Article pubs.acs.org/Macromolecules
Photoinitiated ATRP in Inverse Microemulsion Mustafa Ciftci,† Mehmet Atilla Tasdelen,‡ Wenwen Li,§ Krzysztof Matyjaszewski,§ and Yusuf Yagci*,† †
Department of Chemistry, Istanbul Technical University, Maslak, Istanbul 34469, Turkey Department of Polymer Engineering, Faculty of Engineering, Yalova University, 77100 Yalova, Turkey § Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, United States ‡
ABSTRACT: The recently developed photoinitiated atom transfer radical polymerization has been implemented in the inverse microemulsion polymerization of oligo(ethylene glycol) monomethyl ether methacrylate. Two different mechanisms including a simultaneous reverse and normal initiated ATRP and a combination of activators generated by electron transfer and initiators for continuous activator regeneration ATRP are investigated under UV light. The effects of various reaction parameters on the resulting particles have been systematically investigated, and it has been demonstrated that the particle size can be tuned based on UV light, initial stoichiometry, and aqueous phase fraction. The formed particles contained well-defined watersoluble polymers with relatively narrow molecular weight distribution (Mw/Mn < 1.4).
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INTRODUCTION Advancements in polymer material science, particle engineering is a term coined to encompass means of producing particles having a defined morphology, functionality, particle size distribution, and composition.1 The polymer nanoparticles have many applications in various areas ranging from electronics to photonics, conducting materials to sensors, medicine to biotechnology, pollution control to environmental technology, and so forth.2 Generally, two main strategies are employed for their preparation: (i) dispersion of preformed polymers and (ii) in situ polymerization of monomers (e.g., emulsion, microemulsion, miniemulsion, interfacial polymerizations, etc.).3 Among these techniques, microemulsion polymerization received a special interest, since it allows synthesis of ultrafine polymer latex particles in the size range of 10−50 nm via a free radical mechanism. Depending on the polarity of the continuous medium and the dispersed phase, both oil-in-water (o/w, direct) and water-in-oil (w/o, inverse) microemulsions can be prepared.4 An inverse microemulsion (w/o) system with surfactant encapsulated water droplets suspended in a continuous oil phase has recently attracted considerable attention and become an efficient method for the synthesis of water-soluble polymers and hydrogel nanoparticles.5 Despite the many advantages of microemulsion polymerization, the high concentration of surfactant relative to monomer and the control of molecular weight limit the industrial viability of microemulsion polymerization. Controlled radical polymerization (CRP) techniques that have been implemented so far are mainly nitroxide-mediated polymerization (NMP), atom transfer radical polymerization (ATRP), and reversible addition−fragmentation chain transfer (RAFT) polymerization.6 Among these methods, ATRP is one of the widely studied technique in the aqueous dispersed media, including microemulsion,7 miniemulsion,8 emulsion,9 etc. The diameter of the nanoparticles can be varied from tens of © 2013 American Chemical Society
nanometers up to several hundred nanometers by adjusting the polymerization conditions. Recently, various initiation techniques involving in situ generation of Cu(I) have been reported. These are simultaneous reverse and normal initiated (SR&NI),8d,e,10 activators generated by electron transfer (AGET),11 activators regenerated by electron transfer (ARGET),11b,12 initiators for continuous activator regeneration (ICAR),13 and supplementary activator and reducing agent (SARA) ATRP.13b,14 In these systems, the required copper(I) catalyst for the ATRP can be generated by several approaches involving the in situ reduction of Cu(II) to Cu(I) by (i) various reducing agents,15 (ii) photochemical16 and (iii) electrochemical17 redox processes, and (iv) copper-containing nanoparticles (Scheme 1).18 Although conventional free radical inverse microemulsion polymerization is well documented in the literature, to the best of our knowledge there are only a few examples of the use of CRP methods in inverse microemulsion polymerization known Scheme 1. General Mechanism for in Situ Generation of CuI in ATRP Processes
Received: October 6, 2013 Revised: December 7, 2013 Published: December 11, 2013 9537
dx.doi.org/10.1021/ma402058a | Macromolecules 2013, 46, 9537−9543
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Scheme 2. Schematic Illustration of Photoinitiated SR&NI and AGET&ICAR ATRP of OEOMA in Inverse Microemulsion
to exist today.6a,b The successful examples of inverse microemulsion polymerization of N,N-dimethylacrylamide and oligo(ethylene glycol) monomethyl ether methacrylate were conducted via RAFT19 and AGET ATRP7b mechanism, respectively. The use of light in controlled radical polymerization also brings several distinct advantages, including temporal and spatial control over chain growth, rapid and energy efficient initiation, and minor risk of colloidal destabilization as the reaction is carried out at ambient temperature.16d,g,20 The commonly used photoinitiated CRP methods are adapted from thermal counterparts including iniferter,21 nitroxide-mediated radical polymerization (NMRP),22 atom transfer radical polymerization (ATRP),16a−c,e,f,23 cobalt-mediated radical polymerization (CMRP),24 organoiodine-mediated radical polymerization (OMIP),25 organotellurium-mediated radical polymerization (TERP),26 and reversible addition−fragmentation chain transfer polymerization (RAFT),27 etc. Among them, photoinitiated ATRP has been studied extensively because of the broad range of monomers and mild polymerization conditions. The photoinitiated ATRP is based on photoredox reactions of copper catalysts under various radiation sources with or without various photoinitiators.16a−c,e,f,23a−e Many UV and visible light free radical photoinitiators were reported to be powerful promoters for light-induced ATRP. In this study, we present details of an inverse microemulsion polymerization of oligo(ethylene glycol) monomethyl ether methacrylate via photoinitiated ATRP providing evidence that light can be an additional controlling factor for achieving welldefined polymer chains. In particular, the influence of the initiation mechanism, the concentration of the photocatalyst, and the organic phase content are some of the experimental parameters that will be investigated in detail.
