Precipitation Polymerization in Acetic Acid - American Chemical Society

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J. Phys. Chem. B 2008, 112, 6914–6922

Precipitation Polymerization in Acetic Acid: Synthesis of Monodisperse Cross-Linked Poly(divinylbenzene) Microspheres Qing Yan, Yaowen Bai, Zhe Meng, and Wantai Yang* State Key Laboratory of Chemical Resource Engineering, Beijing and College of Materials Science and Engineering, Beijing UniVersity of Chemical Technology, Beijing 100029, P. R. China ReceiVed: NoVember 30, 2007; ReVised Manuscript ReceiVed: March 24, 2008; In Final Form: March 24, 2008

This paper reports two important results with cross-linked precipitation polymerization. (1) Acetonitrile, a substance harmful to human health, is the most commonly used solvent for the synthesis of crosslinked polymeric microspheres by precipitation polymerization. Here, the much safer acetic acid replaced acetonitrile as a solvent in the precipitation polymerization of monodisperse cross-linked poly(divinylbenzene) (PDVB-55) microspheres. Pumpkin-like particles and microspheres were obtained. XPS results displayed a significant amount of double bonds on the surface of the particles. The effect of monomer content, temperature, and initiator amount on the formed particles were studied. For a DVB loading below 1 vol % at 70 °C, monodisperse microspheres with smooth surfaces and narrow diameters were successfully obtained. With a DVB loading of 2 vol % and by observing the shapes of particles obtained with three different temperature(60, 70, and 80 °C), we found that more spherical particles were obtained at higher temperatures and pumpkin-like particles were obtained at lower temperatures. No significant differences in morphology or the coefficient of variation (CV) of the particles were obtained for different initiator loadings, whereas the particle diameters could be increased with increased initiator concentrations. (2) In order to obtain a better understanding of the formation mechanism of these particles, time-dependent experiments, for the first time, were conducted in a hydrophobic monomer system. By tracing the whole polymerization process, some important results were found. First, with the polymerization time at 70 °C, the particle diameters were found to increase from 800 nm to 3.0 µm, the CV displayed a decrease, and the amount of spheres and the spherical evenness of the particle surfaces improved. Second, by quantitatively calculating the particle number from the yields and diameters data, it is found that starting from 3.1% yield or two hours reaction time the total amount of particles in the system is almost a constant (about 9.6 × 108/L), which means that no homocoagulation occurred and no new particles were generated after nucleation, and there is a linear relation between cubic diameters and yields. These two results give us a distinct impression that particle growth almost comes from capturing of newly formed oligomers. Based on the above results, a scheme for the particle formation is proposed, which shows that that pumpkin-like particles are caused by a prolonged nucleation including the homocoagulation of primary nuclei. The growth of the particles includes two modes, an in situ surface polymerization of monomer and the adsorption of PDVB-55 oligomers. The differences between results in acetonitrile and in acetic acid (higher yields, smaller size, not spherical but pumpkin-like particles in acetic acid) were due to the lower solubilizability of acetic acid which is the so-called proton-containing solvent with the hydrogen bonding structure. Introduction Monodisperse cross-linked polymeric microspheres are receiving significant attention in applications such as coatings, instrument calibration standards, templates for the preparation of porous materials, chromatographic support materials, etc.1,2 Numerous polymerization methods, including seeded emulsion polymerization, dispersion polymerization, and precipitation polymerization, have been adopted to produce monodisperse micron-sized microspheres. Among these methods, seeded emulsion polymerization3 and dispersion polymerization4–7 require surfactants to stabilize the resulting particles. Dispersion polymerization leads to the single step preparation of monodisperse polymeric microspheres with diameters in the range of 0.1-15 µm. The most important contribution in the synthesis of cross-linked polymer micro* To whom correspondence should be addressed. E-Mail: yangwt@ mail.buct.edu.cn. Tel: +86-10-64432262. Fax: +86-10-64416338.

