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Heterogenous polymerization of acrylonitrile initiated by alkali metal hydroxides
404
I . M . PANAIOTOVand A. T. OBRESHKOV
natured DI~A at various temperatures. I t is, however, possible to hope t h a t the use of strong covalent cross links would make it possible to carry out measurements over a wide interval of values of ionic strength. CONCLUSIONS A m e t h o d is p r o p o s e d for d e t e r m i n i n g t h e b o n d i n g e n e r g y o f t h e c o m p l e m e n t a r y chains of D N A , b a s e d on t h e m e a s u r e m e n t o f t h e t e m p e r a t u r e a n d i n t e r v a l w i d t h for t h e m e l t i n g of D N A w i t h clips. T h e m e t h o d m a k e s it possible to d e t e r m i n e b o n d i n g e n e r g y for w e a k solutions of D N A ( a p p r o x i m a t e l y 10-4M). M e a s u r e m e n t s , c a r r i e d o u t a t low ionic s t r e n g t h s , g a v e for t h e b o n d i n g e n e r g y c a l c u l a t e d for a p a i r of nucleotides, a v a l u e of 2 " 7 i 0 . 7 kilocalories/mole.
Translated by G. MODLEN REFERENCES 1. P. L. PRIVALOV, K. A. KAFIANI and D. R. MONASELIDZE, Dokl. Akad. Nauk SSSR 156: 951, 1964 2. M. A. RAWITSCHER, T. D. ROSS and J. M. STURTEVANT, J. Amer. Chem. Soc. 85: 1915, 1963 3. T. M. BIRSHTEIN, Biofizika 7: 513, 1962 4. B. N. SUKHORUKOV, Yu. Sh. MOSHKOVSKII, T. M. BIRSHTEIN and V. N. LYSTSOV, Biofizika 8: 294, 1963 5. L.P. PRIVALOV, Dokl. na I Vscs. biokhimich, s'yezde. (Paper to the First All-Union Biochemical Congress.) Leningrad, 1964 6. M. D. FRANK-KAMENETSKII, Dokl. Akad. Nauk. SSSR 157: 187, 1964 7. M. D. FRANK-KAMENETSKII, Vysokomol. soyed. 7: 354, 1965 8. V. I. PERMOGOROV and Yu. S. LAZURKIN, Biofizika 10: 17, 1965 9. V. I. PERMOGOROV, A. A. PROZOROV, M. F. SHEMYAKIN, Yu. S. LAZURKIN and R. B. KHESIN, Molekulyarnaya biofizika, sb. statci. (Molecular Biophysics, a Symposium), Moscow, 1965
HETEROGENOUS POLYMERIZATION OF ACRYLONITRILE I N I T I A T E D BY A L K A L I METAL H Y D R O X I D E S * I. M. PANAIOTOV and A. T. OBRESHKOV Institute for Organic Chemistry, Bulgarian Academy of Sciences, Sofia
(Received 28 July 1964)
ACRYLONITRILE may be polymerized by an anionic mechanism under the action of various substances: alkali metal amides [1], alkali metals [2], organometallic compounds [3], alkali metal alcoholates [4], quaternary ammonium bases [5], * Vysokomol. soyod. 7: No. 2, 366-371, 1965.
Heterogeneous polymerization of acrylonitrile
405
etc. T h e r e is also m e n t i o n in t h e l i t e r a t u r e of t h e p o l y m e r i z a t i o n of acrylonitrile d u r i n g negligent c y a n e t h y l a t i o n in t h e presence o f alkali m e t a l h y d r o x i d e s [6]. T h e r e h a s also b e e n a r e p o r t o f t h e p o l y m e r i z a t i o n o f m e t h a c r y l o n i t r i l e b y p o t a s s i u m h y d r o x i d e in liquid a m m o n i a [7]. I n t h e p r e s e n t w o r k t h e p o l y m e r i z a t i o n o f acrylonitrile u n d e r t h e action o f t h r e e alkali m e t a l h y d r o x i d e s i n N , N - d i m e t h y l f o r m a m i d e a n d in c e r t a i n o t h e r s o l v e n t s is described.
EXPERIMENTAL
Acrylonitrile. Commercial acrylonitrile (AN) was washed with dilute sulphurie acid, then with dilute alkali, dried with calcium chloride and fractionated. Alkali metal hydro~dde~. Lithium, sodium and potassium hydroxides were used. Chemically pure hydroxides were melted, commlnuted under absolute benzene and left in it for One day in the presence of a lump of the alkali metal (respectively, lithium, sodium or potassium), after which the benzene was distilled off. The dimension of the particles depended on the method of comminution, but generally it was in the range 0.07 to 0.175 cm. These particles are easily crumble d on stirring in the solvent, and grains are thus obtained with a dimension of approximately 0.008 cm. ~olven~. Commercial, N,N-dimethylformamide was fractionated, dried above calcined potassium carbonate and fractionated again; n~, 1.4296; d~*, 0-9498. In certain cases, formamide was used (fractionated in vacuum), pyridine (dried above potassium hydroxide and fractionated), benzene, ether, and petroleum ether (dried above sodium and then redistilled) were used. Polyme~,-~ion. All the experiments in polymerization were carried out in an atmosphere of dry nitrogen, free from oxygen. Polymerization was carried out in a 3-necked vessel with a double wall, into which water from a thermostat passed. The vessel was equipped with a stirrer, a tube for introducing the gas, and a reflux condenser. A weighed quantity of the alkali metal hydroxide and the dimethylformamlde (or other solvent) were placed in the vessel, and the mixture was purged with nitrogen for 15 minutes, after which the AN, which had been purged beforehand with nitrogen, was added with vigorous stirring. At the end of polymerization, the polymer was generally precipitated with an 0-36~o solution of hydrochloric acid, or sometimes with ether, ethanol or water. The precipitated polymer was filtered off, washed with water until chlorine ions were absent, and dried in vacuum. Of the three hydroxides used, only sodium and potassium hydroxides brought about the polymerization of AN. Lithium hydroxide did not have any initiating action. Experiments in the polymerization of methacrylonitrile turned out to be unsuccessful; this monomer did not polymerize under these conditions. Determination of molecular weighS. The molecular weight of the polyacrylonitrile (PAN) obtained viscometrically (a 0.25 solution of the polymer in dimethylformamide at 20°C, Ostwald visoometer). The intrinsio viscosity was determined from the formula 7rap/c= [7] +0.3417]1c [8], and the molecular weight by the formula of Cleland and Stockmayer [9] [7] = 2 . 4 3 × 10 -~ M °'75.
