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Sulfonated Styrene-Divinybenzene Resins: Optimizing Synthesis and Estimating Characteristics of the Base Copolymers and the Resins Muhammad Arif Malik,*,†,‡ Syed Wasim Ali,† and Imtiaz Ahmed† Applied Chemistry Laboratories, PINSTECH, PO Nilore, Islamabad 44000, Pakistan, and Frank Reidy Research Center for Bioelectrics, Old Dominion UniVersity, 4211 Monarch Way, Suite 300, Norfolk, Virginia 23508
Maximum sulfonation and minimum carboxylation are desirable in sulfonated styrene-divinylbenzene cation-exchange resins. These characteristics may be achieved with greater control of porosity of the base copolymers. Optimization of the porosity by varying both the nature and amount of porogen, and crosslinking for maximizing sulfonation and minimizing carboxylation, are reported. Predictors of the characteristics obtained by statistical analysis of the data, and the ranges of conditions in which they can be employed, are discussed. The optimum conditions are cross-linking 20% to 23% and pore volume 0.14 mL/g to 0.3 mL/g. The relationship of the particle size with porosity of the base copolymer and capacity of the resins is discussed. The effects of residual porogen/homopolymer on the sulfonation of these copolymers are also discussed. Introduction Macroporous (also called macroreticular) styrene-divinylbenzene (St-DVB) copolymer beads produced by suspension polymerization were first reported in the 1960s.1,2 The monomers are diluted with porogen, an inert material, which is removed at the completion of the polymerization, leaving behind permanent pores. The porosity is usually controlled by three experimental parameters: (1) nature of the porogen (solvent, nonsolvent, etc.), (2) amount of porogen, and (3) cross-linking.3,4 The copolymers are converted to different products for use in various industrial processes.5-7 Exploration of properties and optimization of synthesis of macroporous copolymers is an active area of research, which is evident from review articles2,8-13 and recent research reports.14-19 Strong acid cation-exchange resins carrying SO3H groups are commercially important materials produced by sulfonation of macroporous St-DVB copolymers with concentrated sulfuric acid at high temperature (∼100 °C).5,20-25 A recent study has revealed that the quantitative extent of sulfonation reaction, i.e., percentage of aromatic rings in a St-DVB copolymer that have incorporated the sulfonic acid group, decreases with an increase of cross-linking.24 We have reported some side-reactions yielding COOH groups during sulfonation of St-DVB copolymers.25 We have reported dependence of ion-exchange capacity per unit volume of the resins on pore volumes of the base copolymers.22 Greater extent of sulfonation, higher capacity, and lower carboxylation are desirable. The present manuscript describes optimization of these three properties by fine-tuning the composition of the polymerization mixture at the time of synthesis of the copolymers. We have reported that the porosity varies with particle size within a single batch of 4-vinylpyridine-divinylbenzene copolymers.26 The dependence of porosity on particle size of StDVB copolymers and its effects on the sulfonation reaction is also reported in this manuscript. Experimental Section Synthesis and Characterization of the Copolymers. StDVB copolymers were synthesized by the suspension polym* To whom correspondence should be addressed. E-mail: MArifMalik@ gmail.com. † PINSTECH. ‡ Old Dominion University.
