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Kinetic study of the reactions of cyclohexane monoisocyanate and cis-, trans-1,3 and 1,4-cyclohexane diisocyanates with alcohols
KINETIC STUDY OF THE REACTIONS OF CYCLOHEXANE MONOISOCYANATE AND cis-, trans-l,3 AND 1,4-CYCLOHEXANE DIISOCYANATES WITH ALCOHOLS* A. G. KOZHEVOV, YE. V. GENKINA and YA. A. SB-M-IDT State Institute of Nitrogen Industry and Products of Organic Synthesis
(Received 5 August 1970)
MUCH research has been carried o u t in t h e field of a r o m a t i c a n d aliphatic diisoc y a n a t e chemistry, b u t practically no i n f o r m a t i o n has been published in r e g a r d to c y c l o h e x a n e mono- a n d diisocyanates. I n view of the electron-donor properties of t h e h y d r o a r o m a t i c ring the react i v i t y o f cyclohexane isocyanates m i g h t be e x p e c t e d to be lower t h a n t h a t o f t h e a r o m a t i c analogues. I t was therefore our aim to d e t e r m i n e the r e a c t i v i t y o f cycloh e x a n e isocyanate (CHIC) and cyclohexane diisocyanates (CHDIC) a n d to investigate t h e r e a c t i v i t y in relation to the conformational s t r u c t u r e o f the l a t t e r in reactions with mono- a n d bifunctional alcohols (methanol, n-butanol, 1,4-butanediol). A f u r t h e r aim was to reach certain conclusions regarding the mechanism o f u r e t h a n e a n d p o l y u r e t h a n e f o r m a t i o n using the results of the kinetic measurements. Since a h y d r o a r o m a t i c ring i n t r o d u c e d into the linear p o l y m e r chain could i m p r o v e the t h e r m a l stability of p o l y u r e t h a n e s a s t u d y was first m a d e of some of the physical properties o f the p r o d u c t polymers, and t h e need for f u r t h e r investigations to discover practical applications o f these polymers was examined. EXPERIMENTAL The original isocya~tes containing 99.8-99.9% basic substance were prepared by the same method as in [1]. The alcohols were dehydrated and double-distilled. Benzene which had first been distilled over metallic sodium was used as the solvent. The use of the latter insured the dissolution of the forming urethanes and made it possible for the reaction to be carried out in a homogeneous medium. The absence of any specific interaction of the reactants with benzene [2, 3] tended to reduce the possibility of distortion of the results of the kinetic measurements. Many authors have reported on the reaction of aromatic and aliphatic isocyanates with alcohols in benzene medium, so we had the opportunity of comparing our findings with those of other authors. * Vysokomol. soyed. A14: No. 3, 662-668, 1972. 740
Kinetic study of reactions of cyclohexane monoisoeyanate
741
The kinetics of the reaction of cyclohexane isocyanates with alcohols were investigated by the m e t h o d of taking samples with the initial concentrations of the reactants as shown in Table 1. The reaction was carried out at 40, 50 and 60 ° owing to the need to obtain high cor~versions in acceptable time intervals, and to reduce the secondary reactions (dimerization, trimerization of isocyanates, allophanate formation). The temperature of the reaction mixture was maintained w i t h an accuracy of ±0.1% The course of the reaction was verified through the amount of unconverted isocyanate as determined by infrared spectroscopy.
TABLE
1. R A T E
CONSTANTS,
ACTIVATION
ENERGIES
ORDER REACTION OF ISOCYANATES
AND ENTROPIES
FOR THE
SECOND
W I T H A L C O H O L S I N BEl~ZE17E
k × 106, 1./mole..see
Reactants
O
,..J 0
0
40 ~
50 °
60 °
o
.g CHIC + methanol
0.464 0-232 0.464 0.232 0-232
4"64 4-64 4.64 4.64 4.64
3.00 5.56 10.6 2.801 5.50 10.1 2.40 5.00 7.90 6.00 10.7 20-2 2-40 5.40 9.80
1 0-9 0.8 2 0"9
13.1 12.8 12.4 12.6 14.6
32.9 32-4 35.8 33.1 28.7
5.0 2.4 1
11.0 9.8 14.1
36.6 44"5 29"9
57-1 58.3 27.8
6.6 5.5 2.8
9-5 9.0 11.0
40.6 43.4 37.5
20-8
2.2
10.4
39.8
33.1 63.1 85.3 27.8* 48.1! 95.7
10 9.0
9.8 12.9
38.7 29.4
4.5 5-2
7.4 8.8
47.7 43.3
21.2
14.6
22-3
0.232 0.232 0.232
0.232 4.64 4.64
10
CHIC -4-n -butanol Cis- 1,4-CHDIC + methanol + n-butanol + 1,4-butane diol Trans- 1,3-CHDIC + methanol -4-n-butanol + 1,4-butanediol Trans- 1,4-CHDIC + methanol
0-232 0.232
I1
+n-butanol
0.232
0.232 23.0 36"5 4.64 16-5 30.6 9-7* 14.6 4.64 7-7 12-1
12
+ 1,4- butanediol
0.232
0.23~
0.232
4.64
0.232
4.64
17.6 19.6
i 0-232
0.232
59.5 125
7 8 9
13
G/s- 1,3-CDHIC + m e t h a n o l
14 15
-1-n-butanol + 1,4-butanediol * Numerator~rate
15.6 27.8 44.8 9.10:13.1 20.3 3.00 5.90 11-6
23.8 35.7 26"4 I 45.5
!
i230
c o n s t a n t o f first isocyanate g r o u p kz; denominator--kffi.
