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Cyclization of aromatic poly(amic acids) to polyimides. Cyclization kinetics of a model compound of N-phenylphthalamic acid
1212
S . V . LAVROV e~ al. REFERENCES
1. J. N. PITTS, R. L. LETZINGER, R. P. TAYLOR, J. M. PATTERSON, G. RECKTENW,&T,]i a n d R. B. MARTIN, J. Amcr. Chem. Soc. 81: 1068, 1959 2. W. M, MOORE, G. S. HAMMOND and R. P. FOOS, J, Amer. Chem. Soc. 83: 2789, 1961 3. G. S. HAMMOND, W. P. B A K E R and W . M. MOORE, J. Amer. Chem. See. 83: 2795, 1961 4. W. M. MOORE and M. KETCHUM, J. Amer. Chem. See. 84: 1368, 1962 5. A. BEKETT and G. PORTER, Trans. F a r a d a y Soc. 59: 2039, 1963 4}. M. WARNOCK and P. GARDNER, J. Appl. P o l y m e r Sci. 12: 2325, 1968 7. E. G. ROZANTSEV, Svobodnye iminoksil'nye r a d i k a l y (Free I m i n o x y l Radicals). p. 190, Izd. " K h i m i y a " , 1970 8. A. V. KUTSENOVA, V. M. ANISIMOV, O. N. K A R P U K H I N and V. Ya. SHLYAPINTOKH, Vysokomol. soyed. BI5: 222, 1973 (Not translated in Polymer Soi. U.S.S.R.) 9. A. CHARLESBY, G. S. (;RACE and F. B. PILKINGTON, Prec. Roy. Soc. 268: 1333, 1962 10. O. N. K A R P I f K H I N and E. M. SLOBODETSKAYA, Vysokomol. soyed. A16: 1624, 1974 (Translated in Polymer Sci. U.S.S.R. 16: 7, 1882, 1974) 11. V. M. PATEL, C. K. PATEL, K. C. PATEL and R. D. PATEL, Makromolek. Chem. 158: 65, 1972 12. E. V. BYSTRITSKAYA, T. S. KARPOVICH and O. N. KARPUKHIN, Dokl. A N SSSR 228: 632, 1976 13. V. M. ANISIMOV, O. N. K A R P U K H I N and A. M. MATTUCHI, Dold. A N SSSR 214: 828, 1974 14. O. N. K A R P U K H I N and A. V. KUTSENOVA, Izv. AN SSSR, seriya khimich., 1301, 1976
~CYCLIZATION OF AROMATIC POLY(AMIC ACIDS) TO POLYIMIDES. CYCLIZATION KINETICS OF A MODEL COMPOUND OF N-PHENYLPHTHALAMIC ACID * S . V. LAYROV, A. YA. ARDASHI~IKOV, I . YE. KARDASH a n d A . N . PRAVEDI~IKOV L. Ya. K a r p o v Physico-chemical Research I n s t i t u t e
(Received 8 September 1976) A kinetic s t u d y of the thermal cyclization of N-phenylphthalamic acid to :Nphenylphthalimide has been carried out in various solvents; the reaction is described b y a second order equation and is catalyzed b y acids. A t 40-60~o conversion the rate constant is reduced as a result of side-reactions t a k i n g place. I t was found t h a t as the basicity of the solvent increases the rate constant is reduced, and the relative contribution of side reactions increases. The experimental results were used to interpret d a t a on the cyclization kinetics of aromatic polyamic acids. * Vysokomol. soyed. A19: :No. 5, 1052-1057, 1977.
Cyclization of aromatic poly (amic acids) to polyimides
I218
ALT~OtTOH a r e l a t i v e l y l a r g e n u m b e r o f p a p e r s [ 1 - 7 ] h a v e a p p e a r e d i n c o n n e c t i o n with kinetic investigations of the thermal cyclization of aromatic polyamic acids to polyimides, no definite conclusions have as yet been reached in regard to regularities of formation of the imide ring. This may be because the cyclization r e a c t i o n t a k e s p l a c e i n t h e s o l i d p h a s e , in t h e p o l y m e r c h a i n s , a n d b o t h o f t h e s e factors may distort the true character of the process of imide ring formation. In view of this our aim in the present work was to investigate kinetic parameters of the eyclization of a low molecular model compound, namely N-phenylphthalamic acid COOH
CO,H-- ~ _ j Cyclization of the N-phenylphthalamic acid was carried out under a variety of conditions, varying the concentration, temperature, and solvent. N-phenylphthalamic acid was prepared b y reacting phthalic anhydride with aniline in chloroform. The precipitate was filtered, washed with chloroform and recrystallized from ethanol; m.p. 170 °. N-Phenylphthalimide was prepared b y heating N - p h e n y l p h t h a lamic acid at 170 ° for 3 hr a n d was recrystallized from ethanol; m.p. 210 °. N,N-Dimethylphthalamic acid was prepared b y passing gaseous dimethylamino t/hrough a solution o f phthalie anhydride in benzene. The precipitate was filtered, washed with benzene, sncl recry.~tatlized from ethanol; m.p. 124.5-125 °. Phthalic anhydride was recrystallized from benzene, a n d phthalic acid, from water. o-Chlorobenzoic acid was sublimed in vacuum at 100 °. Aniline was distilled at reduced pressure. DMF, DMAA and m-cresol were di'ied over Call2 and distilled i n v a c u o . Dimethyl: ether of diethylene glycol (diglyme) was boiled and distilled over K O H , and then boiled and distilled over sodium wire; the fraction with b.p. 160'5-161 ° was collected. Chloroform was washed repeatedly ~dth water, dried over CaCl~ and Call2 and distilled. Benzene w a s distilled over sodium wire. Ethanol and ethyl acetate m~derwent distillation. The melting (boiling) points of t h e reagents and solvents agreed with published information. The kinetics of N-phenylphthalimide formation were investigated b y I R spectroscopy through changes in the optical density of solutions at 721 cm -~ (1780 cm -~ when m-cresol was used as solvent). The reaction was run in glass ampoules in an argon current; the ampoules: were placed in a thermostat, a n d the temperature was maintained to within ± 0 . 2 °. T h e time for the commencement of reading was the moment when the temperature of the ampoule contents reached a level 5 ° below the required temperature. Sampling was carried o u t after specified intervals of time. The spectra were recorded on a UR-10 instrument. The N-phenylphthalimide concentration in solution was determined from a calibration plot of the optical density of N-phenylphthalimide solutions versus concentration (the dependence is a linear one, i.e. the Lambert~-Becr law is satisfied). The amount of unreaeted acid was determined b y potentiometric titration on a pH-340 instrument. Collected samples were diluted with D M F a n d t i t r a t e d with a 0.1 ~ solution of K O H in absolute ethanol; the working electrode and comparative electrode were r e s p e c t i vely of glass and silver chloride.
