Development of a Robust Photocatalyzed ATRP Mechanism

Article pubs.acs.org/Macromolecules

Development of a Robust Photocatalyzed ATRP Mechanism Exhibiting Good Tolerance to Oxygen and Inhibitors Qizhi Yang, Jacques Lalevée, and Julien Poly* Université de Strasbourg − Université de Haute-Alsace (UHA) − Centre National de la Recherche Scientifique (CNRS), Institut de Science des Matériaux de Mulhouse (IS2M), UMR 7361 − CNRS/UHA, 15 rue Jean Starcky, 68057 Mulhouse, France S Supporting Information *

ABSTRACT: A new photocatalyzed atom transfer radical polymerization (ATRP) procedure starting directly from a copper(II) bromine/ phenanthroline (phen) mixture in the presence of triethylamine as a reducing agent is described. Under the irradiation of a compact blue LED lamp, the polymerization of methyl methacrylate (MMA) conducted to PMMAs with narrow molecular weights distributions (Mw/Mn ∼ 1.10). The good chain end fidelity of the products was validated in subsequent chain-extension experiments, using them as macroinitiators, either by conventional thermal ATRP or by photocatalyzed ATRP. The efficient reinitiation under light irradiation was also evidenced by a “light ON/ OFF” experiment. The respective effects of several parameters on the polymerization kinetics were studied, including light intensity, the nature of the solvent, the molar ratio of the ligand, and the nature of the counterion. Besides the essential generation of the excited species [Cu(phen)2]+*, which will undergo an oxidative quenching as the key step of this photocatalytic cycle, supplementary investigations by UV−vis spectroscopy revealed an additional role of light, which also favored the regeneration of the activator. This complementary contribution may consist in a light-triggered exchange of ligands involving minor Cu(II) species, which absorb light in the blue wavelengths domain and are in equilibrium with [Cu(phen)2Br]+ as the predominant Cu(II) complex. Interestingly, this photocatalyzed ATRP mechanism exhibited a good tolerance to oxygen and inhibitors, as demonstrated by the efficient synthesis of PMMAs with relatively narrow molecular weights distributions (Mw/Mn < 1.30) in the presence of air and/ or 4-methoxyphenol (MEHQ).

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INTRODUCTION

contributes to reducing the required catalyst amount and enhancing the tolerance to oxygen during the polymerization. More recently, major advances have been also achieved in the development of atom transfer radical photopolymerizations (ATRP2).8 Such mechanisms combine the aforementioned advantages of ATRP with the ones of photopolymerizations such as spatial and temporal controls under irradiation.9−11 To introduce the effect of light into ATRP, a first possibility consists in the addition of photoinitiators/photosensitizers, resulting in a photoinitiated ATRP mechanism.12,13 Though these additives are expected to only contribute to the in situ generation of Cu(I) activator, the reactive species generated by these additives can also sometimes directly generate new propagating radicals,14,15 which could partially affect the controllability of ATRP. Alternatively, the effect of light can be introduced in ATRP through a photocatalyzed mechanism, which involves the absorption properties of the complex. Metal-based photocatalysts have been widely used in organic synthesis, but it was

The rapid development of reversible-deactivation radical polymerizations (RDRPs) such as NMP,1 ATRP,2,3 and RAFT4,5 has revolutionized the field of macromolecular engineering over the past decades. Among these methods, ATRP, which is based on the reversible termination of propagating radicals induced by a transition metal complex, has been widely embraced in many fields thanks to its simple experimental procedures and variety of commercially available reagents.6 However, in conventional ATRP, a high catalyst loading amount (0.1−1 equiv of the initiator) was required due to the accumulation of the Cu(II) deactivator during the polymerization. Additionally, Cu(I) species are often oxygensensitive, which has been one of the major obstacles for an easy practical application. To tackle these problems, improved ATRP mechanisms such as activators regenerated through electron transfer (ARGET), initiators for continuous activator regeneration (ICAR), supplemental activators and reducing agents (SARA), and electrochemically mediated ATRP (eATRP) were proposed.3,7 For example, in ARGET ATRP, the Cu(I) activator is (re)generated from the accumulated Cu(II) deactivator in the presence of a reducing agent, which © XXXX American Chemical Society

Received: August 19, 2016 Revised: September 23, 2016

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DOI: 10.1021/acs.macromol.6b01808 Macromolecules XXXX, XXX, XXX−XXX

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

[MMA]0/[EBPA]0/[TEA]0

time (h)

convb (%)

Mtheoc (kg/mol)

Mn,SECd (kg/mol)

Mw/Mnd

1 2e 3f 4 5e 6g 7g 8h

200/1/0.85 200/1/0.85 200/1/0.85 200/1/0 200/1/0 200/1/0 200/1/0.85 200/1/0.85

20 114 6.5 71 66 64 64 64

36 0 10 20 0 7 67 20

7.45

7.92

1.12

2.25 4.25

334 3.76

2.51 1.18

1.58 13.6 4.17

5.59 18.5 29.7

1.21 1.15 1.28

a

Catalyst loading: [EBPA]0:[CuBr2]0:[phen]0 = 1:0.016:0.032. bMonomer conversion measured by 1H NMR. cTheoretical molecular weight, calculated from monomer conversion. Mtheo = Minitiator + α[M]0/[I]0; α represents monomer conversion. dNumber-average molecular weight (Mn,SEC) and polydispersisty (Mw/Mn), determined by SEC. ePolymerization carried out in darkness. fNo catalyst was added. gMBiB was used as initiator. hMBrP was used as initiator. MMA/DMF = 1/1 vol LED 1 (75 mW/cm2; λem,max = 465 nm).