erization technique previously reported.25 The polymerization mixture was composed of monomers, porogen, and a polymerization initiator. Monomers were composed of styrene and divinylbenzene. Divinylbenzene is composed of 60% divinylbenzene isomers and 40% impurities, which were mostly ethyl vinyl benzene isomers. Cross-linking is expressed as the percentage of pure divinylbenzene isomers in the total monomers. Porogen is composed of petroleum ether (PE) having a boiling range of 140 to 180 °C and/or diethyl phthalate (DET). Dilution is expressed as the percentage of porogen in the polymerization mixture. Benzoyl peroxide (the polymerization initiator) was 1% of the polymerization mixture. The aqueous phase was prepared by dissolving 1% by weight of each, gum arabic and gelatin, in demineralized water. One part polymerization mixture was poured into five parts by volume of the aqueous phase in a jacketed reaction vessel and stirred with a twin-blade turbine at about 200 rpm at room temperature for 30 min. The temperature was raised to 80 °C in 30 min, maintained at 80 °C for the next 20 h, and finally raised to about 98 °C for 2 h for curing the copolymer. If needed, the speed of the turbine was readjusted to obtain more than 90% of the particle diameters in the range of 40-110 µm in all experiments except when otherwise stated. The copolymer beads were filtered out and washed with hot water. Excess water was removed by centrifuge and the volume of the copolymer beads (Vp) loaded with porogen was measured. The porogen and homopolymer were then extracted with acetone. The copolymer beads were left in acetone for about 20 h. Excess acetone was removed by centrifuge, and the acetone-swollen volume (Vace) was measured. The beads were air-dried then oven-dried at 110 °C for about 20 h. Weight (Wdry) and volume (Vdry) of the dried beads were measured. The density (d), swelling coefficient of the copolymers loaded with the porogen employed for their synthesis (SCp), and swelling coefficient in acetone (SCa) were calculated by using the following relations: d ) Wdry/Vdry; SCp ) 100(Vp s Vdry)/ Vdry; SCa ) 100(Vace s Vdry)/Vdry. Pore volume, surface area, and pore size distribution of macroreticular pores of the dried copolymers were determined by the mercury penetration method using an Autopore II 29220 mercury porosimeter from Micromeritics. It should be mentioned here that ‘macroreticular porosity’ or ‘macroporosity’ mean pores of a permanent nature
10.1021/ie902057x 2010 American Chemical Society Published on Web 02/19/2010
Ind. Eng. Chem. Res., Vol. 49, No. 6, 2010
that do not disappear upon drying of the copolymers. The pores defining ‘macroporosity’ may be macropores and/or mesopores. Sulfonation of the Copolymers and Characterization of the Resins. The dried copolymer beads of known weight (Wd) and volume (Vd) were stirred into 98% sulfuric acid (H2SO4), in a 1:5 weight-to-volume ratio, at 98 °C for 2 h. The mixture was then poured slowly along the inner sidewall of a beaker and stirred into ice-cold demineralized water; the volume of the water was about 20 times that of the resin slurry. The resin beads were filtered and washed with demineralized water until the effluent was free of acid. The resin was washed with dilute HCl and again with demineralized water. Excess water was removed by centrifuge, and the volume of resin in H-form swollen with water (Vr) was measured. The swelling coefficient of the H-form of the resin in water (SCr) was calculated by using the following relationship: SCr ) 100(Vr - Vd)/Vd. All the resin volume (Vr) was packed in a column and the number of moles of SO3H and COOH groups in it were determined based on moles of HCl produced and moles of NaOH consumed in the following reactions, respectively: P-φ-COOH + P-φ-SO3H + NaCl(aq) f P-φ-COOH + P-φ-SO3Na + HCl(aq) P-φ-COOH + P-φ-SO3Na + NaOH(aq) f P-φ-COONa + P-φ-SO3Na + H2O where P represents polymer backbone, and φ represents aromatic rings attached to the copolymer backbone. The capacity of the resins in moles per litter (mol/L) was calculated from the moles of the functional groups and volumes of the resins as described in more detail in our earlier report.25 The quantitative extent of the sulfonation reaction (sulfonation %) is expressed in terms of percentage of aromatic rings of the copolymer that were sulfonated, which is calculated by using the following relationship: sulfonation % ) (moles of SO3H/Wd)/((fstydsty/104) + (fdvbddvb/131)) where fsty and fdvb are volume fractions of styrene and divinylbenzene, respectively, in the monomer mixture, dsty and ddvb are the densities of styrene and divinylbenzene, 104 is molecular weight of styrene, and 131 is average molecular weight of divinylbenzene (about 60% divinylbenzene isomers and about 40% ethylvinylbenzene isomers). The model curves shown in some figures were calculated by ordinary least-squares regression for the purpose of predicting a dependent variable from the values of an easily measurable dependent variable. The predictor equations are shown in the figure inserts. The R2 coefficient of determination is also shown along with the predictor equation, where R2 is a statistical measure of how well the regression line approximates the real data points. R2 varies from 0.0 to 1.0, where 0.0 indicates that the regression line does not fit and 1.0 indicates that it perfectly fits the data.
