Successful Miniemulsion ATRP Using an Anionic Surfactant

Sep 7, 2014 - To date, it has been generally assumed, based on early experimental work, that ATRP in aqueous dispersed systems is incompatible with ...
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Successful Miniemulsion ATRP Using an Anionic Surfactant: Minimization of Deactivator Loss by Addition of a Halide Salt Victoria L. Teo,† Brad J. Davis,‡ Nicolay V. Tsarevsky,*,‡ and Per B. Zetterlund*,† †

Centre for Advanced Macromolecular Design (CAMD), School of Chemical Engineering, The University of New South Wales, Sydney, NSW 2052, Australia ‡ Department of Chemistry and Center for Drug Discovery, Design, and Delivery at Dedman College, Southern Methodist University, 3215 Daniel Avenue, Dallas, Texas 75275, United States S Supporting Information *

ABSTRACT: To date, it has been generally assumed, based on early experimental work, that ATRP in aqueous dispersed systems is incompatible with anionic surfactants. In the present work, it is clarified that this incompatibility originates in the anionic surfactant (sodium dodecyl sulfate, SDS) displacing the halide ligand from the CuII bromide-based deactivator, converting it to a CuII complex, unable to deactivate radicals. This results in a very high polymerization rate as well as essentially no control over the molecular weight distribution. It is demonstrated how such loss of deactivator can be minimized by the addition of a source of halide ions, thus enabling one to conduct ATRP in aqueous dispersed systems using commonly available and inexpensive anionic surfactants such as SDS.

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INTRODUCTION Controlled/living radical polymerization (CLRP),1 which was initially developed for homogeneous systems, has led to a renaissance in the field of polymer chemistry by rendering a wide range of well-controlled and complex polymeric architectures inaccessible via conventional radical polymerization. Soon after the groundbreaking work in this area, significant efforts were devoted to implementation of CLRP in (aqueous) heterogeneous systems (emulsion, miniemulsion, etc.).2−4 Such systems are of great importance because of their extensive use in industry as well as for synthesis of polymeric nanoparticles of various shape and morphology with a wide range of applications. Atom transfer radical polymerization (ATRP)5−8 has been conducted successfully in various dispersed polymerization systems, including (seeded) emulsion,9,10 miniemulsion,11−14 microemulsion,15−17 and dispersion polymerization.18 However, to date, there are to the best of our knowledge no reports on successful implementation of ATRP in aqueous dispersed systems using anionic surfactants, despite such surfactants being the most commonly available and of great importance from an industrial perspective. This is in sharp contrast with other CLRP techniques such as nitroxide-mediated radical polymerization (NMP) and reversible addition−fragmentation chain transfer (RAFT) polymerization, which are routinely employed in connection with anionic surfactants.2,19,20 Makino et al.21 attempted emulsion ATRP of methyl methacrylate with ethyl 2bromoisobutyrate/CuIBr/bpy (bpy = 2,2′-bipyridyl) at 80 °C with the anionic surfactant sodium dodecyl sulfate (SDS); however, the molecular weight distributions (MWDs) were © 2014 American Chemical Society

consistently broad with Mw/Mn > 1.5. Reverse ATRP was also conducted as an emulsion polymerization, also using SDS, with potassium persulfate/CuIIBr2/bpy at 80 °C, but Mw/Mn < 1.5 was only achieved under conditions where the monomer conversion was ∼1%. The use of CuIBr/CuIIBr2/bpy/K2S2O8 at 80 °C did result in low Mw/Mn in some cases, but only under conditions where the monomer conversion was very low. Matyjaszewski et al.22 reported that emulsion polymerization of butyl acrylate with ethyl 2-bromoisobutyrate/CuIBr/dNbpy (dNbpy = 4,4-di(4-nonyl)-2,2′-bipyridyl) using SDS resulted in uncontrolled polymerization. In the absence of surfactant, or using poly(ethylene oxide) as surfactant, extensive coagulation occurred, but control/livingness was observed. It was thus concluded that the anionic surfactant was incompatible with this ATRP system, speculated to be caused by the CuIIBr2based deactivator, reacting with the sulfate anion of SDS to form CuII sulfate and sodium bromide,23 thus preventing the ATRP deactivation step from proceeding. “Microemulsion” ATRP using SDS has also been conducted (based on the recipes, the system in ref 24 is more likely to have been a hybrid between emulsion/miniemulsion polymerization, and not microemulsions, contrary to what the authors claim), but satisfactory control/livingness was not reported in these studies.24,25 As a result of the relatively early works of Matyjaszewski et al.22 and Makino et al.21 around the year 2000, it has been Received: July 7, 2014 Revised: August 15, 2014 Published: September 7, 2014 6230

