JOURNAL OF POLYMER SCIENCE VOL. VIII, NO. 5, PAGES 519-527

Solubility and Fractionation of Polyvinylpyrrolidone

B. JIRGENSONS, Department of Chemistry, Texas Lutheran College, Sequin, Texas

INTRODUCTION

Polyvinylpyrrolidone (PVP) was synthesized from acetylene by Reppe and his associates' in the I. G. laboratories in Germany in 193941, and as early as 1943 it was applied successfully as artificial blood plasma. PVP is easily soluble in water, and these solutions, with certain salts added, are used in surgery. The polymer very probably has the structure shown in (1).

According to recent determinations of Dialer and Vogler2 the major part of the polymer has the molecular weight of 10,000-90,000. The linear molecules are probably not branched, and in solutions they possibly are much coiled. PVP is, like the proteins, precipitated by ammonium sul- fate, tannic acid, and trichloroacetic acid.

The writer has been interested in this substance mainly because it is a simple model of proteins. The studies of this polymer may be promising in understanding of some colloidochemical phenomena of proteins, as for instance the protective action and the mechanism of adsorption. Impor- tant in this respect is the discovery of Meidinger3 that PVP acts as a protec- tive colloid in vitro and i n vivo. The writer found recently that the medium viscous fractions of PVP have a higher protective power than the high viscous fractions.4

The purpose of this work was to investigate the solubility and precipita- bility of PVP fractions. The fractionation of PVP is an important prob- lem, since there are indications that the very large and very small mole- cules are undesirable in the plasma applications. Moreover, the low molec- ular fractions of M = 3000-12,000, if introduced into blood, are excreted too quickly by the kidneys. However, these fractions might be applied as protective colloids elsewhere.

519

520 B. JIRGENSONS

Another important problem treated in this paper is the possibility of applying the method of precipitation titration in the molecular weight de- termination of PVP.6 The author found recently that titration with 2 M ammonium sulfate renders very sharp turbidity points, and the amount of ammonium sulfate. depends on the molecular weight of the PVP fraction.' Another purpose of this work was to h d the quantitative dependence which exists between the molecular weight and precipitability.

EXPERIMENTAL PART

1. Materials and Methods

Two original samples of PVP were used in the following experiments. One (PVP-G) was kindly presented by General Aniline & Film Corpora- tion, Central Research Laboratories, Easton, Pa.; this was a Germanprod- uct. Another sample the author obtained directly from Badische Anilin & Sodafabrik, Ludwigshafen, Germany. In their appearance both samples were similar: the fist (PVP-G) was of a more greyish color; the second (PVP-B) was almost white. Both contained about 3.4% water. Both were easily soluble in water and in the lower alcohols, as well as in chloro- form, pyridine, and glacial acetic acid, only slightly soluble in acetone, and almost completely insoluble in carbon tetrachloride, ether, benzene, and toluene. The intrinsic viscosity, as expressed according to Kraemer, Mark, and others, was 0.210 for PVP-G and 0.192 for PVP-B (both in water) at 34.8'. (According to Staudinger the values are 0.0210 and 0.0192, re- spectively, since concentration is expressed not in grams per 100 cc. but in grams per liter.) The solutions in water had a pH of 5-6; they do not change on heating, but flocculate easily upon addition of 2 M ammonium sulfate, acetone, or trichloroacetic acid.

The acetone, ether, chloroform, ammonium sulfate, and other substances were pure (C.P.) chemicals.

Viscwity was determined with an Ostwald capillary viscometer a t 34.8'. The upper bulb of the viscometer had a volume of 1.9 cc.; the flowing time of 5.0 cc. water a t the same temperature was 87.0 seconds. Only very dilute solutions of 0 .248% PVP were measured. At such low concentrations the dependence qSJc = f (c) for all investigated fractions is linear (see also ref. 4). The lines representing the high viscous fractions in the graphs are lying higher, and their slopes are greater than those of the lower viscous fractions. The reduced viscosity qSt/c of the latter changes very little with concentration, so that the lines are almost parallel with the abscissa. The intrinsic viscosity mentioned in the tables was obtained by extrapolation to zero concentration.

