Preparation of Poly(acrylic acid) Particles by Dispersion

Dec 31, 2009 - Moreover, we succeeded in producing PS particles by thermal polymerization in the absence of a radical initiator at 130 °C in [DEME][T...
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Preparation of Poly(acrylic acid) Particles by Dispersion Polymerization In an Ionic Liquid Hideto Minami,* Akira Kimura, Keigo Kinoshita, and Masayoshi Okubo Graduate School of Engineering, Kobe University, Kobe 657-8501, Japan Received October 29, 2009. Revised Manuscript Received December 16, 2009 Poly(acrylic acid) (PAA) particles were successfully prepared by dispersion polymerization of acrylic acid in ionic liquid, N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoro-methanesulfonyl)amide ([DEME][TFSA]) at 70 °C with low hydrolysis grade (35.4%) poly(vinyl alcohol) as stabilizer. Interestingly, the PAA particles were easily extracted as particle state with water. Thus, the PAA particles had a cross-linked structure during the polymerization without cross-linker. Moreover, it was also noted that the cross-linking density of the PAA particles could be controlled by thermal treatment at various temperatures in [DEME][TFSA] utilizing the advantages of nonvolatility and high thermal stability of the ionic liquid.

Introduction Ionic liquids are novel solvents for green chemistry and electrolytes, which are entirely ions and are liquid state at ambient temperature. Ionic liquids have attractive properties such as ionic conductivity, thermal stability, nonflammability, and nonvolatility, which are considered to give environmentally friendly solvents.1-4 The physical properties of ionic liquids have been extensively investigated.5-7 In the field of polymer chemistry, application of ionic liquids as solvents for polymerization processes *Corresponding author. E-mail: [email protected]. Telephone and Fax: (þ81) 78 803 6197.

(1) Welton, T. Chem. Rev. 1999, 99, 2071 . (2) Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39, 3772. (3) Gordon, C. M. Appl. Catal., A 2001, 222(1-2), 101. (4) Sheldon, R. Chem. Commun. 2001, 2399. (5) Endres, F.; Abedin, S. Z. E. Phys. Chem. Chem. Phys. 2006, 8, 2101. (6) Ludwig, R.; Kragl, U. Angew. Chem. 2007, 119, 6702. (7) Earle, M. J.; Esperanca, J. M. S. S.; Gilea, M. A.; Lopes, J. N. C.; Rebelo, L. P. N.; Magee, J. W.; Seddon, K. R.; Widegren, J. A. Nature 2006, 439, 831. (8) Fuller, J.; Breda, A. C.; Carlin, R. T. J. Electrochem. Soc. 1997, 144(4), L67. (9) Noda, A.; Watanabe, M. Electrochim. Acta 2000, 45, 1265. (10) Hong, K.; Zhang, H.; Mays, J. W.; Visser, A. E.; Brazel, C. S.; Holbrey, J. D.; Reichert, W. M.; Rogers, R. D. Chem. Commun. 2002, 1368. (11) Zhang, H.; Hong, K.; Mays, J. W. Macromolecules 2002, 35, 5738. (12) Vygodskii, Y. S.; Lozinskaya, E. L.; Shaplov, A. S. Macromol. Rapid Commun. 2002, 23, 676. (13) Perrier, S.; Davis, T. P.; Carmichael, A. J.; Haddleton, D. M. Chem. Commun. 2002, 2226. (14) Harrisson, S.; Mackenzie, S. R.; Haddleton, D. M. Macromolecules 2003, 36(14), 5072. (15) Vijayaraghavan, R.; MacFarlane, D. R. Chem. Commun. 2004, 700. (16) Biedron, T.; Kubisa, P. J. Polym. Sci., Part A: Polym. Chem. 2004, 42(13), 3230. (17) Ryan, J.; Aldabbagh, F.; Zetterlund, P. B.; Yamada, B. Macromol. Rapid Commun. 2004, 25, 930. (18) Cheng, L.; Zhang, Y.; Zhao, T.; Wang, H. Macromol. Symp. 2004, 216, 9. (19) Benton, M. G.; Brazel, C. S. Polym. Int. 2004, 53, 1113. (20) Kubisa, P. Prog. Polym. Sci. 2004, 29(1), 3. (21) Kubisa, P. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 4675. (22) Strehmel, V.; Laschewsky, A.; Wetzel, H.; Gornitz, E. Macromolecules 2006, 39, 923. (23) Basko, M.; Biedron, T.; Kubisa, P. Macromol. Symp. 2006, 240, 107. (24) Pringle, J. M.; Ngamna, O.; Chen, J.; Wallace, G. G.; Forsyth, M.; MacFarlane, D. R. Synth. Met. 2006, 156, 979. (25) Winterton, N. J. Mater. Chem. 2006, 16, 4281. (26) Vygodskii, Y. S.; Mel’nik, O. A.; Lozinskaya, E. I.; Shaplov, A. S.; Malyshkina, I. A.; Gavrilova, N. D.; Lyssenko, K. A.; Antipin, M. Y.; Golovanov, D. G.; Korlyukov, A. A.; Ignat’ev, N.; Welz-Biermann, U. Polym. Adv. Technol. 2007, 18(1), 50. (27) Ueki, T.; Watanabe, M. Langmuir 2007, 23, 988.