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because CuIIBr2 is poorly soluble in the monomer and tends to increase the viscosity of the organic phase to unusable levels (cannot be pipetted, not homogeneous). The CuIIBr2 solution was added to the emulsion just before it was sealed and degassed using nitrogen for 10 min. The ascorbic acid solution was added to the emulsion by nitrogen pressure using a cannula and then degassed for a further 10 min. It was at this point that the emulsion changed from a pale emerald green to varying shades of brown, indicating that the copper was reduced from CuII to CuI. This “masterbatch” emulsion was now transferred by cannula into several degassed flasks as necessary (depending on number of polymerization samples required), and then a further 10 min of degassing was undertaken. Once that time had elapsed, the nitrogen was removed, and the septa were painted with vacuum grease. Polymerization was conducted at 40 °C with magnetic stirring using a heated oil bath and stirrer plate. After the prescribed time, the wire wrap/septa/Parafilm was removed and the emulsion agitated by pipette in the flask to introduce air into the emulsion and thus oxidize the CuI complexes back into CuII, hence stopping the polymerization. Determination of Halogenophilicity (Bromidophilicity). Solutions containing the complex cation [CuII(bpy)2]2+ with a weakly coordinating counterion, trifluoromethanesulfonate (triflate, OTf−), were titrated with a solution containing the same concentration of [CuII(bpy)2]2+ triflate but also a large excess (>100 equiv) of Bu4NBr. This convenient alternative to the addition of solid Bu4NBr (which was used previously28) was employed because the total concentration of CuII does not change throughout the experiment due to changes of the volume. Spectra of the original solution containing no bromide and of solutions containing n equivalents of bromide vs CuII were collected. The bromide-free solution was prepared by dissolving Cu(OTf)2 (0.0225 g, 6.25 × 10−5 mol) and bpy (0.0196 g, 1.25 × 10−4 mol) in 25 mL of the solvent. Portions of the bromide salt-containing solution in the same solvent were added until no further change in the spectrum was observed (i.e., the entire amount of [CuII(bpy)2]2+ was converted to [CuII(bpy)2Br]+). The equilibrium constant of bromide association to [CuII(bpy)2]2+, KBr−, was determined using the equation

generally assumed that ATRP is incompatible with anionic surfactants, and consequently ATRP in aqueous dispersed systems is typically conducted using nonionic surfactants9,10,12,14,23 but has also been reported to work well with cationic surfactants.13,15 In the present work, we have revisited the subject of ATRP in aqueous dispersed systems (specifically miniemulsion) in the presence of the anionic surfactant SDS. It is confirmed, in agreement with previous work, that the polymerization does not proceed in a controlled/living manner under such conditions. However, it is shown that miniemulsion ATRP with an anionic surfactant such as SDS can be conducted successfully by judicious addition of a halide salt such as sodium bromide, and the present results thus demonstrate a new technique for ATRP in aqueous dispersed systems.

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EXPERIMENTAL SECTION

Materials. tert-Butyl methacrylate (t-BMA; Sigma-Aldrich; 98%) was purified by passing through a column of activated basic aluminum oxide (Ajax). Ethyl 2-bromoisobutyrate (EBiB; Sigma-Aldrich; 98%), hexadecane (HD; Sigma-Aldrich; 99%), dNbpy (Sigma-Aldrich; 97%), sodium dodecyl sulfate (SDS; Sigma-Aldrich BioXtra; >99%), tetradecyltrimethylammonium bromide (TTAB; Sigma-Aldrich; 99%), NaBr (Univar; 99.5%), KBr (Alfa Aesar; ultrapure, spectroscopy grade), tetrabutylammonium bromide (Sigma-Aldrich; >98%), CuIIBr2 (Sigma-Aldrich; 99%), bpy (Aldrich, 99%), CuII trifluoromethanesulfonate (CuII(OTf)2; Fluka; 97%), acetic acid (J.T. Baker; 99.9%), Lascorbic acid (Sigma-Aldrich; >99%), and anhydrous ethanol (Decon Laboratories, Inc.; 200 proof) were all used as received. Miniemulsion Polymerizations. The miniemulsion recipes employed are listed in Table 1. The amounts given correspond to a

