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Equilibrium peculiarities in the complexing of polymeric acids with poly (ethylene glycols)
EQUILIBRIUM PECULIARITIES IN THE COMPLEXING OF POLYMERIC ACIDS WITH POLY (ETHYLENE GLYCOLS)l* A. D. AI~TIPII~A, V. Y•. BARANOVSKII, I. M. PAPISOV and V. A. KABAIfOV
M. V. Lomonosov Moscow State Un~verslty
(Received 17 September 1970) MANY examples of cooperative interaction of complimentary structures are to be found in nature, and it is of fundamental importance in living organisms. Binary systems consisting of cooperatively linked polymer chains are of definite interest from a practical standpoint as this type of interaction involving ~he loss of the properties of the original components gives rise to new sets of properties and thus opens the way to the development of completely new materials based on many known polymers [1]. It was recently reported that the cooperative interaction of growing polymer chains with macromolecnlar matrices m a y have a marked effect upon the kinetics of polymerization; moreover, the possibility of a matrix forming stable associates with polymer chains is realized only if the degree of polymerization of the! matrix exceeds a certain critical value [2]. It is obvious that if the stability of the complexes depends on the~ length of the macromolecnles included in their composition, the same parameter will similarly have a marked on other properties of the complexes. This paper relates to a study of the effect of the molecular weight Of polyethylene glycol (PEG) and other factors (concentration, p H of the n~edium, temperature) on the complexing of P E G with polymethacrylic (PMA~) and polyacrylic acid (PAA). Some information about the complexes in question is available in [3] where it is suggested that the association of P E G with p01yacids takes place through the formation of intermolecnlar hydrogen bonds. PMA_A_and PA~ were prepared by radical polymerization of the appropriate monomers, using hydrogen peroxide as the initiator [4]. Aqueous solutions of the polyaclds m~derwent dialysis and were dried first lyophihcally and then to constant weight in a vacuumtthermostat at 80°. The viscosity-average molecular weight of the PMAA determined m jabsolute methanol [4] was 100,000. The molecular weight of the PAA was also determined mscometrically m an aqueous solution of sodmm hydroxide [5], and was found to be! 120,000. Isotaetle PAA was prepared by hydrolyms of lsotactlc polylsoprepyl acrylate syrktheslzed by aniomc polymerization of isopropyl acrylate (wath a phenyl magnesmm bromide ~atalyst) at --60 °. The molecular weight of the isotactlc PAA determined viscoraetrlcally was[ 330,000. Narrow fractions of the PEG (produced by the Schuchardt Company of West Germany) were not subjected to further purification. * Vysokomol. soyed. A14: No. 4, 941-949, 1972. 1047
1048
A. D. ANTIPINA ¢~ al.
The solutions intended for mvest~gatlonwere prepared lm_medmtolyprior to the measurements by mixing solutions of the Individual components of higher concentrations. The concentrations of the solutions were generally reckoned m terms of the amount of polyacid m a mixture; PEG was added to the polyacJd m the appropmate molar ratios. The wscoslty of the solutions was determined on an Ubbelohde wscometer, the outflow time of the solvent (water) being 87.7 see. The pH of the solutions was measured on an LPM60M laboratory pH-meter with glass and chlorosolver electrodes. All the measurements were carried out with thermostat control of the systems (thermostat accuracy W-0.05°). A JEM-5Y electron-microscope was used for the electron-microscope investigations. The samples were prepared by means of "thermal attachment" [7] from solutions of an equnnolar mixture of PMAA and PEG (PMAA concentration 0.05 g/dl), time of attachment 5 mm at 40°. The samples were tinted with tungsten oxide on a JEE-4B vacuum apparatus The potentiometric and viscometric methods were used to investigate the eomplexing of the polyaeids with PEG. The potentiometrie method was selected because, as was suggested in [3], the interaction between the complex components takes place through hydrogen bonds of the undissociated carboxyl groups of the polyacid interacting with the oxygen of the PEG. I f this is so, the eomplexing reaction must be accompanied by reduction in the concentration of hydrogen ions, and this will increase the pH of the solution. The potentiometric method m a y be used to determine the amount of uneomplexed polyacid, and in this w a y the concentrations of the complexes m a y be determined. The association of the polyacid and PEG must be accompanied by a reduction in the density of the charge on the polyacid macromolecules, and by a marked increase in the hydrophobic nature of the latter owing to the screening of the hydrophilic groups of the complex. This may lead to big changes in the density and size of the polyacid coils, as is apparent from the viscometric measurements. For investigations of this type it is naturally better to use dilute solutions: complexing in concentrated solutions is very likely to lead to the formation of a continuous three-dimensional network. Figure 1 shows the results of titration of PMAA and PAA with polyethylene glycols of different molecular weights M (henceforward the concentrations in the PEG/polyacid ratios are in basic moles per litre). I t is seen from the Figure t h a t at 25 ° the addition of P E G with M = 1 0 0 0 to the PMAA solution of 0.1 g/dl concentration has no effect on pH. The addition of P E G with M--~2000 and 3000 gradually reduces the pH values of the solutions, and the rise in pH is more rapid when P E G with M = 3000 is added. The addition of P E G with M = 6000, 15,000 and 40,000 is accompanied by a rapid rise in pH, reaching a maximum at PEG/PMAA----1; further increase in the P E G concentration has only a small effect on pH. Similar behaviour is observed for the PAA solutions also, but the rise in p H occurs with P E G of higher molecular weight (6000)~ and it is less marked t h a n in the case of PMAA. The results of measuring the reduced viscosity of the polyacid solutions contahnlng P E G (Fig. 2) are in complete agreement with the results of potentiometric titration; the PMAA and PAA concentrations are 0.1 g/dl in every case.
