Synergic Effect between Nucleophilic Monomers and Cu(II) Metal

Article Cite This: Chem. Mater. 2017, 29, 9445-9455

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Synergic Effect between Nucleophilic Monomers and Cu(II) Metal− Organic Framework for Visible-Light-Triggered Controlled Photopolymerization Hui-Chun Lee,† Marco Fantin,‡ Markus Antonietti,*,† Krzysztof Matyjaszewski,*,‡ and Bernhard V. K. J. Schmidt*,† †

Department of Colloid Chemistry, Max Planck Institute of Colloids and Interfaces, 14424 Potsdam, Germany Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, United States

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‡

S Supporting Information *

ABSTRACT: A Cu(II) metal−organic framework (MOF), Cu2(bdc)2(dabco), was found to be an efficient heterogeneous catalyst for controlled photopolymerization under visible light. The MOF, composed of photostimulable metal sites, was used to catalyze both photoinitiation as well as radical chain control and required no external photoinitiator, dye sensitizer, or ligand. A simple light trigger allowed the photoreduction of Cu(II) to the active Cu(I) state, enabling controlled atom transfer radical polymerization (ATRP). Compared to conventional ATRP with homogeneous catalysts, the ionic-bonded framework imparts high stability and robustness to the catalytic Cu(II) species. Therefore, the polymerization of vinylpyridines (2-vinylpyridine, 2VP; and 4-vinylpyridine, 4VP), usually challenging by traditional polymerization strategies, was controlled up to high conversion (>85%) in 90 min, forming polymers with Đ < 1.3. Methacrylates such as 2-(dimethylamino)ethyl methacrylate (DMAEMA) and methyl methacrylate were polymerized by the Cu(II) MOF with good control as well. Moreover, as a heterogeneous catalyst, the MOF was easily separated, recovered, and repeatedly used for several photopolymerizations of 2VP and DMAEMA.

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INTRODUCTION Reversible deactivation radical polymerization (RDRP) has attracted significant attention in polymer and materials sciences, owing to its application to the synthesis of well-defined polymers with predetermined molecular weight, narrow dispersity, tailor-made functionalities, and designed topology.1−5 Current advanced protocols, e.g., atom transfer radical polymerization (ATRP) or activators regenerated via electron transfer (ARGET) ATRP,3 have made significant contributions in the manipulation of the activation−deactivation equilibrium between the active (Pn•) and dormant (Pn−X) groups to obtain control in radical polymerization (as depicted in Scheme 1). Since the ATRP equilibrium is mediated by the Cu(II)/Cu(I) catalytic couple, precise selection of appropriate copper catalysts with amine ligands is important to optimize the properties of the polymer product.6 Several different external stimuli can trigger and control a polymerization process, such as pressure,7 electrical current,8,9 mechanical forces,10 and light.11−18 Special interest is imparted to photomediated polymerization because of several advantages over traditional polymerization, including adjustable photoactive catalysts, fast reaction rates, and spatiotemporal control. Even though photoinduced catalytic reactions19−24 or polymerizations12,13,17,25−29 have recently been widely developed, their © 2017 American Chemical Society

Scheme 1. Schematic Illustration of the Cu(II) MOFMediated ATRP

use is often limited by the requirement of specialized photoinitiators and photosensitizers,25,30 and, most importantly, by the tedious removal of the non-recyclable catalysts.31−33 In the case of Cu catalysts, the polymerization of strongly coordinating monomers (i.e., 4-vinylpyridine and 2vinylpyridine) is limited by both inevitable association with the Received: August 22, 2017 Revised: October 2, 2017 Published: October 3, 2017 9445

