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Expanding the ATRP Toolbox: Methacrylate Polymerization with an Elemental Silver Reducing Agent Valerie A. Williams and Krzysztof Matyjaszewski* Department of Chemistry, Carnegie Mellon University, 4400 Fifth Ave., Pittsburgh, Pennsylvania 15213, United States

Downloaded by UNIV OF PENNSYLVANIA on September 10, 2015 | http://pubs.acs.org Publication Date (Web): September 9, 2015 | doi: 10.1021/acs.macromol.5b01696

S Supporting Information *

ABSTRACT: The atom transfer radical polymerization (ATRP) of methacrylates using Ag0 as a reducing agent was carried out. Optimized reaction conditions enabled the controlled preparation of polymethacrylates using elemental silver and a CuBr2/ PMDETA catalyst, with polymer dispersity values down to Đ = 1.06. In these reactions, the formation of AgBr was observed as a dark coating on the surface of the silver wire; however, the AgBr coating had minimal effect on polymerization, and the same silver wire could be used in several consecutive reactions without elaborate cleaning. Different polymethacrylates were prepared with good control, and a poly(methyl methacrylate)-b-poly(benzyl methacrylate)-b-poly(ethyl methacrylate)-Br triblock copolymer was prepared with a molecular weight of 32 200 and a dispersity of Đ = 1.07. Additionally, it was shown that silver can act as a supplemental activator in the generation of radicals from ethyl α-bromophenylacetate, with a rate constant of surface −6 activation of kapp cm s−1. a0 = 9.1 × 10

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eversible deactivation radical polymerization (RDRP)1−3 is one of the most powerful and versatile tools developed for polymer synthesis. In the past decades, RDRP techniques have been used to generate homo- and block (co)polymers with highly tailored architectures, topologies, and chain lengths, while maintaining highly preserved chain-end functionalities and narrow molecular weight distributions (MWD).3−10 The most commonly utilized RDRP techniques include nitroxidemediated polymerization (NMP),11−13 reversible addition− fragmentation chain transfer polymerization (RAFT),14−18 and atom transfer radical polymerization (ATRP),7,8,19−23 all of which rely on a fast and dynamic equilibrium between a dormant species and a propagating radical species to achieve good control over polymerization. In ATRP, polymer chain growth is typically mediated by a transition-metal-catalyzed activation/deactivation cycle, in which dormant alkyl halides are activated to alkyl radicals by a low-valent transition-metal catalyst and alkyl radicals are deactivated by high-valent metal catalysts (Scheme 1a).24−26 Originally, a high concentration of transition metal catalyst, on the order of thousands of parts per million (ppm), was required in ATRP to achieve good control over polymerization. This necessity was due to unavoidable radical−radical termination, which resulted in a buildup of deactivator species and therefore a decrease of low-valent metal catalyst available for activation.24,28 In an effort to develop more sustainable, © XXXX American Chemical Society

Scheme 1. (a) Normal Atom Transfer Radical Polymerization with a Transition Metal Catalyst and (b) Proposed Mechanism of ATRP in the Presence of Ag0 27

Received: July 30, 2015 Revised: August 21, 2015

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DOI: 10.1021/acs.macromol.5b01696 Macromolecules XXXX, XXX, XXX−XXX

