCE). The free energy change ΔG for an electron transfer between QXTP (or PQXTP) and Iod can be calculated from the classical Rehm−Weller equation (eq I, where Eox, Ered, ES (or ET), and C are the oxidation potential of QXTP or PQXTP, the reduction potential of Iod, the excited singlet (or triplet) state energy of QXTP or PQXTP, and the electrostatic interaction energy for the initially formed ion pair, generally considered as negligible in polar solvents):55

Scheme 1. Chemical Structures of the Studied Thiophene and Polythiophene Derivatives (QXTP and PQXTP)

merization43 under UV lights39,43 or high intensity visible light sources.40,41 QXTP and PQXTP, associated with an iodonium salt (and optionally N-vinylcarbazole NVK), will be used here for the photoinduced polymerization of an epoxide, an acrylate, and an epoxide/acrylate blend. Moreover, the known coupling reaction of thiophene radical cations44,45 should ensure a low PI/ photolysis product migration. The migration of unreacted species to the surface of materials is actually a serious problem in different applications;46−48 several strategies for the design of low migration UV-sensitive PISs were reported (see e.g. refs 1, 49−51). The photochemical mechanisms of the initiation species formation will be investigated by steady state photolysis, fluorescence, cyclic voltammetry, electron spin resonance spin trapping, and laser flash photolysis techniques.

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EXPERIMENTAL SECTION ΔG = Eox − Ered − ES (or E T) + C

Materials. The investigated thiophene and polythiophene derivatives (QXTP and PQXTP) and other chemical compounds are shown in Schemes 1 and 2. QXTP and PQXTP were prepared according to

(I)

Laser Flash Photolysis. Nanosecond laser flash photolysis (LFP) experiments were carried out using a Q-switched nanosecond Nd/ YAG laser at λexc = 355 nm (9 ns pulses; energy reduced down to 10 mJ; minilite Continuum), and the analyzing system (for absorption measurements) consisted of a pulsed xenon lamp, a monochromator, a fast photomultiplier, and a transient digitizer (Luzchem LFP 212).56 ESR Spin Trapping (ESR-ST) Experiments. ESR-ST experiments were carried out using an X-band spectrometer (MS 400 Magnettech). The radicals were generated at RT upon the halogen lamp exposure under N2 and trapped by phenyl-N-tert-butylnitrone (PBN) according to a procedure57 described in detail. The ESR spectrum simulations were carried out with the WINSIM software. Migration Study. 0.1 g of EPOX/TMPTA blend (50%/50%, w/ w) in the presence of QXTP/Iod/NVK (0.5%/2%/3%, w/w/w) was polymerized under air or under nitrogen, and the produced interpenetrated polymer networks (IPNs) were immersed in 3 mL of acetone for 2 h. The amount of the extracted QXTP was determined using UV−vis spectroscopy. Computational Procedure. Molecular orbital calculations were carried out with the Gaussian 03 suite of programs. The electronic absorption spectra for the different compounds were calculated with the time-dependent density functional theory at the B3LYP/6-31G* level on the relaxed geometries calculated at the UB3LYP/6-31G* level.58,59

Scheme 2. Chemical Structures of Additives and Monomers

the procedures presented in detail in the Supporting Information. PQXTP is characterized by Mn ∼ 5000 g/mol (see Supporting Information). Diphenyliodonium hexafluorophosphate (Iod), N-vinylcarbazole (NVK), methyldiethanolamine (MDEA), phenacyl bromide (R-Br), and solvents were purchased from Sigma-Aldrich or Alfa Aesar and used as received without further purification. The monomers (3,4epoxycyclohexane)methyl 3,4-epoxycyclohexylcarboxylate (EPOX) and trimethylolpropane triacrylate (TMPTA) were obtained from Cytec with the highest purity available and used as benchmark monomers for cationic and radical polymerization. Irradiation Sources. Different visible irradiation sources were used for the photopolymerization experiments: laser diodes at 405 nm (∼8 mW cm−2), 457 nm (100 mW cm−2), 473 nm (100 mW cm−2), 532 nm (100 mW cm−2), and 635 nm (100 mW cm−2), polychromatic light from a halogen lamp (Fiber-Lite, DC-950; incident light