DOI: 10.1021/acs.chemmater.7b03541 Chem. Mater. 2017, 29, 9445−9455

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Chemistry of Materials

basic aluminum oxide (Brockman type I, Acros, 50−200 μm, 60 Å) column to remove the inhibitor. The 2,2′-azobis (isobutyronitrile) (AIBN, Sigma-Aldrich, 98%) was recrystallized from methanol. Bromoacetonitrile (Sigma-Aldrich, 97%), copper(II) acetate monohydrate (Roth, ≥98%), CuBr2 (Alfa Aesar, 99%), diazabicyclo[2.2.2]octane (dabco, Alfa Aesar, 97%), N,N-dimethylformamide (DMF, VWR, ACS grade), ethyl α-bromoisobutyrate (EBIB, Sigma-Aldrich, 98%), 4-ethylpyridine (Sigma-Aldrich, 98%), NaNO3 (Acros, ACS grade), 1,1,4,7,7-pentamethyldiethylentriamine (PMDETA, Acros, >98%), 1,4-terephthalic acid (H2bdc, Alfa Aesar, >98%), triethylamine (TEA, Acros, 99%), methanol (MeOH, Fisher Scientific, for analysis), dichloromethane (DCM, Sigma-Aldrich, HPLC grade), and tetrahydrofuran (THF, VWR, HPLC grade) were used as received. As light source, a 50 W LED chip (Bridgelux BXRA-50C5300; λ > 410 nm) connected to a homemade circuit and cooling system is utilized. The emission spectrum is shown in Figure S1. Cu2(bdc)2(dabco) was fabricated and characterized on the basis of the literature (Figures S2 and S3).42,59 Electrochemical Characterization of Cu(II) MOF. Electrochemical analyses were performed on a PARC 263A potentiostat/ galvanostat using a three-electrode cell with deionized water/0.1 M NaNO3 as solvent. The reference electrode was saturated calomel (SCE), the counter electrode a Pt wire, and the working electrode a glassy carbon disc (Metrohm, 3 mm diameter) coated with a film of Cu(II) MOF. The MOF was dropcast from dry acetone or TEA/dry acetone 1:4 mixtures (5 drops of a dispersion of 10 mg of MOF in 5 mL of solvent, sonicated for 30 min before casting). The film was protected by dropcasting two drops of a solution of 1 wt % Paraloid B72 in acetone, which forms a porous layer incorporated with 0.02 ± 0.01 mg of MOF.60 Prior to each surface deposition, the electrode was polished with a 0.1 μm alumina paste, and rinsed in an ultrasonic bath. Photoelectrochemistry experiments were performed by illuminating the electrode surface with either a UV−vis lamp or a white LED from 1 cm distance. Typical Procedure of Cu(II) MOF-Mediated Photopolymerization. In a glass vial Cu(II) MOF (0.08 g) was added to a bulk solution of 4VP (2 g) and ethyl α-bromoisobutyrate (EBIB; 0.03 g) under argon. The polymerization was carried out via exposing the reaction mixture to a visible-light LED at a distance of 15 cm with stirring, and was terminated by adding DCM. After centrifugation, the polymer was precipitated in water, and the solid product was dried under vacuum. For kinetic analysis, the reaction was terminated by DCM at a specific time, and after centrifugation the clear suspension was filtered for GC−MS to evaluate monomer conversion. Other monomers were polymerized via a similar method (see the Supporting Information for details). Characterization. The microstructure of the Cu(II) MOF was characterized by a Bruker D8 powder X-ray diffractometer (PXRD) using Cu Kα radiation (λ = 0.154 nm) and a scintillation counter (KeveX detector). Nitrogen adsorption and desorption experiments were performed using Quantachrome Quadrasorb instrument at the temperature of liquid nitrogen, and the results were analyzed on the basis of the Brunauer−Emmett−Teller (BET) method. All the samples were degassed at 110 °C for 20 h before measurements and analyzed with the QuadraWin software (version 5.05). The observations of field-emission scanning electron microscopy (FESEM) were performed on a LEO 1550 Gemini instrument. Samples were located on a carbon-coated aluminum holder, and measured without any additional coating. The UV−vis spectra were recorded on a UV-2501PC/2550 instrument (Shimadzu Corporation) at room temperature. Residual of copper in polymer products was determined by inductively coupled plasma optical emission spectrophotometry (ICP−OES); the measurements were performed on a PerkinElmer Optima 8000 instrument, calibrated with standard solutions. Gas chromatography−mass spectrometry (GC−MS) analysis was performed using an Agilent Technologies 5975 gas chromatograph equipped with an MS detector and a capillary column (HP-5MS, 30 m, 0.25 mm, 0.25 μm) for conversion determination. All 1H and 13C nuclear magnetic resonance (NMR) spectra were performed on a Bruker Ascend 400 NMR spectrometer in chloroform-d at a concentration of 1 wt % for proton,