Table 1. Recipes for Miniemulsion Polymerizations Conducted in This Work component organic phase tBMA EBiB hexadecane dNbpy aqueous phase water (deionized) surfactant NaBr (if needed) added later copper(II) bromide (in 0.3 g of H2O) ascorbic acid (AA) (in 0.6 g of H2O)

mass (g)

comments

1.8446 0.0101 0.0922 0.0531

10 wt % rel to aqueous phase tBMA:EBiB = 250:1 (molar ratio) 5 wt % of tBMA dNbpy:CuBr2 = 1.25:1 (molar ratio)

19.678 0.1148 0.2068

0.0234 0.00736

K Br− =

ΔA /Δε ([Cu II]tot − ΔA /Δε)(n[Cu II]tot − ΔA /Δε)

where ΔA is the difference between the absorbance (measured at a given wavelength, in this case, 748 nm) of a given solution containing bromide ions (n equivalents vs CuII) and that of the starting solution before addition of the bromide. The total copper concentration was [CuII]tot. The apparent extinction coefficient Δε is defined as the difference between the extinction coefficients of the complex [CuII(bpy)2Br]+ (calculated from the spectrum of a solution of the “saturated” with bromide copper complex) and the starting tetracoordinated [CuII(bpy)2]2+. Values of KBr− were determined from each of the separate solutions containing different concentrations of bromide (except those that were very close to saturation, and in some cases, the most dilute with respect to bromide solutions), and then the average value was calculated. Several solvent compositions were studied, namely C2H5OH−H2O (75/25 (v/v)), and similar solvents containing small amounts of acetic acid, namely C2H5OH− H 2 O−CH 3 CO 2 H (75/24/1 (v/v/v)) and C 2 H 5 OH−H 2 O− CH3CO2H (75/23/2 (v/v/v)). Studies of the Interaction of [Cu II (bpy) 2 ] 2+ and/or [CuII(bpy)2Br]+) with SDS. CuIIBr2 (0.1117 g, 0.5 mmol) and bpy (0.1562 g, 1 mmol) were dissolved in deionized water in a 100 mL volumetric flask. Various amounts of SDS were measured out in five vials: 0.2163 g (0.75 mmol) in vial 1, 0.4326 g (1.5 mmol) in vial 2, 0.6489 g (2.25 mmol) in vial 3, 0.8651 g (3.0 mmol) in vial 4, and 1.0814 g (3.75 mmol) in vial 5. To each vial a 15 mL aliquot of the solution of the CuII complex was added, and the vials were heated in an oil bath at 50 °C for 20 min to dissolve the SDS. The amounts of SDS in the vials were 10, 20, 30, 40, and 50 equiv relative to CuII in vials 1, 2, 3, 4, and 5, respectively. The spectra of a solution containing no SDS and of the solutions in vials 1−5 were collected at 50 °C.

6.2 wt % rel to tBMA NaBr:SDS = 5:1 (molar ratio; varied) CuBr2:tBMA = 1:125; CuBr2:EBiB = 2:1 (molar ratio) AA:CuBr2 = 1:1 (molar ratio)

total mass of 22 g of emulsion. Each individual polymerization comprised approximately 5.5 g of emulsion. t-BMA, dNbpy, hexadecane, and EBiB were combined to form the organic phase and stirred at 30 °C as required to dissolve the dNbpy. Surfactant, water, and NaBr (if required) were added to the aqueous phase, which was also stirred to dissolve the surfactant as required. These two phases were mixed together at 10 wt % organic phase and subjected to ultrasonication by a Branson Sonifier probe with microtip at 40% amplitude for 10 min while on ice. After the mixture was ultrasonicated, it was left to stand to remove bubbles formed during sonication. CuIIBr2 and ascorbic acid were dissolved separately in water (i.e., generating two separate solutions). The vials containing the CuIIBr2 and ascorbic acid solutions were sealed with rubber septa, copper wire, and Parafilm and degassed using nitrogen. Contrary to the technique used in some of the literature,26,27 the CuIIBr2 was dissolved in water and added to the emulsion af ter ultrasonication 6231

