Synthesis of Tetranitrobenzylpolystyrene Resin for Chromatographic Stationary Phase JAMES T. AYRES and CHARLES K. MANN Department of Chemistry, Florida State University, Tallahassee, Fla. 32306
b The synthesis of a nitrated polystyrene resin is reported. It consists of Friedel-Crafts benzylation of crosslinked divinylbenzene-polystyrene copolymer, followed by nitration. Results of IR, NMR, and elemental analysis indicate that the degree of benzylation is about 0.5 and that an average of two nitro groups is introduced on each ring. The reaction sequences are reproducible and result in production of about 30 mole % of intact beads in the final product. The resin is stable to heat to at least 160" C., not affected by prolonged exposure to the atmosphere, and exhibits the chemical properties of 2,4-dinitrophenyls. The swelling properties of the resin in a number of solvents are reported.
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(1) the authors described the use of tetranitrobenzylpolystyrene (TNBP) resin as a stationary phase for column chromatography. This is a detailed report of the synthesis, and properties of T N B P resin. A discussion of the chromatographic evaluation of the resin will follow ( 2 ) . A more exact structure and nomenclature for T N B P are shown below: N A PRELIMINARY REPORT
H H H
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l
l
poly [3-(2,4dinitrobenzyl)dinitrostyrene resin]
PNAC-PAH molecular complexes are described as charge-transfer ( C T ) complexes (6) in which PNAC functions as a Lewis-acid electron acceptor and PAH behaves as an electron donor. One of the most characteristic features of this type of complex is the CT absorption spectrum. The excitation energy of the CT spectral transition is a function of both the electron affinity of the acceptor and the ionization potential of the donor; the stability of the complex also depends upon these two quantities (3,@' T N B P resin has a soluble linear analog (linear TNBP) and numerous small molecule analogs such as 2,4dinitrotoluene and 2,2', 4,4'-tetranitrodiphenylmethane (TNDPM) which facilitate extrachromatographic studies as to the nature of the chromatographic adsorption mechanisms on TiYBP resin; chromatographic behavior on T X B P resin should parallel the CT equilibria typical of both the macromolecular and micromolecular analogs. Thus, T N B P resin is a chromatographically active, pi-acid stationary phase which contains a well characterized electron acceptor (a PNAC moiety) as an integral part of the molecular structure. As expected, T N B P resin displays activity toward PAH and related donor molecules. SYNTHESIS OF TNBP
T N B P was prepared by FriedelCrafts benzylation of divinylhenzenepolystyrene resin using benzyl chloride in nitrobenzene solvent, followed by nitration with a fuming acid mixture. Benzylation. I n a typical experiment, 0.96 mole (100 grams) of 50-100 mesh 2% divinylbenzene-polystyrene resin was swelled for 4 hours a t room temperature in 400 ml. of nitrobenzene contained in a 1-liter, 3-necked flask H H H
I l l
-c-c-cThe molecular complexes between polynitro aromatic compounds (PNAC) and polycyclic aromatic hydrocarbons (PAH) have been used as a basis of separation] purification] and identification. Therefore, a polynitro matrix such as T N B P resin is a logical stationary phase for the chromatographic fractionation of PAH mixtures.
equipped with a stirrer, condenser, dropping funnel, and drying tube. The reaction mixture was cooled to 0" C., and 0.26 mole of anhydrous AIClr added. The contents were vigorously stirred for 1 hour a t this temperature to allow diffusion of the catalyst into the resin beads. A mixture of 1.4 moles of reagent grade benzyl chloride and 140 ml. of nitrobenzene was added dropwise over a period of 3 hours a t 0" C. The reaction was continued for an additional hour after removal of the ice bath; reaction was evidenced by discharge of HC1. These moderate conditions are necessary to minimize the AlC13-catalyzed cross-linking of the product; higher temperatures favor complete decomposition. The reaction was quenched by pouring the mixture into 1 liter of methanol; the product settles as a viscous mass contaminated with the benzyl chloride self-condensation products. The methanol was decanted, and the product stirred with benzene to remove the byproducts. The benzylated resin was then collected in a filtering funnel and further washed with benzene, and then with a dioxane-HC1 solution to remove residual catalyst. The resin was washed with methanol to cause shrinkage, air dried with an aspirator, and then dried overnight in vacuo a t 80" C. The yield was 150 grams. X sample of low molecular weight linear polystyrene was benzylated in like manner pursuant to the synthesis of linear TXBP. The weight increase (50 grams) on going from polystyrene resin t o poly(benzylstyrene) resin indicates a degree of benzylation of approximately 0.5. There was no evidence of para substitution in the infrared spectra of the benzylated products. The N M R spectrum (4) of the linear poly(benzy1styrene) indicated that substitution did not occur in the ortho position. The product can then be more accurately described as shown below : H H H
CHZCL
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l
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poly (m-benzylstyrene) resin VOL. 38, NO. 7, JUNE 1966
859
Table
I.
Swelling Ratios of TNBP Resin in Various Solvents
Solvent Dimethylsulfoxide Dimethylformamide Xitromethane Acetone Tetrahydrofuran 3 : 1 Dioxane-acetonitrile Acetone 0.5F in NaC104 Tetrahydrofuran 0.5F in
Approximate swelling ratio
IiaC104
Tetrahydrofuran a t 0"
c. us. 25" c.
