Article pubs.acs.org/Macromolecules
RAFT Polymerization in a Miniemulsion System Using a Novel Type of Amphiphilic RAFT Agent with Poly(ethylene glycol) Bound to a Dithiobenzoate Group Hideto Minami,†,* Kengo Shimomura,† Toyoko Suzuki,† Keiichi Sakashita,‡ and Tetsuya Noda‡ †
Department of Chemical Science and Engineering, Graduate School of Engineering, Kobe University, Rokko, Nada, Kobe 657-8501, Japan ‡ Otake Research Laboratories, Mitsubishi Rayon Co., Ltd., Miyuki-cho, Otake, Hiroshima 739-0693, Japan
ABSTRACT: In this study, reversible addition−fragmentation chain transfer (RAFT) miniemulsion polymerization using a novel type of amphiphilic RAFT agent was investigated. The novel amphiphilic RAFT agent has a specific chemical structure in which a hydrophilic poly(ethylene glycol) chain is directly bonded to the “Z-group” position, and not the leaving group (R group), of the thiocarbonylthio group (RAFT group). As a result, the RAFT groups are localized at the interface of the water/ monomer droplets (polymer particles) throughout the polymerization, unlike with a conventional amphiphilic RAFT agent. Polystyrene (PS) particles with a broad molecular weight distribution and 69% degree of livingness were successfully prepared using the novel RAFT agent in a manner similar to that for a conventional RAFT system. Notably, after the completion of polymerization, the RAFT groups could be easily removed from the dispersed PS particles via treatment with an excess of potassium persulfate, because the RAFT groups only exist near the particle surfaces.
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INTRODUCTION In 1982, the first controlled/living radical polymerization (CLRP) was discovered by the Otsu group, and the technique was named “iniferter.”1 Thereafter, several methods were developed as CLRP techniques, such as nitroxide-mediated living radical polymerization,2−4 atom transfer radical polymerization,5−8 and reversible addition−fragmentation chain transfer (RAFT) polymerization.9−12 Discovered by Rizzardo and co-workers in 1998, RAFT is one of the most investigated CLRPs. The exchange reaction during RAFT polymerization is very rapid, which leads to well-controlled polymer synthesis,13 and a wide range of monomers can be used, such as methacrylates, acrylates, styrenes, and vinyl acetate. The CLRP techniques listed above were mainly developed for homogeneous polymerization in bulk or in solutions. Recently, the application of CLRP to aqueous polymerizations, including emulsion polymerization, miniemulsion polymerization, and microemulsion polymerization, has been strongly demanded from the viewpoint of industrial applications and environmental concerns.14−17 In the case of emulsion polymerization, one of the several problems is the partitioning of the © 2013 American Chemical Society
control agent between monomers and water phases. In miniemulsion systems, monomer droplets are converted to polymer particles as polymerization proceeds; therefore, it is not necessary to consider the transport of control agents from the monomer phase to the polymer particles through the aqueous phase.18−20As a result, adaptation of CLRP methods in miniemulsion polymerizations has received significant attention. Many studies of miniemulsion RAFT polymerization using water-insoluble RAFT agents have been reported.21−26 Miniemulsion polymerization using an amphipathic macro RAFT agent was also reported by Hawkett et al., in which a macro RAFT agent served as the sole stabilizer, and the obtained polymer particles had a narrow size distribution with a controlled molecular weight (MW).27,28 Charleux et al. reported the miniemulsion polymerization of styrene mediated by poly(ethylene glycol) (PEG)-based macromolecular RAFT Received: September 16, 2013 Revised: December 21, 2013 Published: December 31, 2013 130
dx.doi.org/10.1021/ma4019203 | Macromolecules 2014, 47, 130−136
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Article
Scheme 1. Structures and Images of Amphiphilic RAFT Agents
(Merck Corp., Tokyo Japan) purification system and had a resistivity of 18.2 MΩ cm−1. Reagent grade 2,2′-azobis(isobutyronitrile) (AIBN) (Wako Pure Chemical Industries, Ltd., Osaka Japan) and potassium persulfate (KPS) (Wako Pure Chemical Industries, Ltd., Japan) were purified via recrystallization using methanol. The conventional and novel PEG−RAFT (MW of PEG: approximately 2000) agents were synthesized by Mitsubishi Rayon Co., Ltd., Tokyo Japan. nHexadecane (HD, Nacalai Tesque, Inc.), tetrahydrofuran (THF, Nacalai Tesque, Inc.), and 1-pyrenylmethyl methacrylate (MMApy, Polysciences, Inc.) were used as received. Syntheses of PEG−RAFT Agents. A conventional amphiphilic RAFT agent (conventional PEG−RAFT) (Scheme 2a) was synthe-
