Frida
Nzulu
a,
Sofia
Telitel
b,
François
Stoffelbach
a,
Bernadette
Graff
b,
Fabrice
Morlet-Savary
b,
Jacques
Lalevée
*b,
Louis
Fensterbank
*a,
Jean-Philippe
Goddard
*c and
Cyril
Ollivier
*a
aUMR CNRS 8232, Institut Parisien de Chimie Moléculaire, Sorbonne Université UPMC Univ Paris 06, 4 place Jussieu, CC 229, F-75252 Paris Cedex 05, France. E-mail: louis.fensterbank@upmc.fr; cyril.ollivier@upmc.fr
bInstitut de Science des Matériaux de Mulhouse IS2M, UMR 7361 CNRS, Université de Haute Alsace, 15 Rue Jean Starcky, 68057 Mulhouse Cedex, France. E-mail: jacques.lalevee@uha.fr
cLaboratoire de Chimie Organique et Bioorganique EA 4566, Université de Haute-Alsace, Ecole Nationale Supérieure de Chimie de Mulhouse, 3 bis rue Alfred Werner, 68093 Mulhouse Cedex, France. E-mail: jean-philippe.goddard@uha.fr
First published on 11th May 2015
Controlled/living atom transfer radical polymerization of methacrylates and acrylates initiated by ethyl α-bromophenylacetate (EBPA) as the initiator in the presence of low loadings (1.25 mol% vs. initiator) of a dinuclear gold(I) complex based photocatalyst [Au2(μ-dppm)2]Cl2 has been accomplished in solution and in laminate under UVA and visible-light photoreductive conditions. In solution, the linear increase of molecular weights with methyl methacrylate (MMA) conversion and the low dispersity are consistent with a controlled/living process. In a film, trimethylolpropane triacrylate (TMPTA) was polymerized and the ethyl α-bromophenylacetate (EBPA)/[Au2(μ-dppm)2]Cl2 system resulted in a faster rate of polymerization compared to EBPA/Ir(ppy)3. Chain extensions of polymers were successfully conducted leading to block copolymers, which also confirms the living character of this new system. Photophysical experiments support a conventional photoredox-catalyzed ATRP mechanism. Finally, this approach utilizes a gold catalyst featuring better solubility and lower cost than the well-known Ir(ppy)3 complex.
The use of photochemical stimuli had a tremendous impact not only in organic chemistry,7 molecular biology and electronics but also in polymer chemistry since the pioneer work of Otsu.8 Accordingly some controlled radical photopolymerization (CRP2) methodologies have been recently proposed. The most substantial advantage is that photo-induced processes are extremely fast, and give access to spatially controlled reactions for photografting, photoinduced modification or (micro)patterning of surfaces. They also represent an environmentally friendly alternative to thermal processes allowing reactions to proceed under mild conditions and to use functional groups and materials that decompose at high temperatures. Several CRP2 systems including transition metal-mediated CRP,9 RAFT,10 NMP,4,11 ITP12 and ATRP have been applied. Of all the CRPs, ATRP13 has been the focus of intense research in the polymer community owing to its versatility for the synthesis of well-defined polymers. Quite recently, Matyjaszewski's,14 Yagci's,15 and Hawker's16 groups have successfully devised reactive systems exploiting the redox properties of copper and iridium catalysts respectively in the presence of light. In this field, the development of very efficient photosensitive systems for UVA/visible light remains a huge challenge. Indeed, these types of radiation are safer than the more-energetic UVB and UVC light for the operator, more chemoselective causing less side reactions with reactants, and also eco-friendly (low energy consumption, no release of ozone or harmful UV rays and very little heat emission). Among the few examples of gold catalyzed polymerization,17 the use of a gold photocatalyst to promote photo-ATRP upon UVA/visible light has not been reported so far.