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is gradually receiving increased attention.8-32 Many studies on homogeneous polymerization systems in ionic liquids have been reported. It has been reported that radical polymerization using ionic liquids as solvents provided advantageous phenomena: higher polymerization rates and higher molecular weights than in bulk or organic solvents (homogeneous systems) due to a reduced termination rate because of the high viscosity of the ionic liquids10,14,19,22,26 and also as a result of an increase in the propagation rate coefficient in some cases.14,30 In a heterogeneous system using ionic liquid as solvents, Pringil et al. prepared conducting polymer nanoparticles by chemical oxidative polymerization in an ionic liquid.24 Kim et al. reported that polypyrrole particles were successfully prepared in a magnetic ionic liquid, in which various nanostructures, such as nanoparticles, nanorods, and nanotubes were prepared by just adding monomer to a magnetic ionic liquid.33 Zheng et al. carried out the direct anodic oxidation electropolymerization of 3,4-ethylenedioxythiophene in an ionic liquid microemulsion.29 Landfester et al. prepared polyamide nanoparticles by heterophase polycondensation in ionic liquids.34 Recently we succeeded in preparing polystyrene (PS) particles by dispersion polymerization in an ionic liquid, N, N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium bis(trifluoromethanesulfonyl)amide ([DEME][TFSA]) for the first time.35 The particle size and particle size distribution in the ionic liquid could be controlled in a manner similar to in organic solvents. Moreover, we succeeded in producing PS particles by thermal polymerization in the absence of a radical initiator at 130 °C in [DEME][TFSA] using a conventional reactor (not autoclave) utilizing the advantages of nonvolatility and thermal stability of the ionic liquid. Moreover, successful preparation of composite (28) Mallakpour, S.; Rafiee, Z. Polymer 2008, 49, 3007. (29) Dong, B.; Zhang, S.; Zheng, L.; Xu, J. J. Electroanal. Chem. 2008, 619/ 620, 193. (30) Woecht, I.; Naake, G. S.; Beuermann, S.; Buback, M.; Garcia, N. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 1460. (31) Ueki, T.; Watanabe, M. Macromolecules 2008, 41, 3739. (32) Lu, J.; Yan, F.; Texter, J. Prog. Polym. Sci. 2009, 34, 431. (33) Kim, J.; Kim, J.; Song, E.; Min, Y.; Hamaguchi, H. Macromolecules 2008, 41, 2886. (34) Frank, H.; Ziener, U.; Landfester, K. Macromolecules 2009, 42, 7846. (35) Minami, H.; Yoshida, K.; Okubo, M. Macromol. Rapid Commun. 2008, 29, 567.

Published on Web 12/31/2009

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polymer particles36 and Nylon-6 particles37 in ionic liquids has been demonstrated. Poly(acrylic acid) (PAA) is produced in large quantities for industrial materials such as dispersants, thickeners and super absorbent polymer. In particular, cross-linked PAA absorbs a large number of water and is used in humidity control and baby diapers. PAA is produced by radical polymerization in heterogeneous systems (inverse suspension38,39/miniemulsion polymerizations40) as particle state as well as homogeneous system (solution polymerization). However, in the case of the heterogeneous systems, large amounts of volatile organic compounds are employed because PAA is water-soluble. In this article, to extend the applications of ionic liquids as media for the preparation of PAA particles, dispersion polymerization of acrylic acid (AA) will be carried out in an ionic liquid, [DEME][TFSA], in which AA is soluble, but PAA is insoluble.