EXPERIMENTAL RESULTS
Effect of hydroxide concentration.
W i t h a n increase in t h e h y d r o x i d e concent r a t i o n , t h e yield o f p o l y m e r rises t o a c e r t a i n value, a f t e r w h i c h it does n o t increase f u r t h e r (Fig. 1). T h e h y d r o x i d e c o n c e n t r a t i o n effecta t h e colour of t h e p o l y m e r s o b t a i n e d .
406
I. M. PANAIOTOVand A. T. OBR~.StIKOV
80
S
~ "-2 -.0- ~ ~ ~-0
l
0"10 Amount of h~dpoxide, #
I
l~20
FIG. 1. Dependence of the yield of polyacrylonitrile on concentration of hydroxide. 1 - With the use of NaOH; 2--KOH. Conditions: initial temperature, 20°C; 10 ml dimethylformamide; 1.2 ml (0.02 mole) acrylonitrile; duration of polymerization, 5 min. W i t h ' a small c a t a l y s t c o n c e n t r a t i o n t h e y are almost colourless, b u t with a large concentration, yellow. I t is difficult t o c o m p a r e the catalytic a c t i v i t y of two hydroxides, since the degree of their dispersion is n o t the same, a n d t h e r e are g r e a t difficulties in obtaining d r y h y d r o x i d e s of the same degrees of dispersion. T h e t e m p e r a t u r e of the reaction m i x t u r e rises during polymerization. T h e m a x i m u m t e m p e r a t u r e ( a p p r o x i m a t e l y 27°C, m e a s u r e d w i t h a thermocouple) is r e a c h e d a p p r o x i m a t e l y 30 sec a f t e r t h e addition o f t h e m o n o m e r ; a f t e r this it falls r a p i d l y because of the cooling of th~ r e a c t i o n m i x t u r e b y the w a t e r fed f r o m the t h e r m o s t a t . Effect of temperature. I n order t o follow the effect of t e m p e r a t u r e , all o t h e r conditions were m a i n t a i n e d constant. The results o b t a i n e d are shown below: Temperature of water from thermostat, °C 0 5 10 15 20 25 30 40 Yield of PAN, % 80"8 8 6 " 0 66"0 8 1 " 3 7 8 " 3 76.3 8 0 " 3 62"1 Molecular weight 1850 2000 2400 -- 2550 2800 3550 6000 Note: Conditions, as Fig. 1. catalyst--potassium hydroxide (0"05 g). The eolour of the p o l y m e r s deepens w i t h a n increase in t e m p e r a t u r e : a t 0°C the p r o d u c t s are colourless, a n d a t 40°C orange. Effect of duration of polymerization. T h e d e p e n d e n c e of yield on polymerization d u r a t i o n is shown below, a n d also in Fig. 2a, a n d b. Polymerization time, seconds 6 15 30 60 300 Yield of PAN, ~o 15"7 29"4 42"4 5 0 " 2 78"0 Note: Remaining conditions, as Fig. 1. Catalyst--sodium hydroxide (0"05 g). Polymerization time,, minutes 1 2 5 10 15 20 Yield of PAN, ~/o 71"9 69"9 78"3 85.1 8 4 " 0 87"5 Molecular weight 1950 -- 2250 2900 3300 3950 Note: Remaining conditions, as Fig. 1. Catalyst--potassium hydroxide (0"05 g). :
~7
Heterogeneous polymerization of acrylonitrile
!
I
7
]
3 Time, rain
I
I
5
10 Time, sec
J
20
Fio. 2. Yield of polyacrylonitrile with various times of polymerization, a--NaOH (0.05 g) catalyst; b--KOH (0.05 g) catalyst. Remaining conditions as in Fig. 1.
The eolours of the polymers obtained are different. The polymers obtained from short polymerization durations are colourless, but from large durations, they are orange. They also have different solubilities in concentrated hydrochloric acid. Polymers obtained with a polymerization duration of up to 1 minute are insoluble, as is PAN obtained by a radical mechanism. Products obtained from long polymerization durations are soluble in concentrated hydrochloric acid. If the polymer is precipitated with ether, and not with hydrochloric acid, and the hydroxide is not removed, then it rapidly becomes coloured and is transformed into a viscous red oil. After dissolving again in dimethylformamide and precipitation with ether, a solid product with a red colour is obtained. i
TABLE 1. DEPENDENCE OF THE YIELD OF P A N ON AMOUNT OF ADDED WATER (l'$ml AN, 0"05g potasium hydroxide;
temperature, 20°C) AddedmlWater, I formamide,Dimethylm '] l YieldpAN,of ~o 0-1 0-2 0.4 0.7 1.0
9.9 9.8 9.6 9-3 9.0
53 25 9-2 1"4"
ot
* Amount of undiuoived hydroxide- very small. t Hydroxide completely dksoived.
Polymerizatio~ in the presence of water. In the presence.of water, the yield of polyacrylonitrile is reduced. With an increase in the amount of water added the amount of solid hydroxide falls a~ a result of solution. We noted that polymerization takes place only in the presence of solid hydroxide. Result8 of polymerization experiments in the presence of water are shown in Table 1.
408
I . M . PANAIOTOVa n d A. T . OBRESHKOV
Dependence of molecular weight on monomer concentration. The molecular •weight of PAN remained almost constant with different concentrations of the monomer, as is shown below: Aerylonitrile, ml 0"5 0"9 1"2 1.5 1"8 2"2 Molecular weight 3200 2700 2600 3200 2600 2600 Note: Conditions, 0.05 g potassium hydroxide, 10 ml dimethylformamide, duration 5 min., temperature 20°C.