In this context, we showed that photoactivated digold bis(diphenylphosphino)methane dichloride [Au2(μ-dppm)2]Cl21 first reported by Schmidbaur,18 was able to catalyze ATRP processes (Scheme 1). Photoredox properties of the corresponding photoactivated cationic complex [Au2(μ-dppm)2][ClO4] were studied by Che who highlighted its ability to generate carbon-centered radicals by single electron transfer (SET) from alkyl halides.19 In 2013, Barriault nicely demonstrated the synthetic potential of 1 in combination with Hünig's base as a sacrificial electron donor for the photoreduction of unactivated aryl and alkyl bromides.20 Inspired by these seminal studies, we anticipated that the unique properties of 1 would permit SET reduction of activated alkyl bromides, initiate and control ATRP processes. Therefore, we investigated the catalytic properties of 1 in different solvents at rt under light activation to generate a carbon-centered radical from ethyl α-bromophenylacetate (EBPA, 2) and trigger polymerization of rather hydrophilic monomers such as methyl methacrylate (MMA, 3) in solution and trimethylolpropane triacrylate (TMPTA, 5) in laminate. Chain extension experiments with benzyl methacrylate (BnMA, 4) and hydrophobic 2,2,3,3,4,4,4-heptafluorobutyl acrylate (HFBA, 6) were also studied (Scheme 1).
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Scheme 1 Atom transfer radical polymerization of various (meth)acrylates catalyzed by digold bis(diphenylphosphino)methane dichloride [Au2(μ-dppm)2]Cl21 as a photoredox catalyst. |
We used laser flash photolysis (LFP) experiments in order to know if the initiator 2 was able to interact with the excited state of complex 1 (AuII*2). The transient absorption spectrum was recorded after laser flash excitation at 355 nm.21 The excited state of 1 is characterized by a transient absorption at about 390 nm in accordance with Che's results.22 Then, LFP allowed the determination of the rate constant of the AuII*2 quenching by 2 (Fig. 1).23 An increasing amount of 2 was added to a solution of 1 under irradiation and a decay of AuII*2 absorption at 390 nm in DCM was observed. When 2 was omitted, the lifetime of 1 was 1.56 μs under a nitrogen atmosphere. This value significantly decreased to 110 ns in the presence of 4 μL of 2 (7.6 × 10−3 M) (Fig. 1). From a Stern–Volmer analysis, we have found that 2 is an excellent oxidative quencher of AuII*2 with a rate constant photolysis of 9.2 × 108 M−1 s−1. In addition, the cyclic voltammogram (CV) of 1 revealed three oxidation waves that appeared at +0.6 V, +1.2 V and +2.0 V in MeCN vs. SCE. The first wave at +0.6 V is assigned to the AuI metal-centered oxidation process while the CV of 2 also gave a reduction potential at −1.3 V (in MeCN vs. SCE). Thus, the oxidation of AuII*2 by 2 would presumably occur as supported by the favorable free energy change ΔG, derived from the Rehm–Weller equation (ΔG° = −1.6 eV) (see the ESI, S-10†). These initial findings suggest that the rate of photoreaction is primarily governed by the homolysis of the C–Br bond of EBPA. These also support that the system 1/2 could be used to initiate radical polymerization with no need of an additional sacrificial donor of electron, as required for the photoreduction of unactivated aryl and alkyl bromides.20
To support this mechanism, we monitored the steady state photolysis of [1] in the presence of 2. We observed a strong decrease of the absorption band at 320 nm upon diode laser exposure concomitantly with the appearance of a band at 420 nm over 10 s of irradiation (Fig. 2). This result confirmed the quenching of the excited state of 1 by the initiator 2 leading to the formation of an ethyl phenylacetyl radical, that could initiate the radical polymerization, and an oxidized gold complex {[Au2(μ-dppm)2Cl2]+, Br−} with an absorption UV signal suspected at 420 nm.
From this system, the generation of a carbon-centered radical intermediate derived from 2 was also confirmed by ESR spin-trapping experiments (Fig. 3).24 A dichloromethane solution of 2/1 and N-tert-butyl-α-phenylnitrone (PBN) was irradiated (Xe–Hg lamp) for 20 s and monitored by ESR. A characteristic signal of a nitroxide adduct which originates from the addition of the phenylacetate radical to PBN (aN = 14.3 G, aH = 2.5 G) was observed. Similarly, 6 minutes of sunlight exposure to the same solution resulted in an identical ESR signal. This set of experiments confirmed that gold catalyst 1 allows a very responsive photo radical process and 2 is an excellent oxidative quencher. Given the perfect adequacy in terms of photochemical properties and reactivity between catalyst 1 and initiator 2, polymerization of MMA 3 in solution was then carried out.
