Precipitation Polymerization in Acetic Acid: Study of the Solvent Effect

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J. Phys. Chem. B 2009, 113, 3008–3014

Precipitation Polymerization in Acetic Acid: Study of the Solvent Effect on the Morphology of Poly(divinylbenzene) Qing Yan, Tongyang Zhao, Yaowen Bai, Fen Zhang, and Wantai Yang* State Key Laboratory of Chemical Resource Engineering, Beijing 100029, People’s Republic of China, and College of Materials Science and Engineering, Beijing UniVersity of Chemical Technology, Beijing 100029, People’s Republic of China ReceiVed: October 10, 2008; ReVised Manuscript ReceiVed: December 23, 2008

This paper reports on two important results regarding the precipitation polymerization of poly(divinylbenzene) (PDVB) in acetic acid (HAc). (1) Acetic acid is a novel kind of solvent worthy of investigation because it is amphipathic and innoxious. Thus, two kinds of model solvents, methyl ethyl ketone (MEK) and n-heptane, were selected to investigate the solvent effect on the particle morphology of PDVB-55 during precipitation polymerization in acetic acid. Monodisperse PDVB-55 microspheres were obtained with an MEK content of 30 vol % and a DVB loading of 2 vol %. Odd-shaped particles were found to almost disappear when MEK was added. For MEK contents up to 90 vol %, space-filling macrogels consisting of small particles with diameters of around 10 nm were obtained. More homocoagulated particles were produced when n-heptane was added, for which concentrations up to 50 vol % gave rise to cauliflower-like particles. Thus, in the acetic acid system, microspheres, pumpkin-like particles, macrogels, and coagulum could be successfully obtained. (2) The preparation of nonpolar PDVB-55 particles could be more predictable. For the first timesbased on the regulation of former studiessthe regularity of the dispersive term (δd) on the particle morphology for a PDVB precipitation polymerization system was reported. The three-dimensional Hansen solubility parameters were utilized to perfect the regularity of the Hildebrand solubility parameter. Microspheres or particles were formed in the range of moderate δ values for both parameters, i.e., δ ) 20.2-24.3 MPa1/2 or δ ) 16 MPa1/2. What was even more important, δd was found to be around 15.4 MPa1/2, and δh should be below 13.5 MPa1/2. Cyclohexane, cyclohexanone, n-butyl acetate, and 1,4-dioxane were used to verify this regularity, and positive results were obtained. Stable, uniform, and well-separated PDVB-55 microspheres and particles were produced as a result of interaction forces between oligomers, polymers, and solvent. Introduction Highly cross-linked polymeric microspheres are attractive for numerous applications such as coatings, instrument calibration standards, templates for the preparation of porous materials, and chromatographic support materials.1-4 Seeded emulsion polymerization, dispersion polymerization, and precipitation polymerization have been adopted in order to produce monodisperse micron-sized microspheres. Among these methods, precipitation polymerization can give rise to such micron-sized microspheres in a single step without the use of any stabilizers or surfactants.4-13 Thanks to this facile process that gives rise to microspheres with clean surfaces and suitable particle sizes, precipitation polymerization has now been applied in numerous fields, such as for the preparation of particles with interesting internal structures, electrophoretic displays, molecularly imprinted polymers, and environmentally responsive polymers.14-19 Poly(divinylbenzene) (PDVB) microspheres are worthwhile to investigate since they are fully cross-linked and present superior thermal properties.28 Many researchers have shown that solvents play extraordinarily important roles in determining the morphology of the particles obtained in precipitation polymerization.12,20-22 The study of the solvent effect on the polymer morphology is thus * Address correspondence to this author at Department of Polymer Science, Beijing University of Chemical Technology, P. O. Box 037, Beijing 100029, People’s Republic of China. Telephone: +86-10-64432262. Fax: +86-10-64416338. E-mail: [email protected].

