Emulsifier-Free, Organotellurium-Mediated Living Radical Emulsion

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Macromolecules 2011, 44, 263–268

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DOI: 10.1021/ma1022346

Emulsifier-Free, Organotellurium-Mediated Living Radical Emulsion Polymerization of Styrene: Effect of Stirring Rate Hirotaka Moribe, Yukiya Kitayama, Toyoko Suzuki, and Masayoshi Okubo* Department of Chemical Science and Engineering, Graduate School of Engineering, Kobe University, Kobe 657-8501, Japan Received September 28, 2010; Revised Manuscript Received November 30, 2010

ABSTRACT: The effect of stirring rate on control/livingness and particle formation in emulsifier-free organotellurium-mediated living radical emulsion polymerization (emulsion TERP) of styrene at 60 °C was investigated at two stirring rates (220 and 1000 rpm), in which styrene phase floated as a layer on an aqueous phase at 220 rpm or dispersed as droplets at 1000 rpm. A water-soluble TERP agent, poly(methacrylic acid) (PMAA)-methyltellanyl (PMAA30-TeMe) (degree of polymerization of PMAA, 30), and a water-soluble thermal initiator, 4,40 -azobis(4-cyanovaleric acid), at high pH were used. The polymerization rate was not affected by the stirring rate, but the control/livingness was significantly improved when the stirring rate was increased from 220 to 1000 rpm. This difference would be caused by a larger amount of consumed PMAA30TeMe in the aqueous phase and higher monomer concentration inside polymerizing particles as polymerization loci at 1000 rpm than at 220 rpm. The stirring rate also affected the particle size distribution: both nanometer-sized and submicrometer-sized particles were prepared at 220 rpm, and mainly nanometer-sized particles were prepared at 1000 rpm. From these results, it is concluded that the stirring rate is an important parameter in emulsion TERP to obtain good control/livingness and control particle formation.

Introduction Controlled/living radical polymerization (CLRP) has dramatically developed as a powerful tool for the preparation of welldefined polymer having a narrow molecular weight distribution (MWD) and complex macromolecular architecture in the past 15 years.1-4 Several methods were developed as CLRP techniques, for example, nitroxide-mediated living radical polymerization (NMP),5-8 atom transfer radical polymerization (ATRP),9-14 and reversible addition-fragmentation chain transfer (RAFT) polymerization.15-17 Recently, Yamago and co-workers developed a novel CLRP technique: organotellurium-mediated living radical polymerization (TERP). TERP has a sufficiently high chain transfer constant (Cex), which is important for the control/livingness, and does not form intermediate radicals, resulting in the less selectivity of macroinitiator for the preparation of block copolymer. Until now, TERP has been developed mainly in homogeneous systems such as bulk and solution polymerizations since 2002.18-26 In recent years, the applications of CLRP techniques to aqueous heterogeneous systems, such as miniemulsion polymerization, microemulsion polymerization, and emulsion polymerization, have been taken notice because of the nontoxic environmentally friendly media.4,27 Because control agent used for CLRP was so far normally hydrophobic, most of the researches were mainly studied in miniemulsion polymerization, which proceeds ideally in monomer droplets.28-32 Emulsion polymerization is industrially useful; however, it had been difficult to apply CLRP to the system from a viewpoint of controlled/livingness because the hydrophobic control agent does not smoothly transfer from monomer droplets into micelles as polymerization loci via an aqueous phase, although a part of hydrophobic control agent was also *Corresponding author. Tel/Fax: þ81-78-803-6161. r 2010 American Chemical Society