Experimental Section AA (Nacalai Tesque, Inc., Kyoto, Japan) was purified by distillation under reduced pressure in a nitrogen atmosphere. Reagent grade 2.20 -azobis(isobutyronitrile) (AIBN) was purified by recrystallization in methanol. Reagent grade 1-pyrenylmethyl methacrylate (PM, Funakoshi, Tokyo, Japan) was used as a fluorescent moiety of PAA without further purification. Poly(vinyl alcohol) (PVA, saponification degree 35.4%, weight-average molecular weight 1.9  105 g/mol, provided by The Nippon Synthetic Chemical Industry Co., Ltd., Osaka, Japan) and poly(vinylpyrrolidone) (PVP, K-30, weight-average molecular weight 3.6  105 g/mol, Nacalai Tesque, Inc., Kyoto, Japan) were used as received. The ionic liquid, [DEME][TFSA] (provided by Nisshinbo Industries Inc., Tokyo, Japan) was used as received.

Dispersion Polymerization in [DEME][TFSA]. Dispersion polymerizations of AA (0.25 g) in [DEME][TFSA] (2.5 g) were carried out in a 10 mL glass vessel at 70 °C for 24 h with magnetic stirring at 400 rpm under nitrogen atmosphere. AIBN (2.5 mg) and PVA (12.5 mg) were used as initiator and stabilizer, respectively. Precipitation polymerization of AA in hexane was carried out for comparison under the same conditions except that the ionic liquid was replaced with hexane without PVA. Heat Treatment of PAA Dispersion. PAA dispersion was diluted to about 1 wt % in solid content with [DEME][TFSA] containing dissolved low hydrolysis grade (35.4%) PVA. The heat treatment was carried out in glass vessels for 1 h at various temperatures: 100, 125, 150, 175, and 200 °C. Moreover, the heat treatment at 200 °C was carried out for various treatment times: 5 min, 15 min, 30 min, 3 h, 5 h, and 10 h. Characterization. PAA particles were observed by optical microscopy (ECLIPSE 80i, Nikon), confocal laser scanning microscopy (CLSM, LSM-GB 200, Olympus) and scanning electron microscopy (SEM, S-2460, Hitachi Science Systems Ltd., Ibaraki, Japan). Particle size distributions were measured by dynamic light scattering (DLS, FPAR-1000, Otsuka Electronics, Osaka, Japan) at the light scattering angle of 90° at room temperature (around 20 °C). The refractive index of [DEME][TFSA] (36) Minami, H.; Yoshida, K.; Okubo, M. Macromol. Symp. 2009, 281, 54. (37) Minami, H.; Tarutani, Y.; Yoshida, K.; Okubo, M., Macromol. Symp. in press. (38) Suda, K.; Pattama, P. J. Appl. Polym. Sci. 1999, 72, 1349. (39) Mayoux, C.; Dandurand, J.; Ricard, A.; Lacabanne, C. J. Appl. Polym. Sci. 2000, 77, 2621. (40) Landfester, K.; Willert, M.; Antonietti, M. Macromolecules 2000, 33, 2370.

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was 1.419 at 20 °C, which was measured with an Abbe 3 L refractometer (Bausch & Lomb Co., Ltd.) under temperaturecontrolled condition. The viscosity of [DEME][TFSA] was 120 mPa at 20 °C (by Kanto Regents’s catalog). Number- (Dn) and weight-average (Dw) particle diameters were obtained with the Marquadt Analysis routine using above values. Monomer conversions were determined by gas chromatography (Shimadzu Corporation, GC-18A) employing helium as the carrier gas, N,Ndimethylformamide as the solvent and p-xylene as the internal standard. Fourier transform infrared measurement was carried out using a JASCO spectrometer FT/IR-6200 applying KBr pellet method. After methyl esterification of PAA,41 molecular weights and molecular weight distributions were obtained by gel permeation chromatography (GPC) using two poly(styrenedivinylbenzene) gel columns (TOSOH Corporation, TSK gel GMHHR-H 7.8 mm i.d.  30 cm) with THF as eluent at a flow rate of 1.0 mL/min employing a refractive index detector (RI-8020). The columns were calibrated with six linear PS samples (1.05  103 to 5.48  106 g/mol, Mw/Mn = 1.01-1.15).