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Spectra were also collected from solutions containing KBr (20 equiv vs CuII) in addition to SDS. Dynamic Light Scattering. A specified volume of the undiluted emulsion was placed into a cuvette (disposable plastic or reusable quartz), taking care to not agitate the solution to avoid the formation of both large and small bubbles. The sample was then analyzed using a Malvern Zetasizer Nano-ZS, set to 173° backscatter angle setting at 25 °C. Five measurements were conducted per sample (runs per measurement is set to automatic), and the three most closely matched distributions were selected. Gravimetry. Monomer conversions were measured by gravimetry. Approximately 1 g of emulsion was weighed into a glass sample tube and covered with perforated foil. The sample was then placed in a vacuum oven at room temperature until dry, and the dried sample was weighed. The weight fractions of monomer and the fraction of nonvolatiles (hexadecane, EBiB, dNbpy, surfactant, NaBr, CuIIBr2, ascorbic acid) that remain after drying were calculated, thus enabling calculation of the conversion. Gel Permeation Chromatography. Molecular weights and molecular weight distributions were determined by gel permeation chromatography (GPC) with a Shimadzu modular system with THF as eluent at 40 °C at a flow rate of 1.0 mL/min with injection volume of 100 μL. The GPC was equipped with a DGU-12A solvent degasser, a LC-10AT pump, a CTO-10A column oven and an ECR 7515-A refractive index detector, and a Polymer Laboratories 5.0 μm bead-size guard column (50 × 7.8 mm) followed by four 300 × 7.8 mm linear Phenogel columns. The system was calibrated against polystyrene standards ranging from 500 to 106 g mol−1. Theoretical numberaverage molecular weights (Mn,th) were calculated based on Mn,th = ([tBMA]0 × α × Mt‑BMA)/([EBiB]0), where [t-BMA]0 is the initial concentration of t-BMA, α is the t-BMA fractional conversion, Mt‑BMA is the molecular weight of t-BMA, and [EBiB]0 is the initial concentration of EBiB. UV/vis spectra were recorded on a Shimadzu UV-3600 spectrophotometer equipped with a Peltier heating device (accurate to 0.01 °C) using 1 cm quartz cuvettes.

Figure 2. MWDs for AGET ATRP miniemulsion polymerization of tBMA at 40 °C using TTAB as surfactant (recipe in Table 1; conversions as indicated in Figure 1).

1.2, except for the data point at the highest conversion of 91% (1.20) (Figure 3). The Mn values increased close to linearly with conversion, although Mn deviated somewhat from Mn,th (Figure 4). Clearly, in this system, the presence of acid (ascorbic acid and the acidic products of its oxidation by CuII) in the system did not lead to poor deactivation efficiency, due to the presence of large excess of a source of bromide relative to CuII (see the discussion below). The TTAB system was in general colloidally stable for several weeks before polymerization. The droplet size distributions before polymerization (Figure S1) indicate that the vast majority of droplets are ∼100 nm in diameter by number (Table 2). The number distribution does not change significantly as the polymerization proceeds, consistent with a miniemulsion polymerization mechanism whereby ideally each monomer droplet is converted to the corresponding polymer particle. In practice, it is well-known that such an ideal system (a so-called “one-to-one copy”) is extremely difficult to achieve,29 although there are claims in the literature of systems that may come close.30−32 There is a significant change in volume average diameter on polymerizationthe large micron sized droplets present before polymerization have disappeared at 44% conversion. The immediate explanation for this type of behavior is a degree of emulsion polymerization-like behavior (i.e., large monomer droplets act as monomer reservoirs, supplying monomer to nucleated droplets). Overall, droplet/ particle size distribution data indicate that the vast majority of the droplets/particles are polymerizing in a miniemulsion manner. Miniemulsion AGET ATRP Using SDS. Miniemulsion AGET ATRP of t-BMA using ascorbic acid/dNbpy was conducted at 40 °C using the anionic surfactant SDS (Table 1). The conversion vs time data (Figure 1) demonstrate that the polymerization proceeded very quickly, reaching ∼60% conversion in 30 min. Inspection of the MWDs (Figure 3) reveals that the control was extremely poorthere is a distinct multimodal nature to all MWDs, and there is very little, if any, trend with increasing conversion. The MWDs remain fairly similar to increasing conversion, and the Mw/Mn values are greater than 10 (Figure 3). The Mn values are relatively close to Mn,th (but do not describe a linear trend with increasing conversion), which indicates that the number of polymer chains in the system is not too far from the expected based on a controlled/living mechanism (Figure 4). This observation, in combination with the fact that the MWDs do not change systematically with increasing conversion, seems to indicate that after initiation occurs via activation of the alkyl halide species chains grow in an uncontrolled manner until irreversible