essential. In this way, the shape of the products can be easily controlled. Sto¨ver et al. have found that acetonitrile is a suitable solvent to obtain monodisperse microspheres, and these authors have created an impressive morphology map of PDVB indicating that in the intermediate or “marginal” solvency range, as well as at certain DVB concentrations, discrete microspheres can be formed during precipitation polymerization.23 In general, good solvents lead to a typical solution-type polymerization, with or without cross-linking, whereas poor solvents give rise to classical precipitation polymerization. The solvent effect on the morphologies of poly(divinylbenzene-alt-maleic anhydride) and poly(methacrylic acid-co-poly(ethylene oxide) methyl ether methacrylate) has also been investigated.9,24 Among these solvents, acetonitrile is the most commonly used for the synthesis of cross-linked polymeric microspheres by precipitation polymerization. In previous research efforts, the Hildebrand solubility parameter (δ), corresponding to the cohesive energy density of solvents and polymers, has been used to interpret the role of solvents in polymerization reactions. It thus represents an indicator for the morphologies of the resultant DVB-55 polymers. Commonly, DVB-55 microspheres have been obtained in marginal solvents, i.e., with δ values of 16 MPa1/2 or around 24 MPa1/2. However, there exist some exceptions. For example, in a first report5 of PDVB-55 microspheres prepared via precipitation polymerization where a mixture of acetonitrile/nbutyl alcohol was used as the solvent, all the δ values of the

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Solvent Effect on Morphology of PDVB mixed solvents were found to be 24.3 MPa1/2, which is identical to the δ of both acetonitrile and n-butyl alcohol (δmixture ) Φ1δ1 + Φ2δ2). It was thus assumed that precipitation polymerization in this mixture of solvents should give rise to PDVB-55 microspheres, but the fact was that only a volume ratio of acetonitrile to n-butyl alcohol of 7/3 resulted in such PDVB-55 microspheres. This example proved that it was not only the Hildebrand solubility parameter that decided the final morphologies of resultant DVB-55 polymers. The regularity thus had limitations, and further investigations on the solvent effect should be conducted. The present interest in solvent effects has also stemmed from a deficiency in our recent work,25,26 where the innoxious acetic acid (HAc) could successfully replace harmful acetonitrile as the solvent in the precipitation polymerization of monodisperse cross-linked PDVB-55 microspheres. Furthermore, as an organic proton-containing solvent, acetic acid is amphipathic, miscible with both hydrophilic and hydrophobic compounds, which enlarges the scope of available monomers. Additionally, acetic acid is a commercially available product with only a weak acidity. Thus, acetic acid is a novel kind of solvent, worthy of more investigation. However, this earlier study had a deficiency: for a DVB loading below 1 vol %, monodisperse microspheres were obtained. However, for a DVB loading over 2 vol % and at 70 °C, only pumpkin-like particles were observed. In order to improve this result, the present work involved the use of two kinds of model solvents to investigate their effects on the PDVB55 particle morphology during precipitation polymerization in acetic acid. Monodisperse microspheres were successfully prepared for a DVB loading of 2 vol %. Further, the threedimensional Hansen solubility parameters were utilized to perfect the regularity of the Hildebrand solubility parameter and the preparation of nonpolar PDVB-55 particles was rendered predictable. Experimental Methods Materials. Divinylbenzene (DVB-55; containing 55% divinylbenzene meta and para isomers) was obtained from TCI and used as received. 2,2′-Azobisisobutyronitrile (AIBN, analytical grade) was obtained from Beijing Chemical Reagent Co. and was recrystallized from ethanol. Acetic acid, methyl ethyl ketone (MEK), n-heptane, cyclohexane, cyclohexanone, n-butyl acetate, and 1,4-dioxane were of analytical grade, purchased from Beijing YILI Fine Chemicals Co., and used as received. Typical Precipitation Polymerization Procedure. The solvent (18 mL), DVB-55 (0.34 mL, 2 vol % solvent), 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 during 1 h and then maintained at 70 °C for 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 and percentage conversions of the monomer were determined by gravimetry. Characterization. The morphologies of the obtained PDVB55 particles were studied with a Hitachi S-4700 scanning electron microscope (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

J. Phys. Chem. B, Vol. 113, No. 10, 2009 3009 TABLE 1: Reported Solvent Effect of PDVB-55 Microsphere Systemsa reaction solvent δ (MPa1/2) δdb (MPa1/2) δp (MPa1/2) δh (MPa1/2) acetonitrile acetonitrile/n-butyl alcohol (7:3 v:v) acetonitrile/ propanol (7:3 v:v) MEK/heptane (2:8 v:v) MEK/acetic acid (3:7 v:v)