Equilibrium peculiarities in the complexing of polymeric acids
1049
As is seen from Fig. 2a, the addition of P E G with M ~ 1 0 0 0 or below 1000 has no effect on viscosity. On adding the P E G with M = 3 0 0 0 or more the viscosity falls rapidly at first, and a minimum is reached at P E G / P M A A = I , b u t i a t t h e minimum point the value of ~ / c is independent from the molecular Weight of P E G and PMAA; the further rise in viscosity is due to the increased conCentrapH #'2
o[ o 2
+4 x5
",," ~ _ ~ _
a
_~'~
3-8
b
3"2
!
z
I 3 [PES]/ [ Po/,~-acid]
I
I 6
Fzo. 1. Titration curves of polymerm ac)ds of PMAA (a) and PAA (b) with PEG of different molecular weights: 1--1000, 2--2000, 3--3000, 4--6000, 5--15,000, 6--40,000. The Continuous curves were plotted at 25° and the fractured ones at 16.5°. Concentratzon of polymeric acids =0.1 g/dl. The points on the ordinate axis relate to solutions of pure polymeric acids. Molecular weight of PMAA--100,000; PAA--120,000. tion of excess uncomplexed PEG. The P E G with M ~ 2 0 0 0 occupies an intermediate position--the addition of the latter reduces the viscosity, b u t the reduction is less marked than in the case of the more high-molecular fractions o~ PEG. A similar pattern is observed with solutions of the P A A - P E G mixtures, b u t the drop in viscosity occurs only when the molecular weight of the P E G is 6000 or more (Fig. 2b). The marked reduction in the viscosity of the P M A A - P E G and P A A - P E G solutions in a narrow molecular weight interval is well illustrated b y Fig. 2c depicting the curves of ~sp/C vs. molecular weight of P E G for the P M A A P E G and P A A - P E G systems when the molar ratio P E G / p o l y a c i d ~ l . Three important conclusions follow from the data in Figs. 1 and 2: 1) the pdlyacids (PAA or PMAA prepared b y radical polymerization) and P E G are present in the composition of the complexes in equimolar amounts, as is shown lby the position of the viscosity lows in Fig. 2a and b, and b y the limiting pHI values attained when P E G / p o l y a c i d = 1 in the case of the high-molecular P E G fractions; 2) the complexing is of a cooperative nature; only P E G molecules whos e length exceeds a certain critical value are involved; 3) the stability of the cbmptex and the critical length of the P E G chain depend on the chemical structur~ of t h e
A. D. ANTn'INA et (~.
1050 /
~sp/ePMAA
Tlsp/cv~ a
2.3 1"6 I" 7
03 0.1
1 '
I
I,,.
3
[ PE~J/ [ PMAAJ
5
~
f
I
I 3
I
[PEG]/[PAA]
I 5
rlsp/OpM~(p~A)
0.3
I i 3.0
~ J~
tog Mpea
0.$
FIG. 2. Reduced vmcomty of solutions of PMAA-PEG (a) and PAA-PEG (b) mixtures vs. PEG/poly~nerm acid ratio for PEG of different molecular weights (conventional notation, as m l%g. 1; the cross-hatched symbols relate to ordinary PAA, 25°; concentration of polymerm acids 0.1 g/dl; molecular weight of PMAA--100,000; PAA--120,000; lsotactic PAA (3", 4"', 5") 330,000; curve 7 m for a mixture of PMAA (M=440,000) and PEG (M~15,000); c--curve of equimolar mixtures of PMAA (1-3) and PAA (2') with PEG versus molecular weight of PEG at 40 (1), 25 (2, 2") and 15° (3). polyacid. F o r a P M A A - P E G c o m p l e x to be f o r m e d it is sufficient t h a t t h e molecular weight o f t h e P E G should be n o t less t h a n 2000, while a n y significant i n t e r a c t i o n b e t w e e n P A A a n d P E G s t a r t s o n l y w h e n t h e molecular weigh~ o f t h e l a t t e r is ~ 6000. Polyacids (PAA and PMAA) prepared by radical polymerization have mainly a Byndiotactie chain s t r u c t u r e [8]. As is seen f r o m Fig. 2b, t h e chain micro-struc-
Equilibrium pecuharities in the eomplexing of polymeric acids
1051
ture has a marked effect on the stoichiometry of the complex: in mixtures qf isotactic PAA and P E G solutions the viscosity low appears at a higher PEG~PAA ratio ( ~ 1-5). I t appears from the results of electron-microscope investigations (see Fig. 3) t h a t the complex particles are spherical coils. The same conclusion is reached on investigating the viscosity of solutions of the complexes. As is seen from Fig. 2a, the reduced viscosity of the solutions when PEG/PMAA-1 is independent from the molecular weight of PEG and PMAA in the case of the high-molecular
FIO. 3. Electron photommrograph of particles of a PMAA-PEG complex. MoleeularWelght of PMAA----100,000; PEG---15,000; PMAA concentration 0-05 g/dl; molar ratio PEGfPMAA 1; tnne of attachment 5 ram, shading with tungsten oxide. PEG fractions. The reduced viscosity of these solutions is similarly independent from the complex concentration (see Fig. 5a, curve 3), i.e. solutions of the complexes obey Einstein's equation for solutions of dense spherical particles
~=~0(1+2.5 ~), where q and qo are the viscosities of the solution and solvent respectively, and q is the volume fraction of dissolved substance. This equation enables us to estimate the approximate amount of solvent in the coils of a complex, assuming the density of the coils to be equal to that of the solvent. In this way we find t h a t the complex particles contain ~ 75% water and 25% polymer. A more accurate estimate is obtainable only by measuring the density of the complex pariicles. However, even in the light of this approximation one may conclude tha~ the particles are dense coils containing very little solvent. The results of the dilution of solutions containing the complex compo aen~ in the equimolar ratio (Fig. 4a) m a y be used to find the degree of conw a'sion during the complexing reaction O ~ = ( C o - - C z ) / c o and its relation to the conc~ ntration of the components (see Fig. 4b), where co is the total concentration of car ~oxyl groups in solution; c~ is the concentration of carboxyl groups unasso( iated with PEG; Co--C ~ is the concentration of carboxyl groups forming-a bond with PEG: % m a y be determined graphically, as is shown in Fig. 4a, assumin t h a t
A.D.
1052
AN~m~A~
a~.
the degree of dissociation of the uncomplexed carboxyl groups remains constant during the complexation. I t is readily apparent from Fig. 4b that in mixtures of PMAA and low-molecular P E G fractions 0~ is reduced with increasing dilution of the solutions. This phi 4.7 0 4~
b
O~~_--.-x--~-x--x-~Lx
3"9
i
t-
2
3"5
O'OZ O ~
0"08 0"08 OlO
O.OZ ~04
006
008
~I0
CP,AA(PAA),g/d!
FIG. 4. Concentration dependence of pH of solutions of polymeric acid complexes with PEG (a), and degree of conversion versus concentration durn~g the eomplexmg of polymeric acids with PEG of different molecular weights (b) at 25°. Molecular weights of PEG: 1, 1'-15,000; 2--3000; 3--2000; 4, 4'--40,000; 5, 5'--no PEG. Curves 1-5 relate to PMAA, and 1', 4', 5' to PAA. Molecular weight of PMAA= 100,000; PAA= 120,000. points directly to dissociation of the complexes through dilution, i.e. the state of equilibrium that exists is of the type of P M A A ~ - P E G ~ complex.
