Copper Complexes in Radical Photoinitiating Systems - American

Article pubs.acs.org/Macromolecules

Copper Complexes in Radical Photoinitiating Systems: Applications to Free Radical and Cationic Polymerization upon Visible LEDs Pu Xiao,*,† Frederic Dumur,‡ Jing Zhang,† Jean Pierre Fouassier, Didier Gigmes,*,‡ and Jacques Lalevée*,† †

Institut de Science des Matériaux de Mulhouse IS2M, UMR CNRS 7361, ENSCMu-UHA, 15, rue Jean Starcky, 68057 Mulhouse, Cedex, France ‡ , Aix-Marseille Université, CNRS, ICR UMR7273, 13397 Marseille, Cedex 20, France S Supporting Information *

ABSTRACT: Three copper complexes (E1, G1, and G2) with different ligands in combination with an iodonium salt (and optionally another additive) were used to generate radicals upon soft visible light exposure (e.g., polychromatic visible light from a halogen lamp, laser diodes at 405 and 457 nm, LEDs at 405 and 455 nm). This approach can be worthwhile and versatile to initiate free radical photopolymerization, ring-opening cationic photopolymerization, and the synthesis of interpenetrating polymer networks. The photochemical mechanisms for the production of initiating radicals are studied using cyclic voltammetry, electron spin resonance spin trapping, steady state photolysis, and laser flash photolysis techniques. The photoinitiation ability of the copper complexes based photoinitiating systems are evaluated using real-time Fourier transform infrared spectroscopy. G1 and G2 are better than the well-known camphorquinone (CQ)-based systems (i.e., TMPTA conversion = 18%, 35%, 48%, and 39% with CQ/iodonium salt, CQ/amine, G1/iodonium salt, and G2/iodonium salt, respectively; halogen lamp exposure). Interestingly, some of these systems are also better than the well-known type I phosphine oxide photoinitiator (BAPO) clearly showing their high performance. These copper complexes can be used as highly efficient catalysts in photoredox catalysis.

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INTRODUCTION Among various kinds of photoinitiators,1−5 metal complexes, e.g. ruthenium6,7 or iridium8−13 complexes, have been successfully used in photoinitiating systems (PISs) for polymer synthesis14,15 as they possess excellent photochemical properties (e.g., intense visible light absorption, long-lived excited states, and suitable redox potentials) and can work through either an oxidation or a reduction cycle to produce reactive species, e.g. radicals or cations. 16−21 Recently, copper complexes are attracting increasing attention in the photopolymerization area due to their comparative cost advantage. Copper complexes with suitable ligands exhibit long excitedstate lifetimes22 which could endow them with potential for various applications such as organic light-emitting diodes (OLED)23−25 or light-mediated photochemical reactions.26−29 It has been reported that radicals could be generated from the interaction between copper complexes and halides under visible light irradiation.27 Interestingly, copper complexes have been reported for the light-induced atom transfer radical polymerization (ATRP) in the presence30,31 or absence of common organic photoinitiators or dyes.32,33 However, to the best of our knowledge, no investigation has been done on the radical generation in copper complex/iodonium salt systems under visible light, and no attempt has been made to use copper © XXXX American Chemical Society

complexes in PISs for both cationic and radical photopolymerization reactions under soft visible light irradiations. Very few papers have only reported, a long time ago, the use of e.g. bis(amino)acid copper(II) chelates to initiate the aqueous photopolymerization of acrylamide at 365 nm, the copper(I) or copper(II) salts for the photopolymerization of tetrahydrofuran, and borate salts of copper for the polymerization of acrylates under visible light (see e.g. in the review paper34). In the present paper, three copper complexes (E1, G1, and G2; Scheme 1) have been incorporated into PISs (containing an iodonium salt and optionally another additive) to photochemically generate reactive species (i.e., radicals and cations). The photoinitiation ability of the copper complex-based PISs for radical or cationic polymerization and interpenetrating polymer network synthesis under 400−460 nm lights (LEDs, laser diodes, halogen lamp) will be investigated and compared to the reference camphorquinone (CQ)-based PISs. The encountered mechanisms will be studied using cyclic voltammetry, electron spin resonance spin trapping, steady state photolysis, and laser flash photolysis techniques. Received: April 2, 2014 Revised: May 19, 2014