24.3 24.3

15.3 15.5

18 14.6

6.1 9.5

23.7

15.5

14.3

9.0

16.0

15.4

1.8

1.0

20.2

15.0

8.3

11.0

a Here, δ corresponds to the Hildebrand solubility parameters. The δ of the mixture of solvents did not agree with δ ) (δd2 + δp2 + δh2)1/2. b δd ) Φ1δd,1 + Φ2δd,2, δp ) Φ1δp,1 + Φ2δp,2, δh ) Φ1δh,1 + Φ2δh,2.

temperature and then coated under vacuum with approximately 4 nm of platinum. In general, 100 individual particles could be measured from the SEM micrographs by using an image-analyzing software to calculate the coefficient of variation (CV).29 The particle size distribution was also measured by a dynamic light scattering particle size analyzer (90Plus, Brookhaven). Results and Discussion Effect of Good and Poor Solvents on PVDB-55 Morphology. In precipitation polymerization, the solubilizability of a solvent is of extraordinary importance in determining the morphology of the particles. In previous studies, for a DVB loading above 2 vol % and at 70 °C, only pumpkin-like particles were obtained. With the purpose of preparing monodisperse PDVB-55 microspheres, it was necessary to tune the solubilizability of the acetic acid system. For this reason, two solvents for styrenic polymers were selected (MEK, δ ) 19.0 MPa1/2, representing a good solvent, and n-heptane, δ ) 15.1 MPa1/2, representing a poor solvent) and their effects on the polymerization and particle morphology was investigated. Figure 1 displays SEM micrographs and particle size distribution of the PDVB-55 particles prepared in a solvent containing acetic acid and MEK. When MEK was mixed with acetic acid, only a few homocoagulated particles existed in the system (Figure 1B). Odd-shaped particles almost disappeared for MEK contents of 20%, but the particle size distribution broadened. Continuing to increase the MEK content to 30 vol % led to monodisperse PDVB-55 microspheres (Figure 1C), but for 60 vol % MEK, macrogels were formed (not shown in Figure 1). A system containing 90 vol % MEK mixed with acetic acid gave rise to the formation of space-filling macrogels (Figure 1D) consisting of small coagulated particles with diameters around 10 nm. A photograph of these space-filling macrogels in a vial can be seen as the inset in the upper-right-hand corner of Figure 1D. Under these conditions the polymer network was in a swollen state, filling the entire reaction volume. Thus, by adding MEK to acetic acid, particle morphologies could be significantly improved. Figure 2 presents SEM micrographs of the PDVB-55 microspheres prepared in a solvent mixture of acetic acid and n-heptane. The n-heptane content was varied from 0 to 50 vol % based on the overall solvent. As seen in the figure, when n-heptane, which is a poor solvent for styrenic polymers, was mixed with acetic acid, a larger amount of homocoagulated particles was produced. For n-heptane concentrations up to 50 vol %, the coagulum was so serious that cauliflower-like particles could be observed (Figure 2D). Figure 3 summarizes the effects of the solvents on the final particle sizes, the particle size distribution, and the yield. It can

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Figure 1. SEM micrographs and particle size distributions of PDVB-55 particles prepared in a mixture of acetic acid and (A, a) 0 vol % MEK, (B, b) 20 vol % MEK, (C, c) 30 vol % MEK, and (D) 90 vol % MEK. DVB loading, 2 vol %; AIBN loading, 3 wt %; 70 °C; 12 h.

be observed that the coefficient of variation (CV) increased from 4.6% to 23.6% and 9.2% with the addition of MEK and n-heptane, respectively. Moreover, also the diameters of the particles increased, from 2.7 to 3.3 and 4.7 µm, respectively. The yield of the particles, on the other hand, decreased from 82% to 62% when the concentrations of MEK and n-heptane were increased.

