Carbonyl Groups in Sulfonated Styrene−Divinylbenzene Macroporous

Ind. Eng. Chem. Res. 2009, 48, 6961–6965

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Carbonyl Groups in Sulfonated Styrene-Divinylbenzene Macroporous Resins Muhammad Arif Malik* Applied Chemistry Laboratories, PINSTECH, PO Nilore, Islamabad, Pakistan

The generation of carbonyl groups during sulfonation of porous styrene-divinylbenzene copolymers is reported. The carbonyl groups and sulfonic acid groups were verified by infrared spectroscopy and quantified by acid-base titrations. A gradual change in sulfonation reaction conditions from 70 °C/30 min to 98 °C/60 min resulted in a gradual increase in sulfonic acid group concentration from ∼1.85 to ∼3.13 mol/kg and in the carboxylic acid group concentration from ∼0 to ∼0.34 mol/kg. Data on the swelling coefficients in water indicate a slight increase in cross-linking during sulfonation. Conversion of ethyl or residual vinyl groups to carboxylic acid groups, followed by conversion of some of the acid groups to cross-links through an acidcatalyzed acylation reaction, is proposed. The proposed reactions also explain the pH-dependent color change of the resins. Sulfonation of copolymers with varied cross-linkages further supports our assertion that, as the concentrations of ethyl and residual vinyl groups increase, the concentration of sCOOH groups also increases and the resin color becomes darker. 1. Introduction Macroporous styrene-divinylbenzene copolymer beads (ST-DVB) are produced by the suspension copolymerization technique.1,2 The macroporosity is controlled by the dilution of monomers with some inert organic liquid (diluent) and cross-linkages,3,4 as discussed in a number of reviews5-7 and research articles.1-4,8-11 The copolymers are employed as supports for enzymes, catalysts, and so on.12,13 Chemical transformations convert the copolymers into ion-exchange resins for a number of applications in industrial processes.13 Strong acid cation-exchange resins are usually derived from ST-DVB by treatment with 95-99% sulfuric acid at 70-100 °C.13-23 The degree of conversion, that is, the percentage of phenyl rings that acquire a sulfonic acid (sSO3H) group, depends on the time and temperature of the sulfonation reaction,13-16 the macroporosity of the copolymers,20 and the divinylbenzene content.23 This study reveals that a small fraction of carbonyl groups (sCOOH, >CdO, etc.) is also formed during the sulfonation of ST-DVB copolymers. From both practical and academic points of view, the study of the generation and reactions of carbonyl groups is important. Carboxylic acid groups (sCOOH) are cation-exchange sites, which have properties significantly different from those of sSO3H groups. For practical applications, it is desirable to determine the experimental conditions under which the concentration of carbonyl groups can be minimized without adversely affecting other desirable properties of the resins. From an academic point of view, it is of interest to understand the side reactions of the sCOOH groups under the sulfonation reaction conditions. The unknown aspects of these side reactions during sulfonation of ST-DVB copolymers are discussed in this article. 2. Experimental Methods Synthesis of the Copolymers. Styrene (ST) (99%) and divinylbenzene (DVB) [60% divinylbenzene isomers, with the rest being mostly ethylvinylbenzene (EVB) isomers] were individually treated with dilute sodium hydroxide solution and * To whom correspondence should be addressed. E-mail: [email protected].