agents. In this case, hydrophilic monomethyl ether PEG units were attached to trithiocarbonate functional groups.29 The amphiphilic RAFT agent provided good control over both colloidal stability and MW. Separately, Luo et al. developed a novel nanoencapsulation strategy via interfacial RAFT radical miniemulsion polymerization.30 An amphiphilic RAFT agent was localized at the water/monomer droplet interface, and polymer chains gradually grew into the monomer droplet/ polymer particle with increasing phase separation, resulting in the formation of a polymer shell. In miniemulsion polymerizations, the amphiphilic RAFT agents should be present at the interface of the water/monomer droplets before polymerization. With the above amphiphilic RAFT agents, which function as both control agents and surfactants, the hydrophilic group, such as PEG, is attached to the dithiocarbonyl group (RAFT group) as a leaving group (conventional amphiphilic RAFT agent). As the polymerization proceeds, the PEG group remains on the particle surface, while the RAFT group departs from the PEG chain and enters the particle (Scheme 1a), contributing to the synthesis of polymers with narrow molecular weight distributions (MWDs). However, removal of such RAFT groups is difficult because they are embedded inside the particle. This retention of the RAFT group causes discoloration of the polymer, which is often a problem for commercial applications. Herein, miniemulsion polymerization was performed using a novel type of an amphiphilic RAFT agent with the hydrophilic component, a PEG chain, directly bonded to the “Z-group” position, which is not a leaving group, and thus the RAFT groups remain at the interface of the water/monomer droplets (polymer particles) throughout the polymerization (Scheme 1b). To the best of our knowledge, there has been no previous report of miniemulsion polymerization using such RAFT agents owing to complicated, multistep syntheses required for their preparation. The controlled/living character of polymers prepared via miniemulsion polymerization using the novel RAFT agent and removal of the RAFT groups from the dispersed polymer particles were investigated.
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Scheme 2. Structural Formulas of the Conventional (a) and Novel (b) PEG−RAFT Agents
sized via esterification of 4-cyano-4-((thiobenzoyl) sulfanyl) pentanoic acid31 with poly(ethylene glycol) monomethyl ether (MW: 2000) in N,N-dimethylformamide (DMF)/toluene (10 mL/20 mL) using N,Ndicyclohexylcarbodiimide (approximately 2 mL) and 4-dimethylaminopyridine (0.53 g, 4.34 mmol) as catalysts at room temperature for 7 h. After the reaction was complete, the precipitate was removed by filtration. The filtrate was concentrated on a rotary evaporator, and then diethyl ether (200 mL) was added dropwise. The precipitate was again collected by filtration, and the target compound was obtained by drying under reduced pressure. This conventional PEG−RAFT agent had an ester-bonded PEG chain, not an amide-bonded PEG, as reported by dos Santos et al.32,33 The novel amphiphilic RAFT agent (novel PEG−RAFT) (Scheme 2b), PEG-modified 4-(2-carboxyethylcarbonyl) oxyethoxyphenyldithiocarbonic acid isobutyronitrile ester, was synthesized as follows: 4-(2-carboxyethylcarbonyl) oxyethoxyphenyldithiocarbonic acid isobutyronitrile ester (1.74 g, 4.57 mmol), which was synthesized in four steps from34 tetrahydropyranyl protection of 4-hydroxyethoxyphenyl bromide, synthesis of 4-tetrahydropyranyloxyethoxyphenyldithiocarbonic acid isobutyronitrile ester, synthesis of 4-hydroxyethoxyphenyl dithiocarbonic acid isobutyronitrile ester, and synthesis of 4-(2-
EXPERIMENTAL SECTION
Materials. Styrene (Nacalai Tesque Inc., Kyoto Japan) were purified by distillation under reduced pressure in a nitrogen atmosphere. The deionized water was obtained using an Elix UV-3 131
dx.doi.org/10.1021/ma4019203 | Macromolecules 2014, 47, 130−136
