Dispersion Polymerization of MMA Using Polystyrene- Block-Poly(4-Vinylpyridine) Prepared by a RAFT

SHORT COMMUNICATION

Dispersion Polymerization of MMA using Polystyrene- block-poly(4-vinylpyridine) Prepared by a RAFT

Jung Min Lee, Sang Eun Shim, and Soonja Choe Department of Chemical Engineering, Inha University, Incheon 402-751, Korea

Received March 21, 2006; Accepted May 28, 2006

Abstract: Poly(methyl methacrylate) (PMMA) particles were produced through dispersion polymerization using poly(styrene)-block-poly(4-vinylpyridine) block copolymer as a steric stabilizer in methanol. Low- molecular-weight-distribution block copolymers were synthesized using 4-toluic acid dithiobenzoate through a reversible addition-fragmentation chain transfer (RAFT) method. Stable polymer particles were obtained when the block copolymer concentration was 1, 4, or 10 wt% relative to monomer. The average particle size (2.4 3.4 µm) varied depending on the concentration of the block copolymer. As the block copolymer concentration increased, the size of the particles decreased. In particular, 4 wt% of the block copolymer produced 2.7 µm monodisperse PMMA (Cv = 3.3 %). The existence of the block copolymer on the PMMA surface was verified through XPS measurements.

Keywords: dispersion polymerization, poly(s-b-4vp), RAFT method, block copolymer

Introduction anchor to the surface of polymer particles and the other 1) lipophilic segment can extend into the continuous phase, Polymer particles have a wide range of applications, can provide a steric barrier to prevent coagulation of including standard samples for instrument calibration, polymer particles. Therefore, control over the molecular column packing materials in chromatography, and micro- weight and molecular weight distribution of block electronic materials [1-5]. In particular, monodisperse copolymer is important for successful dispersion polym- and size-controlled polymer particles are of interest from erization. Most block copolymers have been synthesized both academic and industrial points of view. There are using meticulous and time-consuming living ionic poly- many methods for preparing micron-sized polymer par- merizations [16], but attempts have been made recently ticles. Among them, dispersion polymerization has been to synthesize block copolymers employing other simple used for the past three decades to prepare micrometer- and easy techniques. Among them, controlled/‘living’ scale monodisperse polymer particles in a single step radical polymerization is a particularly useful technique [6-8]. for the preparation of well-defined polymer structures. In dispersion polymerization, steric stabilizers can be The three main techniques capable of inducing living divided into homopolymers, macromonomers [9], and behavior are nitroxide-mediated polymerization (NMP) block [10] or graft copolymers. The commercial homo- [17,18], atom transfer radical polymerization (ATRP) polymer stabilizers are poly(N-vinyl pyrrolidone) (PVP) [19,20], and reversible addition-fragmentation chain [11,12], metharylate polymers [13], and various cellu- transfer polymerization (RAFT) [21,22]. lose derivates [2,14]. It is well known that diblock and In this study, controlled poly(styrene-b-4-vinylpyridine) triblock copolymers are effective stabilizers in hetero- block copolymer was synthesized using the RAFT phase polymerizations [15]. It can be inferred that method and applied to the dispersion polymerization of amphiphilic block copolymers, in which one segment can methyl methacrylate (MMA) as an efficient steric stabilizer. To whom all correspondence should be addressed. (e-mail: [email protected]) Dispersion Polymerization of MMA using Polystyreneblock-poly(4-vinylpyridine) Prepared by a RAFT 649

Experimental Section

The dispersion polymerization ingredients were methyl methacrylate, styrene (Junsei Chemicals, Japan), 4- vinylpyridine (Aldrich, USA), 2,2-azobis(isobutyronitrile) (AIBN, Junsei), methanol (Samchun Chemical, Korea), and 4-toluic acid dithiobenzoate (CTA3). CTA3 was prepared using a modification of a procedure described elsewhere [23]. The block copolymer was synthesized using a RAFT method. This polymerization involved two steps; first, PS homopolymer was prepared by bulk polymerization in the presence of styrene (40 mL), AIBN (0.0112 g), and CTA 3 (0.39 g) as a RAFT agent. The solution was o sealed and completely sunk into an oil bath at 70 C. After polymerization, the reaction product was pre- cipitated in methanol and purified through repeated precipitation. A stock solution of PS (1 g), AIBN (0.02 g), 4-vinylpyridine (7 mL), and DMF (10 mL) was prepared under anhydrous conditions. The polymerization process for the preparation of P(S-b-4VP) was the same as that used for the preparation of PS. Dispersion polymerization was performed in a capped scintillation vial with magnetic stirring under a nitrogen atmosphere. Methanol (25 g), methyl methacrylate (2.5 g), AIBN (0.025 g), and various concentrations of the block copolymer were added to the vial. The polymerization o temperature was fixed at 60 C in an oil bath and after completion of the polymerization, the resultant product was rinsed off with methanol. The chemical structure of the synthesized block copolymer was confirmed by analysis using a Varian 1 400-MHz H-NMR spectrometer with CDCl3 as the solvent. The molecular weight and polydispersity index (PDI) were characterized using a Waters GPC (Gel Permeation Chromatograph). A Hitachi SEM (Scaning Electron Microscope) S-4300 was used to observe the morphology of the PMMA particles. The weight-average diameter (Dw) and coefficient of variation (Cv) were obtained using a Scion Image Analyzer Software by counting 100 of the individual particles from the SEM Figure 1. SEM micrographs of PMMA microspheres prepared with varying concentrations of P(S-b-4VP) in methanol at 60 microphotographs. The surfaces of the PMMA particles o were analyzed using an XPS (X-ray photoelectron C for 12 h. (a) 1 wt% (Cv 5.3 %; Dw 3.4 µm); (b) 4 wt% spectrometer, model ESCALAB 220i-XL); the atomic (Cv 3.3 %; Dw 2.7 µm); (c) 10 wt% (Cv 7.31; Dw 2.4 µm). concentrations were calculated based on the program (spectral data processor, v 4.1). multiplets at 6.1 6.8 and 8.2 8.5 ppm were observed for the four protons on pyridine ring. The P(S-b-4VP) spectrum showed peaks characteristic of both PS and Results and Discussion P4VP, confirming the synthesis of the P (S-b-4VP) block copolymer. The number-average molecular weight and 1 H-NMR spectra were obtained to confirm the chemical polydispersity index (PDI) of P(S-b-4VP) were 98176 structures of the synthesized PS, P4VP, and P(S-b-4VP). g/mol and 1.28, respectively, and those of the PS block The spectrum of PS displayed the five protons of the in P (S-b-4VP) were 11388 g/mol and 1.11, respectively. phenyl ring in the range 6.1 7.1 ppm. For P4VP, two This finding implies that the block copolymer contains a 650 Jung Min Lee, Sang Eun Shim, and Soonja Choe

