DESTRUCTION
()F
POLYAMIDES
IN AGGRESSIVE
MEDIA
COMMUNICATION 1. HYDROLYSIS O} ~ CYCLIC O L I G O M E I ~ S O F POLYCA PROAMIDE IN A 9 P. G.
P. E.
Nechaev, Zaikov,
SOLUTIONS O F S U L F U l t I C ACID Yu. V. Moiseev, and T. E. Petrova
UDC 6"/8.019.3:678.675:542. 938
Polycaproamide in the equilibrium state contains a definite amount of low-molecular substances, which, depending on the method of polymerization, comes to 5-12~: of the weight of the polymer [I]. Duringthe reprocessing of polycaproamide, the low-molecular substances are removed from the polymer; however, in the course of time their equilibrium concentration is restored. Amongthe low-molecular substances, 80~ is accounted for by the monomer, dimer, and trimer. To establish the mechanism of the destruction of polycaproamide it was necessary to know the mechanism of the hydrolysis of e-caprolactam, the cyclic dimer and trimer. This work was devoted to an investigation of the mechanism of the hydrolysis of cyclic oligomers of polycaproamide in aqueous solutions of sulfuric acid. EXPERIMENTAL
METHOD
C y c l i c d i m e r s (mp 340 ~ and the c y c l i c t r i m e r (rap 242 ~ of p o l y c a p r o a m i d e w e r e k i n d l y p r o v i d e d by V. N. M i k h e e v . S u l f u r i c a c i d w a s p u r i f i e d by b o i l i n g with p o t a s s i u m b i c h r o m a t e , f o l l o w e d by r e d i s t i l l a t i o n . The r e a c t i o n of h y d r o l y s i s w a s c o n d u c t e d in g l a s s a m p o u l e s ( P y r e x ) in the t e m p e r a t u r e i n t e r v a l 100160 ~ A f t e r d e f i n i t e p e r i o d s of t i m e , the a m p o u l e s w e r e r e m o v e d f r o m the c o n s t a n t - t e m p e r a t u r e b a t h , c o o l e d r a p i d l y to r o o m t e m p e r a t u r e , o p e n e d , and the r e a c t i o n m i x t u r e t r a n s f e r r e d to the c u v e t t e of an S F D - 1 s p e c t r o p h o t o m e t e r , w h e r e the o p t i c a l d e n s i t y w a s m e a s u r e d at ~ = 2 1 0 - 2 1 4 n m and 25 ~ The p r o c e s s of h y d r o l y s i s i s i r r e v e r s i b l e , s i n c e a t the end of e a c h e x p e r i m e n t the o p t i c a l d e n s i t y c o i n c i d e d with the o p t i c a l d e n s i t y of the r e a c t i o n p r o d u c t e - a m i n o c a p r o i c a c i d at the c o r r e s p o n d i n g m o l a r c o n c e n t r a t i o n . The k elf $
~g 8
0,r
_C~o+~3~+
/2 O,q
0,2
g,2 J
J
fJ 25 h Fig. 1
I
I
I
_A-_
t
5
b"
7
~g(hA~ )
35
Fig. 2
Fig. 1. Kinetic curve (i) and its semilogarithmic plot (2) of the hydrolysis of the cyclic dimer of polycaproamide in 61 H2SQ at 100~ Fig. 2.
Graphical determination of ~ in Eq. (6).
I n s t i t u t e of C h e m i c a l P h y s i c s , A c a d e m y of S c i e n c e s of the USSR. T r a n s l a t e d f r o m I z v e s t i y a A k a d e m i l N a u k SSSR, S e r i y a K h i m i c h e s k a y a , No. 1, pp. 3 3 - 3 8 , J a n u a r y , 1972. O r i g i n a l a r t i c l e s u b m i t t e d May 8, 1970.
9 1972 Consultants Bureau, a division of Plenum Publishing Corporation, 227 West 17th 3treet, New York, N. Y. 10011. All rights reserved. This article cannot be reproduced for any purpose whatsoever without permission of the publisher. A copy of this article is available from the publisher for $15.00.