The polyurethanes prepared from the individual c/s- and trana-isomers of 1,3- and 1,4cyclohexanediisocyanates and 1,4-butanediol were purified by recrystallization from boiling acetone. The molecular weight of the polymers was determined b y measuring the viscosity of 5% polymer solutions in m-cresol at 25 ° in a viscometer with a capillary diameter of 1.2 m m P i n k e v i c h system [3].
742
A. G. KOZHEVOV e~ a/.
DISCUSSION OF RESULTS
Mechanism of the reaction of cyclohexane i8ocyanates with alcohols. No definite conclusions as to the mechanism of this process could be based on the currently held views concerning the interaction of isocyanates with alcohols. The reaction rate is described by second or third order kinetic equations according to the nature of the reactants [2-10]. Y
3
I
1
3
5
7
I
l
I
I
3
I
5
L
I
7
Time , hp FIa. 1. Right-trend portion y of the integral expression of the kinetic equation of the second order reaction of cyclohexane isoeyanates with alcohols plotted against time at 50°. a--for 10-20~/o conversions; b--for 60-70~/o conversions: y =
x
when a~-b;
1 lab(a--x)
when a Cb. The numbering of the curves corresponds to the numbers of the experiments m Table 1. Treatment of our results on the basis of a second order kinetic equation showed t h a t the right-hand portion of the integral expression of this equation is a linear function of time only for the early stages of the process, corresponding to 10-20% conversion of the isocyanate (Fig. la). As the degree of conversion rises, the reaction rate ceases to obey a second order equation, which points to a change in the reaction mechanism. A change of this type which was observed by authors investigating the interaction of aliphatic isocyanates with alcohols was attributed to the development of autocatalysis [6-10]. The following two equations describing these reactions were proposed in papers [6-10]: dx -~=]¢(a--x) (b--x)+l~x(a--x) (b--x), (1)
dx -~--kl (a--x)(b--x)~+k~(a--x)(b--x),
(2)
Kinetie study of reactions of cyclohexane monoisocyanate
743
where a, b are the initial isocyanate and alcohol concentrations, respectively; x is the urethane concentration; k, kl are second and third order rate constants for the initial stage of the reaction; k2 is the rate of the autocatalytic reaction. These equations were used in the treatment of our experimental data. The fact t h a t the initial stage of the process corresponds to the first term in equation (1) was accounted for by the first order of the reactions in respect to each reacting component (see Table 1). Further confirmation was provided by the low activation energies and the high negative activation entropies which allow us to assume t h a t the mechanism of urethane formation could be through a four-membered activated isocyanate-aleohol complex [5]. I f the initial stage o f the process takes place through the bimolecular interaction of an isoeyanate-aleohol complex with a molecule of alcohol [2, 3] or through the isocyanate interacting with a dimerie alcohol complex [6], this stage will then be described by the first term in kinetic equation (2). On integrating this term we obtained the expressions kit
- 1- lna--x (a--b) ~ b--x
1 [- const when a#b (a--x) (b--x)
(3)
and 1
kit= 2~(a--x) ~-const when a=b
(4)
In every case the right-hand portion of equations (3) and (4) is a linear time function. The kl values determined from these curves agree with those calculated by the Sato method [7] within the limits of accuracy of the latter. The activation energies and entropies based on k and kl (see Tables 1 and 2) are similar, and one m a y therefore assume that the first stage of the process m a y involve second and third order competing reactions. The kinetic dependence of the final stage of the reaction of the isocyanates with the alcohols is described by the second term in equations (1) and (2), as is confirmed by the linear dependence of its integral expression on time. The third order of the reaction, as well as the higher rate constants compared with the initial stage, and the lower activation energies and entropies, point to the development of autocatalysis (see Table 2). It should be noted that the activation energies for the final stage of the reaction of 1,4-cis-, 1,3- and 1,4-trans-CHDIC with methanol, and of cis-I,3-CHDIC with n-butanol are higher than the corresponding activation energies for the initial stage. This was also noted in papers [9, 10] dealing with certain aromatic and aliphatic isocyanates. I n the final stage, however, these reactions take place preferentially by an autocatalytic mechanism, probably owing to the less rigid demands being made on the structure of the transition complex, as is shown by the higher activation entropies.
744
A.G. KOZHEVOVe$ ed.
1,4-trans- a n d 1,3-cis-CHDIC w i t h 1,3-trans-CHDIC with n - b u t a n o l , are second order
The reactions of
methanol, a n d of 1,4-cis-, r i g h t t h r o u g h to 6 0 - 7 0 %
and conversion o f t h e diisocyanates (Fig. lb). The i n t e r a c t i o n o f 1,3-cis-CHDIC w i t h m e t h a n o l , a n d of 1,4-trans-CHDIC w i t h n - b u t a n o l is a c c o m p a n i e d b y r e d u c t i o n in t h e reaction rates after degrees of conversion of a r o u n d 50% h a v e been obtained. TABI,E 2.