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lauryl methacrylate (LMA), benzyl methacrylate (BzMA), ethyl αbromoisobutyrate (EBiB), ethyl α-bromophenylacetate (EBPA), N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA), silver bromide, and copper(II) bromide (CuBr2) were purchased from various suppliers. Monomers were passed through basic alumina to remove inhibitors and stored at 0 °C before use. Dimethylformamide (DMF) was purchased from Fisher and used without any purification. Unless otherwise noted, after each reaction silver wire (Strem) was stirred overnight in concentrated NH4OH(aq) to remove AgBr, washed in acetone, air-dried, and used in subsequent reactions. For recycle experiments, silver wire was washed with THF to remove PMMA, airdried, and used directly in subsequent reactions. Tris(2pyridylmethyl)amine (TPMA) was synthesized according to literature procedures.63−65 Characterization. Monomer conversion was determined by 1H NMR in CDCl3 using a Bruker Avance 300 MHz or Bruker 500 MHz spectrometer with DMF (δ = 2.96, 2.88, (CH3)2NCOH) as the internal standard.66 Number-average molecular weight (Mn) and Mw/ Mn values were determined by gel permeation chromatography (GPC) using PSS columns in THF at an eluent at a flow rate of 1 mL/min at 35 °C, calibrated to PMMA using toluene as an internal standard. The GPC system was composed of a Waters 515 HPLC pump and a Waters 2414 refractive index detector. Each sample was dried with MgSO4 and filtered over neutral alumina prior to analysis. A Mark− Houwink correction was applied to obtain the MW of PBMA and PEMA with respect to the PMMA standard. UV−vis spectroscopy was performed on a Varian Cary 5000 UV−vis−NIR spectrometer. Polymerizations. Polymerizations were carried out according to either method (a) or method (b). (a) A 10 mL Schlenk flask was charged with Ag0 wire (d = 2 mm, l = 5 cm) and stir bar, sealed, and evacuated and refilled with N2 six times. A solution of CuBr2/ PMDETA (338 μL of 0.05 M CuBr2 and 0.10 M PMDETA in DMF, 0.017 mmol of CuBr2 and 0.034 mmol of PMDETA), EBPA (103 mg, 0.42 mmol), DMF (3.0 mL), and MMA (9.0 mL, 85 mmol) was prepared and degassed via four freeze−pump−thaw cycles, and 10 mL of this solution was added to the reaction flask under N2 via syringe. The flask was sealed and warmed to 25 °C, and the reaction was monitored by GPC and 1H NMR spectroscopy. (b) A 10 mL Schlenk flask was charged with a solution of EBPA (86 mg, 0.35 mmol), DMF (2.5 mL), MMA (7.5 mL, 71 mmol), and CuBr2/PMDETA (282 μL of 0.05 M CuBr2 and 0.10 M PMDETA in DMF, 0.014 mmol of CuBr2, and 0.028 mmol of PMDETA). Ag0 wire (d = 2 mm, l = 5 cm) and a stir bar were added, and the flask was immediately sealed and immersed in liquid nitrogen. The reaction mixture was degassed via four freeze−pump−thaw cycles, warmed to 25 °C, and monitored by GPC and 1H NMR spectroscopy. Synthesis of PMMA-b-PBzMA-b-PEMA-Br Triblock Copolymer. (i) A PMMA-Br macroinitiator (Mn = 8000, Đ = 1.09) was synthesized according to procedure (b) above and purified by precipitation in ethanol/water (40/60 by v/v), redissolved in THF, dried over MgSO4, filtered, and reprecipitated from hexanes. The product was collected and dried under vacuum for 12 h. (ii) Solid PMMA-Br macroinitiator (5.66 g, 0.71 mmol) was dissolved in DMF (8.00 mL). To this was added MMA (24.0 mL, 142 mmol), CuBr2/ PMDETA (567 μL of 0.05 M CuBr2 and 0.10 M PMDETA in DMF, 0.028 mmol of CuBr2, and 0.057 mmol of PMDETA), silver wire (3 × l = 5 cm, d = 2 mm), and a stir bar, immediately sealed, and immersed in liquid nitrogen. The reaction mixture was degassed via four freeze− pump−thaw cycles, warmed to 25 °C, and allowed to react for 6 h. The reaction was exposed to oxygen to stop, and the resulting polymer was purified by precipitation in ethanol/water (60/40 by v/v), redissolved in THF, dried over MgSO4, filtered, and reprecipitated from hexanes (Mn = 21 000, Đ = 1.05). (iii) PMMA-b-PBzMA-Br macroinitiator (8.45 g, 0.40 mmol) was dissolved in DMF (10.0 mL). To this was added MMA (10.0 mL, 80 mmol), CuBr2/PMDETA (322 μL of 0.05 M CuBr2 and 0.10 M PMDETA in DMF, 0.016 mmol of CuBr2, and 0.032 mmol of PMDETA), silver wire (2 × l = 5 cm, d = 2 mm), and a stir bar, immediately sealed, and immersed in liquid nitrogen. The reaction mixture was degassed via four freeze−pump− thaw cycles, warmed to 25 °C, and allowed to react for 6 h. The