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RESULTS AND DISCUSSION 1. Light Absorption Properties of QXTP and PQXTP. The absorption spectra of the investigated thiophene and polythiophene derivatives (QXTP and PQXTP) in tetrahydrofuran are given in Figure 1. As the molecular weights being very different for QXTP and PQXTP, the extinction coefficients are reported in g−1 L cm−1 for more convenient comparisons. As expected, the absorption maximum of PQXTP 6787


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Table 1. EPOX Final Conversions Obtained upon a 800 s Exposure to Different Visible Light Sources in the Presence of QXTP or PQXTP Based PISs EPOX final conversion (%) irradiation sources laser diode at 405 nm laser diode at 457 nm laser diode at 473 nm laser diode at 532 nm laser diode at 635 nm halogen lamp

Figure 1. UV−vis absorption spectra of QXTP and PQXTP in tetrahydrofuran.

found at 531 nm (14.9 g−1 L cm−1) is red-shifted compared to that of QXTP (443 nm; 13.3 g−1 L cm−1; ∼8000 M−1 cm−1) which endows the PQXTP absorption with a better overlapping with the emission spectra of the halogen lamp. Interestingly, the absorption spectrum of QXTP and PQXTP quite well matches the laser diode lines at 457 nm (mass extinction coefficient: 12.6 and 9.1 g−1 L cm−1 for QXTP and PQXTP, respectively), 405 nm (9.5 and 9.7 g−1 L cm−1), and 473 nm (10.1 and 10.7 g−1 L cm−1) and the blue LED at 462 nm (12.0 and 9.6 g−1 L cm−1). In addition, PQXTP exhibits very interesting mass extinction coefficients at 532 nm (14.9 g−1 L cm−1 vs only 0.6 g−1 L cm−1 for QXTP) and 635 nm (1.6 g−1 L cm−1 vs no absorption for QXTP): PQXTP behaves as a multicolor PI and confers a panchromatic character on the associated polymerizable film. The red-shift of the PQXTP absorption is accounted for by an extended conjugation: the HOMO and LUMO are strongly delocalized over the whole skeleton of PQXTP (Figure S1 in the Supporting Information). 2. Photoinitiating Abilities of QXTP and PQXTP under Different Visible Light Irradiations. The photopolymerization of EPOX in the presence of QXTP (or PQXTP)/Iod and QXTP (or PQXTP)/Iod/NVK photoinitiating systems (NVK is known as a suitable additive60) under air were carried out using several different visible irradiation sources (Iod alone cannot initiate the polymerization as it works below 300 nm).54,61 Typical conversion profiles are given in Figure 2, and the final conversions are summarized in Table 1. For example, upon the 457 nm laser diode exposure, the QXTP/Iod

a

QXTP/Iod/NVK (0.5%/2%/3%, w/w/w)

PQXTP/Iod/NVK (0.5%/2%/3%, w/w/w)

34

21

57a|62

48

61

49

53

30

0

35

61

41

QXTP/Iod (0.5%/2%, w/w).

combination efficiently initiates the polymerization of EPOX (final conversion of 57%; Figure 2a, curve 0). The addition of NVK slightly enhanced the polymerization profile (final conversion of 62%; Figure 2a, curve 2 vs curve 0; tack-free coating). Using QXTP/Iod/NVK with the laser diodes at 405, 473, 532 nm and the halogen lamp leads to final conversions of 34%, 61%, 53%, and 61%, respectively. The amounts of absorbed light Iabs are in a ratio ∼0.5/10/9/0.5 at 405, 457, 473, and 532 nm, respectively. The excellent polymerization profiles found at 457 and 473 nm are in line with the high and almost similar Iabs. The polymerization profiles are less favorable at 405 and 532 nm; this is ascribed to the low light intensity of the source for 405 nm (∼8 mW cm−2) and the low extinction coefficient of QXTP at 532 nm (Figure 1), respectively. The polymerization efficiency in the presence of PQXTP/Iod/NVK was slightly lower than that of QXTP based PISs (Table 1), which is attributed to the fact that the lower mobility of the polythiophene chains reduces the PQXTP/Iod interaction. Nevertheless, good conversion−time profiles were obtained showing the efficiency of this new proposed system upon purple to red light irradiation (Figure 2b). Interestingly, a ∼35% final conversion was reached upon the laser diode exposure at 635 nm. The polymerization efficiencies at 405, 457, 473, 532, and 635 nm roughly follow the evolution of the Iabs (0.7/11/13/17/2 ratio). In both QXTP and PQXTP, the reactivity seems the same whatever the excitation wavelength.