RATE
COI~STANTS~ A N D ACTIVATIOI~ E N E R G I E S A N D
ENTROPIES
FOR
THE
THIRD
O R D E R R E A C T I O N S OF I S O C Y A N A T E S W I T H ALCOHOLS
k~ × 10e,l.2/molo2sec
kl × 10612/rnole2 sec
40 o
50°
4:
60 °
40 °
50 °
60°
,~'~
2~ I d
1
2 3 4 5 6 7 8 9 10 11 12 13 14 15
0-72 0.70 0.53
1.26 1.20 1.06 1.29 2.31 0.52 1.14 67.3 ] 120 1.96[ 2.82 0.651 12.7 99.1 157.2 3.56 6.7 2.1 3.14 143 1272 6.58 I 9.92 4.23 5.68 256 539
2"28 2"20 1"71 4"35 1 2"1 193 4"4 2"5 246"1 12'~ 6"~ 368 18'2 9"8 991
12"0 15'8 12'4 12"6 14"6 10"8 8"4 14'4 9"5 9"6 10"9 9"8 10"6 8"7 13"5
39-4 39"0 38"8 35'8 31"7 34"2 49"0 30"4 39"5 44"4 40'7 35"8 39"5 46"3 22"9
46"3 45"8 38"1 80"5
87.2 86-7 63.5 '222
9.4 9.7 9.6 21-9
40"1 39"7 40"0 0"6
1250 77 "9
543 162
4-54 15.8
48.8 19.4
1543 139
339 259
9.0 14.6
34.5 22.2
1130
1234
34O
3.13
52.3
126-2 1230
222 2085
557.2 3267
15.4 10.2
18.2 30.4
35"4 35'0 25"3
63"5
The c h a n g e in the rate c o n s t a n t m a y be a t t r i b u t e d to the difference in the r e a c t i v i t y o f t h e first i s o c y a n a t e g r o u p g e n e r a l l y characterized b y the c o n s t a n t kl, a n d t h e second g r o u p c h a r a c t e r i z e d b y k. Table 1 gives t h e r a t e c o n s t a n t s k 1 a n d k2 of the second order reaction (exp e r i m e n t s 11 a n d 13). As is seen f r o m Table 1, the k 1 values are higher t h a n those of k2, which m e a n s t h a t t h e a c t i v a t i n g effect o f t h e second i s o c y a n a t e g r o u p is g r e a t e r t h a n t h a t of t h e u r e t h a n e group. I n view o f t h e difficulties t h a t arise in q u a n t i t a t i v e d e t e r m i n a t i o n of t h e rate of t h e final stage of the r e a c t i o n t h e reactivities of t h e investigated c o m p o u n d s were c o m p a r e d on t h e basis of t h e results of kinetic m e a s u r e m e n t s a t low degrees o f conversion o f the diisocyanates. Reactivity of the isocyanates with alcohols. According to t h e k n o w n m e c h a n i s m
Kinetic study of reactions of cyclohexane monoisocyanate
745
of nucleophilic addition of an alcohol to an isocyanate [2-10] one could have expected a reduction in the reactivity of the investigated compounds compared with the aromatic analogues owing to the electron-donor properties of the cyclohexane ring. 9
-0.06!_0.~\..... 0M -0.005
! ~.
2
-O.M7 7.2
F/.z/ 21.6
36
"l ,, f O z, sec
a--x
1
Fro. 2. Right-hand portion of equation (3) plotted against time t for the initial step of the reaction of CHIC with methanol: y =
1
(a--b) z
In
b--x
(a--x) (b--x)
~-const; a=0.464; b--
4.64raole/1.; 1--40°; 2--50°; 3--60 °. A comparison of the rate constants obtained by us for the reaction of cyclohexane isocyanate with methanol, and n-butanol on the one hand, with the published data for phenyl isocyanate on the other hand, confirmed this assumption, and shows t h a t the reactivity of the latter is 30-40 times higher.
KCHD/C KCHIC
CL
20
3
I A
S B
C OL, A
2
6
10
f~/~e
Fzo. 3. Relative reactivity of CHDIC versus distance between carbon atoms L of the isocyanate groups in diisocyanatos (a), and versus acidity of alcohols/~e (b). a: 1--n-butanol; 2--mothaaxol; 3--1,4-butanediol. The points on the abscissa have the following meanings; A--1,3-cis-; B--1,4-t~ane-; C--1,3-trans-; D--1,4-cis-CHDIC; b: 1--cia-l,4-, 2--trans-l,3-; 3--trans-l,4-; 4--c/s- 1,3-CHDIC. The introduction of a second isocyanate group into the cyclohexane ring is accompanied by a rise in the reactivity of the diisocyanates in reactions with alcohols, and this agrees with the d a t a published in the literature for the corresponding aromatic derivatives.
746
A.G.
KOZHEVOV et al.
It was found on the basis of kinetic measurements that the reactivity of the diisoeyanates under review increases in the order cis-l,4~trane-l,3~trans-l,4~cis1,3-CHDIC, and it occupies an intermediate position among the reactivity vaiues of the corresponding aliphatic and aromatic compounds. Moreover, the reactivity of the first compound in the above series (i.e. c/s-I,4-CHDIC) differs little from that of 1,6-hexamethylene diisocyanate [10], whereas the reactivity of the cis-l,3-isomer is 20-30 times higher than that of 1,6-hexamethylene diisocyanate. A quantitative relationship between the reactivity and structure of the diisocyanates in the reactions with n-butane, methanol and 1,4-butanediol was found by correlating the relative reactivity kc~Dm/~CH~C with the distance between the carbon atoms of the isocyanate groups L, • in stable configurations (Fig. 3a). The values of L were calculated using the known values for the bond lengths and the angles between the bonds [12]. The relationships obtained by us obey a hyperbolic equation of the type of y ~ m zn (see Fig. 3a), where y~kCSDIC//CCsID, x ~ L , and m and n are empirical coefficients. By analysing this equation one may determine the reactivity of the diisocyanates both in relation to the position of the 1,3- and 1,4-isocyanate groups, and in relation to the conformation of the moiecules. The higher reactivity of the cis- and trans-l,3 isomers compared with the trans- and cis-l,4-isomers may be attributed to the higher positive charge on the carbon atom and to the higher negative charge on the nitrogen in the isocyanate groups owing to the reduced positive inductive effect of the cyclohexane ring. On comparing the reactivity of the cis-l,3- and tra~-I,4-CHDIC with the trans-l,3 and the cis-l,4-CHI)IC one may conclude that a change in the position of a single isocyanate group from the equatorial to the axial position results in considerably increased reactivity. The reactivity of the alcohols changes in the order n-butanol~methanol~ 1,4-butanediol. The reactivity of n-butanol appears to be somewhat lower than that of methanol, and this conflicts with published information regarding the reactivity of these alcohols in reactions with aromatic isocyanates [6]. This could probably be due to the amphoteric nature of (the) alcohols which have more highly acidic properties in interactions with the less electrophilic carbon of the isocyanate groups in cyclohexane diisocyanates. This is confirmed by the curves in Fig. 3b showing that the reactivity of the diisocyanates is a linear function of the acidity of the alcohols. Physical pro,petrie8 of the polyurethanes. The polyurethanes prepared by reacting the geometrical isomers of CHDIC with 1,4-butanediol are products with molecular weights of 3400-3700, and are white crystalline substances soluble in acetone, methanol and m-cresol. The relatively low molecular weight of the
