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Ind. Eng. Chem. Res. 2008, 47, 6358–6361
Preparation of Functional Nanoparticles by Assembling Block Copolymers Formed by Living Radical Polymerization Tomomi Sato, Sakiko Tsuji, and Haruma Kawagauchi* Graduate School of Science and Technology, Keio UniVersity, 3-14-1 Hiyoshi, Yokohama 223-8522, Japan
A series of poly(glycerol methacrylate) (PGLM)-block-poly(N-isopropylacrylamide) (PNIPAM)s were prepared by quasi-living radical polymerization using a photosensitive iniferter. PGLM core/PNIPAM shell particles were prepared by self-assembling of the block copolymers in a methanol-tetrahydrofuran solution followed by cross-linking of the core. Magnetite nanoparticles were prepared in situ using penetrated ferric and ferrous ions in the core. The composite particles were stably dispersed in water at room temperature but easily collected by a magnet at the temperature above the lower critical solution temperature of PNIPAM. Introduction Conventional radical polymerization has a lot of advantages such as simple conditioning, easy processing, applicability to aqueous system, etc. However, it has a fatal disadvantage, that is, a randomly occurring termination reaction between radicals coming by each other. This termination made it impossible to get polymers with monodispersed chain lengths and controlled molecular architecture. The development of living radical polymerization enabled the preparation of well-designed polymers and block copolymers. A variety of block copolymers were prepared by different living radical polymerizations, that is, stable radical polymerization,1 atom-transfer radical polymerization (ATRP),2 and polymerizations using reversible addition fragmentation transfer (RAFT).3 “Iniferter”, developed by Otsu et al.,5 is a general term of dithiocarbamates that contribute to radical polymerization as an initiator, transfer reagent, and terminator and exhibit quasi-living radical polymerization behavior.6 It was also confirmed that polymerization using iniferters gave block copolymers at will. Among the block copolymers prepared by any living radical polymerizations, amphiphilic block copolymers attracted much attention because they form functional micelles.1–4,7,8 One block in a diblock copolymer contributes to assembling a group of blocks to form micelles, and the other contributes to stabilization of micelles in water by hydration. Either of the core or shell parts of micelles were cross-linked when the micelles were required to be converted to stiff particles. Some functional materials such as metal nanoparticles, catalysts, drugs, etc., or their precursors might be introduced to either the core or shell parts to get functional composite micelles or particles.9,10 In this study, poly(glycerol methacrylate) (PGLM)-blockpoly(N-isopropylacrylamide) (PNIPAM) was prepared by an iniferter. The iniferter used in our study was a water-soluble, UV-mediated one, even though Otsu et al. tried only oil-soluble iniferters.4 The products were characterized in terms of molecular weight and its distribution and micelle-forming ability and employed for composite formation with magnetite, which was in situ prepared via the Mazzart process.11 Experimental Section Materials. Sodium N,N-diethyldithiocarbamate (NaDC; Wako Pure Chemicals Co., Tokyo, Japan) was used to synthesize a * To whom correspondence should be addressed. E-mail: haruma@ applc.keio.ac.jp.