Figure 2. Photopolymerization profiles of EPOX under air under different irradiations in the presence of (a) (curve 0) QXTP/Iod (0.5%/2%, w/w) for a laser diode at 457 nm; (curves 1−5) QXTP/Iod/NVK (0.5%/2%/3%, w/w/w) and (b) (curve 0) PQXTP/Iod (0.5%/2%, w/w/w) for a laser diode at 532 nm; (curves 1−6) PQXTP/Iod/NVK (0.5%/2%/3%, w/w/w). Selected irradiations: laser diodes at 405 nm (curve 1), 457 nm (curve 2), 473 nm (curve 3), 532 nm (curve 4), 635 nm (curve 6), and halogen lamp (curve 5). 6788


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Figure 3. Photopolymerization profiles of (a) neat TMPTA and (b) EPOX/TMPTA blend (10%/90%, w/w) in laminate. Halogen lamp exposure in the presence of QXTP/Iod (0.5%/2%, w/w) (1), QXTP/Iod/NVK (0.5%/2%/3%, w/w/w) (2), QXTP/MDEA (0.5%/2%, w/w) (4), and QXTP/ MDEA/R-Br (0.5%/2%/3%, w/w/w) (5). Laser diode exposure at 457 nm in the presence of QXTP/Iod/NVK (0.5%/2%/3%, w/w/w) (3) and QXTP/MDEA/R-Br (0.5%/2%/3%, w/w/w) (6).

Table 2. TMPTA Final Conversions Obtained upon a 800 s Exposure to Different Visible Light Sources in the Presence of QXTP or PQXTP Based PISs TMPTA final conversion (%)

a

irradiation source

QXTP/Iod/NVK (0.5%/2%/3%, w/w/w)

QXTP/MDEA/R-Br (0.5%/2%/3%, w/w/w)

PQXTP/Iod/NVK (0.5%/2%/3%, w/w/w)

laser diode at 457 nm halogen lamp

36 18a|27

54 37b|44

21 14c|24

QXTP/Iod (0.5%/2%, w/w). bQXTP/MDEA (0.5%/2%, w/w). cPQXTP/Iod (0.5%/2%, w/w).

5 (halogen lamp exposure under air). The decrease of the QXTP absorption at 443 nm is accompanied by the appearance of a new band at 603 nm that can be ascribed to QXTP•+ (reactions 1 and 2) and photoproducts generated from the coupling reaction of the QXTP cation radicals (reaction 3) as observed in related systems.41 As to PQXTP, the lower mobility of the polythiophene chains reduces the reactivity of the coupling reaction. Although the Iabs is obviously higher when using PQXTP, the photolysis of PQXTP/Iod was slower. These behaviors are in line with the trend observed in the polymerization ability.

The QXTP/Iod and QXTP/Iod/NVK combinations can also initiate the radical polymerization of TMPTA in laminate, but the efficiency was low (Figure 3a, curves 1−3). As found in cationic polymerization, the photoinitiation abilities of PQXTP based PISs were still lower than those of QXTP (Table 2). In addition, the QXTP/methyldiethanolamine MDEA (0.5%/2%, w/w) and QXTP/MDEA/phenacyl bromide R-Br (0.5%/2%/ 3%, w/w/w) systems can also initiate the FRP of TMPTA (higher TMPTA final conversions but lower polymerization rates; Figure 3a, curves 4−6). The low efficiency of these PISs is probably due to the poor solubility of QXTP in TMPTA (in contrast with its excellent solubility in EPOX). Therefore, 10% of EPOX was added into the formation to improve the solubility of QXTP (Figure 3b). As expected, the polymerization profiles were enhanced with final conversions of e.g. 36% (QXTP/Iod; halogen lamp; Figure 3b, curve 1), 48% (QXTP/Iod/NVK; halogen lamp; Figure 3b, curve 2), and 51% (QXTP/Iod/NVK; 457 nm laser diode; Figure 3b, curve 3). An interpenetrated polymer network (IPN) can also be prepared through a concomitant cationic/radical photopolymerization of an EPOX/TMPTA blend (50%/50% w/w; one-step hybrid cure process) using the QXTP/Iod/NVK or PQXTP/ Iod/NVK system in laminate or under air upon exposure to the halogen lamp, the laser diode at 457 nm, or the blue LED bulb at 462 nm (Figure 4). As a consequence of the oxygen inhibition effect (which is reduced in laminate) and the predominant consumption of the radicals in the FRP of TMPTA in laminate (rather than by the FRPCP of EPOX), the final conversions of TMPTA are higher in laminate than under air and the situation is opposite when considering the EPOX final conversions (Table 3). The household blue LED bulb (462 nm) irradiation is also very efficient as tack free IPN coatings can be obtained within 30 s. The PQXTP/Iod/NVK combination is less efficient but also leads to tack-free coatings. 3. Photochemistry of the Studied Thiophene Derivatives. The steady state photolysis of QXTP/Iod and PQXTP/Iod solutions in tetrahydrofuran is displayed in Figure