Kinetic study of reactions of cyclohexane monoisocyanatc T A B L E 3. E F F E C T OF THE C H D I C
Isomers of CHDIC G/s- 1,4 Trane- 1,3
Mol. wt. 3400 3450
747
STRUU'±'u~E ON T ~ E POLYUlltETHANE MELTING POINT
m.p. °C
Isomers of CHDIC
130-159 G/a-l,3 1 6 0 - 1 8 0 Tran~-l,4
1~ol. wt. 3500 3750
m.p. °C 220-240 >400
polyurethanes m a y be explained by the low temperatures of polymerization at which the kinetics of this process were investigated. The tabulated data (Table 3) show t h a t the melting points of the polyurethanes based on the unstable isomers (tie-l,4 and tran~-l,3) of CHDIC are lower compared with the stable isomers. This is probably due to the greater stereoregularity of the latter owing to the particular conformational structure of the cis-l,3- and trans-l,4-disubstituted cyclohexanes. The results of this investigation show t h a t the thermal stability of the polymers based on the stable isomers is better than that of the corresponding polyurethanes based on aliphatic and aromatic diisocyanates. The explanation for this probably lies in the greater rigidity of the chain elements and in the stronger forces of intermolecular interaction resulting from the introduction of the hydroaromatic ring into the polymer chain. I n view of these preliminary results it is recommended that similar polyurethanes of higher molecular weight should be prepared, and that their thermomechanical behaviour should be investigated in order to determine the limits of application of these polymers. CONCLUSIONS
(1) A kinetic investigation of the reactions of cyclohexane mono- and diisocyanates with methanol, n-butane and 1,4-butanediol has been carried out in benzene at 40, 50 and 60 °. It is found that the initial reaction step is mainly a second order reaction, the final step being an autocatalytic one. (2) The second and third order rate constants, and the activation energies and entropies have been determined. The reactivity of isocyanates increases in the order: cyclohexane i s o c y a n a t e < c i z - l , 4 ~ t r a n s - l , 3 - ~ t r a n s - l , 4 - < : c i s - l , 3 cyclohexane diisocyanates. An equation has been derived for the reactivity of these diisocyanates in relation to the distance between the carbon atoms of the isocyanate groups. The reactivity of the alcohols increases in. the order n - b u t a n o l ~ m e t h a n o l ~ 1,4butanediol, and is a linear function of their acidity. (3) Polyurethanes based on stable geometric isomers of 1,3- and 1,4-cyclohexane diisocyanates and 1,4-butanediol have high melting points, which could increase the range of their practical applications. TranalaSed by R. J. A. HEND]aY
748
V.Z. KOMPANIETSet al. REFERENCES
1. A. G. KOZHEVOV, Ye. V. GENKINA and Ya. A. SCHMIDT, J. of Mendeleyev All-Union Chem. Soc. 15: 460, 1970 2. I. W. BAKER, M. M. DAVIES and J. GAUNT, J. Chem. Soc., 9, 1949 3. I. W. BAKER and I. B. HOLDSWORTH, J. Chem. Soc. 713, 1947 4. Yu. A. STREPIKHEYEV, Plast. massy, No. 7, 13, 1961 5. E. G. LOVERING and K. I. LAIDLER, Canad. J. Chem. 40: 31, 1960 6. B. EPHRAIM, A. E. WOODWORD and R. B. MESROBIAN, J. Amer. Chem. Soc. 80: 1326, 1958 7. M. SATO, J. Amer. Chem. See. 82: 3893, 1960 8. T. N. GREENSCHIELDS, R. H. PETERS and R. E. STEPTO, J. Chem. Soc. 5101, 1964 9. H. OKADA and Y. YWAKURA, Makromolek. Chem. 66: 91, 1963 10. L. A. BAKALO, S. D. LYUKAS, S. S. ISHCHENKO and T. E. LOPATOVA, Vysokomol. Soyed., A12: 901, 1970 (Translated in Polymer Sei. U.S.S.R. 12: 4, 1023, 1970) 11. T. E. LIPATOVA, L. A. BAKALO, A. L. SIROTINSKAYA and V. S. LOPATINA, Vysokomol. soyed. A12: 911, 1970 (Translated in Polymer Sci. U.S.S.R. 12: 4, 1036, 197U) 12. L. SUTTON (Ed.), Tables of Interatomic Distances and Configurations in Molecules and Ions, London, p. 129, 205, 1958 13. J. HINE and M. HINE, J. Amer. Chem. Soc. 74: 5266, 1952
ANALYSIS OF THE NORMAL VIBRATIONS OF MACROMOLECULES BY MEANS OF ELECTRONIC DIGITAL COMPUTERS* V. Z. KOMPAI~IETS, E. F. OLEINIK a n d N. S. YENTKOLOPYAN Chemical Physics Institute, U.S.S.R. Academy of Sciences (Received 6 August 1970)
INFRARED s p e c t r o s c o p y is one of the m o r e i m p o r t a n t physical m e t h o d s of a n a lysing t h e s t r u c t u r e a n d c o m p o s i t i o n of macromolecules. T h e v a r i o u s p r o b l e m s t h a t are solved b y I R analysis include d e t e r m i n a t i o n o f t h e energetically m o s t f a v o u r a b l e c o n f o r m a t i o n s o f p o l y m e r molecules, calculation of t h e r m o d y n a m i c functions, i n v e s t i g a t i o n of t h e effect o f different s u b s t i t u e n t s on t h e v i b r a t i o n a l s p e c t r a a n d hence on t h e physical p r o p e r t i e s of p o l y m e r s , etc. Spectral p a t t e r n s are u s u a l l y r a t h e r complex, a n d a huge n u m b e r of different f a c t o r s m a y influence t h e position o f t h e b a n d s a n d t h e f o r m s of n o r m a l v i b r a t i o n s , so t h e n e e d arises for t h e o r e t i c a l analysis o f t h e v i b r a t i o n s of s a m p l e s u n d e r i n v e s t i g a t i o n [1]. H o w e v e r , despite t h e obvious need for this sort o f analysis t h e a m o u n t of m a t h e m a t i c a l w o r k i n v o l v e d m a k e s it v e r y difficult. I t would t h e r e f o r e be v e r y desirable if p r o g r a m m e s could be d e v e l o p e d to facilitate t h e use o f c o m p u t e r s * Vysokomol. soyed. A14: No. 3, 669-679, 1972.