water-soluble iniferter. N-Isopropylacrylamide (NIPAM; Kojin Co., Tokyo, Japan) was recrystallized from hexane-toluene (1/ 1, v/v). Glycerol methacrylate (GLM; Tokyo Kasei Co., Tokyo, Japan) was used as received. Divinylsulfone (DVS; Wako Pure Chemicals Co.) was used without further purification. Ferrous chloride tetrahydrate and ferric chloride hexahydrate were purchased from Wako Pure Chemicals Co. and used without further purification. Solvents, methanol (MeOH) and tetrahydrofuran (THF), were of extra-pure grade. Doubly distilled water was used as a polymerization medium. Preparation of a Water-Soluble Iniferter. NaDC (10 g), sodium chloroacetate (5.17 g), and water (100 g) were mixed in a 300 mL flask under mild stirring at room temperature for 2 days. Then the mixture was adjusted to pH 4 to precipitate the product. The product, N,N-diethyldithiocarbamate acetic acid (DCAA), was recrystallized from acetone. Polymerization of GLM Using DCAA. A total of 0.05 g of DCAA was dissolved in 1.5 mL of 1 N NaOH in a polymerization vessel equipped with a condenser, stirrer, and serum rubber. A total of 2 g of GLM and 200 mL of water were added to this vessel. The contents was bubbled with nitrogen gas to purge oxygen at 20 °C. Then polymerization was started by irradiating the mixture with UV light. Polymers were taken out of the vessel during the polymerization and analyzed. Unreacted monomer and other contaminants were dialyzed, and thus purified PGLM was used for the next step. Preparation of PGLM-block-PNIPAM (GN) Using DCAA. A total of 2.8 × 10-2 mol of a NIPAM monomer was added to a solution containing 2.8 × 10-2 mol of PGLM in the reaction vessel used for the polymerization of GLM. The product taken out at a certain interval of the polymerization course was termed as GNX, in which X indicates the time (minutes) when the sample was taken out of the reaction vessel. Formation of GN Micelles. GN was expected to form micelles in a mixed solvent of MeOH and THF. A total of 12 mg of GN90 was dissolved in 120 µL of MeOH. To this solution was added dropwise under stirring 1080 µL of THF. The rate of THF addition was changed from 1 to 5 mL/min. Cross-Linking of GN Micelles. A micellar solution of GN90 was adjusted to pH 12 with NaOH. Then DVS was added to the solution, and it was kept there for 24 h. The ratio of added DVS to GN was changed from 0.1 to 1.0. The solution was dialyzed through a semipermeable membrane, Spectra/Por, to remove unreacted chemicals.
10.1021/ie7017614 CCC: $40.75 2008 American Chemical Society Published on Web 06/25/2008
Ind. Eng. Chem. Res., Vol. 47, No. 17, 2008 6359 Scheme 1. Schematic Mechanism for Living Radical Polymerization Using DCAA
Preparation of a Magnetite/GN Composite Particle. Nitrogen was bubbled into the dispersion of GN particles for 30 min. Then 0.2 M FeCl2 · 4H2O and 0.2 M FeCl3 · 6H2O (1/2, v/v) was added to the dispersion under mild stirring for 30 min. After that, the mixture was adjusted to pH 12 with 0.5 N NaOH, and it was kept there for 60 min. Characterization of GN. Molecular weights of polymers were measured by size exclusion chromatography with a TSKgel G4000PW (Toso Co.) in a 0.05 M phosphate buffer solution at a flow rate of 1.0 mL/min at 15 °C. Thermosensitivity of GN was assessed by transmittance of an aqueous or 0.5 M NaOH solution of GN as a function of the temperature. NMR is known to give some information of the micelle structure. The homogeneous solution of a block copolymer in deuterated MeOH and the micellar solution in deuterated MeOH-THF were put into NMR tubes, and 1H NMR spectra were obtained with a Varian 300 at 20 and 40 °C. The diluted GN micellar solution in MeOH-THF was employed for DLS measurement to determine the hydrodynamic diameter of micelles. The scattering angle for the measurement was 90°. The hydrodynamic diameter of cross-linked GN nanoparticles was also measured by DLS. Results and Discussion Evaluation of Living Radical Polymerization. GLM was polymerized using DCCA in an aqueous medium under nitrogen at 20 °C with the aid of UV irradiation. The polymerization was expected to proceed in the manner presented in Scheme 1. Scheme 1 shows basic reactions of an iniferter, DCCA, with monomer (M). M is consumed by initiation and propagation reactions. Dormant oligomers and polymers are formed by termination and chain-transfer reactions. The molecular weight and distribution of PGLM were measured as a function of the polymerization period or conversion. The result is shown in Figure 1. The molecular weight increased with increasing conversion. The proportionality of the molecular weight with conversion is evidence of the realization of living radical polymerization. However, the linearity is not so good, and the ratio of weight- to-number-average molecular weights is not close to 1.0 in our polymerization systems. These results suggest that the reaction equation presented as Scheme 1 is imperfect. Some unexpected chain-transfer and coupling reactions seem to be unavoidable in this iniferter system. The second block-forming polymerization was carried out by adding NIPAM to a GLM polymerizing system. The molecular weight was measured in the course of the second block-forming polymerization. The result is shown in Figure 2. The figure clearly shows that all of the polymer chains restarted the second-stage polymerization and the molecular
Figure 1. Molecular weight of a polymer and its polydispersity in aqueous polymerization of NIPAM using a photoiniferter.