:1214
S.V. LAv~ov e~ at.
The amount of N-phenylphthalimide formed was also determined by a gravimetrie method [1] by fractionating the prodUcts according to their solubility. The composition of the reaction products was determined by thin-layer chromatography "with a witness". Plates with a "Silufol" layer attached were used; a benzeneethyl acetate mixture (2:5) was used as the eluent. Specimens intended for chromatography were prepared as follows. A solution of N-phenylphthalamic acid (phthalie anhydride or phthalic acid) in D!~IF was heated for 2 hr at 145°, DMF was distilled off under vacuum, and the dry residue was dissolved in ethyl acetate. 1 #g
3
2
I
I
I
I
lo0 20o 300 qgo" Time, rain FIG. 1. Degree of conversion of N-phenylphthalamic acid to ~T-phenylphthalimide
vs. time: I -- determined by potentiometrie titration of the acid; 2 -- determined gravimetricaUy; 3 -- by I1% spectroscopy. Solvent -- DMF, temperature 145°, initial concentration 0.14 mole/1. F i g u r e 1 shows t h e kinetic curves for t h e cyclization o f N - p h e n y l p h t h a l a m i c acid in solution in DM_F a t 145 °, c o n c e n t r a t i o n 0.14 mole/1, p l o t t e d b y m e a n s o f t h e various methods. M a x i m u m values for the cyclization r a t e were o b t a i n e d w h e n t h e N-phenylphthalimide c o n c e n t r a t i o n was d e t e r m i n e d b y IR-spectroscopy. T h e u n d e r e s t i m a t e d values o b t a i n e d g r a v i m e t r i c a l l y m a y be t h e result o f t h e i n e v i t a b l e losses occurring during f r a c t i o n a t i o n of the reaction products. T h e lowest r a t e of cyclization was o b t a i n e d b y t h e m e t h o d of t i t r a t i o n of t h e u n r e a c t e d l ~ - p h e n y l p h t h a l a m i c acid, i.e. it was f o r m a l l y f o u n d t h a t t h e r a t e o f d i s a p p e a r a n c e o f N - p h e n y l p h t h a l a m i c acid is lower t h a n the r a t e o f N - p h e n y l p h t h a l i m i d e f o r m a t i o n . Since it was t h o u g h t t h a t t h e u n d e r e s t i m a t e d results o b t a i n e d b y p o t e n t i o m e t r i c t i t r a t i o n were due to t h e f o r m a t i o n o f carboxyl-conr a i n i n g p r o d u c t s in solution, t h e m e t h o d o f I R s p e c t r o s c o p y was Used b y us t o investigate t h e kinetics o f t h e imidization reaction. I t can be seen t h a t a n inflexion appears on t h e kinetic curves in Fig. 1 a t 4 0 % conversion. This inflexion is p a r t i c u l a r l y e v i d e n t if t h e kinetic d a t a are p l o t t e d as curves o f 2.3 log [a/(a--x)] a n d x/a (a--x) versus time, where a is t h e initial conc e n t r a t i o n o f t h e N - p h e n y l p h t h a l a m i e acid, a n d x is t h e c u r r e n t c o n c e n t r a t i o n o f N - p h e n y l p h t h a l i m i d e (Fig. 2a). I t seems t h a t a t a definite degree of conversion t h e r e is a m a r k e d r e d u c t i o n in t h e r a t e constant, a p p a r e n t l y as a result of a change
Cyclization of aromatic poly (amic acids) to polyimides
1215
in the mechanism of the process. The plot of 2.3 log [a/(a--x)] versus time is a straight line only in the initial portion prior to the point of inflection. At the same time, if we plot the curve of x / a ( a - - x ) versus time, we obtain two intersecting straight lines with different slopes, i.e. the process is described b y two second order equations with different rate constants. Since no conclusion as to the order
z.alog[a/(a-=)]
x/a(a:=)
- _~I0
1"0
-Io~!u 0"5 /
b
-E
V
I I l
I00
200 300 Time, rain
3 40g
0"6 0"8 1.0 1.2 -10g c
FIG. 2. a -- Plots of 2-3 log [a/(a--x)] (1) and x/a(a--x) (2) vs. time, using the IR spectroscopic data in Fig. 1; b -- logarithm of the rate (log v) of cyelization (mole/1..rain) of N-phenylphthalamic acid vs. the logarithm of the initial DMF concentration log c. of the reaction can be reached through the data in Fig. 2a on the basis of initial portions of the curves, we measured the initial rates of the reaction with different initial concentrations of N-phenylphthalamic acid (Fig. 2b); the slope of the curve is 1.87. This means t h a t the cyclization reaction of N-phenylphthalamic acid is described by a second order equation both in the initial stage of the process and in subsequent stages. The rate constants for a second order reaction of cyclization of N-phenylphthalamie acid in DMF at 145 °, determined from the slope of the x/a (a--x) versus time curve are 0.0487 1./mole. min in the initial stage of the process and 0.0122 1./mole.min in subsequent stages, i.e. there is approximately a fourfold difference in the rate constants. The bimolecular character of the cyclization reaction m a y be due to a protonated form of N-phenylphthalamic acid, or of a dimer of the latter, undergoing cyclization. The formation of dimers in highly polar basic solvents such as DM:F is considered doubtful [8]. I t appears more probable t h a t an intermediate auotoprotonated product m a y be formed. This supposition is borne out by the fact t h a t the reaction is catalysed b y earboxylie acids. I t was found ~hat additions of o-chlorobenzoic acid within the concentration interval 0.01-0.15 mole/1, leads to
1216
S.V. LivRov et ed.