(Received 5 August 1970)
MUCH research has been carried o u t in t h e field of a r o m a t i c a n d aliphatic diisoc y a n a t e chemistry, b u t practically no i n f o r m a t i o n has been published in r e g a r d to c y c l o h e x a n e mono- a n d diisocyanates. I n view of the electron-donor properties of t h e h y d r o a r o m a t i c ring the react i v i t y o f cyclohexane isocyanates m i g h t be e x p e c t e d to be lower t h a n t h a t o f t h e a r o m a t i c analogues. I t was therefore our aim to d e t e r m i n e the r e a c t i v i t y o f cycloh e x a n e isocyanate (CHIC) and cyclohexane diisocyanates (CHDIC) a n d to investigate t h e r e a c t i v i t y in relation to the conformational s t r u c t u r e o f the l a t t e r in reactions with mono- a n d bifunctional alcohols (methanol, n-butanol, 1,4-butanediol). A f u r t h e r aim was to reach certain conclusions regarding the mechanism o f u r e t h a n e a n d p o l y u r e t h a n e f o r m a t i o n using the results of the kinetic measurements. Since a h y d r o a r o m a t i c ring i n t r o d u c e d into the linear p o l y m e r chain could i m p r o v e the t h e r m a l stability of p o l y u r e t h a n e s a s t u d y was first m a d e of some of the physical properties o f the p r o d u c t polymers, and t h e need for f u r t h e r investigations to discover practical applications o f these polymers was examined. EXPERIMENTAL The original isocya~tes containing 99.8-99.9% basic substance were prepared by the same method as in [1]. The alcohols were dehydrated and double-distilled. Benzene which had first been distilled over metallic sodium was used as the solvent. The use of the latter insured the dissolution of the forming urethanes and made it possible for the reaction to be carried out in a homogeneous medium. The absence of any specific interaction of the reactants with benzene [2, 3] tended to reduce the possibility of distortion of the results of the kinetic measurements. Many authors have reported on the reaction of aromatic and aliphatic isocyanates with alcohols in benzene medium, so we had the opportunity of comparing our findings with those of other authors. * Vysokomol. soyed. A14: No. 3, 662-668, 1972. 740
Kinetic study of reactions of cyclohexane monoisoeyanate
741
The kinetics of the reaction of cyclohexane isocyanates with alcohols were investigated by the m e t h o d of taking samples with the initial concentrations of the reactants as shown in Table 1. The reaction was carried out at 40, 50 and 60 ° owing to the need to obtain high cor~versions in acceptable time intervals, and to reduce the secondary reactions (dimerization, trimerization of isocyanates, allophanate formation). The temperature of the reaction mixture was maintained w i t h an accuracy of ±0.1% The course of the reaction was verified through the amount of unconverted isocyanate as determined by infrared spectroscopy.
TABLE
1. R A T E
CONSTANTS,
ACTIVATION
ENERGIES
ORDER REACTION OF ISOCYANATES
AND ENTROPIES
FOR THE
SECOND
W I T H A L C O H O L S I N BEl~ZE17E
k × 106, 1./mole..see
Reactants
O
,..J 0
0
40 ~
50 °
60 °
o
.g CHIC + methanol
0.464 0-232 0.464 0.232 0-232
4"64 4-64 4.64 4.64 4.64
3.00 5.56 10.6 2.801 5.50 10.1 2.40 5.00 7.90 6.00 10.7 20-2 2-40 5.40 9.80
1 0-9 0.8 2 0"9
13.1 12.8 12.4 12.6 14.6
32.9 32-4 35.8 33.1 28.7
5.0 2.4 1
11.0 9.8 14.1
36.6 44"5 29"9
57-1 58.3 27.8
6.6 5.5 2.8
9-5 9.0 11.0
40.6 43.4 37.5
20-8
2.2
10.4
39.8
33.1 63.1 85.3 27.8* 48.1! 95.7
10 9.0
9.8 12.9
38.7 29.4
4.5 5-2
7.4 8.8
47.7 43.3
21.2
14.6
22-3
0.232 0.232 0.232
0.232 4.64 4.64
10
CHIC -4-n -butanol Cis- 1,4-CHDIC + methanol + n-butanol + 1,4-butane diol Trans- 1,3-CHDIC + methanol -4-n-butanol + 1,4-butanediol Trans- 1,4-CHDIC + methanol
0-232 0.232
I1
+n-butanol
0.232
0.232 23.0 36"5 4.64 16-5 30.6 9-7* 14.6 4.64 7-7 12-1
12
+ 1,4- butanediol
0.232
0.23~
0.232
4.64
0.232
4.64
17.6 19.6
i 0-232
0.232
59.5 125
7 8 9
13
G/s- 1,3-CDHIC + m e t h a n o l
14 15
-1-n-butanol + 1,4-butanediol * Numerator~rate
15.6 27.8 44.8 9.10:13.1 20.3 3.00 5.90 11-6
23.8 35.7 26"4 I 45.5
!