Figure 2. Gel permeation chromatography (GPC) traces of samples taken at different polymerization periods. GPC column: TSKgel G4000PW. Moving phase: 0.05 M phosphate buffer. Flow rate: 1.0 mL/min.
weight gradually increased with increasing conversion. However, it cannot be ignored that the peaks gradually broadened with conversion. It was concluded that livingness of radical polymerization in an iniferter system is not necessarily perfect, but an iniferter has the potential to give an almost satisfactory block copolymer. The usefulness of DCCA as a quasi-living polymerization species is obvious because conventional radical polymerization gave neither polymers whose molecular weight increased with conversion nor block copolymers by the sequential addition of monomers. Micelle Formation. Aqueous solutions of GNs having different G/N ratios exhibited different temperature dependences of transparency. All of the solutions were transparent at room temperature. The solution of GN containing a smaller amount of NIPAM kept the transparency up to a higher temperature. For example, a 1% GN30 solution was transparent even at 37 °C. However, with an increase of the NIPAM fraction in a polymer, the solution became turbid at lower temperature, e.g., at 33 °C. The turbid appearnce was evidence for the formation of aggregates. A 0.1% GN150 solution began to be opaque from a temperature of around 32 °C, which was the lower critical solution temperature (LCST) of PNIPAM. This opaque solution returned to a transparent one when the temperature fell below 32 °C. This was attributed to reversible micelle organization and disorganization with temperature change through LCST. The micelle formed in this way is composed of a collapsed PNIPAM core and hydrated PGLM hairs. The condition for the formation of inverse micelles, that is, micelles composed of a PGLM core and a PNIPAM shell, was studied next. A mixed-solvent method was applied for this. THF is a poor solvent for PGMA, whereas it is a good solvent for PNIPAM. So, the gradual addition of THF into a MeOH solution of GN90 resulted in the formation of a micelle, in which
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Figure 3. 1H NMR spectra of PGLM-block-PNIPAM in deuterated MeOH and in deuterated MeOH-THF.
Figure 4. SEM view of cross-linked GN particles.
PGMA and PNIPAM composed the core and shell of the micelle, respectively. The dropping rate of THF was a critical factor in the formation of monodisperse micelles. Dropping of THF at 2.5 mL/min into 5 mL of a MeOH solution of GN90 for 18 min gave the best result. The weight-average hydrodynamic diameter of the micelle was about 170 nm, and its polydispersity (the ratio of weight- to number-average diameter) was less than 1.05. Micelle formation was confirmed by NMR spectra of MeOH and MeOH-THF (1/9, v/v) solutions of GN, as shown in Figure 3. The NMR chemical shifts of different 1H’s in PNIPAM-blockPGLM micelles were assigned from them in individual homopolymers of PNIPAM and PGLM. The peaks of j (methyl of GLM, 0.94 ppm), m (methylene of GLM, 3.7 ppm), and l (methyne of GLM, 3.91 ppm) were clearly observed in MeOH but became negligible compared with the obvious peaks assigned to NIPAMs. This was attributed to solidification of GLM in the core of the micelle. Particle Formation by a Cross-Linking Micelle. DVS is a cross-linking reagent for hydroxyl-group-containing polymers. In fact, treatment of the GN micelle with DVS in an alkaline solution resulted in the formation of a particle that cannot separate into individual molecules even in MeOH and water. The scanning electron microscopy (SEM) view of the particles is shown in Figure 4. The number ratio of a DVS molecule to an OH group in GN to give the preferable degree of crosslinking was about 0.1. Too dense cross-linking suppressed sufficient swelling of the particle in water and prevented permeation of foreign materials into the particle. The hydrodynamic diameters of the GN90 particle were 202 nm in MeOH and 250 nm in water at room temperature. The dispersion of the particles lost the transparency above 32 °C and included tiny aggregates. This was because a collapsed PNIPAM shell
Figure 5. Transmittance of dispersions of magnetic composite microspheres and bare magnetite particles as a function of the aging time at room temperature.