“greener” catalytic systems, in the past decade new ATRP methods have been established to reduce catalyst loadings down to the low ppm level through regeneration of the lowvalent activator species from the deactivator complex.29,30 These techniques include addition of a reducing agent (activator regenerated by electron transfer, ARGET ATRP)31−33 or azo radical initiator (initiators for continuous activator regeneration, ICAR ATRP),31,34 as well as the use of electrical current to generate the reduced activator complex (eATRP),35 application of light (photoATRP),36−41 or addition of a zerovalent metal (supplemental activator and reducing agent, SARA ATRP).42−47 The use of zerovalent metals in ATRP dates as far back as 1997,45 when Cu0 was first shown to both reduce CuIIBr2/L to CuIBr/L activator complex (major pathway) and simultaneously activate alkyl halides directly from the metal surface (minor pathway),30,42 and was straightforward enough in setup to be used in advanced undergraduate laboratories.48,49 Since then, many other metals including Fe0, Mg0, Zn0, and Sn0 and lanthanides such as Sm0 and Yb0 have been shown to heterogeneously activate alkyl halides from the metal surface.50−53 Recently, the use of Ag0 as a reducing agent in the coppercatalyzed ARGET ATRP of various acrylates was reported (Scheme 1b).27 Very good control over polymerization was observed in these reactions, resulting in dispersity values of as low as Đ = 1.03 for poly(n-butyl acrylate) and facilitating the synthesis of highly regular block copolymers. It was proposed that this exceptional control was likely due to several reasons: (i) Ag0 only has two common oxidation states (0 and +1), so the proposed reduction process is likely to only involve one electron;54,55 (ii) because both Ag0 and the oxidized species AgIBr are fairly insoluble in most reaction media, side reactions due to these components during polymerization are less probable; and (iii) Ag0 is relatively inert toward typical polymerization reagents, minimizing or even eliminating entirely the undesirable radical generation/termination side reactions often observed in standard SARA ATRP reactions.30,56 It was shown that under the reaction conditions studied alkyl halide activation from Ag0 was negligible, simplifying the Ag0-mediated ARGET reaction relative to traditional heterogeneous ATRPs while retaining the added benefit of simplified purification procedures characteristic of heterogeneous reactions. In the present work, we exploit the benefits of Ag0 as a reducing agent in ATRP and apply this reducing agent to the copper-catalyzed ATRP of methacrylates (Scheme 1b). The polymerization of methacrylates pose additional challenges relative to acrylates: because the propagating tertiary methacrylate radical is more stable than the propagating secondary acrylate radical, activation to form the propagating species occurs much more readily for methacrylates. Propagation rate constants are lower for methacrylates, so to achieve good control over the ATRP of methacrylates, an alkyl halide initiator with a weak C−Br bond must be selected such that activation of initiator occurs more readily than activation of dormant polymer chains. Additionally, a less-active copper catalyst must be selected to slow the rate of activation relative to propagation.57−62 These challenges will be addressed below.