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RESULTS AND DISCUSSION Miniemulsion AGET ATRP Using TTAB. Prior to developing an anionic surfactant ATRP system, conditions were confirmed under which miniemulsion ATRP proceeds satisfactorily using a surfactant type that is known to be compatible with ATRP. Cationic surfactants have previously been employed successfully in connection with miniemulsion ATRP systems.13 Miniemulsion AGET ATRP of t-BMA using ascorbic acid/dNbpy was thus conducted at 40 °C using the cationic surfactant TTAB (Table 1). The polymerization proceeded to ∼90% conversion in 24 h (Figure 1), and the MWDs shifted to higher molecular weights with increasing conversion (Figure 2) with all Mw/Mn values remaining below

Figure 1. Conversion vs time data for AGET ATRP miniemulsion polymerizations of t-BMA at 40 °C using TTAB, SDS, and SDS with added NaBr (molar ratio NaBr:SDS = 5:1; full recipes in Table 1). 6232

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Figure 3. Mw/Mn values and MWDs for AGET ATRP miniemulsion polymerizations of t-BMA at 40 °C using TTAB, SDS, and SDS with added NaBr (molar ratio NaBr:SDS = 5:1; full recipes in Table 1).

rendering it inactive.22,21,23 This is also consistent with the very high polymerization rate observed. From a colloidal stability point of view, the SDS systems were generally acceptable. Prior to polymerization in the absence of stirring, the miniemulsions tend to start phase separating somewhat earlier (12−24 h) than the corresponding systems based on TTAB. The droplet/particle size distributions were very similar before polymerization and at 90% conversion (Figure S2), consistent with predominant monomer droplet nucleation. Mechanistic Studies Related to AGET ATRP in the Presence of SDS and Acidic Additives. The fast and uncontrolled polymerization observed when AGET ATRP was carried out in miniemulsion using SDS as the surfactant and ascorbic acid as the reducing agent was most likely due to poor efficiency of deactivation. It has been indeed reported28,33 that in aqueous or other protic solvents CuII halide-based deactivators of the type X−CuIILm (X is halide and L is a ligand, which may or may not be charged; the charges of the complexes depend upon the charges of L and are not shown for simplicity) tend to dissociate with the formation of a free halide ion and the complex CuIILm that is unable to deactivate radicals due to the absence of a coordinated halide ligand. In other words, the halidophilicity34 of typical CuIILm complexes used in ATRP is low in protic media and, in mixed solvents containing water, decreases as the amount of water is increased. The effect is due to the ability of water and protic solvents to solvate very well halide anions (with formation of hydrogen bonds) and to some extent due to the ability of water to coordinate to CuII complexes with an open coordination site such as CuIILm. The rate of ATRP is inversely proportional to the concentration of deactivator X−CuIILm, and in systems where a significant part of the deactivator dissociates by losing a halide ligand, fast polymerizations and significant radical termination are observed.35 Further, the width of the MWD increases as the average number of monomer units added during each active state of the chains increases, and as a result, polymers with broad MWDs are obtained in reaction systems containing low concentration of deactivator, i.e., in systems using very low overall concentration of catalyst or in systems where deactivator dissociation is pronounced.35 The solubility of water in t-BMA

Figure 4. Number-average molecular weights (Mn) vs conversion for for AGET ATRP miniemulsion polymerizations of t-BMA at 40 °C using TTAB, SDS, and SDS with added NaBr (molar ratio NaBr:SDS = 5:1; full recipes in Table 1). The full line is the theoretical Mn (Mn,th).

Table 2. Droplet/Particle Diameters as Measured by DLS before polymerization polymerization SDS TTAB SDS/NaBra NaBr:SDS 2.5:1 4:1 5:1 6:1 7.5:1 a

after polymerization

dn (nm)

dv (nm)

conv (%)

dn (nm)

dv (nm)

124 121 68

211 865 155

90 44 77

118 101 105

222 227 230

118 30 68 36 39

183 110 155 141 179

70 63 77 58 87

79 58 105 63 137

134 107 230 216 952

Molar ratio NaBr:SDS = 5:1.

termination occurs. As such, the number of chains would be close to that expected based on a true controlled/living mechanism, and thus Mn not too far from Mn,th. It seems likely that the problem is indeed related to the deactivation step, consistent with previously reported assertions that the anionic surfactant interacts with the deactivating CuII complex, 6233