partitioned in the monomer-swollen micelles.33-35 For example, in the case of RAFT polymerization in emulsion system, the unreacted RAFT agent remained even after polymerization.34 In order to solve this problem, Hawkett and co-workers proposed a novel efficient method, which is an emulsifier-free RAFT emulsion polymerization utilizing the self-assembly mechanism.36,37 Since then, some successful researches of applying NMP38-44 and RAFT45-51 in emulsion polymerization systems have been reported. Hawkett and co-workers employed monomer feed method throughout the polymerization where there was no monomer droplet in the system to prevent the adsorption of RAFT agent having surface activity at monomer/water interface.37 The adsorption would hinder RAFT agent from entering polymerization loci smoothly, resulting in bad control of molecular weight distribution. However, it needs a long polymerization time to carry out emulsion polymerization without monomer droplet because monomer must be slowly added to the system. In a previous work, employing the self-assembly approach, we succeeded to apply TERP of n-butyl acrylate (BA) to a batch emulsifier-free emulsion polymerization system (emulsion TERP) using a highly hydrophilic TERP agent, which is poly(methacrylic acid) (PMAA)-methyltellanyl (PMAA30-TeMe) (degree of polymerization of PMAA: 30) to suppress the adsorption of control agent at monomer/water interface.52 It was confirmed that stirring rate significantly affected the polymerization rate (Rp), MWD of the obtained polymers, and particle size distribution. Such phenomena due to the stirring rate have not been revealed yet. Styrene is commonly used as a monomer in a typical model system of emulsion polymerization and poly(styrene) (PS) particles with a higher glass temperature (Tg) than room temperature, which is very convenient to estimate accurately the particle size with a transmission electron microscope (TEM), whereas poly(BA) (PBA) particles with much lower Tg than room temperature flatten out on a TEM grid. It was already confirmed that Published on Web 12/31/2010

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emulsion TERP of styrene at 220 rpm proceeded, maintaining livingness.53 In this article, emulsion TERP of styrene will be carried out at two different stirring rates of 220 and 1000 rpm to clarify the effects of stirring rate in the emulsion TERP on Rp, control/ livingness, and particle size distribution, in comparison with the previous results obtained in the emulsion TERP of BA.52 Experimental Part Materials. Styrene (Nacalai Tesque, Japan) was purified by distillation under reduced pressure in a nitrogen atmosphere. Deionized water used in all experiments was obtained using an Elix UV (Millipore Japan) purification system and had a resistivity of 18.2 MΩ cm-1. 4,40 -Azobis(4-cyanovaleric acid) (V-501, Wako Pure Chemicals, Japan) was purified by recrystallization in water. PMAA30-TeMe (ratio of weight-average molecular weight (Mw) to number-average molecular weight (Mn) (Mw/Mn) ∼ 1.1) was supplied from Otsuka Chemical Co., Ltd., Osaka, Japan, and trimethylsilyldiazomethane (TMSD, Nacalai Tesque) was used as received. Emulsion TERP of Styrene. A typical procedure is described below. Water and styrene were separately deoxygenized by nitrogen bubbling. First, V-501 (10.5 mg, 37.8 μmol) and NaOH aqueous solution (45 g, 1.68  10-3 M, equivalent to carboxyl groups of V-501) were added into a round-bottom Schlenk flask, which was sealed off with a silicon rubber septum and then degassed using several N2/vacuum cycles. PMAA30-TeMe (295 μL, 0.127 M aqueous solution neutralized by NaOH (pH = 13), 37.8 μmol) was injected into the system using a syringe. After styrene (1.57 g, 15 mmol) had been injected, and the mixture was degassed again using several vacuum/N2 cycles, the flask was then placed in a water bath at 60 °C (taken to be the start of the polymerization, t = 0). The polymerization was allowed to proceed for a given time at two different stirring rates with a magnetic stirrer. Characterization. Conversion was measured by gravimetry. Mn and MWD were measured by gel permeation chromatography (GPC) using two styrene/divinylbenzene gel columns (TOSOH Corp., TSKgel GMHHR-H, 7.8 mm i.d  30 cm) using THF as an eluent at 40 °C at a flow rate of 1.0 mL/min employing refractive index (RI) (TOSOH RI-8020/21) and ultraviolet (UV) detectors (TOYO SODA UV-8II). The columns were calibrated with six standard PS samples (1.05  103-5.48  106, Mw/Mn = 1.01-1.15). Before GPC measurement, PMAA30-TeMe and PMAA30-b-PS-TeMe were modified by methylation of the carboxyl group using TMSD as follows. After acidification of the carboxyl group, each polymer was recovered by drying the polymer emulsion. The polymer was dissolved in a mixture of dimethylformamide and methanol at room temperature. A yellow solution of TMSD was added at room temperature into the polymer solution, and the reaction was allowed to continue overnight. After excess TMSD was destroyed by acetic acid, poly(methyl methacrylate) (PMMA)30TeMe and PMMA30-b-PS-TeMe solutions were diluted with a large amount of THF and used for GPC measurement. Theoretical molecular weight (Mn,th) was calculated by the following equation: Mn, th ¼ MWPMAA30 -TeMe þ