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Fig. 3 ESR spin trapping (PBN) spectrum. [PBN] = 7.90 × 10−4 M, [1] = 1.57 × 10−4 M, [2] = 1.91 × 10−3 M in 1.5 mL of DCM/tert-butylbenzene 1![]() ![]() |
The influence of the irradiation wavelength and the solvent was first examined. Typical conditions involved a controlled concentration of MMA (2.08 M) with an initiator/monomer/catalyst (2/3/1) ratio fixed at 1/500/0.0125 (mol/mol). Conversion of 3, molecular weight and dispersity of the so-formed PMMA were measured by gravimetry and GPC analysis and reported in Table 1.
Entry | Light (nm) | 2 Conv.b (%) | M n,theo (kg mol−1) | M n,exp (kg mol−1) | Đ |
---|---|---|---|---|---|
a Polymerizations were performed with [MMA] = 2.08 M (vol MMA + vol solvent) at rt with 2/3/1 = 1/500/0.0125 mol/mol. All reactions were degassed before setting the reaction. b Conversion measured by gravimetric analysis. c Determined after one precipitation in MeOH. d Reaction performed in DCM. e Reaction in a mixture 2/3 (1/500) in DMF without the gold catalyst. | |||||
1 | 300 | 84 | 42.3 | 34.0 | 1.81 |
2 | 350 | 75 | 37.5 | 34.0 | 1.67 |
3 | 380 | 54 | 27.4 | 42.0 | 1.77 |
4 | 419 | 63 | 31.7 | 48.4 | 1.85 |
5 | 350d | 55 | 27.8 | 24.7 | 1.62 |
6 | Dark | — | — | — | — |
7 | 350e | 100 | 50.0 | 25.0 | 2.1 |
Experiments were performed in N,N′-dimethylformamide (DMF) or dichloromethane (DCM) under UV to visible-light activation at 25 °C. At 300 nm in DMF, 84% of conversion was reached in 24 h (Table 1, entry 1). PMMA with lower number-average molar mass than the theoretical value was obtained with a dispersity of 1.81. Under these reaction conditions, effective initiation proceeded but the physical data of the PMMA can be indicative of side reactions. Lower conversions were observed at 380 and 419 nm (54 and 63% respectively) and the resulting polymers have higher molar mass, revealing a failure of the initiation system (Table 1, entries 3 and 4). A good control of the polymerization was achieved when the irradiation was done at 350 nm (Table 1, entry 2). The conversion of MMA was 75%, the molar mass distribution (Đ) of PMMA decreased to 1.67 with a number-average molar mass closer than the theoretical value. Well controlled polymerization was also obtained in DCM under irradiation at 350 nm (Table 1, entry 5). Molar mass fitted with the theoretical value and the dispersity was 1.62. These last results indicated that the initiation process was efficient and side reactions giving birth to dead chains were very limited. In order to argue in the favor of a controlled radical polymerization, the same mixture of 2/3/1 was placed in the dark and no polymer was formed (Table 1, entry 6). If the gold catalyst was omitted, polymerization occurred but it could not be controlled (Table 1, entry 7).
To evaluate the living character of this polymerization process, chain extension experiments were conducted using MMA or BnMA as the second monomer (Scheme 2). An original sample of PMMA obtained by the present protocol and purified by precipitation in methanol (Mn = 18.0 kg mol−1, Đ = 1.32), was considered as a macroinitiator and placed in DMF with MMA 3 and the gold photocatalyst 1 in a 1/500/0.0125 proportion respectively (Scheme 2A). After 24 h of irradiation at 350 nm, a PMMA homopolymer was isolated showing an important increase of Mn up to 30.9 kg mol−1. From a different PMMA sample (Mn,exp = 14.2 kg mol−1, Đ = 1.50), a block copolymer was obtained with BnMA 4 (PMMA/4/1 1/500/0.0125 mol/mol) (Scheme 2B). Similarly, a considerable Mn,exp increase was observed and has been determined to be 43.8 kg mol−1. The corresponding SEC traces of both experiments showed the evolution of the polymer populations. This clearly demonstrated the possibility of reactivating the bromide terminus of the polymer. Thus, it appears reasonable to postulate a photoATRP pathway for this novel new gold-catalyzed polymerization.