The precipitation titrations with ammonium sulfate or acetone were car- ried out by means of a microburet in a thermostat a t 25'. The turbidity point was determined visually. Adding of the precipitant was stopped when a weak but quite definite turbidity appeared.

POLY VINY LPY RROLIDONE 521

2. Fractionation

Fractionation of PVP can be performed in several different ways, choos- ing different solvents and nonsolvents.

The precipitability is now usually expressed by the y value, i.e., the ratio of the volume of precipitant (nonsolvent) to the total volume of the solu- tion at the point of starting precipitation. Thus, if the original volume of the solution of PVP in water is vo cc., and after addition of v cc. acetone the ikst turbidity appears:

Y = v/(vo + 8 )

The results of precipitability (appearance of the first fraction) are pre- sented in Table I.

TABLE I PRECIPITABILITY OF

Solvent

Water Water Water Ethanol (95%) Ethanol (95%) Chloroform Ethanol, abs.

DRY PVP-G FROM DIFFERENT SOLVENTS BY SEVERAL NONSOLVENTS

c %OfPVP a't turbidity

Nonsolvent point y value

. Acetone 0.6 0.715 Acetone 2.2 0.685 Acetone 5.4 0.677 Benzene 2.7 0.728 Benzene 4.7 ' 0.672 Ether 3.9 0.495 Ether 1.9 0.798

The precipitability in water-acetone mixtures thus depends only a little on the concentration of the PVP. Preliminary experiments showed that the temperature, too, has only a slight influence on these fractionations (see section 5). In all the cases mentioned in the table PVP separates out as a liquid phase; solid precipitates are formed, e.g., in precipitation from eth- anol with toluene, or from water with ammonium sulfate.

Some fractionation results are presented in Tables I1 to V.

TABLE I1 FRACTIONATION OF PVP-G FROM WATER WITH ACETONE

8 g. PVP dissolved in 112 cc. water

Fraction

I I1

I11 IV V

VI VII

Precipitated with aC0tOne

255 cc. +10 cc. +20 cc. f30 cc. +40 cc. +60 cc. +80 cc.

Per cent of fraction in polymer*

6.2 5.7

31.6 20.5 9.3 6.2 4.9

Intrinsic viscoeity

0.43 0.39 0.27 0.17 0.13 0.10 0.09

* Both the starting substance and the fractions were dried in the same fashion: for a The same short time at 105-115°, and then in a vacuum desiccator for at least 20 hours.

in all other fractionations.

B. JIRGENSONS

TABLE I11 FRACTIONATION OF PVP-G FROM WATER WITH ACETONE

16 g. PVP dissolved in 84 cc. water

Fraction

I I1

I11 IV V

VI VII

VIII

Per cent of +ecipitated fraction in w t h acetone polymer

201 cc. +10 cc. +I0 cc. +I0 cc. +10 cc. +I5 cc. 4-20 cc. +30 cc.

6.9 29.2 23.2 9 . 5 5 .0 4 .7 4 .6 3 .2

Intrinsic viscosity

0.472

0.235 0.145 0.115 0.106 0.092 0.090

0.280

TABLE IV FRACTIONATION FROM CFILOROFORM WITH ETHER

8 g. PVP-B dissolved in 100 cc. chloroform Per cent of Intrinsic

Fraction With ether fraction visoosity

r I1

111 IV v

vr VXI

vm

104 cc.

+I0 cc. +10 cc. +15 cc. +20 cc. +30 cc. +50 cc. +80 cc.

4.7

17.0 21.4 17.8 12.7 3 .9

6 .0 8 .5

Insol. in water

0.300 0.175 0.132 0.120 0.103 0.090 0.082

TABLE V FRACTIONATION FROM 95% ETHANOL WITH BENZENE

16 g. PVP-G dissolved in 100 cc. ethanol. Fraction I precipitated by 224 cc. benzene Per cent of fraction in Intrinsic

Fraction polymer viaoosity

Ia Ib

I1 I11

Ic IV

Id V

15.9 9.7

19.7 16.5 5 .9

13.3 3 .2 8 . 3

0.525 0.223 0.196 0.158 0.141 0.095 0.070 0.045

In the first example (TabIe 11), @.4% of the polymer was precipitated, in the second (Table 111) 86.3%. The major part of the polymer, accord- ing to these results, has the intrinsic viscosity of 0.11-0.28. In the frac- tionation from ethanol with benzene a very large first fraction precipitated. This was dissolved once more in etbanol and reprecipitated with benzene. The results are presented in Table V.