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Table 1. Experimental Conditions of Synthesis, Pore Volumes, and Surface Areas of St-DVB Copolymers Synthesized in This Study exp. no.
porogen (PE:DET)
dilution (%)
cross-linking (%)
pore volume (mL/g)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
0:100 40:60 50:50 60:40 75:25 90:10 100:0 0:100 0:100 100:0 100:0 0:100 0:100 0:100 0:100 75:25 75:25 75:25 75:25 100:0 100:0 100:0 100:0
50 50 50 50 50 50 50 40 56 40 56 50 50 50 50 50 50 50 50 50 50 50 50
30 30 30 30 30 30 30 30 30 30 30 15 22 45 60 15 22 38 60 15 22 45 60
0.5610 0.2854 0.2815 0.3478 0.5886 0.8036 0.9658 0.3357 1.2372 0.6150 0.7009 0.0903 0.2237 0.8228 0.8967 0.0901 0.2440 0.6259 0.8918 0.3700 0.8290 1.0361 1.0265
porogen resulted in a decrease in pore volume until the porogen was a 50:50 mixture of PE and DET. A further increase of PE in the porogen increased the pore volume. Usually a nonsolvent component, such as PE, increases the pore volume.3,12 Exceptions to this general trend can be expected,18 because the porosity does not depend on the solubility parameter of the porogen alone; it also depends on the difference between the averaged solubility parameter of the polymerization liquid (including unreacted monomers) and the solubility parameter of the polymer being formed.27 Dilution of the monomers was gradually increased in experiment nos. 8, 1, and 9 and in experiment nos. 10, 7, and 11, resulting in a gradual increase in pore volume. The only exception to this general trend was experiment no. 11. An excess amount of nonsolvent PE was employed in this experiment, which may have resulted in a loss of mechanical strength and, consequently, the collapse of some of the pores during drying. Cross-linking was gradually increased from 15% to 60% in experiment nos. 12 to 15, 16 to 19, and 20 to 23, resulting in a gradual increase in pore volume. Pore size distribution shifted toward larger pores with an increase in pore volume, irrespective of how the pore volume was increased. This trend is illustrated in Figure 1 for the case of experiment nos. 1 to 7. A similar trend was observed when the pore volume was increased as a consequence of increasing the amount of porogen or increasing the cross-linking. When the pore volume was the same, the pore size distribution was also the same under our experimental conditions. Experiment nos.
Results and Discussion Effect of Porogen and Cross-Linking on Macroreticular Porosity of the Base Copolymers. Changing the composition of the polymerization mixture in 23 experiments altered the porosity of the copolymers as listed in Table 1. In the first 7 experiments, the composition of the porogen was varied, keeping all other parameters constant. A gradual increase of PE in the
Figure 1. Pore size distribution curves showing a shift toward large pores as the pore volume was increased gradually from experiment nos. 1 to 7: (a) cumulative pore size distributions, and (b) pore volume density distributions.
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Figure 2. Extent of sulfonation versus cross-linking curve (experiment nos. 12-23).
Figure 4. Curve showing ion-exchange capacity of resins versus pore volume of their base copolymers. Table 2. Experimental Conditions of Synthesis and Characteristics of St-DVB Copolymers and Resins Derived from Them with Improved Capacities exp. no.
porogen (PE:DET)
dilution (%)
cross-linking (%)
pore volume (mL/g)
capacity (mol/L)
24 25 26 27 28
100:0 100:0 0:100 0:100 n-heptane
30 40 30 40 30
30 20 30 20 30
0.2201 0.4666 0.2061 0.1416 0.1909
1.97 2.10 2.21 2.26 2.17
Figure 3. Extent of carboxylation versus cross-linking curve (experiment nos. 12 to 23).
