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HAROLD H. ROTH Physical Research Laboratory, The Dow Chemical Co., Midland, Mich.
Sulfonation of Poly(viny1 Aromatics)
The principal objective of the discussion presented here is to point out many of the pitfalls of polymer sulfonation, without releasing guidance for economic commercialization. Further data will be added by several patents from the author's laboratory. The information presented should kelp to elucidate many an experimenter's frustrating experience in polymer sulfonation and promote further research activity. T H E sulfone cross-linking side reaction has, perhaps, been most responsible for the slow progress in the sulfonation of polymers. Many factors require control when a water-soluble, non-cross-linked, color-free product is desired. Baer (7, 2), Breuers, Mark, and Konrad (4), Roth (70, 77), Signer (73), Signer and Demagistri (74, Soday (76), Wulff (27), and others have discussed the utility of water-soluble sulfonation products of poly(viny1 aromatics). This article attempts to explore many of the problems encountered in the sulfonation reaction. I t points in particular to problems encountered in the preparation of non-cross-linked, water-soluble products. I n his discussion of experimental results Demagistri (5) made an earlier contribution in this direction, although limited to work with one polymer (molding grade polystyrene, German Trolitul). More recently Signer, Demagistri, and Muller ( 7 5 ) have further expanded the initial work of Demagistri, but their work is also limited to molding grade polystyrene. During experimental work the author found many factors to play a part in the sulfonation reaction. By far the most important problem to control was that of cross linking-presumably by
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sulfone links. (Insoluble, water-swellable products are obtained when the sulfone content is probably no higher than 0.170.) The factors found important in sulfonation may be itemized as: Sulfonating agent species Polymer species Molecular weight of polymer Solvent species Concentration of reactants in solution Agitation Purity of solvent and reactants Temperature Molar ratios Modes of reactant feeding Size of sulfonating vessel Aging of finished reaction mixture With such a host of factors affecting the reaction (particularly cross-linking tendencies), one can visualize slow progress and many delays on the road to understanding and control of sulfonation. The factor in the author's experience which, perhaps, contributed most to progress was discovery of the importance of low concentrations oi reactants as an aid to minimizing cross linking. An early experiment at a 17 0 concentration might be credited with much of the impetus given to the entire research program. During continuing research efforts about 1000 sulfonation experiments have been performed in the laboratory. Seldom, in the earlier work, were the immediate goals clearly outlined or the progress anticipated. The following discussion of the factors outlined above is presented without reference to specific experimental data because of the great mass and variety of data collected, and because many additional data would be needed before the data could be subjected to statistical analyses. Sulfonating Agent Species Concentrated Sulfuric Acid. High sulfonation temperatures were used (140" to 200" C.) for varying periods (5
INDUSTRIAL AND ENGINEERING CHEMISTRY
minutes to several hours), depending upon the thickness of the polymer film or granule (8). The product may be colored and is usually water-swellable (not water-soluble). Ford (6) claimed soluble sulfonates from powdered telomers at lower temperatures. Young, Sniyers, and Sparks (22) claimed soluble products from styrene-isobutylene copolymers in the presence of solvents. Polyvinyltoluenes are usually sulfonated in about one fifth the time required for polystyrene under the same conditions. For best results the sulfuric acid should be of 95% concentration or higher. Below 9570 the sulfonation proceeds at a slower rate, little sulfonation occurring below 9070. Chlorosulfonic Acid. A solvent was necessary (preferably chlorinated hydrocarbon) to minimize charring. Highly swellable, white products were usually obtained at dilute concentrations (1 to 570)of the reactants at room temperature (9). Turbulent agitation during the reaction favored production of water-soluble products from polymers of low molecular weight (below about 100,000). Sulfur Trioxide. A solvent was necessary (preferably chlorinated hydrocarbon) to minimize charring. Watersoluble, white products were obtained more easily with dilute concentrations (about 1 to 10%) of the reactants at -20" to 45" C., according to Roth (70, 17), Teot (78), and Teot and Wiggins (79). Sulfur trioxide was used both as a liquid dissolved in a solvent and as a vapor diluted with an inert gas. Liquid sulfur trioxide showed a lesser tendency to cross-link the polymer when it was free of its polymeric form. This quality was most easily and safely attained with liquefied Sulfan (stabilized liquid sulfur trioxide) (General Chemical Division, Allied Chemical and Dye Corp., New York, N. Y.). Sulfur Trioxide Addition Complexes.