29
TA BLE
1 keff, rain "I
II2S0~, %
4,96 9,89 20,2 29,7 40A 50,8 60,6
i00 o
4,9.t0 -.~
t,8.10 -3 3,8.t0-s 3,6.t0 -3 2,9.t0 -a t,3.10 -a 2,2.t0-4
t,3.t0-~ 8,9.t0-a
TA B L E 2
5,22 i0,5 20,95 30,54 40,8 52,6 6t
100~
3,0.10-3 5,8.10-~ 7,6.10-a 5,5.10-z 3,5.t0 -a 1,7.t0 -3 0,8.t0-~
1t0 ~
6,2.t0-8 t,3.10 .2 t,4.10-~ t,3.t0-2 7,2.10-~ 2,7.10-a t,5.10 -3
120.~
t , 4 . t 0 -~
2,6.t0 -2 3,5.t0-'~ 2,4.t0 -~ t,5.t0 -e 6,3.10 -3 3,6.t0 -3
1~5~
6,7.t0-s t,2.t0 -~
3,3-10-~
1 , 3 . t 0 -2
8,7.t0 -a 5,2.10-s t , 2 . t 0 -a 5,9.10-4
100~
t , 8 . 1 0 -~
~,5.t0-~ 2,5.10 -~ 1,4.t0 -~ 4,7.10 -a 1 , 2 " t 0 -a
5,2.10 -e t , t . t 0 -2 4,4.t0 -s
c o n c e n t r a t i o n of t h e c y c l i c d i m e r a n d t r i m e r w a s (1-3) 9 10 -3 M, w h i c h a l m o s t e n t i r e l y e x c l u d e d t h e i n f l u e n c e of t h e r e a g e n t s on t h e m e d i u m .
~eff, ain "1 H~SO4, %
t39~
130~
The h y d r o l y s i s of the c y c l i c d i m e r and t r i m e r of p o l y c a p r o a m i d e is a c o m p l e x p r o c e s s , c o n s i s t i n g of t h e r e a c t i o n of o p e n i n g of t h e r i n g , f o l l o w e d by
2,7.t0 -2 5,5.t0 -~ 5,5.t0-2 5,3.t0 -2 3,2.10-~ t , t . t 0 -2 5,6.10-z
b r e a k d o w n of the l i n e a r o l i g o m e r s . A c c o r d i n g to the d a t a of [2], i n t h e c o u r s e o f t h e h y d r o l y s i s o f t h e cyclic dimer, practically, only the cyclic dimer and r ~ - a m i n o c a p r o i e a c i d a r e p r e s e n t in the r e a c t i o n m i x -
ture. On this basis it can be concluded that the reaction of ring opening is far slower than the subsequent reaction of hydrolysis of the linear dimer of polycaproamide, and, consequently, we measure only the rate constant of the first reaction. The effective rate constant was calculated according to the equation for a f i r s t - o r d e r equation
2.31g(.DoDt--D~)=--keff, Do
(i)
where Dt, D~, and DO are the current, final, and initial optical densities, respectively. Table 1 presents the effective rate constants at various temperatures and concentrations of H2SO4 for the cyclic dimer of polycaproamide. In the hydrolysis of the cyclic trimer, intermediate products - the linear dimer and t r i m e r of polycaproamide - are formed in the reaction mixture in addition to the starting material and reaction product. Two assumptions were made to calculate the rate constant of hydrolysis of the cyclic trimer: a) the absorption coefficients of the linear dimer and trimer of polyeaproamide were determined according to the equation =n
e~tr
_4_~aca,
w h e r e n i s t h e n u m b e r of a m i d e b o n d s i n t h e l i n e a r o l i g o m e r ;
of the cyclic t r i m e r and ~-aminocaproic acid,
(2) ~c.tr and eac a are the absorption coefficients
respectively.
b) the rate constants of hydrolysis of the linear dimer and t r i m e r are the same. * Calculations performed on an electronic computer indicated that the kinetic curve of the hydrolysis of the cyclic t r i m e r of polycaproamide is not satisfactorilydescribed in the case when the rate constants of hydrolysis of the cyclic trimer and of the linear t r i m e r and dimer were equal. Consequently, in the three compounds indicated above, the amide bonds have the same reactivity, and the hydrolysis of the cyclic t r i m e r of polycaproamide should be described by one effective rate constant, determined from Eq. (1). Figure 1 presents the kinetic curve and its semilogarithmie plot for the hydrolysis of the cyclic t r i m e r of polycaproamide in 61~c H2SO4 at 100~ Table 2 presents the effective rate constants at various temperatures and concentrations of H2SO4 for the cyclic t r i m e r of polycaproamide. The effective rate constants of the hydrolysis of the cyclic dimer and t r i m e r are satisfactorily described by an Arrhenius equation. The effective activation energies, calculated according to the method of least squares, are given in Tables 3 and 4. 9In [3] the equality of the rate constants of hydrolysis of the linear dimer and t r i m e r of polycaproamide in 25% HCI at 110~ was demonstrated experimentally.