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is likely low, and therefore the halidophilicity of the CuII complex of dNbpy in the monomer droplets is not lowered solely due to the presence of water in the organic (oil) phase. It is however expected that the presence of even small amounts of protic acids in ATRP systems could lead to a marked decrease of the CuII complex halidophilicity. Ascorbic acid and the products of its oxidationascorbate radical (transient species) and dehydroascorbic acidare present in the AGET ATRP reaction mixtures described above and are sufficiently strong acids36 to lead to a decrease in the CuII halidophilicity. Moreover, the ascorbate and dehydroascorbate anions are likely coordinate to CuII,37 which would further decrease the conditional38−40 halidophilicity of the CuII complex. To test this hypothesis, the halidophilicity of the [CuII(bpy)2]2+ complex was determined in the presence of small amounts of an acidic additive (acetic acid) that also dissociates, yielding a weakly coordinating anion (acetate). Acetic acid was chosen as a model compound because it is unable to participate in additional redox reactions under the specified conditions, which would introduce complications in the halidophilicity measurements. In order to be able to compare the data with reported values of halidophilicity, the complexes of bpy rather than those of the structurally similar dNbpy were studied. The halidophilicities (more precisely, the bromidophilicities) were determined by titration of solutions of the bpy complex of CuII(OTf)2 with bromide in various mixed solvents, as described in the Experimental Section. The spectral data are presented in the Supporting Information (Figures S3− S5), and the calculated values of the bromidophilicities are gathered in Table 3.

It should be noted that all Cu complexes present in the miniemulsion system ([CuI(dNbpy)2]+, [CuII(dNbpy)2Br]+, and [CuII(dNbpy)2]2+) as well as the reducing agent and the products of its oxidation partition between the aqueous and the organic phase. Even if the dissociation of the deactivating complex [CuII(dNbpy)2Br]+ to [CuII(dNbpy)2]2+ does not happen appreciably in the organic phase due to the low solubility of water and ascorbic, and dehydroascorbic acid in tBMA, it is very pronounced in the aqueous phase containing acid. This is likely to increase the partitioning of [CuII(dNbpy)2Br]+ from the organic to the aqueous phase, leading to inefficient deactivation in the organic phase. The poor polymerization control previously observed in ATRP carried out in miniemulsion employing SDS as the surfactant but in the absence of ascorbic acid41,42 indicates that SDS itself may displace the halide ligand from the CuII bromide-based deactivator, converting it to a CuII complex, unable to deactivate radicals. To qualitatively determine the ability of SDS to displace coordinated bromide from the ATRP deactivator, a solution of CuIIBr2 and bpy (1:2) in water was prepared, and various amounts of solid SDS were added. The observed changes in the spectra were very minor (Figure S6a) and were most likely due to the increased volume of the solutions. At these conditions, a very low concentration of deactivator [CuII(bpy)2Br]+ was present in the solution; i.e., due to the very low halidophilicity of [CuII(bpy)2]2+ in pure water, all originally dissolved [CuII(bpy)2Br]+ dissociated to [CuII(bpy)2]2+, so no significant displacement of bromide could be discerned. It was necessary to ensure that at least some measurable amount of [CuII(bpy)2Br]+ was present in the solution before SDS was added. This was accomplished by preparing an aqueous solution of CuIIBr2 and bpy (1:2) and adding 20 equiv of KBr in addition to SDS (Figure S6b). As seen, the addition of SDS to the solutions containing [CuII(bpy)2Br]+ led to a decrease in the absorbance, indicating most likely the displacement of the bromide ligand from the complex. The effect of SDS is consistent with the observed loss of polymerization control. It has been demonstrated that in homogeneous ATRP reactions carried out in protic media, where the halidophilicity of CuIILm complexes is low, the polymerization control can be markedly improved and the polymerization can be slowed down by the addition of sources of halide anions,28,43 e.g., tetraalkylammonium or alkali metal halides. The reason is that in the presence of free halide part of the deactivator, which would otherwise be “lost” due to halide ligand dissociation or displacement, is regenerated. Thus, in order to carry out wellcontrolled ATRP in the presence of acidic compounds (ascorbic acid and the products of its oxidation) and in the presence of compounds able to partially displace the halide ligand from the deactivator (SDS), it is necessary to add to the system a source of halide ions. Testing of Interaction between SDS and Cu II Complex. Given that both the CuIIBr2 and SDS are initially dissolved in the aqueous phase, any interactions that may limit the availability of the CuII complex for transport to the organic phase should be investigated and ideally controlled. To this end, SDS and CuIIBr2 were added to water at various ratios, followed by addition of NaBr (1−10 wt % relative to water; Figure 5). Without NaBr, a flaky precipitate formed immediately, indicating that there was some merit to the theory that SDS does limit copper availability due to some interaction. Moreover, the foam height/longevity increased