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EXPERIMENTAL SECTION

Materials. All manipulations were carried out under an inert atmosphere of dry N2 unless otherwise noted. Methyl methacrylate (MMA), ethyl methacrylate (EMA), n-butyl methacrylate (BMA), B


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Table 1. Optimization of Reaction Conditions for the Polymerization of Methyl Methacrylate (MMA) with Ag0-Mediated ARGET ATRPa entry

ligand

initiator

monomer concn (M)

temp (°C)

time (h)

conv (%)

Mn,obs

Mn,th

Đ

kapp (h−1)

linear evolution of MW?

initiation efficiency

1b 2 3 4 5c 6

TPMA PMDETA PMDETA PMDETA PMDETA PMDETA

EBiB EBiB EBPA EBPA EBPA EBPA

3.5 4.7 4.7 4.7 7.1 7.1

50 50 50 25 50 25

6 6 6 6 6 8

9 45 69 30 33 24

10400 14900 24500 8200 7600 5200

5800 9000 13800 6000 6600 4800

1.64 1.35 1.16 1.12 1.13 1.11

0.044 0.085 0.219 0.058 0.038 0.058

N N Y Y Y Y

n.d. n.d. 0.56 0.73 0.87 0.92

TPMA = tris(2-pyridylmethyl)amine, PMDETA = N,N,N′,N″,N″-pentamethyldiethylenetriamine, EBiB = ethyl α-bromoisobutyrate, EBPA = ethyl α-bromophenylacetate. Reaction conditions: [MMA]0:[Initiator]0:[CuBr2]0:[ligand]0 = 200:1:0.04:0.08 (200 ppm of CuBr2) in DMF, in the presence of 5 cm Ag0 wire (d = 2 mm, Vtot = 10 mL). n.d. = not determined. b[MMA]0:[EBiB]0:[CuBr2]0:[TPMA]0 = 500:1:0.05:0.1 (200 ppm of CuBr2). cNo additional monomer conversion was observed after 6 h.


a

reaction was exposed to oxygen to stop, and the resulting polymer was purified by precipitation in ethanol/water (80/20 by v/v), redissolved in THF, dried over MgSO4, filtered, and reprecipitated from hexanes (Mn = 32 200, Đ = 1.07).

explanation for this observation could be the rapid loss of initiator in the very early stages of polymerization, which would result in higher than expected MW values but still allow for linear evolution of MW. By comparing observed to calculated MW values, an initiation efficiency (f) of 0.56 was obtained, implying nearly 50% loss of initiator in the initial stages of polymerization. This initiator loss is likely due to fast activation of EBPA by CuI, followed by radical coupling or decay at a rate comparable to that of MMA addition. Thus, to improve initiation efficiency, one could feasibly modulate the Cu/Ag reduction equilibrium (eq 1) to reduce the amount of CuI present in the initial stages

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RESULTS AND DISCUSSION As a starting point, polymerization of methyl methacrylate (MMA) was carried out under the optimal conditions reported for the ATRP of n-butyl acrylate (BA, Table 1, entry 1);27 however, poor control over polymerization was observed. Although monomer consumption followed first-order kinetics, evolution of MW as a function of conversion was nonlinear and indicated slow initiation as expected. To properly tailor polymerization conditions for the successful ATRP of methacrylates, a less-active transition metal catalyst and more active initiator must be used, so the polymerization was carried out first with a CuBr2/PMDETA catalyst (PMDETA = N,N,N′,N″,N″-pentamethyldiethylenetriamine) and then with the very active ATRP initiator EBPA (EBPA = ethyl αbromophenylacetate, Figure 1). Utilization of a CuBr2/

of reaction, decreasing rate of EBPA activation and loss of initiator. To this end, polymerizations with added AgBr or decreased amounts of Ag0 wire were carried out (Table S1). In all cases an improvement in initiation efficiency was observed, but these polymerizations typically stopped at low conversions. Gratifyingly, decrease of reaction temperature from 50 to 25 °C allowed for a higher initiation efficiency of f = 0.73, while maintaining pseudo-first-order consumption of monomer over time and a linear increase of MW with conversion (Table 1, entry 4). To determine the effect of temperature on the CuI/ CuII equilibrium, the reduction of CuBr2/PMDETA by Ag0 in polymerization media at 25 °C in the absence of initiator was monitored by UV−vis spectroscopy as a function of time (Figure 2 and Figure S2). A rapid initial reduction was observed, with a rate constant of reduction of kred = 2.1 × 10−3

Figure 1. ATRP initiators and ligands used in this work.