30
DISCUSSION
9
4-,-}.~-~!4' .J.,+!
Z
52
l;
OF
F o r the acid h y d r o l y s i s of a m i d e s , t h e r e is c u r r e n t l y no m e c h a n i s m that s a t i s f a c t o r i l y d e s c r i b e s the e x p e r i m e n t a l data in a b r o a d r a n g e of acid c o n c e n t r a t i o n s . T h e r e a r e two c e n t e r s of p r o t o n a t i o n in the amide bond: the amino n i t r o g e n and the c a r b o n y l oxygen. Data on H - D exchange in the amide g r o u p in acid m e d i a a r e usually cited in s u p p o r t of N - p r o t o n a t i o n [6]; s o m e data on UV [7], NMR [8], and IR s p e c t r o s c o p y [9[ a r e c i t e d in s u p p o r t of O - p r o t o n a t i o n . At the p r e s e n t t i m e , on the basis of the data obtained by the m e t h o d s of NMR [10, 11] and IR s p e c t r o s c o p y [12], it can be c o n s i d e r e d e s t a b l i s h e d that only the G - p r o t o n a t e d f o r m is b a s i c a l l y f o r m e d ; m o r e o v e r , it was shown in [13] that the p r o t o n a t i o n of a m i d e s is s a t i s f a c t o r i l y d e scribed by the acidity function H~ CB
KB
~lr2
llii
-~o0
= ~
As is well known, for amides kef f passes through a maximum relative to the acid concentration (see Table I). On the basis of the aforementioned, it can be assumed that in the limiting step of the hydrolysis of amides there is an attack of the hydroxonium ion on the nitrogen atom. Then protonation of the amide will prevent the formation of an activated complex.
r e = koff Co =
'
of the amide as 7 =
bond can be expressed
d d
f~fl{~o+
(4)
w h e r e ktrue is the t r u e r a t e c o n s t a n t ; a ~ and f~ r e p r e s e n t the a c t i v i t y and the a c t i v i t y c o e f f i c i e n t of the a c t i v a t e d c o m p l e x , r e s p e c t i v e l y ; CH30+ is the h y d r o x o n i u m ion c o n c e n t r a t i o n ; C o is the c o n c e n t r a t i o n of a m i d e bonds. Substituting the e x p r e s s i o n f o r C B found in Eq. (3) into (4), c o n s i d e r i n g the balance equation, we obtain k trueCii.o+ keff = i -~hA/K B
0 0 0 0 ~ 0 ~
(3)
hA
where K B is the thermodynamic basicity constant; CB and CBH + are the concentrations of nonionized and ionized forms of the amide bond, respectively; and H A = - logh A.
The rate of the hydrolysis by the equation
1111111
RESULTS
fB/H30+
(5)
f~
In [141 an o r i g i n a i m e t h o d was developed f o r d e t e r m i n i n g the change in pc. Using this m e t h o d and e x p r e s s i n g the a c t i v i t y c o e f f i c i e n t of the nonionized f o r m fB in t e r m s of the solubility S, we obtain the g e n e r a l equation Keff =
I I ! [ l [ l
(1 -~- hA/KB) (~ F i g u r e 2 p r e s e n t s a s t r a i g h t line in a plot of l o g k e f f S / ( C H ~ O + fH30 +)
L~O
< b-
v e r s u s log (hA f H o o + / S ) with a slope of 1.1 :~ 0.1, c o n s t r u c t e d a c o c o r d i n g to the data f o r the h y d r o l y s i s of the d i m e r of p o l y c a p r o a m i d e (see Table 3). C o n s i d e r i n g the p a r a m e t e r oz is c l o s e to one, we find that the r a t i o f B f i t a o + / f r- is p r a c t i c a l l y unchanged with the acid conc e n t r a t i o n , and c o n s e q u e n t l y , Eq. (6) can be r e d u c e d to the f o r m ktrueC~o+
keff = { T ~
"
(7)