spheres has resulted from the work of Sto¨ver et al.,8 where a series of polymer particles in the range 2-5 µm with a narrow particle size distribution could be obtained in organic media, usually acetonitrile, via a single step precipitation polymerization without the use of any stabilizers or surfactants. In addition, they proposed an impressive stabilization mechanism of poly(divinylbenzene) microspheres in precipitation polymerization, including the autosteric stabilization from a surfacegel layer, the continuous capture of oligomers from solution, and ultimately a yield of monodisperse microspheres of 0.2-6 µm diameter.9–19,29,30 Bai and Huang developed a novel polymerization technique, i.e., distillation-precipitation polymerization, and prepared highly cross-linked monodisperse polymeric microspheres with diameters between 1.10 and 3.41 µm.20,21 Choe et al. synthesized a series of cross-linked copolymer microspheres, including poly(styrene-co-divinylbenzene), poly(methyl methacrylate-co-divinylbenzene) and poly(acrylamide-

10.1021/jp711324a CCC: $40.75  2008 American Chemical Society Published on Web 05/20/2008

Polymerization in Acetic Acid

J. Phys. Chem. B, Vol. 112, No. 23, 2008 6915

Figure 1. A SEM micrograph of a PDVB-55 powder prepared in acetic acid at 70 °C for 12 h with a DVB loading of 2 vol % and an AIBN loading of 3 wt %. Figure 3. The yield of particles as a function of the polymerization time at three temperatures. DVB loading ) 2 vol %, AIBN loading ) 3 wt %, solvent ) acetic acid.

is a harmful substance, capable of inducing toxic effects similar to those observed in acute cyanide poisoning. Blood cyanide and thiocyanate levels become elevated during acute acetonitrile intoxication of humans,27 and it is therefore necessary to replace acetonitrile in precipitation polymerization by innocuous solvents. Acetic acid is a commercially available product that was selected for the present study as a potential solvent for precipitation polymerization based on the following considerations: first, it is harmless to human beings except for its unpleasant odor; second, it is miscible with water and organic compounds similar to acetonitrile; and third, the boiling point of acetic acid is 117.87 °C, which is high enough for precipitation polymerization. After our former investigation,28 this paper presents the preparation of highly cross-linked poly(divinylbenzene) monodisperse microspheres by using acetic acid as the solvent and discusses the differences between acetic acid and acetonitrile systems based on the physical properties of solvents. Time-dependent experiments were carried out, for the first time, to trace the whole polymerization process to obtain a better understanding of the formation mechanism and evolution of these particles in precipitation polymerization in hydrophobic monomer system. Some principal factors affecting the precipitation polymerization are also investigated. Experimental Methods

Figure 2. XPS results of the PDVB-55 particles displaying (A) the whole spectrum and (B) the carbon 1s spectrum. DVB loading ) 2 vol %, AIBN loading ) 3 wt %, 70 °C, 12 h.

co-divinylbenzene), by precipitation copolymerization.22–24 Knut Irgum et al. have developed a novel photoinitiated precipitation polymerization which can avoid coagulum in thermal initiation and can get exceptionally high monodisperse microspheres.25 Leonie Barner et al. have also developed a precipitation polymerization Initiated by UV-Irradiation at ambient temperature and got well-defined microspheres.26 The most commonly used solvent in precipitation polymerization is acetonitrile due to its excellent miscibility with various solvents and monomers, as well as its proper ability to solubilize the resulting polymers. However, acetonitrile

Materials. Divinylbenzene (DVB-55, containing 55% divinylbenzene m- and p-isomers) was obtained from TCI and used as received. 2,2′-Azobis (isobutyronitrile) (AIBN, analytical grade) was obtained from Beijing Chemical Reagent Company and was recrystallized from ethanol. Acetic acid and tetrahydrofuran (THF) were of analytical grade, from Beijing YILI Fine Chemicals Co., and were used as received. Typical Precipitation Polymerization Procedure. In a typical precipitation polymerization procedure, acetic acid (18 mL), DVB-55 (0.34 mL, 2 vol % of acetic acid), and AIBN (0.01 g, 3 wt % relative to DVB-55) were charged to a 20 mL glass vial. The vial was placed on a set of steel rollers in a closed air chamber and rotated around its horizontal axis at a speed of 5 rpm. The temperature of the air chamber was ramped from room temperature to 70 °C in 1 h and then held at 70 °C for 12 h (24 h for the investigation of polymerization evolution).

6916 J. Phys. Chem. B, Vol. 112, No. 23, 2008

Yan et al.

Figure 4. SEM micrographs displaying the effect of polymerization temperature on the particle morphology at polymerization temperatures of (A) 60 °C, (B) 70 °C, and (C) 80 °C. DVB loading ) 2 vol %, AIBN loading ) 3 wt %, 12 h.