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dx.doi.org/10.1021/ma5006793 | Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. Chemical Structures of the Studied Copper Complexes (E1, G1, and G2)

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Irradiation Sources. Different visible lights were used for the irradiation of photocurable samples: (i) polychromatic light from the halogen lamp (Fiber-Lite, DC-950; incident light intensity: ∼12 mW cm−2 in the 370−800 nm range), (ii) purple laser diode at 405 nm (∼8 mW cm−2) or blue laser diode at 457 nm (∼80 mW cm−2), and (iii) LED at 405 nm (ThorLabs; ∼110 mW cm−2) or LED at 455 nm (ThorLabs; ∼80 mW cm−2). The emission spectrum of the halogen lamp is given in the Supporting Information (Figure S1). Redox Potentials. The oxidation potentials (Eox vs SCE) of the studied copper complexes were measured in acetonitrile 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 the studied copper complexes and the iodonium salt can be calculated from the classical Rehm−Weller equation: ΔG = Eox − Ered − ES + C, where Eox, Ered, ES, and C are the oxidation potential of the studied copper complexes, the reduction potential of the iodonium salt, the excited state energy of the studied copper complexes, and the electrostatic interaction energy for the initially formed ion pair, generally considered as negligible in polar solvents.37 ESR Spin Trapping (ESR-ST) Experiment. The ESR-ST experiment was carried out using an X-band spectrometer (MS 400 Magnettech). The radicals were generated at room temperature upon the halogen lamp exposure under N2 and trapped by phenyl-N-tertbutylnitrone (PBN) according to a procedure38 described elsewhere in detail. The ESR spectra simulation was carried out using the WINSIM software. Steady State Photolysis Experiments. The copper complex (and optionally with the iodonium salt) solutions were irradiated with the laser diode at 405 nm, and the UV−vis spectra were recorded

EXPERIMENTAL SECTION

Materials. The investigated copper complexes (E1, G1, and G2) and other chemical compounds are shown in Schemes 1 and 2. E1, G1,

Scheme 2. Chemical Structures of Additives and Monomers

and G2 were prepared according to the procedures presented in detail in the Supporting Information. These compounds were prepared (see in the Supporting Information) with analytical purity up to accepted standards for new compounds (>98%) which were checked by high field NMR analysis. [Methyl-4-phenyl(methyl-1-ethyl)-4-phenyl]iodonium tetrakis(pentafluorophenyl) borate (Iod1)35,36 was obtained from Bluestar Silicones-France. Diphenyliodonium hexafluorophosphate (Iod2), N-vinylcarbazole (NVK), and solvents were purchased from Sigma-Aldrich or Alfa Aesar and used as received without further purification. Bisacylphosphine oxide (Irgacure 819 or BAPO) was obtained from BASF. Trimethylolpropane triacrylate (TMPTA) and (3,4-epoxycyclohexane)methyl 3,4-epoxycyclohexylcarboxylate (EPOX) were obtained from Allnex and used as benchmark monomers for radical and cationic photopolymerization.

Figure 1. (a) UV−vis absorption spectra of E1, G1, and G2 in DCM. (b) UV−vis absorption spectra of G1 in DCM and ACN. B


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Figure 2. UV−vis spectra of G1 in (a) DCM and (b) ACN at different irradiation time (laser diode at 405 nm).