½M0 MWM R ½PMAA-TeMe0

ð1Þ

where R is the conversion of monomer, MWPMAA30-TeMe and MWM are the molecular weights of PMAA30-TeMe and styrene, respectively, and [PMAA30-TeMe]0 and [M]0 are the initial concentrations of PMAA30-TeMe and monomer, respectively. The consumption of the PMAA30-TeMe was estimated by transforming from the MWDs to the number distributions (w(log M)/M2) vs M. The weight distributions (w(log M)/M) vs M were normalized with respect to each conversion before the

Figure 1. Photographs indicating mixing states at (a) 220 and (b) 1000 rpm before emulsion TERP. Styrene phase was dyed with Oil Blue.

Figure 2. Conversion vs time plots for emulsion TERP using PMAA30TeMe and V-501 at 60 °C with stirring at 220 rpm (open circles) and 1000 rpm (closed circles). Styrene/PMAA30-TeMe/V-501 (molar ratio) = 400/1/1.

transformation. The number of chains could be calculated as the integral of the number distributions; therefore, the number of remaining PMAA30-TeMe was obtained. Monomer concentration in polymer particles ([M]p) was estimated using gas chromatography (GC, GC-18A, Shimadzu Co.) according to a previous article.54 Particle size distribution and number-average particle diameter (dn) were measured using a TEM (JEOL JEM-1230) and dynamic light scattering (DLS, FPAR-1000 RK, fiber-optics particle analyzer, Photal Otsuka Electronics, Osaka, Japan). Before the TEM observation, each emulsion was diluted to ∼50 ppm, and then a drop was placed on a carbon-coated copper grid and dried at room temperature in a desiccator. In the DLS measurement, 1-2 droplets of emulsion samples withdrawn from reactor were diluted with ∼8 mL of distilled water before measurement in the dilution mode at the light scattering angle of 90° at 25 °C using the Contin analysis routine.

Results Polymerization Rate. In this article, emulsion TERP of styrene was carried out at 60 °C at two different stirring rates of 220 and 1000 rpm. Figure 1 shows photographs indicating mixing states at 220 (a) and 1000 rpm (b) before starting polymerization, where monomer phases were dyed with Oil Blue for clear observation of the dispersion states. Actual polymerizations were conducted without dyeing. The stirring rate greatly affected the dispersed state of styrene. At 220 rpm, most of styrene floated as an upper layer; on the other hand, at 1000 rpm a large number of styrene droplets were dispersed in the aqueous medium. Figure 2 shows conversion vs time plots for emulsion TERP of styrene at 60 °C with stirring at 220 and 1000 rpm. Both polymerizations smoothly proceeded without an induction period with a similar rate, and the polymerizations were almost completed in 30 h. In emulsion TERP of BA in

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the previous work, Rp at 1000 rpm was faster than at 220 rpm.52 Control/Livingness. Parts a and b of Figure 3 show MWDs (RI detector) of PS at various conversions of the emulsion TERP at 220 and 1000 rpm, respectively. In both systems, MWDs shifted to higher molecular weight with increasing conversion, indicating that both polymerizations maintained living nature. The MWD of PS obtained was narrower at 1000 rpm than at 220 rpm. These behaviors were similar to those in emulsion TERP of BA at the two stirring rates.52 At 220 rpm, a peak due to PMAA30-TeMe was observed at a low molecular weight (log M = 3.5) until high conversion. On the other hand, at 1000 rpm, PMAA30-TeMe was consumed faster than at 220 rpm. This consumption of PMAA30-TeMe behavior will be discussed later. Parts a and b of Figure 4 show Mn (open circles) and Mw/ Mn (closed circles) of PS at different conversions of the

Figure 3. MWDs (RI detector) of PMMA30-b-PS-TeMe (after methylation) at different conversions (as indicated in %) of emulsion TERP using PMAA30-TeMe and V-501 at 60 °C with stirring at 220 (a) and 1000 rpm (b). Styrene/PMAA30-TeMe/V-501 (molar ratio) = 400/1/1.