1 and 5 and experiment nos. 2 and 3 have the same pore volumes, which is why Figure 1 appears to show five curves instead of seven. These results are in accordance with the earlier literature.3,4,14-19 Effect of Porogen and Cross-Linking on Sulfonation Reaction. The quantitative extent of the sulfonation reaction (sulfonation %) remained constant at around 75% when the cross-linking was fixed at 30% and pore volume was varied in the range of 0.2 mL/g to 1.2 mL/g by varying the nature or amount of porogen (experiment nos. 1 to 11). A gradual increase in cross-linking from 22% to 60% resulted in the gradual decrease in sulfonation from about 85% to about 55%, irrespective of porogen, as illustrated in Figure 2. The optimum crosslinking was 22%. Decreasing the cross-linking to 15% resulted in a sharp decrease in the extent of sulfonation as shown by solid symbols in Figure 2. Sulfonation was accompanied by carboxylation as a sidereaction.25 The extent of carboxylation remained constant at around 2.3% when the cross-linking was fixed at 30% and pore volume was varied in the range of 0.2 mL/g to 1.2 mL/g by varying the nature or amount of porogen (experiment nos. 1 to 11). A gradual increase in cross-linking from 15% to 60% resulted in a gradual increase in the extent of carboxylation from about 0% to about 8%, irrespective of the porogen, as illustrated in Figure 3. These results support the hypothesis that the COOH groups are formed as a result of oxidation of pendant vinyl or ethyl groups of divinylbenze and ethylvinylbenzene isomers.25 Ion-exchange capacity per unit volume of the resins is another characteristic of interest. The capacity is not directly related to the extent of sulfonation alone because the volumes of the resins vary depending on porosity and degree of swelling. Results of this study show that the capacity increases with a decrease in pore volume, irrespective of how the pore volume is varied, within the limit where pore volume > 0.1 mL/g, as illustrated in Figure 4. From the data in Table 1, the optimum pore volume with respect to the capacity, was 0.2237 mL/g. Some additional
Table 3. Effect of Bead Size Synthesized under Identical Experimental Conditions on Pore Volume of St-DVB Copolymers and Capacity of the Resins Derived from Them exp. porogen dilution cross-linking particle diam pore volume capacity no. (PE:DET) (%) (%) (µm) (mL/g) (mol/L) 29a 30a 31 32 33 34
50:50 50:50 50:50 50:50 50:50 50:50
50 50 50 50 50 50
30 30 30 30 30 30
45-150 150-250 20-60 20-60 100-200 400-500
0.2643 0.3471 0.2267 0.2250 0.3981 0.4617
2.00 1.77 1.92 2.01 1.72 1.56
a Copolymer from a single batch was segregated into size fractions mentioned in experiment nos. 29 and 30.
Figure 5. Surface area versus pore volumes curve for St-DVB copolymers synthesized in this study.
experiments were done to find the lowest limit of pore volume beyond which the capacity increases. The results of the additional experiments are listed in Table 2. First, experiment no. 10 was repeated using a lower dilution of monomers (experiment no. 24) and with lower cross-linking (experiment no. 25). Both changes resulted in lower pore volume and higher capacity, as shown in Table 2. Then experiment no. 8 was repeated using a lower dilution of monomers (experiment no. 26) and with lower cross-linking (experiment no. 27). As before,
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Figure 6. Swelling coefficients versus pore volumes curves: (a) swelling coefficient of the copolymers in acetone, (b) swelling coefficient of copolymers loaded with porogen employed for their synthesis, (c) swelling coefficient of H-form-resins in water.
the pore volume decreased and the capacity increased. n-Heptane was also tested in one experiment (experiment no. 28). All the experiments in Table 2 were repeated under the same conditions with the exception that the porogen was not extracted from the copolymer prior to sulfonation. The capacity of the resin was the same in both cases using PE and n-heptane as the porogen. The capacity was better and reproducible in the presence of the porogen when DET was used. The idea of subjecting the copolymer to sulfonation without prior extraction of the porogen appears attractive from an economical point of view, but it cannot be recommended as a general procedure, because different porogens may react differently with sulfuric acid. DET might have been hydrolyzed to phthalic acid and ethanol by sulfuric acid. Part of the additional capacity may be due to the ion-exchange groups on the reacted porogen/homopolymer, which may leak out during the use of the resins. The copolymers in this study were subjected to repeated cycles of treatment with dilute NaOH followed by dilute HCl. No signs of loss of the capacity were observed. For example, the standard deviation was only 0.051 in six repeated analyses of resin from experiment no. 26 with no sign of decrease in the capacity. The reacted residue, being hydrophilic, was apparently leached out during the washing of the resins. The macropores in these resins may have allowed the fast leachout of the residues. Still this cannot be recommended as a general procedure, as the residues may not be so easily washed out from every copolymer. It is possible that some porogen, e.g., n-heptane, may be extracted by a steam-distillation process during the curing of the copolymer after the polymerization reaction. This possibility was tested in experiment no. 28 for six repeated syntheses with a standard deviation in the capacities of only 0.060. Effect of Bead Size on the Porosity of the Base Copolymers and Ion-Exchange Capacity of the Resins Derived from Them. We have observed different porosities in smaller and larger particles coming from a single batch in the case of 4-vinylpyridine-divinylbenzene copolymers.26 A comparison of experiment no. 29 with experiment no. 30 in Table 3 shows the same trend in the case of St-DVB copolymers. Comparison of experiment nos. 31, 32, 33, and 34 in Table 3 shows that the copolymers obtained from the same polymerization mixture have different porosities if their average particle sizes are different. Varying the speed of the turbine in the suspension polymerization reactor varied the particle size ranges in these repeated experiments. It can be observed from the last column in Table 3 that the capacities of the resins also varied when the pore volumes of the base copolymers varied. The variation of porosity in different particle sizes may be explained on the basis of Laplace pressure, i.e., the effect caused by the surface tension of the interface between the aqueous phase and the oil phase droplets during the suspension polymerization.