Copolymerization. Almost pure PAN (nitrogen concentration, 24~) was obtained by copolymerization of AN and methylmethaerylate in equimoleeular ratios. Polymerization in other solvents. Polymerization under the action of solid alkali metal hydroxides can proceed only in a solvent. Polymerization did not take place upon mixing AN with the solid hydroxide in the absence of a solvent. Polymerization took place only in the presence of dimethylformamide, formamide, acetone and pyridine, and did not take place in such solvents as ether, benzene and petroleum ether. The highest yields were observed in dimethylformamide and ~n formamide, amd the lowest in pyridine. UV and IR-spevtra of the laolymers. In the UV spectra of PAN, obtained by a radical mechanism an absorption maximum is observed at 271 m/~ [10]. An absorption maximum in the UV" spectra of the polymers obtained by us (Table 2) is found in the 273-311 m~ region. Upon increasing the polymerization temperature, and also with an increase in its duration, the absorption maximum is shifted towards higher wave lengths. The IR spectrum of PAN, obtained by a radical mechanism [11] is almost identical to the spectra of polymers, which we obtained with durations of polymerization of up to 10-15 seconds (see above) (the most characteristic maxima were 1360, 1456, 2242 and 2924 era-l). The spectra of all the remaining polymers differ from this. For example, the IR spectrum of the polymer obtained in the presence of KOH as catalyst with a polymerization duration of 10 minutes, had the following most characteristic absorption maxima: 1380, 1525, 1580, 1620, 1665, 2210, 2245, 2940, 2960, 3350, and 3630 cm -1. DISCUSSION OF RESULTS
Polymerization mechanism. The results of experiments in copolymerization (almost pure PAN was obtained) points to the fact that the polymerization of AN under these conditions takes place by an anionic mechanism. Polymerization initiation is probably accomplished in the following way: K O H + CH 2 = CH -+ H O - - C H s - - C H + K +
f
CN
r
CN
The molecular weight is almost independent of the monomer concentration (see above), and this points to polymer chain termination chiefly by the me~hod of
Heterogeneous polymerization of acrylonitrile
409
chain transfer to the monomer. In principle, this may take place by two paths: a) by transfer of a proton from the monomer to the polymer ion [4]; b) by transfer of a hydride ion from the polymer ion to the monomer [12]. . . . - - C H s - - C H I ~- CH~ =-C ...--CH~---CH ~- CI-I2 = CH
CN
/
CN
I
CN
(a)
I
CN
...--CH = CH ~- CH3~CH (b) I I CN CN
The IR spectra point to the presence of conjugated nitrile groups (maximum at 2210 era-l), formed by the reaction equations shown above. Some spectrum data point to the fact that, in our case, chain termination prol~ably proceeds by reaction (b). In the IR spectra of our polymers, there is no maximum at about 900 cm -1, which is typical of the vinyl group, which would be formed if chain termination took place by reaction (a). On the other hand, there is a maximum in the spectra at 1380 cm -1, which is typical of the methyl group (deformation vibration), which may be formed in reaction (b). The maxima at 2940-and 2960 em -1 are probably caused by the methylene and methyl groups respectively (valency vibrations). TABLE 2. UV-sPECTRA OF POLYMERS* Conditions of synthesis of polymer 0% 5 min, 0"05 g K O H . 20 °, 5 min, 0"05 g K O H 40 °, 5 rain, 0-05 g K O H
Absorption maximum, m~ 273, 297 273, 283, 306 273, 284, 288, 297, 304
* Spectra were recorded in dimethylformamide,using a Sl~ctrophutometre enreglatreur Leres, type T2D.
A polymer is obtained only in the presence of the solid hydroxide (see above), and the yield is proportional to the concentration of hydroxide (Fig. 1). Polymerization does not take place in a saturated solution of the hydroxide. The rate of polymerization depends on the size of the catalyst surfaced: with the same polymerization duration, the yield is lower in the presence of more finely divided hydroxide. All this points to heterogeneous polymerization. Potassium hydroxide has a certain solubility in dimethylformamide (approximately 0.042~ at 20°C). If it is assumed that polymerization initiation takes place in a homogeneous medium (that is, under the action of dissolved hydroxide), the yields of polymer ought to be higher at higher temperatures, at which the solubility of the hydroxide rises; however, in our case, this is not observed. If polymerization is heterogeneous, it ought to take place on the hydroxide surface and not occur if this surface is blocked. In fact, polymerization did not take place in benzene, ether and petroleum ether, or in pure AN, in which the
410
I. M. PANAIOTOV and A. T. OBRESHKOV
polymer does not dissolve; however, it occurred in acetone, pyridine, dimethylformamide and formamide, since these dissolved the polymer and maintained a clean catalyst surface. The comparatively high yield in dimethylformamide and in formamide, and the low yields in acetone and especially in pyridine, may be explained by the dielectric constants of these compounds (Table 3). TABLE3. DIELECTRICCONSTANTSOF THE SOLVENTSUSEDAT20° [13] Solvent Pentane Benzene Pyridine Acetone N,l~-Dimethylforman~de Acrylonitrile [14] Formamide
Dieletric constan~ 1"84 2"28 13"23 21"5
37.65 38 77-7