When the effect of the polymerization temperature was studied, the temperatures were set at 60, 70, and 80 °C (DVB loading ) 2 vol %, AIBN loading ) 3 wt %, 12 h.). When the effect of the total monomer loading was studied, the DVB-55 loadings were from 0.5 to 4 vol % (AIBN loading ) 3 wt %, 70 °C, 12 h). When the effect of the initiator loading was investigated, the AIBN loadings were from 2 to 8 wt %. (DVB loading ) 2 vol %, 70 °C, 12 h.) At the end of the reaction, the PDVB-55 particles were separated from the solvent by centrifugation, washed three times with methanol, and dried under vacuum at 50 °C overnight. The yields of particles were determined by gravimetry. Characterization. The morphologies of the obtained PDVB55 particles were studied with a Hitachi S-4700 scanning electron microscopy (SEM). SEM specimens were prepared by directly placing powders of the samples on electric tapes or by diluting the particle dispersions with ethanol and placing one drop of each on a cover glass. The drops were dried at room temperature and then coated under vacuum with approximately 4 nm of platinum. In general, 100 individual particles could be measured from the SEM microphotographs by using an image analyzer software to calculate the coefficient of variation (CV) according to the following equation

CV )

[

]

N

∑ (di - djn)2 i)1

dn

1⁄2

× 100

(1)

Here, N is the total number of counted particles; dn is the average diameter and di is the diameter of the i th particle. The surface chemical composition of the powder samples was analyzed by X-ray photoelectron spectroscopy (XPS, ThermoVG ESCALAB 250).

Results and Discussion Typical results. Monodisperse, cross-linked PDVB-55 particles could be prepared via a precipitation polymerization with acetic acid as the solvent and with 2 vol % DVB loading. After centrifugation, washing, and drying, a white powder was obtained. Figure 1 illustrates a SEM micrograph of the powder, and as can be seen from the figure, the PDVB-55 particles were uniform and well separated. No weight loss occurred after extraction of the particles with THF at room temperature for 24 h, which means that these particles were highly cross-linked. The average diameter of the PDV-55 particles was measured to 2.69 µm and the CV was 4.66%. Figure 2 presents the XPS results of the PDVB-55 sample. In Figure 2A, peaks at 284.7 and 531.55 eV representing C and O, respectively, can be observed. In Figure 2B, on the other hand, three kinds of chemical carbon bonds could be distinguished in the carbon 1s spectrum. These were a carbon-carbon single bond, a carbon-carbon double bond, and a carbon-oxygen bond. The ratio of the fitting area of the double bond to the single bond was approximately 11, a value larger than 3, which is the ratio in a styrene group. This result indicates that there were many unreacted vinyl groups at the surface of the crosslinked particles.19 Effect of the Polymerization Temperature. Figure 3 shows the relationship between the particle yield and the polymerization time at three temperatures (at 60, 70, and 80 °C; DVB loading ) 2 vol %, AIBN loading ) 3 wt %, 12 h.). It can be seen that the polymerization rate increased when the reaction temperature was increased. This is in agreement with the general principle of free radical polymerization. The initial polymerization rate was fairly low for the polymerization reaction that was carried out at 60 °C when compared to those at higher temperatures, and the final yield of this reaction was also the lowest when the polymerization was stopped after 12 h.



Figure 5. SEM micrographs displaying the effect of the total monomer loading on the precipitation polymerization in acetic acid: (A) 0.5, (B) 1, (C) 1.5, (D) 2, (E) 3, and (F) 4 vol % DVB. AIBN loading ) 3 wt %, 70 °C, 12 h.

Figure 4 shows the effect of the polymerization temperature on the morphology of the particles. At 60 °C, the particles were cauliflower-like. At 70 °C, the particles were pumpkin-like. When the polymerization temperature was increased, particles were more spherical. The diameters of the particles decreased with the increase in polymerization temperature as a result of the lower polymerization temperature giving rise to particle aggregation. When measuring the particle size, a flower-like aggregation was recognized as one particle with a larger diameter. The effects of the polymerization temperature on the particle morphologies can be explained by the stabilization mechanism of precipitation polymerization,19 where the solventswollen gel layer at the particle surface acted as a steric stabilizer.31 When at a lower temperature, the solubilizability of acetic acid was reduced and the solvent-swollen gel layer was not thick enough to prevent the aggregation of small particles, since acetic acid is not a marginal solvent for styrenic polymers. Thus, the homocoagulation period was prolonged,30 resulting in the formation of irregularly cauliflower-like shaped particles. When the polymerization temperature was increased, the solubilizability of acetic acid was enhanced and caused an increase in the thickness of the layer, thereby allowing for well-