The effects of the good vs poor solvents on the PVDB-55 morphology can be explained by our particle formation mechanism for the precipitation polymerization in acetic acid, as previously reported.26 The pumpkin-like particles in acetic acid were caused by a prolonged nucleation, including the homocoagulation of primary nuclei into the final nuclei. Solventswollen gel layers at the particle surfaces acted as steric

Solvent Effect on Morphology of PDVB

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Figure 2. SEM micrographs of PDVB-55 particles prepared in a mixture of acetic acid and (A) 0 vol % n-heptane, (B) 10 vol % n-heptane, (C) 40 vol % n-heptane, and (D) 50 vol % n-heptane. DVB loading, 2 vol %; AIBN loading, 3 wt %; 70 °C; 12 h.

Figure 3. Effect of addition of MEK and n-heptane on (A) particle diameter and CV and (B) the yield of the particles, during DVB-55 polymerization. DVB loading, 2 vol %; AIBN loading, 3 wt %; 70 °C; 12 h.

stabilizers. In acetic acid, due to the low solubilizability, it is believed that barely any of these efficient swollen layers could form, for which reason primary nuclei could not be efficiently stabilized. Consequently, the prolonged nucleation took place and pumpkin-like particles were obtained. For the DVB monomer and PDVB-55, the addition of MEK to acetic acid increased the solubilizability of the solvent mixture while the addition of n-heptane decreased it. At the beginning of the nucleation, the insoluble oligomers aggregated to form primary particle nuclei with the vinyl groups on the surface. For the DVB monomer and PDVB-55, the MEK/HAc mixture was a better solvent than pure acetic acid, and thus, for a given monomer loading, this higher solubilizability could afford longer oligomers than the system with only acetic acid and could thus give rise to a decrease in the amount of primary nuclei formed.

When the nuclei have grown to a certain size, the surface energy decreases and the swollen layer on the surface is formed, thus stabilizing the particle. This swollen surface layer is beneficial to the formation of smooth spherical particles. In the MEK/ HAc mixture, due to the higher solubilizability of MEK as opposed to acetic acid, it is believed that such efficient swollen layers could be more readily formed and, therefore, offer an efficient stabilization of the particles. Moreover, the lower concentration of nuclei in the system should lead to a lower possibility of collisions among the primary nuclei. As a result, with the proper recipe, these primary nuclei should not aggregate to form irregularly shaped nuclei as in the acetic acid system. Thus, the addition of MEK should reduce odd-shaped particles and promote regularly shaped microspheres, assuming that the recipe is appropriate.

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TABLE 2: Solubility Parameters of Solvents Used for Testing the Regularitya polymerization solvent

δ (MPa1/2)

δdb (MPa1/2)

δp (MPa1/2)

δh (MPa1/2)

observation

cyclohexane/HAc (2.5:7.5 v:v) cyclohexanone/HAc (1.5:8.5 v:v) n-butyl acetate/HAc (3.5:6.5 v:v) 1,4-dioxane/HAc (1:9 v:v) ethanol/HAc (6.7:3.3 v:v)

19.7 20.5 19.5 20.7 24.3

15.1 15.0 15.0 15.0 15.4

6.0 7.7 6.5 7.4 8.5

10.2 12.2 11.0 12.9 17.5

microspheres/particles microspheres/particles microspheres/particles microspheres/particles coagulum

a Here, δ is the Hildebrand solubility parameters and did not agree with δ ) (δd2 + δp2 + δh2)1/2. b δd ) Φ1δd,1 + Φ2δd,2, δp ) Φ1δp,1 + Φ2δp,2, δh ) Φ1δh,1 + Φ2δh,2.