not until recently that their application in the field of RDRPs has attracted great attention.16−27 Photocatalyzed ATRP can follow two pathways, namely the reductive quenching or the oxidative quenching mechanisms, whether it involves an excited state generated under irradiation from the deactivator or the activator, respectively. In the former case, the photocatalysis consists in a faster deactivation process, thanks to a deactivator being more oxidative at the excited state than at the ground state. So far, photocatalyzed ATRP using ruthenium-based catalysts28 and most of the copper-based catalysts followed the reductive quenching mechanism.22,23 On the contrary, the oxidative quenching mechanism involves the activator, which generates under irradiation an excited state being more reductive than at the ground state, making faster the activation step. Such a mechanism was reported for iridium,19,29 iron,30,31 and gold32 containing catalysts. Additionally, organophotocatalysts (OPCs) involving reductive quenching such as fluorescein33 or oxidative quenching such as perylene34 or phenothiazine-based compounds10,35−39 were also reported. Although the application of OPCs can be attractive, for instance regarding the possible issue of residual metal catalyst or the broader accessible wavelength domain, it is still difficult to combine high initiation efficiency with narrow molecular weight distribution.33,39 Moreover, the development of an OPC-based ATRP mechanism exhibiting tolerance to oxygen could be quite challenging. On the other hand, for metal-based photocatalyzed ATRP, the expensive prices of some catalysts (iridium or ruthenium catalysts) or the need for high energy irradiation sources (high intensity and/or short wavelengths) could still be potential obstacles for their practical application.8 Considering these factors, we reported previously the use of [Cu(phen)2]+ (phen = 1,10-phenanthroline) as a photocatalyst for the ATRP of methyl methacrylate (MMA) under low intensity blue light irradiation (λem,max = 465 nm).40 The polymerization involved an oxidative quenching process as evidenced by the faster reaction between the alkyl halide and [Cu(phen)2]+ under irradiation. In the first experiment, it turned out that the polymerization kinetics was quite slow, although well controlled. The polymerization kinetics was then greatly improved with the introduction of triethylamine (TEA) as a reducing agent, ensuring a complementary regeneration of the activators, similarly to the ARGET process. Although our attempts at controlling the polymerization of acrylates using the same catalyst were unsuccessful, this photocatalyzed ATRP mechanism was then also effectively applied to the synthesis of (co)polymers based on glycidyl methacrylate, with a regeneration of the activators being directly ensured by the reducing

behavior of the epoxide groups.41 Recently, Ma introduced Cu(dap)2Cl (dap = 2,9-bis(p-anisyl)-1,10-phenanthroline) as a photoredox catalyst under blue light (λem,max = 420 nm) for ATRP, involving also an oxidative quenching mechanism.42 However, it turned out that controlled polymerization was observed only when tris[2-(dimethylamino)ethyl]amine (Me6TREN) was added, which acted both as a reducing agent and as an exchangeable ligand. Regarding these last examples, it is worth reminding that in ATRP procedures starting directly from the Cu(I) activator only, a limited control can be sometimes observed at the early stage of the polymerization due to the insufficient amount of the Cu(II) deactivator. Optimized procedures in which Cu(I) species are generated in situ are thus expected to be better controlled and more reproducible. In this contribution, we reported the synthesis of welldefined PMMA (PDI ∼ 1.10) starting from the air-stable CuBr2/phen mixture (Scheme 1) using household blue LEDs as light sources. In the presence of TEA as a reducing agent, a faster polymerization kinetics was observed. The temporal control of the polymerization was also achieved under intermittent light irradiation. But more importantly, the polymerization remained well controlled in the presence of air and/or inhibitor (4-methoxyphenol, MEHQ). Additionally, a deeper investigation in the mechanism of this photocatalyzed ATRP, relying predominantly on the reactivity of the activator at its excited state ([Cu(phen)2]+*), suggested also possible secondary reactions, involving minor species formed by ligand exchange.

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RESULTS AND DISCUSSION Polymerization with TEA as Reducing Agent. The preliminary experiments were performed using CuBr2/phen (molar ratio 1:2) and additional TEA as catalyst and external reducing agent, respectively. After irradiation during 20 h (LED 1 at 75 mW/cm2 as light source, λem,max = 465 nm), the monomer conversion reached 36% and PMMA with a narrow molecular weight distribution was obtained (Mw/Mn = 1.12) (Table 1, entry 1). To verify the respective effects of light irradiation, catalyst, and reducing agent, controlled experiments were carried out by removing one of these elements. It turned out that no polymerization occurred in darkness (entry 2). Polymerization took place in the absence of catalyst; however, only PMMA with broad molecular weight distribution (Mw/Mn = 2.51) was obtained, thus indicating an uncontrolled polymerization. For the polymerization carried out in the absence of reducing agent, to our surprise, polymerization was B


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Macromolecules

resulting in a much lower conversion of MMA. This observation can be explained by the partial photosensitivity of EBPA or/and the lower efficiency of MBiB. A lower efficiency can also account for the results of the comparative experiments performed in the presence of TEA, but replacing EBPA by MBiB or MBrP (entries 7 and 8, respectively), which showed a larger deviation from the theoretical molecular weights. In order to investigate the controllability throughout the polymerization, its kinetics was monitored using the same conditions as for entry 1 (Figure 1, [M]0/[I]0 = 200). It turned out that the polymerization followed a first-order kinetics, consistently with a constant concentration of reactive species. The measured molecular weights (Mn,SEC) of the resulting polymers increased linearly with monomer conversion and were in good agreement with the theoretical values. The polydispersity indexes were lower than 1.20. These results evidenced that the polymerization was well controlled. This was still the case when a lower maximal degree of polymerization was targeted ([M]0/[I]0 = 100) using the same catalyst and initiator concentrations in more dilute conditions for the monomer (MMA: 25% vol instead of 50% previously). One of the advantages of photoRDRP is the possible temporal control under intermittent irradiation. In our system, it turned out that once the light was switched off during the polymerization, the polymerization dramatically slowed down, resulting in a very limited further increase of monomer conversion (Figure 2a). This slower polymerization was also supported by the small shift of the molecular weight as determined by SEC (Figure 2b,c). More importantly, when the solution was re-exposed to light, the polymerization was

observed (entry 4), although it was much slower (monomer conversion of 20% only after 71 h under irradiation). But when Scheme 1. Main Reactants Used in This Studya

a MMA = methyl methacrylate; EBPA = ethyl α-bromophenylacetate; MBiB = methyl α-bromoisobutyrate; MBrP = methyl 2- bromopropionate; phen = 1,10-phenanthroline; bpy = 2,2′-bipyridine; MEHQ = 4-methoxyphenol; Cu(OTf)2 = copper(II) triflate.