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Figure 1. Conversion vs time plots (a) for RAFT miniemulsion polymerization of styrene using the conventional (open circle) and novel (closed circle) PEG−RAFTs at 65 °C. Styrene/PEG−RAFT (molar ratio) = 500/1 and (b and c) visual appearance of the PS emulsions prepared via RAFT miniemulsion polymerization using the conventional (b) and novel (c) PEG−RAFTs, respectively. carboxyethylcarbonyl) oxyethoxyphenyldithiocarbonic acid isobutyronitrile ester, was reacted with poly(ethylene glycol) monomethyl ether (MW: 2000, 8.56 g, 4.28 mmol) in DMF/toluene using the same catalyst system and conditions as described above for the preparation of conventional PEG−RAFT. The overall yield was 26.7%. RAFT Miniemulsion and Suspension Polymerizations of Styrene. A solution of AIBN (2.5 mg, 0.015 mmol), styrene (2.5 g, 24 mmol), and HD (125 mg, 552 mmol) was added to an aqueous PEG− RAFT solution (conventional; 112 mg, dissolved in 22.5 g of water). The combined mixture was ultrasonicated using an ultrasonic homogenizer (NISSEI CORPORATION, US-600T, 12-mm diameter tip, set at “Power 10”) for 3 min in an ice−water bath. Approximately 25 mL of the miniemulsion was then charged to a round-bottom Schlenk flask, which was then sealed off with a silicon rubber septum, and the solution was degassed using several N2/vacuum cycles. Miniemulsion polymerization was conducted at 65 °C, and the flask was rapidly cooled in water after an appropriate time interval. RAFT suspension polymerization was carried out in the same manner, except for the absence of HD and the formation of the monomer droplets using a NISSEI ABM-2 homogenizer at 3000 rpm for 5 min in a glass vial. Measurements. Conversion was determined by gas chromatography (GC-2014, Shimadzu Corp., Kyoto, Japan) with helium as the carrier gas, tetrahydrofuran (THF) as the solvent, and p-xylene as the internal standard. The zeta-potentials of the PS emulsions (diluted with 10 mM NaCl aq) were measured using a ζ-potential and particle size analyzer (ELSZ-1M, Otsuka Electronics Co., Ltd., Osaka, Japan). MW and MWDs were analyzed via gel permeation chromatography (GPC) at 40 °C using two styrene/divinylbenzene gel columns [Tosoh Corp., TSK gel GMHHR-H, 7.8 mm i.d. × 30 cm] with THF as the eluent, a flow rate of 1.0 mL min−1, a refractive index (RI) detector (TOSOH RI-8020/21), an ultraviolet (UV) detector (TOSOH UV8II), and a fluorescence (FL) detector (JASCO Corp., FP-2020). The columns were calibrated with PS calibration standards (MW = 1.05 × 103−5.48 × 106, Mw/Mn = 1.01−1.15). The theoretical numberaverage MW Mn (Mn,th) values were calculated using the following equation:
M n,th = MPEG−RAFT +
diluted with approximately 8 mL of distilled water before measurement in the dilution mode. Fluorescent spectra of ethanol solutions of the conventional and novel PEG−RAFTs were recorded using a fluorescence spectrophotometer (F-2500, Hitachi High-Technologies Corp., Tokyo Japan). To observe the localization of the RAFT groups in the styrene droplets and PS particles, the styrene droplets and PS particles were observed using a C2si confocal laser scanning microscope (CLSM, Nikon Corp., Tokyo Japan) with excitation by a diode laser at 405 nm and an emission bandpass of 417−477 nm.
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RESULTS AND DISCUSSION Control/Livingness of Miniemulsion Polymerization Using the PEG−RAFT Agent. RAFT miniemulsion polymerization of styrene was performed in the presence of the conventional and novel PEG−RAFTs using AIBN as the initiator at 65 °C. In both systems, it is likely that the miniemulsion polymerizations smoothly proceeded without an induction period, although no data for the conversion were obtained until 5 h after initiation of the reaction. However, both reactions required nearly 35 h of polymerization time to complete (Figure 1a), suggesting that significant retardation occurred.35 Previously reported RAFT polymerizations, which were performed in emulsion and miniemulsion systems, were generally completed within 1 h with good control/livingness.36,37 Despite the absence of additional surfactants or colloidal stabilizers, the monomer droplets and PS emulsions after polymerization (Figure 1b,c) exhibited good colloidal stability in both PEG−RAFT systems. The diameters (Dn) of the monomer droplets and PS particles were 171 nm (variation coefficient: Cv = 26%) and 175 nm (Cv = 12%) in the conventional PEG−RAFT system and 217 nm (Cv = 16%) and 279 nm (Cv = 26%) in the novel PEG−RAFT system, respectively. In both cases, the particle sizes were nearly unchanged with colloidal stabilization before and after polymerization, indicating that miniemulsion polymerizations successfully proceeded, and that the PEG−RAFTs serve as adequate emulsifiers. Figure 2 shows the MWDs, Mn, and Mw/Mn at various conversions for RAFT miniemulsion polymerizations using the conventional and novel PEG−RAFTs. In both systems, the Mw/Mn values were not small (