When the solvent was comprised of acetic acid and n-heptane, it is believed that the efficient swollen layer on the surfaces of primary nuclei could barely form due to the low solubilizability. Moreover, with the higher possibility of collisions among the primary nuclei, the homocoagulation period was prolonged, leading to an even more significant aggregation than in the acetic acid system. Furthermore, the solvents were able to affect the yield of particles. With the addition of MEK, the solubilizability increased and fewer oligomers could precipitate from the solvent to form particles. Thus, the particle yield decreased. On the other hand, when n-heptane was added, the solvent-swollen gel surface layers became thinner, and the swellability was also lowered. It thus became more difficult for the oligomers to enter the solvent-swollen gel layers, which resulted in the oligomers remaining in the solvent and decreasing the particle yield. When MEK was added, the critical chain length increased due to its good solubilizability, thereby leading to a decrease in the amount of nuclei formed. Thus, each nucleus could hold a larger number of oligomers, and bigger particles were created. When n-heptane was added, the increase in particle size for increasing n-heptane concentrations resulted from an increasing coagulation between the particles. Regularity of Hansen Solubility Parameters with PDVB55 Morphology. Besides the effect of good and poor solvents on the PVDB-55 morphology, an investigation was also carried out on the regularity of solubility parameters of the solvents. Recently, the Hildebrand solubility parameter corresponding to the cohesive energy density of the solvents and polymers has become an indicator of the morphologies of the resulting DVB55 polymers. Most PDVB-55 microspheres were obtained in marginal solvents, i.e., with δ of 16 MPa1/2 or around 24 MPa1/ 2. However, certain exceptions existed which did not follow the regularity above. This indicated that the overall solubility parameter was unable to exactly represent the effects of the solvent on the precipitation polymerization. For example, in the acetonitrile/n-butyl alcohol system mentioned above,5 the final morphologies of the resultant DVB-55 polymers were not determined solely by the Hildebrand solubility parameter. Although the overall solubility parameter of a solvent has mostly been considered and discussed in order to evaluate a certain solvent, the one-dimensional Hildebrand solubility parameter reflects only the overall solvent properties. It does not distinguish between specific effects such as dispersive forces, permanent dipole-dipole interactions, and hydrogen bonding forces. Several investigators have decomposed the Hildebrand parameter into several terms representing different contributions to the free energy of mixing.27 The three-dimensional Hansen solubility parameters proved to be more useful than the Hildebrand parameter in understanding the role of the solvent in the DVB-55 system. The Hansen solubility parameters are an attempt to break the overall interactions between a solute and a solvent into dispersive forces (δd), permanent dipole-dipole interactions (δp), and hydrogen bonding forces (δh). On the other

hand, the solubility parameters of the mixed solvents (δmixture) can be calculated using

δmixture ) Φ1δ1 + Φ2δ2

(1)

Here, Φ1 and Φ2 represent the volume fractions of each solvent in the solution. In this report, eq 1 was used to calculate the three subparameters of the Hansen solubility parameters of the mixed solvents. Although this might introduce some negligible inaccuracy, it could disclose a certain regularity. Table 1 shows the Hildebrand parameter and the Hansen solubility parameters of the reported PDVB-55 system from which it was possible to successfully obtain monodisperse microspheres. It can be seen from Table 1 that all the δ parameters were almost in the marginal solvent bound just as reported. Furthermore, when focusing on the three subparameters of the Hansen solubility parameters, it was found that δp and δh displayed no distinct regularities. However, it could also be observed that the microspheres were formed in the range of moderate values for both parameters, i.e., δ ) 20.2-24.3 MPa1/2 or δ ) 16 MPa1/2. What was even more interesting, δd was found to be approximately 15.4 MPa1/2. Solvents with δ between 16 and 20.2 MPa1/2 might give rise to microgels or macrogels because of the overly high solubilizability, whereas solvents with δ below 16 MPa1/2 or over 24.3 MPa1/2 might promote coagulum because of the low solubilizability. With the purpose of testing this regularity, a group of mixtures, with solubility parameters following the regularity, were selected to act as polymerization solvents. Recipes and observed results are listed in Table 2. Prior to the polymerization, the reaction system was homogeneous and transparent. After nucleation, the transparent solution started to become slightly turbid. Then, the polymerization system gradually turned milky, and finally, a stable milky latex was obtained. Figure 4 displays SEM micrographs and particle size distributions of PDVB-55 particles prepared in these test solvents, and an amplified image was placed in the upperright-hand corner of each micrograph. From Table 2 and Figure 4, it can be seen that the test result was positive and that the regularity could be verified. Figure 4 illustrates that the PDVB55 microspheres and particles were uniform and well separated after polymerization for 12 h in selected solvents. Stable monodisperse PDVB-55 microspheres or particles were obtained. The existence of doublet- or triplet-shaped particles was due to the low solubilizability mentioned above. The only exception was the system containing ethanol in which only coagulum was observed. In fact, even a small amount of ethanol in acetic acid (5 vol %) was enough to give rise to coagulum. This exception was caused by the high δh of ethanol (19.4 MPa1/2), which meant that the hydrogen bonding forces in ethanol were extremely strong. The mixture of acetic acid and ethanol was believed to follow the trend of being assembled