POLYVINY LPYRROLIDONE 523

3. Solubility of PW in Acetone

According to a private communication from the research laboratory 'of Badische Anilin & Sodafabrik, dry PVP is soluble in dry acetone. The American C.P. acetone does not contain much water, but the little moisture present in it apparently is sufficient to form an insoluble hydrate. Even in a sample of C.P. acetone which was distilled over anhydrous sodium sul- fate, the well dried samples of PVP did not dissolve very much.

The solubility experiments were carried out a t room temperature of 24- 26°C. To a definite amount of PVP was added a definite amount of solvent. In the case of acetone the polymer in a short time became soft, half liquid; and the system was then thoroughly stirred with a glass rod for 2 minutes. Then the samples were left to stand in closed vessels until the next day. 10 CC. of the clear solvent phase was then removed by a pipet, evaporated, and weighed. The rest of the solvent phase, too, was decanted, and the PVP phase mixed with a new portion of solvent.

The solubility results are presented in Table VI, The results clearly indicate that acetone dissolves comparatively easily only the low viscous components. These solutions become turbid if a little water is added, but the turbidity disappears on addition of more water. The results of Table VI indicate that fraction I1 contains several per cent of low viscous com- ponents; they dissolve in acetone easily, and are extracted first. In sev- eral other series of experiments it was found that all the fractions contain 1-5% low molecular weight components of intrinsic viscosity about 0.07, and these small molecules can easily be removed by successive 2-3 extrac- tions with acetone.

TABLE VI SUCCESSIVE EXTRACTION OF 4 G. OF DRY FRACTION I1 (TABLE 111)

Bv 30 cc. acetone in 24 hours.

Extract

Solubility, g. Per cent in 100 cc. diaS0lve.d acetone

Intrin$c visooslty (in water)

I 1.30 0.173 0.065 I1 1 .za 0.165 0.076

111 1.24 0.160 0.102 IV 1.98 0.255 0.120 v 2.63 0.330 0.112

VI 1.51 0.183 0.116

4. Precipitability with Ammonium Sulfate. Dependence of Precipitability on Molecular Weight

According to Dialer and Vogler,2 who determined the molecular weight of fractionated PVP by ultracentrifuge, the following relationship exists be- tween intrinsic viscosity and molecular weight:

1111 = 6.4 X X Momm

The molecular weight of the fractions treated in the presented work was

5% B. JIRGENSONS

calculated according to this equation. The data on precipitation with ammonium sulfate are presented in TabIe VII. The precipitability value, C, of ammonium sulfate was calculated in moles per liter of the mixture at the turbidity point. Usually 5.0 cc. of a 1% PVP solution was titrated by 2 M ammonium sulfate. The titration values are well reproducible.

TABLE VII PREcmrrmm~ WITH AMMONIUM SULFATE OF FRACTIONS OF TABLE V

5.0 cc. of 1% PVP titrated 6th 2 M ammonium sulfate at 25O 6.

Intrinsic Fraction viaoosity

prai .tabitig, c &p?rltter Molecular (Unmon1lllll

weight sulfate

Ia 0.525 105,000 0.760 Ib 0.223 24,000 0.885 11 0.1% 19,000 0.932

111 0 .' 158 13,500 0.964 Ic 0.141 11,200 0.988

IV 0.095 5,600 1.056 Id 0.070 3,300 1.094

Analysis of the data shows that the precipitability dependence on molec- ular weight is not a linear function if C is plotted against molecular weight, degree of polymerization, or its reciprocal. In this respect PVP behaves like the system cellulose acetate in acetone with water as precipitant,' but not like, e.g., glycogen-water, precipitant methanol,' or nitrocellulose-

i , 3.5 4.0 4.5 . 5.0

log M Fig. 1. Dependence of precipitability of fractionated P W on the molecular weight of the fractions. C = mole per liter ammonium sulfate at the turbidity point.