PMDETA catalyst (Table 1, entry 2) resulted in better control over polymerization, as evidenced by the decrease of Đ from 1.64 to 1.35. Unfortunately, slow initiation was still observed with the EBiB initiator, resulting in a nonlinear evolution of MW with conversion. When EBPA was used as the initiator with a CuBr2/PMDETA catalyst (Table 1, entry 3), good control over polymerization was observed (Đ = 1.16) and evolution of MW was linear with conversion. For this polymerization, however, at all conversions the obtained MW values were 1.8× greater than the theoretically predicted values for a living polymerization (Figure S1). One possible

Figure 2. Reduction of CuBr2/PMDETA with Ag0 wire in the absence of alkyl halide initiator. Concentration of CuII determined by monitoring absorbance of copper d−d band at 792 nm. Reaction conditions: [MMA]0:[CuBr2]0:[PMDETA]0 = 200:0.04:0.08 (200 ppm of CuBr2) in DMF, in the presence of 5 cm Ag0 wire (d = 2 mm, Vtot = 10 mL). C


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Macromolecules cm s−1 over the first 15 min.67 Equilibrium between CuII/Ag0 and CuI/AgI was reached after 60 min, at which point roughly 60% of initial CuII had been converted to CuI. After equilibrium was established, the mixture was heated to 50 °C, and a new equilibrium was achieved after approximately 60 min. At 50 °C in the absence of alkyl halide initiator, roughly 80% of CuII was converted to CuI. It is important to note that under standard polymerization conditions a significantly lower CuI/CuII ratio is typically present due to the consumption of CuI through alkyl halide activation and possible CuI-based termination pathways.30,56 Nevertheless, this result indicates that the reaction shown in eq 1 is driven toward products at higher temperatures. The reverse reaction was considerably slower, presumably due to the insoluble nature of AgBr, and the system only reached equilibrium again after cooling to 25 °C for approximately 24 h (Figure 2). Initiation efficiency was also improved by increasing concentration of monomer to 75% v/v in DMF (Table 1, entry 5, Figure 3). This enhancement could be due to two

components in less polar media. In addition, because AgBr is moderately soluble in DMF,71,72 we suspect that a small amount of DMF is necessary to dissolve some of the AgBr formed on the surface of the Ag0 wire (vide inf ra). Activation of less than 0.005% of initiator in any given polymerization would produce enough AgBr to form a monolayer on the surface of the Ag0 wire (Figure S3), so it is essential to solvate enough of the AgBr such that reduction of CuBr2 can easily occur throughout the course of reaction. Thus, all subsequent polymerizations were carried out at 75% v/v monomer in DMF at 25 °C. Recycling of Ag0. Ag0 wire is highly reusable over many reactions without the necessity of extensive cleaning procedures in the Ag0-mediated ARGET ATRP of acrylates.27 This was a very significant observation, particularly because Ag0 is more expensive than alternative reducing agents such as ascorbic acid or Cu0, and indeed, one of the greatest benefits of a heterogeneous reaction is the reusability of reaction components. In the polymerization of acrylates, the same Ag0 wire could be used in five sequential reactions without any decrease in performance, and between each reaction the Ag0 visually appeared unchanged. In contrast, during five sequential polymerizations of MMA with the same Ag0 wire, a visible darkening of the Ag0 was observed (Figure 4), presumably due to the formation of a layer of AgBr on the surface of the wire. Inherently, the polymerization of methacrylates would result in formation of more AgBr than the polymerization of acrylates because a greater amount of methacrylate radicals would lead to more pronounced radical−radical termination relative to acrylates. This, in turn, would necessitate more regeneration of activator complex through reduction of CuII with Ag0 (Scheme 1b). Therefore, more AgBr would be observed during the polymerizations of MMA. Although the Ag0 wire used in each polymerization was visually changed, performance in polymerizations did not significantly decrease. When fresh Ag0 was used, a relatively fast polymerization was observed after a long induction period (Figure 5, red squares), but after this first cycle, very similar polymerization rates were obtained for all subsequent polymerizations. In all reactions, MW increased linearly with conversion and dispersities down to Đ = 1.06 were obtained at high conversion. Initiation efficiencies ranged from 0.81 to 0.92, with the general trend of longer induction periods correlating to higher initiation efficiencies. Substrate Scope. The Ag0-mediated ARGET ATRP of MMA was further expanded to include a range of methacrylates, summarized in Table 2. With butyl (BMA), ethyl (EMA), lauryl (LMA), and benzyl methacrylate (BzMA) monomers, polymethacrylates with quite narrow MW distributions were obtained. Initiation efficiencies ranged from ∼0.7 to nearly 1 for poly(benzyl methacrylate).

