Porous monodisperse poly(divinylbenzene) microspheres by precipitation polymerization

Porous Monodisperse Poly(divinylbenzene) Microspheres by Precipitation Polymerization WEN-HUI LI, HARALD D. H. STÖVER Department of Chemistry, McMaster University, Hamilton, Ontario, L8S 4M1 Canada Received 14 July 1997; accepted 17 December 1997 ABSTRACT: The precipitation polymerization of commercial divinylbenzene in acetoni- trile containing up to 40 vol. % toluene or other cosolvents is shown to produce novel porous monodisperse poly(divinylbenzene) microspheres. These microspheres have diameters between 4 and 7 mm, total pore volumes of up to 0.52 cm3 /g, and surface areas of up to 800 m2 /g. As no surfactant nor stabilizer was used in the preparation of these particles, their surfaces are free of any such residues. The particles were slurry-packed into stainless steel columns for size exclusion chromatography evaluation, and the results show an exclusion limit at molecular weights of 500 g/mol. q 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1543–1551, 1998 Keywords: polymer microspheres; monodisperse; crosslinked; porosity; porogen; precipitation polymerization; divinylbenzene INTRODUCTION Highly crosslinked macroporous styrene/divinylbenzene microspheres have been much studied since their first use in size exclusion chromatography by Moore at Dow Chemical in 1964.1 They combine good mechanical strength with excellent chemical stability and can be prepared with a wide variety of diameters and pore sizes. As a result, crosslinked polystyrene particles have become the most widely used insoluble polymeric supports for ion-exchange, chromatographic, and biosynthetic applications.2 – 8 They are largely produced by aqueous suspension polymerization of commercial divinylbenzene containing about 55% divinylbenzene isomers (DVB-55), wherein suspended monomer droplets are converted into polymer beads. Permanent small and large pores can be introduced into these particles through the use of solvent or polymeric porogens as reviewed by Guyot and Bartholin.2 Such suspension polymer particles can be preCorrespondence to: H. D. H. Stöver Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 36, 1543–1551 (1998) q 1998 John Wiley & Sons, Inc. CCC 0887-624X/98/101543-09 pared with average diameters as small as 5–10 mm, 8,9 though with usually broad size distributions arising from the coalescence and shearing of the monomer droplets during the polymerization. This broad size distribution can reduce their performance in chromatographic applications through band spreading and increased column back-pressure. In recent years, mono- or narrow-dispersed porous resins have been developed for use in highperformance chromatography and elsewhere. Best known perhaps are the monodisperse polymer particles developed by Ugelstad’s group using an activated two-step swelling process.10,11 In their process, monodisperse seed particles are prepared by aqueous emulsion polymerization and are then swollen with low-molecular weight hydrocarbons (activation) followed by a mixture of monomer, crosslinker, initiator, and optional porogen. The resulting suspensions are polymerized to form monodispersed porous particles suitable for liquid chromatography.3,12 Several studies reported in detail the preparation of these separation resins prepared using porogenic solvents 6,13 – 15 or polymeric porogens.16 – 18 We are studying styrenic polymer particles 1543 / 8G7A$$135P 8g7a 05-28-98 08:56:55 polca W: Poly Chem 97135p 1544 LI AND STÖVER formed by precipitation polymerization in organic solvents. In a previous paper, we described the formation of highly crosslinked and monosized poly(divinylbenzene) particles by polymerization of commercial divinylbenzene in neat acetonitrile using AIBN as initiator.19 As this process requires no surfactants or steric stabilizers, the resulting particles have clean, stabilizer-free surfaces and carry no ionic charges. In this paper we report the introduction of porosity into these precipitation polymer particles. Toluene and xylenes were used as cosolvents with acetonitrile, to give mono- or narrow-dispersed, highly crosslinked divinylbenzene particles that have up to 0.52 cm3 /g total pore volume and