washed three times with water. The polymerization mixture was prepared by mixing styrene, divinylbenzene, petroleum ether (boiling range of 140-180 °C), diethylphthalate, and 1% (w/ v) benzoylperoxide. The aqueous phase was prepared by dissolving 1% by weight of each gum arabic and gelatin in demineralized water. One part of polymerization mixture was poured into five parts of aqueous phase by volume 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. The copolymer beads were filtered out and washed with hot water. Unreacted monomers and diluents were removed by washing the polymer beads with acetone until the effluent formed no turbidity upon mixing with water. Characterization of the Copolymers. The washed polymer beads were dried in an oven at 110 °C for 24 h. Their dry weight (Wpolym) was determined. Dry volume (Vpolym) was determined by packing the beads in a measuring cylinder under tapping. The density (d) was calculated using formula d ) Wpolym/Vpolym. The pore volume of the dried copolymers was determined by the mercury penetration method using an Autopore II 29220 mercury porosimeter from Micromeritics, Norcross, GA. FTIR spectra of the dried copolymers, mixed with KBr, in the form of disks were recorded on a MIDAC, M-Series FTIR instrument. Sulfonation of the Copolymers. The dried copolymer beads, in a 1:4 weight to volume ratio, were stirred in 98% sulfuric acid (H2SO4). The mixture was then poured slowly along the inner side wall of a beaker with stirring 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. Characterization of the Resins. The resin was soaked in water for more than 24 h. Excess water was removed by centrifuge, and the wet volume of the resin (Vwet) was measured in the tube used to hold the resin in the centrifuge machine. The resin was dried in air and then in an oven at 110 °C, until a constant dry weight (Wdry) was achieved. The dry weight was measured immediately after the resin had been cooled in a desiccator because it can absorb moisture from air.24 The procedures for the determination of the dry volumes (Vdry), densities, FTIR spectra, and pore volumes of the resins were

10.1021/ie900681n CCC: $40.75  2009 American Chemical Society Published on Web 06/05/2009

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Table 1. Experimental Conditions and Characteristics of Resins Synthesized by Varying Temperature or Time of Sulfonation Reaction sSO3H

sCOOH

expt no.

temperature (°C)

time (min)

swelling coefficient (%)

density (g/mL)

pore volume (mL/g)

mol/L

mol/kg

mol/L

mol/kg

1a 2 3 4 5 6

70 90 98 98 98

30 30 30 60 180

25 41 60 61 61 57

0.51 0.68 0.69 0.68 0.72 0.74

0.2712 0.1669 0.1544 0.1398 0.1361 0.1386

0.89 1.20 1.29 1.40 1.77

1.85 2.78 3.05 3.13 3.75

NDb ND 0.05 0.15 0.12

ND ND 0.12 0.34 0.22

a Represents styrene-divinylbenzene base copolymer synthesized from polymerization mixture comprising 20% styrene, 20% divinylbenzene (60% pure), 30% petroleum ether, and 30% diethylphthalate. b ND, not detected.

the same as described for the copolymers. The swelling coefficients of the resins in water (SCs) were calculated using formula SC ) (Vwet - Vdry)100/Vdry. The resin was soaked in water, packed in a column (of diameter D), and washed with water repeatedly until a constant resin bed height (h) was attained. The resin bed volume (RBV) was calculated using the formula RBV ) πD2h/4. The resin was treated with 2 M HCl in a volume of about 10RBV and was then washed with demineralized water until the effluent was neutral. A volume of about 4RBV of 2 M NaCl was passed through the resin, followed by about 3RBV of demineralized water, and as a result, a volume of 7RVB (VHCl) of effluent was collected therefrom. After that, about 2RBV (VNaOHin) of 0.2 M NaOH ([NaOH]in) was passed through the same resin, followed by about 3RBV of demineralized water, and as a result, the combined effluent of about 5RBV (VNaOH) was collected therefrom. The molar concentration of hydrochloric acid ([HCl]) in the first combined effluent (VHCl) and the molar concentration of sodium hydroxide ([NaOH]eff) in the second combined effluent (VNaOH) were analyzed by acid-base titrations. The concentration of sSO3H groups was calculated in moles per liter and in moles per kilogram as ([HCl]VHCl)/RBV and ([HCl]VHCl)/Wdry, respectively. The concentration of sCOOH groups was calculated in moles per liter and in moles per kilogram as ([NaOH]inVNaOHin - [NaOH]effVNaOH)/RBV and ([NaOH]inVNaOHin - [NaOH]effVNaOH)/Wdry, respectively. Averages of three analyses are reported. The individual values lie within (5% of the average. 3. Results and Discussion The compositions of the polymerization mixtures, the experimental conditions of the sulfonation reactions, and the results of the characterizations are listed in Table 1 for experiments 1-6 and in Table 2 for experiments 7-11. Experiment 1 describes the ST-DVB copolymer chosen for subsequent sulfonation under varying conditions. The degree of sulfonation was varied by changing the temperature (experiments 2-4) and the duration of the sulfonation reaction (experiments 4-6). The sSO3H groups were quantified based on an analysis of the HCl produced in the reaction PsφsCOOH + PsφsSO3H + NaCl(aq) f PsφsCOOH + PsφsSO3Na + HCl(aq) where P represents the polymer backbone and φ represents the phenyl ring. Then, the sCOOH groups were quantified based on the consumption of NaOH in the reaction PsφsCOOH + PsφsSO3Na + NaOH(aq) f PsφsCOONa + PsφsSO3Na + H2O During sulfonation, when the temperature was set at 70, 90, and 98 °C, the evaluated concentrations of sSO3H groups were