catalyst34,35 and SN2-type nucleophilic substitution, which affects the halide end-groups of polymers and results in the formation of branched structures.36 Therefore, such difficult monomers would benefit from the development of a significantly more stable and heterogeneous catalyst. Metal−organic frameworks (MOFs) are highly ordered frameworks constructed from organic linkers and metal ions. MOFs are well-defined and tunable porous materials with ultrahigh specific surface area. These properties allow for diverse applications, such as gas storage,37 organic catalysis,38,39 polymerization catalysis,40−42 and even nanoreactors for polymerization.43−45 Furthermore, optical properties of the MOFs can be designed: The choice of organic linkers affected chemical bonding and electronic structures, adjusting the optical properties and the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).46−48 Namely, with the manipulation of conjugated molecules as linkers, diverse MOFs have been prepared as universal photosensitizers with adjustable chemical and optical properties.46,49−51 Once irradiated with light, the metal ions act as electronically tunable photocatalytic centers for efficient oxidation or reduction processes.52−54 For example, theoretical and experimental investigations revealed that MOFs containing unsaturated metal sites acted as Lewis-acid centers with enhanced catalytic activity toward electron-rich molecules.55,56 Moreover, nitrogen-containing ligands were introduced as potential reducing agents to activate the metal catalytic centers under light irradiation.12,19,57,58 In this regard, the Cu(II) ion-based MOF, [Cu2(bdc)2(dabco)]n, is herein demonstrated as an excellent catalyst for visible-light photoinduced controlled radical polymerization (PCRP) of various nucleophilic monomers, namely, 4-vinylpyridine (4VP), 2-vinylpyridine (2VP), 2(dimethylamino)ethyl methacrylate (DMAEMA), and methyl methacrylate (MMA) (Scheme 1). The applied MOF is determined as a 4/mmm primitive symmetry cell with channel size 7.5 × 7.5 Å,59 which is composed of dication Cu2 segments, with terephthalic acid (H2bdc) as two-dimensional linker and with 1,4-diazabicyclo[2.2.2]octane (dabco) as both pillar and potential reducing motif under light irradiation. A series of photochemical and electrochemical properties of the Cu(II) MOF were investigated first. Then, PCRP was carried out, under visible-light irradiation to reduce Cu(II) to the active Cu(I), i.e., without photospecialized initiators, sensitizers, or complexed ligands. Surpassing the conventional homogeneous catalyst, the Cu(II) MOF was found to tolerate strongly coordinating monomers. The presence of monomer affected the MOF’s properties, influencing both photoabsorption and polymerization behaviors. Being a heterogeneous catalyst, the MOF could be easily separated from the polymerization mixture by centrifugation, and then reused for the PCRP of 2VP and DMAEMA. Moreover, the on/off character of photoinduced reactions was exploited to obtain temporal control in the polymerization of 4VP.

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EXPERIMENTAL SECTION

Materials. The 4-vinylpyridine (4VP, Sigma-Aldrich, ≥95%), 2vinylpyridine (2VP, Alfa Aesar, 97%), 2-(dimethylamino)ethyl methacrylate (DMAEMA, Sigma-Aldrich, 98%), methyl methacrylate (MMA, Alfa Aesar, 99%), poly(ethylene glycol) methyl ether methacrylate (OEGMA, Sigma-Aldrich, average Mn 300), and isobornyl acrylate (IBA, Alfa Aesar, 85%) were filtered through a 9446


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Figure 1. Images of Cu(II) MOF−monomer complexes with variation in emission color: (a) before and (b) after the exposure to visible light for 30 min. (The as-synthesized Cu(II) MOF was dispersed in dry MeOH as a reference.) Ultraviolet−visible diffuse reflectance spectra of the Cu MOF in association with different monomers: (c) during polymerization process (after 30 min of light exposure, mostly reduced state); and (d) after termination of the polymerization and washing with DCM (mostly oxidized state). and 10 wt % for carbon analysis. Size exclusion chromatography (SEC) for P4VP, P2VP, and PDMAEMA was conducted in 1-methyl-2pyrrolidinone (NMP; Fluka, GC grade) with 0.05 mol/L LiBr and BSME as internal standard at 70 °C using a column system by a PSS GRAM 100/1000 column (8 × 300 mm, 7 μm particle size) with a PSS GRAM precolumn (8 × 50 mm), a Shodex RI-71 detector, and a PMMA calibration with standards from PSS. SEC for PMMA copolymers was conducted in THF with toluene as internal standard at 25 °C using a column system by a PSS SDV 1000/10000/1000000 column (8 × 300 mm, 5 μm particle size) with a PSS SDV precolumn (8 × 50 mm), a SECcurity RI detector, a SECcurity UV−vis detector, and a calibration with PMMA standards from PSS. The polydispersity is defined as Đ = Mw/Mn. The theoretical number-averaged molecular mass Mn,theo was calculated as MMinitiator + MMmonomer × ([monomer]0/[initiator]0) × conversion.

transition was enhanced for 4VP and 2VP, yet to an insignificant extent for the less coordinating DMAEMA and MMA. The shift in light absorption was also traced by solid-state UV−vis spectroscopy. The corresponding optical band gap, EG, was estimated from the UV−vis profiles by the empirical Tauc plot formula: (αhν)2 = A(hν − EG)

(1)

where α is the absorption coefficient, h is the Planck constant, ν is the light frequency, and A is a constant that is a function of the refractive index of the material.62,63 The as-synthesized Cu(II) MOF has a maximum absorption around 250−350 nm and EG of ∼3.5 eV (black line in Figure 1c). Absorption properties changed upon forming a coordination complex with monomers and 30 min of light irradiation (colored lines in Figure 1c). For 4VP and 2VP, the spectrum red-shifted toward the visible-light region (300−600 nm); the band gap decreased to 2.6 eV, and the overall absorbance increased, especially in the case of 4VP. These changes were less prominent with DMAEMA and MMA, and the band gap decreased only to 3.1 eV. The red shifted absorption indicated a smaller energy requirement, while the diminished band gap indicated stronger light-harvesting. These effects on the absorption spectra are not caused by the isolated monomers, which do not absorb visible light (Figure S4). As a result, it appears that 4VP and 2VP strongly associated with Cu(II) ions, tuning the band gap and reinforcing the light use by extending absorption into the visible area. In contrast, molecules like MMA, which show less affinity to metallic ions, lead to insignificant variation in MOF light absorption. The absorption spectra also gave an indication about the redox state of the MOF. For reference as shown in Figure S4a, the spectrum of Cu(I)Br in solution was very red-shifted compared to that of Cu(II)Br2, indicating that the red shift in the UV−vis spectra can be ascribed to the reduction of MOFcomprised Cu(II) to the Cu(I) state. Reduction efficiency was