surface areas of up to 800 m2 /g. The reactions are again carried out in absence of steric and ionic stabilizers. Agitation must be gentle in order to prevent particle coagulation. EXPERIMENTAL Materials Commercial divinylbenzene (DVB-55, 55% divinylbenzene isomers, technical grade, Aldrich Chemical Co.) was freed from inhibitor by passage through a short column of silica gel prior to polymerization. Solvents (acetonitrile, HPLC grade, Aldrich Chemical Co.; toluene, analytical reagent grade, BDH Inc.; xylenes, 1,4-dioxane, methyl ethyl ketone, Baker Analyzed) were used as received. Initiator (2,2*-azobis(2-methylpropionitrile), AIBN, Eastman Kodak Co.) was recrystallized from methanol. General Procedure for Preparation of Microspheres For a typical polymerization, 0.6 mL of commercial divinylbenzene (DVB-55), 0.109 g of AIBN, and 30 mL of reaction medium (acetonitrile/toluene mixtures) were placed into a 30 mL HDPE bottle. Up to 12 such bottles were attached to a rotor plate. The rotor plate was submerged in a water bath and rotated around its horizontal axis at 30 rpm to provide gentle agitation. The temperature of the bath was ramped from room temperature to 707C over 2 h and then kept constant at 707C for 24 h. At the end of the reaction, a trace of hydroquinone was added to each of the bottles, the particles were separated by centrifugation or by filtration through a polymer membrane, washed three / 8G7A$$135P 8g7a 05-28-98 08:56:55 times each with tetrahydofuran and acetone, and dried under vacuum at 507C overnight. The filtrate from the reaction was concentrated and an excess amount of methanol was added to precipitate the soluble polymer (sol) fraction. This sol fraction was filtered off, washed with methanol, and dried under vacuum at room temperature. Conversion of DVB-55 to soluble polymer and particles was determined by gravimetry. The preparation of the DVB-55 suspension polymer particles was described previously. It involved Methocel as stabilizer and toluene or a toluene/dodecanol mixture as porogen.8 Particle Characterization Particle diameters and particle size distributions were measured on a 256-channel Coulter Multisizer II using Isoton II as electrolyte. An orifice tube with a 30 mm aperture was used for all the measurements reported here. The surface morphology of the particles was studied using an ISI DC130 scanning electron microscope. The particles were deposited onto aluminum studs from an acetone dispersion, dried under vacuum, and sputtercoated with 15 nm gold. The internal texture of the particles was studied using a JEOL 1200EX transmission electron microscope. Here, the particles were embedded in Spur epoxy resin and microtomed to produce 40–60 nm thick slices. Pore volume, pore size distribution, and specific surface area of the particles were measured with a Quantachrome Autosorb-1 automated gas adsorption system using nitrogen at 77 K as adsorbate, as described previously.8 The polymer microspheres were dried under vacuum at 1207C overnight just prior to BET measurements. The sol fractions were analyzed by size exclusion chromatography to measure molecular weight averages and molecular weight distributions. A Waters 590 programmable pump equipped with four PL columns and a Waters 410 differential refractometer were used with tetrahydrofuran as the mobile phase. Porous particles prepared in the presence of 25% toluene were slurry packed into a stainless steel column (4.6 mm i.d. and 250 mm in length) as described earlier.8 A set of narrow disperse polystyrene standards, as well as toluene and 1,2-diphenylethane, were used to measure the separation efficiency of the column. The set-up included a Waters Module 590 programmable solvent delivery model with a model 441 UV/vis absorbance detector operated at 254 nm, with tetra- polca W: Poly Chem 97135p POROUS MONODISPERSE MICROSPHERES 1545 Table I. Precipitation Polymerization of DVB-55 in Toluene/Acetonitrile % Conversion to Toluene (vol %) DVB-55a (vol %) Dn (mm) Coef. Var. (%) Mn (sol)b 0 5 10 15 20 25 30 35 40 2 2 2 2 2 2 2 2 2 4.0 4.7 4.5 5.3 5.5 6.6 6.1 4.7 4.6 3.4 4.1 3.9 3.2 2.9 2.9 3.4 3.9 6.4 2,600 1,550 45 50 100 2 2 2 1 1 1 Gel Gel Solution 4,500 6,200 7,190 40 40 40 3 4 5 5.1 5.7 8.3 6.0 6.1 5.5 7,000 