(l)
The dissociation of the complexes of low-molecular P E G with PMAA due to dilution is confirmed b y the viscometric measurements (see Fig. 5a). In the case of complexes of PMAA with high-molecular P E G there is an absence of the rise in reduced viscosity accompanying dilution (see curve 3) that is characteristic of polyelectrolytes, i.e. the solutions in question contain no free PMAA. In the case of the P M A A - P E G complexes where M for P E G = 2 0 0 0 and 3000 the reduced viscosity of the solutions rises on dilution, approaching that of PMAA solutions of like concentrations containing no PEG. (It is remarkable that dilution not only results in a rise in ~/CPMAAb u t also increases the time of outflow of these solutions through the capillary, i.e. the absolute viscosity of the solutions is increased). The existence of a true state of equilibrium (1) in the solutions contain-
Equilibrium peculiarities in the oomplexing of polymeric acids
1053
ing PMAA and low-molecular P E G is also indicated by the results of an investigation of the stability of the complexes at different temperatures. In view Of the data in Fig. 1 we would conclude that the stability of the P M A A - P E G complexes increases with rising temperature (see the curves corresponding to 16.5 an d 25°). This is also confirmed b y the viscometric measurements. Figure 5b shows the temperature dependence of the reduced viscosity of the P M A A - P E G solUtions ~Sp/OPH/Ul 2.2
f'8
b
1"0 0"9
•
"
1
0"6
0"2 0.02
0"06
CpMAA , ~/~l
0.I0
0.I1
L_.×3.
15
~
20
~
•
25 30 T,°O
1"
35
"~ #0
FIG. 5. Reduced solution viscosity of PMAA-PEG complexes versus solution concentration at 25° (a) and versus temperature with a PMAA concentration of 0.1 g/dl (b) without I~EG (1) and PEG wzth M=2000 (2); 15,000 (3); 3000 (4). Molecular weight of PMAA----lO0,000. ( M = 15,000 and 2000). The complex with P E G ( M = 15,000) is stable over practically the entire temperature range studied, as is shown by the temperatureindependence of the reduced viscosity. At 15° the viscosity of the solution of the P M A A - P E G (M=2000) complex approaches that of the PMAA solution , and falls as the temperature rises, so t h a t at 40 ° it coincides with the viscosity of the complex containing P E G with M=15,000. Curve 2 is reproducibl~either for heating or cooling of the solutions, i.e. the system in question is reversible. The increased stability of P M A A - P E G bonding with rising temperature means t h a t hydrophobic interactions play a major role in stabilizing the com-
1054
A.
D.
ANTIPINA
e$ 6~/.
plex. The same conclusion is reached on comparing the properties of the PMAAPEG and P A A - P E G complexes. The methyl groups in PMAA make it more hydrophobic, and favour stabler association of PEG with PMAA. The interaction of high-moleculax PMAA with low-molecular PEG is therefore a reversible reaction, and as the molecular weight of PEG rises, the equilibrium of this reaction is shifted in the direction of complexation (a similar sort of pattern would probably emerge in the case of the association of high-molecular PEG with low-molecular ~olyacids). The stability of the complex increases rapidly • o t ° with nsmg molecular weight of PEG, and when M ----15,000 the quilibrium in reaction (1) is shifted almost completely to the right, or in other words the complex that is formed by the high-moleculax PMAA and PEG does not dissociate into macromolecular components. This is confirmed by the following experimental findings: a) the reduced viscosity of solutions of the complexes remains constant in the case of dilution over a wide range of temperatures; b) the degree of conversion ~ is independent from the ratio of the components in solution (in actual fact a maximum pH is obtained for P E G / P M A A = I on titrating PMAA solutions with high-molecular PEG; further increase in the PEG concentration thereafter has virtually no effect on pH); c) the degree of conversion is independent from the concentration of the solution. There would appear to be a contradiction here, seeing that if a solution contains equimolar amounts of high-molecular PMAA and PEG, all the macromolecules form a stable complex, i.e. the yield of reaction (1) is practically 100%, while on the other hand the experimentally found maximum degree of conversion t?~ maxis ~ 1, and amounts to ~ 0.7. However, this contradiction is readily resolved by taking into account that a state of equilibrium m a y exist inside the actual particles of the complex
•CH2\ /
~rCHi
*t~CH, 0...HOOC--v~
\ /
~CH2
O-~--COOH ~ - - C 0 0 - + H ++O
/ \
~" CHI
CH,
The,possibility of equilibrium of this sort existing in polyelectrolyte complexes was pointed out by Zezin and coworkers [9]. We now come to the important conclusion t h a t two types of equilibrium m a y exist in cooperative interactions between macromolecules. The first type of equilibrium is found within the particles of the complex (in a PMAA-PEG system this equilibrium is described by equation (2)). Theoretically, ff we know the degree of conversion 81-~nk/no inside an associated pair of macromoleeules (n o is the average number of groups in a complex t h a t axe capable of forming a bond, n~ is the average number of groups t h a t have formed a bond) we m a y find the internal equilibrium constant ~Y1 for the complex, and also the corresponding
Equilibrium peculiarities in the complexing of polymeric acids
1055
thermodynamic parameters of reaction (2). 01 is apparently a function of temperature and dependent on the geometrical similarity of the macromolecules forming a complex, and is independent from the ratio or the concentration of the compohents in solution, as the equilibrium is realized within the coils of the complex, and in the case under consideration these coils must be regarded as a separate phase. 01 m a y be determined experimentally as the maximum degree of conversion obtainable by increasing the degree of polymerization of the components Of the complex. Some difficulties arise in the experimental determination of 01 for the system studied here; 0~max=0"73, found on the basis of the data in Fig. 45 is merely an effective value, seeing t h a t if the particles of the complex are fairly dense coils, most of the hydrogen ions will appear on the surface of these coils. In this case 0~maxwill be a function of the specific surface of the coils, and the magnitude of this surface m a y depend on the length of the chains of the most high-moleCular component, or on the degree of aggregation of the particles of the eomplexi etc. It would therefore appear to be impossible to determine the relationship between 01 and 0~maxwithout further investigations. Another type of equilibrium is realized at macromolecular level (equation (1)). I t appears t h a t the equilibrium constant K~ for this reaction must be dependent on the molecular weight of the reacting macromolecules. The reactive lunit in these systems is in fact the cooperative chain segment, and an increase in the degree of polymerization is analogous to increase in the number of associated reactive units. The increase in molecular weight m a y therefore lead to ~ sort of "hypercooperative" s~ate, and to still further reinforcement of the ~ond in the complex. As is seen from Figs. 1, 2, 4 and 5, there is in fact a rise in the bond strength of the PMAA and PEG macromolecules as the molecular weight Of the P E G rises. However, we do not know what relationship exists between K~ and the concentrations of the components. To obtain at least a rough estimate of complex stability in relation to molecular weight of P E G we will calculate Some effective constants K 2' for the reaction CH2 ~ / ---COOH + O ~ complex ~CHz ~ i.e. we will take the above reaction as being conventionally bimolecular! disregarding the chain structure of the macromolecules. Then if we restric~ our, 2 , where c~ ~s the selves to the equimolar ratio of the components K~=ck/(c0--ck) concentration of --COOH groups (mole/L) entering the complex (irrespective of whether the groups are in the associated form or in "loops"), ~o is the t o t a l concentration of carboxyl groups, mole/1.; in the calculations, we dlaregard the dissociation of carboxyl groups as the degree of iorlization of the PMAA is (airly low, and this will not introduce a. major error into the calculations. Then if 0z--ok/% is the degree of conversion in reaction (1), we have K~=OZ/%(l+Oo) 2.