QXTP → 1Q XTP (hv) and 1Q XTP → 3Q XTP

(1)

Q XTP + Ph 2I+ → QXTP•+ + Ph 2I•

1,3

→ QXTP•+ + Ph• + Ph−I 2QXTP•+ → QXTP−QXTP + 2H+

(2) (3)

Fluorescence quenching experiments lead to very high QXTP/Iod and 1PQXTP/Iod interaction rate constants: kq = 1.3 × 109 and 5.5 × 108 M−1 s−1 for QXTP (kqτ0 = 36.6, fluorescence lifetime τ0 = ∼28.3 ns) and PQXTP (kqτ0 = 2.1, τ0 = ∼3.8 ns), respectively. The free energy changes ΔG for the QXTP/Iod and PQXTP/Iod electron transfer reaction are highly negative and make the processes favorable: ΔG = −1.31 and −1.22 eV, respectively (oxidation potentials Eox of the QXTP and PQXTP = 0.94 and 0.64 V, respectively, as measured by cyclic voltammetry; reduction potential Ered = −0.2 V1 for Iod; singlet state energy ES (2.45 and 2.06 eV for QXTP and PQXTP) extracted from the UV−vis absorption and fluorescence emission spectra as usually done62). The QXTP/Iod and PQXTP/Iod electron transfer quantum yields ΦeT calculated according to eq II1 are 0.63 and 0.09, respectively (for [Iod] = 4.7 × 10−2 M). These relative ΦeT can explain the lower efficiency of PQXTP/Iod compared to 1

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Figure 4. Photopolymerization profiles of an EPOX/TMPTA blend (50%/50%, w/w) in the presence of (i) QXTP/Iod/NVK (0.5%/2%/3%, w/w/ w) under air (a, c, e) and in laminate (b, d, f); halogen lamp exposure (a, b); laser diode exposure at 457 nm (c, d); blue LED bulb exposure at 462 nm (e, f); and (ii) in the presence of PQXTP/Iod/NVK (0.5%/2%/3%, w/w/w) in laminate upon the (g) halogen lamp exposure and (h) laser diode at 457 nm exposure.

rate constant of 3QXTP/Iod (reaction 2) cannot be safely determined. The process is, however, favorable (ΔG = −0.41 eV using the calculated triplet state energy of 3QXTP (1.55 eV at B3LYP/6-31G* level)). The low intensity of this transient probably indicates a rather limited triplet quantum yield probably due to a low intersystem crossing (reaction 1) from

QXTP/Iod in the ROP of EPOX or the FRP of TMPTA, i.e., less radicals and cations are generated in reaction 2 for PQXTP. ΦeT = kqτ0[Iod]/(1 + kqτ0[Iod])

(II)

In LFP experiments (laser excitation at 355 nm), the transient absorption is very low and noisy; i.e., the interaction 6790


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Table 3. EPOX and TMPTA Final Conversions Obtained from the Photopolymerization of an EPOX/TMPTA Blend (50%/50%, w/w) under Air or in Laminate upon Exposure to the Halogen Lamp or the Laser Diode at 457 nm for 800 s or the LED at 462 nm for 300 s in the Presence of QXTP/ Iod/NVK (0.5%/2%/3%, w/w/w) or PQXTP/Iod/NVK (0.5%/2%/3%, w/w/w) QXTP/Iod/NVK (0.5%/2%/3%, w/w/w) EPOX conversion (%) under air halogen lamp laser diode at 457 nm LED at 462 nm

in laminate

under air

in laminate

59 46 26 78 55 44 40 78 53 45 37 79 PQXTP/Iod/NVK (0.5%/2%/3%, w/w/w) EPOX conversion (%)

halogen lamp laser diode at 457 nm

TMPTA conversion (%)

Figure 6. Kinetic monitored at 660 nm for a solution of QXTP in the presence of [Iod] = 16.5 mM, laser excitation at 355 nm; in nitrogensaturated tetrahydrofuran.