POLY VlNYLPYRROLIDONE 525

acetone, precipitant water? A linear dependence however exists between C and log M, as shown in Figure 1. This permits checking the approxi- mate mean molecular weight of a PVP fraction.

5. Dependence of Precipitability on Temperature

Data on the dependence of precipitability on concentration of PVP have been published previo~sly.~ The common relation holds that the amount of precipitant increases with increasing dilution (compare also Table I of this paper). In respect to temperature changes, the PVP, if precipitated from water with acetone, behaves like-the many other macromolecular sub- stances hitherto investigated: i.e., the amount of acetone needed for pre- cipitation increases with increasing temperature. For instance, at + 5°C. the y value of fraction 111 (Table 11) was 0.713, but at +34.8" it was 0.725, if the concentration of PVP at the point of turbidity was 0.55 g. PVP in 100 cc. of the acetone-water mixture. Quite different, however, is the dependence on temperature in the precipi-

tation by ammonium sulfate. Contrary to the usual behavior, PVP can be precipitated by a smaller concentration of ammonium sulfate a t a higher than at a lower temperature. The results with fraction I11 (Table 11) are presented in Figure 2. There is a linear dependence of C on tempera- ture. The titrations were carried out with 2.0 and 0.4% solutions of the

1.1 { 1.0.

0.9 .

'0.8 -

,0.7 -

0.6 -

C

I 10 20 90 40 50 60 70

Fig. 2. Dependence of precipitability of fractionated PVP on temperature. T

526 B. JIRGENSONS

PVP. The upper line represents the more dilute solutions, the lower the more concentrated PVP.

CONCLUSIONS

SufEcient fractionation of PVP can be achieved by precipitation from water with acetone, from chloroform with ether, or from ethanol with ben- zene. All the fractions, even if refractionated, contain the low viscous components, usually 1-4% (intrinsic viscosity about 0.07). They can be removed by extraction with acetone.

Precipitation titration with ammoflium sulfate is a very convenient means of characterizing the fractions. The turbidity point is sharp and well reproducible if the temperature is kept constant. The abnormal tempera- ture dependence is very peculiar; the writer knows only one more similar example, namely, precipitation of albumin with acetone,g but in that in- stance a sufEciently higher temperature alone produces turbidity.

The precipitation titration with ammonium sulfate is proposed as an independent method to estimate the mean molecular weights of PVP samples. The concentration of ammonium sulfate that produces turbidity is a linear function of the log of molecular weight. This is the same rela- tion which the writer found in 1942, in the precipitation titration of pep- tides.l0

Noteworthy is the similarity of PVP and the proteins in precipitation with ammonium sulfate. The albumins, which have a lower molecular weight than the globulins of the same source, require for precipitation more ammonium sulfate than the globulins. In proteins this dependence is only qualitative, since they differ in composition and constitution. In a series of PVP fractions of different mean molecular weight the dependence is quantitative.

The author wishes to thank General Aniline & Film Corporation, Central Research Laboratory and Sales Department, Easton, Pa., and Badische Anilin & Sodafabrik, Hauptlaboratorium, Ludwigshafen, Germany, for the samples of polyvinylpyrrolidone.

References

(1) W. Reppe, N e w Edwicklungen auf dem Gebiete der Chemie des Acetykm und Kohknozyds, Springer, Berlii, 1949, p. 21. W. Reppe, G. Hecht, and H. Weese, I.G.. German Pat. 738,994 (1941). H. Fikentscher and Herrle, Modern Plastics, 23, No. 3, 157-61, 212, 214, 216, 218 (1945).

