Photocatalyzed Cu-Based ATRP Involving an Oxidative Quenching

Article pubs.acs.org/Macromolecules

Photocatalyzed Cu-Based ATRP Involving an Oxidative Quenching Mechanism under Visible Light Qizhi Yang,† Frédéric Dumur,‡ Fabrice Morlet-Savary,† Julien Poly,*,† and Jacques Lalevée† †

Institut de Science des Matériaux de Mulhouse (IS2M), UMR 7361 − CNRS/UHA, 15 rue Jean Starcky, 68057 Mulhouse, France Aix-Marseille Université, CNRS, ICR UMR7273, 13397 Marseille, Cedex 20, France

‡

S Supporting Information *

ABSTRACT: A new type of photocatalyzed Cu-based atom transfer radical polymerization (ATRP) is described, involving directly the light absorption of the activator form of the copper complex Cu(I). The selected catalyst was bis(1,10phenanthroline)copper(I), Cu(phen)2+, due to its intense absorption in the visible domain, which permitted to use very soft irradiation conditions, consisting of a simple household blue LED at 0.9 W. An excellent control over the polymerization of methyl methacrylate (MMA) in dimethylformamide (DMF) was observed under irradiation in these conditions, using ethyl α-bromophenylacetate (EBPA) as the initiator, with polydispersity indexes (PDI) as low as 1.10 while using low catalyst content (80 ppm). The proposed mechanism implies first the formation under irradiation of the excited state of the activator form of the complex Cu(I)*. It can then rapidly undergo the oxidative quenching of the alkyl bromide, which results in its conversion into the deactivator form of the complex Cu(II)−Br along with the generation of a propagating radical. The setting up of the ATRP equilibrium ensues. Additionally, it was possible to complete the catalysis mechanism by adding triethylamine (TEA), which permitted a faster polymerization, thanks to a faster regeneration of the activator Cu(I). An excellent control over the polymerization was also observed in the presence of TEA, with PDI as low as 1.06. The addition of TEA allowed also to use a catalyst loading as low as 20 ppm, while maintaining a good controllability.

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thermal counterparts7,8 or in the access to spatial and temporal controls.9 In particular, the development of atom transfer radical photopolymerization (ATRP2) has been a very active domain of research in the past few years.10 The effect of light on ATRP was first demonstrated by Guan et al., who observed an acceleration of the polymerization rate as well as a better control under visible light.11 Considering the well-known ATRP equilibrium, ATRP2 can recover actually several different strategies to introduce an effect of light in this mechanism, and a clear distinction between photoinduced and photocatalyzed mechanisms can be proposed. In photoinduced ATRP, the equilibrium between active and dormant species results either from the photoreduction of the deactivator or from radicals generated under irradiation, similarly to AGET (activators generated by electron transfer) ATRP and SR&NI (simultaneous reverse and normal) ATRP, respectively.2,3 In photocatalyzed ATRP, light absorption by the catalyst in its oxidized or reduced form results into more reactive activators or deactivators in their excited state and thus into faster activation or deactivation, respectively. A faster catalytic cycle ensues.

INTRODUCTION

The development of controlled radical polymerization (CRP) mechanisms has indisputably revolutionized the field of macromolecular engineering, offering an easy and unprecedented access to miscellaneous well-defined polymer architectures in terms of composition, topology, and functionalization.1 Atom transfer radical polymerization (ATRP) has been one of the most applied and studied CRP mechanisms, thanks to the excellent controllability and versatility that it can provide, while implying only simple experimental procedures based on commercially available reactants.2,3 In particular, the potential limitations inherent to the need for relatively high catalyst concentrations, encountered in the early developments of ATRP, have been overcome since then, thanks to improved mechanisms such as ARGET (activators regenerated by electron transfer) ATRP or ICAR (initiators for continuous activator regeneration) ATRP.2,3 Many current developments of CRPs have been directed toward the possibility of playing on their control mechanism through external stimuli.4 Among them, regulations based on light appear especially promising,5,6 since they can allow to combine the aforementioned advantages of CRPs with some typical features of photopolymerizations. This can consist for instance in the possibility to perform the reaction at ambient temperature by resorting to photoinitiators instead of their © 2015 American Chemical Society