Solvent Effect on Morphology of PDVB

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Figure 4. SEM micrographs and particle size distributions of PDVB-55 particles prepared in a mixture of solvents: (A, a) cyclohexane/HAc (2.5:7.5 v:v), (B, b) cyclohexanone/HAc (1.5:8.5 v:v), (C, c) n-butyl acetate/HAc (3.5:6.5 v:v), and (D, d) 1,4-dioxane/HAc (1:9 v:v). DVB loading, 2 vol %; AIBN loading, 3 wt %; 70 °C; 12 h.

into dimers or trimers with hydrogen bonding. Consequently, this trend was thought to weaken the solvation effect of PDVB55, giving rise to a decrease in the solubilizability of the solvent and resulting in coagulum. Since stable latexes could be obtained in neat acetic acid, and the δh of acetic acid was 13.5 MPa1/2, an additional note was added to the regularity that the δh of the mixture should be below 13.5 MPa1/2. The final regularity is

presented in Figure 5. This investigation thus renders the preparation of nonpolar PDVB-55 particles more predictable. The regularity of the dispersive term (δd) in the PDVB precipitation polymerization system could be explained by interaction forces between the oligomers, polymers, and solvent. As for each precipitation polymerization processed in a certain solvent system, the property of the solvent greatly determines

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Figure 5. Morphology of PDVB-55 as a function of δ and δd. Microspheres or particles could only be found in solvents with moderate δd values around 15.5 MPa1/2 with a δh below 13.5 MPa1/2.

the final outcome of the morphology. Both the formation and stabilization of particles are concerned in the interaction between reactant and solvent. The precipitation polymerization includes a phase separation process that is partly driven by solvency of the continuous phase as determined by the interaction forces between the oligomers, polymers, and solvent. The stabilization of microspheres relates to a surface-swollen mechanism, and the swelling process is decided by the interactions between the surface layer polymers and the solvent. In some nonpolar monomer polymerization systems, the permanent dipole and hydrogen bonding forces among monomers, oligomers, and polymers can be very weak. Therefore, the dipole and hydrogen bonding interactions between reactant and solvent would be insignificant. On the contrary, the dispersive term contributes distinctly in almost every situation, so it is unreasonable to simply take the overall solubility parameter into account, without discriminating various terms. On the other hand, in some polar monomer systems, such as that of poly(methacrylic acid-copoly(ethylene oxide) methyl ether methacrylate),24 the effect of the permanent dipole and hydrogen bonds has been shown to be of great importance. In the present DVB polymerization system, the monomers, oligomers, and polymers all constituted almost nonpolar molecules, and the permanent dipole and hydrogen bond interactions between reactant and solvent were thus very weak Therefore, dispersive forces were the main interaction forces between the reactant and solvent, and such dispersive terms are thus believed to have the main influence on the final polymer morphologies. Conclusions Acetic acid is a novel kind of solvent worthy of investigation because it is amphipathic and innoxious. Two kinds of solvents, MEK and n-heptane, were selected in order to investigate the solvent effect on the PDVB-55 particle morphology during a precipitation polymerization in acetic acid. Odd-shaped particles almost disappeared with the addition of MEK, and monodisperse PDVB-55 microspheres were obtained with MEK contents of 30 vol % and DVB loadings of 2 vol %. For MEK contents up to 90 vol %, space-filling macrogels consisting of small particles with diameters around 10 nm were obtained. With the addition of heptane, more homocoagulated particles were produced. For n-heptane concentrations up to 50 vol %, cauliflower-like particles could be observed, and it was thus possible to successfully prepare microspheres, pumpkin-like particles, macrogels, and coagulum in an acetic acid system. The threedimensional Hansen solubility parameters were utilized to perfect the regularity of the Hildebrand solubility parameter.