Results and Discussion Dispersion polymerization starts as homogeneous system, in which monomer, initiator and steric stabilizer are soluble in the medium, but formed polymer is insoluble. Because AA is soluble in [DEME][TFSA] but PAA is insoluble therein, dispersion polymerization should be expected to proceed in this ionic liquid. In our previous work for the preparation of PS particles in [DEME][TFSA], PVP operated effectively as a steric stabilizer.35 However, when PVP was used in this work, the system was colloidally unstable and a large amount of coagulum was formed. Although the amount of PVP increased 2-fold, colloidal stability was not improved. As a suitable stabilizer in dispersion polymerization, it is generally important to consider the affinity for both the medium and the polymer particles. PVP is soluble in [DEME][TFSA], but it would not be adsorbed onto formed hydrophilic PAA particles. Low hydrolysis grade (35.4%) PVA was thus used as a steric stabilizer instead of PVP. Because poly(vinyl acetate) segments of the PVA is soluble in [DEME][TFSA] but poly(vinyl alcohol) segments is insoluble therein, they are expected to be a desirable stabilizer for the dispersion polymerization system. Before starting the polymerization, the system was homogeneous, that is, AA, PVA and AIBN were soluble in the ionic liquid. During the polymerization, the system became turbid, which indicates the formation of colloidal stable PAA particles. Figure 1 shows conversion-time plots for dispersion polymerization of AA in the ionic liquid and precipitation polymerization of AA in hexane for comparison. Both polymerizations proceeded rapidly. The polymerization rate of the dispersion polymerization in the ionic liquid was higher than that of the precipitation polymerization of AA in hexane. This trend is also observed in the heterogeneous system, in which PS particles were prepared by dispersion polymerization in [DEME][TFSA],35,36 and homogeneous systems.10,14,22 The reason is considered to be the reduced termination rate because of slow termination reaction due to the high viscosity of ionic liquids and/or the increase of the propagation rate coefficient. The weight-average molecular weight (Mw) and polydispersity (Mw/Mn) of PAA, which were measured by GPC after methyl esterification, prepared in ionic liquid was 5.3  105 and 4.01, respectively, and the molecular weight distribution was monomodal. Mw was higher than that prepared in hexane (Mw = 2.7  105, Mw/Mn = 4.71). (41) Sugihara, Y.; Kagawa, Y.; Yamago, S.; Okubo, M. Macromolecules 2007, 40, 9208.

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Figure 1. Conversion-time plots for dispersion polymerization of acrylic acid in [DEME][TFSA] (O) and precipitation polymerization in hexane (b) at 70 °C.

Figure 2. SEM photograph of PAA particles prepared by dispersion polymerization in [DEME][TFSA] at 70 °C for 24 h with magnetic stirring at 400 rpm.

Figure 2 shows SEM photograph of PAA particles prepared by dispersion polymerization in [DEME][TFSA] at 70 °C for 24 h. About 100 nm-sized PAA particles were observed, which indicates that PAA particles were successfully prepared by dispersion polymerization in ionic liquid for the first time. Because the product seemed to be aggregates of the 100 nm-sized particles, the diameter of PAA particles were measured with DLS in ionic liquid to clarify the state of the PAA particles in the ionic liquid before drying them for the preparation of their SEM sample, in which PAA particles were washed with isoamyl acetate. The numberaverage diameter and Cv (coefficient of variation) of PAA particles in [DEME][TFSA] were about 115 nm and 26%, respectively (Figure 3). This result indicates the 100 nm-sized PAA particles were dispersed individually and did not extensively coagulate at the completion of the dispersion polymerization in the ionic liquid. To separate the obtained PAA particles for the preparation of a SEM sample as well as PS particles in our previous works35,36 from the dispersed system in [DEME][TFSA] medium after the polymerization, the medium was replaced with an organic solvent such as isoamyl acetate or methanol, which is miscible with the ionic liquid but unable to dissolve the PAA or PS particles, respectively. In this work, the product could be desirably extracted only by adding water to the dispersion as shown in Figure 4. Just after addition of water (Figure 4a), the upper water phase is clear, but after 24 h, the water phase became turbid and in turn the lower ionic liquid phase became clear (Figure 4b). This indicates almost all PAA particles were extracted with water, which was confirmed by FT-IR analysis of ionic liquid phase after the extraction. Lodge et al. have reported that the micelles Langmuir 2010, 26(9), 6303–6307

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Figure 3. Particle size distribution of PAA particles (number fraction) in [DEME][TFSA] obtained by DLS at room temperature. Dn = 115 nm; Cv = 26%.