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RESULTS AND DISCUSSION Optical Properties of Monomer−Cu2(bdc)2(dabco) Complexes. It was reported that N-containing monomers can reduce Cu(II) complexes in solution to the active Cu(I) oxidation state under light irradiation.61 In the following sections, it is shown that nucleophilic N-containing monomers can both reduce the MOF to its Cu(I) state and enhance the light absorption of the MOF. The optical properties of the Cu(II) MOF were thereby modified by the inclusion of different monomers in the crystal structure, which altered the coordination microenvironment of the Cu(II) ions. As shown in Figure 1a, the as-synthesized Cu(II) MOF in methanol (MeOH) exhibited the characteristic cyan blue of the Cu(II) ions, but pronounced color variations were observed once monomers were added. The suspensions with 4VP and 2VP both turned dark green because of the high affinity of these two monomers to the metal centers. In contrast, addition of DMAEMA caused only small color transition to cyan-green, because of its lower coordination strength; similarly, a limited color transition was observed for the least coordinating monomer, MMA. After exposure to visible light for 30 min (Figure 1b), the phenomenon of color 9447


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Figure 2. Electrochemical characterization of glassy carbon coated with Cu(II) MOF in deionized water with 0.1 M NaNO3 as supporting electrolyte, at room temperature. (a) CV at scan rate = 0.2 V/s. Inset: CV with smaller electrochemical window to highlight the reversible monoelectronic reduction of Cu(II). (b) Chronoamperometry at +0.4 V vs SCE under intermittent UV−vis irradiation (peak intensity at 365 nm, 9 W power).

Figure 3. Evolution of Mn with conversion for Cu(II) MOF-mediated photopolymerization of (a) P4VP, (b) P2VP, (c) PDMAEMA, and (d) PMMA. [4VP]0/[EBIB]0 = 125:1; [2VP]0/[EBIB]0 = 370:1; [DMAEMA]0/[EBIB]0 = 500:1; and [MMA]0/[bromoacetonitrile]0/[4-ethylpyridine]0 = 325:1:8, with Cu(II) MOF 0.08 and 2 g of monomer.

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Table 1. Properties of (Co)Polymers Obtained via Conventional Strategies and Cu2(bdc)2(dabco) Catalysis, At Ambient Temperature under Visible Light homo polymer P4VP P2VP PDMAEMA PMMA

catalyst free radical Cu(II) MOFa free radical Cu(II) MOFa CuBr2/PMDETAa Cu(II) MOFa CuBr2/PMDETAa Cu(II) MOF/TEAa Cu(II) MOF/4-ethylpyridinea Cu(II) MOF/4-ethylpyridineb

kpapp (h−1)

Mn,theo (g/mol) (conversion)

1.32

2800 (22%)

0.32

4900 (13%) 18 500 (79%) 2300 (3%) 21 900 (93%) 4200 (13%) 4500 (14%)

0.029

0.068

3400 (11%)

time (h)

Mn,SEC (g/mol)

Đ

24 0.25 24 0.33 24 0.5 24 2 6

11 300 3300 87 300 5300 15 200 3200 20 000 19 400 13 800

6.4 1.2 9.7 1.4 1.2 1.2 1.2 1.3 1.3

3700

1.2

1.5

copolymerc

Mn,theo (g/mol)

Mn,SEC (g/mol)

Đ

P4VP-b-POEGMA

15 500

13 900

1.2

P2VP-b-PIBA

15 900

13 800

1.3

PDMAEMA-b-POEGMA

10 800

9700

1.2

PMMA-b-PIBA

39 200

33 600

1.2

a Using EBIB as initiator. bUsing bromoacetonitrile as initiator. c[macroinitiator]0/[OEGMA/IBA]0/[CuBr2]0/[PMDETA]0/[dabco]0 = 1:340:4:8.8:20, at 50 °C for 24 h in 50 vol % methanol (P4VP and PDMAEMA) or DMF (P2VP and PMMA).