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hydroxyethoxy)phenyl]-2-hydroxy-2-methyl-1-propane-1-one (Irgacure 2959, Ciba), hexane (Aldrich, 95%), 2-bromoisobutyryl bromide (Aldrich, 98%), and tris(2-pyridylmethyl)amine (TPMA, 98%; Aldrich) were used as received. Polyoxyethylene (3) oleyl ether (PEO3C18) and polyoxyethylene (6) oleyl ether (PEO6C18) surfactants were kindly donated by Nihon Emulsion, Tokyo, Japan. Water-soluble poly(ethylene glycol)-based macroinitiator PEO-Br (average Mn ∼ 2000 g mol−1) was synthesized according to the previously published procedures using Me-PEO and 2-bromoisobutyryl bromide.28 Photoinitiated ATRP of OEOMA in Inverse Microemulsion. A series of microemulsion ATRP reactions, using OEOMA, were carried out under different experimental conditions at room temperature. An example detailing a typical procedure for the synthesis of P(OEOMA) follows: CuIIBr2 (0.0029 g, 0.013 mmol), ligand (PMDETA) (2.7 μL, 0.013 mmol), and photoinitiator (Irgacure 2959) (0.0029 g, 0.013 mmol) were dissolved in a mixture of OEOMA (0.8 g, 2.67 mmol), water (1.1 g), and PEG-550 costabilizers (0.8 g) at room temperature to form an aqueous solution containing stable copper complexes and initiators. Meanwhile, PEO3C18 and PEO6C18 as surfactant (2 g) were dissolved in 15 g of hexane to form organic phase. The aqueous solution was added to organic solution under stirring. The resulting mixture was put into a Schlenk flask (i.d. = 9 mm), and the reaction mixture was degassed by three freeze−pump−thaw cycles and purged with nitrogen. The mixture was irradiated by a photoreactor (Rayonet) equipped with 16 lambs emitting light nominally at 350 nm at room temperature. The light intensity was 45 mW cm−2 as measured by Delta Ohm model HD-9021 radiometer. After given time (3−24 h) solvents were removed by rotary evaporation, and the remaining residues were dissolved in a minimum amount of THF. The resulting THF solution was added dropwise into cyclohexane, precipitating the polymers, which were isolated by decantation, and then dried in a vacuum oven at for 12 h. In this way unreacted OEOMA and PEO3C18 and PEO6C18 can be removed from reaction mixture, since they are miscible with cyclohexane. Conversion of the monomer was determined gravimetrically. Characterization. Gel permeation chromatography (GPC) measurements were obtained from a Viscotek GPCmax autosampler system consisting of a pump, a Viscotek UV detector, and a Viscotek differential refractive index (RI) detector. Three ViscoGEL GPC columns (G2000HHR, G3000HHR, and G4000HHR, 7.8 mm i.d., 300 mm length) were used in series. The effective molecular weight ranges were 456−42 800, 1050−107 000, and 10 200−2 890 000, respectively. Tetrahydrofuran was used as an eluent at flow rate of 1.0 mL min−1 at 30 °C. Both detectors were calibrated with polystyrene standards having narrow molecular weight distribution. Data were analyzed using
EXPERIMENTAL SECTION
Materials. N,N,N′,N″N″-Pentamethyldiethylenetriamine (PMDETA, 99%; Aldrich) was distilled before use. Oligo(ethylene glycol) monomethyl ether methacrylate (OEOMA, average Mn = 300, Aldrich), poly(ethylene glycol) (PEG 550, average Mn = 550, Alfa Aesar), poly(ethylene oxide) methyl ether (Me-PEO, Mn ∼ 2000 g mol−1, Aldrich) copper(II) bromide (CuIIBr2, 99%; Acros), 1-[4-(29538
dx.doi.org/10.1021/ma402058a | Macromolecules 2013, 46, 9537−9543
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Viscotek OmniSEC Omni-01 software. The average diameter of the polymer particles (Dn) was determined by dynamic light scattering (DLS) on a Malvern Nano-S instrument at room temperature. A hundred measurements each having 10 s correlation times were recorded for each sample.