Figure 3. Evolution of Mn and Mw/Mn with conversion in the ATRP of MMA with selected reaction conditions given in Table 1. Reaction c o nd i ti o ns : [ M M A ] 0 : [ E B P A ] 0 : [ C u B r 2 ] 0 : [ P M D ET A ] 0 = 200:1:0.04:0.08 with [MMA]0 = 4.7 or 7.1 M in DMF at 25 or 50 °C, in the presence of 5 cm Ag0 wire (d = 2 mm, Vtot = 10 mL).

possible reasons: (i) an increase in monomer concentration may decrease the amount of activated EBPA that undergoes rapid termination (rate of propagation is increased relative to rate of EBPA decomposition), or (ii) a higher MMA concentration changes the polarity of the reaction medium, affecting KATRP and Kred favorably for efficient propagation.68−70 As anticipated, carrying out the polymerization of MMA at high monomer concentration and low temperature gave the best result (Table 1, entry 6, and Figure 3), with an initiation efficiency of up to 0.92. However, it is important to note that the polymerization did not proceed at significantly higher concentrations of monomer or in bulk (Table S1). This is presumably due to the poor solubility of the reaction

Figure 4. Photographs of Ag0 wire (a) before polymerization, (b) after cycle 1, (c) after cycle 2, (d), after cycle 3, (e) after cycle 4, and (f) after cycle 5. D


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Figure 6. GPC traces for the synthesis of PMMA-b-PBzMA-b-PEMABr. Thick lines correspond to purified polymer obtained after precipitation; thin lines correspond to crude polymer sampled after 2 or 4 h of polymerization. Reaction conditions: [monomer]0: [initiator]0:[CuBr2]0:[PMDETA]0 = 200:1:0.04:0.08 (200 ppm of CuBr2), 75% v/v monomer in DMF, in the presence of 5 cm Ag0 wire (d = 2 mm, Vtot = 10 mL), 25 °C.

acrylates with a CuBr2/TPMA catalyst under the reaction conditions studied.27 However, some zerovalent metals can be directly involved in radical generation, albeit slowly, with activation observed on the time scale of hours or days.50−52 In this system, a less active catalyst, CuBr2/PMDETA, was specifically chosen to decrease the rate of CuI-based activation. Therefore, supplemental radical generation from the surface of Ag0 was measured to determine whether this mode of activation was a significant competing process with activation by CuI.

