A new method of preparation of fluorodesoxycellulose

A NEW METHOD OF PREPARATION OF FLUORODESOXYCELLULOSE* B. N.

GORBUNOV, A_. A. NAZAROV, P. A. PROTOPOPOV and A. P. K H A R D I N Volgograd Polytechnic Institute (Received 25 January 1971)

THERE has been a recent increase in the amount of research directed toward the production of fluoro-derivatives of cellulose. Most of the papers published have however been concerned with the preparation of cellulose ethers and esters containing fluorine, or of graft copolymers in which the fluorine atom is in the side chain. Meanwhile there has been practically no study of cellulose derivatives in which a fluorine atom is joined directly to a carbon atom of the pyran ring. The few attempts to synthesize such derivatives b y nucleophilic substitution in mesyl and tosyl cellulose esters with metal fluorides [1, 2] resulted in the production of materials with a low degree of substitution, containing only about 2% of fluorine.

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Fro. 1. Effect of the quantity of sulphur tetra fluoride on the degree of fluorhaatiort of cellulose at 20°. Number of moles of SF~ per repeating unit of the cellulose molecule: 1--1; 2--5; 3-- 10; 4-- 20; 5-- 30; accepter--sodium fluoride. We have prepared fluorodesoxycellulose b y reaction of cellulose with sulphur tetrafluoride. It was shown that the reaction of cellulose with this fluorinating agent proceeds as follows [C6HTO~(OH)~]~-~-SF 4 -~ [CsHTO~(OH)3_~Fx]n~-SOF~-~HF The degree of fluorination is dependent on the ratio of the reactants and oa reaction time. Analysis of the results presented in Fig. 1 shows t h a t the rate of fluorination and the fluorine content of the polymer increase as the molar excess * Vysokomol. soyed. AI4: No. 12, 2527-2530, 1972. 2941

2942

B.N.

G O R B U N O V el a,L

of sulphur tetrafluoride per cellulose unit is increased. The greatest q u a n t i t y of fluorine can be introduced with a thirty-fold molar excess of sulphur tetrafluoride in a reaction time of 6 hr. Replacement of the hydroxyl groups by fluorine does not occur when only a small excess of the fluorinating agent is used. In those circumstances all t h a t occurs is adsorption of the sulphur tetrafluoride by the cellulose, as the results of earlier work have also shown [3]. T A B L E 1. E F F E C T OF T H E N A T U R E O F T H E ACCEPTOR AND OF T E M P E R A T U R E ON T H E D E G R E E OF F L U O R I N A T I O N OF C E L L U L O S E

Acceptor

Ratio of acceptor: cellulose,

Tempera-

mole

°C

%

20 20 20 60 100 20 20 60

14.5 14-7

1:1 2.5:1 5:1 5:1 5:1 1:1 2.5:1 1: 1

NaF NaF NaF NaF NaF C~HsN

C,H,N C,H,N

ture,

Fluorine content,

14-8 18.5 21.0 15.9 2.3 19"0

Colour of polymer Browi~

Yellow White Yellow Brown Yellow White Brown

l~rnarks. Cotton celluJose was fluorinated with 30 moles of 8F4, calculated on the cellulose repeating unit; reaction time 2 hr.

Since in the course of substitution hydrogen fluoride is evolved, which causes degradation, as far as carbonization, of cellulose, the successful conduct of this reaction is dependent on the e xt e nt to which the hydrogen fluoride is removed TABLE2.