Received: November 25, 2014 Revised: March 6, 2015 Published: March 19, 2015 1972

DOI: 10.1021/ma502384y Macromolecules 2015, 48, 1972−1980

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Macromolecules

Scheme 1. Types of Photocatalyzed ATRP, Involving the Reductive or the Oxidative Quenching of the Excited State Derived from the Deactivator or the Activator, Respectively; Additional Quenchers Can Also Be Used

in the formation of an Ir(IV)+ complex with a Br− counterion. This deactivator will finally react with a propagating chain to regenerate Ir(III). This photocatalysis was successfully applied under visible light to the CRP of methacrylates36,37,39 and acrylates.42 On the contrary, contributions reported so far on photocatalyzed ATRP resorting to complexes based on Cu or Ru have evidenced catalytic cycles involving a reductive quenching. A first study about photocatalyzed ATRP was reported by Yagci et al., who used Cu complexes with pentamethyldiethylenetriamine (PMDETA) ligand under UV irradiation.25 They noticed that light played a determining role when copper was introduced initially at the II oxidation state as CuBr2: polymerization of methyl methacrylate (MMA) occurred only in the presence of light. On the contrary, only a minor effect was observed when the copper salt was CuBr. These observations are consistent with a reductive quenching, though the corresponding mechanism was not proposed in this paper. It can be suggested that the quencher was the excess PMDETA: amines are well-known quenchers in photocatalysis that can be easily oxidized into radical cations.47 The role of extra amine as a quencher was indeed clearly established by Choi et al. in the case of photocatalyzed ATRP using [Ru(bpy)3]2+ as the catalyst.34 Yagci’s contribution was followed by two studies which reported excellent controllability of MMA polymerization under UV/vis irradiation, using Cu catalysts based on PMDETA or TPMA at low concentration.26,27 The reported results were also consistent with a possible reductive quenching, either by the solvent26 or by the excess of ligand.27 The possible effect of an amine onto Cu-based photocatalyzed ATRP was then more explicitly described by Haddleton et al.31 before its role as a reductive quencher was recently demonstrated by Matyjaszewski et al.33 Beyond ATRP2, photoredox catalysis has been already applied in the field of photopolymerizations.48,49 For instance, several catalysts based on ruthenium,50,51 iridium,52−56 or copper57,58 were designed and successfully used to initiate radical and radical promoted cationic photopolymerizations. More recently, an original control radical photopolymerization, consisting of the combination of the reversible addition− fragmentation chain transfer (RAFT) mechanism with a photocatalysis cycle based on Ir(ppy)3 or [Ru(bpy)3]2+, was also proposed.59−61