consisting of poly(1,2-butadiene-b-ethylene oxide) block copolymers transferred between hydrophobic ionic liquid and water reversibly (“micelle shuttle”) by the change of temperature.42,43 It was revealed that the micelle shuttle was due to entropy-driven process and the transfer mechanism was proposed. The transfer mechanism in the case of PAA particles would be analogous to above case. However, in this work, the particle size was quite different and there was no reversibility, which will be discussed in detail in a future article. After the extraction, unexpectedly PAA particles did not dissolve aqueous phase. Micrometer-sized highly swollen particles were observed in the aqueous phase with an optical microscope (Figure 4c). Since PAA is water-soluble, it seemed that the PAA particles underwent some sort of crosslinking reaction during the polymerization without cross-linker. In the case of precipitation polymerization in hexane, the obtained PAA dissolved in water, that is, the PAA did not have a cross-linking structure. DeSimon et al. reported that PAA particles prepared by precipitation polymerization in supercritical carbon dioxide did not have a cross-linking structure.44,45 These results suggest that ionic liquid, [DEME][TFSA] may have a great influence on the formation of the cross-linking structure. This point will be discussed later. The size distribution of extracted PAA particles became significantly broader as compared to that of dispersed PAA particles in ionic liquid, which seems to indicate heterogeneity of cross-linking density among the PAA particles. The size of the swollen particles was mainly around ∼10 μm, which corresponds to the absorption of one million times the volume of water by 100 nm-sized PAA particles. From the viewpoint of application as a super absorbent polymer, this result is desirable. However, the contrast of the swollen particles with respect to medium in the optical microscope observation was too high to consider highly swollen particle with water. If the PAA particles would be swollen by one million times the volume of water, the refractive index of the swollen particles should be almost same as that of the medium, which means the boundary of the swollen particles could not be observed. Thus, micrometersized swollen particles would not be so highly swollen with medium and should be aggregates formed by coagulation of submicrometer-sized particles during the extraction process. Figure 5 shows the particle size distribution (number fraction) of the PAA particles extracted to the aqueous phase. The DLS result indicates that there were not only micrometer-sized particles (42) Bai, Z.; He, Y.; Lodge, T. P. Langmuir 2008, 24, 5284. (43) Bai, Z.; Lodge, T. P. J. Phys. Chem. B 2009, 113, 14154. (44) Romack, T. J.; Maury, E. E.; DeSimone, J. M. Macromolecules 1995, 28, 912. (45) Liu, T.; Garner, P.; DeSimone, J. M.; Roberts, G. W.; Bothun, G. D. Macromolecules 2006, 39, 6489.

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Figure 4. Visual appearance of polymerization mixtures just after (a) and 24 h after (b) addition of water (no mixing) and optical micrograph of PAA particles extracted from [DEME][TFSA] to water at pH 2.2 (c).

Figure 7. Optical (a) and fluorescence (b) micrographs of PAA particles, heat treated at 200 °C for 1 h in [DEME][TFSA], dispersed in water at pH 12.

Figure 5. Particle size distribution of PAA particles (number

Scheme 1. Formation of the Acid Anhydride of the Carboxyl Group and the Elimination of Carbon Dioxide

fraction) in water at pH 3.8 obtained by DLS.

Figure 6. Optical micrographs (a, b) and fluorescence micrographs (a0 , b0 ) of P(AA-PM) particles extracted by water at pH 3 (a, a0 ) and pH 11 (b, b0 ).

but also submicrometer-sized particles, although weight percentage of the latter particles was extremely low. Micrometer-sized swollen particles could be considered as coagulation of submicrometer-sized particles during extraction process as mentioned above. On the other hand, submicrometer-sized particles seem to be swollen from 100 nm-sized particles. The variation of swelling ratios of submicrometer-sized PAA particles with pH values of extracted water phase was investigated. The swelling ratios were estimated by the volume ratio calculated from Dn values observed in water and the ionic liquid by DLS measurements. The swelling ratio increased (from 7 to 12) with increasing pH value (from 3.8 to 4.8) of the water phase due to the ionization of carboxyl groups of PAA. Moreover, at pH 11, the swelling ratio increased, and then the dispersed system became transparent. In order to check if 6306 DOI: 10.1021/la904115s