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emulsion TERP at 220 and 1000 rpm, respectively. Mn and Mw/Mn values were estimated from the peak due to obtained polymer in the MWDs shown in Figure 3 after being divided into the peaks due to PMAA30-TeMe and the obtained polymer. Since, at 1000 rpm, the two peaks in the MWDs at low conversions (18 and 37%) could not be divided because of overlapping peaks with each other, the Mw/Mn values of these polymers look slightly high compared to the other polymers. At 1000 rpm, Mn increased linearly throughout the polymerization, while at 220 rpm, Mn increased linearly up to 80% conversion but decreased at the final stage of the polymerization. The decreasing Mn was caused by newly formed polymers having low Mn.53,55 Experimental Mn at 1000 rpm was very close to theoretical Mn (Mn,th) compared to that at 220 rpm. The Mw/Mn values at 1000 rpm were sufficiently low (∼1.3) relative to those at 220 rpm except for the first stage. Particle Size Distribution. Figure 5 shows particle size distributions (weight distribution) measured by DLS of PS emulsions prepared by the emulsion TERP at 60 °C with stirring at 220 rpm (a: 98% conversion) and 1000 rpm (b: 90% conversion). The size distribution of PS particles obtained at 220 rpm was bimodal distribution consisting of nanometer- and submicrometer-size (dn = 40 and 150 nm), and the volume ratio of nanometer-sized particles to submicrometer-sized ones was approximately unit, while that obtained at 1000 rpm was a monomodal distribution consisting of nanometer size (dn = 50 nm). TEM photographs of the PS particles are shown in Figure 6. Similarly to the DLS measurements (Figure 5), nanometer- and submicrometersized particles were observed at 220 rpm, and most of the particles were nanometer-sized at 1000 rpm. Both final particle size distributions in the different stirring systems in the emulsion TERP of styrene were similar to those in the emulsion TERP of BA.52 Discussion

Figure 4. Mn (open circles) and Mw/Mn (closed circles) of PMMA30-bPS-TeMe (after methylation) at different conversions of emulsion TERP using PMAA30-TeMe and V-501 at 60 °C with stirring at 220 (a) and 1000 rpm (b). Styrene/PMAA30-TeMe/V-501 (molar ratio) = 400/1/1. Full line is Mn,th.

In general emulsion polymerization, high [M]p increases Rp. In emulsion TERP of styrene, [M]p values were always significantly higher at 1000 rpm than at 220 rpm except for the final stage of the polymerization (Figure 7). Because total monomer/water interfacial area was much larger at 1000 rpm than at 220 rpm, the monomer should smoothly transfer to polymerizing particles across the aqueous phase. Although it was presumed that higher [M]p at 1000 rpm induces faster Rp than at 220 rpm, which was actually observed in the emulsion TERP of BA,52 the Rp values in both systems of styrene were similar. This phenomenon might be caused by the lower [M]p at 220 rpm. In our unpublished work, Tg values of styrene-methacrylic acid copolymer (P(S-MAA),

Figure 5. Particle size distributions (weight distribution) (measured by DLS) of PS emulsion prepared by emulsion TERP of styrene using PMAA30-TeMe and V-501 at 60 °C with stirring at 220 rpm (a: 98% conversion) and 1000 rpm (b: 90% conversion). Styrene/PMAA30-TeMe/ V-501 (molar ratio) = 400/1/1.

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Figure 6. TEM photographs of PS particles prepared by emulsion TERP using PMAA30-TeMe and V-501 at 60 °C with stirring at 220 rpm (a: 98% conversion) and 1000 rpm (b: 90% conversion). Styrene/PMAA30-TeMe/V-501 (molar ratio) = 400/1/1.

Figure 7. Monomer concentrations in the PS particles ([M]p) at different conversions of emulsion TERP using PMAA30-TeMe and V-501 at 60 °C with stirring at 220 rpm (open circles) and 1000 rpm (closed circles). Styrene/PMAA30-TeMe/V-501 (molar ratio) = 400/1/1.