had no steric stabilizing effect above the LCST of PNIPAM.12 However, the aggregates were so small that they did not easily sediment. Preparation of Magnetite/GN Composite Particle. Ferric and ferrous ions interact with polymers having OH groups via a coordination reaction.13 The core of a GN particle includes large amounts of OH groups. So, GN particles can invite ferric and ferrous ions from the medium to their core. Standing the dispersion at pH 12 for 30 min resulted in conversion of ferric and ferrous ions to magnetite. The ratio of GN to ferric and ferrous chlorides was the important factor to get a magnetite/ GN composite particle in a stable state. The selected conditions were [GN] g 0.067% and GN/magnetite ) 3 in weight. A too small amount of GN gave insufficient composite formation, and a too large amount of GN resulted in the formation of a large amount of unconjugated polymer. The composite particles prepared under the best conditions dispersed very stably and kept the turbidity of dispersion for a long time, as shown in Figure 5. When they were centrifuged at 15 000 rpm for 20 min, all of the particles settled down without leaving free GN particles in the serum. X-ray diffraction analysis for the prepared composite particle revealed that the composite particle included just Fe3O4. The technology of the in situ formation of magnetite in a polymer particle was adopted to form composite particles starting from latex particles prepared by emulsifier-free emulsion copolymerization of glycidyl methacrylate and NIPAM.14 In the
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Literature Cited
Figure 6. Transmittance of the dispersion of magnetic microgels contacting with a magnet at different temperatures.
study, successful entrapment of magnetite in the core of particles was clearly observed on the transmission electron microscopy view.14 When magnetite was formed from ferric and ferrous ions under alkaline conditions in the absence of GN, a coarse magnetite aggregate precipitated spontaneously (Figure 5). Therefore, it was confirmed that GN served as a stabilizer for magnetite nanoparticles at a temperature lower than LCST of NIPAM. The dispersion of magnetite/GN composite particles in a cell did not show any change even if a magnet was applied to the cell wall at a temperature lower than the LCST of PNIPAM. The dispersion stability of composite particles decreased at a temperature above the LCST of PNIPAM for the same reason as that for destabilization of GN particles. The resulting tiny aggregates could be easily collected from the dispersion by a magnet, as presented in Figure 6. This result clearly shows that an individual magnetite/GN composite particle cannot be collected by a magnet but aggregated particles can. Kondo et al. reported a similar result using magnetic particles prepared by emulsion polymerization.15 Conclusions A designed block copolymer of PGLM-block-PNIPAM was prepared by quasi-living radical polymerization using a photosensitive iniferter. The block copolymer self-assembled in a MeOH-THF solution to form PGLM core/PNIPAM shell micelles. After the core was cross-linked, the micelles were mixed with a solution of FeCl2 and FeCl3. Following an alkali treatment brought about in situ magnetite nanoparticle formation in the core. The composite particles thus obtained exhibited temperature-dependent magnetic behavior.
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ReceiVed for reView December 25, 2007 ReVised manuscript receiVed April 25, 2008 Accepted May 2, 2008 IE7017614