(Figure 2b). This latter absorption at ∼450 nm is probably due to the presence of a Cu(dmp)2 complex (where dmp = 2,9dimethyl-1,10-phenanthroline) in the solution as proposed in ref 41; the irradiation accelerates the generation of the Cu(dmp)2 complex. It can be observed that for G1 the solvent affects its stability upon light irradiation. Indeed, in the case of G1 in DCM, there was no change in the UV−vis spectra upon light irradiation (Figure 2a), which indicates that G1 exhibits a good photostability in DCM.40 Similarly, E1 and G2 are also photostable in DCM (Figure S2 in the Supporting Information). Copper Complexes in Radical Initiating Systems. The free energy change of the E1/Iod1 electron transfer reaction ΔG = −1.72 eV is highly negative and makes the process favorable (the oxidation potential of E1, Eox = 1.25 V, was measured by cyclic voltammetry (Figure S3 in the Supporting Information); ES = 3.17 eV was evaluated from the average values of maximum absorption and fluorescence wavelengths of E1 (from this work); and Ered = −0.20 V was used for Iod1,1 which is the same as the Ered of Iod2 (−0.20 V)).12 As a result, the counteranions of iodonium salts would not affect the results of the photochemical mechanism studies for the initiation process. For the G1/Iod1 interaction, ΔG = −1.17 eV, which also supports a favorable electron transfer process (Eox = 1.35 V40 or 1.36 V,42 as previously reported, was used for the oxidation potential of G1 and ES = 2.72 eV, from this work). In ESR spin trapping experiments (Figure 3; irradiation of G1/Iod1; halogen lamp exposure), aryl radicals Ar• were observed (hyperfine coupling constants of the PBN spin adduct: aN = 14.2 G; aH = 2.2 G in agreement with that for Ar• 6,43). They result from reactions 1 and 2 and can initiate the radical polymerization of TMPTA.

using a JASCO V-530 UV/vis spectrophotometer at different irradiation times. 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 consisted of a ceramic xenon lamp, a monochromator, a fast photomultiplier, and a transient digitizer (Luzchem LFP 212).7 Photopolymerization Experiments. The experimental conditions for the photopolymerization reaction are given in the figure captions. The photocurable formulations were deposited on a BaF2 pellet under air or in laminate (the formulation is sandwiched between two polypropylene films to avoid a reoxygenation in the course of the photopolymerization; 25 μm thick) for irradiation with different lights. The evolution of the double-bond content of TMPTA and the epoxy group content of EPOX was continuously followed by real-time FTIR spectroscopy (JASCO FTIR 4100)13,39 at about 1630 and 790 cm−1, respectively.

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RESULTS AND DISCUSSION Light Absorption Properties of the Copper Complexes. The absorption spectra of the investigated copper complexes (E1, G1, and G2) in dichloromethane (DCM) are given in Figure 1a. For G1 and G2, the absorption maxima at 380 nm (ε380 nm ∼ 3200 M−1 cm−1) and 383 nm (ε383 nm ∼ 2300 M−1 cm−1), respectively, correspond to a metal-to-ligand charge-transfer (MLCT) transition; more intense intraligand transitions appear at shorter wavelengths.40 Interestingly, the light absorption of E1, G1, and G2 allows an efficient covering of the emission spectra of the laser diode or the LED at 405 nm (i.e., ε405 nm ∼ 2400, 2200, and 1800 M−1 cm−1 for E1, G1, and G2, respectively). For the laser diode at 457 nm, the overlapping with E1 (ε457 nm ∼ 670 M−1 cm−1) is higher than that with G1 (ε457 nm ∼ 70 M−1 cm−1) or G2 (ε457 nm ∼ 70 M−1 cm−1). The absorptions of the studied copper complexes also ensure a good overlapping with the emission spectrum of the halogen lamp. The absorption spectrum of G1 was also studied in a more polar solvent (acetonitrile ACN) and compared to that of G1 in DCM (Figure 1b). The MLCT maximum shifts from 380 nm in DCM to 374 nm in ACN, which is in agreement with the previously reported blue-shifts in more polar solvents.40 More interestingly, an absorption band at approximately 450 nm is observed in ACN, which could be attributed to the solventdependent ligand redistribution reactions of G1 in ACN as already noted in ref 40. Furthermore, when following the evolution of the UV−vis spectra of G1 in DCM and ACN as a function of time (upon the laser diode at 405 nm exposure; Figure 2), it can be clearly seen that, in ACN, the absorption band of G1 at 374 nm decreases, whereas the absorption at ∼450 nm increases

Figure 3. ESR spectra of the radical generated in G1/Iod1 upon the halogen lamp exposure and trapped by PBN in toluene: (a) experimental and (b) simulated spectra. C