1.85, 2.78, and 3.05 mol/kg, respectively (experiments 2-4). At a constant temperature of 98 °C, when the duration of the reaction was set at 30, 60, and 180 min, the determined concentrations of sSO3H groups were 3.05, 3.13, and 3.75 mol/ kg, respectively (experiments 4-6). The concentration of sSO3H in the resin (i.e., the degree of sulfonation) increased with increasing temperature and/or duration of sulfonation reaction, as expected.13-16 The concentration of sSO3H groups usually ranges from 2 to 4 mol/kg in commercial macroporous resins.13 The same range was also reported in a recent study on the sulfonation of macroporous ST-DVB copolymers.23 Lightly cross-linked ST-DVB, such as 4% divinylbenzenes, yields an average weight of 105.2 g/unit, where unit means styrene or divinylbenzene or ethylvinyl benzene component incorporated in the copolymer. Substitution of one sulfonic acid group per aromatic ring yields an average weight of 185.2 g/unit or 5.40 mol of sSO3H groups/kg.17 Highly cross-linked ST-DVB copolymer, such as 60% divinylbenzene isomers and 40% ethylvinylbenzene isomers, yields an average weight of 130.9 g/unit. Substitution of one sSO3H group per phenyl ring yields an average weight of 210.9 g/unit or 4.74 mol of sSO3H groups/kg. The results of this study are in total agreement with the calculations and literature values.13,17,23 In general, the density of the resins increases with increasing degree of sulfonation (experiments 2-6). The pore volume of the resins decreases with increasing degree of sulfonation. Replacement of lightweight sH (1 amu) with bulky sSO3H (81 amu) explains the increase in the density and decrease in the pore volume with increasing degree of sulfonation. The results in Table 1 show that sCOOH groups were detected when the sulfonation was carried out at g98 °C. When the sulfonation was carried out at 98 °C for 60 min, the sCOOH groups were at their maximum. Infrared spectra of a typical ST-DVB base copolymer and the sulfonated resins derived from it are compared in Figure 1. The spectra of the rest of the base copolymers and resins were, in general, similar to those shown in Figure 1 and reported in the literature.22 Sulfonation of ST-DVB copolymers results in the appearance of three broad and strong peaks: one at 1050-1300 cm-1 assigned to sSO3H groups; the second, comprising multiple and overlapped peaks, at 1630-1850 cm-1 assigned to carbonyl groups, i.e., sCOOH, >CdO, etc.; and the third at 3200-3700 cm-1 assigned to -OH groups and/or moisture.25 Infrared spectra of the ST-DVB base copolymer (experiment 1) and the sulfonated resins derived from it (experiments 2-6) are compared in Figure 2. The signal intensity of the sSO3H signal at 1050-1300 cm-1 was found to increase in the order experiment 2 < experiment 3 < experiment 4 < experiment 5 < experiment 6, which is in accordance with the observed increase in the concentration of sSO3H groups analyzed by acid-base titration. The signal intensity of carbonyl groups in the range

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Table 2. Experimental Conditions and Characteristics of Resins Derived from the Base Copolymers Synthesized by Varying the Styrene (St)/Ethylvinylbenzenes (EVB)/Divinylbenzenes (DVB)/Petroleum Ether (PE) Ratio in the Polymerization Mixture density (g/mL) expt no. 7 8 9 10 11 a