i230
c o n s t a n t o f first isocyanate g r o u p kz; denominator--kffi.
The polyurethanes prepared from the individual c/s- and trana-isomers of 1,3- and 1,4cyclohexanediisocyanates and 1,4-butanediol were purified by recrystallization from boiling acetone. The molecular weight of the polymers was determined b y measuring the viscosity of 5% polymer solutions in m-cresol at 25 ° in a viscometer with a capillary diameter of 1.2 m m P i n k e v i c h system [3].
742
A. G. KOZHEVOV e~ a/.
DISCUSSION OF RESULTS
Mechanism of the reaction of cyclohexane i8ocyanates with alcohols. No definite conclusions as to the mechanism of this process could be based on the currently held views concerning the interaction of isocyanates with alcohols. The reaction rate is described by second or third order kinetic equations according to the nature of the reactants [2-10]. Y
3
I
1
3
5
7
I
l
I
I
3
I
5
L
I
7
Time , hp FIa. 1. Right-trend portion y of the integral expression of the kinetic equation of the second order reaction of cyclohexane isoeyanates with alcohols plotted against time at 50°. a--for 10-20~/o conversions; b--for 60-70~/o conversions: y =
x
when a~-b;
1 lab(a--x)
when a Cb. The numbering of the curves corresponds to the numbers of the experiments m Table 1. Treatment of our results on the basis of a second order kinetic equation showed t h a t the right-hand portion of the integral expression of this equation is a linear function of time only for the early stages of the process, corresponding to 10-20% conversion of the isocyanate (Fig. la). As the degree of conversion rises, the reaction rate ceases to obey a second order equation, which points to a change in the reaction mechanism. A change of this type which was observed by authors investigating the interaction of aliphatic isocyanates with alcohols was attributed to the development of autocatalysis [6-10]. The following two equations describing these reactions were proposed in papers [6-10]: dx -~=]¢(a--x) (b--x)+l~x(a--x) (b--x), (1)
dx -~--kl (a--x)(b--x)~+k~(a--x)(b--x),
(2)
Kinetie study of reactions of cyclohexane monoisocyanate
743
where a, b are the initial isocyanate and alcohol concentrations, respectively; x is the urethane concentration; k, kl are second and third order rate constants for the initial stage of the reaction; k2 is the rate of the autocatalytic reaction. These equations were used in the treatment of our experimental data. The fact t h a t the initial stage of the process corresponds to the first term in equation (1) was accounted for by the first order of the reactions in respect to each reacting component (see Table 1). Further confirmation was provided by the low activation energies and the high negative activation entropies which allow us to assume t h a t the mechanism of urethane formation could be through a four-membered activated isocyanate-aleohol complex [5]. I f the initial stage o f the process takes place through the bimolecular interaction of an isoeyanate-aleohol complex with a molecule of alcohol [2, 3] or through the isocyanate interacting with a dimerie alcohol complex [6], this stage will then be described by the first term in kinetic equation (2). On integrating this term we obtained the expressions kit
- 1- lna--x (a--b) ~ b--x
1 [- const when a#b (a--x) (b--x)
(3)
and 1
kit= 2~(a--x) ~-const when a=b
(4)
In every case the right-hand portion of equations (3) and (4) is a linear time function. The kl values determined from these curves agree with those calculated by the Sato method [7] within the limits of accuracy of the latter. The activation energies and entropies based on k and kl (see Tables 1 and 2) are similar, and one m a y therefore assume that the first stage of the process m a y involve second and third order competing reactions. The kinetic dependence of the final stage of the reaction of the isocyanates with the alcohols is described by the second term in equations (1) and (2), as is confirmed by the linear dependence of its integral expression on time. The third order of the reaction, as well as the higher rate constants compared with the initial stage, and the lower activation energies and entropies, point to the development of autocatalysis (see Table 2). It should be noted that the activation energies for the final stage of the reaction of 1,4-cis-, 1,3- and 1,4-trans-CHDIC with methanol, and of cis-I,3-CHDIC with n-butanol are higher than the corresponding activation energies for the initial stage. This was also noted in papers [9, 10] dealing with certain aromatic and aliphatic isocyanates. I n the final stage, however, these reactions take place preferentially by an autocatalytic mechanism, probably owing to the less rigid demands being made on the structure of the transition complex, as is shown by the higher activation entropies.
744
A.G. KOZHEVOVe$ ed.
1,4-trans- a n d 1,3-cis-CHDIC w i t h 1,3-trans-CHDIC with n - b u t a n o l , are second order
The reactions of
methanol, a n d of 1,4-cis-, r i g h t t h r o u g h to 6 0 - 7 0 %
and conversion o f t h e diisocyanates (Fig. lb). The i n t e r a c t i o n o f 1,3-cis-CHDIC w i t h m e t h a n o l , a n d of 1,4-trans-CHDIC w i t h n - b u t a n o l is a c c o m p a n i e d b y r e d u c t i o n in t h e reaction rates after degrees of conversion of a r o u n d 50% h a v e been obtained. TABI,E 2.