10 mol % MAA) particles containing various amounts of styrene (that is, at various [M]p) were measured with a power compensation-type highly sensitive differential scanning calorimeter. Because Tg,P(S-MAA) at 1.4 mol/(L particles) of [M]P was ∼60 °C, the viscosity of polymerizing particles in the emulsion TERP of styrene at 60 °C with stirring at 220 rpm where [M]p was 1-2 mol/(L particles) in the conversion range of 10-70% would be extremely high, resulting in slow diffusion of polymer chains and decrease of rate constant of termination reaction (kt) between two polymer radicals.56 The decrease of kt increases average number of radicals per particle. On the other hand, the rate constant of propagation reaction (kp) between radical and monomer should barely change below Tg even under high viscosity.57 As a result, Rp at 220 rpm became faster than that presumed as above. In the emulsion TERP of BA, because Tg,PBA is much lower than polymerization temperature: 60 °C (Tg,PBA = -54 °C, Tg,PS = 100 °C),58 viscosity in polymerizing particles would not be so high even if it proceeds at low [M]p at 220 rpm that kt does not change much. Consequently, the effect of stirring rate on Rp in emulsion TERP depends on Tg of polymerizing particles. TERP proceeds via two control mechanisms, which are thermal dissociation (TD) and degenerative chain transfer (DT), in which the main control mechanism is DT at 60 °C.20 Two factors could be considered about the reason for the improving control/livingness by increasing stirring rate from 220 to 1000 rpm (Figure 4), in which the total monomer/water interfacial area markedly increased. One would be lower viscosity inside polymerizing particles due to higher [M]p at 1000 rpm than 220 rpm (Figure 7). Although the chain transfer reaction in the TERP is also a reaction between radical and dormant species having a certain level of molecular weight, high diffusion of the reactants (radical and dormant species) at 1000 rpm should allow chain transfer reaction to

Moribe et al.

Figure 8. Consumptions of PMAA30-TeMe at different conversions of emulsion TERP using PMAA30-TeMe and V-501 at 60 °C with stirring at 220 rpm (open circles) and 1000 rpm (closed circles). Styrene/ PMAA30-TeMe/V-501 (molar ratio) = 400/1/1.

proceed smoothly, resulting in good control/livingness. However, because the viscosity inside polymerizing particles was higher due to lower [M]p at 220 rpm than 1000 rpm, slow diffusion of the reactants results in decrease of the chain transfer rate constant (ktr) similarly to the decrease of kt as mentioned above. It seems that higher [M]p leads to a greater number of monomer units added per activation-deactivation step and became less control over the MWD at 1000 rpm. [M]p at 1000 rpm (ca. 5.3 mol/(L particles)) was ∼3 times as high as that at 220 rpm (ca. 1.9 mol/(L particles)), which leads that the Rp at 1000 rpm was 3 times larger than that at 220 rpm. On the other hand, chain transfer reaction would be diffusion controlled similarly to termination reaction because both chain transfer reaction and termination reaction were polymer-polymer reactions. The diffusion controlled kt in bulk system was investigated by Zetterlund and coworkers.56 In the paper, the kt decreased 100 times from conversion 45% ([M]: 5.5 mol/L) to 80% ([M]: 2.0 mol/L). Therefore, Rtr would be significantly (ca. 100 times) larger at 1000 rpm than at 220 rpm. Thus, lower Mw/Mn value at 1000 rpm is based on higher Rtr. On the other hand, the kp barely changes even under high viscosity as described above. As a result, control/livingness became bad at 220 rpm. The other factor to obtain the high control/livingness at 1000 rpm is the high consumption of the control agent, which is important for maintaining a narrow MWD. Figure 8 shows the consumption of PMAA30-TeMe at different conversions of the emulsion TERP at 220 and 1000 rpm. At 220 rpm, more than 30% PMAA30-TeMe still remained even at 98% conversion. On the other hand, at 1000 rpm, 90% PMAA30-TeMe was consumed until 90% conversion, indicating that larger amount of PMAA30TeMe could participate in the polymerization. In a DT system, higher concentration of control agent in polymerization loci would lead to narrower MWD,59 which accorded with the result that MWD was sufficiently controlled at 1000 rpm compared to 220 rpm (Figure 3). In Figure 4a,b, Mn,th was calculated using eq 1 assuming that PMAA30-TeMe is completely consumed at the beginning of the polymerization. Because the large differences between Mn,th and Mn at 220 rpm might be caused by the gradual consumption of PMAA30-TeMe, theoretical Mn, which is estimated by defining M*n,th, was calculated using eq 2. M n, th ¼ MWPMAA30 -TeMe þ