this was caused by the refractive indices of the swollen particles and the medium being similar or dissolution of the particles, the following experiment was carried out. A small amount (0.52 mg) of fluorescent monomer (1-pyrenylmethyl methacrylate) (PM) was added in the dispersion polymerization of AA under the same conditions. After the polymerization, the obtained particles were extracted with water in the same way and observed with a fluorescence microscope. Figure 6 shows optical and fluorescence micrographs of P(AAPM) particles. When the pH value was low (pH 3), the bright particles were observed with a fluorescence microscope (Figure 6a0 ) at the same place of the corresponding optical micrograph (Figure 6a). On the other hand, when the pH value was high (pH 11), particles were not observed with both microscopes (Figure 6b, b0 ). These indicate that the cross-linking structure formed during the polymerization could be easily broken at pH 11. Previously, the formation of acid anhydride of carboxyl group of PAA (solid state) by heat treatment has been reported.46 This acid anhydride link can be easily cloven at an alkaline aqueous solution and eliminates carbon dioxide at higher temperature (Scheme 1). When the acid anhydride link is formed by the intermolecular (interchain) reaction, a cross-linking structure should be generated. Just after the polymerization in this work, cross-linking structure by acid anhydride group would be formed (46) Maurer, J. J.; Eustace, D. J.; Ratcliffe, C. T. Macromolecules 1987, 20, 196.

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Figure 8. FT-IR spectra of PAA particles before (a) and after heat treatment for 1 h at various temperatures (°C), 100 (b), 125 (c), 150 (d), 175 (e), and 200 (f), and at 200 °C for various times (min), (g) 15, (h) 30, (i) 60, (j) 180, and (k) 600.

in ionic liquid at 70 °C. In order to clarify the possibility of the formation of the latter type of cross-linking structure (elimination of carbon dioxide) in PAA particles, heat treatment at higher temperature of the PAA particles was carried out in the ionic liquid at 200 °C for 1 h utilizing advantages of the ionic liquid: nonvolatile and high thermal stability. Figure 7 shows optical and fluorescence micrographs of P(AAPM) particles dispersed in alkaline aqueous solution (pH 12) after the heat treatment. That is, submicrometer-sized particles were observed with both of optical and fluorescence microscopes even at pH 12, the treated P(AA-PM) particles did not coagulate and were insoluble in the alkaline aqueous solution. This indicates that the irreversible cross-linking structure was formed by elimination of carbon dioxide from the acid anhydride. Figure 8 shows FT-IR spectra of PAA particles treated at various temperatures for 1 h and at 200 °C at various times. The changes in the structure of PAA were observed in the case of high temperature treatment. The peaks of 1050 and 1800 cm-1 can be assigned to the acid anhydride. The intensities of both peaks increased with increasing temperature, which indicated that the acid anhydride link, that is, the cross-linking density, increased. On the other hand, at 200 °C, these peaks initially increased with the heat treatment time, and then, the peaks gradually decreased after 3 h. This indicates that the acid anhydride link increased with the time; afterward, it would transform to an acyl group with irreversible cross-linking. These results were consistent with the decrease in swelling ratio of PAA particles with water as an increase in treatment temperature and time took place. Furthermore, in order to investigate the possibility of recycling of the ionic liquid, second run polymerization was carried out under the same conditions using a lower ionic liquid phase (Figure 4b) as medium, in which a small amount of dissolved water, undecomposed initiator, and PVA were

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remaining. Before the second run, heat treatment of the [DEME][TFSA] at 100 °C for 10 h was carried out in order to eliminate water and decompose the initiator. In the case of the second run, PAA particles were also observed without coagulation though the number-averaged diameter of PAA (270 nm) was larger than that of the first run (115 nm). Obtained PAA particles were also able to extract with water from the ionic liquid, and the ionic liquid phase became clear again. This result insists that [DEME][TFSA] can easily be reused after the extraction of PAA particles prepared by dispersion polymerization.

Conclusion We have demonstrated the preparation of PAA particles by dispersion polymerization in [DEME][TFSA]. Interestingly, the PAA particles were easily extracted from the ionic liquid to water, and the PAA particles had a cross-linked structure during the polymerization without cross-linker. The cross-linking density of the PAA particles could be controlled by heat treatment in [DEME][TFSA] utilizing the advantages of nonvolatility and high thermal stability of ionic liquid. The results described in this study will further expand the utility of ionic liquids as heterogeneous polymerization media for preparation of unique polymer particles. Moreover, after the extraction of PAA particles, [DEME][TFSA] could be reused for second polymerization medium. Acknowledgment. This work was partially supported by Grant-in-Aid for Scientific Research (Grant 21655082) from the Japan Society for the Promotion of Science (JSPS) and Hyogo Science and Technology Association. The authors thank Nisshinbo Industries Inc. for supplying [DEME][TFSA].

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