RATE
COI~STANTS~ A N D ACTIVATIOI~ E N E R G I E S A N D
ENTROPIES
FOR
THE
THIRD
O R D E R R E A C T I O N S OF I S O C Y A N A T E S W I T H ALCOHOLS
k~ × 10e,l.2/molo2sec
kl × 10612/rnole2 sec
40 o
50°
4:
60 °
40 °
50 °
60°
,~'~
2~ I d
1
2 3 4 5 6 7 8 9 10 11 12 13 14 15
0-72 0.70 0.53
1.26 1.20 1.06 1.29 2.31 0.52 1.14 67.3 ] 120 1.96[ 2.82 0.651 12.7 99.1 157.2 3.56 6.7 2.1 3.14 143 1272 6.58 I 9.92 4.23 5.68 256 539
2"28 2"20 1"71 4"35 1 2"1 193 4"4 2"5 246"1 12'~ 6"~ 368 18'2 9"8 991
12"0 15'8 12'4 12"6 14"6 10"8 8"4 14'4 9"5 9"6 10"9 9"8 10"6 8"7 13"5
39-4 39"0 38"8 35'8 31"7 34"2 49"0 30"4 39"5 44"4 40'7 35"8 39"5 46"3 22"9
46"3 45"8 38"1 80"5
87.2 86-7 63.5 '222
9.4 9.7 9.6 21-9
40"1 39"7 40"0 0"6
1250 77 "9
543 162
4-54 15.8
48.8 19.4
1543 139
339 259
9.0 14.6
34.5 22.2
1130
1234
34O
3.13
52.3
126-2 1230
222 2085
557.2 3267
15.4 10.2
18.2 30.4
35"4 35'0 25"3
63"5
The c h a n g e in the rate c o n s t a n t m a y be a t t r i b u t e d to the difference in the r e a c t i v i t y o f t h e first i s o c y a n a t e g r o u p g e n e r a l l y characterized b y the c o n s t a n t kl, a n d t h e second g r o u p c h a r a c t e r i z e d b y k. Table 1 gives t h e r a t e c o n s t a n t s k 1 a n d k2 of the second order reaction (exp e r i m e n t s 11 a n d 13). As is seen f r o m Table 1, the k 1 values are higher t h a n those of k2, which m e a n s t h a t t h e a c t i v a t i n g effect o f t h e second i s o c y a n a t e g r o u p is g r e a t e r t h a n t h a t of t h e u r e t h a n e group. I n view o f t h e difficulties t h a t arise in q u a n t i t a t i v e d e t e r m i n a t i o n of t h e rate of t h e final stage of the r e a c t i o n t h e reactivities of t h e investigated c o m p o u n d s were c o m p a r e d on t h e basis of t h e results of kinetic m e a s u r e m e n t s a t low degrees o f conversion o f the diisocyanates. Reactivity of the isocyanates with alcohols. According to t h e k n o w n m e c h a n i s m
Kinetic study of reactions of cyclohexane monoisocyanate
745
of nucleophilic addition of an alcohol to an isocyanate [2-10] one could have expected a reduction in the reactivity of the investigated compounds compared with the aromatic analogues owing to the electron-donor properties of the cyclohexane ring. 9
-0.06!_0.~\..... 0M -0.005
! ~.
2
-O.M7 7.2
F/.z/ 21.6
36
"l ,, f O z, sec
a--x
1
Fro. 2. Right-hand portion of equation (3) plotted against time t for the initial step of the reaction of CHIC with methanol: y =
1
(a--b) z
In
b--x
(a--x) (b--x)
~-const; a=0.464; b--
4.64raole/1.; 1--40°; 2--50°; 3--60 °. A comparison of the rate constants obtained by us for the reaction of cyclohexane isocyanate with methanol, and n-butanol on the one hand, with the published data for phenyl isocyanate on the other hand, confirmed this assumption, and shows t h a t the reactivity of the latter is 30-40 times higher.
KCHD/C KCHIC
CL
20
3
I A
S B
C OL, A
2
6
10
f~/~e
Fzo. 3. Relative reactivity of CHDIC versus distance between carbon atoms L of the isocyanate groups in diisocyanatos (a), and versus acidity of alcohols/~e (b). a: 1--n-butanol; 2--mothaaxol; 3--1,4-butanediol. The points on the abscissa have the following meanings; A--1,3-cis-; B--1,4-t~ane-; C--1,3-trans-; D--1,4-cis-CHDIC; b: 1--cia-l,4-, 2--trans-l,3-; 3--trans-l,4-; 4--c/s- 1,3-CHDIC. The introduction of a second isocyanate group into the cyclohexane ring is accompanied by a rise in the reactivity of the diisocyanates in reactions with alcohols, and this agrees with the d a t a published in the literature for the corresponding aromatic derivatives.
746
A.G.
KOZHEVOV et al.