a marked increase in the reaction rate; in addition, the initial rate of the reaction is a linear function of the amount of acid added (see Fig. 3). To shed light on the factors underlying the reduced rate of the cyclization reaction at ~40~/o conversion, we investigated the composition of reaction products formed on heating N-phenylphthalamic acid for 90 min in solution in DMF. gO
4 0
70 Z'
I 50
LIj~ lole/L.m[n O'OO2
10
0-00I 0.05
07O
0.15
[csAJ ' mole~/.
FIG. 3
I
I
1oo
3oo
Time, min
FIG. 4
FIG. 3. initial rate of cyclization of l~-phenylphthalamic acid vs. the concentration of o-chlorobenzoic acid (CBA) added. DMF, initial concentration 0.14 mole/1. FzG. 4. Degree of conversion of N-phenylphthalamic acid to l~-phenylphthalimide vs. time in different solvents: 1 -- DMAA, 2 -- DMF, 3 -- diglyme, 4 -- m-cresol. Aniline, phthalic acid and N,N-dimethylphthalic acid were detected by thin layer chromatography. Aniline and phthalic acid are apparently formed through intramolecular dissociation of N-phenylphthalamic acid together v/ith s u b s e q u e n t hydrolysis of phthalic anhydride by water evolved during imidization. Possible factors giving rise to the formation of N,l~-dimethylphthalamic acid could be reactions of phthalie anhydride, t h a t is at equilibrium with N-phenylphthalamie acid, or of phthalic acid with DMF. Such a view is supported by other results [10, 11] relating to the interaction of anhydrides with DMF. I n addition, we found t h a t the heating of solutions of phthalic acid or its anhydride in DMF for 1-3 h r at 145 ° leads to t.he formation of N,N-dimethy]phthalamic acid.
Cyclization of aromatic poly (amic acids) to polyimides
Thus a complex
mixture of products
appears in the cyclization
1217
of N-phenyl-
phthslamic acid. At the same time with prolonged heating of the reaction mixture ( N 70 hr) the N-phenylphthalamide yield determined by IR-spectroscopy is close to lOOoh, i.e. the products formed during the reaction are nevertheless converted to N-phenylphthalamide. Certainly, on heating equimolar mixtures (~~0.15 mole/l.) of phthalic acid with aniline in DMF at 145” it was found that N-phenylphthrtlimide is formed. The rate constant for the latter reaction is 0.019 I./mole* min, which is only insignificantly higher than the rate constant for the slow stage of cyclization of N-phenylphthalamic acid (k= 0.012 I./mole - min). Similar results were also obtained on heating a mixture of N,N-dimethylphthalamic acid and aniline (c=O*15 mole/l.) in DMF at 145”. The rate constant for N-phenylphthalamide formation in the latter case is 0,032 I./mole*min, i.e. it is slightly lower than the rate constant in the initial stage of the cyclization of N-phenylphthalamic acid, and considerably higher than in the slow stage. This means that the change in the rate constant during the cyclization reaction of N-phenylphthalamic acid is mainly due to the formation of phthalic acid, the interaction of which with aniline takes place at an appreciably lower rate. In view of the experimental results, the processes occurring during the cyclizntion of N-phenylphthalsmic acid may be represented by the following scheme:
t
It should be noted that under the conditions adopted in this investigation thecyclization reaction of N-phenylphthalamic acid is an irreversible one. No change in the N-phenylphthalimide concentration appears on heating the latter in DMF solution (c==O*12 mole/l.) in the presence of 2% water at 148” for 7 hr. Phthalic acid formation enables one to account for differences in the results obtained by the methods of potentiometric titration and IR spectroscopy. The absence of agreement is due to the fact that the forming phthalic acid is titrated as a dibasic acid, which leads to overestimated results in determinations of the unreacted acid, which in turn leads to errors in determining the N-phenylphthalimide concentration. Starting with the temperature depedence of the cyclization rate constant in theinitial stages of N-phenylphthalamic acid formation in DMF solution determined
S . V . L A v ~ o v et al.