St/EVB/DVB/PE 75:10:15:100 62:15:23:100 50:20:30:100 25:30:45:100 00:40:60:100

base polymer 0.41 0.36 0.33 0.30 0.30

sSO3H a

resin

0.53 0.49 0.45 0.41 0.40

swelling coefficient (%) 111 79 63 18 15

mol/L 1.32 1.47 1.33 1.14 0.87

sCOOH

mol/kg 5.26 5.37 4.93 3.28 2.5

mol/L b

ND 0.02 0.05 0.09 0.15

mol/kg ND 0.06 0.17 0.25 0.44

Sulfonation was carried out at 98 °C for 60 min in experiments 7-11. b ND, not detected.

Scheme 1. Plausible Mechanism for the Generation and Further Reactions of Carbonyl Groups during Sulfonation of Styrene-Divinylbenzene Copolymers

Figure 1. Comparison of FTIR spectra of styrene-divinylbenzene base copolymer from experiment 9 and sulfonated styrene-divinylbenzene resin derived from it.

Figure 2. FTIR spectra of styrene-divinylbenzene copolymer (experiment 1), and the derived resins (experiments 2, 4, and 5). Only parts of the spectra from experiments 3 and 6 are shown.

of 1630-1850 cm-1 was in the order: experiment 2 < experiment 3 < experiment 4 < experiment 6 < experiment 5. Despite the very low concentration of sCOOH groups in experiments 2 and 3, even below the detection limits as analyzed by acid-base titrations, the strong IR absorption in the carbonyl region indicates the presence of other types of carbonyl groups as well. The swelling coefficient in water increased from 25% in the case of base copolymer (experiment 1) to 41% in the case of sulfonated resin (experiment 2) having 1.85 mol of sSO3H groups/kg. Addition of 0.93 mol of sSO3H groups/kg in experiment 3 as compared to experiment 2 resulted in an increase in the swelling coefficient from 41% to 60%. However, in experiments 3-6, 0.97 mol of sSO3H groups/kg was added along with 0.22 mol of sCOOH groups/kg, but the swelling coefficient remained almost constant, i.e., around 60%. These results suggest that cross-linking increased slightly in experiments 3-6. The basis of this deduction is that the only force that counteracts the osmotic pressure developed by the ionic/ hydrophilic groups is the stress developed along the cross-links in the swelling polymer matrix. A reaction mechanism is proposed (Scheme 1) to explain the formation of carbonyl groups and other experimental facts

observed in this study. There are two possible ways of generating the carbonyl groups: (a) cleavage of a CsC bond in the polymer backbone followed by oxidation of the terminal carbons of the fragments and (b) oxidation of sCH2CH3 groups of EVB units and/or residual sCH2dCH2 vinyl groups of DVB units. The first proposed reaction involves breakage of a crosslink, which is not supported by swelling coefficient data, whereas the second proposed reaction is represented by the conversion of structure I to structure II in Scheme 1. Being unstable under the experimental conditions, the sCH2COOH group of the structure II eliminates CO2, and the residual methylene carbon is oxidized to a sCOOH group, as represented by the conversion of structure II to structure III in Scheme 1. Structure III accounts for the sCOOH groups analyzed in this study by acid-base titration. Some of the sCOOH groups can react further in the presence of 98% sulfuric acid by (a) condensation of two sCOOH groups to form acid anhydride (PsφsCOOOCsφsP) or (b) acidcatalyzed acylation to form ketone (PsφsCOsφsP). Both of these proposed reactions account for an increase in cross-linking, as suggested by swelling coefficient data. Being present in low concentration, it is less likely for a sCOOH group to find another sCOOH group nearby on a neighboring chain for the formation of acid anhydride. On the contrary, acid-catalyzed acylation is more likely because the phenyl rings are numerous.

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Figure 3. Photographs of resins from experiments 2, 3, 4, 5, and 6 (from left to right). The top portion of lighter color is the resin in 0.1 M NaOH solution, and the corresponding bottom portion of darker color is the same resin in neutral water.