As initial experiments revealed that 1 efficiently catalyzes the polymerization of methacrylates 3 and 4 in solution, comparative kinetic experiments were carried out. A laminate system was chosen to compare the present system (2/1) with the known 2/fac-Ir(ppy)3 (Fig. 4). To address this question, the polymerization of TMPTA 5 in laminate with both systems was monitored by RT-FTIR spectroscopy at 405 nm. The disappearance of a characteristic absorption band of 5 (1600–1650 cm−1) served to measure the conversion of this monomer.25 The 2/fac-Ir(ppy)3 polymerization profile (black curve) showed a rapid initial rate to reach 33% conversion in 800 s. After an activation period of 50 s, the 2/1 system gave a faster polymerization rate than with iridium to reach 38% conversion in 400 s. The iridium catalyst was already known to be very efficient but to our delight, this new gold photocatalyst 1 proved to be even more active for the polymerization of acrylates in bulk.
The dependence of this polymerization process on light was further studied by employing a periodic “on/off” light event during the TMPTA polymerization in laminate 2/5/1 (1/100/0.0125 mol/mol) (Fig. 5). The film was irradiated with LED bulbs (405 nm) with alternating irradiation periods (30 s) and blackout (30 s). The conversion of 5 was monitored by RT-FTIR spectroscopy and is reported in Fig. 5 according to the sequential irradiation. The polymerization rates were dramatically impacted and turning off the light resulted in a significantly low rate. Thus, a clear acceleration of the polymerization was observed under irradiation. This experiment also showed the living character of the polymerization and turning off the light did not induce the interruption of the growing chains.
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Fig. 5 TMPTA conversion as a function of time for intermittent blue LED bulb irradiation. Polymerization was performed from a 2/5/1 (1/100/0.0125 mol/mol) mixture. |
An investigation of the living character was extended to surface functionalization. PhotoATRP of TMPTA 5 in laminate with the 2/1 system (2/5/1 1/100/0.0125 mol/mol) allowed obtaining a well defined film after Xe–Hg lamp irradiation for 3 min (Fig. 6A). The residual monomer was removed by washing with acetone and the surface was analyzed by XPS (see the ESI† for the full spectrum). This analysis evidenced the presence of gold and bromine atoms at the surface of the film. A drop of water on the layer made a 55° contact angle in agreement with the wettability character of PMMA. Then, HFBA 6 was deposited on the surface of the first layer and irradiated under the same conditions for 10 minutes (Fig. 6B). The contact angle dramatically changed from 55° to 115°, illustrating the surface modification to a more hydrophobic property due to the presence of perfluoroalkyl chains. This change proved the reactivation of the first layer for the synthesis of a block copolymer. Moreover, XPS of the second layer still showed the presence of gold and bromine atoms and opened the possibility of a third reactivation process (Fig. 6C). TEM (Transmission Electron Microscopy) analysis allowed visualization of the layer surfaces and so helped to exclude the presence of gold nanoparticles which could have been the active catalyst.
All these experiments completed by further mass spectrometry analysis by MALDI-TOF/ESI-TOF and ATRA studies (see the ESI, S17 and S18†) drove us to the following mechanism proposal for this new photoATRP process (Scheme 3). The polymerization relies on the in situ photogeneration of excited state [1]* and its subsequent reaction with alkyl halide (Pn-X) resulting in the formation of the active radical (Pn˙) and [Au2]IIIX. Then, the radical adds onto a monomer and the resulting radical is rapidly deactivated by [Au2]IIIX to form a dormant species and regenerate 1.
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Scheme 3 Proposed mechanism for photoATRP of (meth)acrylates using [Au2(μ-dppm)2]Cl2 as a photocatalyst. |
Footnote |
† Electronic supplementary information (ESI) available: Characterization data of the catalyst, organic compounds and polymers are provided. Photochemical and photophysical properties of the catalytic system are available. See DOI: 10.1039/c5py00435g |
This journal is © The Royal Society of Chemistry 2015 |