1056
A. D. ANTIPINA e$ a~.
Knowing that 100~ association of the macromoleeular chains of PMAA (M = 100,000) and PEG corresponds to ~x----0.73, and assuming that the degree of completeness ofreaction (1) has no effect uponthe degree of completeness of reaction (2) in the complexed segments we may find 8~ and ~ (~=0.73 ~s). K~ is readily found by plotting ~ / ( 1 - - ~ ) 2 against co. In this way we obtained for PEG with M----2000 K~(~ooo)-~l.3× 102, and for PEG with M----3000 K~(s00o)---=9.6 × 10~1./mole. Taking into account that the error in the pH determination amounts to -~0.02 pH, and using the data in Figs. 1 and 4 we may show that for PEG M----1000 K~(100o) will be of the order of 10 L/mole or less, and for PEG with M-=lS,000 K~(ls,00o) will be more than 104 L/mole, i.e. the stability of the complex does fall rapidly as the length of the reactive maeromolecules increases: with a 15-fold rise in molecular weight (from 1000 to 15,000) the stability constant is increased by a factor exceeding 1000. A thermodynamic description of the investigated systems will be the subject of a later report, including the effect of the medium on the complexing process. CONCLUSIONS
(1) The complexing of polyacrylic and polymethacrylic acids with polyethylene glycols (PEG) of different molecular weights has been investigated by the potentiometric and viscometric methods in dilute solutions. It is shown that the components are present in the complexes in equlmolar amounts. (2) It is suggested that two types of equilibrium exist during the formation of the macromolecular complexes, i.e. at the macromolecular level, and inside the associated pair of macromolecules. It is shown that the equilibrium association of the macromolecules increases rapidly with increase in their molecular weight, and at high molecular weights the equilibrium is shifted almost completely in the complexing direction. The equilibrium existing inside the associated pair of macromolecules should probably be regarded as an equilibrium in a separate phase. (3) Hydrophobic interactions arc an important factor influencing the stabilization of the complexes. The complex stability increases with rising temperature. Translated by R. J. A. HE~-DRY
REFERENCES 1. K. L. SMITH, A. E. WINSLOW and D. E. PETERSEN, Industr. a~d Engng. Chem. 51: 1361, 1959 2. Ye. OSADA, A. D. ANTIPINA, I. M. PAPISOV, V. A. KABANOV and V. A. KARGIN, Dokl. A N SSSR 191: 399, 1970 3. F. E. B),II.Ey, R. D. LUNDBERG and R. W. C~T.I.ARD, J. Polymer Sci. A2: 845, 1964 4. S. S. URAZOVSKII and I. T. SLYUSAROV, Vysokomol. soyed. 3: 420, 1961 (Translated m Polymer Sin. U.S.S.R. 3: 1, 128, 1962) 5. S. RYUTI, J. Chem. Soc. Japan, Pure Chem. Sect. 88: 386, 1962 6. V. A. KARGIN, V. A. KABANOV and A. V. VLASOV, Vysokomol. soyed. 3: 134, 1961 (Translated in Polymer Sci. U.S.S.R. 3: 1, 28, 1962)
Fluorination of polyvinyl alcohol with sulphur tetrafluorlde
1057
7. S. B. STEFANOV, Biofizika 7: 725, 1962 8. G. SClHt0DEg, Makromolek. Chem. 97: 232, 1966 9. A. B. ZEZIN, V. V. LUTSENKO, V. B. ROGACHEVA, O. A. ALEKSINA and V. A. K~kRGI IN, Vysokomol. soyed. A14: 772, 1972
FLUORINATION OF POLYVINYL ALCOHOL WITH SULPHUR TETRAFLUORIDE* V. P. BEZSOLITSEI~,B. N. GORBUNOV,A. A. NAZAROVand A. P. ] ~ A ~ D I ~ Volgograd Polytechnical Institute
(Received 21 September 1970) FLUORII~ATION is a method of modifying the properties of high-molecular hydrocarbons which could well lead to promising developments. Only a small number of papers have been published b y authors investigating the fluorination of polyethylene and chlorine-contalning polymers with elementary fluorine and h~drofluoric acid [1-4], or the fluorination of polyvinyl alcohol (PVA) with metal acid fluorides [5]. H6wever, it appears from the papers cited that up to 20% of fluorine m a y be introduced .into the polymers. The only w a y of further increasing the degree of fluorination is through reactions with elementary fluorine, b u t such processes invariably involve marked degradation of the polymer. Degradative processes have been observed even during the fluorination of Ifilms with a thickness of not more than 0.075 mm coated on special substrat~s [6]. Recently an a t t e m p t was made to fluorinate polymeric materials containing OH groups. In view of the interaction of sulphur tetrafluoride with alcohols [7, 8] which takes place at 140-200 ° and is accompanied b y the substit~ution of fluorine for O H groups, it was suggested that this might similarly be poSsible with polymers. However, attempts to substitute O H groups in polymeric!compounds have been unsuccessful: the fluorinating agent was only adsorbed b y the polymer, and did not enter into chemical interactions [9]. This paper gives the results of an investigation of the interaction of PVA with sulphur tetrafluoride. EXPERIMENTAL
The fluorination reaction was carried out m 1Kh18N96 grade 12.5 cm' stainlesS steel test tubes. 0.5 g of PVA (PVA-3, GOST (State Standard) 10779-64) along with 1 g of sodium chloride were placed m a test tube that had first been vacuum-dried at 50° for 48 h~. The test tube was sealed with a stopper provided with an outlet tube, and was purge~ with nitrogen for 15 rain to remove air, after which the purge was maintained and the test tube * VysokomoL soyed. A14: No. 4, 950-954, 1972.