the light was off (entry 5), no polymerization was observed. The polymerization observed in entries 3 and 4 could be ascribed to a slight photosensitivity of EBPA, as already reported by Matyjaszewski.21 However, we observed in our previous contribution that its possible absorption is extremely weak at the wavelengths used.40 An alternative or complementary explanation for entry 4 will be proposed in the last section of this article. When EBPA was replaced by the initiator MBiB (entry 6), polymerization still occurred, although

Figure 1. Kinetic monitoring of the photocatalyzed ATRP of MMA using CuBr2/phen mixture: [EBPA]0/[CuBr2]0/[phen]0/[TEA]0 = 1/0.016/ 0.032/0.85. (a) Polymerization kinetics. (b) Evolution of Mn,SEC and polydispersity with conversion. [M]0/[I]0 = 200 or 100 (circles or squares, respetively) with DMF/MMA = 1/1 or 3/1 in volume, respectively. (c, d) SEC chromatograms (RI detector) for [M]0/[I]0 = 200 or 100, respectively. LED 1 (75 mW/cm2; λem,max = 465 nm). C


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Macromolecules Table 2. Effects of Ligand on the Photocatalyzed ATRP of MMAa entry

[ligand]0/[CuBr2]0

ligand

time (h)

conv (%)

Mtheo (kg/mol)

Mn,SEC (kg/mol)

Mw/Mn

1 2 3 4

2/1 6/1 2/1 2/1

phen phen bpy phen

160 116 13 13

26 33 34 45

5.45 6.85 7.07 9.25

4.91 7.14 12.9 10.5

1.14 1.07 1.39 1.07

a

Polymerization conditions: [MMA]0/[EBPA]0/[CuBr2]0 = 200/1/0.016, MMA/DMF = 1/1 in volume. Entries 1 and 2: the polymerizations were carried out without TEA, LED 1 at 75 mW/cm2. Entries 3 and 4: the polymerizations were carried out in the presence of TEA ([TEA]0/[EBPA]0 = 0.85), LED 3 (150 mW/cm2; λem,max = 450 nm).

Figure 2. Kinetic monitoring of the photocatalyzed ATRP of MMA with intermittent exposure to LED 1 (75 mW/cm2; λem,max = 465 nm). (a) Evolution of conversion with reaction time. (b) Evolution of Mn,SEC and polydispersity with monomer conversion. (c) SEC chromatograms (RI detector). [MMA]0/[EBPA]0/[CuBr2]0/[phen]0/[TEA]0 = 200/1/0.016/0.032/0.85; DMF/MMA: 1/1 in volume.

As expected, a faster polymerization was observed as evidenced by the higher value for the slope of the ln([M]0/[M]t) vs time curve (Figure 4). However, slightly higher molecular weights and PDI values were measured at similar monomer conversion under more intense irradiation. This could be due to a higher concentration of active species generated under more intense light irradiation, thus increasing the possibility of side reactions such as irreversible terminations. Effect of the Nature of Solvent. It has been reported that more polar solvents lead to a higher ATRP equilibrium constant and consequently to faster polymerization kinetics.43 For example, a faster polymerization was observed in DMSO than in DMF, DMSO being a more polar solvent than DMF (relative polarities of 0.444 and 0.236, respectively).44 The polymerization kinetics in DMSO and DMF were thus compared (Figure 5). It turned out that monomer conversion surpassed 50% within 17 h for the polymerization in DMSO, whereas it took about 30 h to reach similar monomer conversion in DMF. Moreover, narrow molecular weight distributions (Mw/Mn ∼ 1.10) were also obtained when the polymerization was conducted in DMSO, despite the significantly faster kinetics. However, a slight deviation from

effectively reinitiated as evidenced by the SEC chromatograms. Additionally, the polydispersity indexes of the obtained polymer were kept lower than 1.20 during all the light “ON/ OFF” experiment. The fact that the polymerization does not stop immediately but more progressively could be ascribed to a relatively slow deactivation process with this catalyst. In order to investigate the end group fidelity, a PMMA synthesized with the polymerization procedure described previously was used as a macroinitiator for a chain extension experiment. Under irradiation ([macroinitiator]0/[CuBr2]0/ [phen]0/[TEA]0 = 1:0.016:0.032:0.85) (Figure 3a), the macroinitiator was efficiently extended, and only a marginal tail peak was observed on the SEC curve, evidencing a good preservation of the bromine chain end functionality. In another experiment, the chain extension experiment was carried out under thermal conditions with a similar PMMA precursor, also synthesized by photocatalyzed ATRP ([macroinitiator]0: [CuBr]0:[CuBr2]0:[PMDETA]0 = 1:0.6:0.2:2.8) (Figure 3b). A significant shift of the peak toward higher molecular weights was also observed after the polymerization. Effect of Light Intensity. The polymerization of MMA at higher light intensity (LED 3 at 150 mW/cm2) was also tested. D


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Figure 3. Chain-extension experiments of PMMAs synthesized by photocatalyzed ATRP: (a) photocatalyzed ATRP under blue light irradiation using LED 2 (100 mW/cm2; λem,max = 470 nm) with CuBr2/phen as catalyst and TEA as reducing agent; (b) under thermal conditions with CuBr/ CuBr2/PMDETA as catalyst.

Figure 4. Polymerization under different light intensities (squares: LED 1 at 75 mW/cm2; circles: LED 3 at 150 mW/cm2). (a) Polymerization kinetics. (b) Evolution of Mn and polydispersity with conversion. [MMA]0/[EBPA]0/[CuBr2]0/[phen]0/[TEA]0 = 200/1/0.016/0.032/0.85; DMF/ MMA: 1/1 in volume.