stabilized particles. Hence, microspheres with a regular shape could be produced. Effect of the Monomer Loading. The SEM micrographs in Figure 5 display the effect of the total monomer loading on the precipitation polymerization of DVB-55(with DVB-55 loading from 0.5 to 4 vol %, AIBN loading ) 3 wt %, 70 °C, 12 h) It can be observed that when the DVB loading was below 1 vol % monodisperse microspheres were successfully obtained. As the DVB loading increased, doublet- or triplet-shaped coagulated particles appeared. For a DVB loading of 4 vol %, a secondary nucleation was observed, and when 6 vol % DVB was employed, coagulum was obtained. These results are logical, since a higher particle concentration in the medium should lead to a higher possibility of collision among the particles.32 When the DVB loading was very high, the system was crowded with oligomers in the solvent, and the higher concentration of DVB was able to increase the solubilizability of the system, since DVB is a good solvent for styrenic polymers. In this way, the nucleation period was prolonged, resulting in a secondary nucleation. This trend can also be confirmed in Figure 6, where the CV can be seen to increase with DVB loading.


Figure 6. Effect of the total monomer loading on (A) the particle diameter and CV and (B) yield of particles. AIBN loading ) 3 wt %, 70 °C, 12 h.

Figure 7. The effect of AIBN loading in neat acetic acid on the yield of particles and the particle diameter. DVB loading ) 2 vol %, 70 °C, 12 h.

Figure 6 also portrays that the particle yield and particle diameter increases with the DVB loading, from 17% to 95% and from 1.7 to 3.8µm, respectively. For a fixed solvent concentration, the amount of PDVB oligomers dissolved in the solvent was more or less constant. The higher the DVB loading, the lower was the fraction of dissolved oligomer with respect to PDVB. Thus, more PDVB was able to precipitate, resulting in the increase of the yields and the diameters.

Yan et al. Effect of AIBN Loading. SEM micrographs of PDVB-55 particles obtained after polymerization with various AIBN loadings are not shown in this paper in order to save space (with AIBN loading from 2 to 8 wt %, DVB loading ) 2 vol %, 70 °C, 12 h.) However, the micrographs displayed that there were no significant differences in the morphology of the particles. When acetic acid was used as the solvent, monodisperse cross-linked PDVB-55 microspheres and doublet- or triplet-shaped pumpkin-like particles were observed. From Figure 7, it can be seen that the initiator loading has no significant effect on the particle distribution whereas the particle diameters increased from 2.65 to 2.88 µm with an increase in AIBN loading. In the nucleation period, an increased amount of initiator (from 2 to 8%) resulted in a higher polymerization rate. Thus, during the nucleation period, many small particles were formed in a short amount of time and then aggregated before the swollen surface could reach the proper thickness. This explains why an increased AIBN loading induced increased particle diameters. The figure also shows that the particle yield could be increased from 74 to 92% by increasing the initiator loading from 2 to 8%. This result is meaningful since the polymerization rate, which is directly related to the yield in the same reaction time, increased with the concentration of free radicals.30 Moreover, the polymerization system had not undergone deoxygenation by purging with nitrogen. The higher initiator loading could thus increase the initiating efficiency and higher yields could be obtained. Nucleation and Particle Formation Mechanisms. In terms of the mechanisms of nucleation and growth of cross-linked particles for DVB-55 precipitation polymerization, Sto¨ver et al.,19 who had made great contribution in this field, proposed that nucleation occurred by aggregation of soluble oligomers to form colloidally stable nuclei and particle growth involved the capture of oligomers by reaction of vinyl groups on the particle surface. However, there has been no report about the close tracing of the whole particle formation process. Here, to obtain a better understanding of the formation mechanism and evolution of these particles prepared in precipitation polymerization in hydrophobic monomer system, for the first time, a set of time-dependent experiments were carried out with acetic acid as the solvent, where the samples toward different polymerization times were collected and the morphology and yields of each sample was examined at the same time by SEM and gravimetry. The results of the time-dependent experiments are displayed in Figures 8 and 9. Figure 8 show a series of SEM micrographs illustrating the morphological evolution of PDVB-55 particles with polymerization time. Moreover, in order to follow the shape evolution of the particles, an amplified image was placed in the upper righthand corner of each micrograph (Figure 8). By seriously observing Figure 8, some interesting results could be extracted. (1) The PDVB-55 particles are not spherical but display an odd multiple-coalescent pumpkin-like shape, which seems to be a result of the aggregation of smaller particles. As the polymerization proceeded, the multiplecoalescent shape is more or less maintained. (2) Interestingly, even though the PDVB-55 particles are not regularly spherical in shape, the average sizes of the particles increased with the polymerization time, and the relationship curves of the corresponding average diameter (statistically calculated based on image in Figure 8) with polymerization time are plotted in Figure 9. (3) The size distribution of particles continuously decreased and the CV curve of the PDVB-55 particles with polymerization