F 1.5 7.0 15.4

E L E M E N T A R Y COMPOSITION OF F L U O R I N A T E D C E L L U L O S E

Elementary composition, ~o C O H found I ealc. found ] calc. found [ calc. 44.5 44"7 44.2

44.3 44.4 43.7 I

47.6 44.4 35.4

48.0

43.2 35.0

5.8 5.4 4.9

6.1 5'8 5.3

Formula of product C6HTOm(OH)HTF0.13 CeHTOI(OH)2.,0Fo., CeH~OI(OHh.,FI.,

from the system. The most efficient aeceptors of hydrogen fluoride in this reaction are sodium fluoride and pyridine. I t is seen from Table 1 t h a t the quant i t y of fluorine combined with the cellulose is dependent on the nature of the acceptor. When sodium fluoride is used as aceeptor the products are powders, but with pyridine the fibrous nature of the material is preserved. The degree of substitution of fluorine for h y d r o x y l groups is practically independent of the source of the cellulose used for fluorination. An i m p o r t a n t factor

Preparation of fluorodesoxycellulose

2943

controlling the q u a n t i t y of fluorine in the derivatives is the molecular-weight distribution of the cellulose. A high degree of substitution is achieved in fluorination of high-quality cellulose, with the highest degree of molecular uniformity. F,% g% 15 - x f . ~ _

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FIG. 2. Effect of preliminary treatment of cellulose on the quantity combined fluorine~ for untreated (1) and regenerated cellulose (2); cellulose "included" with benzene (3) and mercerized cellulose (4). Fzo. 3. Effect of the degree of "inclusion" on the hydrogen-bond strength and on the quantity of combined fluorine: /--variation in the relative, differential optical density of the band of the valency vibrations of OH groups during the course of "inclusion"; 2--the fluorine content of "included" cellulose samples after fluorination. In contrast to etherification or esterification, preliminary treatment of the cellulose to loosen its structure does not have a beneficial effect on replacement o f hydroxyl groups. I t is seen t h a t the use of such widely used methods as "inclusion" (swelling of cellulose with an alcohol-water mixture followed by gradual replacement of the swelling mixture by a solvent such as benzene--Translator) or mercerization results in reduction in the degree of fluorination. Comparison o f the spectroscopic data on the variation in hydrogen-bonding in samples of cellulose with different degrees of "inclusion" with the results of fluorination of these samples (Fig. 3), shows t h a t hydrogen-bonded hydroxyl groups have a higher reactivity in reaction with SF 4. Therefore the decrease in the degree of ordering brought about by mercerization and "inclusion" in fact leads to decrease in the degree of substitution of fluorine for hydroxyl groups. This interesting fact is explained by the specific nature of the reaction of sulphur tetrafluoride with alcohols, namely t h a t increase in the electron density on the oxygen atom being attacked favours substitution. Examination from this p o i n t of view of two hydroxyl groups of cellulose joined by a hydrogen bond, shows that. ~he oxygen atom of the group acting as the proton donor carries an increased negative charge. Therefore a t t a c k on ~his group by a molecule of sulphur t e t r a fluoride will be favoured.

B. •. GORBUNOV et al.

2944

T h e p o l y m e r o b t a i n e d b y r e a c t i o n o f c e l l u l o s e w i t h s u l p h u r t e t r a f l u o r i d e is a f l u o r o d e s o x y c e l l u l o s e , w i t h a d e g r e e o f s u b s t i t u t i o n (7) u p t o 150. T h i s is c o n f i r m e d by elementary and infrared spectroscopic analysis. Table 2 shows the elementary composition of samples of fluorinated cellulose containing various amounts of fluorine. The experimentally found amounts of the elements are in good agreement with the quantities calculated for fluorodesoxycellulose. •

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FiG. 4. I n f r a r e d spectra of untreated cellulose (1) and of fluorodesoxycelluloses with ? = 17 (2); 60 (3) and 118 (4).