Figure 5. (a) Kinetics and (b) evolution of Mn and Mw/Mn with conversion in the ATRP of MMA with the same silver wire in five sequential reactions. Reaction conditions: [MMA] 0 :[EBPA] 0 : [CuBr2]0:[PMDETA]0 = 200:1:0.04:0.08 with [MMA]0 = 7.1 M in DMF at 25 °C, in the presence of 5 cm Ag0 wire (d = 2 mm, Vtot = 10 mL).

app ka0

Ag 0(s) + RX + L ⎯⎯⎯→ Ag IX/L + R•

To determine whether acceptable retention of chain-end functionality was achieved, several successive chain extension experiments were carried out. First, PMMA-Br (Mn = 8000, Đ = 1.09) was synthesized and purified according to the procedure outlined in the Experimental Section and used as a macroinitiator in the polymerization of BzMA. The PMMA-bPBzMA-Br diblock polymer (Mn = 21 000, Đ = 1.05) was then purified via precipitation and used for the subsequent polymerization of a block of PEMA. The triblock copolymer PMMA-b-PBzMA-b-PEMA-Br ultimately formed was highly uniform, with a Mn = 32 200 and a dispersity of Đ = 1.07. Unfortunately, the appearance of a high molecular weight shoulder and a small amount of low molecular weight tailing after addition of a third block in the copolymer (Figure 6) indicated a non-negligible fraction of dead chains, precluding further efficient chain extension. Nevertheless, chain extension experiments demonstrate the power of Ag0-mediated ATRP as a tool for the highly controlled synthesis of short block copolymers of methacrylates. Surface Activation (SA) by Ag0. Because of the versatility of this methodology, we closely examined the mechanism of this reaction. Previously, it was reported that alkyl halide activation by Ag0 was negligible in the polymerization of

(2) 0

To quantify direct activation by Ag , a useful metric to examine is the rate coefficient of activation, kapp a0 (eq 2). The polymerization of MMA with an EBPA initiator was therefore carried out in the absence of CuBr2. As shown in Figure 7, polymerization was quite slow, with only 12% conversion reached after 24 h and 29% conversion reached after 48 h. Molecular weight data were quite high relative to the predicted values for standard controlled radical polymerization, and dispersity values ranged from 1.5 to 1.7, indicative of radical generation by Ag0 followed by standard free radical polymerization in the absence of a deactivator complex (Table 3). −6 These values provide a kapp cm s−1 (see a0 = 9.1 × 10 Supporting Information), corresponding to a rate of radical generation of 1.0 × 10−7 M s−1. In standard ARGET-only ATRP reactions (Scheme 1b), the rate of polymerization is dictated by the rate of reduction of CuII. Therefore, one could reasonably use the rate of reduction of CuBr2/PMDETA with Ag0 wire in the absence of alkyl halide initiator (eq 1, kred = 2.1 × 10−3 cm s−1, rate of reduction = 9.5 × 10−7 M/s−1) as a model for the ARGET-only rate of radical generation in the polymerization of MMA initiated by EBPA. In this model, it

Table 2. Polymerization of Various Methacrylates with Ag0-Mediated ARGET ATRP monomer

time (h)

conv (%)

Mn,obs

Mn,th

Đ

kapp (h−1)

initiation efficiency

MMA BMA EMA LMA BzMA

8 8 8 8 8

13 10 31 30 50

3600 4200 8910 17900 17300

2600 2840 7068 15240 17600

1.12 1.12 1.06 1.13 1.07

0.032 0.043 0.066 0.082 0.114

0.72 0.68 0.79 0.85 ∼1.0

E


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polymerization of MA was carried out under the same conditions in the presence of Ag0 wire (Table 3, entry 3). −7 The calculated rate constant of kapp cm s−1 for Ag0 a0 = 2.9 × 10 is approximately 3 orders of magnitude less than that of Cu0, illustrating that although Ag0 can participate in the activation of alkyl halides, this process is slow. Based on these data, radical generation by Ag0 is negligible at short reaction times, such as in the copper-catalyzed polymerization of acrylates,27 but in the presence of highly active alkyl halide initiators such as EBPA,60 Ag0 can slowly generate radicals over time, as in the ATRP of methacrylates.