Figure 8. SEM micrographs of PDVB-55 particles formed in acetic acid after (A) 2, (B) 3, (C) 4, (D) 6, (E) 8, (F) 10, (G) 12, and (H) 24 h of polymerization. DVB loading ) 2 vol %, AIBN loading ) 3 wt %, 70 °C.

time can also be seen in Figure 9. Consequently, after enough polymerization time, i.e. 12-24 h, quite monodisperse PDVB55 particles could be obtained. (4) The amount of spheres and the spherical evenness of the particle surfaces improved, which can be seen from the amplified image in the corner of each micrograph. The yield of PDVB-55 particles was plotted with polymerization time. Prior to the polymerization, the reaction system was homogeneous and transparent. After 2 h of polymerization, the transparent solution started to become

slightly turbid, and at this point, the yield was about 3.1% (stage I). The appearance of this turbidity indicated a phase separation of PDVB from the polymerization medium. In stage II, the polymerization system gradually turned milky and the yield increased smoothly to 80%, where it reached a plateau. During stage III, the rate of polymerization slowed down due to the low concentration of monomer. After 24 h of polymerization, the final yield leveled off at 85%, since a certain amount of PDVB-55 oligomer was believed to be dissolved in the polymerization medium.


Yan et al.

Mt ) m0Yt

(2)

Here, m0 is the total mass of monomer. If the resulting particles were spheral, the mass of each microsphere at t is mt

( )

Dt 4 mt ) πF 3 2

3

(3)

Here, F is the density of resulting polymer. Dt is the average diameter of microspheres at time t. From (1) and (2), we can get:

Nt )

Mt m0Yt 6 ) × 3 mt πFV D

(4)

t

Figure 9. Polymerization evolution of the PDVB-55 particles: yields of particles, particle size diameters, and CV as a function of the polymerization time. DVB loading ) 2 vol %, AIBN loading ) 3 wt %, 70 °C.

Here, Nt is the total amount of particles per liter in the system at time t and V is the system volume. Based on eq 3, from Yt values and Dt values on the two curves of Figure 9 at different polymerization times t, Nt could be obtained, and the result is displayed in Figure 10. It was surprising to observe that starting from the first sample, taken at 2 h, Nt is almost a constant, about 9.6 × 108/L. Sine 2 h just respond an appearance of slightly turbid and as low as 3.1% yield, we could say that no homocoagulation occurred and no new particles were generated after nucleation. From eq 4 we can get another equation

Yt )

Figure 10. The total amount Nt of particles at different times. DVB loading ) 2 vol %, AIBN loading ) 3 wt %, 70 °C.

Figure 11. Relationship between cube of diameter and yield at different times. DVB loading ) 2 vol %, AIBN loading ) 3 wt %, 70 °C.

From two curves, i.e., yield evolution curve and diameter evolution curve, by quantitatively calculation, we could obtain some interesting and important results. For the polymerization time t, if Yt is the yield of particles and Mt is the mass of total particles,

πFVNt × Dt3 ) kDt3 6m0

(5)