The proposed acylation reaction is represented by the reaction of structure III with structure IV to form structure V in Scheme 1. Sulfonation of one or both of the phenyl rings of structure V is possible under the experimental conditions, as shown by the conversion of structure V into structure VI in Scheme 1. The long conjugated system of structure VI can potentially absorb radiation in the visible spectrum. This deduction is supported by the fact that sulfonated ST-DVB resins are invariably colored. The colors of the resins in experiments 2-6 are shown in Figure 3. As expected, the color becomes darker in the following order: experiment 2 < experiment 3 < experiment 4 < experiment 5 < experiment 6. The carbonyl group of structure VI can potentially protonate and deprotonate in acidic and basic media, respectively, as shown by the equilibrium between structures VI and VII in Scheme 1. Structure VII is one resonance contributor, and another is represented by structure VIII. The rest of the resonance contributors are omitted. The proposed equilibrium is supported by noticeable color changes of the resin in 0.1 M NaOH and in neutral water, as shown in Figure 3. A set of experiments was carried out in which the proportions of ethylvinylbenzenes and divinylbenzenes were increased gradually in the following order experiment 7 < experiment 8 < experiment 9 < experiment 10 < experiment 11, as listed in Table 2. The sCH2CH3 and sCH2dCH2 groups in the copolymers were qualitatively analyzed by infrared spectroscopy as shown in Figure 4. The peak assignments were based on previous literature.26 The intensity of the signals of residual sCH2dCH2 groups at 1630 and 990 cm-1 were in the following order: experiment 7 < experiment 9 < experiment 11. The intensity of the signals at 1510 and 838 cm-1 assigned to p-disubstituted phenyl rings and at 795 cm-1 assigned to m-disubstituted phenyl rings were also in the same order, namely, experiment 7 < experiment 9 < experiment 11. These trends support our view that the concentrations of sCH2dCH2 and sCH2sCH3 groups increase with increasing numbers of DVB and EVB units in the polymer. The intensity of signals at 1493, 1028, and 760 cm-1 assigned to monosubstituted phenyl rings (i.e., styrene units) were in the order experiment 7 > experiment 9 > experiment 11. As expected, this trend reflects the decreasing proportion of styrene units in the polymers. The copolymers from experiments 7-11 were treated with 98% sulfuric acid under same conditions, that is, 98 °C for 120 min. A gradual increase in the amount of EVB + DVB units in the polymers from 25% to 100% caused a gradual decrease in the concentration of sSO3H groups from 5.26 to 2.5 mol/kg, which is in accordance with the literature.23 The concentration of sCOOH groups gradually increased from 0.00 to 0.44 mol/

Figure 4. Comparisons of FTIR spectra of styrene-divinylbenzene copolymers with 25% DVB + EVB (bottom), 50% DVB + EVB (middle), and 100% DVB + EVB (top) in the copolymer, from experiments 7, 9, and 11, respectively.

Figure 5. Photographs of resins from experiments 8, 9, and 10 (from left to right). The top row of darker color is in 1 M HCl, and the bottom row is the same resin in 1 N NaOH.

kg with the gradual increase in the proportion of EVB + DVB from 25% to 100%. The color of the resins became darker with increasing concentration of EVB + DVB units in the polymers, as shown in Figure 5. A pH-dependent color change was also noticed in these resins as shown in Figure 5. These results support our proposal that the carbonyl groups originate from the oxidation of sCH2CH3 and/or residual sCH2dCH2 groups of EVB and DVB units in the copolymers. 4. Conclusions The following conclusions are supported by the present study: (1) sCOOH and >CdO groups are generated during the sulfonation of porous styrene-divinylbenzene copolymers using concentrated sulfuric acid. (2) An increase in the temperature or time of sulfonation reaction causes an increase in the proportion of carbonyl groups in the resins. (3) The swelling coefficients in water show that cross-linking increases during sulfonation.