TMPTA conversion (%)

under air

in laminate

under air

in laminate

47 48

28 27

22 22

63 59

4. Migration Study. A satisfactory migration stability is already found when using QXTP in photoinitiating systems. Indeed, the free QXTP in the IPNs was extracted using acetone. As shown in Figure 8, less than 0.01% of QXTP (IPN prepared under nitrogen) and 99.5% of QXTP remains inserted in the polymer). In the formulation, the generated QXTP•+ is incorporated in the IPNs matrix through the ring-opening addition to the epoxide and the selfcoupling reaction (reaction 3), which improves the migration stability of QXTP in the IPNs.

the singlet to the triplet state.40,41 As a consequence, the singlet route is probably the most important for reaction 2. Upon excitation of a QXTP/Iod solution, however, a very long-lived transient absorption at 660 nm was observed (Figure 6): it can be assigned to a thiophene derived radical cation in line with reaction 2, the photolysis experiment (see above), and other works on somewhat similar systems.41 In line with reaction 2, phenyl radicals are also observed in ESR spin trapping experiments on QXTP/Iod (Figure 7A) and PQXTP/Iod (Figure 7B). The multicomponent QXTP/MDEA or QXTP/MDEA/RBr systems work according to reactions 4 and 5 as already proposed in other systems.30,53 On the basis of the above investigations, the generated QXTP•+ and Ph-NVK+ species initiate the CP/FRPCP of EPOX and the Ph•, Ph-NVK•, aminoalkyl, and phenacyl radicals initiate the FRP of TMPTA.

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CONCLUSION Thiophene derivatives QXTP and PQXTP in combination with an iodonium salt Iod and N-vinylcarbazole NVK can be used as high performance purple to red light sensitive PISs to efficiently initiate the free radical promoted cationic photopolymerization of EPOX and the radical photopolymerization of TMPTA under various different visible light irradiation sources. The QXTP based photoinitiating system was also efficient for the cationic and radical photopolymerization of an epoxide/acrylate blend in a one-step hybrid cure and led to the formation of an interpenetrated polymer network. Interestingly, the migration stability of QXTP is excellent. PQXTP is an interesting compound exhibiting a visible absorption better than that of QXTP and extending up to 635 nm. While keeping a high light absorption, its worse solubility in acrylates could be improved by synthesizing lower molecular weight derivatives (indeed, an electron delocalization of a few monomer units is enough to

Q XTP + MDEA → QXTP•− + MDEA•+

1,3

→ QXTP − H• + MDEA (−H)•

(4)

QXTP•− + R−Br → QXTP + (R−Br)•− → QXTP + R• + Br −

(5)

Figure 5. Steady state photolysis of (a) QXTP/Iod and (b) PQXTP/Iod in tetrahydrofuran upon the halogen lamp exposure; [Iod] = 2 × 10−2 M. UV−vis spectra recorded at different irradiation times. 6791