In eq 5, when Nt is a constant, k is also a constant. It means that there should be a linear relation between yield and cube of diameter. Figure 11 shows this expected relation. The linear relation gives us a distinct impression that particle growth almost comes from capturing of newly formed oligomers. ∆D3 in Figure 11 can be viewed as the newly formed polymer layers at the particle surface while the ∆Y as the newly formed oligomers. Some inaccuracy in Figures 10 and 11 may come from the unevenness in particle shape and due to experimental errors. By systematically following the growth of particles (Figure 8) and quantitatively analyzing the relationship of particle amount, particle size and yield (Figures 9, 10 and 11), an unambiguous growth mechanism and reaction scheme could be extracted as follows. For acetic acid system, it has a similar process to that of acetonitrile system, i.e., the particle formation consists of two stages, nucleation and particle growth by capturing cross-linked oligomers. Moreover, compared with acetonitrile system,8 the polymerization of DVB-55 in acetic acid behaved with the following unique characteristics: under the same polymerization conditions, higher yields (85 to 35%), smaller size (2.8 to 3.3 µm), not spherical but pumpkin-like particles (the particles become more and more spherical with the increased diameters and polymerization time). Based on the results and phenomena above, we present a scheme for the formation of PDVB-55 cross-linked particles in acetic acid. The beginning of nucleation involves conversion of monomer to oligomer as a classical solution polymerization. The insoluble oligomers then aggregate to form primary particle nuclei with the vinyl groups on the surface. For DVB monomer and PDVB-55, acetic acid is a poorer solvent than acetonitrile due to being a so-called proton-containing solvent and therefore has lower solubility. So, for a given monomer loading, this lower solubility only could afford shorter oligomers than in the acetonitrile system and, at last, would result in an increase in the amount of primary nuclei formed. In acetonitrile solvent,



SCHEME 1: Particle Formation Process

when the nuclei grow to a certain size, the surface energy decreases and the swollen layer on the surface is formed and stabilizes the particle. This swollen layer on the surface is beneficial to the formation of smooth spherical particles. But in acetic acid, because of low solubility, it is believed that such efficient swollen layer can hardly be formed and, therefore, can not stabilize the particles well. Moreover, the higher nuclei concentration in the medium should lead to a higher possibility of collision among the primary nuclei. As a result, these primary nuclei will aggregate again to form irregular shape nuclei, which have larger sizes and low surface energy like doublet- or tripletshaped particles with vinyl groups on the surface. This is the end of prolonged nucleation. This process can be proved by Figure 8A, as there were already odd-shape particles at the end of nucleation. Particle growth includes two modes: an in situ surface polymerization of monomer and the adsorption of PDVB-55 oligomers. After nucleation, the particle amounts remain constant because of the absence of homocoagulation (Figure 10). Soluble oligomer radicals are captured from solution to gel layer by reactive or entropic capture caused by surface vinyl groups, followed by cross-linking and desolvation to become another layer (Figure 11), and the repeat of the process results in forming larger pumpkin-like particles. With polymerization proceeding, according to “lowest energy rationale (the lowest surface area and the lowest surface energy of a particle)”, the oligomers should prevailingly be absorbed and cross-linked at the bottom-groove of pumpkin-like particles so that these particles become more and more spherical or close to spherical in shape. Conclusions Monodisperse cross-linked PDVB-55 particles could be prepared by precipitation polymerization in innocuous acetic acid. These particles consist of pumpkin-like particles and microspheres. With the DVB loading at 2 vol % and by observing the shapes of particles obtained with three different temperature(60, 70, and 80 °C), we found that more spherical particles were obtained at higher temperatures and pumpkinlike particles were obtained at lower temperatures. For DVB loading below 1 vol %, monodisperse microspheres with smooth surfaces and narrow diameters were successfully obtained. By tracing the whole polymerization process, i.e., time-dependent experiments, we found that the after two hours or 3.1% yield,