It is known that the rate of ionic polymerization depends strongly on the dielectric constant of the solvent [15]. Independent of the high dielectric constant of AN, polymerization does not take place in it because of the solubility of the polymer in it. Polymer structure. The properties of the polymers obtained by us depend on the conditions of synthesis. With an increase in catalyst concentration and in polymerization duration, and with a rise in temperature, the colour of the polymer changed from white to orange. Their solubility in concentrated hydrochloric acid also changed, as did their UV and IR spectra. The UV spectra of PAN, obtained by the radical mechanism, has an absorption maximum at 271 m/~, and after treatment with a solution of sodium hydroxide, a new maximum at about 330 m/~ appears [li]. The spectrum of the polymer obtained by the reduction of acrylonitrile by sodium or by the electrochemical method [16], also has maxima in the region 305-315 m~. Takata (17) reported that 2,4,6, 8-tetraeyan-n-nonane had an absorption maximum at 265.5m/~, and after treatment with alcoholic potassium hydroxide, another maximum appeared at about 310 m/~; the colour of the solution thus became orange. Takata found that under these conditions cyelization takes place with the formation of naphthylidine rings, and that octahydro-2,7-diimino-3,6-dimethyl-l,8-naphthylidine was structurally similar to t h e coloured PAN obtained by. treatment of colourless PAN with alkali. The retention of the absorption maximum at 273 m~ and the appearance of a new maximum above 300 m~ in the polymers obtained by us may be explained in the following way: initially, PAN is formed having a chain which does not differ from the chain of the polymer obtained by the ordinary radical polymeriza-
Heterogeneous polymerization of acrylonitrile
411
tion. With longer polymerization, under the action of the hydroxide, rings of the naphthylidine type are also formed in the polymer which also are the cause of the colour of the product [12]: ,~HqI CHs ...--CH2--CH--CH2--C
Crl uH--CHz--CH--... % HO -~
CN CHI CHz A A -~ ...--CH 2 CH--CH2--CH CH CH--CHI--CH--... I I I I I CN C C C CN /\/~/\_ HO N N N In all probability, this reaction takes place more easily at higher temperatures. The negatively charged nitrogen atom may give a start to a new chain, and this should lead to an increase in the molecular weight of the polymer. In fact, at higher temperatures the molecular weight was higher. The I R spectra are also evidence in favour of the structure presented above. The reduction in the absorption maximum at 2245 cm -1 (characteristic of the CN group) in the coloured polymers is also an indication of the fact that the nitrile group also takes part in the reaction. The appearance of a maximum at 1580 cm -1 points to the presence of conjugated - - C = N - - groups. The presence of these groups also explains the solubility of the polymers in concentrated hydrochloric acid. The authors wish to express their thanks to B. Iordanov for interpreting the I R spectra of the polymers. CONCLUSIONS
(1) The polymerization of acrylonitrile in solvents under the action of solid alkali metal hydroxides has been studied. (2) It has been found that this polymerization takes place on the surface of the hydroxide by an anionic mechanism. (3) On the basis of UV and IR spectra, a proposal has been made for the polymerization mechanism and for the structure of the polymers obtained. Trans/aSed by G. MODLRN REFERENCES 1. M. G. EVANS, W. C. HIGGINSON'and N. S. WOODING, Recueil tray. chim., 68; 1069, 1949; W. C. mGGINSON and N. S. WOODING, J. Chem. Soc., 760, 1952 2. F. C. FOSTER, J. Amer. Chem. Soc. 74: 2299, 1952 3. M. IMOTO and M. KINOBHITA, J. Chem. Soc. Japan, Industr. Chem. Sect., 61: 452, 1958; M. FRANKEL, A. OTTOLENGHI, M. ALBEC'K and A. ZlLKH&, J. Chem. Soc., 3858, 1959 4. A. Z ~ . ~ U ~ , , B. A. FEIT and M. FRANKEL, J. Chem. Soc., 928, 1959 5. A. ZILKHA, B. A. FEIT and M. FRANKEL, J. Polymer Sci. 49: 231, 1961
412 ' 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.
L e t t e r to the E d i t o r A. Z A H N and P: S(~JKt~FER, Makromolek. Chem. 80: 225, 1959 C. O. OVERRERGER, H. J U K I a n d N. U R A K A W A , J. Polymer Sci., 45: 127, 1960 M. L. MILLER, J. Polymer Sei. 56: 203, 1962 R. L. CLELAND and W. H. STOCKMAYER, J. P o l y m e r Sci. 17: 473, 1955 E. TREIBER, W. BERNDT and H. TOPLAK, Angew. Chem., 67:69 1955; J. SCHURZ, H. BAYZER and H. STt~BCHEN, Makromolek. Chem. 2S: 152, 1957 H. BAYZER and J, SCHURZ, Z. Phys. Chem. 18: 30, 1957 R. B. CUNDALL, D. D. ELEY and J. V~ORRALL, J. P o l y m e r Sci. 58: 869, 1962 LANDOLT.BORNSTEIN, Zahlenwerte und Funktionen, Springer-Verlag, Berlin -G6ttingen -- Heidelberg, Bd. I I T . 6, S. 618 MONOMERY, Sb statei (Monomers, Symposium) Russian translation, Foreign Languages Publishing House, P 17, 1951 D. C. PEPPER, Nature 158: 789, 1946 M. MURPHY, M. O. CARANGELO, M. B. ~RINAINE and M. C. MARKHAM, J. Polymer, Sci. 54: 107, 1961 T. TAKATA, Kobunshi K a g a k u , 16: 693, 1959; 18: 235, 1961; 19: 641, 628, 1962; T. TAKATA, J. HIROI and M. TANIYAMA, J. Polymer Sci., A2: 1567, 1964
Letter to the Editor HYDROPHOBIC INTERACTION AND CONFORMATIONAL TRANSITIONS IN POLYMETHACRYLIC ACID T. M. B I R S H T E I N , Y E . V. A N U F R I Y E V A , T. N . N E K R A S O V A , O. B. P T I T S Y N a n d T. V. S H E V E L E V A
(Received 19 October 1964) IT IS known [1, 2], t h a t unionized and weakly ionized molecules of polymethacrylie acid (PMAA) have, in aqueous solutions, a n elementary structure which is practically absent from molecules of polyacrylic acid (PAA). F r o m our point o f view, this difference is explained b y the fact t h a t the presence of methyl groups in the ~-position in PMAA leads to the creation of hydrophobic interactions between the groups in aqueous solutions, and these reactions m a y also stabilize intramolecular hydrogen bonds. I n fact, the structure of unionized PMAA is much more m a r k e d in aqueous solutions t h a n in the organic solvent methanol [1, 2]. The intrinsic viscosity [t/] of P M A A in water is several times less t h a n the values observed in methanol, although both solvents are ideal. F r o m the suggestion made above, it follows t h a t the forces stabilizing the compact structure of P M A A molecules are in m a n y respects similar to the forces which stabilize the compact structure of globular proteins. The dependence of [t/] for P M A A on the composition of the solvent in mixtures of water (0.002 1~ HC1) a n d methanol, which we have investigated (curve 1 in the Figure), clearly points to the presence of a co-operative conforma* Vysokomol. soyed. 7: No. 2, 372-373, 1965.