Regarding first of all photoinduced ATRP, all the contributions deal with usual copper catalysts. Several methods were reported to reduce in situ Cu(II) into Cu(I) under irradiation. For instance, radicals were directly generated under irradiation by oxidation of the solvent12 or by using photoinitiators under UV irradiation13,14 or visible light.15−17 More generally, several photosensitive reactants were used to generate electron donator species under UV or visible (vis) irradiation, prone to reduce the catalyst in its deactivator form into its activator counterpart: dimanganese decacarbonyl (vis),18,19 semiconductors based on doped ZnO nanoparticles (UV)20 or mesoporous carbon nitride (UV/vis)21 and TiO2 nanoparticles (UV).22 The coating of TiO2 nanoparticles with a dye allowed also the use of visible light.23 Camphorquinone was also used as a photoinitiator: its excited state resulting from light absorption can generate radicals by H abstraction from quenchers such as amines.24 The development of photocatalyzed ATRP and related CRP mechanisms has been also a very active domain of research over the past few years. Several studies have been reported, which deal with catalysts based on copper,25−33 ruthenium,34,35 or iridium.36−42 Two catalytic cycles are possible, depending on the properties of the different oxidation states of the metal complex, namely, their absorbance at the ground state as well as their lifetime and redox potential at the excited state (Scheme 1).43−46 The first possibility implies the deactivator, which generates after light absorption an excited state being more oxidant, prone to be rapidly converted into the activator via a fast reductive quenching with a reductant. The reductant can be for instance a propagating radical. Conversely, an excited state resulting from the activator can undergo a fast oxidative quenching, resulting in the deactivator form of the catalyst. The oxidant quencher can be for instance the initiator or the dormant species. This distinction between these two pathways clearly appears in the aforementioned references, depending on the nature of the catalyst implemented. All the references dealing with iridium catalysts correspond to tris(2-phenylpyridinato-C2,N)iridium(III) (Ir(ppy)3), an Ir(III) complex which implies a photocatalytic cycle based on an oxidative quenching. Ir(III) will generate an excited state Ir(III)* by absorption in the UV− vis domain. Ir(III)* can then rapidly reduce a dormant brominated chain, which will act as a quencher. This results 1973


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Scheme 2. Structure of the Reactants Used in This Contribution: EBPA = Ethyl α-Bromophenylacetate; MMA = Methyl Methacrylate; [Cu(phen)2]BF4 = Bis(1,10-phenanthroline)copper(I) Tetrafluoroborate; TEA = Triethylamine

Figure 1. (a) UV−vis absorption spectra of the compounds used in this work in solution in a DMF/MMA mixture (1/1 in volume; distilled MMA): Cu(phen)2+ at 7.4 × 10−5 mol/L, EBPA at 3.0 × 10−2 mol/L, and TEA at 5.9 × 10−2 mol/L. (b) Emission spectrum of the LED lamp used in this work (arbitrary units). atmosphere of nitrogen. The solution was then stirred in front of the LED lamp. For all the experiments, the tube was placed at the same position and at the same distance to the lamp. The polymerization was stopped by opening the tube to the air. An aliquot was withdrawn to determine the conversion by 1H NMR. The polymer was purified by precipitation in methanol before being analyzed by SEC. The same procedure was followed for the all the experiments except for the kinetic studies for which the polymer was not purified by precipitation: each aliquot was diluted into chloroform before being passed through a short neutral alumina column so as to remove the catalyst. Characterizations. Nuclear Magnetic Resonance (NMR) Spectroscopy: 1H NMR spectra were recorded in solution in CDCl3 on a Bruker Avance spectrometer at 400 MHz and on a Varian Mercury spectrometer at 300 MHz. Size Exclusion Chromatography (SEC): Absolute molecular weights were determined by SEC in THF. Solutions of samples with precise concentrations around 5.00 mg/mL were prepared and filtered (PTFE membrane; 0.20 μm) before injection. The flow rate was 1.0 mL/min (35 °C). The following Agilent 1260 Infinity series setup was used: a G1310B isocratic pump, a G1322A degasser, a G1329B autosampler, a G1316A thermostated column compartment equipped with a set of Polymer Laboratories ResiPore columns (nominal particle size: 3 μm; porosity: 2 μm) composed of a guard column (50 × 7.5 mm) and two columns (300 × 7.5 mm), a G1314B variable wavelength detector, a G7800A multidetector suite equipped with a MDS refractive index detector, and a MDS viscosimeter detector. Universal calibration was performed using a set of EasiVial polystyrene PS-M standards. Agilent GPC/SEC software and multidetector upgrade were used to determine molar masses values and distributions. UV−vis Absorption Spectroscopy: Spectra were acquired on a Cary 3 UV/vis spectrometer (Agilent).