t218
a t 130, 135, 140 and 145 ° , we found that the activation energy for the process is 30.5 kcal/mole. Figure 4 shows the kinetic curves for the cyclization of N-phenylphthalamic acid in the different solvents at 145 °. I t can be seen that the cyclization rate decreases in the order m - c r e s o l ~ d i g l y m e ~ D M F ~ D M A A as the basicity of the solCYCLIZATION RATE CONSTANTS FOR N-PHENYLPHTHALAI~IIC ACID IN DIFFEREI~T SOLVENTS AT 145 ° (kl AT THE START OF THE REACTION, ]Ca IN THE SUBSEQUENT SLOW STAGE) Solvent DMAA DMF Diglyme m-Cresol
kl, l./mole, min
ka, L / m o l e - r a i n
0-0408 0.0487 0.1010 0-1606
0.0001 0.0122 -0.0010
v e n t increases. The shape of the curves is identical for all the solvents. The
x]a (a--x) versus time plots appear as two intersecting straight lines; in addition, t h e degree of conversion at which the discontinuity appears varies with the nature o f the solvent, and rises in the order D M A A < D M F < m - c r e s o l . The Table gives t h e rate constants for the reaction in the different solvents. I t appears t h a t the rate constant largely depends on the basicity of the solvent; the m a x i m u m rate is obtained in the acidic m-cresol. Moreover, there is at the same time a reduction in the relative contribution of the reaction of l~-phenylphthalamide formation via a stage of phthalic acid formation. The results of kinetic investigations of the cyclization of N-phenylphthalamic acid to N-phenylphthalimide may be used to interpret data on the cyclizationl~inetics of polyamic acids. In particular, the change in the rate constant in the cyclization of polyamic acids [1-3, 5, 7] is apparently similar in character to that occurring in the case of the model compound. However, the mobility of the polymer chains decreasing in the course of the reaction impedes interaction between terminal carboxyl groups and amino-groups, and results in a continuous reduction in the cyclization rate constant after a definite degree of conversion has been reached [7]. The influence of the nature of a solvent remaining in polyamic acids [7, 8] is similar to the influence which a solvent has upon the cyclization of N-phenylphthalamic acid. I n either case this is related to the fact t h a t autoprotonation of amic acid groups is facilitated when the basicity of the solvent is reduced. I n view of this we have, of course, the acid catalysis occurring in the cyclization of the model compound and of polyamic acids [6]. Translated by R. J. A. H E N D E Y
Polymerization of benzyl thiocyanate
1219
REFERENCES 1. J. A. KREUZ, A. L. ENDREY, F. P. GAY and C. E. SROOG, ft. Polymer Sci. 4, A - I , 2607, 1966 2. L. A. LAIUS, M. I. BESSONOV, E. V. KALT.TSTOVA, N. A. ADROVA and F. S. FLORINSKII, Vysokomol. soyed. A9: 2185, 1967 (Translated in Polymer Sci. U.S.S.R. 9: 10, 2475, 1967) 3. L. A. LAIUS, M. I. BESSONOV a n d F. S. FLORINSKII, Vysokomol. soyed. A13: 2006, 1971 4. V. D. IVIOISEYEV and L. S. BUBLIK, Abstracts of I V Symposium "Polymers-73", Varna, vol. 1, p. 133, 1973 5. A. N. PRAVEDNIKOV, I. E. KARDASH, N. P. GLIfKHOYEDOV and A. Ya. ARDASHNIKOV, Vysokomol soyed. A I 5 : 349, 1973 (Translated in Polymer Sci. U.S.S.R. 15: 2, 399, 1973) 6. A. G. CHERNOVA, L. S. BUBLIK, L. P. RODIONOV, A. V. IVANOVA and V. D. VOROBYEVA, Plast. massy, ~ o . 3, 11, 1975 7. I. E. KARDASH, A. Ya. ARDASHNIKOV, F. S. YAKUSHIN and A. N. PRAVEDNIKOV, Vysokomol. soyed. A17: 598, 1975 (Translated in Polymer Sci. U.S.S.R. 17: 3, 689, 1975) 8. A. Ya. ARDASHNIKOV, Dissertation, 1974 9. A. Ya. ARDASHNIKOV, I. E. K A R D A S H and A. N. PRAVEDNIKOV, Vysokomol. soyed. A13: 1863, 1971 10. C. COPPINGER, J. Amer. Chem. Soc. 76: 1372, 1954 I1. A. Ya. ARDASHNIJKOV, I. E. K A R D A S H and A. N. PRAVEDNIKOV, Vysokoraol. soyed. B16: 368, 1974 (Not translated in P o l y m e r Sci. U.S.S.R.)
THE POLYMERIZATION OF BENZYL THIOCYANATE* B. A. ZHUBA~COV,A. I. KOLESI~IKOVA and T. YA. SMIRI~OVA Chemical Science Institute, Kaz. S.S.R. A c a d e m y of Sciences
(Rceeived 12 September 1976) A p o l y m e r containing - - C = N - - conjugated bonds has been synthesized from benzyl thiocyanate. The kinetics a n d mechanism of the polymerization reaction have been investigated. I n the light of the results of I R , NMR and chemical analyses the structure of the polybenzylthiocyanate has been elucidated. Some properties of the p o l y m e r have been investigated.
O~ an earlier occasion [1] we showed that the formation of polymers from organic thioeyanates is feasible. In a comparative analysis of the reactivity of aromatic, f a t t y aromatic and aliphatie thioeyanates [2] it was found ~hat the presence of a labile H atom in the * Vysokomol. soyed. A19: No. 5, 1058-1060, 1977.