Figure 5. Polymerization in different solvents (squares: DMF; circles: DMSO). (a) Polymerization kinetics. (b) Evolution of Mn,SEC and polydispersity (Mw/Mn) with conversion. [MMA]0/[EBPA]0/[CuBr2]0/[phen]2/[TEA]0 = 200/1/0.016/0.032/0.85; solvent/MMA: 1/1 in volume; LED 1 (75 mW/cm2; λem,max = 465 nm).

the theoretical molecular weights can be observed in the case of DMSO. This observation could be related to the faster polymerization rate, indicating a higher concentration of active species and thus a higher probability for the occurrence of some irreversible termination reactions. These observations will be commented in the light of the more detailed mechanism presented in the last discussion section of this article. Effects of Ligands. The effect of the ligand was investigated by adjusting the feeding ratio ligand/copper salt. In the absence of reducing agent (entries 1 and 2, Table 2), when the ratio increased from 2 (entry 1) to 6 (entry 2), a higher conversion (33 % vs. 26 %) was reached in a shorter time (116 h vs. 160 h), indicating either a faster polymerization

and/or a shorter induction time, although the polymerization remained slow. The excess of ligand is expected here to favor the formation of a Cu(II) complex with two phen ligands, which could not be otherwise fully quantitative, due to possible competitive complexation, for instance with DMF. Compared with phenanthroline, 2,2′-bipyridine (bpy) is more commonly used in ATRP. The Cu(II)/bpy complex exhibits an intense absorption around 440 nm.45 However, in previous reports, when Cu(II)/bpy was used as a catalyst for photoATRP, the polymerizations were conducted under ultraviolet irradiation or heating conditions.46−48 Herein, when a blue LED was used as the light source with TEA as a reducing agent (entry 3), the polymerization occurred. E


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Figure 6. Effect of [phen]0/[CuBr2]0 ratio on the polymerization: [MMA]0/[EBPA]0/[CuBr2]0 = 200/1/0.016; LED 1 (75 mW/cm2; λem,max = 465 nm); MMA/DMF = 1/1 in volume. (a, b) Polymerization without TEA. (c, d) Polymerization with TEA ([TEA]0/[EBPA]0 = 0.85).

presence of photoredox catalyst such as fac-[Ir(ppy)3], which can effectively convert oxygen to inactive species.24,51,52 In order to investigate the effect of oxygen on the polymerization with our catalytic system, experiments without deoxygenation process were conducted at different air/solution volume ratios (Table 3). To control the amount of oxygen

However, a slower polymerization rate was observed compared to entry 4 where phen was used as the ligand. Moreover, the molecular weights of the obtained polymer exhibited a much broader distribution (Mw/Mn: 1.39 vs 1.07). The effect of the ratio ligand/copper salt on the polymerization kinetics in the absence or presence of TEA was also investigated (Figure 6). Without TEA (Figure 6a), the increased ligand/Cu(II) feeding ratio from 2:1 to 6:1 contributed to a faster polymerization. However, in both cases, the polymerization gradually slowed down. Regardless of the ligand/Cu(II) feeding ratio, the molecular weights of the obtained polymers were in good agreement with the theoretical values, and the polydispersities were smaller than 1.2 (Figure 6b). The decreased polymerization rate could be due to the side products generated by the reduction of Cu(II) to Cu(I), which can affect the stability of the copper catalyst.49 In the presence of TEA, much faster polymerization kinetics were observed (Figure 6c). In the polymerization with the higher ligand/Cu(II) feeding ratio, an induction time (ca. 5 h) was observed (Figure 6c). In both cases, the polydispersities of the obtained PMMAs were always lower than 1.20. An explanation for the observed induction time will be proposed in the last section of this contribution. Effects of Oxygen and Inhibitor. One of the major challenges of RDRPs is to achieve tolerance to oxygen. Mosnácě k reported that in the photochemically mediated ATRP using CuBr2/TMPA (TMPA = tris(2-pyridylmethyl)amine) as catalyst, the polymerization can be carried out in the presence of a limited amount of air.50 It was supposed that the Cu(I)/TPMA species generated under photoirradiation could consume all the dissolved oxygen before the equilibrium of ATRP was established. Tolerance to oxygen also was observed in the mechanism of PET-RAFT developed by Boyer in the

Table 3. Effect of Oxygen on the Photocatalyzed ATRP of MMAa entry

air/solution ratio

conv (%)

Mtheo (kg/mol)

Mn,SEC (kg/mol)

Mw/Mn

1 2 3 4b

0.17 0.48 1.00 0.48

80 75 86 84

16.3 15.2 17.5 17.0

18.3 18.4 21.1 23.4

1.17 1.16 1.26 1.22

a

The polymerizations were carried out in 2 mL standard vials equipped with a stirring bar. Polymerization conditions: [MMA]0/ [EBPA]0/[CuBr2]0/[phen]0/[TEA]0 = 200/1/0.010/0.020/0.85. Lamp: LED 3 (150 mW/cm2; λem,max = 450 nm); polymerization time: 20 h. bPolymerization was carried out after storage for 8 days in darkness.

more accurately, polymerization was carried out in standard vials equipped with identical magnetic stirring bars. Although the amount of air seemed to affect the initiation efficiency, as suggested by values of the molecular weights measured by SEC higher than the theoretical ones calculated from conversions, it turned out that the polydispersity of the obtained polymer remained narrow (Mw/Mn < 1.3). An inhibition period (4 ± 1 h) was also observed in a supplementary experiment with an air/solution ratio of 0.48 (Table S1). Additionally, after storage in the dark for 8 days, the polymerization could still be controlled under light irradiation, although a larger deviation F


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Macromolecules from the theoretical molecular weight was observed. The results indicated that the CuBr2/phen/TEA catalytic system has good tolerance to oxygen. To facilitate their storage and transportation, most of the commercial vinyl monomers contain inhibitors at low concentration. For example, the methyl methacrylate used in this work contains less than 30 ppm of 4-methoxyphenol (MEHQ) before being purified by distillation. However, it was reported that phenol compounds can act as efficient reducing agents in the presence of an excess of tertiary amine.53 Herein the effect of the inhibitor was first investigated by introducing different fractions of unpurified monomer (Table 4). It turned out the controllability of the polymerization was

Table 5. Photocatalyzed ATRP in the Presence of Oxygen and the Inhibitora

fraction of unpurified monomer (%)

conv (%)

Mtheo (kg/mol)

Mn,SEC (kg/mol)

Mw/Mn

1 2 3 4b 5 6b

25 50 100 100 100 100

75 62 64 23 28 ∼0

14.5 12.2 13.0 4.74 5.95

17.3 15.0 15.5 5.54 8.60

1.09 1.08 1.11 1.19 1.11

air/solution ratio

conv (%)