½M0 MWM R ½PMAA-TeMe0 β

ð2Þ

where β is the fraction of consumed PMAA30-TeMe. However, contrary to our expectation, the difference between M*n,th and Mn only became a little lower and still persisted. In a previous

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Table 1. Number-Average Particle Sizes (dn), Mn,th, Mn, and Mw/Mn of Centrifugally Separated Fraction of PS Particles, Which Were Prepared by Emulsion TERP Using PMAA30-TeMe and V-501 at 60 °C with Stirring at 220 rpm (Conversion 98%) and 1000 rpm (Conversion 90%) stirring rate (rpm) 220 1000

size submicrometer nanometer submicrometer nanometer

weight fraction (%) 51 49 1 99

article, it is already revealed that nanometer- and submicrometersized particles were generated from self-assembly and homogeneous nucleations, respectively.55 On the basis of the particle size distributions (Figures 5a and 6a), it would be an alternative explanation that the large difference between Mn,th and Mn at 220 rpm is attributed to the presence of the two particle nucleation mechanisms. Thus, polymerizing particles generated from the homogeneous nucleation, polymerization was barely controlled by TERP because of low concentration of TeMe derivatives in the polymerizing particles, resulting in polymer having much higher molecular weight than Mn,th, although it was well controlled in polymerizing particles generated from the selfassembly nucleation because of high concentration of TeMe derivatives. This explanation was supported by results (Table 1) that Mn of polymer obtained from submicrometer-sized particles after separation by centrifugation was higher than that obtained from nanometer-sized particles at both stirring rates. The interfacial tensions of a styrene/water interface and styrene/ PMAA30-TeMe aqueous solution interface measured using the pendant drop method were 35.1 and 21.4 mN/m, respectively. These results indicate that a certain amount of PMAA30-TeMe had been adsorbed at the interface between styrene and water until the monomer phase disappeared. It seems to be extremely low in the adsorption state to react with initiator (and oligomer) radicals formed in the aqueous phase, which should delay the consumption of PMAA30-TeMe. The total monomer/water interfacial area was much larger at 1000 rpm than at 220 rpm. Accordingly, it presumed that the percentage of PMAA30-TeMe adsorbed at the monomer/water interface must be much larger at 1000 rpm, resulting in worse living control. However, the obtained result was quite contrary as shown in Figure 5. Considering the result, there must be other factors induced by the different stirring rates affecting the consumption of PMAA30-TeMe. As indicated in Figure 7, [M]p values were higher at 1000 rpm than at 220 rpm; in other words, transfer of monomer to polymerizing particles at 1000 rpm was faster than at 220 rpm. As a result, the monomer phases disappeared at a lower conversion (40%) at 1000 rpm than at 220 rpm (70%). Therefore, PMAA30-TeMe that had been adsorbed at the monomer/water interface moved into the aqueous phase at lower conversion at 1000 rpm than at 220 rpm, resulting in the rapid consumption of PMAA30-TeMe and better living control at 1000 rpm. At 1000 rpm, the consumption of PMAA30-TeMe was also faster than at 220 rpm in the early stage of the polymerization around 20% conversion (Figure 8). This might be induced by difference of monomer concentration in aqueous phase in the early stage of the polymerization at the different stirring conditions. The difference of monomer concentration would affect particle nucleation in an early stage of the emulsion TERP. This will be discussed in detail in the near future. Conclusions Emulsion TERP of styrene was successfully carried out in the two different stirring systems (220 and 1000 rpm) using PMAA30TeMe and V-501 at 60 °C. In both stirring systems, Rp values were of the same degree, which was different from the result in emulsion TERP of BA. The difference based on the kind of monomer seems to depend on Tg of polymerizing particles.

dn (nm)