S . V . LAVROV e~ al. REFERENCES
1. J. N. PITTS, R. L. LETZINGER, R. P. TAYLOR, J. M. PATTERSON, G. RECKTENW,&T,]i a n d R. B. MARTIN, J. Amcr. Chem. Soc. 81: 1068, 1959 2. W. M, MOORE, G. S. HAMMOND and R. P. FOOS, J, Amer. Chem. Soc. 83: 2789, 1961 3. G. S. HAMMOND, W. P. B A K E R and W . M. MOORE, J. Amer. Chem. See. 83: 2795, 1961 4. W. M. MOORE and M. KETCHUM, J. Amer. Chem. See. 84: 1368, 1962 5. A. BEKETT and G. PORTER, Trans. F a r a d a y Soc. 59: 2039, 1963 4}. M. WARNOCK and P. GARDNER, J. Appl. P o l y m e r Sci. 12: 2325, 1968 7. E. G. ROZANTSEV, Svobodnye iminoksil'nye r a d i k a l y (Free I m i n o x y l Radicals). p. 190, Izd. " K h i m i y a " , 1970 8. A. V. KUTSENOVA, V. M. ANISIMOV, O. N. K A R P U K H I N and V. Ya. SHLYAPINTOKH, Vysokomol. soyed. BI5: 222, 1973 (Not translated in Polymer Soi. U.S.S.R.) 9. A. CHARLESBY, G. S. (;RACE and F. B. PILKINGTON, Prec. Roy. Soc. 268: 1333, 1962 10. O. N. K A R P I f K H I N and E. M. SLOBODETSKAYA, Vysokomol. soyed. A16: 1624, 1974 (Translated in Polymer Sci. U.S.S.R. 16: 7, 1882, 1974) 11. V. M. PATEL, C. K. PATEL, K. C. PATEL and R. D. PATEL, Makromolek. Chem. 158: 65, 1972 12. E. V. BYSTRITSKAYA, T. S. KARPOVICH and O. N. KARPUKHIN, Dokl. A N SSSR 228: 632, 1976 13. V. M. ANISIMOV, O. N. K A R P U K H I N and A. M. MATTUCHI, Dold. A N SSSR 214: 828, 1974 14. O. N. K A R P U K H I N and A. V. KUTSENOVA, Izv. AN SSSR, seriya khimich., 1301, 1976
~CYCLIZATION OF AROMATIC POLY(AMIC ACIDS) TO POLYIMIDES. CYCLIZATION KINETICS OF A MODEL COMPOUND OF N-PHENYLPHTHALAMIC ACID * S . V. LAYROV, A. YA. ARDASHI~IKOV, I . YE. KARDASH a n d A . N . PRAVEDI~IKOV L. Ya. K a r p o v Physico-chemical Research I n s t i t u t e
(Received 8 September 1976) A kinetic s t u d y of the thermal cyclization of N-phenylphthalamic acid to :Nphenylphthalimide has been carried out in various solvents; the reaction is described b y a second order equation and is catalyzed b y acids. A t 40-60~o conversion the rate constant is reduced as a result of side-reactions t a k i n g place. I t was found t h a t as the basicity of the solvent increases the rate constant is reduced, and the relative contribution of side reactions increases. The experimental results were used to interpret d a t a on the cyclization kinetics of aromatic polyamic acids. * Vysokomol. soyed. A19: :No. 5, 1052-1057, 1977.
Cyclization of aromatic poly (amic acids) to polyimides
I218
ALT~OtTOH a r e l a t i v e l y l a r g e n u m b e r o f p a p e r s [ 1 - 7 ] h a v e a p p e a r e d i n c o n n e c t i o n with kinetic investigations of the thermal cyclization of aromatic polyamic acids to polyimides, no definite conclusions have as yet been reached in regard to regularities of formation of the imide ring. This may be because the cyclization r e a c t i o n t a k e s p l a c e i n t h e s o l i d p h a s e , in t h e p o l y m e r c h a i n s , a n d b o t h o f t h e s e factors may distort the true character of the process of imide ring formation. In view of this our aim in the present work was to investigate kinetic parameters of the eyclization of a low molecular model compound, namely N-phenylphthalamic acid COOH
CO,H-- ~ _ j Cyclization of the N-phenylphthalamic acid was carried out under a variety of conditions, varying the concentration, temperature, and solvent. N-phenylphthalamic acid was prepared b y reacting phthalic anhydride with aniline in chloroform. The precipitate was filtered, washed with chloroform and recrystallized from ethanol; m.p. 170 °. N-Phenylphthalimide was prepared b y heating N - p h e n y l p h t h a lamic acid at 170 ° for 3 hr a n d was recrystallized from ethanol; m.p. 210 °. N,N-Dimethylphthalamic acid was prepared b y passing gaseous dimethylamino t/hrough a solution o f phthalie anhydride in benzene. The precipitate was filtered, washed with benzene, sncl recry.~tatlized from ethanol; m.p. 124.5-125 °. Phthalic anhydride was recrystallized from benzene, a n d phthalic acid, from water. o-Chlorobenzoic acid was sublimed in vacuum at 100 °. Aniline was distilled at reduced pressure. DMF, DMAA and m-cresol were di'ied over Call2 and distilled i n v a c u o . Dimethyl: ether of diethylene glycol (diglyme) was boiled and distilled over K O H , and then boiled and distilled over sodium wire; the fraction with b.p. 160'5-161 ° was collected. Chloroform was washed repeatedly ~dth water, dried over CaCl~ and Call2 and distilled. Benzene w a s distilled over sodium wire. Ethanol and ethyl acetate m~derwent distillation. The melting (boiling) points of t h e reagents and solvents agreed with published information. The kinetics of N-phenylphthalimide formation were investigated b y I R spectroscopy through changes in the optical density of solutions at 721 cm -~ (1780 cm -~ when m-cresol was used as solvent). The reaction was run in glass ampoules in an argon current; the ampoules: were placed in a thermostat, a n d the temperature was maintained to within ± 0 . 2 °. T h e time for the commencement of reading was the moment when the temperature of the ampoule contents reached a level 5 ° below the required temperature. Sampling was carried o u t after specified intervals of time. The spectra were recorded on a UR-10 instrument. The N-phenylphthalimide concentration in solution was determined from a calibration plot of the optical density of N-phenylphthalimide solutions versus concentration (the dependence is a linear one, i.e. the Lambert~-Becr law is satisfied). The amount of unreaeted acid was determined b y potentiometric titration on a pH-340 instrument. Collected samples were diluted with D M F a n d t i t r a t e d with a 0.1 ~ solution of K O H in absolute ethanol; the working electrode and comparative electrode were r e s p e c t i vely of glass and silver chloride.