Mtheo (kg/mol)

Mn,SEC (kg/mol)

Mw/Mn

1 2 3

0.18 0.48 1.00

54 71 56

11.3 14.8 11.5

13.2 17.8 14.2

1.13 1.13 1.15

a

Polymerization conditions: [MMA]0/[EBPA]0/[CuBr2]0/[phen]0/ [TEA]0 = 200/1/0.010/0.020/0.85. Polymerization time: 20 h. LED 3 (150 mW/cm2; λem,max = 450 nm). Polymerizations were carried out in 2 mL standard vials, and unpurified MMA was used directly.

ization with purified monomer (data in Table 3), but the polymerization remained very well controlled judging from the PDIs of the obtained polymers, which were lower than 1.15. The great tolerance of this photocatalyzed ATRP mechanism to oxygen and inhibitors makes it very robust and especially attractive for applications implying simple experimental procedures, such as surface functionalization. Mechanism Investigation. In a previous contribution, we described a photocatalyzed ATRP mechanism based on the oxidative quenching of the excited state of the [Cu(phen)2]+ complex, generated under blue light irradiation.40 It can be reminded that such copper-based photocatalyst were already commonly used in organic chemistry, before being applied in ATRP.54 Besides this reported reactivity, the proposed mechanism was supported by two observations. The first one was the comparison of the emission spectrum of the blue LED lamp used with the respective absorption spectra of the [Cu(phen)2]+ complex and its conjugate oxidant [Cu(phen)2Br]+ (Figure 8). A good overlap was observed for the Cu(I) species whereas almost no absorption could occur for Cu(II), evidencing the possible selective excitation of Cu(I) in this wavelengths domain. The second and more conclusive observation was the effect of light on the reaction occurring between [Cu(phen)2]+ and EBPA: the oxidation of Cu(I) was really faster under light irradiation, in agreement with the generation of the more reducing excited species [Cu(phen)2]+*. This observation explained why no polymerization occurred in dark at ambient temperature. The photocatalyzed ATRP was then made faster thanks to an additional regeneration of the activators, similar to the ARGET process, using TEA as a reducing agent. Although the polymerization was well controlled, it was started directly from the Cu(I) complex. We then decided to

Table 4. Effect of Inhibitor: Photocatalyzed ATRP with Unpurified Monomera entry

entry

a

Polymerization conditions: [MMA]0/[EBPA]0/[CuBr2]0/[phen]0/ [TEA]0 = 200/1/0.010/0.020/0.85. Polymerization time: 20 h; entries 1−4: LED 3 (150 mW/cm2); entries 5 and 6: LED 1 (75 mW/cm2). b Polymerization carried out in the absence of TEA.

not significantly affected by MEHQ, judging from the polydispersity (Mw/Mn < 1.20). However, the polymerization without TEA was much slower (entry 4, 150 mW/cm2) or even totally inhibited (entry 6, 75 mW/cm2). This confirms the importance of TEA as a reducing agent in the presence of MEHQ. The polymerization kinetics in the presence of a large amount of MEHQ (3000 ppm vs monomer) was also studied. In comparison with the polymerization without MEHQ, the polymerization kinetics was not significantly affected (Figure 7). Moreover, judging from the molecular weights and the polydispersities (Mw/Mn < 1.20), the polymerization was also well controlled. Finally, polymerizations were also tested in the presence both of inhibitor and oxygen (Table 5). It turned out that the polymerization was much slower compared to the polymer-

Figure 7. Polymerization in absence (squares) and presence (circles) of MEHQ. (a) Polymerization kinetics. (b) Evolution of Mn and polydispersity with conversion. [MMA]0/[EBPA]0/[CuBr2]0/[phen]2/[TEA]0 = 200/1/0.016/0.032/0.85; MMA/DMF: 1/1 in volume; lamp: LED 1 (75 mW/ cm2; λem,max = 465 nm). G


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Cu(I), confirming its role as a reducing agent. By the way, it can be noted that the solvent DMF itself cannot reduce the [Cu(phen)2Br]+ complex. TEA does not absorb light in the blue wavelengths domain used, and as for the predominant [Cu(phen)2Br]+ complex, its absorption is not strictly null but extremely weak, making very improbable the direct formation of a derived excited state under the irradiation of a blue LED. In this regard, we can mention that a preliminary experiment aiming at introducing a complementary reductive quenching mechanism, exploiting the specific absorption properties of the [Cu(phen)2Br]+ complex through the use of an additional LED emitting in the red wavelengths domain, was unsuccessful (Figure S1). Therefore, these results rather suggest that other absorbing species, not considered until now, formed and played an additional role in this photocatalyzed ATRP mechanism. It was reported previously that for a catalytic system prepared from CuBr2 using 2 equiv of bipyridine (bpy) as the ligand the predominant complex [Cu(bpy)2Br]+ is in equilibrium with other minor Cu(II) species, which can become predominant at other CuBr2/bpy ratios.51 By analogy, we investigated by UV− vis spectroscopy the influence of the stoichiometry between CuBr2 and phen in DMF (Figure 10a). A supplementary absorption band (λabs,max ≈ 497 nm) clearly appeared when only 1 equiv of phen was introduced, evidencing the coexistence with [Cu(phen)2Br]+ of another Cu(II) complex. This band was still detectable when 2 equiv of phen was used but no longer visible at lower [CuBr2]0/[phen]0 ratios. Importantly, this band overlaps well with the emission spectrum of the blue LEDs used, suggesting that this minor species could play an additional role in the photocatalyzed mechanism by accounting for the possible generation of Cu(I) under irradiation in the absence of TEA. By analogy with complexes based on bpy, we can use the simplified notation [Cu(phen)Br2] to describe this compound, based on its assumed stoichiometry. Still considering other possible minor Cu(II) species which could absorb light in the blue wavelengths domain, we simply studied the absorption properties of a solution of CuBr2 in DMF (Figure 10b). We found that the Cu2+ and/or CuBr+ species complexed and solvated by DMF absorb light significantly in this domain. When TEA was added, the absorption spectrum was significantly modified, evidencing the complexation by TEA as a better ligand. Here also there was still a noticeable absorption in the wavelengths range used. However, the characteristics absorptions of these Cu(II) species were no longer observed when phen was added, which conducted immediately to the spectrum of the [Cu(phen)2 Br]+ complex shown previously. Therefore, although several minor Cu(II) species in equilibrium with the predominant [Cu(phen)2Br]+ complex can exhibit significant light absorption in the wavelengths domain of the LEDs used, [Cu(phen)Br2] seems to be the most important one. Regarding the role of this species in the photocatalyzed mechanism, a first possibility could be a direct reductive quenching between an excited state generated under irradiation and TEA or DMF acting as the reducing agent. Alternatively, another pathway could imply a preliminary phototriggered ligand exchange, which would then make more favorable the subsequent reduction of Cu(II). This possible mechanism was suggested indeed by several previous contributions. For instance, Alexandrova found that Ru complexes containing a 2-phenylpyridine, a phen (or bpy), and two acetonitrile