Mn,th

130 34 110 31

4.4  10 4.4  104 4.1  104 4.1  104

Mw/Mn

Mn 4

1.4  10 9.0  104 6.9  104 4.5  104 5

2.9 2.7 1.5 1.3

MWD was sufficiently controlled at 1000 rpm compared to that at 220 rpm, where the disappearance of monomer phase was earlier at 1000 rpm than at 220 rpm. After the disappearance of monomer phase, PMAA30-TeMe that had been adsorbed at monomer/water interface should work as control agent. The stirring rate seriously affected the particle size distribution and MWD similarly to the emulsion TERP of BA. The final particle size distribution at 220 rpm was bimodal with nanometer- and submicrometer-sized, whereas that at 1000 rpm was monomodal with nanometer-sized. From these results, it is concluded that the stirring rate is an important factor to control MWD and particle size distribution in emulsion TERP. In the following work, the differences of the MWD and particle size distribution due to the stirring rate will be discussed in terms of the particle nucleation, focusing on the early stage of the polymerization. Acknowledgment. The authors are grateful to Otsuka Chemical Co., Ltd., for supplying organotellurium compounds. This work was partially supported by Grant-in-Aid for Scientific Research (A) (Grant 21245050) from the Japan Society for the Promotion of Science (JSPS) and by Research Fellowship of JSPS for Young Scientists (given to Y.K.). References and Notes (1) Matyjaszewski, K. Controlled/Living Radical Polymerization: Progress in ATRP, NMP, and RAFT; American Chemical Society: Washington, DC, 2000. (2) Matyjaszewski, K. Advances in Controlled/Living Radical Polymerization; American Chemical Society: Washington, DC, 2003. (3) Braunecker, W.; Matyjaszewski, K. Prog. Polym. Sci. 2007, 32, 93– 146. (4) Zetterlund, P. B.; Kagawa, K.; Okubo, M. Chem. Rev. 2008, 108, 3747–3794. (5) Georges, M. K.; Veregin, R. P. N.; Kazmaier, P. M.; Hamer, G. K. Macromolecules 1993, 26, 2987–2988. (6) Hawker, C. J.; Bosman, A. W.; Harth, E. Chem. Rev. 2001, 101, 3661–3688. (7) Nakamura, T.; Zetterlund, P. B.; Okubo, M. Macromol. Rapid Commun. 2006, 27, 2014–2018. (8) Zetterlund, P. B.; Okubo, M. Macromolecules 2006, 39, 8959–8967. (9) Kato, M.; Kamigaito, M.; Sawamoto, M.; Higashimura, T. Macromolecules 1995, 28, 1721–1723. (10) Wang, J.; Matyjaszewski, K. J. Am. Chem. Soc. 1995, 117, 5614–5615. (11) Kamigaito, K.; Ando, T.; Sawamoto, M. Chem. Rev. 2001, 101, 3689–3745. (12) Matyjaszewski, K.; Xia, J. Chem. Rev. 2001, 101, 2921–2990. (13) Kagawa, Y.; Zetterlund, P. B.; Minami, H.; Okubo, M. Macromol. Theory Simul. 2006, 15, 608. (14) Kagawa, Y.; Zetterlund, P. B.; Minami, H.; Okubo, M. Macromolecules 2007, 40, 3062–3069. (15) Chiefari, J.; Chong, Y. K.; Ercole, F.; Krstina, J.; Jeffery, J.; Le, T. P. T.; Mayadunne, R. T. A.; Meijs, G. F.; Moad, C. L.; Moad, G.; Rizzardo, E.; Thang, S. H. Macromolecules 1998, 31, 5559– 5562. (16) Moad, G.; Chong, Y. K.; Postma, A.; Rizzardo, E.; Thang, S. H. Polymer 2005, 46, 8458–8468. (17) Moad, G.; Rizzardo, E.; Thang, S. H. Aust. J. Chem. 2006, 59, 669– 692. (18) Yamago, S.; Iida, K.; Yoshida, J. J. Am. Chem. Soc. 2002, 124, 2874–2875. (19) Yamago, S.; Iida, K.; Yoshida, J. J. Am. Chem. Soc. 2002, 124, 13666–13667. (20) Goto, A.; Kwak, Y.; Fukuda, T.; Yamago, S.; Iida, K.; Nakajima, M.; Yoshida, J. J. Am. Chem. Soc. 2003, 125, 8720–8721.

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