I . M . PANAIOTOVand A. T. OBRESHKOV
natured DI~A at various temperatures. I t is, however, possible to hope t h a t the use of strong covalent cross links would make it possible to carry out measurements over a wide interval of values of ionic strength. CONCLUSIONS A m e t h o d is p r o p o s e d for d e t e r m i n i n g t h e b o n d i n g e n e r g y o f t h e c o m p l e m e n t a r y chains of D N A , b a s e d on t h e m e a s u r e m e n t o f t h e t e m p e r a t u r e a n d i n t e r v a l w i d t h for t h e m e l t i n g of D N A w i t h clips. T h e m e t h o d m a k e s it possible to d e t e r m i n e b o n d i n g e n e r g y for w e a k solutions of D N A ( a p p r o x i m a t e l y 10-4M). M e a s u r e m e n t s , c a r r i e d o u t a t low ionic s t r e n g t h s , g a v e for t h e b o n d i n g e n e r g y c a l c u l a t e d for a p a i r of nucleotides, a v a l u e of 2 " 7 i 0 . 7 kilocalories/mole.
Translated by G. MODLEN REFERENCES 1. P. L. PRIVALOV, K. A. KAFIANI and D. R. MONASELIDZE, Dokl. Akad. Nauk SSSR 156: 951, 1964 2. M. A. RAWITSCHER, T. D. ROSS and J. M. STURTEVANT, J. Amer. Chem. Soc. 85: 1915, 1963 3. T. M. BIRSHTEIN, Biofizika 7: 513, 1962 4. B. N. SUKHORUKOV, Yu. Sh. MOSHKOVSKII, T. M. BIRSHTEIN and V. N. LYSTSOV, Biofizika 8: 294, 1963 5. L.P. PRIVALOV, Dokl. na I Vscs. biokhimich, s'yezde. (Paper to the First All-Union Biochemical Congress.) Leningrad, 1964 6. M. D. FRANK-KAMENETSKII, Dokl. Akad. Nauk. SSSR 157: 187, 1964 7. M. D. FRANK-KAMENETSKII, Vysokomol. soyed. 7: 354, 1965 8. V. I. PERMOGOROV and Yu. S. LAZURKIN, Biofizika 10: 17, 1965 9. V. I. PERMOGOROV, A. A. PROZOROV, M. F. SHEMYAKIN, Yu. S. LAZURKIN and R. B. KHESIN, Molekulyarnaya biofizika, sb. statci. (Molecular Biophysics, a Symposium), Moscow, 1965
HETEROGENOUS POLYMERIZATION OF ACRYLONITRILE I N I T I A T E D BY A L K A L I METAL H Y D R O X I D E S * I. M. PANAIOTOV and A. T. OBRESHKOV Institute for Organic Chemistry, Bulgarian Academy of Sciences, Sofia
(Received 28 July 1964)
ACRYLONITRILE may be polymerized by an anionic mechanism under the action of various substances: alkali metal amides [1], alkali metals [2], organometallic compounds [3], alkali metal alcoholates [4], quaternary ammonium bases [5], * Vysokomol. soyod. 7: No. 2, 366-371, 1965.
Heterogeneous polymerization of acrylonitrile
405
etc. T h e r e is also m e n t i o n in t h e l i t e r a t u r e of t h e p o l y m e r i z a t i o n of acrylonitrile d u r i n g negligent c y a n e t h y l a t i o n in t h e presence o f alkali m e t a l h y d r o x i d e s [6]. T h e r e h a s also b e e n a r e p o r t o f t h e p o l y m e r i z a t i o n o f m e t h a c r y l o n i t r i l e b y p o t a s s i u m h y d r o x i d e in liquid a m m o n i a [7]. I n t h e p r e s e n t w o r k t h e p o l y m e r i z a t i o n o f acrylonitrile u n d e r t h e action o f t h r e e alkali m e t a l h y d r o x i d e s i n N , N - d i m e t h y l f o r m a m i d e a n d in c e r t a i n o t h e r s o l v e n t s is described.
EXPERIMENTAL
Acrylonitrile. Commercial acrylonitrile (AN) was washed with dilute sulphurie acid, then with dilute alkali, dried with calcium chloride and fractionated. Alkali metal hydro~dde~. Lithium, sodium and potassium hydroxides were used. Chemically pure hydroxides were melted, commlnuted under absolute benzene and left in it for One day in the presence of a lump of the alkali metal (respectively, lithium, sodium or potassium), after which the benzene was distilled off. The dimension of the particles depended on the method of comminution, but generally it was in the range 0.07 to 0.175 cm. These particles are easily crumble d on stirring in the solvent, and grains are thus obtained with a dimension of approximately 0.008 cm. ~olven~. Commercial, N,N-dimethylformamide was fractionated, dried above calcined potassium carbonate and fractionated again; n~, 1.4296; d~*, 0-9498. In certain cases, formamide was used (fractionated in vacuum), pyridine (dried above potassium hydroxide and fractionated), benzene, ether, and petroleum ether (dried above sodium and then redistilled) were used. Polyme~,-~ion. All the experiments in polymerization were carried out in an atmosphere of dry nitrogen, free from oxygen. Polymerization was carried out in a 3-necked vessel with a double wall, into which water from a thermostat passed. The vessel was equipped with a stirrer, a tube for introducing the gas, and a reflux condenser. A weighed quantity of the alkali metal hydroxide and the dimethylformamlde (or other solvent) were placed in the vessel, and the mixture was purged with nitrogen for 15 minutes, after which the AN, which had been purged beforehand with nitrogen, was added with vigorous stirring. At the end of polymerization, the polymer was generally precipitated with an 0-36~o solution of hydrochloric acid, or sometimes with ether, ethanol or water. The precipitated polymer was filtered off, washed with water until chlorine ions were absent, and dried in vacuum. Of the three hydroxides used, only sodium and potassium hydroxides brought about the polymerization of AN. Lithium hydroxide did not have any initiating action. Experiments in the polymerization of methacrylonitrile turned out to be unsuccessful; this monomer did not polymerize under these conditions. Determination of molecular weighS. The molecular weight of the polyacrylonitrile (PAN) obtained viscometrically (a 0.25 solution of the polymer in dimethylformamide at 20°C, Ostwald visoometer). The intrinsio viscosity was determined from the formula 7rap/c= [7] +0.3417]1c [8], and the molecular weight by the formula of Cleland and Stockmayer [9] [7] = 2 . 4 3 × 10 -~ M °'75.