Both the dioxane and the bis-(/3-chloro-
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ethyl) ether complexes are shown by Baer (7, Z), Demagistri ( 5 ) , Signer (73), Signer and Demagistri (74, and Signer, Demagistri, and Muller (75) to produce water-soluble products in the presence of solvents. In the author's experience the bis-(B-chloroethy1)ether complex produced soluble products more easily. The water-soluble products produced by the author were not free of cross linking, however, as indicated by the high water viscosities obtained (above 1000 cp. at 0.501, when the starting polymer was molding grade polystyrene). The limited experimental evidence gathered by the author indicates that addition complexes may require rather stringent control during sulfonation. The recent work of Signer, Demagistri, and Muller (75) is an important contribution. Blaser and Tischbirek (3) claim watersoluble sulfonates from a sulfur trioxideketone complex. Fuming Sulfuric Acid or Oleum. Twenty per cent oleum charred the polymers when used without solvents. In the presence of solvents considerable cross linking usually occurred. Its sulfonation activity was studied by Spryskov (77), who reported greater rates using 30 to 40y0oleum in comparison to sulfur trioxide.
Polymer Species Polystyrene. Early sulfonation research progress was apparently slowed by preoccupation with molding grade polystyrene. Except for the work of Soday (76),this was an almost universal approach, probably prompted by the large supply of styrene at a relatively low cost. Erratic cross linking confused most early results of the author to the point where the quality of the sulfonation product could not be predicted from experiment to experiment. Polyvinyltoluene. Work with vinyltoluene polymers resulted in a more rapid progress, compared to work with polystyrene. Polyvinyltoluene sulfonated to high substitution about five times more rapidly in sulfuric acid experiments and with a lesser tendency to cross link during sulfonation. Other Polymers. Poly(a-methylstyrene) cross-linked more readily than polystyrene during sulfonation. Polychlorostyrenes were somewhat more difficult to sulfonate to a high degree of substitution-as would be expected. Copolymers with methyl methacrylate sulfonated with a lesser tendency to crosslink than the homopolymers at high molecular weight.
Molecular Weight of Starting Polymer As a general rule, increasing the molecular weight produced an increased
tendency to cross-link during sulfonation. In some sulfonation methods the cross linking increased much more than in others. Polymers above 200,000 were particularly difficult to work with. With most sulfonation methods there was a molecular weight limitation, above which only swellable products were produced. The non-cross-linked sulfonate of polymers of above 800,000 molecular weight have shown potential value in soil conditioning, as claimed by Mussel and Roth (7). This has been one of the greater benefits arising out of the work with polymers of high molecular weight. Methyl methacrylate and vinyltoluene copolymers were advantageously used a t high molecular weights.
tion was found to be dependent upon molecular weight, temperature, polym:r species, solvent species, efficiency of agitation, and sulfonating agent species.
Degree of Agitation With rapid sulfonating agents, such as chlorosulfonic acid or sulfur trioxide, efficient agitation was a requirement. The Waring Blendor and the Eppenbach Homomixer are examples of good mixing equipment. With sulfonating agents such as the sulfur trioxide complexes or sulfuric acid agitation efficiency was not so important. Inefficient agitation usually promoted cross linking or gave a poorly sulfonated product.
Solvent Smecies' Methyl chloroform was preferred when the sulfonating agent was chlorosulfonic acid, according to Roth (9). Sulfur dioxide in a mixture with chlorinated hydrocarbons was used to advantage by Roth (70, 77). Perchlorinated hydrocarbons were preferred with sulfur trioxide by Teot and Wiggins (79). Methylene chloride had advantages, according to Teot (78). Baer (7, 2) showed examples with ethylene dichloride, while Signer (73) showed examples with perchlorinated hydrocarbons. The effect of the solvent is not clearly understood. There is first a need for chemical stability to the sulfonating agent. One hypothesis might concern the distribution factor of free sulfonating agent between the sulfonate and the solvent. If the solvent is not favored, there may be a cross-linking tendency, as free sulfonating agent absorbed by the sulfonate may cause dehydration between adjacent phenyl and sulfonic acid groups to form a sulfone group:
Purity of Reactants and Solvents
I
ArH
+ ArSOaH + (SOaX
h S O z h f HzSOi
+ (sO8)s-i
The viscosity and solvent power of the solvent may also affect the expansion or contraction of the polymer chains in solution.