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RESULTS AND DISCUSSION The photoinitiated ATRP is based on photoredox reactions of copper catalysts under UV and visible light irradiation. The required ATRP catalysts are generated by direct irradiation or indirect activation by using a photoinitiator. In the direct system, the polymerization activator, CuIX/L, was generated from CuIIX2/L under UV light without any photoinitiator, and the polymerization was initiated by the reaction of the CuIX/L with alkyl halide.16a,b,e,23d In the indirect system, the polymerization activators, CuIX/L, can be generated from CuIIX2/L under UV light through the help of photoinitiators. Many UV and visible light free radical photoinitiators were reported to be powerful promoters for photoinitiated ATRP.16c,e,23a Scheme 2 presents the basic components used for photoinitiated simultaneous reverse and normal initiated (SR&NI) ATRP and combination of activator generated by electron transfer (AGET) and initiators for continuous activator regeneration (ICAR) ATRP of OEOMA in inverse microemulsion media. First CuIIBr2, PMDETA, or TPMA as a ligand and PEO-Br as a macroinitiator were dissolved in a mixture of OEOMA, water, and PEG 550 costabilizers to form an aqueous solution containing catalyst precursors for ATRP, which was then slowly added to the organic solution composed of hexane and a mixture of polyoxyethylene (3) oleyl ether (PEO3C18) and polyoxyethylene (6) oleyl ether (PEO6C18) surfactants. A stable and bluish w/o inverse microemulsion was generated after mixing these solutions, forming small aqueous monomer swollen “micelles”/droplets with the copper catalyst precursors encapsulated inside. The photoinitiated SR&NI ATRP was initiated by activating the catalysts via UV light irradiation of water-soluble photoinitiator (Irgacure 2959). Under UV light irradiation, Irgacure 2959 generated benzoyl and α-substituted alkyl radicals, which readily reduced Cu(II)/ PMDETA into Cu(I)/PMDETA ion and both being instrumental in the initiation step of ATRP. On the other hand, direct photolysis of Cu(II)/TPMA deactivator complex can also induce a redox reaction of between Cu(II)/TPMA and Cu(I)/ TPMA activator plus bromine radical to start the polymerization and then continue to regenerate the activator lost in the biradical termination process. This mechanism was a hybrid of ICAR ATRP and AGET ATRP. Because the radiation induces the reduction of Cu(II)/TPMA and Cu(I)/TPMA, the mechanism resembles AGET ATRP.16e However, a halogen radical is formed, which resembles the generation of radicals in ICAR ATRP. In both system, the catalyst complexes in some of the droplets were activated and these droplets served as the nuclei for the polymer particles, while the remaining droplets functioned as monomer reservoirs. As the polymerization continued, monomers diffused from the unactivated droplets to the growing polymer particles, resulting in the formation of polymer particles with larger size compared to the size of the original monomer swollen “micelles”/droplets. The absorption spectra of reaction mixture (OEOMA/ CuIIBr2/PMDETA/PEO-Br) with or without Irgacure 2959 (I2959) and (OEOMA/CuIIBr2/TPMA/PEO-Br) are displayed in Figure 1, together with the spectral emission of a photoreactor (Rayonet) equipped with 16 lambs emitting
Figure 1. UV−vis spectra of the CuIIBr2/PMDETA with or without I2959 and CuIIBr2/TPMA in water. The concentration of all components was 6 × 10−7 M.
light nominally at 350 nm. Previous spectroscopic studies pointed out that CuII/L complex has three distinct absorption bands at 250, 300, and 640 nm.29 As shown in Figure 1, the strong absorption at 300 nm is attributed to the ligand-to-metal charge-transfer transition of the copper catalysts.30 In our irradiation conditions (