3.0 3.0 2.0 2.0 1.8 1.4 3.3
0.4
2.7 1.1
Nitration. I n a 2-liter, 3-necked flask equipped as before, 10001200 nil. of 90% HSO, was cooled t o about -50" C. with a d r y ice bath. T o this vigorously stirred cold solution was then added 0.33 mole (100 grams) poly(m-benzylstyrene) resin (polystyrene resins react exothermically with rapid swelling in fuming HX03 a t room temperature). The dry ice bath was removed, and the contents were allowed to reach room temperature. The contents were heated to 35O C. to ensure complete swelling. The mixture was again cooled as before, and 300 ml. of fuming sulfuric acid, 65% added dropwise; the addition rate and stirring rate were maintained a t the level necessary for effective heat dissipation. The mixture was slowly brought t o a temperature of 80" C.; reaction was continued at this temperature for 6 hours. The contents were again cooled with dry ice, and then poured into 4 liters of crushed ice. The product settles as nitrated beads contaminated with degradation products (such as carboxy acids and low molecular weight nitropolymer). hIost of the acids and byproducts were removed by decantation washing with water. The product was collected on a filter and thoroughly washed with acetone; this solvent readily swells T N B P resin and readily dissolves residual acids and degradation fragments. The resin was washed with methanol to cause shrinkage, air dried, and then dried overnight in vacuo a t 100" C. X yield of 0.28 mole of T K B P resin u-as obtained. A sample of linear poly(m-benzylstyrene) was nitrated in like manner to form linear TXBP. The resin was free of bead fragments: initial bead fragmentation during nitration is evidently followed by complete degradation t o an acetone-soluble form. Any nitrating medium that swells polystyrene resin can be used with equal effectiveness. Polystyrene has not been nitrated beyond the di-
860
ANALYTICAL CHEMISTRY
nitro stage; exhaustive nitration results in degradation. The infrared spectrum of T N B P had bands at 1540, 1350, 1070, 915, and 840 ern.-' which are characteristic of 2,4-dinitrophenyls ( 7 ) . The average of duplicate elemental analyses are as follows: calculated, 48 59% C, 2.84% H, and 14.78% N ; found, 49.13% C, 318% H, and 14.45% N. The elemental analyses and spectral data support the designated T N B P structure. The reaction sequences are reproducible. PROPERTIES OF TNBP RESIN
T N B P resin is composed of rustcolored spherical beads which are chemically and thermally stable to at least 160" C. The dry density is 1.38 grams/ml. These beads are quite rigid, but can be pulverized with a mortar. T N B P exhibits behavior typical of 2,4dinitrophenyls, such as the blue color formation with methanolic base. Prolonged contact with various solvents, and standing for one year in contact with the atmosphere, produced no change in its infrared spectrum. Nitrogen bases are strongly adsorbed, but can easily be removed with HC1. The resin swells, and linear T N B P dissolves, in the solvents listed in Table I. The swelling ratio is expressed as the swelled volume/dry volume. Swelling is also effected by nitroaromatics, amides, and related polar solvents. Dioxane-acetonitrile mixtures swell TNBP, but not the neat solvents. hlixtures composed of nonpolar solvents and those listed in Table I will swell T X B P to some extent. T N B P will not swell in hydrocarbons, halocarbons, alcohols, esters, or carboxy acids. The addition of electrolytes enhances swelling in acetone and tetrahydrofuran, but does not affect swelling in dimethylsulfoxide or dimethylformamide. T K B P is hydrophobic. Particles acquire static charges during manipulation; sizing is best accomplished by a water-methanol washing technique. Static charges persist even when the resin is in contact with hexane; the resin exists as lumps rather than discrete particles. However, this tendency is not present when higher dielectric solvents such as acetone are used. I n the chromatographic studies on T N B P resin, acetone and acetonehexane solutions were used as eluents. Dry T N B P resin will swell in acetone and acetone-rich eluents, but not in hexane-rich eluents. I n the chromatographic conditioning process, the ace-
0.8 0.7 0.8 as MOLE FRACTION ACETONE, X
0.5
1.0
Figure 1 . TNBP swelling ratio as a function of eluent composition
tone-swelled state was produced first and then the swollen resin was allowed to equilibrate with the given eluent prior to packing the column. Thus, the swelling was always unidirectional, from the acetone-swelled state to the eluentswelled state. The swelling ratio, internal volume per unit weight of resin, determined under chromatographic conditions is plotted as a function of the eluent composition in Figure 1. The portion of the curve below mole fraction 0.84 represents the region in which swelling behavior is irreversible. Dry resin will not swell in solvent mixtures in this range, but previously swelled resin does not lose all occluded solvent. Above mole fraction 0.84, behavior is reversible. The same results are obtained, whether one starts with dry or swelled resin. ACKNOWLEDGMENT
The authors thank The Dow Chemical Co. for the divinylbenzene-styrene copolymer. LITERATURE CITED
(1) Ayres, J. T., Mann, C. K., ANAL. CHEM.36, 2185 (1964). (2) Ayres, J. T., Mann, C. K., Ibid., 38, 861 (1966). (3) Basu, S., Chem. & Ind. (London)1956,
-.
7I " AA
(4) Bovey, F. A., Tiers, G. V. D., Filipovich, G., J . Polymer Sci. 38, 73 (1959). (5) Chakrabarti, S. K., Basu, S., Trans. Faraday SOC.60, 465 (1964). (6) Mulliken, R. S., J . Am. Chem. SOC. 72. 600 (1950): Rec. Truv. Chim. 75. 845 (1956). ( 7 ) Pristera, F., Halik, M.,Castelli, A., Fredericks, W., ANAL. CHEM.32, 495 (1960). "
RECEIVEDfor review October 7, 1965. Accepted March 21, 1966. Work suported under grant 5368 from the Petrceum Research Fund and grant GM 10064 from the National Institutes of Health, U. S. Public Health Service.
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