Table 3. Bromidophilicity, KBr−, of [CuII(bpy)2]2+ in Various Mixed Solvents solvent C2H5OH−H2O (75/25 (v/v)) C2H5OH−H2O−CH3CO2H (75/24/1 (v/v/v)) C2H5OH−H2O−CH3CO2H (75/23/2 (v/v/v))

[CH3CO2H]a [M]

KBr−b [M−1]

0 0.175

620 ± 105 (4) 484 ± 69 (4)

0.35

302 ± 66 (6)

a

Calculated assuming no contraction upon mixing of the solvent components. bThe number in the parentheses is the number of spectra acquired during the titration that were used to calculate the average bromidophilicity value and the error.

Even the addition of small amounts of acetic acid (1−2 vol %) leads to a small but noticeable decrease in the halidophilicity. It is therefore reasonable to assume that the presence of an acidic reducing agent (ascorbic acid) and the acidic products of its oxidation are at least partially responsible for decreased stability of the deactivator and therefore for the observed fast polymerizations and poor deactivation, both of which result in polymers with broad MWDs. It should be noted that if the coordination of continuously accumulating dehydroascorbate anions to CuII is stronger than that of acetate anions, the decrease in halidophilicity will be more significant than in the model solvent systems containing acetic acid. The presence of small amounts of protic additives in the miniemulsion system using TTAB as the surfactant did not lead to poor polymerization control due to the presence of large excess of a bromide source (the surfactant itself). Owing to the presence of bromide, the deactivator that would otherwise dissociate was regenerated. 6234

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Figure 6. MWDs for AGET ATRP miniemulsion polymerizations of tBMA at 40 °C using SDS/NaBr (molar ratios NaBr:SDS as indicated; black trace shows case of no NaBr) (recipe in Table 1).

Figure 5. Photographs of aqueous solutions of SDS (0.5 wt % relative to water) and CuBr2 (0.12 wt % relative to water) (same concentrations as in actual polymerization (Table 1)) containing different amounts of NaBr: From left to right SDS:NaBr = 1:0 (no NaBr), 1:2.5, 1:5, 1:15, 1:25, and 1:50.

with increasing NaBr content up to a point, indicating increased surfactant activity. The molar ratio SDS:NaBr ≈ 1:5 appeared optimal for suppression of the formation of the precipitate and inhibited surfactant activity the least as determined by foam height and foam longevity. Miniemulsion AGET ATRP Using SDS: Addition of NaBr. Miniemulsion AGET ATRP of t-BMA using ascorbic acid/dNbpy was conducted using SDS as the sole surfactant at 40 °C in the presence of NaBr. The molar ratio NaBr:SDS initially employed was 5:1, as determined by using the aqueous phase interaction experiments as a guide (see above; Figure 5). The polymerization proceeded smoothly to high conversion, reaching ∼80% conversion in 16 h (Figure 1). The MWDs (Figure 3) are monomodal and relatively narrow (Mw/Mn ≈ 1.2 and 1.5 at low and high conversion, respectively) and shift to higher molecular weights with increasing conversion, although a high-MW shoulder appears at 70% conversion. The Mn values increase linearly with conversion (Figure 4) but lie somewhat below Mn,th. This may be (partly) caused by the GPC system being calibrated with linear polystyrene standards (based on the Mark−Houwink−Sakurada coefficients for these two systems,44 the Mn values are underestimated by ∼15% in this molecular weight range). Overall, there is a dramatic improvement in control/livingness compared to the SDS system without NaBr. This is, to the best of our knowledge, the first time ATRP has been conducted successfully in an aqueous dispersed system with an anionic surfactant. From a colloidal perspective, the addition of NaBr appeared to exert no significant influence on the system. The droplet/particle size distributions before and after (77% conversion; dn = 105 nm; dV = 230 nm) polymerization were similar, consistent with a miniemulsion polymerization (Figure S7; predominant droplet nucleation). Miniemulsion AGET ATRP Using SDS with NaBr: Optimization. Miniemulsion AGET ATRP of t-BMA using ascorbic acid/dNbpy was conducted using SDS as the sole surfactant at 40 °C (Table 1) at various levels of NaBr addition. The polymerization time was 24 h, except for the sample without NaBr which was polymerized for 2 h. Figure 6 shows MWDs obtained at various levels of NaBrthe presence of NaBr has a dramatic narrowing effect on the MWD. Even a relatively small amount of NaBr (NaBr:SDS = 2.5:1 molar ratio) has a very significant effect on the Mw/Mn (Figure 7). There does not appear to be a clear optimum amount of NaBr over the range of NaBr:SDS = 2.5:1 to 7.5:1. Given that the