:1214
S.V. LAv~ov e~ at.
The amount of N-phenylphthalimide formed was also determined by a gravimetrie method [1] by fractionating the prodUcts according to their solubility. The composition of the reaction products was determined by thin-layer chromatography "with a witness". Plates with a "Silufol" layer attached were used; a benzeneethyl acetate mixture (2:5) was used as the eluent. Specimens intended for chromatography were prepared as follows. A solution of N-phenylphthalamic acid (phthalie anhydride or phthalic acid) in D!~IF was heated for 2 hr at 145°, DMF was distilled off under vacuum, and the dry residue was dissolved in ethyl acetate. 1 #g
3
2
I
I
I
I
lo0 20o 300 qgo" Time, rain FIG. 1. Degree of conversion of N-phenylphthalamic acid to ~T-phenylphthalimide
vs. time: I -- determined by potentiometrie titration of the acid; 2 -- determined gravimetricaUy; 3 -- by I1% spectroscopy. Solvent -- DMF, temperature 145°, initial concentration 0.14 mole/1. F i g u r e 1 shows t h e kinetic curves for t h e cyclization o f N - p h e n y l p h t h a l a m i c acid in solution in DM_F a t 145 °, c o n c e n t r a t i o n 0.14 mole/1, p l o t t e d b y m e a n s o f t h e various methods. M a x i m u m values for the cyclization r a t e were o b t a i n e d w h e n t h e N-phenylphthalimide c o n c e n t r a t i o n was d e t e r m i n e d b y IR-spectroscopy. T h e u n d e r e s t i m a t e d values o b t a i n e d g r a v i m e t r i c a l l y m a y be t h e result o f t h e i n e v i t a b l e losses occurring during f r a c t i o n a t i o n of the reaction products. T h e lowest r a t e of cyclization was o b t a i n e d b y t h e m e t h o d of t i t r a t i o n of t h e u n r e a c t e d l ~ - p h e n y l p h t h a l a m i c acid, i.e. it was f o r m a l l y f o u n d t h a t t h e r a t e o f d i s a p p e a r a n c e o f N - p h e n y l p h t h a l a m i c acid is lower t h a n the r a t e o f N - p h e n y l p h t h a l i m i d e f o r m a t i o n . Since it was t h o u g h t t h a t t h e u n d e r e s t i m a t e d results o b t a i n e d b y p o t e n t i o m e t r i c t i t r a t i o n were due to t h e f o r m a t i o n o f carboxyl-conr a i n i n g p r o d u c t s in solution, t h e m e t h o d o f I R s p e c t r o s c o p y was Used b y us t o investigate t h e kinetics o f t h e imidization reaction. I t can be seen t h a t a n inflexion appears on t h e kinetic curves in Fig. 1 a t 4 0 % conversion. This inflexion is p a r t i c u l a r l y e v i d e n t if t h e kinetic d a t a are p l o t t e d as curves o f 2.3 log [a/(a--x)] a n d x/a (a--x) versus time, where a is t h e initial conc e n t r a t i o n o f t h e N - p h e n y l p h t h a l a m i e acid, a n d x is t h e c u r r e n t c o n c e n t r a t i o n o f N - p h e n y l p h t h a l i m i d e (Fig. 2a). I t seems t h a t a t a definite degree of conversion t h e r e is a m a r k e d r e d u c t i o n in t h e r a t e constant, a p p a r e n t l y as a result of a change
Cyclization of aromatic poly (amic acids) to polyimides
1215
in the mechanism of the process. The plot of 2.3 log [a/(a--x)] versus time is a straight line only in the initial portion prior to the point of inflection. At the same time, if we plot the curve of x / a ( a - - x ) versus time, we obtain two intersecting straight lines with different slopes, i.e. the process is described b y two second order equations with different rate constants. Since no conclusion as to the order
z.alog[a/(a-=)]
x/a(a:=)
- _~I0
1"0
-Io~!u 0"5 /
b
-E
V
I I l
I00
200 300 Time, rain
3 40g
0"6 0"8 1.0 1.2 -10g c
FIG. 2. a -- Plots of 2-3 log [a/(a--x)] (1) and x/a(a--x) (2) vs. time, using the IR spectroscopic data in Fig. 1; b -- logarithm of the rate (log v) of cyelization (mole/1..rain) of N-phenylphthalamic acid vs. the logarithm of the initial DMF concentration log c. of the reaction can be reached through the data in Fig. 2a on the basis of initial portions of the curves, we measured the initial rates of the reaction with different initial concentrations of N-phenylphthalamic acid (Fig. 2b); the slope of the curve is 1.87. This means t h a t the cyclization reaction of N-phenylphthalamic acid is described by a second order equation both in the initial stage of the process and in subsequent stages. The rate constants for a second order reaction of cyclization of N-phenylphthalamie acid in DMF at 145 °, determined from the slope of the x/a (a--x) versus time curve are 0.0487 1./mole. min in the initial stage of the process and 0.0122 1./mole.min in subsequent stages, i.e. there is approximately a fourfold difference in the rate constants. The bimolecular character of the cyclization reaction m a y be due to a protonated form of N-phenylphthalamic acid, or of a dimer of the latter, undergoing cyclization. The formation of dimers in highly polar basic solvents such as DM:F is considered doubtful [8]. I t appears more probable t h a t an intermediate auotoprotonated product m a y be formed. This supposition is borne out by the fact t h a t the reaction is catalysed b y earboxylie acids. I t was found ~hat additions of o-chlorobenzoic acid within the concentration interval 0.01-0.15 mole/1, leads to
1216
S.V. LivRov et ed.