M. V. Lomonosov Moscow State Un~verslty
(Received 17 September 1970) MANY examples of cooperative interaction of complimentary structures are to be found in nature, and it is of fundamental importance in living organisms. Binary systems consisting of cooperatively linked polymer chains are of definite interest from a practical standpoint as this type of interaction involving ~he loss of the properties of the original components gives rise to new sets of properties and thus opens the way to the development of completely new materials based on many known polymers [1]. It was recently reported that the cooperative interaction of growing polymer chains with macromolecnlar matrices m a y have a marked effect upon the kinetics of polymerization; moreover, the possibility of a matrix forming stable associates with polymer chains is realized only if the degree of polymerization of the! matrix exceeds a certain critical value [2]. It is obvious that if the stability of the complexes depends on the~ length of the macromolecnles included in their composition, the same parameter will similarly have a marked on other properties of the complexes. This paper relates to a study of the effect of the molecular weight Of polyethylene glycol (PEG) and other factors (concentration, p H of the n~edium, temperature) on the complexing of P E G with polymethacrylic (PMA~) and polyacrylic acid (PAA). Some information about the complexes in question is available in [3] where it is suggested that the association of P E G with p01yacids takes place through the formation of intermolecnlar hydrogen bonds. PMA_A_and PA~ were prepared by radical polymerization of the appropriate monomers, using hydrogen peroxide as the initiator [4]. Aqueous solutions of the polyaclds m~derwent dialysis and were dried first lyophihcally and then to constant weight in a vacuumtthermostat at 80°. The viscosity-average molecular weight of the PMAA determined m jabsolute methanol [4] was 100,000. The molecular weight of the PAA was also determined mscometrically m an aqueous solution of sodmm hydroxide [5], and was found to be! 120,000. Isotaetle PAA was prepared by hydrolyms of lsotactlc polylsoprepyl acrylate syrktheslzed by aniomc polymerization of isopropyl acrylate (wath a phenyl magnesmm bromide ~atalyst) at --60 °. The molecular weight of the isotactlc PAA determined viscoraetrlcally was[ 330,000. Narrow fractions of the PEG (produced by the Schuchardt Company of West Germany) were not subjected to further purification. * Vysokomol. soyed. A14: No. 4, 941-949, 1972. 1047
1048
A. D. ANTIPINA ¢~ al.
The solutions intended for mvest~gatlonwere prepared lm_medmtolyprior to the measurements by mixing solutions of the Individual components of higher concentrations. The concentrations of the solutions were generally reckoned m terms of the amount of polyacid m a mixture; PEG was added to the polyacJd m the appropmate molar ratios. The wscoslty of the solutions was determined on an Ubbelohde wscometer, the outflow time of the solvent (water) being 87.7 see. The pH of the solutions was measured on an LPM60M laboratory pH-meter with glass and chlorosolver electrodes. All the measurements were carried out with thermostat control of the systems (thermostat accuracy W-0.05°). A JEM-5Y electron-microscope was used for the electron-microscope investigations. The samples were prepared by means of "thermal attachment" [7] from solutions of an equnnolar mixture of PMAA and PEG (PMAA concentration 0.05 g/dl), time of attachment 5 mm at 40°. The samples were tinted with tungsten oxide on a JEE-4B vacuum apparatus The potentiometric and viscometric methods were used to investigate the eomplexing of the polyaeids with PEG. The potentiometrie method was selected because, as was suggested in [3], the interaction between the complex components takes place through hydrogen bonds of the undissociated carboxyl groups of the polyacid interacting with the oxygen of the PEG. I f this is so, the eomplexing reaction must be accompanied by reduction in the concentration of hydrogen ions, and this will increase the pH of the solution. The potentiometric method m a y be used to determine the amount of uneomplexed polyacid, and in this w a y the concentrations of the complexes m a y be determined. The association of the polyacid and PEG must be accompanied by a reduction in the density of the charge on the polyacid macromolecules, and by a marked increase in the hydrophobic nature of the latter owing to the screening of the hydrophilic groups of the complex. This may lead to big changes in the density and size of the polyacid coils, as is apparent from the viscometric measurements. For investigations of this type it is naturally better to use dilute solutions: complexing in concentrated solutions is very likely to lead to the formation of a continuous three-dimensional network. Figure 1 shows the results of titration of PMAA and PAA with polyethylene glycols of different molecular weights M (henceforward the concentrations in the PEG/polyacid ratios are in basic moles per litre). I t is seen from the Figure t h a t at 25 ° the addition of P E G with M = 1 0 0 0 to the PMAA solution of 0.1 g/dl concentration has no effect on pH. The addition of P E G with M--~2000 and 3000 gradually reduces the pH values of the solutions, and the rise in pH is more rapid when P E G with M = 3000 is added. The addition of P E G with M = 6000, 15,000 and 40,000 is accompanied by a rapid rise in pH, reaching a maximum at PEG/PMAA----1; further increase in the P E G concentration has only a small effect on pH. Similar behaviour is observed for the PAA solutions also, but the rise in p H occurs with P E G of higher molecular weight (6000)~ and it is less marked t h a n in the case of PMAA. The results of measuring the reduced viscosity of the polyacid solutions contahnlng P E G (Fig. 2) are in complete agreement with the results of potentiometric titration; the PMAA and PAA concentrations are 0.1 g/dl in every case.
Equilibrium peculiarities in the complexing of polymeric acids
1049
As is seen from Fig. 2a, the addition of P E G with M ~ 1 0 0 0 or below 1000 has no effect on viscosity. On adding the P E G with M = 3 0 0 0 or more the viscosity falls rapidly at first, and a minimum is reached at P E G / P M A A = I , b u t i a t t h e minimum point the value of ~ / c is independent from the molecular Weight of P E G and PMAA; the further rise in viscosity is due to the increased conCentrapH #'2
o[ o 2
+4 x5
",," ~ _ ~ _
a
_~'~
3-8
b
3"2
!
