Macromol. Rapid Commun. 16, 673 - 678 (13'95) 613

A polarographic study of propene-maleic anhydride copolymers

Kirsten Werner, Volker Steinert? Steffi Reinhardt

Institute of Polymer Research Dresden, Hohe StraRe 6, 01069 Dresden, Germany

(Received: April 20, 1995; revised manuscript of June 8, 1995)

SUMMARY The polarographic behaviour of hydrolyzed propene-maleic anhydride copolymers in the

interval pH 2.0- 1 1 .O has been investigated by differential pulse polarography in aqueous lithium chloride solution at 25 "C. In dependence on the pH-value, one, two or three signals, belonging to the acid groups in the copolymer, appear in the polarograms. Three apparent polarographic acid constants have been determined graphically from the changes in the half- wave potential of the separate peaks in correlation with pH.

Introduction

Although electrochemical methods are not as frequently employed in polymer analysis as the various kinds of spectroscopy, their application is diverse. The utili- zation of the electrochemical methods in polymer chemistry includes the determination of low-molecular-weight compounds as additives I) , studies of initiator decomposi- tion '2 3, as well as the acquisition of analytical data of macromolecules, particularly the determination of functional groups '1, investigations with respect to structural changes in polymers and macro-ionkounter-ion systems themselves.

Polarography can be used to study fast chemical reactions taking place at the elec- trode surface, as well as slower reactions taking place in the bulk of solution. More opportunities are offered by polarography in the determination of functional groups.

For many organic substances, it has been observed that each one or more of three parameters, the half-wave potential, the peak height and the peak shape, depend on the acidity of the electrolyzed solution. These changes result from the effect of acidity on the acid-base equilibrium or on the rates of chemical reactions, which can be antece- dent to the electrode process or interposed between two electrochemical steps. When changing the acidity it is possible to observe one, two or more peaks in polarographic curves. Only a few publications have dealt with the polarographic investigation of polymers 5,6) but hardly has anyone investigated the behaviour of polymers containing carboxylic groups.

From investigations on 2,3-diethylsuccinic acid7-' l ) and -anhydride it is known that the reactivity and acidity depend on the configuration. Differences in the reactivities of the succinic anhydride basic units in the maleic anhydride copolymers have been proved experimentally 12) in dependence on their configuration, whereas for the characterization of the acidities of these copolymers only two pK,-values for the first and the second dissociation step have been reported 13* ").

0 1995, Hiithig & Wepf Verlag, Zug CCC 1022-1 336/95/$02.50

674 K. Werner, V. Steinert, S. Reinhardt

The present work is a study of the polarographic behaviour of polyelectrolytes applied to the copolymer of maleic anhydride and propene in aqueous LiCI-solutions in dependence on the pH.

Experimental part

Materials

Alternating copolymer of maleic anhydride and propene was synthesized by precipitation polymerization. It was performed in a steel autoclave under pressure (P,,, = 5,38 bar) in anhydrous dichloroethane using 2,T-azoisobutyronitrile as initiator at 70 "C. The precipi- tated copolymer was filtered and washed with anhydrous dichloroethane and dried in vacuum. After solving in acetone, reprecipitation was carried out in 2-propanol. The molecular weights were determined by gel permeation chromatography in dimethylform- amide with 0.5 v01.-% water and 2 g/L LiC1'5). The weight-average molecular weight of the investigated poly(propene-co-maleic anhydride) is 22 100 g/mol.

The stock solution was prepared by dissolving the copolymer in water at 60°C in the presence of a slight excess of sodium hydroxide for 24 h. Subsequently, the solution was deionized with an ion-exchange column and adjusted to the desired polymer concentration (Cp = 0.0122 mol/L refering to the content of carboxylic groups in the solution) by addition of water.

Solutions for polarography were prepared from the stock solution by adding water and the required amounts of NaOH and LiCl (anh., purum p. a., Fluka Chemie AG). Before the current was measured the solutions were allowed to stand for another 24 h at room tempera- ture.

Methods

Measurements were carried out by differential pulse polarography using the polarograph PA4 (Laboratorni (Pfistroje Praha), electrode unit SMDE 1 and the x-y recorder 4105.

The dropping mercury electrode with a drop time of 2 s was used as indicator electrode and the saturated calomel electrode was used as reference electrode. The accuracy of the potential measurements was periodically checked by potential measurements with standard cadmium chloride solutions.