9,000 5,400 Particlec Sol Total 47 50 49 54 56 54 50 52 51 3 6 50 56 7 11 19 16 20 18 61 67 73 66 72 69 75 81 82 19 14 15 94 95 97 1,600 4,300 4,600 3,700 4,700 4,700 a 2 wt % AIBN relative to DVB-55. Polydispersity of MWD for the sol fractions is around 2. c The conversions to particles at 2% monomer loading are the averages of up to six experimental series ({8%). b hydrofuran as mobile phase at a flow rate of 0.5 mL/min. RESULTS AND DISCUSSION Pore Formation in Suspension and Seeded Swelling Polymerizations Most porous particles are formed by polymerization in the presence of porogens. In both suspension polymerizations and seeded swelling polymerizations, the porogen and monomer(s) together constitute the dispersed organic phase. As the polymerization proceeds, phase separation occurs in the organic phase. This leads to porogenrich domains, or pores, distributed within the final polymer beads. With a nonsolvent porogen such as dodecanol, phase-separation occurs early during the polymerization and leads to large pores. With toluene as a porogen on the other hand, phase separation occurs late and leads to small pores. Mixtures of solvent and nonsolvent porogens can be used to give intermediate pore sizes.8 Pore formation in suspension polymerization of divinylbenzene is often described from the polymer’s perspective.2,6,7,20 – 23 Initiation takes place inside the droplets and leads to the formation of crosslinked nuclei dispersed within the droplet. / 8G7A$$135P 8g7a 05-28-98 08:56:55 Subsequently, these nuclei aggregate into larger clusters or microspheres. Finally, these overlap or aggregate to form the final porous polymer bead. The different pores (micropores, mesopores, and macropores) correspond to the interstitial volumes between the nuclei and their aggregates. This explanation of the pore generation process accounts well for the broad pore size distribution generally observed in suspension polymerization.2 Precipitation Polymerization The mechanisms of precipitation polymerization of DVB-55 have been discussed earlier.19 The polymerization mixtures are initially homogeneous solutions of monomer (DVB-55) and initiator (AIBN) in acetonitrile. Monomer loadings must be kept low (2–5 vol. % of DVB-55) to prevent coagulation. Polymerization starts in the homogeneous phase, and the oligomers aggregate to form colloidally stable particles. These particles grow by capturing oligomers and monomer, to ultimately form monodispersed, crosslinked particles having diameters between 1 and 8 mm. In this paper we report the introduction of pores into divinylbenzene precipitation polymer particles by using 5–40% toluene as a cosolvent during the polymerization. Below we describe the resulting porous particles and compare them with polca W: Poly Chem 97135p 1546 LI AND STÖVER Figure 1. Particle size distributions of precipitation poly(divinylbenzene) particles prepared in the presence of different toluene volume fractions. the suspension polymer particles prepared earlier using toluene and toluene/dodecanol as porogens.8 Particle and Sol Fractions in Precipitation Polymerization Most of the polymerizations were carried out at 2 vol. % DVB-55 loading and 2 wt. % AIBN loading relative to monomer. The toluene content in the toluene/acetonitrile solvent mixture was varied from 0 to 100%, and the results are shown in Table I. Mono- or narrow-dispersed particles were obtained in presence of 0–40 vol. % toluene. The average particle diameter increased from 4.0 mm in neat acetonitrile to 6.6 mm in presence of 25 vol. % toluene and then decreased to 4.6 mm at 40 vol. % toluene. All particle batches have narrow size distributions, with coefficients of variation 24 near or below 5%. Figure 1 shows the Coulter particle size distributions of the particles pre- / 8G7A$$135P 8g7a 05-28-98 08:56:55 pared at the same monomer loading but with various toluene contents. The particle size maximum around 25 vol. % toluene is quite noticeable. As well, the shape of the particle size distribution changes from columnar to a more rounded form at higher toluene fractions. The total conversion to particles is around 50% at 2% monomer loading. For comparison, Figure 2 shows the broad particle size distributions for suspension