vis light, a sudden increase in oxidation current was observed, which indicates the formation of Cu(I) species. In fact, it has already been shown that photoexcited amine ligands, such as dabco, can reduce Cu(II) to Cu(I).61 When the light was switched off, the current decayed to the background value quickly. The on/off cycles could be repeated for an arbitrary amount of times, which is an indication of the good reversibility of the redox reaction and stability of the MOF structure under irradiation. When the MOF was deposited from a TEA/acetone 1:4 mixture instead of pure acetone, the photocurrent increased 5fold (Figure S7). This confirmed that N-containing compounds enhanced the photoreduction property of the MOF. Unfortunately, the effect of monomers could not be directly probed by electrochemistry, because as such they prevent the deposition of compact MOF layers on the electrode. Visible-Light-Triggered Photopolymerization via Cu2(bdc)2(dabco). The Cu(II) MOF was used to photopolymerize the four monomers mentioned above, in the order of coordinating ability: 4VP, 2VP, DMAEMA, and MMA. A linear region in the semilogarithmic plots of monomer conversions for both P4VP and P2VP indicated a constant concentration of propagating radicals (Figure S5a,b). In both cases, molecular weights increased linearly with conversion and agreed well with theoretical values (Figure 3a,b). The polymerization rate of 4VP was the fastest, with nearquantitative conversion (>85%) in 90 min. The slower rate with 2VP, 40% conversion in 90 min, was attributed to the different microstructure. The different position of the heteroatom in 2VP could affect the binding affinity toward the MOF due to steric hindrance,67 diminishing the polymerization rate.36,68 Table 1 shows that well-defined polymers were synthesized, giving controlled P4VP with Mn,SEC = 3300 and Đ = 1.2, and P2VP with Mn,SEC = 5300 and Đ = 1.4. The control over 2VP polymerization indicated limited side reactions such as intramolecular displacement of Br by the penultimate 2VP unit.36,68 For both monomers, the non-moderated free-radical polymerization gave uncontrolled polymers with dispersity up to 9.7. The polymerization behavior of the two vinylpyridine monomers supported the proposed concept of a synergistic effect resulting from a highly nucleophilic monomer and metal ions. The MOF, which intrinsically harvests only near-UV light, was sensitized to accept visible light, and thus improved polymerization efficiency.

enhanced in agreement with the coordination ability of the monomers, with 4VP and 2VP promoting the strongest reduction. Interestingly, monomer coordination was reversible. Via facile solvent washing, the absorption shift resulting from monomer association was reversed (Figure 1d), with all monomer−MOF absorption profiles shifting back into the blue wavelengths, matching well with the initial Cu(II) MOF. Electrochemical and Photoelectrochemical Properties of Cu2(bdc)2(dabco). The redox properties of the Cu(II) MOF were investigated by cyclic voltammetry (CV), after depositing the insoluble MOF on a glassy carbon electrode as described in the Experimental Section. The CV curve of the Cu(II) MOF presented two reduction waves, associated with two consecutive monoelectronic reductions of the transition metal to Cu(I) and Cu(0) (Figure 2a). Upon reaching more negative potentials (∼−0.45 V versus SCE), a film of metallic copper deposited on the electrode, a result of the decomposition of the MOF structure. The Cu(0) film was stripped from the electrode during the oxidation scan at ∼0.2 V versus SCE. A similar behavior was reported in other Cu(II) MOFs containing bdc as ligand.60 To avoid the formation of Cu(0), the CV scan was repeated in a smaller electrochemical window (inset in Figure 2a). Interestingly, this CV curve clearly showed the reversibility of the Cu(II)/Cu(I) redox couple, whose reduction potential was calculated as E⊖ ≈ E1/2 = (Epc + Epa) = −0.084 V versus SCE (where Epc and Epa are the cathodic and anodic peak potentials, respectively). The redox potential of the Cu(II)/Cu(I) couple in the MOF is more negative than that of active ATRP catalysts in aprotic solvent, e.g., E⊖ = −0.02 V versus SCE for Cu(II)/ tris(2-pyridylmethyl)amine2+ (TPMA) in CH3CN,64 but more positive than the redox potential for Cu(II)/TPMA in water, E⊖ = −0.34 V versus SCE.65 The Cu(II) MOF is a good ATRP catalyst due to the following properties: (i) an accessible Cu(I) oxidation state, separated by ∼0.4 V from the reduction to Cu(0); (ii) a reversible Cu(II)/Cu(I) redox behavior, required for the ATRP equilibrium; and (iii) a reducing power (i.e., ATRP activity) comparable to that of the most active ATRP catalysts.66 The effect of light on the redox state of the Cu(II) MOF was subsequently studied (Figure 2a). In the dark, application of a steady potential of +0.4 V versus SCE generated a small background current of ca. 1 nA, indicating no reactions at the electrode. Upon irradiation of the electrode surface with UV− 9449