A polarographic cell with water-jacket was used. All measurements were carried out at 25 f 0.1 "C.

Oxygen was removed from the investigated solution by bubbling purified nitrogen for 5 min. An inert atmosphere above the solution during measurements was maintained by flushing with nitrogen.

A pH-meter CG 804 with a pH combination electrode type N 6280 (Schott-Gerate GmbH) was used for all pH measurements.

Results and discussion

Fig. 1 shows two polarograms of a hydrolyzed copolymer of maleic anhydride and propene obtained by polarographic investigations at different pH-values. Usually, organic acids are polarographically active in the potential range between - 1 and - 2 V 16). The signals appearing in this range belong to the reduction of the proton after the dissociation step. The keto-group of the carboxylic group is polarographically

A polarographic study of propene-maleic anhydride copolymers 615

Q, 10

c 0 8 - 0 6

Fig. 1 . Polarograms of a

anhydride copolymer in de- pendence on the pH-value;

1.26-10-3 mol/L; (. . .)

hydrolyzed propene-maleic 0 1

0.1 mol/LiCl/L, cpOlymer - -

pH 4.5; (- ) P H 5 0

0 2

-1 2 -0 9 -0 6 -0 3 0 U in V

inactive. If the elec troactive compound bears two or more groups with acidic proper- ties, the polarogram will show two or more peaks when the pH is varied over a suffi- ciently wide pH-range.

The different reactivities of both configurations of a maleic anhydride copolymer suggest that four signals caused by the different acid constants should be detectable by polarographic investigations of a hydrolyzed copolymer. Actually only three peaks could be detected due to overlapping of the two first dissociation steps.

This behaviour is in accordance with results of investigations on maleic and fumaric acid. Both acid configurations show tow peaks at pH of 5 . In a polarogram of a mixture of both acids four single peaks should appear, but only three peaks have been seen, whereby the first peak is the sum of the first dissociation steps of maleic acid as well as fumaric acid. The acid constants of these dissociation steps are so close to each other that the polarographic signals of both are superimposed.

These findings can be transfered to the polarographic behaviour of a hydrolyzed maleic anhydride copolymer. The first peak (A) in the polarogram (Fig. I), analogous to maleic and fumaric acid, is the signal for the first dissociation step of both stereo configurations. The second (B) and the third (C) peaks belong to the second dissoci- ation step. During the hydrolyzation of a maleic anhydride copolymer the anhydride ring (Fig. 2) is opened and the erythro-form transformed into a thermodynamically favourable position by rotation around the C-C bonding. For this reason the peak (B) belongs to the second dissociation step of the erythro-form whereas the peak (C) belongs to the threo-form. The signal (C) is shifted to more negative potential ranges, because of formation of a hydrogen bond between the carboxylate group and the neighbouring carboxylic group.

The three peaks observed in the polarogram of hydrolyzed propene-maleic anhydride copolymer, which was recorded in solutions of varied acidity, change the half-wave potential as well as the height of the peaks. If both quantities change with pH, this indicates that both the electroactive and the electroinactive form, which are both

676 K. Werner, V. Steinert, S. Reinhardt

H

0

1 R'; 2 S*

32 percent

0 do 1 R'; 2 R'

68 percent

Fig. 2. threo-succinic anhydride basic units in propene-maleic anhydride copolymers

Content of erythro- and

present in the solution, are transported to the electrode surface. The electroactive one can be generated from the electroinactive form at a rate that depends on pH.

A remarkable circumstance is the fact that the peaks for the copolymer are about three times higher than those of the low-molecular-weight dicarboxylic acids, in the case of constant concentrations in the solution. It has to be paid attention to the fact that the local concentration of the acid groups in a solution of a low-molecular-weight acid is not the same as in the solution of a copolymer containing acid groups. The local acid concentration in a copolymer solution is determined by the distance between two neighbouring carboxylic groups in the copolymer and by the coil dimension of the polymer. Therefore the local acid concentration in a copolymer solution is much higher and the polarography is more sensitive to the determination of acid copolymers than for organic acids.