Yan et al. Monodisperse PDVB-55 microspheres or particles were formed in the range of moderate values for both parameters, i.e., δ ) 20.2-24.3 MPa1/2 or δ ) 16 MPa1/2 and δd was found to be around 15.4 MPa1/2. It was also determined that δh of the mixture should be below 13.5 MPa1/2. Cyclohexane, cyclohexanone, n-butyl acetate, and 1,4-dioxane were used to verify this regularity, and positive results in the form of stable, uniform, and well-separated PDVB-55 microspheres and particles could be obtained. This could be explained by the interaction forces between oligomers, polymers, and solvent. Based on the present study, the preparation of nonpolar particles should now be more predictable. Acknowledgment. The authors 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, 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) Thibault, R. J.; Hotchkiss, P. J.; Gray, M.; Rotello, V. M. J. Am. Chem. Soc. 2003, 125, 11249–11252. (4) Huang, G.; Hu, Z. B. Macromolecules 2007, 40, 3749–3756. (5) Li, K.; Sto¨ver, H. D. H. J. Polym. Sci., Part A: Polym. Chem. 1993, 31, 3257–3263. (6) Li, W. H.; Sto¨ver, H. D. H. Macromolecules 2000, 33, 4354–4360. (7) Zheng, G.; Sto¨ver, H. D. H. Macromolecules 2002, 35, 7612–7619. (8) Zheng, G.; Sto¨ver, H. D. H. Macromolecules 2002, 35, 6828–6834. (9) Frank, R. S.; Downey, J. S.; Yu, K.; Sto¨ver, H. D. H. Macromolecules 2002, 35, 2728–2735. (10) Zheng, G.; Sto¨ver, H. D. H. Macromolecules 2003, 36, 1808–1814. (11) Zheng, G.; Sto¨ver, H. D. H. Macromolecules 2003, 36, 7439–7445. (12) Bai, F.; Yang, X.; Huang, W. Macromolecules 2004, 37, 9746– 9752. (13) Shim, S. E.; Yang, S.; Choi, H. H.; Choe, S. J. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 835–845. (14) Li, G. L.; Yang, X. L. J. Phys. Chem. B 2007, 111, 12781–12786. (15) Takekoh, R.; Li, W. H.; Nicholas, A. D. B.; Sto¨ver, H. D. H. J. Am. Chem. Soc. 2006, 128, 240–244. (16) Bousquet, A.; Perrier-Cornet, R.; Ibarboure, E.; Papon, E.; Labruge`re, C.; He´roguez, V.; Rodrı´guez-Herna´ndez, J. Macromolecules 2007, 40, 9549–9554. (17) Suzuki, D.; Tsuji, S.; Kawaguchi, H. J. Am. Chem. Soc. 2007, 129, 8088–8089. (18) Priego-Capote, F.; Ye, L.; Shakil, S.; Shamsi, S. A.; Nilsson, S. Anal. Chem. 2008, 80, 2881–2887. (19) Werts, M.; Badila, M.; Brochon, C.; He´braud, A.; Hadziioannou, G. Chem. Mater. 2008, 20, 1292–1298. (20) Downey, J. S.; Frank, R. S.; Li, W.-H.; Sto¨ver, H. D. H. Macromolecules 1999, 32, 2838–2844. (21) Frank, R. S.; Downey, J. S.; Sto¨ver, H. D. H. J. Polym. Sci., Part A: Polym. Chem. 1998, 36, 2223–2227. (22) Shim, S.; Yang, S.; Jin, M.-J.; Chang, Y. H.; Choe, S. J. Colloid Polym. Sci. 2004, 283, 41–48. (23) Downey, J. S.; McIsaac, G.; Frank, R. S.; Sto¨ver, H. D. H. Macromolecules 2001, 34, 4534–4541. (24) Goh, E. C. C.; Stover, H. D. H. Macromolecules 2002, 35, 9983– 9989. (25) Yan, Q.; Bai, Y. W.; Meng, Z.; Yang, W. T. Acta Polym. Sin. 2007, 11, 1102–1104. (26) Yan, Q.; Bai, Y. W.; Meng, Z.; Yang, W. T. J. Phys. Chem. B 2008, 112, 6914–6922. (27) Hansen, C. M. Ind. Eng. Chem. Prod. Res. DeV. 1969, 8, 2–11. (28) Shim, S. E.; Yang, S.; Choi, H. H.; Choe, S. J. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 835–845. (29) Ray, B.; Mandal, B. M. Langmuir 1997, 13, 2191–2196.

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