EXPERIMENTAL RESULTS
Effect of hydroxide concentration.
W i t h a n increase in t h e h y d r o x i d e concent r a t i o n , t h e yield o f p o l y m e r rises t o a c e r t a i n value, a f t e r w h i c h it does n o t increase f u r t h e r (Fig. 1). T h e h y d r o x i d e c o n c e n t r a t i o n effecta t h e colour of t h e p o l y m e r s o b t a i n e d .
406
I. M. PANAIOTOVand A. T. OBR~.StIKOV
80
S
~ "-2 -.0- ~ ~ ~-0
l
0"10 Amount of h~dpoxide, #
I
l~20
FIG. 1. Dependence of the yield of polyacrylonitrile on concentration of hydroxide. 1 - With the use of NaOH; 2--KOH. Conditions: initial temperature, 20°C; 10 ml dimethylformamide; 1.2 ml (0.02 mole) acrylonitrile; duration of polymerization, 5 min. W i t h ' a small c a t a l y s t c o n c e n t r a t i o n t h e y are almost colourless, b u t with a large concentration, yellow. I t is difficult t o c o m p a r e the catalytic a c t i v i t y of two hydroxides, since the degree of their dispersion is n o t the same, a n d t h e r e are g r e a t difficulties in obtaining d r y h y d r o x i d e s of the same degrees of dispersion. T h e t e m p e r a t u r e of the reaction m i x t u r e rises during polymerization. T h e m a x i m u m t e m p e r a t u r e ( a p p r o x i m a t e l y 27°C, m e a s u r e d w i t h a thermocouple) is r e a c h e d a p p r o x i m a t e l y 30 sec a f t e r t h e addition o f t h e m o n o m e r ; a f t e r this it falls r a p i d l y because of the cooling of th~ r e a c t i o n m i x t u r e b y the w a t e r fed f r o m the t h e r m o s t a t . Effect of temperature. I n order t o follow the effect of t e m p e r a t u r e , all o t h e r conditions were m a i n t a i n e d constant. The results o b t a i n e d are shown below: Temperature of water from thermostat, °C 0 5 10 15 20 25 30 40 Yield of PAN, % 80"8 8 6 " 0 66"0 8 1 " 3 7 8 " 3 76.3 8 0 " 3 62"1 Molecular weight 1850 2000 2400 -- 2550 2800 3550 6000 Note: Conditions, as Fig. 1. catalyst--potassium hydroxide (0"05 g). The eolour of the p o l y m e r s deepens w i t h a n increase in t e m p e r a t u r e : a t 0°C the p r o d u c t s are colourless, a n d a t 40°C orange. Effect of duration of polymerization. T h e d e p e n d e n c e of yield on polymerization d u r a t i o n is shown below, a n d also in Fig. 2a, a n d b. Polymerization time, seconds 6 15 30 60 300 Yield of PAN, ~o 15"7 29"4 42"4 5 0 " 2 78"0 Note: Remaining conditions, as Fig. 1. Catalyst--sodium hydroxide (0"05 g). Polymerization time,, minutes 1 2 5 10 15 20 Yield of PAN, ~/o 71"9 69"9 78"3 85.1 8 4 " 0 87"5 Molecular weight 1950 -- 2250 2900 3300 3950 Note: Remaining conditions, as Fig. 1. Catalyst--potassium hydroxide (0"05 g). :
~7
Heterogeneous polymerization of acrylonitrile
!
I
7
]
3 Time, rain
I
I
5
10 Time, sec
J
20
Fio. 2. Yield of polyacrylonitrile with various times of polymerization, a--NaOH (0.05 g) catalyst; b--KOH (0.05 g) catalyst. Remaining conditions as in Fig. 1.
The eolours of the polymers obtained are different. The polymers obtained from short polymerization durations are colourless, but from large durations, they are orange. They also have different solubilities in concentrated hydrochloric acid. Polymers obtained with a polymerization duration of up to 1 minute are insoluble, as is PAN obtained by a radical mechanism. Products obtained from long polymerization durations are soluble in concentrated hydrochloric acid. If the polymer is precipitated with ether, and not with hydrochloric acid, and the hydroxide is not removed, then it rapidly becomes coloured and is transformed into a viscous red oil. After dissolving again in dimethylformamide and precipitation with ether, a solid product with a red colour is obtained. i
TABLE 1. DEPENDENCE OF THE YIELD OF P A N ON AMOUNT OF ADDED WATER (l'$ml AN, 0"05g potasium hydroxide;
temperature, 20°C) AddedmlWater, I formamide,Dimethylm '] l YieldpAN,of ~o 0-1 0-2 0.4 0.7 1.0
9.9 9.8 9.6 9-3 9.0
53 25 9-2 1"4"
ot
* Amount of undiuoived hydroxide- very small. t Hydroxide completely dksoived.
Polymerizatio~ in the presence of water. In the presence.of water, the yield of polyacrylonitrile is reduced. With an increase in the amount of water added the amount of solid hydroxide falls a~ a result of solution. We noted that polymerization takes place only in the presence of solid hydroxide. Result8 of polymerization experiments in the presence of water are shown in Table 1.
408
I . M . PANAIOTOVa n d A. T . OBRESHKOV
Dependence of molecular weight on monomer concentration. The molecular •weight of PAN remained almost constant with different concentrations of the monomer, as is shown below: Aerylonitrile, ml 0"5 0"9 1"2 1.5 1"8 2"2 Molecular weight 3200 2700 2600 3200 2600 2600 Note: Conditions, 0.05 g potassium hydroxide, 10 ml dimethylformamide, duration 5 min., temperature 20°C.