(2) K. Dialer and K. Vogler, M ~ k r o m ~ l . Chem., 6 (Staudinger Festband), 191 (1951). (3) F. Meidinger, Bull. soc. chim. biol., 29, 411 (1947). (4) B. Jirgemns, Mukroml. Chem., 6 (Staudinger Festband), 30 (1951). (5) G. V. Schulz, 2. physik. Chem., A179, 321 (1937). G. V. Schulz and B. Jirgen-

L. H. Cragg and H. Harnmerschlag, Chem. sons, Z . physik. Chem., B46, 105 (1940). Revs., 39, 261 (1946).

(6) D. R. Morey and J, W. Tamblyn, J . Phys. 4 Colloid Chem., 51, 721 (1947). (7) E. Husemann, J. prakt. Chem., 158, 163 (1941). (8) G. V. Schulz and B. Jirgensons, 2. physik. Chem., B46, 105 (1940). (9) B. Jirgensons, Biochem. Z., 311, 332 (1942).

(10) B. Jirgensons, J. prakt. Chem., 161,30 (1942).

521

Synopsis

Polyvinylpyrrolidone (PVP) can be convedently fractionated from water by suc- cessive precipitation with acetone, from chloroform with ether, or from ethanol with benzene. The major part (60-70%) of the common PVP has the intrinsic viscosity of 0.12-0.30. The fractions can be liberated from the low molecular components by ex- traction of the solid polymer with acetone. Precipitability with ammonium sulfate is a very convenient means of characterizing the fractions. The amount of ammonium sul- fate needed to produce turbidity decreases with increasing mean molecular weight of the fractions, with the increasing concentration of the polymer, and with increasing temperature. The molar concentration of ammonium sulfate that produces turbidity is a linear function of the logarithm of the mean molecular weight of the fractions. The amount of ammonium sulfate which produces turbidity decreases linearly with increasing temperature.

RBsumt5

La polyvinylpyrmlidone peut Btre fractionnhe convenablement au d6part de ses solu- tions aqueuses par addition d’achtone, de ses solutions chloroformiques par I’bther, et de I’alcool par le benz8ne. La majeure partie (60 A 70%) de la PVP ordinaire a une vis- cositb intrinseque de 0.12 B 0.30. Les fractions peuvent &re purifiks de leurs compo- sants ?I faible poids molkculaire par extraction du polymhre solide t3 I’ac6tone. La prb- cipitabilitb an moyen du sulfate ammonique est un moyen trhs facile en vue de caractbr- iser les fractions. La Fantit6 de sulfate ammonique nkcessaire pour produire uu trouble dkroit avec un poids mol6culaire moyen croissant, avec une augmentation de concentration en polymhre et avec une augmentation de tempbrature. La concentra- tion molaire du sulfate ammonique nkessaire B provoquer l’apparit ion d’un trouble est m e fonction lin6aire du logarithme du poids molkculaire moyen des fractions considbhs. La quantit6 de sulfate ammonique qui produit une turbidit6 d6croit en raison inverse de l’accroissement de la tempbrature.

Zusammenfassung

Polyvinylpyrrolidon (PVP) kann bequem durch sukzessives Ausfallen mit Aceton aus Wasser fraktioniert werden, wie auch mit &her aus Chloroform, oder mit Benzol aus Xthanol. Der Hauptanteil (60-70 %) des gewohnlichen PVP hat eine Grenzviskositat von 0,10-0,30. Die Fraktionen konnen durch Auszug des festen Polymeren mit Aceton von den niedermolekularen Bestandteilen befreit werden. Die Niederschlagbarkeit mit Ammoniumsulfat ist ein sebr geeignetes Mittel zur Charakterisierung der Frak- tionen. Die Menge Ammoniumsulfat, die zur Erzeugung von Triibung niitig ist, nimmt mit zunehmender mittleren Molekulargewicht der Fraktionen, mit zunehmender Poly- merkonzentration und mit zunehmender Temperatur ab. Die molare Konzentration von Ammoniumsulfat die Triibung erzeugt, ist eine lineare Funktion des Logarithmus des mittleren Molekulargewichtes der Fraktionen. Die Menge Ammoniumsulfat, die Triibung erzeugt, nimmt linear mit zunehmender Temperatur ab.

Received November 2,1951