Concentration of Reactants Early experiments at 1% concentrations in a solvent gave unexpectedly favorable results. This has been considered one of the more valuable discoveries in the course of the research. Cross linking in many sulfonation methods was extremely dependent on concentration. Upon analysis, this may not be unexpected because of the large size of the polymer molecules. Dilute solutions would be expected to allow greater freedom for the polymer molecule to become extended, making it more vulnerable to sulfonic acid substitution. When linearity in the product was desired, the maximum operable concentra-
Results with sulfur trioxide were improved when it was free of its polymerized form. This was accomplished by maintaining it in its liquid form. The purity of the solvent used affected both its stability to the sulfonating agent and its tendency to cause cross linking. Anhydrous conditions were always desirable. Ninety-nine per cent sulfuric acid gave a considerably more rapid rate of sulfonation than 95%. When the chlorinated hydrocarbon solvents were used in mixtures with liquid sulfur dioxide, there was less need for high purity.
Temperature of Sulfonation Higher temperatures usually favored increased cross li&ing and color in the sulfonates. With chlorosulfonic acid the reaction was more sensitive to temperature. Little sensitivity was shown by sulfur trioxide under laboratory conditions from 40' to -15' C. Under sulfonation conditions that suppressed the tendency to cross-link, higher temperatures caused molecular weight deeradation, evidenced by the lower than expected water viscosities obtained in some cases. To complicate some studies at higher temperatures, there has been evidence that cross linking and degradation can occur simultaneously.
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Molar Ratio of Polymer to Sulfonating Agent This affected the cross-linking tendencies of some methods. Usually enough sulfonating agent was used to cause about 80% substitution. With sulfuric acid a higher equivalence was usually required to attain the desirable degree of substitution. Chlorosulfonic acid appeared to require closer control of molar ratio to obtain a good yield of useful products. Sulfuric acid required little control over molar ratio above the minimum amount, except for economic reasons. VOL. 4 9 , NO. 1 1
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30,000 j
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GELS APPEARING
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m Wotir Soluble W m r Swelloble
trolled or adjusted in the direction to favor reduction in cross-linking tendencies. Obviously, economic considerations *limit the extent to which dilution, agitation, purity, temperature, etc., can be carried. Interest in scaleup potential of a particular method usually presumes interest in commercialization-where economic considerations become an important controlling factor.
Aging of Finished Reaction Mixture 0
5
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15
20
25
30
35
AGING T I M E (HOURS) AT O’C
Figure 1. Effect of controlled cross linking on viscosity of a linear sulfonated polyvinyltoluene
Mode of Feeding Reactant to Reactant Vessel
Adding the polymer to the highly active sulfonating agents-e.g., sulfur trioxide or chlorosulfonic acid-tended to cause both degradation and cross linking. With good agitation it was possible to add diluted sulfur trioxide to the polymer solution with considerable success. With chlorosulfonic acid greater difficulty was encountered; a large, rubbery mass of partially sulfonated polymer tended to form when the sulfonation was carried to about 3070 substitution. With less active sulfonating agentse.g., sulfuric acid and dioxane-sulfur trioxide complexes-the mode of addition was of less importance. T o counteract the shortcomings of batchwise addition, much work was done with continuous flowing, concurrent addition. Improved sulfonation products usually resulted, all other conditions being equal. This mode was most valuable for the polymers of higher molecular weight or for scale-up with active sulfonating agents. The improvements with the concurrent mode may be due to a more rapid and even distribution of the sulfonic acid groups along the polymer chain, while the molar excess of sulfonating agent is continuously held a t a low value.
Size of Sulfonating VesseI Scale-up of a sulfonation method to produce water-soluble, non-cross-linked products has been difficult. Where the products were produced with relative ease in a small vessel, erratic cross linking often occurred at scale-ups of five to ten times. Polystyrene was usually more troublesome than polyvinyltoluene, particularly a t high molecular weight. Concurrent reactant feeding in some cases gave a degree of success. Lowering the concentration of the reactants also improved the results. As a general rule, scale-up successes depended upon how stringently the various factors were con-
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Baer (7, 2) dhcusses the increasing cross linking when a finished reaction mixture is aged. This occurs with most methods, particularly at higher temperatures. With the mixed solvent method of Roth (70, 7 7 ) , which used liquid sulfur dioxide, there was little tendency to cross-link a t -10” C. for several hours after sulfonation was completed. In some of the other methods simultaneous degradation and cross linking appeared to occur. To determine the effect of stepwise increases in cross linking upon the water viscosity and solubility of a non-crosslinked sulfonate of high molecular weight the following experiment was conducted : The dry product from the sulfonation of a polymer of about 800,000 molecular weight was dispersed in liquid sulfur dioxide containing free sulfur trioxide a t -10’ C. The slurry was divided, sealed in several glass From bombs, and aged a t 0 ” C. time to time the bombs were opened and the products were recovered, purified by ether extraction (72), and measured for their 0.5% water viscosity as the sodium salt (Figure 1). The water viscosity and solubility behavior follows closely that suggested by a Wall and Beste (20) concept of increasing interand intramolecular cross linking. The maximum occurred after the product appeared to have become insoluble (only water-swellable), Roth has discussed the application of the Wall and Beste concept to sulfonation products (77).