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Possible leakage of Cu ions from the MOF template was investigated by ICP−OES. With a comparison to homogeneous ARGET, catalyzed by CuBr2/PMDETA, Cu contamination in the polymer product was reduced significantly in both Cu(II) MOF-synthesized PDMAEMA and PMMA, with Cu concentration decreasing from 8.7 to 0.26 mg/g, and from 0.53 to 0.062 mg/g, respectively (as shown in Table S2). The products from crude bulk solutions of Cu(II) MOF-mediated polymerization showed almost 30 times lower copper ion concentration compared to ARGET ATRP under homogeneous conditions. A slightly higher Cu concentration was detected for poly(vinylpyridines), which strongly coordinate to the Cu(II) ions of the MOF (2.5 mg/g for P4VP and 1.3 mg/g for P2VP; Table S2). However, it has to be noted that this strong association is also the basis of the successful polymerization of vinylpyridines, which is difficult with conventional catalysts. In fact, CuBr2/ PMDETA was unstable, and formed insoluble products in the presence of vinylpyridines, as can be seen in Figure S4c. The crystalline MOF is significantly more stable and avoids the chelating effect of nucleophilic monomers. In conclusion, the ICP and polymerization results supported the stability of the Cu(II) MOF during the photopolymerization, particularly in comparison with traditional copper catalysts. Proposed Mechanism of Cu(II)-MOF-Catalyzed Photopolymerization. The photocatalytic nature of Cu(II) MOF was strongly supported by both UV−vis absorption (Figure 1) and photoelectrochemical analyses (Figure 2). Irradiating the MOF by light caused reduction of comprised Cu(II) to Cu(I), promoting controlled polymerization by ATRP in an ARGETtype process. Moreover, under irradiation, nucleophilic monomers (or additives, such as TEA and 4-ethylpyridine) further drove the (re)generation of activators (Scheme 2).61

Subsequently, DMAEMA, an alkyl amine-containing monomer with lower coordination strength toward the MOF, was polymerized. Although slower, i.e., 36% conversion in 15 h (Figure 3c), the polymerization of DMAEMA was controlled (Figure S5c), yielding PDMAEMA with Mn = 18 900 and Đ = 1.4 (Table 1). The slower rate might be due to the weaker coordination between DMAEMA and embedded Cu ions, resulting in less significant photoharvesting ability, as evaluated by UV−vis spectra in Figure 1. Finally, the non-nucleophilic MMA was attempted to be polymerized. Under conditions that were successful for 4VP, 2VP, and DMAEMA, MMA did not polymerize even after 20 h of photoirradiation, as shown in Figure S6a. This indicated that MOF-comprised dabco alone is an insufficient photoreducing agent to trigger polymerization; instead, nucleophilic Ncontaining monomers are required. For enhancement of the light-harvesting properties during MMA polymerization, TEA and 4-ethylpyridine were added to trigger the polymerization. TEA and 4-ethylpyridine are nucleophiles and reducing agents that mimic DMAEMA and vinylpyridines, respectively.61 Figure S7 shows that, in the presence of TEA, the photocurrent related to the reduction of Cu(II) to Cu(I) increased by 5 times. Indeed, addition of TEA promoted successful polymerization of MMA, giving 20% conversion (Figure S6b), but a polymer with a molecular weight (Mn,SEC = 194 000, Đ = 1.3) higher than predicted (Mn,theo = 4200). The 4-ethylpyridine was more effective, and promoted polymerization up to 30% conversion (Figure S6c), resulting in PMMA with Đ = 1.3 and Mn,SEC = 13 800 (Mn,theo = 4500, Figure S7c). These high values of Mn,SEC are attributed to a low initiation efficiency by EBIB, which is a poor initiator for MMA due to a penultimate effect.61,69,70 When bromoacetonitrile was used, a more active initiator, molecular weights were controlled with Mn,SEC = 3700, Mn,theo = 3400, and Đ = 1.2 (Figure 3d). Moreover, the polymerization rate increased, with 20% conversion after 3 h instead of 10 h (Figure S6d). The improved efficiency with 4ethylpyridine and bromoacetonitrile is in line with the enhanced polymerization behavior of vinylpyridines compared to those of DMAEMA or the TEA/MMA system, advocating the versatility of PCRP catalyzed by the Cu(II) MOF. SEC chromatograms and all polymerization kinetic plots are shown Figure S10 and Table S1. Dispersity decreased with conversion, which is a typical behavior of ATRP with low catalyst loadings. Monomodal molecular weight distributions were obtained for both 4VP and 2VP, suggesting a living chain end and limited nucleophilic substitution which could replace the bromide chain end with pyridine groups. Stability of Cu2(bdc)2(dabco). The electrochemical measurements suggested the stability of the MOF under photoirradiation (Figure 2b). Moreover, the conservation of the MOF structure during each polymerization was confirmed by PXRD (Figure S2a). Nevertheless, PXRD cannot account for changes on the catalytically active surface. Variations in relative peak intensities were attributed to the incorporation of the guest molecules in the host nanochannel.43 Indeed, because of monomer incorporation in the porous framework, BET profiles showed decreased surface area (m2/g)/pore volume (cc/g) from 2280/0.91 to 230/0.15, 560/0.25, 450/0.29, and 1050/ 0.36 after polymerization with P4VP, P2VP, PDMAEMA, and PMMA, respectively. Interestingly, the decrease corresponds to the coordination ability of the utilized monomer (i.e., 4VP > 2VP > DMAEMA > MMA).