we believe that the copolymer molecules are located preferentially on the surfaces of the particles. Therefore, the poly(4-vinylpyridine) segments can provide a steric barrier to prevent coagulation of the polymer particles. Figure 2 shows the XPS, spectrum of the PMMA microspheres prepared using 10 wt% block copolymer o for the dispersion polymerization in methanol at 60 C. Samples for surface analysis were prepared by dispersing the particles ultrasonically in pure methanol. A droplet of the suspension was deposited on aluminum foil and left to dry at room temperature. Two strong peaks appear for carbon and oxygen and a small peak for nitrogen at 280, 520, and 400 eV, respectively. The existence of a nitrogen atom peak implies that the particle surface contains P(S-b-4VP), because nitrogen atoms exist only in 4- Figure 2. XPS spectrum of PMMA microspheres prepared by vinylpyridine. The atomic concentrations were calcu- dispersion polymerization in the presence of 10 wt% block o lated using a program; the calculated fractions of each copolymer in methanol at 60 C. element in the PMMA microspheres were 58.1, 36.8, and 5.1 % for carbon, oxygen, and nitrogen, respectively. long poly(4-vinylpyridine) block and a short polystyrene In summary, narrow-molecular-weight-distribution am- block. The block copolymer dissolved well in methanol, phiphilic block copolymers consisting of hydrophilic even though methanol is a precipitator of PS. Thus, all of 4-vinylpyridine and hydrophobic styrene, using 4-toluic the reaction ingredients dissolved in the medium at the acid dithiobenzoate, were synthesized by a RAFT method. beginning of the reaction and the block copolymer is 1 Based on H NMR spectra, we confirmed that P(S-b- believed to behave as a stabilizer in the dispersion poly- 4VP) was successfully synthesized. This P(S-b-4VP) merization. block copolymer was used as a steric stabilizer for the Figure 1 displays SEM images of the PMMA micro- dispersion polymerization of MMA. As a result, 2.4 3.4 spheres prepared by dispersion polymerization in meth- o -µm microspheres were obtained when using 1, 4, and 10 anol at 60 C using various concentrations of P(S-b- wt% of the block copolymer. In particular, when 4 wt% 4VP). By observing the spherical particles, the P(S- of the block copolymer was used relative to the b-4VP) was proved to be used as a stabilizer. The monomer, a very narrow distribution of particle sizes average particle sizes varied within the range 2.4 3.4 (2.72 µm; 3.31 % Cv) was obtained. µm depending on the concentration of the block copolymer. As the block copolymer concentration increased, the PMMA particle diameter decreased; this tendency of Acknowledgment decreasing particle size upon increasing the stabilizer concentration has been observed by others [11,14,24,25]. This study was supported by a Korean a Korea Research In particular, monodisperse microspheres (2.72 µm; 3.31 Foundation Grant (KRF-2004-042-D00055) funded by %Cv) were obtained when using 4 wt% P(S-b-4VP). In the Government (MOEHRD). dispersion polymerization using conventional stabilizers, such as PVP or hydroxylpropyl cellulose (HPC), monodisperse, micron-sized polymer particles result only References when using a high concentration of the stabilizer. Typically, the window is 10 15 wt% of stabilizer for styrene and 30 wt% for methyl methacrylate, based on 1. J. Ugelstad, J. Polym. Sci. Polym. Symp., 72, 225 the amount of monomer [3,11,26]. However, our (1985). experimental results shows that monodisperse PMMA 2. C. K. Ober, K. P. Lok, and M. L. Hair, J. Polym. Sci. microspheres were obtained without coagulation or the Polym. Lett. Ed., 23, 103 (1985). formation of secondary particles when using as little as 4 3. K. P. Lok and C. K. Ober, Can. J. Chem., 64, 209 wt% of stabilizer. This finding implies that using a small (1985). amount of P(S-b-4VP) is more efficient than using a 4.J.G.Park,J.W.Kim,S.G.Oh,andK.D.Suh,J. conventional stabilizer, not only for producing PMMA Appl. Polym. Sci., 87, 420 (2003). microspheres but also for obtaining them in a very 5. T. Isao, Y. Yutaka, N. Atsuo, and G. Yasushi, US narrow distribution of particle size. Because P(S- b-4VP) Patent, 5,001,542 (1991). consists of hydrophobic PS and hydrophilic P4VP units, 6. K. E. Barrett, Dispersion polymerization in organic Dispersion Polymerization of MMA using Polystyreneblock-poly(4-vinylpyridine) Prepared by a RAFT 651

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