Contrary to the aforementioned studies of Cu-based photocatalyzed ATRP, we describe in this contribution an oxidative quenching mechanism, involving the excited state of the Cu(I) form of the complex. Such a mechanism is thus original for a Cu-based system, while being similar to the one described previously for Ir-based complexes. For this purpose, we resorted to a bis(1,10-phenanthroline)copper(I) complex Cu(phen)2+ as the catalyst (Scheme 2). Moreover, this complex absorbs strongly in the visible domain (λmax,abs = 440 nm), which allowed the use of very soft irradiation conditions (1 household blue LED lamp at 0.9W; λmax,em = 465 nm). An excellent control of the polymerization of MMA was observed. In particular, the effect of light was confirmed by the possibility to slow down dramatically the polymerization rate by switching off the lamp. Finally, it was also possible to accelerate the polymerization while maintaining a good controllability by adding TEA which enables a faster regeneration of the activator form of the catalyst.

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EXPERIMENTAL SECTION

Materials. N,N-Dimethylformamide (DMF, ≥99.8%, SigmaAldrich) and methyl methacrylate (MMA, 99%, ≤30 ppm in inhibitor, Sigma-Aldrich) were purified by distillation over CaH2. Ethyl αbromophenylacetate (EBPA, 97%, Alfa Aesar) and triethylamine (TEA, >99%, TCI) were used as received. [Cu(phen)2]BF4 was synthesized following a procedure described elsewhere.58 Photocatalyzed ATRP Procedure. For all the experiments, EBPA, MMA, and DMF were added successively into a Schlenk tube equipped with a stirring bar. TEA was also added for experiments described in Table 2 and in Figure 4. The mixture was degassed by three freeze−pump−thaw cycles. The mixture was frozen again, and the catalyst [Cu(phen)2]BF4 was introduced in the tube under nitrogen flush. This addition was followed by three more freeze− pump−thaw cycles before conditioning the tube under an inert

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RESULTS AND DISCUSSION Aiming at developing a photocatalyzed ATRP, the UV−vis spectra of Cu(phen)2+ and the other reactants were recorded. It 1974


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Macromolecules Table 1. Photocatalyzed ATRP of MMA: Conditions Tested and Characterizationsa entry 1 2 3 4

[M]0/[I]0/[Cu(I)]0 200/1/0.016 200/1/0.016 200/0/0.016 200/1/0

t (h)

conversionb (%)

Mnc (g/mol)

Mw/Mn

Mthd (g/mol)

28 ∼0e ∼0e 9

5.67 × 10

3

1.10

5.85 × 103

1.61 × 105

2.46

2.41 × 103

82 82 126 10

a

[M]0, [I]0, and [Cu(I)]0: initial concentrations in monomer (M = MMA), initiator (I = EBPA), and catalyst (Cu(I) = Cu(phen)2+), respectively. The solvent was DMF (DMF/MMA: 1/1 in volume). The experiments were all conducted at room temperature under the irradiation of a household blue LED lamp (0.9 W; 465 nm), except for entry 2 which was carried out in the absence of light. bConversion determined by 1H NMR. c Determined by SEC. dTheoretical M, calculated from conversion. eNo conversion measured, in the limits of 1H NMR detection.

Figure 2. Kinetic monitoring of the photocatalyzed ATRP of MMA using Cu(phen)2+. (a) SEC chromatograms (RI detector). (b) Polymerization kinetics. (c) Evolution of Mn with conversion (dashed line: theoretical M calculated from conversion). (d) Evolution of the polydispersity with conversion. [MMA]0/[EBPA]0/[Cu(phen)2+]0 = 200/1/0.016; DMF/MMA: 1/1 in volume. One LED lamp at 0.9 W.

sample corresponding to entry 1 revealed a narrow polydispersity index, which suggested an efficient control of the polymerization. This first comparison clearly evidences a light-driven polymerization mechanism. No polymerization occurred when the initiator was removed (entry 3), whereas in the absence of the catalyst, high molecular weights polymers with a broad polydispersity were formed at low monomer conversion (entry 4). This can be ascribed to the partial dissociation of the initiator under irradiation, the sensitivity to light of halogenated compounds being well-known. Additionally, for entry 1, the measured value of the molecular weight is close to the theoretical one calculated from conversion, which indicates an initiating efficiency close to 1 for EBPA. Regarding initiating efficiencies, two other initiators, namely diethyl 2bromo-2-methylmalonate (DBMM) and methyl 2-bromopropionate (MBrP), were also tested in preliminary experiments. The efficiency was high also (∼1) for DBMM, but it was very low for MBrP (∼15%). These results are presented in the Supporting Information (Scheme S1, Table S1, and Figure S1).