Copolymerization. Almost pure PAN (nitrogen concentration, 24~) was obtained by copolymerization of AN and methylmethaerylate in equimoleeular ratios. Polymerization in other solvents. Polymerization under the action of solid alkali metal hydroxides can proceed only in a solvent. Polymerization did not take place upon mixing AN with the solid hydroxide in the absence of a solvent. Polymerization took place only in the presence of dimethylformamide, formamide, acetone and pyridine, and did not take place in such solvents as ether, benzene and petroleum ether. The highest yields were observed in dimethylformamide and ~n formamide, amd the lowest in pyridine. UV and IR-spevtra of the laolymers. In the UV spectra of PAN, obtained by a radical mechanism an absorption maximum is observed at 271 m/~ [10]. An absorption maximum in the UV" spectra of the polymers obtained by us (Table 2) is found in the 273-311 m~ region. Upon increasing the polymerization temperature, and also with an increase in its duration, the absorption maximum is shifted towards higher wave lengths. The IR spectrum of PAN, obtained by a radical mechanism [11] is almost identical to the spectra of polymers, which we obtained with durations of polymerization of up to 10-15 seconds (see above) (the most characteristic maxima were 1360, 1456, 2242 and 2924 era-l). The spectra of all the remaining polymers differ from this. For example, the IR spectrum of the polymer obtained in the presence of KOH as catalyst with a polymerization duration of 10 minutes, had the following most characteristic absorption maxima: 1380, 1525, 1580, 1620, 1665, 2210, 2245, 2940, 2960, 3350, and 3630 cm -1. DISCUSSION OF RESULTS
Polymerization mechanism. The results of experiments in copolymerization (almost pure PAN was obtained) points to the fact that the polymerization of AN under these conditions takes place by an anionic mechanism. Polymerization initiation is probably accomplished in the following way: K O H + CH 2 = CH -+ H O - - C H s - - C H + K +
f
CN
r
CN
The molecular weight is almost independent of the monomer concentration (see above), and this points to polymer chain termination chiefly by the me~hod of
Heterogeneous polymerization of acrylonitrile
409
chain transfer to the monomer. In principle, this may take place by two paths: a) by transfer of a proton from the monomer to the polymer ion [4]; b) by transfer of a hydride ion from the polymer ion to the monomer [12]. . . . - - C H s - - C H I ~- CH~ =-C ...--CH~---CH ~- CI-I2 = CH
CN
/
CN
I
CN
(a)
I
CN
...--CH = CH ~- CH3~CH (b) I I CN CN
The IR spectra point to the presence of conjugated nitrile groups (maximum at 2210 era-l), formed by the reaction equations shown above. Some spectrum data point to the fact that, in our case, chain termination prol~ably proceeds by reaction (b). In the IR spectra of our polymers, there is no maximum at about 900 cm -1, which is typical of the vinyl group, which would be formed if chain termination took place by reaction (a). On the other hand, there is a maximum in the spectra at 1380 cm -1, which is typical of the methyl group (deformation vibration), which may be formed in reaction (b). The maxima at 2940-and 2960 em -1 are probably caused by the methylene and methyl groups respectively (valency vibrations). TABLE 2. UV-sPECTRA OF POLYMERS* Conditions of synthesis of polymer 0% 5 min, 0"05 g K O H . 20 °, 5 min, 0"05 g K O H 40 °, 5 rain, 0-05 g K O H
Absorption maximum, m~ 273, 297 273, 283, 306 273, 284, 288, 297, 304
* Spectra were recorded in dimethylformamide,using a Sl~ctrophutometre enreglatreur Leres, type T2D.
A polymer is obtained only in the presence of the solid hydroxide (see above), and the yield is proportional to the concentration of hydroxide (Fig. 1). Polymerization does not take place in a saturated solution of the hydroxide. The rate of polymerization depends on the size of the catalyst surfaced: with the same polymerization duration, the yield is lower in the presence of more finely divided hydroxide. All this points to heterogeneous polymerization. Potassium hydroxide has a certain solubility in dimethylformamide (approximately 0.042~ at 20°C). If it is assumed that polymerization initiation takes place in a homogeneous medium (that is, under the action of dissolved hydroxide), the yields of polymer ought to be higher at higher temperatures, at which the solubility of the hydroxide rises; however, in our case, this is not observed. If polymerization is heterogeneous, it ought to take place on the hydroxide surface and not occur if this surface is blocked. In fact, polymerization did not take place in benzene, ether and petroleum ether, or in pure AN, in which the
410
I. M. PANAIOTOV and A. T. OBRESHKOV
polymer does not dissolve; however, it occurred in acetone, pyridine, dimethylformamide and formamide, since these dissolved the polymer and maintained a clean catalyst surface. The comparatively high yield in dimethylformamide and in formamide, and the low yields in acetone and especially in pyridine, may be explained by the dielectric constants of these compounds (Table 3). TABLE3. DIELECTRICCONSTANTSOF THE SOLVENTSUSEDAT20° [13] Solvent Pentane Benzene Pyridine Acetone N,l~-Dimethylforman~de Acrylonitrile [14] Formamide
Dieletric constan~ 1"84 2"28 13"23 21"5
37.65 38 77-7
