Makromol. Chem. 182,279-281 (1981) 279

Rapid Communication

A New Convenient Method for the Synthesis of Poly(styrenesu1fonic acid)

Hans Vink

Institute of Physical Chemistry, University of Uppsala, P.O. Box 532, Uppsala, Sweden

(Date of receipt: November 27, 1980)

Poly(styrenesu1fonic acid) (PSSA) is an often used model compound in polyelec- trolyte chemistry. Welldefined samples of PSSA may be prepared from narrov molecular weight polystyrene (PS) samples (which are commercially available). If the sulfonation is carried out under conditions which do not degrade or crosslink the polymer molecules the narrow molecular weight distribution of the parent polymer is preserved. In the past two different methods have been used to accomplish the sulfonation under the stipulated conditions. In the method of Turbak2- 3), the sulfonation is performed homogeneously using a complex between triethyl phosphate and sulfur trioxide. In the method of Carrol and Eisenberg4) the sulfonation is carried out heterogeneously with 100% sulfuric acid, in the presence of Ag2S04 as catalyst. However, these methods require several preparatory steps, which make the synthesis a tedious procedure. Here, a much simplified procedure is described, which gives a welldefined product in high yields. The sulfonation is carried out in conc. sulfuric acid to which PS, dissolved in an inert solvent, is added. In this work cyclohexane was used as solvent for PS and the reaction was carried out at slightly elevated tempera- ture (40°C) to prevent phase separation in the cyclohexane solution. The reaction was sluggish in ordinary sulfuric acid (95 - %'yo), but was accelerated by Ag2S04 or phosphorus pentoxide. In the present work the latter accelerator was in general used.

Experimental Part

Two narrow molecular weight polystyrene (PS) samples (from Pressure Chem. Comp., F'JttsbJrgh) were used, having the molecular weights 37000 (I) and 860000 (11), with the ratio MJM, less than 1,06 and 1,1, respectively.

Sulfuric acid (95 - 97%). phosphorus pentoxide, and cyclohexane (all from Merck, Darm- stadt) had reagent grade purity.

Dialysis tubing was from Union Carbide Corp., Chicago. A typical sulfonation was performed as follows. To a 500 cm3 round bottle, provided with a

heavy teflon-covered magnetic stirrer, 50 cm3 of sulfuric acid was transferred. 1 1 g of P20, was slowly added with stirring and the acid was allowed to cool to ~ 4 0 ° C . 1,5 g of PS, dissolved in 75 cm3 of cyclohexane, was then added with stirring. After a few minutes the reaction mixture took a highly viscous, seemingly homogeneous form. About 10 min later the reaction mixture began to separate into pure cyclohexane and a faintly yellow phase containing the mineral acids and poly(styrenesu1fonic acid) (PSSA). After 30 min the stirring was stopped and the reaction

280 H. Vink

mixture was allowed to stand at 40°C for about I h. The bottle was then cooled in an ice-bath and 25 g of crushed ice was slowly added with stirring. In this process the polyacid was precipitated in the form of a yellowish-white sticky mass. The mixture was transferred to a separating funnel and the (heavier) mineral acids were separated from the polyacid/cyclohexane mixture. To the latter 150 cm3 of water was added, which dissolved the polyacid. After separation from cyclohexane, the polyacid solution was filtered through a porous glass filter and dialyzed in cellophane tubing against distilled water. The water was frequently changed and the dialysis was continued until the conductivity of the dialyzate remained close to the conductivity of the distilled water used.

The equivalent concentration of the PSSA stock solution was determined by titrating aliquots of it with NaOH, in the presence of an excess of NaCI, with phenolphtalein as indicator. Taking the degree of substitution (DS) to be unity, the yield in the synthesis was found to be in the range of 90- 95%.

A part of the stock solution was converted into the sodium salt (NaPSS) by adding an equivalent amount of a standardized NaOH solution. This solution was used for the further characterization of the product.

Tab. 1. Data for the polyelectrolyte samples

Sample Equiv. weight DS [VMdl. g- '1

NaPSS I 207,9 0,98 NaPSS I1 205,8 1 ,00

Tab. 2. UV-spectra of the polyelectrolyte samples

0,23 2,28

Sample ~ / ( ~ . e q u i v . - ' . c m - ' ) at A/nm h r 7

225 248 255,5 25 8 261,5

NaPSS I 10 300 244 323 304 374 NaPSS I1 9 930 240 320 301 310

Equivalent weight: The equivalent weight (Me) of NaPSS was determined gravimetrically, by evaporating aliquots of the solution to dryness at 110°C. The mean values of the determina- tions for each sample are listed in Tab. 1 . These values indicate that both samples are practically monosubstituted (theoretical Me = 206,2).

Viscosity: The NaPSS samples were characterized by determining their intrinsic viscosities in 0,5 M NaCl solutions at 25 "C. The measurements were carried out with an Ostwald type of viscometer, yielding the rate of shear 1400 s- ' at the capillary wall for water at 25 "C. The results are listed in Tab. 1 .

UV-spectra: The samples were further characterized by their UV-spectra, determined by a Zeiss PMQII model spectrophotometer in 1 cm quartz cells. The equivalent absorption coefficients E for the samples in the acid and salt form were identical within experimental error. The &-values for the NaPSS samples at the main maxima and minima of the spectrum are listed

A New Convenient Method for the Synthesis of Poly(styrenesu1fonic acid) 28 1

in Tab. 2. For the maximum at 225 nm the absorption coefficient is in good agreement with the data reported by others5). In the higher wavelength region the absorption coefficients are close to the &-values reported by Butler et a1.@, but substantially lower than those reported by other^^*^- ’). It was found by Reddy and Marinsky’) that the absorbance in the high-wavelength region increases when PSSA deteriorates. The low absorption coefficients of the present samples indicate the absence of undesirable side reactions in the synthesis. Also, the stock solution of PSSA was found to be completely stable at room temperature, and no change in the spectrum could be detected after several weeks of storing.

’) R. Waack, A. Renbaum, J. D. Coombes, M. Szwarc, J. Am. Chem. SOC. 79, 2026 (1957) *) A. F. Turbak, Ind. Eng. Chem. Prod. Res. Dev. 1, 275 (1962) 3, U.S. 3072618; A. F. Turbak; Chem. Abstr. 58, 1 3 8 5 1 ~ (1963) 4, W. R. Carroll, H. Eisenberg, J . Polym. Sci., Part A-2, 4, 599 (1966) ’) M. Reddy, J. A. Marinsky, J. Phys. Chem. 74, 3884 (1970) 6, J. A. V. Butler, A. B. Robins, K. V. Shooter, Proc. R. SOC. London, Ser. A 241,299 (1957) ’) D. 0. Jordan, T. T. Kurucsev, M. L. Martin, Trans. Faraday SOC. 65, 606 (1969) *) D. Kozak, J. Kristan, D. Dolar, Z. Phys. Chem. (Frankfurt am Main) 76, 85 (1971) ’) J. Span, A. GaEeEa, Z. Phys. Chem. (Frankfurt am Main) 90, 26 (1974)