Figure 7. Pore volume versus density curve for St-DVB copolymers synthesized: in this study (×), and in an earlier study (∆).17
Because of the Laplace pressure, all of the particles might have squeezed out a fraction of the porogen during the polymerization reaction. This effect might be greater in the case of smaller particles, because of the larger pressure on them and the larger surface-tovolume ratio as compared with the larger particles. This hypothesis is supported by the fact that an oily layer is usually observed on top of the reaction mixture at the completion of the polymerization. Simple Methods of Estimating Characteristics of the Copolymers and Resins Derived from Them. The best-fit equations that can be used for predicting the extent of sulfonation and carboxylation and the capacity of the resins are shown in Figures 2, 3, and 4. Figure 5 shows that the model curve of the relationship of pore volume to surface area17 is a good fit to the present data. Figure 6 shows swelling coefficients versus pore volumes curves. It can be observed from Figure 6 that, in general, the swelling decreases with an increase in pore volume. The relationship of the swelling coefficient in acetone (SCa) with the pore volume taken from an earlier study,17 predicts the trend of SCa (Figure 6a), the swelling of porogen-loaded copolyemrs (SCp, Figure 6b), and the swelling of the H-form of resins in water (SCr, Figure 6c) in the present study. However, it should be kept in mind that the predicted values are reasonably close to the actual values in the case of SCa but not in the case of SCp and SCr. Figure 6c shows a lot of deviation of the experimental values from the model curve. The swelling of the resins is controlled by multiple factors, such as the porosity of the copolymer matrix, the extent of sulfonation, etc., which explains the deviations. Figure 7 shows the relationship of densities with the pore volumes of the copolymers. The model curve from an earlier study28 was refined on the basis of the data of the present study. Data points from an earlier study17 (triangle symbols in figure 7) also fit well on the refined model, confirming its greater accuracy. Conclusions 1. Sulfonation increases and carboxylation decreases with a decrease in cross-linking, up to 22% cross-linking.
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2. Ion-exchange capacity per unit volume of the resin increases with a decrease in pore volume, up to the pore volume of 0.14 mL/g. 3. The porosity varies with the particle size of the St-DVB copolymers within the same batch. 4. Several characteristics of the macroporous St-DVB copolymers and the resins derived from them can be predicted with a reasonable degree of accuracy from cross-linking or easily measurable densities of the copolymers. Acknowledgment The authors thank the reviewers for their valuable suggestions, which improved the clarity and quality of the manuscript. The authors also thank Barbara C. Carroll and Thomas Camp of Frank Reidy Research Center for Bioelectrics for proofreading the manuscript. Literature Cited (1) Kun, K. A.; Kunin, R. Macroreticular resins. III. Formation of macroreticular styrene-divinylbenzene copolymers. J. Polym. Sci., Part A: Polym. Chem. 1968, 6 (10), 2689. (2) Seidl, J.; Malinsky´, J.; Dusˇek, K.; Heitz, W. Makroporo¨se StyrolDivinylbenzol- Copolymere und ihre Verwendung in der Chromatographie und zur Darstellung von Ionenaustauschern. AdV. Polym. Sci. 1967, 5, 113. (3) Sederel, W. L.; De Jong, G. J. Styrene-divinylbenzene copolymers. Construction of porosity in styrene divinylbenzene matrices. J. Appl. Polym. Sci. 1973, 17, 2835. (4) Jacobelli, H.; Bartholin, M.; Guyot, A. Styrene divinylbenzene copolymers. 1. Texture of macroporous copolymers with ethyl-2-hexanoic acid in diluent. J. Appl. Polym. Sci. 1979, 23, 927. (5) Dorfner, K. Ion Exchangers; Walter de Gruyter: New York, 1991. (6) Alexandratos, S. D. Ion-Exchange Resins: A Retrospective from Industrial and Engineering Chemistry Research. Ind. Eng. Chem. Res. 2009, 48, 388. 388. (7) Barbaro, P.; Liguori, F. Ion Exchange Resins: Catalyst Recovery and Recycle. Chem. ReV. 2009, 109, 515. (8) Guyot, A.; Bartholin, M. Design and properties of polymers as materials for fine chemistry. Prog. Polym. Sci. 1982, 8, 277. (9) Arshady, R. Beaded polymer supports and gels: I. Manufacturing techniques. J. Chromatogr. 1991, 586, 181. (10) Arshady, R. Beaded polymer supports and gels: II. Physico-chemical criteria and functionalization. J. Chromatogr. 1991, 586, 199. (11) Svec, F.; Frechet, J. M. J. New designs of macroporous polymers and supports: Form separation to biocatalysis. Science 1996, 273, 205. (12) Okay, O. Macroporous copolymer networks. Prog. Polym. Sci. 2000, 25, 711. (13) Liu, Q.; Wang, L.; Xiao, A. Research progress in macroporous styrene-divinylbenzene co-polymer microspheres. Des. Monomers Polym. 2007, 10, 405.