Figure 8. UV−vis absorption spectra of Cu(I) ([Cu(phen)2]BF4) and Cu(II) ([CuBr2]0:[phen]0 = 1:2) and emission spectrum of a blue LED used as light source (LED 1).

investigate different possible improvements of this photocatalyzed ATRP and especially the development of a procedure starting from Cu(II) with the in situ generation of Cu(I), for which a better control at the early stage of the polymerization and a better reproducibility can be expected. In the preliminary experiments described in Table 1, a very good control of the polymerization was observed for such a procedure, still using TEA as a reducing agent. As expected, no polymerization occurred in the absence of light, which was ascribed to a too slow activation process with [Cu(phen)2]+ as the activator at ambient temperature, compared to its corresponding more reactive excited state [Cu(phen)2]+* which forms under irradiation. However, it was surprisingly observed that polymerization occurred even when no TEA was added and that it was still well controlled, although much slower, indicating that Cu(I) was generated in these conditions. This possible reduction of Cu(II) under light irradiation in the absence of TEA was confirmed by UV−vis spectroscopy measurements on solutions of CuBr2 and phen (2 equiv) in DMF, on which the respective effects of light and/or TEA were tested (Figure 9). Although it was apparently not fast, the

Figure 9. Evolution of Cu(I) from Cu(II) under different conditions (with/without TEA, under irradiation/in darkness). [CuBr2]0:[phen]0: [TEA]0 = 1:2:0 or 1:2:12, [Cu2+] = 3.7 × 10−3 mol/L, t = 96 h. Irradiation using LED 1 (75 mW/cm2; λem,max = 465 nm).

generation of Cu(I) in the presence of TEA in darkness was observed as expected. Surprisingly, the generation of Cu(I) was more marked without TEA but under irradiation. However, the most efficient reduction was obtained when light and TEA addition were combined. Whether in darkness or under irradiation, the addition of TEA favored the generation of H


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Figure 10. UV−vis spectra of Cu(II) solutions. (a) CuBr2/phen in DMF ([CuBr2]0 = 2.5 × 10−3 mol/L). (b) CuBr2 salt, CuBr2/TEA (1:5.7), CuBr2/phen/TEA (1:2:5.7) in DMF ([CuBr2]0 = 3.64 × 10−3 mol/L). The emission spectrum of LED 1 (arbitrary units) is also reminded on (a).

Figure 11. Evolution of absorbance at 443 nm for CuY2/phen/TEA solution: (a) in dark [CuY2]0/[phen]0/[TEA]0 = 1:2:12, [Cu2+]0 = 3.7 × 10−3 mol/L; (b) under irradiation using LED 3 (150 mW/cm2; λem,max = 450 nm) [CuY2]0/[phen]0/[TEA]0 = 1:2:5.7, [Cu2+]0 = 3.7 × 10−3 mol/L (squares: Y = TfO−; circles: Y = Br−).

Figure 12. Kinetic monitoring of the photocatalyzed ATRP of MMA using CuY2/phen mixtures (molar ratio 1:2). Y = Br− (squares) or TfO− (triangles). [MMA]0/[EBPA]0/[CuY2]0/[phen]0/[TEA]0 = 200/1/0.016/0.032/0.85; DMF/MMA: 1/1 in volume. LED 1 (75 mW/cm2; λem,max = 465 nm). (a) Polymerization kinetics. (b) Evolution of Mn and polydispersity with conversion.

(MeCN) ligands were efficiently activated under visible light by losing one of the MeCN molecules.56 More recently, Sawamoto reported Ru catalysts which were made more efficient through the exchange of a chlorine ligand by a more weakly bound MeCN solvent molecule.57 Haddleton also recently reported an exchange of a bromine ligand accelerated under light irradiation.58 The effect of the nature of the ligands on the reactivity of the Cu(II) species, which can be modified through the use of other counterions than bromine, was also clearly evidenced by Matyjaszewski22 and Mosnácě k.59

In this view, we investigated the effect of the counterion of the copper salt be comparing CuBr2 with Cu(OTf)2 (OTf = triflate), triflate being a noncoordinating anion, contrary to bromine. A first experiment was performed in dark, on solutions containing the copper salt, phen, and TEA (Figure 11a). A really faster generation of the Cu(I) complex was observed when Cu(OTf)2 was used instead of CuBr2, evidencing that Cu(II) can be more easily reduced when it is not coordinated with a Br− ligand. The formation of Cu(I) was observed even in the presence of oxygen, which reminds how the relative stabilities of Cu(I) and Cu(II) species can be I


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Macromolecules subtlety affected by their coordination.60 On the contrary and very interestingly, when the comparison was performed under irradiation, a faster reduction of Cu(II) was observed with CuBr2 (Figure 11b). Regarding the polymerization reaction under light irradiation, the replacement of CuBr2 by Cu(OTf)2 led apparently to an induction time (Figure 12). These differences can be ascribed to the aforementioned specific absorption of the [Cu(phen)Br2] species in equilibrium with the predominant [Cu(phen)2Br]+ complex. This supplementary band, which corresponds to a ligand to metal charge transfer, was not observed when the counterion was TfO− (Figure 13). It can be assumed that the excited state