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G2) alone in DCM is very photostable; see above). On the other hand, a very slow bleaching occurred in the E1/Iod2 system (Figure 4a), which supports the much lower photoinitiation ability of these latter systems (see below). Laser flash photolysis experiments were also carried out. The short lifetime of the excited state of E1 (τ0 < 6 ns) is unfavorable for an efficient E1/Iod1 interaction and likely explains a low yield in initiating radicals in full agreement with the poor reactivity of this two-component system. The luminescence observed at 600 nm after the laser excitation of G1 at 355 nm in dichloromethane (long lifetime: approximately 3 μs; a little bit shorter than that previously reported: 14.3 μs;40 Figure 5) was quenched by Iod1. Moreover, a very long-lived

(1)

*Cu I + Ar2I+ → Cu II + Ar2I• → Cu II + Ar • + Ar−I (2)

By addition onto the N-vinylcarbazole (NVK) double bond, these aryl radicals can be converted into NVK-based radicals that can be further easily oxidized by an iodonium salt, thereby leading to efficient initiating cations for the ring-opening cationic polymerization (ROP) of epoxides (Scheme 3).44 Scheme 3. Conversion of Easily Oxidizable NVK-Based Radicals Obtained from Aryl Radicals; Generation of Initiating Cations from the Oxidization of These NVK-Based Radicals by an Iodonium Salt

The steady state photolysis of E1/Iod2, G1/Iod2, and G2/ Iod2 in dichloromethane is shown in Figure 4 (upon the laser diode at 405 nm exposure under air). A very fast photolysis was observed for G1 (or G2)/Iod2 under the irradiation (Figure 4b,c). More specifically, the absorption at 380 nm for G1 (or 383 nm for G2) decreased fast during the irradiation, and a small shoulder peak at ∼450 nm appeared (as in the case of G1 alone in ACN). It means that a photochemical reaction between G1 (or G2) and Iod2 happens, and the addition of Iod2 accelerates the photolysis of G1 (or G2) in DCM (G1 (or

Figure 5. Transient traces of G1 upon addition of different amount of Iod1 as monitored at 600 nm immediately after the laser excitation at 355 nm in nitrogen-saturated dichloromethane.

transient absorption was observed: it can be assigned to newly generated photoproducts as confirmed by the change of the

Figure 4. Steady state photolysis of (a) E1/Iod2, (b) G1/Iod2, and (c) G2/Iod2 ([Iod2] = 2.2 mM) in DCM upon the laser diode at 405 nm exposure; UV−vis spectra recorded at different irradiation times. D


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Figure 6. Photopolymerization profiles of TMPTA in the presence of (a) E1/Iod2 (0.2%/2%, w/w) upon the laser diode at 405 or 457 nm exposure in laminate; (b) G1/Iod2 (0.2%/2% w/w) upon the laser diode at 405 nm (curve 1) or 457 nm (curve 2), or halogen lamp (curve 3) exposure in laminate; G1/Iod2/NVK (0.2%/2%/3%, w/w/w) upon the halogen lamp exposure in laminate (curve 4) or under air (curve 5); and CQ/Iod2 (0.5%/2%, w/w) (curve CQ/Iod2) or CQ/MDEA (0.5%/2%, w/w) (curve CQ/MDEA) upon the halogen lamp exposure as references; (c) G1/ Iod2/NVK (0.2%/2%/3%, w/w/w) upon the LED at 405 nm (curve 1) or LED at 455 nm (curve 2) exposure in laminate; BAPO (0.2 wt %) upon the LED at 405 nm (curve BAPO) exposure in laminate as reference; (d) G2/Iod2 (0.2%/2% w/w) upon the laser diode at 405 nm (curve 1) or 457 nm (curve 2), or halogen lamp (curve 3) exposure in laminate.