Equation (7) d e s c r i b e s the e x p e r i m e n t a l data on the h y d r o l y s i s of the d i m e r and t r i m e r of p o i y c a p r o a m i d e in the e n t i r e i n v e s t i g a t e d i n t e r v a l *The acidity function was d e m o n s t r a t e d in [4] using a s e r i e s of a r o m a r i e a m i d e s as the i n d i c a t o r s . 31
TA BLE 4 CITsO+ .10-~ lg
H2SO4, %
hA
5,22 t0,5 20,95 30,54 40,8
t,095 2,78 10,2 3t,6 t05
tggeff 25* lgCH304 ~eff '5---'--T --5,8 --5,5 --5,4 --5,6 --5,8
3,9 4,1 6,7 15,3 32,7
--0,25 0,07 0,385 0,575 0,745
CH*O+
E
..,
kcal/
keff 13~176 ~ff 13o" [e~{ e --t,56
2t,7 [ 20+t t8,6 20:1=t 44,4 20• 7t 20• t73 20.+_1
--t,26
--t,26
--t,28 --t,49
TA BLE 5 ASeentropy
Etrue ' kcal/ mole
Compound
ktrue, min "l
Cyclic dimer of polycaproamide [
2't0-7
t
--0,9
--t2•
23_i
Cyclic trimer of polycaproamide t
5"t0-8
I
--0,7
--t6•
20+t
PK B
units
of H2SO 4 c o n c e n t r a t i o n s . C o n s i d e r i n g the fact that e x t r a p o l a t i o n of keff to 25 ~ f r o m the h i g h - t e m p e r a t u r e r e g i o n leads to l a r g e e r r o r s , Figs. 3 and 4 p r e s e n t the data both f o r 25 ~ and f o r 130 ~ The values of ktrue and pK B a r e given in Table 5. The data obtained p e r m i t a c a l c u l a t i o n of the t r u e activation e n e r g y and of the activation. Table 5 p r e s e n t s E t r u e and AS ~ f o r the c y c l i c d i m e r and t r i m e r of p o l y c a p r o a m i d e . F r o m Table 5 it is evident that the c y c l i c d i m e r d i f f e r s s u b s t a n t i a l l y in its r e a c t i v i t y f r o m the c y c l i c t r i m e r . In [15] it was h y p o t h e s i z e d that the a n o m a l o u s r e a c t i v i t y of the c y c l i c d i m e r is due to the p r e s e n c e of i n t r a m o l e c u l a r h y d r o g e n bonds between the two a m i d e groups. To t e s t this h y p o t h e s i s we obtained the IR s p e c t r a of the c y c l i c d i m e r and t r i m e r of p o l y c a p r o a m i d e , as well as e - c a p r o l a c t a m in d i c h l o r o a c e t i c acid* at 25 ~ in q u a r t z c u v e t t e s {length 10 ram) on an IKS-14 s p e c t r o p h o t o m e t e r . The s p e c t r a w e r e obtained in the r e g i o n of 6000-7000 c m - i ; the a m i d e c o n c e n t r a t i o n was 0.2 g - e q / l i t e r . In an i n v e s t i g a t i o n of the IR s p e c t r a of a n u m b e r of p o l y a m i d e s in acid m e d i a , it was shown [16] that the a b s o r p t i o n band of the f i r s t o v e r t o n e of the N H - v a l e n c e v i b r a t i o n at 6325 and 6500 c m - i c o r r e s p o n d s to the g r o u p bonded by h y d r o g e n bonds, while the a b s o r p t i o n band at 6625 c m -1 c o r r e s p o n d s to the p r o t o n a t e d a m i d e group. T h e r e a r e no significant d i f f e r e n c e s in the IR s p e c t r a of the c y c l i c d i m e r and t r i m e r . This p e r m i t s us to a s s u m e that the c o n c e p t of H e r m a n s of the p r e s e n c e of i n t r a m o l e c u l a r bonds in the c y c l i c d i m e r , in all probability, is i n c o r r e c t [17]. In o u r opinion, the a n o m a l o u s r e a c t i v i t y of the c y c l i c d i m e r m a y be due * D i c h l o r o a e e t i e acid, while a c o m p a r a t i v e l y s t r o n g acid, in c o n t r a s t to H2SO4 is optically m o r e t r a n s p a r e n t and d i s s o l v e s c y c l i c o l i g o m e r s of p o l y c a p r o a m i d e b e t t e r .