polymer particles prepared in presence of a 40/60 ratio of toluene/dodecanol porogen (Fig. 2A) and in presence of neat toluene as porogen (Fig. 2B).8 In both cases, the ratio of (total) porogen to monomer was 1.4 : 1. In presence of 45 or 50 vol. % toluene as cosolvent the whole reaction mixture gelled. Only soluble polymer was isolated from polymerizations in neat toluene, without any precipitate being formed. These results are in agreement with the formation of soluble polymer microgels reported by Walczynski et al. during polymerization of commercial divinylbenzene in neat toluene.25 Soluble polymer fractions were isolated at all the toluene volume fractions. The molecular weight and the amount of the sol fractions, in general, increase with toluene volume fraction, reflecting the increasing solvency of the medium (Table I). This sol fraction resembles the Staudinger microgels recently reviewed by Antonietti.26 They will be discussed further in a subsequent paper. The last three entries in Table I indicate that the average particle size increases with the monomer loading. Monomer loadings above 5 vol. % commonly resulted in coagulation. The reaction media contain residual monomer and soluble oligomers and can be reused directly. In ‘‘recycling’’ experiments we used the filtrates of polymerizations having toluene volume frac- Figure 2. Particle size distributions of suspension poly(divinylbenzene) particles prepared using (A) toluene/dodecanol (40 : 60) and (B) toluene as porogens. In both cases, the total porogen comprises 60% of the organic phase, the remainder being monomer. polca W: Poly Chem 97135p POROUS MONODISPERSE MICROSPHERES 1547 Figure 3. Scanning electron micrographs of precipitation poly(divinylbenzene) particles prepared with toluene volume fractions of (A) 0% and (B) 40% and of suspension poly(divinylbenzene) particles prepared with (C) toluene as porogen and with (D) a mixture of toluene/dodecanol (40 : 60) as porogen. In both (C) and (D) the total porogen amounts to 60%, the remainder being monomer. tions of 0, 20, and 40%, as reaction media for subsequent polymerizations, by simply adding the appropriate amounts of DVB-55 and AIBN. The resulting microspheres are again mono-, or narrow-disperse, indicating that the residual monomer and soluble oligomer did not affect the particle formation. External Particle Morphology by Scanning Electron Microscopy (SEM) Figure 3 shows the SEM images of poly(divinylbenzene) particles prepared by precipitation polymerization in neat acetonitrile (Fig. 3A) and in a toluene/acetonitrile (40 : 60) mixture (Fig. 3B). As well, the SEM images of the related suspension poly(divinylbenzene) resins described above are shown for comparison (Fig. 3C, using neat toluene as porogen; Fig. 3D, using a toluene/dodecanol (40 : 60) porogen mixture). No apparent pores are seen in Figure 3A–C, though different morphologies are noticeable. The precipitation particles prepared in neat acetonitrile (Fig. 3A) have near perfect spherical shapes. In contrast, the particles prepared at 40% toluene volume fraction have nonspherical shapes (Fig. 3B), including several instances where two or more particles appear to have fused together at an early stage dur- / 8G7A$$135P 8g7a 05-28-98 08:56:55 ing their growth. The suspension microspheres in the last sample (Fig. 3D, using a comparatively nonsolvent porogen) have a visibly porous structure. Internal Particle Morphology by Transmission Electron Microscopy The four types of particles described above were embedded in Spur epoxy resin and microtomed to produce 50–60 nm thick sections for transmission electron microscopy. Figure 4A,B corresponds to the precipitation particles prepared in neat acetonitrile and in acetonitrile/toluene (60 : 40), respectively. Figure 4C,D shows the suspension polymer particles prepared using toluene and toluene/dodecanol (40 : 60), respectively, as porogens. The nonporous precipitation polymer particles (Fig. 4A) show significant cutting artifacts arising from the microtome knife passing through the hard, crosslinked particles. The porous precipitation polymer particles (Fig. 4B) show a mottling effect at the limit of resolution, indicating the presence of pores in the low nanometer range. The