was found that only the copper complex exhibits significant light absorption, with a maximal absorption at 440 nm (Figure 1a). This absorption, which is due to a metal to ligand charge transfer,62 will ensure a good overlap with the emission spectrum of the selected LED lamp, with a maximal emission at 465 nm (Figure 1b). After these preliminary characterizations, a first series of experiments aimed at demonstrating the effect of light in the ATRP mechanism involving Cu(phen)2+ as the catalyst. For this purpose, the reactants were introduced in the same proportions in two separate Schlenk tubes. The catalyst was introduced at a level of 80 ppm compared to the monomer concentration. After thorough deoxygenation, the reaction was performed under the light exposure of the LED lamp for the first tube, whereas the second one was entirely covered with an aluminum foil. A determining effect was observed, with polymerization occurring only in the first tube, while no polymer was formed in the absence of light, even after 82 h (Table 1, entries 1 and 2). Moreover, the analysis by SEC of the 1975


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Macromolecules Additional preliminary tests concerned the choice of the monomer: no polymerization was observed when MMA was replaced by butyl acrylate. Experimental conditions corresponding to entry 1 were then more deeply investigated by performing a kinetic monitoring of the polymerization. The evolution of the molecular weights and their distribution were followed by means of SEC equipped with a viscosimetric detector (Figure 2a). Conversions were determined by 1H NMR spectroscopy. The results confirmed the efficient control of the polymerization with a kinetics following an apparent first order (Figure 2b) and a linear relationship between molecular weights and conversions (Figure 2c). The measured molecular weights were in agreement with the values calculated from the monomer conversion. Additionally, narrow polydispersities were obtained and maintained during all the polymerization (Figure 2d). These observations and in particular the comparison between entries 1 and 2 in Table 1 are consistent with a photocatalyzed ATRP mechanism involving an oxidative quenching step. First of all, it is worth mentioning that the Cu(phen)2+ complex was already investigated in several previous papers dealing with the thermal ATRP of styrene63,64 and MMA.65 Though efficient control was observed in these contributions, it was necessary to conduct the polymerization at high temperature so as to counterbalance the relatively high apparent activation energy, which results from the stability of the Cu(phen)2+ complex (Cu(I)).63 To this respect, the experiment corresponding to entry 2 (no light) was also conducted at 70 °C, which did not lead either to any polymerization reaction. The catalytic mechanism under irradiation, leading to an efficient polymerization at ambient temperature, implies therefore necessarily a far more reactive species. Considering previous studies on the photocatalytic activity of Cu complexes based on phenanthrolines as ligands, the formation of an excited state Cu(phen)2+* (Cu(I)*) can be proposed.62 This transient species would be far more reactive than the ground state due to its lower oxidation potential. A fast bromine transfer reaction with the initiator can ensue, resulting in the generation of a propagating radical and the complex corresponding to the Cu(II) oxidation state (Cu(II)-Br). Subsequent reaction of this deactivant form of the catalyst with a propagating radical will regenerate the Cu(I) complex and produce dormant species. This results in the progressive setting up of the classical ATRP equilibrium which can be represented as a catalytic cycle so as to introduce more clearly the effect of light (Scheme 3, black part only). The fact that we did not observe any polymerization in the absence of light is probably ascribable to a reaction between Cu(phen)2+ and EBPA being too slow at ambient or moderate temperature, as it can be suggested by their reported oxidation and reduction potentials.66,67 Compared to Cu(phen)2+, a faster reaction is expected with Cu(phen)2+* due to the lower activation energy which is needed for its oxidation by EBPA. A classical experiment enabling to put in evidence such a reaction consists in a quenching fluorescence study. It was done for instance by Hawker et al. in the case of the mechanism that they described, based on Ir(ppy)3.38 This method cannot be used in this work because the Cu(phen)2+ complex does not exhibit luminescence properties at ambient temperature, contrary to similar compounds implying substituted phenanthroline ligands.62 However, the proposed oxidative quenching mechanism, which is typical of such complexes, can be supported by alternative experiments.