It is known that the rate of ionic polymerization depends strongly on the dielectric constant of the solvent [15]. Independent of the high dielectric constant of AN, polymerization does not take place in it because of the solubility of the polymer in it. Polymer structure. The properties of the polymers obtained by us depend on the conditions of synthesis. With an increase in catalyst concentration and in polymerization duration, and with a rise in temperature, the colour of the polymer changed from white to orange. Their solubility in concentrated hydrochloric acid also changed, as did their UV and IR spectra. The UV spectra of PAN, obtained by the radical mechanism, has an absorption maximum at 271 m/~, and after treatment with a solution of sodium hydroxide, a new maximum at about 330 m/~ appears [li]. The spectrum of the polymer obtained by the reduction of acrylonitrile by sodium or by the electrochemical method [16], also has maxima in the region 305-315 m~. Takata (17) reported that 2,4,6, 8-tetraeyan-n-nonane had an absorption maximum at 265.5m/~, and after treatment with alcoholic potassium hydroxide, another maximum appeared at about 310 m/~; the colour of the solution thus became orange. Takata found that under these conditions cyelization takes place with the formation of naphthylidine rings, and that octahydro-2,7-diimino-3,6-dimethyl-l,8-naphthylidine was structurally similar to t h e coloured PAN obtained by. treatment of colourless PAN with alkali. The retention of the absorption maximum at 273 m~ and the appearance of a new maximum above 300 m~ in the polymers obtained by us may be explained in the following way: initially, PAN is formed having a chain which does not differ from the chain of the polymer obtained by the ordinary radical polymeriza-
Heterogeneous polymerization of acrylonitrile
411
tion. With longer polymerization, under the action of the hydroxide, rings of the naphthylidine type are also formed in the polymer which also are the cause of the colour of the product [12]: ,~HqI CHs ...--CH2--CH--CH2--C
Crl uH--CHz--CH--... % HO -~
CN CHI CHz A A -~ ...--CH 2 CH--CH2--CH CH CH--CHI--CH--... I I I I I CN C C C CN /\/~/\_ HO N N N In all probability, this reaction takes place more easily at higher temperatures. The negatively charged nitrogen atom may give a start to a new chain, and this should lead to an increase in the molecular weight of the polymer. In fact, at higher temperatures the molecular weight was higher. The I R spectra are also evidence in favour of the structure presented above. The reduction in the absorption maximum at 2245 cm -1 (characteristic of the CN group) in the coloured polymers is also an indication of the fact that the nitrile group also takes part in the reaction. The appearance of a maximum at 1580 cm -1 points to the presence of conjugated - - C = N - - groups. The presence of these groups also explains the solubility of the polymers in concentrated hydrochloric acid. The authors wish to express their thanks to B. Iordanov for interpreting the I R spectra of the polymers. CONCLUSIONS
(1) The polymerization of acrylonitrile in solvents under the action of solid alkali metal hydroxides has been studied. (2) It has been found that this polymerization takes place on the surface of the hydroxide by an anionic mechanism. (3) On the basis of UV and IR spectra, a proposal has been made for the polymerization mechanism and for the structure of the polymers obtained. Trans/aSed by G. MODLRN REFERENCES 1. M. G. EVANS, W. C. HIGGINSON'and N. S. WOODING, Recueil tray. chim., 68; 1069, 1949; W. C. mGGINSON and N. S. WOODING, J. Chem. Soc., 760, 1952 2. F. C. FOSTER, J. Amer. Chem. Soc. 74: 2299, 1952 3. M. IMOTO and M. KINOBHITA, J. Chem. Soc. Japan, Industr. Chem. Sect., 61: 452, 1958; M. FRANKEL, A. OTTOLENGHI, M. ALBEC'K and A. ZlLKH&, J. Chem. Soc., 3858, 1959 4. A. Z ~ . ~ U ~ , , B. A. FEIT and M. FRANKEL, J. Chem. Soc., 928, 1959 5. A. ZILKHA, B. A. FEIT and M. FRANKEL, J. Polymer Sci. 49: 231, 1961
412 ' 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.
L e t t e r to the E d i t o r A. Z A H N and P: S(~JKt~FER, Makromolek. Chem. 80: 225, 1959 C. O. OVERRERGER, H. J U K I a n d N. U R A K A W A , J. Polymer Sci., 45: 127, 1960 M. L. MILLER, J. Polymer Sei. 56: 203, 1962 R. L. CLELAND and W. H. STOCKMAYER, J. P o l y m e r Sci. 17: 473, 1955 E. TREIBER, W. BERNDT and H. TOPLAK, Angew. Chem., 67:69 1955; J. SCHURZ, H. BAYZER and H. STt~BCHEN, Makromolek. Chem. 2S: 152, 1957 H. BAYZER and J, SCHURZ, Z. Phys. Chem. 18: 30, 1957 R. B. CUNDALL, D. D. ELEY and J. V~ORRALL, J. P o l y m e r Sci. 58: 869, 1962 LANDOLT.BORNSTEIN, Zahlenwerte und Funktionen, Springer-Verlag, Berlin -G6ttingen -- Heidelberg, Bd. I I T . 6, S. 618 MONOMERY, Sb statei (Monomers, Symposium) Russian translation, Foreign Languages Publishing House, P 17, 1951 D. C. PEPPER, Nature 158: 789, 1946 M. MURPHY, M. O. CARANGELO, M. B. ~RINAINE and M. C. MARKHAM, J. Polymer, Sci. 54: 107, 1961 T. TAKATA, Kobunshi K a g a k u , 16: 693, 1959; 18: 235, 1961; 19: 641, 628, 1962; T. TAKATA, J. HIROI and M. TANIYAMA, J. Polymer Sci., A2: 1567, 1964
Letter to the Editor HYDROPHOBIC INTERACTION AND CONFORMATIONAL TRANSITIONS IN POLYMETHACRYLIC ACID T. M. B I R S H T E I N , Y E . V. A N U F R I Y E V A , T. N . N E K R A S O V A , O. B. P T I T S Y N a n d T. V. S H E V E L E V A
(Received 19 October 1964) IT IS known [1, 2], t h a t unionized and weakly ionized molecules of polymethacrylie acid (PMAA) have, in aqueous solutions, a n elementary structure which is practically absent from molecules of polyacrylic acid (PAA). F r o m our point o f view, this difference is explained b y the fact t h a t the presence of methyl groups in the ~-position in PMAA leads to the creation of hydrophobic interactions between the groups in aqueous solutions, and these reactions m a y also stabilize intramolecular hydrogen bonds. I n fact, the structure of unionized PMAA is much more m a r k e d in aqueous solutions t h a n in the organic solvent methanol [1, 2]. The intrinsic viscosity [t/] of P M A A in water is several times less t h a n the values observed in methanol, although both solvents are ideal. F r o m the suggestion made above, it follows t h a t the forces stabilizing the compact structure of P M A A molecules are in m a n y respects similar to the forces which stabilize the compact structure of globular proteins. The dependence of [t/] for P M A A on the composition of the solvent in mixtures of water (0.002 1~ HC1) a n d methanol, which we have investigated (curve 1 in the Figure), clearly points to the presence of a co-operative conforma* Vysokomol. soyed. 7: No. 2, 372-373, 1965.