suspension polymer particles show pores with diameters of about 20 nm (Fig. 4C) and 100 nm (Fig. 4D), reflecting the different porogens used. polca W: Poly Chem 97135p 1548 LI AND STÖVER Figure 4. Transmission electron micrographs of the same particles as shown in Figure 3A–D: precipitation poly(divinylbenzene) particles prepared with toluene volume fractions of (A) 0% and (B) 40% and suspension poly(divinylbenzene) particles prepared with (C) toluene as porogen and with (D) a mixture of toluene/dodecanol (40 : 60) as porogen. In both (C) and (D) the total porogen amounts to 60%, the remainder being monomer. Porosity Analysis Using Nitrogen Adsorption (BET) The porosity of the precipitation polymer particles prepared with different toluene volume fractions was measured using nitrogen adsorption (BET), the method of choice for small pores.8 Table II shows that the total pore volume and the surface area of the particles increase with increasing toluene volume fractions. Particles prepared using toluene volume fractions of 40% show large specific surface areas of up to 800 m2 /g. Figure 5 compares the pore size distributions of precipitation polymer particles (Figure 5A–D) and suspension polymer particles (Figure 5E,F). Precipitation particles prepared in neat acetonitrile show negligible porosity and a surface area of around 9 m2 /g (Fig. 5A). Addition of toluene / 8G7A$$135P 8g7a 05-28-98 08:56:55 as a cosolvent increases the surface area and the total pore volume. A narrow peak at 15–20 Å pore radius, corresponding to about 20% of the total pore volume, appears for all precipitation polymer particles prepared in the presence of toluene (Figure 5B–D). The overall pore size distribution for these particles is fairly narrow and concentrated below 40 Å pore radius. In contrast, the suspension polymer particles display a much broader range of pore sizes (Figure 5E,F). Mechanism of Pore Formation in Precipitation Polymerization It is instructive to compare precipitation polymerization with suspension polymerization. In both polca W: Poly Chem 97135p POROUS MONODISPERSE MICROSPHERES 1549 Table II. Porosity Analysis Toluene (Cosolvent/ Porogena) (vol %) DVB-55 (vol %) Precipitation particles 0 5 20 30 35 40 40 Suspension particles 60a,b 24 t/36 da,c 2 2 2 2 2 2 4 40 40 AIBN (wt % Rel. to Monomer) Surface Area (Multipoint BET) (m2/g) Total Pore Vol (for Pore Range) (cm3/g) 2 2 2 2 2 2 2 9 30 480 530 650 810 670 0.013 0.029 0.248 0.293 0.363 0.516 0.417 (õ790 (õ660 (õ570 (õ620 (õ600 (õ480 (õ520 3.4 3.4 610 180 1.040 (õ1380 Å) 0.645 (õ1450 Å) Major Peak Radius (Å) Å) Å) Å) Å) Å) Å) Å) 16 15 16 20 15 21 16 a Suspension polymerization.8 60% toluene and 40% DVB-55 in the organic phase. c 24% toluene, 36% dodecanol, and 40% DVB-55 in the organic phase. b systems, polymerization starts as an organic solution polymerization, and the first stage involves formation of lightly crosslinked oligomer radicals (nuclei). These grow and crosslink to form microspheres. At this point the two processes diverge: in suspension polymerizations, with their high monomer loadings of around 40 vol. % in the organic phase, these microspheres continue to grow and overlap until each droplet has become one interconnected porous polymer network. Precipitation polymerizations, on the other hand, have monomer loadings of only 2–5 vol. %. Hence the microspheres do not overlap but continue to grow individually by capturing subsequently formed oligomers and monomer. In a nonsolvent medium such as acetonitrile, phase separation occurs early and leads to nonporous polymer microspheres dispersed within the continuous acetonitrile liquid phase. Rather than creating large interstitial pores as in suspension polymerization, nonsolvents alone do not cause porosity in precipitation polymerization. Replacing some of the acetonitrile with toluene improves the solvency of the reaction medium. Phase separation will now occur later during the polymerization, causing some solvent to remain trapped inside the crosslinking polymer particles and leading to the small pores observed. This particle growth mechanism is not likely to create large pores. Indeed, the TEM photograph (Fig. 4B) and the porosity analysis from BET (Fig. 5A– D) confirm the