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(4) Styrene-divinylbenzene copolymers are either transparent or milky white, but the sulfonated resins derived from them are invariably colored. (5) The sulfonated resins show pH-dependent color changes. (6) An increase in concentration of sCH2CH3 and/or sCH2dCH2 groups in the base copolymers results in an increase in concentration of sCOOH groups, which, in turn, causes a darkening of the resin color. (7) The experimental results can be explained on the basis of a proposed conversion of sCH2CH3 and/or sCH2dCH2 groups to sCOOH groups, followed by the acid-catalyzed acylation of phenyl rings with sCOOH groups and sulfonation of the phenyl rings. Acknowledgment The author thanks Mr. Ejaz ur Rehman of ACL for performing IR spectroscopy. The author also thanks Mr. Ejaz ur Rehman and Dr. Imtiaz Ahmed of ACL for improving the English of the manuscript. Literature Cited (1) Kun, K. A.; Kunin, R. Macroreticular resins. III. Formation of macroreticular styrene-divinylbenzene copolymers. J. Polym. Sci. A 1968, 6 (10), 2689. (2) Watters, J. C.; Smith, T. G. Pilot-Scale Synthesis of Macroporous Styrene-Divinylbenzene Copolymers. Ind. Eng. Chem. Process Des. DeV. 1979, 18 (4), 591. (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) Svec, F.; Frechet, J. M. J. New designs of macroporous polymers and supports: Form separation to biocatalysis. Science 1996, 273, 205. (6) Okay, O. Macroporous copolymer networks. Prog. Polym. Sci. 2000, 25, 711. (7) Liu, Q.; Wang, L.; Xiao, A. Research progress in macroporous styrene-divinylbenzene co-polymer microspheres. Des. Monomers Polym. 2007, 10, 405. (8) 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 (22), 8237.

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(9) Malik, M. A.; Ali, S. W.; Waseem, S. A simple method for estimating parameters representing macroporosity of porous styrene-divinylbenzene copolymers. J. Appl. Polym. Sci. 2006, 99 (6), 3565. (10) Dragan, E. S.; Avram, E.; Dinu, M. V. Organic ion exchangers as beads. Synthesis, characterization and applications. Polym. AdV. Technol. 2006, 17 (7-8), 571. (11) 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. (12) Gelbard, G. Organic Synthesis by Catalysis with Ion-Exchange Resins. Ind. Eng. Chem. Res. 2005, 44 (23), 8468. (13) Dorfner, K. Ion Exchangers; Walter de Gruyter: New York, 1991. (14) 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. (15) Fritz, J. S.; Story, J. N. Selectivity behavior of low-capacity, partially sulfonated, macroporous resin beads. J. Chromatogr. 1974, 90, 267. (16) Hajos, P.; Inczedy, J. Preparation and ion chromatographic application of surface-sulfonated cation exchangers. J. Chromatogr. 1980, 201, 253. (17) 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. (18) Lode, F.; Freitas, S.; Mazzotti, M.; Morbidelli, M. Sorptive and Catalytic Properties of Partially Sulfonated Resins. Ind. Eng. Chem. Res. 2004, 43 (11), 2658. (19) Martin, C.; Cuellar, J. Synthesis of a Novel Magnetic Resin and the Study of Equilibrium in Cation Exchange with Amino Acids. Ind. Eng. Chem. Res. 2004, 43, 475. (20) 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. (21) Coutinho, F.; Souza, R. R.; Gomes, A. S. Synthesis, characterization and evaluation of sulfonic resins as catalysts. Eur. Polym. J. 2004, 40, 1525. (22) 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. (23) 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. (24) Iborra, M.; Tejero, J.; Cunill, F.; Izquierdo, J. F.; Fite, C. Drying of Acidic Macroporous Styrene-Divinylbenzene Resins with 12-20% Cross-Linking Degree. Ind. Eng. Chem. Res. 2000, 39, 1416. (25) Williams, D. H.; Fleming, I. Spectroscopic Methods in Organic Chemistry; McGraw-Hill Book Company Limited: London, 1973. (26) Bartholin, M.; Boissier, G.; Dubois, J. Styrene-divinylbenzene Copolymers, Revisited IR Analysis. Makromol. Chem. 1981, 182, 2075.

ReceiVed for reView April 29, 2009 ReVised manuscript receiVed May 22, 2009 Accepted May 25, 2009 IE900681N