Scheme 2. Proposed Mechanism of Photopolymerization with the Formation of the Monomer−Cu(II) MOF Complexes, Giving Adjustable Photoabsorption and Polymerization Behavior

The faster polymerization and enhanced control in polymerization of vinylpyridines related to their stronger coordination, which enhanced the light-absorption property of the monomer−Cu(II) complex. Polymerization rate in the presence of additives followed the same trend: The 4-ethylpyridine, which mimics 4VP, gave a faster reaction than TEA, which mimics DMAEMA. The reduction of Cu(II) in a mixture with P2VP has been described in the literature albeit at elevated temperature.71 Nevertheless, irradiation might be able to 9450


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Figure 4. SEC chromatograms of (a) P4VP, (b) P2VP, (c) PDMAEMA, and (d) PMMA before and after chain extension with OEGMA or IBA: in 50 vol % methanol (P4VP and PDMAEMA) or DMF (P2VP and PMMA) with [macroinitiator]0/[OEGMA/IBA]0/[CuBr2]0/[PMDETA]0/ [dabco]0 = 1:340:4:8.8:20, at 50 °C and 24 h.

proof of the inactivity of leaked Cu2+ was provided by blank polymerizations as negative control: No polymerization occurred when a typical reaction mixture [Cu(II) MOF, monomer, and initiator] was prepared, and then filtered to remove the MOF (Figure S9a). Additionally, no conversion was detected in the absence of initiator, indicating that the alkyl halide was the only source of radicals (Figure S9b). In summary of this section, the surface of the MOF reacted with polymer chain ends to give controlled polymerization, while both MOF bulk and leaked Cu2+ were inactive. Polymer Microstructure Resulting from Cu(II) MOFMediated Photopolymerization. Considering the strong association between nucleophilic monomers and the wellordered Cu framework, the microstructure of the synthesized polymers was studied.42,75 The tacticity of each synthesized polymer was analyzed by 13C NMR (Figure S10). Results were compared between MOF-mediated photopolymerization and traditional polymerization approaches (i.e., free-radical polymerization or homogeneous ARGET). Isotacticity increased for P4VP, with a content of isotactic triads of 27% by photopolymerization, in comparison to 13% by conventional freeradical polymerization. The increased isotacticity can be

provide the required energy for photoreduction of Cu(II). Overall, the photoreduction of Cu(II) proceeds without pyridine or external amine addition as shown via CV (Figure 2), but pyridine- or amine-supported reduction might take place as well. ATRP activation requires Br atom transfer between chain end and Cu. However, Br should be difficult to accommodate in the rigid structure of the MOF without distorting the coordination sphere. Conversely, vacant or defect sites on the MOF outer surface may be more accessible, acting as catalytic hot spots. Because of the small size of the crystals (∼100 nm), content of Cu on the surface should be a remarkable 5% of total Cu (considering 0.76 nm the average Cu−Cu distance from coordination structure;59 see the SI for details on estimation). Moreover, defects can increase accessibility of catalytic ions and then further enhance the reactivity.72,73 Therefore, we suggest that the bulk polymerization is catalyzed by the MOF surface or defects in MOF crystals, where monomer, polymer, and Br are able to interact with available coordination sites. Aside from Cu ions on the MOF surface, leaked Cu2+ could also deactivate radicals.74 The Cu2+/dabco complex, however, was an inactive ATRP catalyst, as shown in Figure S14. Further 9451


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Figure 5. (a) Conversion of 2VP and DMEAMA in subsequent photopolymerizations with recycled Cu2(bdc)2(dabco). (b) Semilogarithmic kinetic plot of 4VP consumption during intermittent light exposure, with consecutive light (white area) and dark (shaded area) treatments.