z
I 3 [PES]/ [ Po/,~-acid]
I
I 6
Fzo. 1. Titration curves of polymerm ac)ds of PMAA (a) and PAA (b) with PEG of different molecular weights: 1--1000, 2--2000, 3--3000, 4--6000, 5--15,000, 6--40,000. The Continuous curves were plotted at 25° and the fractured ones at 16.5°. Concentratzon of polymeric acids =0.1 g/dl. The points on the ordinate axis relate to solutions of pure polymeric acids. Molecular weight of PMAA--100,000; PAA--120,000. tion of excess uncomplexed PEG. The P E G with M ~ 2 0 0 0 occupies an intermediate position--the addition of the latter reduces the viscosity, b u t the reduction is less marked than in the case of the more high-molecular fractions o~ PEG. A similar pattern is observed with solutions of the P A A - P E G mixtures, b u t the drop in viscosity occurs only when the molecular weight of the P E G is 6000 or more (Fig. 2b). The marked reduction in the viscosity of the P M A A - P E G and P A A - P E G solutions in a narrow molecular weight interval is well illustrated b y Fig. 2c depicting the curves of ~sp/C vs. molecular weight of P E G for the P M A A P E G and P A A - P E G systems when the molar ratio P E G / p o l y a c i d ~ l . Three important conclusions follow from the data in Figs. 1 and 2: 1) the pdlyacids (PAA or PMAA prepared b y radical polymerization) and P E G are present in the composition of the complexes in equimolar amounts, as is shown lby the position of the viscosity lows in Fig. 2a and b, and b y the limiting pHI values attained when P E G / p o l y a c i d = 1 in the case of the high-molecular P E G fractions; 2) the complexing is of a cooperative nature; only P E G molecules whos e length exceeds a certain critical value are involved; 3) the stability of the cbmptex and the critical length of the P E G chain depend on the chemical structur~ of t h e
A. D. ANTn'INA et (~.
1050 /
~sp/ePMAA
Tlsp/cv~ a
2.3 1"6 I" 7
03 0.1
1 '
I
I,,.
3
[ PE~J/ [ PMAAJ
5
~
f
I
I 3
I
[PEG]/[PAA]
I 5
rlsp/OpM~(p~A)
0.3
I i 3.0
~ J~
tog Mpea
0.$
FIG. 2. Reduced vmcomty of solutions of PMAA-PEG (a) and PAA-PEG (b) mixtures vs. PEG/poly~nerm acid ratio for PEG of different molecular weights (conventional notation, as m l%g. 1; the cross-hatched symbols relate to ordinary PAA, 25°; concentration of polymerm acids 0.1 g/dl; molecular weight of PMAA--100,000; PAA--120,000; lsotactic PAA (3", 4"', 5") 330,000; curve 7 m for a mixture of PMAA (M=440,000) and PEG (M~15,000); c--curve of equimolar mixtures of PMAA (1-3) and PAA (2') with PEG versus molecular weight of PEG at 40 (1), 25 (2, 2") and 15° (3). polyacid. F o r a P M A A - P E G c o m p l e x to be f o r m e d it is sufficient t h a t t h e molecular weight o f t h e P E G should be n o t less t h a n 2000, while a n y significant i n t e r a c t i o n b e t w e e n P A A a n d P E G s t a r t s o n l y w h e n t h e molecular weigh~ o f t h e l a t t e r is ~ 6000. Polyacids (PAA and PMAA) prepared by radical polymerization have mainly a Byndiotactie chain s t r u c t u r e [8]. As is seen f r o m Fig. 2b, t h e chain micro-struc-
Equilibrium pecuharities in the eomplexing of polymeric acids
1051
ture has a marked effect on the stoichiometry of the complex: in mixtures qf isotactic PAA and P E G solutions the viscosity low appears at a higher PEG~PAA ratio ( ~ 1-5). I t appears from the results of electron-microscope investigations (see Fig. 3) t h a t the complex particles are spherical coils. The same conclusion is reached on investigating the viscosity of solutions of the complexes. As is seen from Fig. 2a, the reduced viscosity of the solutions when PEG/PMAA-1 is independent from the molecular weight of PEG and PMAA in the case of the high-molecular
FIO. 3. Electron photommrograph of particles of a PMAA-PEG complex. MoleeularWelght of PMAA----100,000; PEG---15,000; PMAA concentration 0-05 g/dl; molar ratio PEGfPMAA 1; tnne of attachment 5 ram, shading with tungsten oxide. PEG fractions. The reduced viscosity of these solutions is similarly independent from the complex concentration (see Fig. 5a, curve 3), i.e. solutions of the complexes obey Einstein's equation for solutions of dense spherical particles
~=~0(1+2.5 ~), where q and qo are the viscosities of the solution and solvent respectively, and q is the volume fraction of dissolved substance. This equation enables us to estimate the approximate amount of solvent in the coils of a complex, assuming the density of the coils to be equal to that of the solvent. In this way we find t h a t the complex particles contain ~ 75% water and 25% polymer. A more accurate estimate is obtainable only by measuring the density of the complex pariicles. However, even in the light of this approximation one may conclude tha~ the particles are dense coils containing very little solvent. The results of the dilution of solutions containing the complex compo aen~ in the equimolar ratio (Fig. 4a) m a y be used to find the degree of conw a'sion during the complexing reaction O ~ = ( C o - - C z ) / c o and its relation to the conc~ ntration of the components (see Fig. 4b), where co is the total concentration of car ~oxyl groups in solution; c~ is the concentration of carboxyl groups unasso( iated with PEG; Co--C ~ is the concentration of carboxyl groups forming-a bond with PEG: % m a y be determined graphically, as is shown in Fig. 4a, assumin t h a t
A.D.
1052
AN~m~A~
a~.
the degree of dissociation of the uncomplexed carboxyl groups remains constant during the complexation. I t is readily apparent from Fig. 4b that in mixtures of PMAA and low-molecular P E G fractions 0~ is reduced with increasing dilution of the solutions. This phi 4.7 0 4~
b
O~~_--.-x--~-x--x-~Lx
3"9
i
t-
2
3"5
O'OZ O ~
0"08 0"08 OlO
O.OZ ~04
006
008
~I0
CP,AA(PAA),g/d!
FIG. 4. Concentration dependence of pH of solutions of polymeric acid complexes with PEG (a), and degree of conversion versus concentration durn~g the eomplexmg of polymeric acids with PEG of different molecular weights (b) at 25°. Molecular weights of PEG: 1, 1'-15,000; 2--3000; 3--2000; 4, 4'--40,000; 5, 5'--no PEG. Curves 1-5 relate to PMAA, and 1', 4', 5' to PAA. Molecular weight of PMAA= 100,000; PAA= 120,000. points directly to dissociation of the complexes through dilution, i.e. the state of equilibrium that exists is of the type of P M A A ~ - P E G ~ complex.