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CONCLUSION Use of Ag0 as a reducing agent in copper-catalyzed ATRP has been optimized for the polymerization of methacrylates. In these reactions, it was necessary to use the highly active initiator EBPA, which formed radicals that terminated at a rate comparable to that of MMA addition during the early stages of polymerization. This problem was largely circumvented through decrease of reaction temperature and increase in monomer concentration. Under these improved conditions, the highly controlled polymerization of methacrylates could be carried out with good initiation efficiency, with resulting polymer dispersity values down to Đ = 1.06. The formation of AgBr was observed during reaction as a dark coating on the surface of the Ag0 wire, but despite this noticeable change, the same silver wire could be used in successive polymerizations without extensive cleaning and with minimal effect on reaction performance. This methodology was expanded to the polymerization of a set of different methacrylates, and a PMMA-bPBzMA-b-PEMA-Br triblock copolymer (Mn = 32 200, Đ = 1.07) was synthesized with this technique. Furthermore, it was shown in these long reactions that alkyl halide activation by Ag0 was a contributor to radicals generated in this system; however, in the presence of CuII deactivator complex, MW distributions remained narrow. Thus, Ag0 can be used as an effective reducing agent in the copper-catalyzed ATRP of methacrylates.

Figure 7. (a) Kinetics and (b) evolution of Mn and Mw/Mn with conversion for the polymerization of MMA in the absence of CuBr2 (radical generation exclusively from Ag0). Reaction conditions: [MMA]0:[EBPA]0:[PMDETA]0 = 200:1:0.08, 75% v/v monomer in DMF, in the presence of 5 cm Ag0 wire (d = 2 mm, Vtot = 10 mL), 25 °C.

must be taken into account that in a standard ATRP reaction initiator is consumed in the early stages of polymerization, and SA during ATRP occurs primarily from poly(methacrylate) rather than alkyl halide initiator. Nevertheless, the rate of radical generation due to reduction of CuII by Ag0 is somewhat higher than the rate of radical generation due to direct SA. Because of the noticeable contribution of radicals generated by Ag0 in the presence of EBPA, we reevaluated the role of surface activation in the polymerizations of acrylates. During a standard polymerization of BA,27 roughly 70% monomer conversion was achieved after 3 h at 50 °C. In the absence of copper catalyst, no polymerization was observed on this time scale. However, if the copper-free reaction was carried out for longer reaction times, the gradual formation of high MW polymer was observed, indicative of slow activation by Ag0 (Table S2). By far the most thoroughly studied metal to date for surface activation is copper. In an extensive study carried out in 2013,67 it was determined that the rate constant for activation of MBP from the surface of Cu0 in DMSO with Me6TREN as a ligand at 25 °C was approximately 1.8 × 10−4 cm s−1 (Table 3, entry 2). To compare this value to activation from the surface of Ag0, the

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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.5b01696. Additional characterization data and calculations (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (K.M.). Notes

The authors declare no competing financial interest.

Table 3. Activation of Alkyl Halides by Mt0 metal 0a

Ag Cu0 b Ag0 c

initiator

ligand

monomer

time (h)

conv (%)

Mn,obs

% initiator consumed

Đ

EBPA MBP MBP

PMDETA Me6TREN Me6TREN

MMA MA MA

24

12

14600

16

1.70

8

16

308000

10

1.99

−1 kapp a0 (cms )

9.1 × 10−6 1.8 × 10−4 2.9 × 10−7

a [MMA]0:[EBPA]0:[PMDETA]0 = 200:1:0.08, with [MMA]0 = 7.05 M. bReference 67. c[MA]0:[MBP]0:[Me6TREN]0 = 200:1:0.1, with [MA]0 = 7.36 M in DMSO.

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ACKNOWLEDGMENTS We thank Pawel Krys and Thomas Ribelli for helpful discussions and the NSF (CHE-1400052) and members of the CRP consortium for financial assistance.


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