DMAEMA, and polymers during the subsequent polymerization tests. Incorporation of reagents was also confirmed by the gradual increment of catalyst weight (Figure S13), by the decline of surface area from 2280 to 320 m2/g, and by the contraction of pore volume from 0.91 to 0.18 cc/g (Figure S2b). However, filling the channels of the MOF and leakage of Cu atoms did not compromise its catalytic properties in the repeated polymerizations, confirming that the surface, not the bulk, is the active site for photopolymerization. Temporal Control of Cu2(bdc)2(dabco) in P4VP Photopolymerization. On/off reactivity is an appealing characteristic of photopolymerization. Using intermittent light exposure and dark for specific times (i.e., 20, 10, and 60 min), the MOFmediated polymerization of 4VP proceeded much faster when light irradiation was applied. Very limited activity was observed during the dark periods, which could be attributed to Cu(I) remaining in the system (Figure 5b). As a result, the photoresponsive property of the Cu(II) MOF was confirmed by the temporal control of polymerization.

attributed to the coordination and alignment of 4VP and P4VP along the MOF framework, because the comprised Cu ions acted as Lewis acids, forming strong coordination with nitrogen atoms from 4VP monomer and/or polymer chains.44,76,77 Except 4VP, the other monomers retained their original tacticity.77 In the case of 2VP, this could result from the steric hindrance on the heteroaromatic ring, enabling only a free form to polymerize.67 For methacrylates, this was attributed to their weak affinity toward Cu centers. Consequently, only the strongly coordinating 4VP showed a significant increase in isotacticity. Another desirable feature in PCRP is chain-end functionality, which was investigated by chain extension of all prepared polymers with either oligo(ethylene glycol) methyl ether methacrylate (OEGMA), or isobornyl acrylate (IBA) by ARGET ATRP under homogeneous conditions (Figure 4 and Table 1). The SEC traces show a monomodal increase of molecular weight, and thus preservation of the halogen endgroup, which is typical of “living” polymerizations. The block copolymers were well-controlled with Đ = 1.2−1.3. The difference in composition between macroinitiators and block copolymers was also traced by 1H NMR (Figure S12). It should be noted that additional dabco is acting as a reducing agent in solution at 50 °C here as shown via the successful block copolymer formation. On the other hand, dabco is an ineffective reducing agent under light irradiation at ambient temperature. Recycling Cu2(bdc)2(dabco). As a heterogeneous catalytic complex with particle size ∼100 nm, the Cu(II) MOF was easily separated via centrifugation, recovered by washing, and repeatedly utilized for subsequent photopolymerizations. As demonstrated in Figure 5a, the catalytic complex was recycled up to six times, conducting alternating polymerizations of P2VP and PDMAEMA. Monomer conversion remained constant in each catalyst recycling step. The preservation of the MOF structure after the repeated tests was confirmed by PXRD in Figure S2a. Nevertheless, PXRD is not sufficient to account for the catalytically active surface. Although a significant amount of copper atoms were lost because of leakage (Table S2), no significant difference in catalytic activity was observed. The slight shift of position and intensity of the peaks resulted from the incorporation of 2VP,

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CONCLUSIONS The MOF Cu2(bdc)2(dabco) was used for controlled photopolymerization under visible light, without the need for external photoinitiators. N-containing monomers and additives (i.e., 4VP, 2VP, TEA, and 4-ethylpyridine) had a dual/synergistic effect on the MOF-mediated polymerization. On one hand, these compounds acted as reducing agents under irradiation; on the other hand, the formation of monomer−MOF complexes improved the photoabsorption property of the MOF, which could harvest more visible light. The photoabsorption property improved when stronger coordinating monomers were used, with 4VP > 2VP > DMAEMA > MMA. Interestingly, the rate of photopolymerization followed the same trend. The effect of the monomer coordination was reversible, as the original Cu(II) MOF could be recovered by simple washing with solvent. Cu2(bdc)2(dabco) presented a well reversible Cu(II)/Cu(I) redox behavior, with E⊖ = −0.084 V versus SCE, suggesting excellent catalytic activity in ATRP. Photoreduction of Cu(II) to the active Cu(I) was also well reversible as traced by chronoamperometry under intermittent light irradiation. 9452


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Controlled photopolymerization of highly nucleophilic monomers was achieved with the Cu(II) MOF, synthesizing polymers with predictable molecular weight and low Đ, ranging from 1.2 to 1.4. After reaction, the catalyst was easily recycled for several polymerizations, avoiding high contamination of catalytic ions. Moreover, photoreaction was temporally controlled, exploiting the stimulus-responsive redox nature of the Cu(II) MOF.

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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.chemmater.7b03541. Additional synthetic procedures, NMR, SEC, CV, and electron microscopy data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *Phone: +01 412 268-3209. Fax: +01 412 268-6897. E-mail: [email protected]. *Phone: +49 331 567-9501. Fax: +49 331 567-9502. E-mail: [email protected]. ORCID

Krzysztof Matyjaszewski: 0000-0003-1960-3402 Bernhard V. K. J. Schmidt: 0000-0002-3580-7053 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

Max-Planck society, National Science Foundation, Deutscher Akademischer Austauschdienst. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support from the German Academic Exchange Service (DAAD) and the Max Planck Society. The authors also acknowledge Marlies Gräwert for SEC experiments, Rona Pitschke and Heike Runge for electron microscopy, and Jeannette Steffen for ICP analysis. M.F. and K.M. acknowledge support through the NSF DMR 1501324 grant.

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Synergic Effect between Nucleophilic Monomers and Cu(II) Metal

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