[Cu(bpy)2]2+ complex as the main species but at a lower predominance.55 By analogy, a mechanism implying the [Cu(phen)2]2+ complex could account for the differences observed for the generation of Cu(I) whether the copper salt used is Cu(OTf)2 or CuBr2. In dark the formation of this complex is favored with Cu(OTf)2, which explains the observed faster reduction of Cu(II). On the contrary, under light irradiation, a complete conversion of [Cu(phen)2Br]+ into [Cu(phen)2]2+, either directly or more probably through the equilibrium with the more strongly absorbing species [Cu(phen)Br2], would lead to a higher concentration in the efficient reactive species in the case of CuBr2, explaining the faster reduction observed. A second argument supporting a ligand exchange mechanism is the minor modification of the coordination sphere that this pathway would imply. On the basis of the complementary investigations presented in this contribution, a more complete mechanism can be proposed that considers the additional effect of light in the (re)generation of Cu(I) catalyst (Scheme 2). This catalytic cycle is in agreement with the experimental results shown and discussed previously. For instance, it can account for the photocatalyzed ATRP observed under irradiation in the absence of TEA, with DMF playing in this case the role of the reducing agent (Table 1, entry 4). As already mentioned, due to its very weak absorption in this range of wavelengths, the direct photolysis of EBPA is also a possible alternative or complementary pathway. This more complete mechanistic scheme can also account for the aforementioned solvent effect. As with DMF (Figure 10a), the UV−vis spectra of the Cu(II) solutions at different CuBr2:phen ratios were also recorded in DMSO (Figure S2). Interestingly, the [Cu(phen)Br2] compound was no longer detected, which could be ascribed to the higher polarity of DMSO, favoring the solvation of the charged Cu(II) complexes, namely [Cu(phen)2Br]+ and [Cu(phen)2]2+. The faster polymerization observed in DMSO could thus be due to a higher concentration of [Cu(phen)2]2+, ensuring the regeneration of Cu(I). Regarding the effect of the ratio [CuBr2]0/[phen]0, the induction time observed with the higher concentration of phen can be ascribed to an equilibrium between the Cu(II) species less displaced toward the formation of the more absorbing [Cu(phen)Br2] compound, leading thus to a slower generation of the Cu(I) species at the beginning of

Figure 13. UV−vis spectra of Cu(II) solutions: ([CuY2]0:[phen]0 = 1:1, [Cu2+] = 2.5 × 10−3 mol/L, Y = Br− or TfO−).

[Cu(phen)Br2]* generated under light irradiation could then either undergo a direct reductive quenching or exchange first its Br− ligands before being reduced. In both cases TEA or less favorably DMF would act as the reducing agent. Although it cannot be definitely concluded on the nature of the predominant mechanism accounting for the reduction of Cu(II), a light-triggered ligand exchange, implying the replacement of the two Br− ligands by a phen one and leading to the formation of the [Cu(phen)2]2+ complex, seems realistic for two main reasons. First of all, if we go back to similar complexes prepared using 2 equiv of bpy ligand in DMF, the counterion had a strong effect on the nature of the Cu(II) species: while [Cu(bpy)2Br]+ was clearly predominant when using CuBr2, the use of Cu(OTf)2 resulted in the reactive

Scheme 2. Proposed Mechanism of Photocatalyzed ATRP Using CuBr2/phen/TEA as Catalytic System

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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 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, and 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). Irradiation Conditions. In all the experiments performed in Schlenk tubes, the blue LED lamp was put directly in contact with the tube. For the tests in standard vials, the lamp was placed at a distance of 1 cm. Three different LED lamps were used. Their emission spectra and irradiance were determined using a Thorlabs CS100 optical spectrum analyzer and a Thorlabs PM100D powermeter at a distance of 2 cm, respectively. Depending on their characteristics, the lamps used in this study will be noted as follows. LED 1 (Lexman; 1.8 W): irradiance of 75 mW/cm2; λem,max = 465 nm. LED 2 (V-Lumtech; 2.5 W): irradiance of 100 mW/cm2; λem,max = 470 nm. LED 3 (V-Lumtech; 3.5 W): irradiance of 150 mW/cm2; λem,max = 450 nm. General Procedure for the Polymerization Starting with Cu(II) Complexes. In a typical experiment (entry 1, Table 1 as an example), a stock solution of the catalyst was prepared by dissolving CuBr2 (6.718 mg, 0.030 mmol), phen (10.834 mg, 0.060 mmol), and TEA (162.0 mg, 1.601 mmol) in DMF (3793.1 mg, 4.0 mL). EBPA (44.5 mg, 0.183 mmol, 1.0 equiv), distillated MMA (3791.8 mg, 37.872 mmol, 200 equiv), DMF (3398.4 mg, 3.6 mL), and 397.4 mg (0.4 mL) of a CuBr2/phen/TEA stock solution (CuBr2/phen/TEA = 0.016/0.032/0.875) were charged into a Schlenk tube with a magnetic stirrer. The mixed solution was degassed by four freeze−pump−thaw cycles and then backfilled with nitrogen. Then the polymerization was started by placing the tube in front of the LED lamp (Figure S3). After settled time, the polymerization was stopped by opening the tube and exposing the solution to air. An aliquot was withdrawn to determine the conversion by 1H NMR. The polymer was purified by passing through a short neutral alumina column and then dried under vacuum, before being analyzed by SEC. In kinetic studies, samples were withdrawn periodically under nitrogen protection, followed by analysis by means of 1H NMR and SEC. In the kinetics with a large amount of inhibitor (MEHQ), MEHQ was introduced to the deoxygenated solution under nitrogen flush. Three more freeze−pump−thaw cycles were performed to remove the oxygen completely. For the polymerization in the presence of air, the mixed solution was directly added to a standard vial equipped with a stirrer without deoxygenation. The container was sealed with a cap and para-film. Chain Extension Using PMMA-Br as a Macroinitiator. Two similar PMMA-Br synthesized by the photocatalyzed ATRP procedure described previously were used as macroinitiators for subsequent chain-extension experiments, either by the photocatalyzed mechanism or by thermal ATRP. For the synthesis of both of them, the polymerization was stopped at a conversion of 50%. The chainextension experiments were performed at a ratio monomer/macroinitiator = 940. For the chain-extension by photocatalyzed ATRP, the macroinitiator (Mn = 11.6 kg/mol; PDI = 1.08) was added into a Schlenk tube charged with 4 mL of a MMA/DMF solution (1/3, volume ratio). When the polymer was fully solubilized, a calculated amount of CuBr2/phen/TEA (molar ratio 1:2:64) stock solution in DMF was added. The final ratio of macroinitiator/CuBr2 was settled at