a marked increase in the reaction rate; in addition, the initial rate of the reaction is a linear function of the amount of acid added (see Fig. 3). To shed light on the factors underlying the reduced rate of the cyclization reaction at ~40~/o conversion, we investigated the composition of reaction products formed on heating N-phenylphthalamic acid for 90 min in solution in DMF. gO
4 0
70 Z'
I 50
LIj~ lole/L.m[n O'OO2
10
0-00I 0.05
07O
0.15
[csAJ ' mole~/.
FIG. 3
I
I
1oo
3oo
Time, min
FIG. 4
FIG. 3. initial rate of cyclization of l~-phenylphthalamic acid vs. the concentration of o-chlorobenzoic acid (CBA) added. DMF, initial concentration 0.14 mole/1. FzG. 4. Degree of conversion of N-phenylphthalamic acid to l~-phenylphthalimide vs. time in different solvents: 1 -- DMAA, 2 -- DMF, 3 -- diglyme, 4 -- m-cresol. Aniline, phthalic acid and N,N-dimethylphthalic acid were detected by thin layer chromatography. Aniline and phthalic acid are apparently formed through intramolecular dissociation of N-phenylphthalamic acid together v/ith s u b s e q u e n t hydrolysis of phthalic anhydride by water evolved during imidization. Possible factors giving rise to the formation of N,l~-dimethylphthalamic acid could be reactions of phthalie anhydride, t h a t is at equilibrium with N-phenylphthalamie acid, or of phthalic acid with DMF. Such a view is supported by other results [10, 11] relating to the interaction of anhydrides with DMF. I n addition, we found t h a t the heating of solutions of phthalic acid or its anhydride in DMF for 1-3 h r at 145 ° leads to t.he formation of N,N-dimethy]phthalamic acid.
Cyclization of aromatic poly (amic acids) to polyimides
Thus a complex
mixture of products
appears in the cyclization
1217
of N-phenyl-
phthslamic acid. At the same time with prolonged heating of the reaction mixture ( N 70 hr) the N-phenylphthalamide yield determined by IR-spectroscopy is close to lOOoh, i.e. the products formed during the reaction are nevertheless converted to N-phenylphthalamide. Certainly, on heating equimolar mixtures (~~0.15 mole/l.) of phthalic acid with aniline in DMF at 145” it was found that N-phenylphthrtlimide is formed. The rate constant for the latter reaction is 0.019 I./mole* min, which is only insignificantly higher than the rate constant for the slow stage of cyclization of N-phenylphthalamic acid (k= 0.012 I./mole - min). Similar results were also obtained on heating a mixture of N,N-dimethylphthalamic acid and aniline (c=O*15 mole/l.) in DMF at 145”. The rate constant for N-phenylphthalamide formation in the latter case is 0,032 I./mole*min, i.e. it is slightly lower than the rate constant in the initial stage of the cyclization of N-phenylphthalamic acid, and considerably higher than in the slow stage. This means that the change in the rate constant during the cyclization reaction of N-phenylphthalamic acid is mainly due to the formation of phthalic acid, the interaction of which with aniline takes place at an appreciably lower rate. In view of the experimental results, the processes occurring during the cyclizntion of N-phenylphthalsmic acid may be represented by the following scheme:
t
It should be noted that under the conditions adopted in this investigation thecyclization reaction of N-phenylphthalamic acid is an irreversible one. No change in the N-phenylphthalimide concentration appears on heating the latter in DMF solution (c==O*12 mole/l.) in the presence of 2% water at 148” for 7 hr. Phthalic acid formation enables one to account for differences in the results obtained by the methods of potentiometric titration and IR spectroscopy. The absence of agreement is due to the fact that the forming phthalic acid is titrated as a dibasic acid, which leads to overestimated results in determinations of the unreacted acid, which in turn leads to errors in determining the N-phenylphthalimide concentration. Starting with the temperature depedence of the cyclization rate constant in theinitial stages of N-phenylphthalamic acid formation in DMF solution determined
S . V . L A v ~ o v et al.