(14) Garcia-Diego, C.; Cuellar, J. Synthesis of Macroporous Poly(styreneco-divinylbenzene) Microparticles Using n-Heptane as the Porogen: Quantitative Effects of the DVB Concentration and the Monomeric Fraction on Their Structural Characteristics. Ind. Eng. Chem. Res. 2005, 44, 8237. (15) Garcia-Diego, C.; Cuellar, J. Determination of the Quantitative Relationships between the Synthesis Conditions of Macroporous Poly(styreneco-divinylbenzene) Microparticles and the Characteristics of Their Behavior as Adsorbents Using Bovine Serum Albumin as a Model Macromolecule. Ind. Eng. Chem. Res. 2006, 45, 3624. (16) Dragan, E. S.; Avram, E.; Dinu, M. V. Organic ion exchangers as beads. Synthesis, characterization and applications. Polym. AdV. Technol. 2006, 17, 571. (17) Malik, M. A.; Ali, S. W.; Waseem, S. A Simple Method for Estimating Parameters Representing Macroporosity of Porous StyreneDivinylbenzene Copolymers. J. Appl. Polym. Sci. 2006, 99, 3565. (18) Scheler, S. A Novel Approach to the Interpretation and Prediction of Solvent Effects in the Synthesis of Macroporous Polymers. J. Appl. Polym. Sci. 2007, 105, 3121. (19) Garcia-Diego, C.; Cuellar, J. Design of polymeric microparticles with improved structural properties: Influence of ethylstyrene monomer and of high proportions of crosslinker. Eur. Polym. J. 2008, 44, 1487. (20) Topp, N. E.; Pepper, K. W. Properties of ion-exchange resins in relation to their structure. Part I. Titration curves. J. Chem. Soc. 1949, 3299. (21) Holboke, A. E.; Pinnell, R. P. Sulfonation of polystyrene: Preparation and characterization of an ion exchange resin in organic laboratory. J. Chem. Educ. 1989, 66 (7), 613. (22) Ahmed, M.; Malik, M. A.; Pervez, S.; Raffiq, M. Effect of porosity on sulfonation of macroporous styrene-divinylbenzene beads. Eur. Polym. J. 2004, 40, 1609. (23) Oliveira, A. J. B.; Aguiar, A. P.; Aguiar, M. R. M. P.; Santa Maria, L. C. How to maintain the morphology of styrene-divinylbenzene copolymer beads during the sulfonation reaction. Mater. Lett. 2005, 59, 1089. (24) Toro, C. A.; Rodrigo, R.; Cuellar, J. Sulfonation of macroporous poly(styrene-co-divinylbenzene) beads: Effect of the proportion of isomers on their cation exchange capacity. React. Funct. Polym. 2008, 68, 1325. (25) Malik, M. A. Carbonyl Groups in Sulfonated Styrene-Divinylbenzene Macroporous Resins. Ind. Eng. Chem. Res. 2009, 48, 6961. (26) Malik, M. A.; Naheed, R. Porous 4-Vinylpyridine-Divinylbenzene CopolymerssVarying Porosity in Different Bead Sizes of a Single Batch. e-Polym. 2007, no. 135. (27) Malik, M. A.; Ahmed, M.; Ikram, M. A new method to estimate pore volume of porous styrenesdivinylbenzene copolymers. Polym. Test. 2004, 23, 835. (28) Malik, M. A.; Rehman, E.; Naheed, R.; Alam, N. M. Pore volume determination by density of porous copolymer beads in dry state. React Funct. Polym. 2002, 50, 125.
ReceiVed for reView December 28, 2009 ReVised manuscript receiVed February 8, 2010 Accepted February 10, 2010 IE902057X