the polymerization (Figure 6c). As for the comparison between CuBr2 and Cu(OTf)2, the apparent induction time observed with Cu(OTf)2 can be ascribed to a lower initial concentration of [Cu(phen)2]2+, whereas irradiation enables the generation of a higher concentration of this reactive complex when CuBr2 is used since the beginning of the polymerization, thanks to the equilibrium with the light absorbing species [Cu(phen)Br2].

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CONCLUSIONS We developed a photocatalyzed ATRP procedure starting from a CuBr2/phenanthroline mixture in the presence of triethylamine as a reducing agent so as to enable the (re)generation of the activators. An excellent control of the polymerization of MMA was observed under blue light irradiation. As expected, the polymerization was made faster under more intense irradiation. The efficient temporal control of the reaction, which is a typical feature of photopolymerizations, was then demonstrated trough a “light ON/OFF” experiment. The possibility to reinitiate the chains under light irradiation was also evidenced by a chain-extension experiment, using a PMMA already synthesized by photoATRP as a macroinitiator. This catalytic cycle relies on the oxidative quenching of the excited state resulting from the intense absorption of the activator [Cu(phen)2]+ under blue light irradiation. But besides this major event, a deeper investigation in the mechanism of this photoATRP revealed an additional role of light, which also affects favorably the regeneration of the activators. This complementary contribution may consist in a light-triggered exchange of ligands in the minor [Cu(phen)Br2] species, which can also absorb light in the blue wavelengths domain, contrary to the predominant [Cu(phen)2Br]+ complex, with which it is in equilibrium. This photocatalyzed ATRP procedure is very robust, as evidenced by its good tolerance to oxygen and/or inhibitor. It is thus especially interesting in the field of materials, such as for the preparation of polymer surfaces through “grafting from” approaches, for which simple experimental procedures, with in particular no tedious degassing process, are always in demand. Moreover, compared to thermal ARGET procedures, this mechanism also brings two characteristics of photopolymerizations, namely the spatial and temporal controls of the reaction. These additional features are especially interesting in the field of surface engineering, since they can enable for instance the preparation of templated or multilayered surfaces.

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EXPERIMENTAL SECTION

Materials and Characterizations. Materials. N,N-Dimethylformamide (DMF, ≥99.8%, Sigma-Aldrich) and methyl methacrylate (MMA, 99%, ≤30 ppm in inhibitor, Sigma-Aldrich) were purified by distillation over CaH2. Ethyl α-bromophenylacetate (EBPA, 97%, Alfa Aesar), methyl 2-bromopropionate (MBrP, ≥97%, Fluka), methyl αbromoisobutyrate (MBiB, ≥99.0%, Sigma-Aldrich), triethylamine (TEA, >99%, TCI), copper(II) bromide (CuBr2, 99%, Sigma-Aldrich), copper(II) triflate (Cu(OTf)2, 98%, Sigma-Aldrich), copper(I) bromide (CuBr, ≥98.0%, Sigma-Aldrich), dimethyl sulfoxide (DMSO, ≥99.8%, CARLO ERBA), 4-methoxyphenol (MEHQ, 98+ %, Alfa Aesar), N,N,N′,N′′,N′′-pentamethyldiethylenetriamine (PMDETA, 99%, Sigma-Aldrich), 2,2’-bipyridyl (bpy, ≥99%, SigmaAldrich), and 1,10-phenanthroline (phen, ≥99%, Sigma-Aldrich) were used as received. Nuclear Magnetic Resonance (NMR) Spectroscopy. 1H NMR spectra were recorded in solution in CDCl3 on a Varian Mercury spectrometer at 300 MHz. K


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Macromolecules (1:0.016). After degassing by four freeze−pump−thaw cycles, the tube was backfilled with nitrogen. The tube was placed in front of the lamp (LED 2), and the polymerization was stopped 67 h later by exposing the solution to air. The polymer was purified by precipitation in an excess of methanol. The obtained polymer product was analyzed by SEC. For the chain extension under thermal conditions, a mixture of CuBr/CuBr2/PMDETA was used as catalyst. The solution was prepared by adding CuBr (35.64 mg, 0.248 mmol) to degassed DMF (2.48 g, 2.62 mL) containing CuBr2 (10.043 mg, 0.046 mmol) and PMDETA (182.0 mg, 1.047 mmol) under nitrogen protection. Another tube was charged with the macroinitiator (Mn = 11.7 kg/mol; PDI = 1.14), MMA and DMF before being degassed by three freeze− pump−thaw cycles. The catalyst was introduced in this tube through a nitrogen purged syringe. The final ratio of the initiator/CuBr/CuBr2 ratio was settled at 1:0.8:0.2. The polymerization was carried out in an oil bath at 50 °C and lasted for 72 h. When the polymerization was stopped, the polymer was purified by precipitation in methanol. UV−Vis Measurements. For the spectra which needed to be taken under nitrogen protection, the quartz cell used for measurement was purged with nitrogen for 20 min, before the degassed sample solution was added under nitrogen flush. Otherwise, the spectra were taken directly.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01808. Additional experiments: investigation of a possible complementary reductive quenching mechanism under red light irradiation; UV−vis absorption spectra of copper(II)/phenanthroline complexes in DMSO; picture of the typical experimental setup for photocatalyzed ATRP; supplementary experiment on the effect of oxygen (PDF)

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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

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REFERENCES

UHA is acknowledged for the PhD scholarship awarded to Qizhi Yang. The authors thank Frédéric Dumur, who provided us with the [Cu(phen)2]+, BF4− complex used in previous contributions,40,41,61 and the absorption spectrum of which was reminded in Figure 8.

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