ground state absorption spectra upon the laser exposure (Figure S4 in the Supporting Information). Photoinitiation Ability of the Investigated Copper Complexes E1, G1, and G2. Free Radical Polymerization FRP of Acrylates. The FRP of TMPTA in laminate upon the laser diodes (405 and 457 nm) led to quite low conversions when using E1/Iod2 (40%; laser diodes at 405 and 457 nm or halogen lamp irradiation). The efficiency of the G1/Iod2/NVK system was even better (higher photopolymerization rate Rp; conversions ∼56%; Figure 6b, curve 4 vs curve 3; Table 1; upon the halogen lamp exposure). It also worked under air (Figure 6b, curve 5) but with much lower Rp and conversion due to the oxygen inhibition effect. Interestingly, G1/Iod2 or G1/Iod2/NVK exhibited a better efficiency than the well-known CQ-based PISs (Figure 6b and Table 1; in laminate; halogen lamp irradiation). Moreover, the performance of the G1/Iod2/NVK system upon LED irradiations (at 405 or 455 nm) is also excellent [Figure 6c, curves 1 and 2 for LED at 405 nm (conversion = 63%) and 455 nm (56%), respectively] and even better than that of the well-known commercial type I photoinitiator BAPO with the LED at 405 nm. The G2/Iod2 PIS (Figure 6d) was not as efficient as the G1/Iod2 system, which could be ascribed to the better light absorption property of G1 (higher extinction coefficients; see in Figure 1a). Cationic Polymerization CP and Free Radical Promoted Cationic Polymerization FRPCP of Epoxides. The cationic polymerization of EPOX in the presence of E1 (0.2 wt %), E1/ Iod1 (0.2%/2%, w/w), or E1/Iod1/NVK (0.2%/2%/3%, w/w/ w) under air was carried out using the laser diode (457 nm) or the halogen lamp irradiation: no polymerization was observed in any case. The same holds true when using G2-based PISs (e.g., G2/Iod1 or G2/Iod1/NVK) and CQ-based systems (i.e., CQ/Iod2 or CQ/Iod2/NVK). However, the initiation ability of the G1/Iod1 PIS was relatively weak (conversion = 11% and 13% upon the halogen lamp and the 405 nm laser diode, respectively). Remarkably, the G1/Iod1/NVK system ensured

Table 1. TMPTA Conversions (in %) Obtained in Laminate or under Air upon Exposure to Different Visible Light Sources for 400 s in the Presence of E1, G1, or G2 Based PISs (E1, G1, or G2:0.2 wt %; Iod2:2 wt %; NVK: 3 wt %); CQ/Iod2 (0.5%/2%, w/w), CQ/MDEA (0.5%/2%, w/w), or BAPO (0.2 wt %) as References

E1/Iod2 G1/Iod2 G1/Iod2/ NVK G2/Iod2 CQ/ MDEA CQ/Iod2 BAPO a

laser diode (405 nm)

laser diode (457 nm)

25a 41a

14a 41a

18a

LED (405 nm)

LED (455 nm)

halogen lamp

63a

56a

48a 56a|13b

26a

39a 35a 18a 53a

In laminate. bUnder air. E


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Figure 7. (a) Photopolymerization profile and (b) IR spectra recorded before and after the photopolymerization of EPOX in the presence of G1/ Iod1/NVK (0.2%/2%/3%, w/w/w) upon the halogen lamp exposure under air. The IR bands of the epoxy, NVK double bond, polyether, and hydroxyl are observed at ∼790, ∼1640, ∼1080, and ∼3500 cm−1, respectively. (c) Photopolymerization profile of EPOX in the presence of G1/ Iod2/NVK (0.2%/2%/3%, w/w/w) upon the LED at 405 nm (curve 1) or LED at 455 nm (curve 2) exposure under air.