absence of pores having radii larger than 4 nm. / 8G7A$$135P 8g7a 05-28-98 08:56:55 Effects of Other Cosolvents We have tested the effects of using benzene, 1,4dioxane, methyl ethyl ketone (MEK), and xylenes as cosolvents with acetonitrile. Spherical particles with bimodal size distributions were obtained from polymerizations in acetonitrile mixtures containing 15, 25, and 35 vol. % benzene or 30% MEK, as cosolvents. Reaction mixtures containing 15, 25, or 35 vol. % 1,4-dioxane gave ‘‘doublet particles’’ and ‘‘triplet particles’’ (two and three particles aggregated early during the polymerization). Only xylenes, used in 15, 25, and 35 vol. % as cosolvent, gave narrow dispersed spherical particles. Particles formed in the presence of 35% xylenes have pore volumes and surface areas comparable to those of the particles prepared in the presence of 35% toluene. Particle Characterization by Size Exclusion Chromatography Polymer microspheres having small pores can be used for size exclusion chromatography of small molecules and oligomers.27,28 The porous precipitation polymer particles prepared in the presence of 25 vol. % toluene were slurry packed into a stainless steel column (4.6 1 250 mm) as described earlier.8 Figure 6 shows a representative detector trace of the separation of polystyrene standard with molecular weight of 580 ( 1), 1,2diphenylethane (2), and toluene (3). The small peak (4) corresponds to oxygen dissolved in the sample solutions.29 The compounds are almost polca W: Poly Chem 97135p 1550 LI AND STÖVER baseline separated, and the small shoulder on the PS580 peak likely corresponds to a trace of the styrene tetramer. This separation is complete in 7 min at a flow rate of 0.5 mL/min and is significant considering the relatively low pore volume of 0.27 cm3 /g. The column has a very sharp exclusion limit of about 500 Da, with all polystyrenes having higher molecular weight being excluded from the pores, and eluting with almost the same retention volume as PS 580. These results are again in agreement with the BET results that show no significant porosity above 4 nm pore radius. This sharp molecular weight cutoff may make these particles useful for the selective retention and separation of small organic molecules. The large surface area may prove useful in reverse phase HPLC applications. Figure 6. Size exclusion chromatogram of a mixture comprising a linear polystyrene standard with a molecular weight of 580 ( 1), 1,2-diphenylethane ( 2), and toluene (3). Peak 4 is an artifact, corresponding to dissolved oxygen. Column: 4.6 mm 1 250 mm. Column packing: precipitation poly(divinylbenzene) particles prepared with 25 vol. % toluene. Mobile phase: tetrahydofuran at 0.5 mL/min. UV detector: 254 nm. CONCLUSION We have shown that mixtures of acetonitrile with good cosolvents such as toluene can be used to form precipitation polymer microspheres having significant porosity. Highly crosslinked monodisperse particles in the micrometer size range, having nanopores, large specific surface areas, and clean surfaces, can thus be prepared in a single step. The total pore volume and the specific surface area increase with the amount of cosolvent in the reaction medium. Total pore volumes of 0.52 cm3 /g and surface areas of 800 m2 /g were obtained for particles prepared with toluene volume fractions of 40%. These particles may have applications as selective separation resins or sorbents. The clean, stabilizer-free surfaces may be useful for protein binding studies in biochemistry. We thank the Ontario Centre for Materials Research and the National Research Council of Canada for their financial support of this work. Figure 5. Pore size distribution of the poly(divinylbenzene) particles prepared by precipitation polymerization with (A) 0% toluene, 2% monomer loading, (B) 20% toluene, 2% monomer loading, (C) 40% toluene, 2% monomer loading, and (D) 40% toluene, 4% monomer loading and prepared by suspension polymerization with (E) toluene as porogen and (F) a mixture of toluene/dodecanol (40 : 60) as porogen. In both (E) and (F) the total porogen amounts to 60%, the remainder being monomer. / 8G7A$$135P 8g7a 05-28-98 08:56:55 REFERENCES AND NOTES 1. J. 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