(l)
The dissociation of the complexes of low-molecular P E G with PMAA due to dilution is confirmed b y the viscometric measurements (see Fig. 5a). In the case of complexes of PMAA with high-molecular P E G there is an absence of the rise in reduced viscosity accompanying dilution (see curve 3) that is characteristic of polyelectrolytes, i.e. the solutions in question contain no free PMAA. In the case of the P M A A - P E G complexes where M for P E G = 2 0 0 0 and 3000 the reduced viscosity of the solutions rises on dilution, approaching that of PMAA solutions of like concentrations containing no PEG. (It is remarkable that dilution not only results in a rise in ~/CPMAAb u t also increases the time of outflow of these solutions through the capillary, i.e. the absolute viscosity of the solutions is increased). The existence of a true state of equilibrium (1) in the solutions contain-
Equilibrium peculiarities in the oomplexing of polymeric acids
1053
ing PMAA and low-molecular P E G is also indicated by the results of an investigation of the stability of the complexes at different temperatures. In view Of the data in Fig. 1 we would conclude that the stability of the P M A A - P E G complexes increases with rising temperature (see the curves corresponding to 16.5 an d 25°). This is also confirmed b y the viscometric measurements. Figure 5b shows the temperature dependence of the reduced viscosity of the P M A A - P E G solUtions ~Sp/OPH/Ul 2.2
f'8
b
1"0 0"9
•
"
1
0"6
0"2 0.02
0"06
CpMAA , ~/~l
0.I0
0.I1
L_.×3.
15
~
20
~
•
25 30 T,°O
1"
35
"~ #0
FIG. 5. Reduced solution viscosity of PMAA-PEG complexes versus solution concentration at 25° (a) and versus temperature with a PMAA concentration of 0.1 g/dl (b) without I~EG (1) and PEG wzth M=2000 (2); 15,000 (3); 3000 (4). Molecular weight of PMAA----lO0,000. ( M = 15,000 and 2000). The complex with P E G ( M = 15,000) is stable over practically the entire temperature range studied, as is shown by the temperatureindependence of the reduced viscosity. At 15° the viscosity of the solution of the P M A A - P E G (M=2000) complex approaches that of the PMAA solution , and falls as the temperature rises, so t h a t at 40 ° it coincides with the viscosity of the complex containing P E G with M=15,000. Curve 2 is reproducibl~either for heating or cooling of the solutions, i.e. the system in question is reversible. The increased stability of P M A A - P E G bonding with rising temperature means t h a t hydrophobic interactions play a major role in stabilizing the com-
1054
A.
D.
ANTIPINA
e$ 6~/.
plex. The same conclusion is reached on comparing the properties of the PMAAPEG and P A A - P E G complexes. The methyl groups in PMAA make it more hydrophobic, and favour stabler association of PEG with PMAA. The interaction of high-moleculax PMAA with low-molecular PEG is therefore a reversible reaction, and as the molecular weight of PEG rises, the equilibrium of this reaction is shifted in the direction of complexation (a similar sort of pattern would probably emerge in the case of the association of high-molecular PEG with low-molecular ~olyacids). The stability of the complex increases rapidly • o t ° with nsmg molecular weight of PEG, and when M ----15,000 the quilibrium in reaction (1) is shifted almost completely to the right, or in other words the complex that is formed by the high-moleculax PMAA and PEG does not dissociate into macromolecular components. This is confirmed by the following experimental findings: a) the reduced viscosity of solutions of the complexes remains constant in the case of dilution over a wide range of temperatures; b) the degree of conversion ~ is independent from the ratio of the components in solution (in actual fact a maximum pH is obtained for P E G / P M A A = I on titrating PMAA solutions with high-molecular PEG; further increase in the PEG concentration thereafter has virtually no effect on pH); c) the degree of conversion is independent from the concentration of the solution. There would appear to be a contradiction here, seeing that if a solution contains equimolar amounts of high-molecular PMAA and PEG, all the macromolecules form a stable complex, i.e. the yield of reaction (1) is practically 100%, while on the other hand the experimentally found maximum degree of conversion t?~ maxis ~ 1, and amounts to ~ 0.7. However, this contradiction is readily resolved by taking into account that a state of equilibrium m a y exist inside the actual particles of the complex
•CH2\ /
~rCHi
*t~CH, 0...HOOC--v~
\ /
~CH2
O-~--COOH ~ - - C 0 0 - + H ++O
/ \
~" CHI
CH,
The,possibility of equilibrium of this sort existing in polyelectrolyte complexes was pointed out by Zezin and coworkers [9]. We now come to the important conclusion t h a t two types of equilibrium m a y exist in cooperative interactions between macromolecules. The first type of equilibrium is found within the particles of the complex (in a PMAA-PEG system this equilibrium is described by equation (2)). Theoretically, ff we know the degree of conversion 81-~nk/no inside an associated pair of macromoleeules (n o is the average number of groups in a complex t h a t axe capable of forming a bond, n~ is the average number of groups t h a t have formed a bond) we m a y find the internal equilibrium constant ~Y1 for the complex, and also the corresponding
Equilibrium peculiarities in the complexing of polymeric acids
1055
thermodynamic parameters of reaction (2). 01 is apparently a function of temperature and dependent on the geometrical similarity of the macromolecules forming a complex, and is independent from the ratio or the concentration of the compohents in solution, as the equilibrium is realized within the coils of the complex, and in the case under consideration these coils must be regarded as a separate phase. 01 m a y be determined experimentally as the maximum degree of conversion obtainable by increasing the degree of polymerization of the components Of the complex. Some difficulties arise in the experimental determination of 01 for the system studied here; 0~max=0"73, found on the basis of the data in Fig. 45 is merely an effective value, seeing t h a t if the particles of the complex are fairly dense coils, most of the hydrogen ions will appear on the surface of these coils. In this case 0~maxwill be a function of the specific surface of the coils, and the magnitude of this surface m a y depend on the length of the chains of the most high-moleCular component, or on the degree of aggregation of the particles of the eomplexi etc. It would therefore appear to be impossible to determine the relationship between 01 and 0~maxwithout further investigations. Another type of equilibrium is realized at macromolecular level (equation (1)). I t appears t h a t the equilibrium constant K~ for this reaction must be dependent on the molecular weight of the reacting macromolecules. The reactive lunit in these systems is in fact the cooperative chain segment, and an increase in the degree of polymerization is analogous to increase in the number of associated reactive units. The increase in molecular weight m a y therefore lead to ~ sort of "hypercooperative" s~ate, and to still further reinforcement of the ~ond in the complex. As is seen from Figs. 1, 2, 4 and 5, there is in fact a rise in the bond strength of the PMAA and PEG macromolecules as the molecular weight Of the P E G rises. However, we do not know what relationship exists between K~ and the concentrations of the components. To obtain at least a rough estimate of complex stability in relation to molecular weight of P E G we will calculate Some effective constants K 2' for the reaction CH2 ~ / ---COOH + O ~ complex ~CHz ~ i.e. we will take the above reaction as being conventionally bimolecular! disregarding the chain structure of the macromolecules. Then if we restric~ our, 2 , where c~ ~s the selves to the equimolar ratio of the components K~=ck/(c0--ck) concentration of --COOH groups (mole/L) entering the complex (irrespective of whether the groups are in the associated form or in "loops"), ~o is the t o t a l concentration of carboxyl groups, mole/1.; in the calculations, we dlaregard the dissociation of carboxyl groups as the degree of iorlization of the PMAA is (airly low, and this will not introduce a. major error into the calculations. Then if 0z--ok/% is the degree of conversion in reaction (1), we have K~=OZ/%(l+Oo) 2.