Conclusions Much experimental work remains to be done, to fit the factors in the sulfonation reaction into a more exact relationship with each other in the many methods. hlore rapid progress was made by quickly applying new knotvledge to subsequent experiments-for example, when the value of liquid sulfur dioxide as a solvent was learned, many other interesting avenues of research were abandoned. After much trial and error it was found that the production of valuable products was favored by low concentrations, efficientagitation, vinyltoluene-containing polymers, low temperatures, pure solvents, concurrent reactant feeding, small sulfonation ves-
INDUSTRIAL AND ENGINEERING CHEMISTRY
sel, low molar excess of sulfonating agent, and rapid handling of the finished product when sulfur trioxide or chlorosulfonic acid were used. Other sulfonating agents might be favored by a somewhat different set of factors, although low concentrations appeared to be favored by all methods.
Acknowledgment The author is indebted to the management of Haco Gesellschaft, A. G., Gumligen, Switzerland, for furnishing a microfilm copy of the dissertation of Demagistri (5) a t an early date.
Literature Cifed Baer, M. (to Monsanto Chemical Co.), U. S. Patent 2,533,210 (Dec. 12, 1950). Ibid., 2,533,211 (Dec. 12, 1950). Blaser, Bruno, Tischbirek, Guenther (to Henkel & Cie.), Zbid., 2,764,576 (Sept. 25, 1956). Breuers, W.,Mark, H., Konrad, E. (to I. G. Farbenindustrie), Ibid., 2,031,929 (Feb. 25, 1936). Demagistri, A,, “Sulfonation of Polystyrene,” doctoral dissertation, University of Bern, Switzerland, March 2, 1950. Ford, T. A. (to E. I. du Pont de Nemours & Co.), U. S. Patent 2,394,761 (Feb. 12, 1946). Mussel, D. R., Roth, H. H. (to Dow Chemical Co.), Ibid., 2,778,809 (Jan. 22,1957). Roth, H. H. (to Doiv Chemical Co.), Ibid., 2,604,461 (July 22, 1952). Ibzd., 2,683,137 (July 6, 1954). Zbid.,2,691,644 (Oct. 12,1954). Roth, H. H., IND. ENG. CHEM.46, 2435-9 (1954). Roth, H. H., Smith, H. B. (to Dow Chemical Co.), U. S. Patent 2,663,700 (Dec. 21, 1953). Signer, R., Ibid., 2,604,456 (July 22, 1952’1.
(14) Signer, R., Demagistri, A , , J. chim. ,bhys. 47, 704-7 (1950). (15) Signer, R., Demagistri, A., Muller, C., Makromol. Chem. 18/19, 139-50 (1956). Soday, F. J. (to United Gas Improvement Co.), U. S. Patent 2,283,236 (May 19,1942). Spryskov, A. A,, J . Gen. Chem. (U.S.S.R.) 25, 1683-5 (1955). Teot, A. S. (to Dow Chemical Co.), U. S. Patent 2,763,634 (Sept. 18, 1956). Teot, A. S., Wig ins: G. C. (to Dow Chemical C O ~ Ibid., , 2,640,820 (June 2,1953). Wall, F. T., Beste, L. F., J . Am. Chem. SOC.69, 1761-4 (1947). Wulff, C. (to I. G. Farbenindustrie), Ger. Patent 580.366 (Julv 13, 1933). ( 2 2 ) Young, D. W., Smyers, W. N.,Sparks, W. J. (to Standard Oil Development Co.), U. S. Patent 2,638,445 (May 12,1953). .
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RECEIVED for review April 25, 1957 ACCEPTEDJuly 25, 1957 Division of Industrial and Engineering Chemistry, Symposium on Engineering Aspects of Polymer Processes and Applications, Joint with Divisions of Paint, Plastics, and Printing Ink and Polymer Chemistry, 131st Meeting, ACS, Miami, Fla., April 1957.