Scheme 3. Catalytic Cycle Proposed for the ATRP2 of MMA Involving Cu(phen)2+ (Cu(I))a

a

The predominant quenching reaction occurs between dormant species PBr (initially EBPA) and the excited state Cu(phen)2+* (Cu(I)*). A faster polymerization is observed in the presence of TEA as an additional amine permitting the faster regeneration of the activator (red part).

We noticed indeed during our experiments that a solution containing both the catalyst and the initiator will discolor in any case: the discoloration is very fast under irradiation, whereas a slower discoloration will be observed in the absence of light. The disappearance of the characteristic brown color of the Cu(phen)2+ complex, which is only observed in the presence of the initiator, puts in evidence the reaction between these two compounds. In order to determine quantitatively the effect of light on this reaction, the evolution of the absorbance of a solution containing the two reactants was followed for two samples, one of them being irradiated under the LED lamp between two measurements. A dramatic difference was observed, with an almost total disappearance of the Cu(phen)2+ complex after 3 min under irradiation, while the concentration was only divided by 2 after 23 min in the absence of light (Figure 3a,b). Comparing these two figures, it can be estimated that the consumption of Cu(phen)2+ is ∼30 times faster in the presence of light (Figure 3c). This comparative experiment supports the photocatalytic cycle proposed previously, which involves an oxidative quenching. No evolution was observed under irradiation when the solution contained only Cu(phen)2+, which confirms the stability of the complex in DMF. By the way, the formation of the Cu(II) complex is also evidenced by the progressive apparition of a broad and weak band, with a maximum absorption at ∼750 nm. The spectra of these species in DMF were already described in a previous study.68 The authors designed the Cu(II) species as a Cu(phen)22+ complex. Therefore, the species described in the current work as a Cu(II)-Br complex could be perhaps more conveniently described as a “Cu(phen)22+, Br−” ion pair. As already mentioned, the spontaneous reaction of Cu(phen)2+ with EBPA is very slow at ambient temperature, which results in a very slow activation rate and thus does not allow to consider realistically any ATRP in these conditions. The generation under irradiation of Cu(phen)2+* as a far more 1976


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Figure 3. Evolution of the UV−vis absorption of Cu(phen)2+/EBPA mixture solutions in DMF ([Cu(phen)2+]0 = [EBPA]0 = 2.5 × 10−4 mol L−1) at ambient temperature. (a) Spontaneous evolution. (b) Evolution under light irradiation (1 blue LED: 0.9 W; 465 nm). (c) Effect of light on the evolution of the concentration of Cu(phen)2+.

Table 2. Photocatalyzed ATRP of MMA in the Presence of TEA: Conditions Tested and Characterizationsa entry

[M]0/[I]0/[Cu(I)]0/[A]0

t (h)

conversionb (%)

1 2 3 4 5 6 7

200/1/0.016/0.85 200/0/0.016/0.85 200/1/0/0.85 200/1/0.008/0.85 200/1/0.004/0.85 100/1/0.016/0.85 100/1/0.016/0.85

39 18 4 112 112 62 62

64 5 15 68 58 67 ∼0e

Mnc (g/mol)

Mw/Mn

Mthd (g/mol)