1056
A. D. ANTIPINA e$ a~.
Knowing that 100~ association of the macromoleeular chains of PMAA (M = 100,000) and PEG corresponds to ~x----0.73, and assuming that the degree of completeness ofreaction (1) has no effect uponthe degree of completeness of reaction (2) in the complexed segments we may find 8~ and ~ (~=0.73 ~s). K~ is readily found by plotting ~ / ( 1 - - ~ ) 2 against co. In this way we obtained for PEG with M----2000 K~(~ooo)-~l.3× 102, and for PEG with M----3000 K~(s00o)---=9.6 × 10~1./mole. Taking into account that the error in the pH determination amounts to -~0.02 pH, and using the data in Figs. 1 and 4 we may show that for PEG M----1000 K~(100o) will be of the order of 10 L/mole or less, and for PEG with M-=lS,000 K~(ls,00o) will be more than 104 L/mole, i.e. the stability of the complex does fall rapidly as the length of the reactive maeromolecules increases: with a 15-fold rise in molecular weight (from 1000 to 15,000) the stability constant is increased by a factor exceeding 1000. A thermodynamic description of the investigated systems will be the subject of a later report, including the effect of the medium on the complexing process. CONCLUSIONS
(1) The complexing of polyacrylic and polymethacrylic acids with polyethylene glycols (PEG) of different molecular weights has been investigated by the potentiometric and viscometric methods in dilute solutions. It is shown that the components are present in the complexes in equlmolar amounts. (2) It is suggested that two types of equilibrium exist during the formation of the macromolecular complexes, i.e. at the macromolecular level, and inside the associated pair of macromolecules. It is shown that the equilibrium association of the macromolecules increases rapidly with increase in their molecular weight, and at high molecular weights the equilibrium is shifted almost completely in the complexing direction. The equilibrium existing inside the associated pair of macromolecules should probably be regarded as an equilibrium in a separate phase. (3) Hydrophobic interactions arc an important factor influencing the stabilization of the complexes. The complex stability increases with rising temperature. Translated by R. J. A. HE~-DRY
REFERENCES 1. K. L. SMITH, A. E. WINSLOW and D. E. PETERSEN, Industr. a~d Engng. Chem. 51: 1361, 1959 2. Ye. OSADA, A. D. ANTIPINA, I. M. PAPISOV, V. A. KABANOV and V. A. KARGIN, Dokl. A N SSSR 191: 399, 1970 3. F. E. B),II.Ey, R. D. LUNDBERG and R. W. C~T.I.ARD, J. Polymer Sci. A2: 845, 1964 4. S. S. URAZOVSKII and I. T. SLYUSAROV, Vysokomol. soyed. 3: 420, 1961 (Translated m Polymer Sin. U.S.S.R. 3: 1, 128, 1962) 5. S. RYUTI, J. Chem. Soc. Japan, Pure Chem. Sect. 88: 386, 1962 6. V. A. KARGIN, V. A. KABANOV and A. V. VLASOV, Vysokomol. soyed. 3: 134, 1961 (Translated in Polymer Sci. U.S.S.R. 3: 1, 28, 1962)
Fluorination of polyvinyl alcohol with sulphur tetrafluorlde
1057
7. S. B. STEFANOV, Biofizika 7: 725, 1962 8. G. SClHt0DEg, Makromolek. Chem. 97: 232, 1966 9. A. B. ZEZIN, V. V. LUTSENKO, V. B. ROGACHEVA, O. A. ALEKSINA and V. A. K~kRGI IN, Vysokomol. soyed. A14: 772, 1972
FLUORINATION OF POLYVINYL ALCOHOL WITH SULPHUR TETRAFLUORIDE* V. P. BEZSOLITSEI~,B. N. GORBUNOV,A. A. NAZAROVand A. P. ] ~ A ~ D I ~ Volgograd Polytechnical Institute
(Received 21 September 1970) FLUORII~ATION is a method of modifying the properties of high-molecular hydrocarbons which could well lead to promising developments. Only a small number of papers have been published b y authors investigating the fluorination of polyethylene and chlorine-contalning polymers with elementary fluorine and h~drofluoric acid [1-4], or the fluorination of polyvinyl alcohol (PVA) with metal acid fluorides [5]. H6wever, it appears from the papers cited that up to 20% of fluorine m a y be introduced .into the polymers. The only w a y of further increasing the degree of fluorination is through reactions with elementary fluorine, b u t such processes invariably involve marked degradation of the polymer. Degradative processes have been observed even during the fluorination of Ifilms with a thickness of not more than 0.075 mm coated on special substrat~s [6]. Recently an a t t e m p t was made to fluorinate polymeric materials containing OH groups. In view of the interaction of sulphur tetrafluoride with alcohols [7, 8] which takes place at 140-200 ° and is accompanied b y the substit~ution of fluorine for O H groups, it was suggested that this might similarly be poSsible with polymers. However, attempts to substitute O H groups in polymeric!compounds have been unsuccessful: the fluorinating agent was only adsorbed b y the polymer, and did not enter into chemical interactions [9]. This paper gives the results of an investigation of the interaction of PVA with sulphur tetrafluoride. EXPERIMENTAL
The fluorination reaction was carried out m 1Kh18N96 grade 12.5 cm' stainlesS steel test tubes. 0.5 g of PVA (PVA-3, GOST (State Standard) 10779-64) along with 1 g of sodium chloride were placed m a test tube that had first been vacuum-dried at 50° for 48 h~. The test tube was sealed with a stopper provided with an outlet tube, and was purge~ with nitrogen for 15 rain to remove air, after which the purge was maintained and the test tube * VysokomoL soyed. A14: No. 4, 950-954, 1972.