Figure 7. Conversion levels, Mw/Mn (numbers indicate Mw/Mn values), and Mn values for AGET ATRP miniemulsion polymerizations (all 24 h, except polymerization without NaBr for 2 h) of t-BMA at 40 °C using different SDS/NaBr ratios (recipe in Table 1).

presence of NaBr presumably prevents loss of deactivator, one might have expected the polymerization rate to decrease with increasing NaBr content, but the effect on polymerization rate appears to be minor. There was no systematic trend in the effect of the amount of NaBr on the level of conversion, which was in the approximate range 60−87% (Figure 7). However, the polymerization with the highest NaBr content (NaBr:SDS = 7.5:1) did exhibit the highest conversion (87%). Figure S8 shows Mn vs conversion for the polymerizations at different NaBr:SDS ratios, all conducted over 24 h. The Mn values fall relatively close to Mn,th, although there is a general trend of Mn < Mn,th for reasons that are currently unclear. On the basis of the mechanistic explanations put forward above, one would expect that an increase in NaBr would lead to a higher concentration of deactivator and thus better control and slower polymerization. However, this may not necessarily affect Mn, since this quantity is only influenced by the number of chains (methacrylates mainly terminate via disproportionation, and as such termination does not influence Mn). There does 6235

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Notes

not seem to be a particularly strong trend when Mn is compared with the molar ratio of NaBr:SDS (Figure 7). The polymerization without NaBr has higher Mn, while the other polymerizations exhibit relatively similar Mn values. From a colloidal perspective, the systems behaved similarly. The use NaBr:SDS in excess of 15:1 resulted in colloidal instability and catastrophic phase separation within minutes, presumably as a result of the high ionic strength leading to a significant reduction of the ionic double layer. A comparison of the particle size distributions reveals no clear trend with regards to particle size and NaBr concentration (Figure S9). Aside from the catastrophic phase separation experienced at high concentrations, the effect of NaBr on the colloidal stability of the system is minimal.

The authors declare no competing financial interest.

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CONCLUSIONS It has to date been generally known that ATRP in aqueous dispersed systems is incompatible with anionic surfactants, speculated to be caused by the surfactant interacting with the CuII deactivator complex. In the present work, AGET ATRP in the presence of the anionic surfactant SDS has been investigated in aqueous miniemulsion under a variety of conditions in conjunction with spectroscopic studies to elucidate the influence of SDS and acidic additives on the ATRP process. AGET ATRP of t-BMA was performed in aqueous miniemulsion at 40 °C with ascorbic acid as the reducing agent and dNbpy as the ligand. It was confirmed that the polymerization proceeds with control/livingness using the cationic surfactant TTAB, but when employing SDS as surfactant, very high polymerization rate is accompanied by severe loss of control/livingness. Spectroscopic investigations revealed a slight decrease in CuII complex (i.e., ATRP deactivator) halidophilicity in the presence of acetic acid (used as model compound), i.e., increased probability of the deactivator dissociating by losing a halide ligand. The addition of SDS to solutions containing [CuII(bpy)2Br]+ leads to a decrease in the absorbance, indicating most likely the displacement of the bromide ligand from the complex, consistent with the observed loss of polymerization control. Miniemulsion AGET ATRP of t-BMA was subsequently performed in the presence of various amounts of NaBr as a source of halide ions. Consistent with the spectroscopic studies, the polymerizations proceeded with good control/livingness under these conditions as a result of the likelihood of halide displacement decreasing in the presence of a source of excess halide ions. The present work thus demonstrates how ATRP can indeed be conducted in aqueous dispersed systems using commonly available and inexpensive anionic surfactants such as SDS by simple addition of a source of halide ions.

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ASSOCIATED CONTENT

S Supporting Information *

Droplet/particle size distributions by dynamic light scattering (DLS), molecular weight data, and UV/vis spectra. This material is available free of charge via the Internet at http:// pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected]; Tel +1-214-768-3259; Fax +1-214-7684089 (N.V.T.). *E-mail [email protected]; Tel +61 2 9385 4331; Fax +61 2 9385 6250 (P.B.Z.). 6236

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