× × × × × ×

1.06 5.78 2.01 1.08 1.14 1.07

1.31 × 104

1.41 2.03 1.57 1.46 1.26 9.60

104 105 105 104 104 103

3.25 1.39 1.19 6.95

× × × ×

103 104 104 103

a

[M]0, [I]0, [Cu(I)]0, and [A]0: initial concentrations in monomer (M = MMA), initiator (I = EBPA), catalyst (Cu(I) = Cu(phen)2+), and amine (A = TEA), respectively. The solvent was DMF (DMF/MMA: 1/1 in volume for entries 1 to 5 and DMF/MMA: 2/1 in volume for entries 6 and 7). The experiments were all conducted at room temperature under the irradiation of a household blue LED lamp (0.9 W; 465 nm), except for entry 7 which was carried out in the absence of light. bConversion determined by 1H NMR. cDetermined by SEC. dTheoretical M, calculated from conversion. eNo conversion detected, in the limits of 1H NMR detection.

was made to increase dramatically by the addition of TEA, while keeping an excellent control of the polymerization (Table 2, entry 1). It is important to mention here that TEA would react even faster with a Cu(II)* species, if it is formed. Such a reductive quenching was already exploited in several studies of photocatalyzed ATRP, as mentioned in the Introduction.25−27,31,33 In the current work, such an additional reductive quenching in the presence of TEA can be ruled out because the Cu(II)-Br complex that is formed does not absorb light at all at the wavelength used for irradiation, as already described. The catalytic cycle proposed previously can be completed so as to consider an additional ARGET process as a second possible pathway for the reduction of Cu(II)-Br in the presence of TEA (Scheme 3, red part). Here again, no controlled polymerization

reductive species ensures a faster activation rate which makes then ATRP possible. However, a satisfactory apparent polymerization rate, which is proportional to the [Cu(I)]/ [Cu(II)-Br] ratio, implies conversely a very fast deactivation reaction, so as to regenerate Cu(I). Despite a very good control, the slow apparent polymerization rate which was observed for the experiments described previously is therefore very probably ascribable to a deactivation reaction not being fast enough to enable a sufficient regeneration of Cu(I). In order to validate this hypothesis, the reaction corresponding to entry 1 in Table 1 was reproduced in the presence of triethylamine (TEA) in order to regenerate faster the activator form of the catalyst. This exploits the principle of ARGET ATRP described previously. The apparent polymerization rate 1977


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Figure 4. Kinetic monitoring of the photocatalyzed ATRP of MMA using Cu(phen)2+. (a) SEC chromatograms (RI detector). (b) Polymerization kinetics. (c) Evolution of Mn with conversion (dashed line: theoretical M calculated from conversion). (d) Evolution of the polydispersity with conversion. [MMA]0/[EBPA]0/[Cu(phen)2+]0/[TEA]0 = 200/1/0.016/0.85; DMF/MMA: 1/1 in volume. One LED lamp at 0.9 W.

Figure 5. Kinetic monitoring of the photocatalyzed ATRP of MMA using Cu(phen)2+ with intermittent exposure to the LED light (1 LED lamp at 0.9 W). (a) Evolution of conversion with reaction time. (b) SEC chromatograms (RI detector). [MMA]0/[EBPA]0/[Cu(phen)2+]0/[TEA]0 = 200/ 1/0.016/0.85; DMF/MMA: 1/1 in volume.

was observed in the absence of initiator or catalyst (Table 2, entries 2 and 3). After these comparative tests putting clearly in evidence the interest of the addition of TEA to speed up the catalytic cycle, a complete kinetic monitoring was also performed (Figure 4). The polymerization followed also a pseudo-first-order kinetics in the presence of TEA (Figure 4b). An induction period can be noticed and ascribed to an ATRP equilibrium which is initially almost totally displaced toward Cu(II)-Br, after the rapid generation of Cu(I)* and its subsequent oxidative quenching. This induction period already existed in the previous experiment without TEA but was less visible at a first time due to the slower polymerization and thus to the

longer reaction time. The controlled behavior was confirmed by the linear relationship between the molecular weights and the conversions (Figure 4c), still indicating an efficient initiation, as well as by the very narrow polydispersities which were measured (Figure 4d). Thanks to the improvement of the photocatalytic ATRP cycle with an additional ARGET mechanism, it was also possible to reduce very significantly the catalyst amount, while keeping an apparent polymerization rate close to the one obtained without TEA (Table 2, entries 4 and 5): it was possible to use a catalyst concentration as low as 20 ppm while maintaining a good control of the polymerization. Besides this additional ARGET mechanism, an increase of the polymer1978