Analysis of the infrared spectra of fluorinated celluloses presented in Fig. 4, shows t h a t as the quantity of fluorine in the polymer increases the band of the valency vibrations of hydroxyl groups at 3600 cm -1 decreases and the contour of the b a n d in the region of 1040 cm -I changes. According to reference [4] the latter can be assigned to vibrations of the carbon--fluorine bond. EXPERIMENTAL

Sulphur tetrafluoride was prepared b y a modification of the method of Tullock [5]. Dimethylformamide was used as the solvent instead of aeetonitrile. Samples of cellulose with different degrees of "inclusion" were prepared as in reference [6], using m e t h a n o l - w a t e r mixtures of different concentration. Cotton cellulose of State Standard Specifcation GOST 5556-66, sulphate wood cellulose, Hercules cotto~ cellulose and wood cellulose with the trade m a r k "Florene", were used for fluorination. The fluorination was carried out in autoclaves of K h l 8 N I O T steel, with polytetrafluoroethylene seals, under autogenous pressure. The autoclaves were charged with 0.5 g of cellulose, dried to constant weight at 105 °, a n d the required amotmt of accepter. After the autoclave had been cooled to --70 ° in a mixture of solid carbon dioxide and acetone, the calculated amount of sulphur tetrafluoride was distilled into it. I t was then sealed a n d heated to the required temperature. A t the end of the reaction the autoclave was again cooled to --70 ° and the reactio~ product was removed.

Preparation of fiuorodesoxycellulose

2945

The resulting fiuorodesoxycellulose was washed with distilled water to remove the accepter(dried to constant weight at 40-50 ° and analysed for fluorine and sulphur by conventional methods [7]. The samples of fluorinated cellulose did not contain sulphur. All the infrared spectra were recorded in an IKS-22 spectrophotometer, with a sodium chloride prism. Samples of the original and "included" cellulose were prepared for recording of the infrared spectra by pelleting the tmground fibre with potassium bromide [8]. The infrared spectra of the fluorinated celluloses were obtained with pellets prepared by mixing 2 mg of the ground polymer with 1.3 g of potassium bromide and compressing the mixture at 9000 kg/cm 2. CONCLUSIONS

(1) T h e r e a c t i o n of cellulose w i t h s u l p h u r tetrafluoride has b e e n i n v e s t i g a t e d . I t is s h o w n t h a t t h e r e a c t i o n results in s u b s t i t u t i o n of fluorine for h y d r o x y l g r o u p s of t h e cellulose, w i t h f o r m a t i o n of fluorodesoxycellulose. (2) I t w a s f o u n d t h a t t h e h y d r o g e n fluoride e v o l v e d during t h e r e a c t i o n causes d e g r a d a t i o n of t h e cellulose. A c c e p t e r s o f h y d r o g e n fluoride t h a t p r e v e n t this d e g r a d a t i o n h a v e b e e n found. . (3) T h e o p t i m a l conditions for t h e fluorination r e a c t i o n h a v e b e e n d e t e r m i n e d . I t is s h o w n t h a t fluorodesoxycellulose containing 1 5 % of fluorine can be o b t a i n e d b y t r e a t i n g cellulose w i t h a t h i r t y - f o l d excess of s u l p h u r tetrafluoride for 6 h r a t 20 °, in t h e presence of s o d i u m fluoride or pyridine.

Translated by E. O. PHILLIPS REFERENCES

1. E. PACSU and R. SCHWENKER, Text. Res. J. 27: 173, 1957 2. L. S. SLETKINA and Z. A. ROGOVIN, Vysokomol. soyed. B9: 37, 1967 (Not translated in Polymer Sci. U.S.S.R.) 3. U.S. Pat. 2983626; RZhKhim, 1T299, 1963 4. L. BELLAMY, In:frakrasnye spektry slozhnykh molekul (Infrared Spectra of Complex Molecules). Foreign Literature Publishing House, 1963 (Russian translation) 5. F. S. FAWCETT and C. W. TULLOCK, Inorg. Syn. 7: 119, 1963 6. H. KR~.SSIG and E. SCHROTT, Bull. Inst. Text. France, No. 49, 47, 1954 7. V. A. KL1MOVA, Osnovnye mikrometody analiza organicheskikh soyodinonii (Principal Micro-methods of Analysis of Organic Compounds). p. 119, Izd. "Khimiya", 1967 8. O. ANT-WUORINEN and A. VISAP~A, Paper and Timber 42: 367, 1960