t218
a t 130, 135, 140 and 145 ° , we found that the activation energy for the process is 30.5 kcal/mole. Figure 4 shows the kinetic curves for the cyclization of N-phenylphthalamic acid in the different solvents at 145 °. I t can be seen that the cyclization rate decreases in the order m - c r e s o l ~ d i g l y m e ~ D M F ~ D M A A as the basicity of the solCYCLIZATION RATE CONSTANTS FOR N-PHENYLPHTHALAI~IIC ACID IN DIFFEREI~T SOLVENTS AT 145 ° (kl AT THE START OF THE REACTION, ]Ca IN THE SUBSEQUENT SLOW STAGE) Solvent DMAA DMF Diglyme m-Cresol
kl, l./mole, min
ka, L / m o l e - r a i n
0-0408 0.0487 0.1010 0-1606
0.0001 0.0122 -0.0010
v e n t increases. The shape of the curves is identical for all the solvents. The
x]a (a--x) versus time plots appear as two intersecting straight lines; in addition, t h e degree of conversion at which the discontinuity appears varies with the nature o f the solvent, and rises in the order D M A A < D M F < m - c r e s o l . The Table gives t h e rate constants for the reaction in the different solvents. I t appears t h a t the rate constant largely depends on the basicity of the solvent; the m a x i m u m rate is obtained in the acidic m-cresol. Moreover, there is at the same time a reduction in the relative contribution of the reaction of l~-phenylphthalamide formation via a stage of phthalic acid formation. The results of kinetic investigations of the cyclization of N-phenylphthalamic acid to N-phenylphthalimide may be used to interpret data on the cyclizationl~inetics of polyamic acids. In particular, the change in the rate constant in the cyclization of polyamic acids [1-3, 5, 7] is apparently similar in character to that occurring in the case of the model compound. However, the mobility of the polymer chains decreasing in the course of the reaction impedes interaction between terminal carboxyl groups and amino-groups, and results in a continuous reduction in the cyclization rate constant after a definite degree of conversion has been reached [7]. The influence of the nature of a solvent remaining in polyamic acids [7, 8] is similar to the influence which a solvent has upon the cyclization of N-phenylphthalamic acid. I n either case this is related to the fact t h a t autoprotonation of amic acid groups is facilitated when the basicity of the solvent is reduced. I n view of this we have, of course, the acid catalysis occurring in the cyclization of the model compound and of polyamic acids [6]. Translated by R. J. A. H E N D E Y
Polymerization of benzyl thiocyanate
1219
REFERENCES 1. J. A. KREUZ, A. L. ENDREY, F. P. GAY and C. E. SROOG, ft. Polymer Sci. 4, A - I , 2607, 1966 2. L. A. LAIUS, M. I. BESSONOV, E. V. KALT.TSTOVA, N. A. ADROVA and F. S. FLORINSKII, Vysokomol. soyed. A9: 2185, 1967 (Translated in Polymer Sci. U.S.S.R. 9: 10, 2475, 1967) 3. L. A. LAIUS, M. I. BESSONOV a n d F. S. FLORINSKII, Vysokomol. soyed. A13: 2006, 1971 4. V. D. IVIOISEYEV and L. S. BUBLIK, Abstracts of I V Symposium "Polymers-73", Varna, vol. 1, p. 133, 1973 5. A. N. PRAVEDNIKOV, I. E. KARDASH, N. P. GLIfKHOYEDOV and A. Ya. ARDASHNIKOV, Vysokomol soyed. A I 5 : 349, 1973 (Translated in Polymer Sci. U.S.S.R. 15: 2, 399, 1973) 6. A. G. CHERNOVA, L. S. BUBLIK, L. P. RODIONOV, A. V. IVANOVA and V. D. VOROBYEVA, Plast. massy, ~ o . 3, 11, 1975 7. I. E. KARDASH, A. Ya. ARDASHNIKOV, F. S. YAKUSHIN and A. N. PRAVEDNIKOV, Vysokomol. soyed. A17: 598, 1975 (Translated in Polymer Sci. U.S.S.R. 17: 3, 689, 1975) 8. A. Ya. ARDASHNIKOV, Dissertation, 1974 9. A. Ya. ARDASHNIKOV, I. E. K A R D A S H and A. N. PRAVEDNIKOV, Vysokomol. soyed. A13: 1863, 1971 10. C. COPPINGER, J. Amer. Chem. Soc. 76: 1372, 1954 I1. A. Ya. ARDASHNIJKOV, I. E. K A R D A S H and A. N. PRAVEDNIKOV, Vysokoraol. soyed. B16: 368, 1974 (Not translated in P o l y m e r Sci. U.S.S.R.)
THE POLYMERIZATION OF BENZYL THIOCYANATE* B. A. ZHUBA~COV,A. I. KOLESI~IKOVA and T. YA. SMIRI~OVA Chemical Science Institute, Kaz. S.S.R. A c a d e m y of Sciences
(Rceeived 12 September 1976) A p o l y m e r containing - - C = N - - conjugated bonds has been synthesized from benzyl thiocyanate. The kinetics a n d mechanism of the polymerization reaction have been investigated. I n the light of the results of I R , NMR and chemical analyses the structure of the polybenzylthiocyanate has been elucidated. Some properties of the p o l y m e r have been investigated.
O~ an earlier occasion [1] we showed that the formation of polymers from organic thioeyanates is feasible. In a comparative analysis of the reactivity of aromatic, f a t t y aromatic and aliphatie thioeyanates [2] it was found ~hat the presence of a labile H atom in the * Vysokomol. soyed. A19: No. 5, 1058-1060, 1977.