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This contribution is expected to open new perspectives for the development of ATRP2, with in particular the possibility to combine this photocatalytic cycle, based on an oxidative quenching, with other possible pathways for a fast and efficient regeneration of the activator.

ization rate can be also expected if a higher light intensity is used. We confirmed this point experimentally by performing additional experiments changing this parameter: two identical LED lamps at 0.9 W were used instead of only one. A faster polymerization was observed when either EBPA or DBMM was used as the initiator (Supporting Information, Figures S2− Figure S6). Additionally, it was checked that the polymerization can apparently occur only under irradiation in the presence of TEA, as demonstrated by the comparison of entries 6 and 7 in Table 2. Taking into account the shorter time scale for kinetics permitted by the regeneration of the activator, the effect of irradiation was investigated further in an experiment in which the reaction medium, containing TEA, was intermittently exposed to irradiation. A clear effect of light was observed once again, with polymerization being obviously reinitiated under irradiation (Figure 5). It can be noticed that the reaction does not stop totally when the light is turned off but still goes on at a very slower apparent polymerization rate. This is consistent with a very slower activation step involving Cu(I) instead of Cu(I)*, as evidenced by the UV−vis spectroscopy measurements described previously. Finally, the fact that no conversion was measured in the two experiments conducted in the absence of light (entry 2 in Table 1 and entry 7 in Table 2) does not mean that no polymerization occur at all, but just that that it is so slow that it cannot be observed over a reasonable time scale. This ON/OFF experiment enables also to estimate that the reinitiating efficiency is close to 1 during this process, by comparing the molecular weight increase observed during a light irradiation sequence with the theoretical one calculated from the increase of conversion. Still regarding reinitiating efficiency and in order to test chain-end fidelity, we also studied the possibility to reuse some products obtained by this photocatalyzed ATRP either with or without TEA as possible macroinitiators for thermal ATRP. In both cases, chain extension occurred (Figure S7). However, a significant amount of the macroinitiator is not chain extended. Though a partial loss of chain end fidelity can be expected in the case of ARGET, it is more surprising in the case of the catalytic cycle without TEA. This is still under investigation.

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ASSOCIATED CONTENT

S Supporting Information *

Additional experiments: effect of the nature of the initiator; effect of light intensity; chain-extension experiments. 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.P.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS UHA is acknowledged for the PhD scholarship awarded to Qizhi Yang. REFERENCES

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CONCLUSIONS A new ATRP2 mechanism was developed using directly Cu(phen)2+ as the catalyst. Contrary to previous studies dealing with Cu-based photocatalyzed ATRP, this contribution exploits for the first time the formation under irradiation of the excited state derived from the activator form Cu(I) of the complex. This far more reactive excited state Cu(I)* can then undergo a rapid oxidative quenching from a dormant chain, resulting in the formation of a propagating radical along with the deactivator form Cu(II)-Br. Considering precisely the absorption domain of Cu(phen)2+, it was possible to resort only to a very soft irradiation source, consisting of a simple household blue LED at 0.9 W, which was selected on the basis of its suitable emission spectrum. An excellent control of the polymerization of MMA was observed under irradiation, with narrow polydispersities and a linear increase of molecular weights with conversion. The addition of TEA resulted in a faster apparent polymerization rate, ascribable to a faster regeneration of the activator. The photocatalyzed ATRP mechanism which was developed in this contribution allowed reaching a catalyst loading as low as 20 ppm, while keeping an excellent control of the polymerization. 1979


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Photocatalyzed Cu-Based ATRP Involving an Oxidative Quenching

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