the amount of particles remained stable and the particle diameters increased linearly with the polymerization time as well as the CV decreased. The spherical evenness of the particle surfaces get better with the growth of particles based on lowest energy rationale. The reason for producing pumpkin-like particles could be attributed to the lower solubilizability of acetic acid, compared to acetonitrile. Acknowledgment. We gratefully acknowledge financial support from the National Natural Science Foundation of China (NSFC), major project No. 50433040, and the Polymer Chemistry and Physics, BMEC (No. XK 100100433 and No. XK 100100540). References and Notes (1) Ugelstad, J.; Berge, A.; Ellingsen, T.; Schmid, R.; Nilsen, T.-N.; Mørk, P. C.; Stenstad, P.; Hornes, E.; Olsvik, Ø. Prog. Polym. Sci. 1992, 17, 87–161. (2) Kawaguchi, H. Prog. Polym. Sci. 2000, 25, 1171–1210. (3) Vanderhoff, J. W.; Elaasser, M. S.; Micale, F. J.; Sudol, E. D.; Tseng, C. M.; Silwanowicz, A.; Kornfeld, D. M.; Vicente, F. A. J. Dispersion Sci. Technol. 1984, 5, 231–246. (4) Barrett, K. E. J.; Thomas, H. R. J. Polym. Sci., Part A-1 1969, 7, 2621–2650. (5) Ober, C. K.; Lok, K. P.; Hair, M. L. J. Polym. Sci., Polym. Lett. Ed. 1985, 23, 103–108. (6) Tseng, C. M.; Lu, Y. Y.; Elaasser, M. S.; Vanderhoff, J. W. J. Polym. Sci., Part A: Polym. Chem. 1986, 24, 2995–3007. (7) Li, K.; Sto¨ver, H. D. H. J. Polym. Sci., Part A: Polym. Chem. 1993, 31, 2473–2479. (8) Li, K.; Sto¨ver, H. D. H. J. Polym. Sci., Part A: Polym. Chem. 1993, 31, 3257–3263. (9) Li, W. H.; Sto¨ver, H. D. H. J. Polym. Sci., Part A: Polym. Chem. 1998, 36, 1543–1551. (10) Frank, R. S.; Downey, J. S.; Sto¨ver, H. D. H. J. Polym. Sci., Part A: Polym. Chem. 1998, 36, 2223–2227. (11) Li, W. H.; Sto¨ver, H. D. H. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 2899–2907. (12) Li, W. H.; Sto¨ver, H. D. H. Macromolecules 2000, 33, 43544360. (13) Zheng, G.; Sto¨ver, H. D. H. Macromolecules 2002, 35, 76127619. (14) Zheng, G.; Sto¨ver, H. D. H. Macromolecules 2002, 35, 68286834. (15) Frank, R. S.; Downey, J. S.; Yu, K.; Sto¨ver, H. D. H. Macromolecules 2002, 35, 2728–2735. (16) Zheng, G.; Sto¨ver, H. D. H. Macromolecules 2003, 36, 1808–1814. (17) Zheng, G.; Sto¨ver, H. D. H. Macromolecules 2003, 36, 7439–7445. (18) Takekoh, R.; Li, W. H.; Nicholas, A. D. B.; Sto¨ver, H. D. H. J. Am. Chem. Soc. 2006, 128, 240–244. (19) Downey, J. S.; Frank, R. S.; Li, W.-H.; Sto¨ver, H. D. H. Macromolecules 1999, 32, 2838–2844.

6922 J. Phys. Chem. B, Vol. 112, No. 23, 2008 (20) Bai, F.; Yang, X.; Huang, W. Macromolecules 2004, 37, 9746– 9752. (21) Qi, D.; Bai, F.; Yang, X.; Huang, W. Eur. polym. J 2005, 41, 2320– 2328. (22) Shim, S. E.; Yang, S.; Choi, H. H.; Choe, S. J. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 835–845. (23) Yang, S.; Shim, S. E.; Choe, S. J. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 1309–1311. (24) Jeong, M. J.; Sunhye, Y.; Sang, E. S.; Choe, S. J. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 5343–5346. (25) Fredrik, L.; Knut, I. Macromolecules 2007, 40, 1962–1968. (26) Joso, R.; Pan, Eh Hau; Stenzel, M. H.; Davis, T. P.; BarnerKowollik, C.; Barner, L. J. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 3482–3487.

Yan et al. (27) Hashimoto, K.; Morimoto, K.; Dobson; S. In Environmental Health Criteria; World Health Organization: Geneva, 1993;Vol. 154, p 110. (28) Yan, Q.; Bai, Y. W.; Meng, Z.; Yang, W. T. Acta Polym. Sin. 2007, 11, 1102–1104. (29) Downey, J. S.; McIsaac, G.; Frank, R. S.; Sto¨ver, H. D. H. Macromolecules 2001, 34, 4534–4541. (30) Goh, E. C. C.; Stover, H. D. H. Macromolecules 2002, 35, 9983– 9989. (31) Drew, Myers In Surfaces, Interfaces, and Colloids: Principles and Applications, 2nd ed.; John Wiley & Sons: New York, 1999; p 234. (32) Shim, S.; Yang, S.; Jin, M.-J.; Chang, Y. H.; Choe, S. J. Colloid Polym. Sci. 2004, 283, 41–48.

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