Figure 8. Photopolymerization profiles of EPOX/TMPTA blend (50%/50%, w/w) in the presence of G1/Iod1/NVK (0.1%/3%/5%, w/w/w) (a) under air or (b) in laminate upon the halogen lamp exposure.

good initiation ability for the polymerization of EPOX under air upon the halogen lamp exposure (Figure 7a). A high EPOX conversion (61% at t = 800 s) was associated with a high NVK double bond consumption (74%) and the concomitant formation of polyether and hydroxyl groups (Figure 7b). It means that the addition of NVK in the formulations improves the polymerization profiles of EPOX as previously reported for other systems.44 Lower conversion (32%) was obtained upon the laser diode at 405 nm exposure. More interestingly, the G1/ Iod2/NVK PIS was also very efficient for the polymerization of EPOX under the LEDs at 405 or 455 nm irradiation (Figure 7c): tack-free coatings (EPOX conversion = 58%) can be obtained after photopolymerization, which demonstrates the excellent ability of the G1/Iod2/NVK PIS to reduce oxygen inhibition effect in the FRPCP process. The cation formation results from reactions 1 and 2 (see above) and reactions 3−5:

Ar • + NVK → Ar−NVK•

(3)

Ar−NVK• + Ar2I+ → Ar−NVK+ + Ar • + Ar−I

(4)

Ar−NVK• + Cu II → Ar−NVK+ + Cu I

(5)

II

The formed Cu in reaction 2 is a quite inefficient species to initiate a ring-opening polymerization process when using a Cu complex/Iod two-component system: this may be attributed to the fact that the active site (CuII) was hidden by the ligands. However, CuII can be a good oxidation agent and can oxidize free radicals;45 this process (reaction 5) regenerates CuI and leads to additional initiating species (Ar−NVK+). Reaction 5 ensures a photocatalyst behavior for the copper complex. As known,44 the NVK-based cations (formed here in the Cu complex/Iod/NVK three-component system) exhibit a high efficiency toward the initiation of the EPOX cationic photopolymerization. For CQ/Iod2 or CQ/Iod2/NVK, no polymerF


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ization occurs upon halogen lamp showing the excellent efficiency of the newly proposed G1-based system. IPN Synthesis: Photopolymerization of EPOX/TMPTA Blends. The G1/Iod1/NVK PIS allows the formation of interpenetrated polymer networks (IPNs) through a concomitant cationic/radical photopolymerization of EPOX/TMPTA blend (50%/50% w/w) under air or in laminate upon the halogen lamp exposure (Figure 8). As elsewhere,46−48 the polymerization conversion of TMPTA is higher in laminate than under air and the situation is opposite when considering the EPOX conversions (Table 2). It is attributed to the fact that

EPOX conversion

TMPTA conversion

45 38

30 74

the FRP of TMPTA is faster than the FRPCP of EPOX, and most of the free radicals are consumed to initiate the FRP when the oxygen inhibition effect is reduced by the laminated condition.

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CONCLUSION Copper complexes G1 or G2 with iodonium salt (and optionally NVK) can work as photoinitiating systems for the radical polymerization of TMPTA in laminate under different visible light irradiations (e.g., polychromatic visible light from halogen lamp, laser diodes at 405 or 457 nm, and LEDs at 405 or 455 nm). More interestingly, the G1/Iod1/NVK system exhibits good initiation ability for the cationic polymerization of EPOX under air upon the halogen lamp, LEDs at 405 or 455 nm exposure. Acrylate/epoxide-based IPNs can also be produced under air with the halogen lamp. The high performance of the G1/Iod/NVK system makes it a versatile PIS especially under the LED (405 or 455 nm) irradiations. The photochemical mechanism studies reveal that the ligands of the copper complexes play an important role (e.g., light absorption properties, luminescence lifetimes, or redox potentials) and affect the photoinitiation ability. The design of other copper complexes (with various ligands) as photoinitiators will be presented in forthcoming papers.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details; Figures S1−S4. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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Table 2. EPOX and TMPTA Conversions (in %) Obtained in the Photopolymerization of EPOX/TMPTA Blend (50%/ 50%, w/w) under Air or in Laminate upon Exposure to the Halogen Lamp (t = 800 s) in the Presence of G1/Iod1/NVK (0.1%/3%/5%, w/w/w) under air in laminate

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (J.L.). *E-mail [email protected] (D.G.). *E-mail [email protected] (P.X.). Notes

The authors declare no competing financial interest. J.P.F.: Formerly, ENSCMu-UHA, 3 rue Alfred Werner, 68093 Mulhouse Cedex, France. G


Macromolecules

Article

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