Fig. 3 shows the dependence of p H on the current (peak height) of a propene-maleic acid copolymer in 0.1 mol/L LiC1. The appearance and disappearance of the acid group peaks in dependence on the pH-value correspond with the findings on low-

Q 1.2

- 1.0

I

C ._

0.8

0.6

0.4

0.2

0

Fig. 3. Dependence of the current (peak height) on pH for a hydrolyzed propene-maleic anhydride copolymer; (0.1 mol/L

1.26. mol/L - LiC1; Cpolymer -

0 2 4 6 8 10 PH

A polarographic study of propene-maleic anhydride copolymers 677

> - 0 8 'w > W

- 0 9 -. Fig. 4. Dependence of the half-wave potential on pH for

polarogram of a hydrolyzed propene-maleic anhydride copolymer; 0.1 mol/L LiCI; cpolymer = 1.26. mol/L,

the first peak (A) in the -1 0 -.

-1 1 -~

pKA (A) = 4.5 -1 2

rn " 0

.

Tab. 1. Polarographic acid constants for a hydrolyzed propene-maleic anhydride copoly- mer

Peak pKA

A 4.5 B 5.4 C 7.1

molecular-weight dicarboxylic acids described by several authors j7, 18). The first peak (A) in the polarogram disappears in the range of pH 6 to 7. It means that the first dissociation step of both configurations is almost completed at this pH. The second peak (B) appears at a pH of 4 and disappears at pH 8. The last peak (C), which is shifted to more negative potential due to the formation of a hydrogen bond, appears at pH 4.5 and is found up to a pH of 11. In the pH-range of 4.5 to 6 all three peaks appear in the polarogram.

The pKA-value of the dissociation steps follows from the pH-dependence on the half-wave potential of the single peaks. Usually, the p#,-value can be derived from the bend point of the vs. pH curve, which is demonstrated in Fig. 4 for the first peak (A). The pK,-values for a propene-maleic anhydride copolymer are shown in Tab. 1. These values are polarographically determined acid constants. For the determination of the thermodynamical constants the values should have to be corrected. Such a correction of the pK,-values, reported only for organic dicarboxylic acids 19), was not carried out in the case, because only the second dissociation step of the configuration can be described completely. Polymer specific effects have also to be taken into account for a correction.

The authors gratefully acknowledge financial support of the Bundesministerium fur Bildung, Wissenschaft, Forschung und Technologie (BMBF). The research is sponsored under project number FKZ 03 C 2016 7.

678 K. Werner, V. Steinert, S. Reinhardt

I ) G. C. Whitnack, Anal. Chem. 20, 658 (1948) *) V. S. Sporykhina, A. L. Mirkind, Lakokras. Mater. Zkh Primen. 5 , 48 (1974) 3, I. M. Kolthoff, L. S. Cuss, D. R. May, A. I. Medalia, J. Polym. Sci. 1, 340 (1946) 4, R. Peter, Angew. Makromol. Chem. 193, 51 (1991) 5 , R. Nastke, K. Dietrich, W. Teige, Acta Polym. 31, 522 (1980) 6, B. Philipp, H. Dautzenberger, B. Lukanoff, Vysokomol. Soedin., Ser. A: 21,2579 (1979) ’) L. Eberson, Acta Chem. S C Q ~ ~ . 18, 1276 (1964) *) L. Eberson, H. Welinder, J. Am. Chem. SOC. 93, 5824 (1971) 9, L. Eberson, L. Landstrom, A C ~ Q Chem. Scand. 26, 239 (1972)

lo) J. M. Bouvier, C.-M. Bruneau, Bull. SOC. Chim. 11, 1093 (1977) “) I. Spirevska, V. Rekalic, GIQs. Hem. Drus. Beograd 49, 45 (1984) 1 2 ) S. Zschoche, Dissertation, Technische Hochschule Merseburg, 1985 13) E. Bianchi, A. Ciferri, R. Parodi, R. Rampone, A. Tealdi, J. Phys. Chem. 74, 1050(1970) 1 4 ) S. Kawaguchi, T. Kitano, K. Ito, Macromolecules 23, 731 (1990) 15) H. Hahmann, E. Brauer, D. Voigt, R. Lahne, Plaste Kautsch. 37, 258 (1990) 1 6 ) G. Henze, R. Neeb, “Elektrochemische Analytik”, Springer Verlag, Berlin 1986 ‘’1 N. H. Furman, C. E. Bricker, J. Am. Chem. SOC. 64, 660 (1942) I s ) P. J. Elving, I. Rosenthal, J. And. Chem. 26, 1454 (1954) 19) P. Stradyn, V. V. Teraud, M. V. Schimanskaja, Zzv. Akad. Nauk. Latv. SSR, Ser. Khim.

5, 547 (1964)