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Figure 7. ESR spectra of the radicals generated in (A) QXTP/Iod and (B) PQXTP/Iod and trapped by PBN in tert-butylbenzene: (a) experimental and (b) simulated spectra. PBN/phenyl radical adducts obtained in QXTP/Iod and PQXTP/Iod: aN = 14.2 G, aH = 2.2 G and aN = 14.2 G, aH = 2.2 G, respectively; reference values.63,64 (2) Crivello, J. V. Photoinitiators for Free Radical, Cationic and Anionic Photopolymerization, 2nd ed.; John Wiley & Sons: Chichester, 1998. (3) Dietliker, K. A Compilation of Photoinitiators Commercially Available for UV Today; Sita Technology Ltd.: Edinburgh, London, 2002. (4) Allen, N. S. Photochemistry and Photophysics of Polymer Materials; Wiley: New York, 2010. (5) Eaton, D. F. In Advances in Photochemistry; Hammond, D. H., Gollnick, G. S., Eds.; Wiley: New York, 1986; pp 427−487. (6) Cunningham, A. F.; Desobry, V. In Radiation Curing in Polymer Science and Technology; Fouassier, J. P., Rabek, J. F., Eds.; Elsevier: Barking, UK, 1993; Vol. 2, pp 323−374. (7) Monroe, B. M.; Weed, G. C. Chem. Rev. 1993, 93, 435−448. (8) Timpe, H. J. In Radiation Curing in Polymer Science and Technology; Fouassier, J. P., Ed.; Elsevier: Barking, UK, 1993; Vol. 2, pp 529−554. (9) Urano, T.; Ito, H.; Yamaoka, T. Polym. Adv. Technol. 1999, 10, 321−328. (10) Urano, T. J. Photopolym. Sci. Technol. 2003, 16, 129−156. (11) Kabatc, J. Polymer 2010, 51, 5028−5036. (12) Matsushima, H.; Hait, S.; Li, Q.; Zhou, H.; Shirai, M.; Hoyle, C. E. Eur. Polym. J. 2010, 46, 1278−1287. (13) Yagci, Y.; Jockusch, S.; Turro, N. J. Macromolecules 2010, 65, 6245−6260. (14) Allonas, X.; Ibrahim, A.; Ley, C.; Saimi, H.; Bugnet, J.; Kawamura, K. J. Photopolym. Sci. Technol. 2011, 24, 531−534. (15) Li, Z.; Siklos, M.; Pucher, N.; Cicha, K.; Ajami, A.; Husinsky, W.; Rosspeintner, A.; Vauthey, E.; Gescheidt, G.; Stampfl, J.; Liska, R. J. Polym. Sci., Part A: Polym. Chem. 2011, 49, 3688−3699. (16) Sun, F.; Zhang, N.; Nie, J.; Du, H. J. Mater. Chem. 2011, 21, 17290−17296. (17) Wei, J.; Wang, B. Macromol. Chem. Phys. 2011, 212, 88−95. (18) Griesser, M.; Dvorak, C.; Jauk, S.; Hofer, M.; Rosspeintner, A.; Grabner, G.; Liska, R.; Gescheidt, G. Macromolecules 2012, 45, 1737− 1745. (19) Karaka-Balta, D.; Temel, G.; Okal, N.; Yagci, Y. Macromolecules 2012, 45, 119−125. (20) Yilmaz, G.; Acik, G.; Yagci, Y. Macromolecules 2012, 45, 2219− 2224. (21) Yilmaz, G.; Iskin, B.; Yilmaz, F.; Yagci, Y. ACS Macro Lett. 2012, 1, 1212−1215. (22) Korkut, S. E.; Temel, G.; Karaca Balta, D.; Arsu, N.; Kasım Şener, M. J. Lumin. 2013, 136, 389−394. (23) Telitel, S.; Dumur, F.; Faury, T.; Graff, B.; Tehfe, M.; Gigmes, D.; Fouassier, J. P.; Lalevée, J. Beilstein J. Org. Chem. 2013, 9, 877−890. (24) Telitel, S.; Dumur, F.; Gigmes, D.; Graff, B.; Fouassier, J. P.; Lalevée, J. Polymer 2013, 54, 2857−2864. (25) Tehfe, M. A.; Dumur, F.; Graff, B.; Clement, J. L.; Gigmes, D.; Morlet-Savary, F.; Fouassier, J. P.; Lalevée, J. Macromolecules 2013, 46, 736−746.

Figure 8. UV−vis absorption spectra of QXTP extracted with acetone from the IPN film prepared from the photopolymerization of an EPOX/TMPTA blend (50%/50%, w/w) in the presence of QXTP/ Iod/NVK (0.5%/2%/3%, w/w/w) (1) under air and (2) under nitrogen.

keep the absorption properties). The approach is expected to pave the way for the future development of novel multicolor photoinitiators with high migration stability.

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis of QXTP and PQXTP; Figure S1: HOMO and LUMO for QXTP and PQXTP. 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: [email protected] (J.L.); [email protected] (D.G.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.L. thanks the Institut Universitaire de France for the financial support. This work has benefited from the facilities and expertise of the Small Molecule Masse Spectrometry platform of IMAGIF.

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REFERENCES

(1) Fouassier, J. P.; Lalevée, J. Photoinitiators for Polymer SynthesisScope, Reactivity, and Efficiency; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, 2012. 6792


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Multicolor Photoinitiators for Radical and Cationic Polymerization

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