C"31___*. lO-.-sJ
~e~3o*.10-z~-lj
2/
.'keff
2
Fig.
r h4./08-1 3
15 i eff
2
# 6 B Fig. 4
/0/
Fig. 3. G r a p h i c a l d e t e r m i n a t i o n of ktrue and KB f r o m Eq. (7) f o r the c y c l i c d i m e r of p o l y c a p r o a m i d e a c c o r d ing to the data of Table 3 at the t e m p e r a t u r e : 1) 25~; 2) 130 ~. Fig. 4. G r a p h i c a l d e t e r m i n a t i o n of ktrue and K B f r o m Eq. (7) f o r the c y c l i c t r i m e r of p o l y c a p r o a m i d e a c c o r d ing to the data of Table 4 at the t e m p e r a t u r e : 1) 25~ 2) 130 ~.
32
to e l e c t r o s t a t i c interaction of the closeiy situated amide groups through space. The creation of a supplem e n t a r y positive charge close to the nitrogen atom prevents the approach of a proton, thereby reducing the reactivity of the cyclic d i m e r of polycaproamide. We should note that the high conformational stability of the cyclic dimer of polyeaproamide was indicated in [18]. CONCLUSIONS 1. The kinetics of the h y d r o l y s i s of cyclic o l i g o m e r s of polyeaproamide was investigated in a broad range of t e m p e r a t u r e s and tI2SO 4 concentrations by a s p e c t r o p h o t o m e t r i c method. 2. The effective rate constant of h y d r o l y s i s passes through a m a x i m u m with i n c r e a s i n g acid concentration. 3. A m e c h a n i s m of h y d r o l y s i s was proposed. 4. The anomalous reactivity of the cyclic d i m e r of polycaproamfde is explained by e l e c t r o s t a t i c i n t e r action of closely situated amide groups through space. LITERATURE 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
15. 16. 17. 18.
CITED
N . D . Katorzhnov, Khimicheskie Volokna, 1, 3 (1966). D. tteikens, P. H. t I e r m a n s , and H. A. Ve[dhoven, Makromolek. Chem., 30, 154 (1959). D. Heikens, J. P o l y m e r Sei., 22, 65 (1956). K. Yates, J. B. Stevens, and A. K. Katritzky, Canad. J. Chem., 42, 1957 (1964). M . I . Vinnik, Usp. Khim., 35, 1922 (1966). J . M . Klotz and B. H. Frank, J. A m e r . Chem. Soc., 87, 272 (1965); S. O. Nielsen, Biochim. et Biophys. Acta, 37, 146 (1960). J. T. Edward, H. S. Chang, K. Yates, and R. Stewart, Canad. J. C h e m . , 38, 1518 (1960). M . L . Bender, Chem. Rev., 60, 66 (1960); D. G. Kowalewski and V. J. Kowalewski, Arkiv. Kemi, 16, 373 (1961); A. Saika, J. A m e r . Chem. Soc., 82, 3540 (1960). C . H . Smith and J. D. Robinson, J. A m e r . Chem. Soc., 79, 1349 (1957). C . A . Bunion, B. N. Figgis, and B. Nayan, Advances Molec. Spectroscopy, 3, 1209 (1962). J . T . Edward, I. B. Leane, and I. C. Wang, Canad. J. Chem., 4_O0,1521 (196-2). Yu. V. Moiseev, G. I. Batyukov, and M. I. Vinnik, Izv. Akad. Nauk SSSR, Ser. Fiz., 26, 1306 (1962). K. Yates, J. B. Stevens, Canad. J. Chem., 43, 529 (1965); L. M. S t e r t i n g a n d K . Yates, Canad. J. Chem., 44, 2395 (1966). N. B. Librovich and M. I. Vinnik, Zh. Fiz. Khim., 42, 2530 (1968); M. I. Vinnik and N. B. Librorich, ibid, 41, 2013 (1967); M. I. Vinnik, I. M. Medvetskaya, L. R. Andreeva, and A. E. Tiger, ibid, 41, 252 (1967); M. I. Vinnik and I. M. Medvetskaya, ibid, 41, 1775 (1967). Modern P r o b l e m s of Physical Organic C h e m i s t r y [Russian translation], Mir, (1967), p. 291. S. Hanlon, Biochemistry, 5, 2049 (1966). P. It. I I e r m a n s , Nature, 177, 127 (1956). J. Dale, J. Chem. Soc., 107 (1963).
33