It was found on the basis of kinetic measurements that the reactivity of the diisoeyanates under review increases in the order cis-l,4~trane-l,3~trans-l,4~cis1,3-CHDIC, and it occupies an intermediate position among the reactivity vaiues of the corresponding aliphatic and aromatic compounds. Moreover, the reactivity of the first compound in the above series (i.e. c/s-I,4-CHDIC) differs little from that of 1,6-hexamethylene diisocyanate [10], whereas the reactivity of the cis-l,3-isomer is 20-30 times higher than that of 1,6-hexamethylene diisocyanate. A quantitative relationship between the reactivity and structure of the diisocyanates in the reactions with n-butane, methanol and 1,4-butanediol was found by correlating the relative reactivity kc~Dm/~CH~C with the distance between the carbon atoms of the isocyanate groups L, • in stable configurations (Fig. 3a). The values of L were calculated using the known values for the bond lengths and the angles between the bonds [12]. The relationships obtained by us obey a hyperbolic equation of the type of y ~ m zn (see Fig. 3a), where y~kCSDIC//CCsID, x ~ L , and m and n are empirical coefficients. By analysing this equation one may determine the reactivity of the diisocyanates both in relation to the position of the 1,3- and 1,4-isocyanate groups, and in relation to the conformation of the moiecules. The higher reactivity of the cis- and trans-l,3 isomers compared with the trans- and cis-l,4-isomers may be attributed to the higher positive charge on the carbon atom and to the higher negative charge on the nitrogen in the isocyanate groups owing to the reduced positive inductive effect of the cyclohexane ring. On comparing the reactivity of the cis-l,3- and tra~-I,4-CHDIC with the trans-l,3 and the cis-l,4-CHI)IC one may conclude that a change in the position of a single isocyanate group from the equatorial to the axial position results in considerably increased reactivity. The reactivity of the alcohols changes in the order n-butanol~methanol~ 1,4-butanediol. The reactivity of n-butanol appears to be somewhat lower than that of methanol, and this conflicts with published information regarding the reactivity of these alcohols in reactions with aromatic isocyanates [6]. This could probably be due to the amphoteric nature of (the) alcohols which have more highly acidic properties in interactions with the less electrophilic carbon of the isocyanate groups in cyclohexane diisocyanates. This is confirmed by the curves in Fig. 3b showing that the reactivity of the diisocyanates is a linear function of the acidity of the alcohols. Physical pro,petrie8 of the polyurethanes. The polyurethanes prepared by reacting the geometrical isomers of CHDIC with 1,4-butanediol are products with molecular weights of 3400-3700, and are white crystalline substances soluble in acetone, methanol and m-cresol. The relatively low molecular weight of the
Kinetic study of reactions of cyclohexane monoisocyanatc T A B L E 3. E F F E C T OF THE C H D I C
Isomers of CHDIC G/s- 1,4 Trane- 1,3
Mol. wt. 3400 3450
747
STRUU'±'u~E ON T ~ E POLYUlltETHANE MELTING POINT
m.p. °C
Isomers of CHDIC
130-159 G/a-l,3 1 6 0 - 1 8 0 Tran~-l,4
1~ol. wt. 3500 3750
m.p. °C 220-240 >400
polyurethanes m a y be explained by the low temperatures of polymerization at which the kinetics of this process were investigated. The tabulated data (Table 3) show t h a t the melting points of the polyurethanes based on the unstable isomers (tie-l,4 and tran~-l,3) of CHDIC are lower compared with the stable isomers. This is probably due to the greater stereoregularity of the latter owing to the particular conformational structure of the cis-l,3- and trans-l,4-disubstituted cyclohexanes. The results of this investigation show t h a t the thermal stability of the polymers based on the stable isomers is better than that of the corresponding polyurethanes based on aliphatic and aromatic diisocyanates. The explanation for this probably lies in the greater rigidity of the chain elements and in the stronger forces of intermolecular interaction resulting from the introduction of the hydroaromatic ring into the polymer chain. I n view of these preliminary results it is recommended that similar polyurethanes of higher molecular weight should be prepared, and that their thermomechanical behaviour should be investigated in order to determine the limits of application of these polymers. CONCLUSIONS
(1) A kinetic investigation of the reactions of cyclohexane mono- and diisocyanates with methanol, n-butane and 1,4-butanediol has been carried out in benzene at 40, 50 and 60 °. It is found that the initial reaction step is mainly a second order reaction, the final step being an autocatalytic one. (2) The second and third order rate constants, and the activation energies and entropies have been determined. The reactivity of isocyanates increases in the order: cyclohexane i s o c y a n a t e < c i z - l , 4 ~ t r a n s - l , 3 - ~ t r a n s - l , 4 - < : c i s - l , 3 cyclohexane diisocyanates. An equation has been derived for the reactivity of these diisocyanates in relation to the distance between the carbon atoms of the isocyanate groups. The reactivity of the alcohols increases in. the order n - b u t a n o l ~ m e t h a n o l ~ 1,4butanediol, and is a linear function of their acidity. (3) Polyurethanes based on stable geometric isomers of 1,3- and 1,4-cyclohexane diisocyanates and 1,4-butanediol have high melting points, which could increase the range of their practical applications. TranalaSed by R. J. A. HEND]aY
748
V.Z. KOMPANIETSet al. REFERENCES
1. A. G. KOZHEVOV, Ye. V. GENKINA and Ya. A. SCHMIDT, J. of Mendeleyev All-Union Chem. Soc. 15: 460, 1970 2. I. W. BAKER, M. M. DAVIES and J. GAUNT, J. Chem. Soc., 9, 1949 3. I. W. BAKER and I. B. HOLDSWORTH, J. Chem. Soc. 713, 1947 4. Yu. A. STREPIKHEYEV, Plast. massy, No. 7, 13, 1961 5. E. G. LOVERING and K. I. LAIDLER, Canad. J. Chem. 40: 31, 1960 6. B. EPHRAIM, A. E. WOODWORD and R. B. MESROBIAN, J. Amer. Chem. Soc. 80: 1326, 1958 7. M. SATO, J. Amer. Chem. See. 82: 3893, 1960 8. T. N. GREENSCHIELDS, R. H. PETERS and R. E. STEPTO, J. Chem. Soc. 5101, 1964 9. H. OKADA and Y. YWAKURA, Makromolek. Chem. 66: 91, 1963 10. L. A. BAKALO, S. D. LYUKAS, S. S. ISHCHENKO and T. E. LOPATOVA, Vysokomol. Soyed., A12: 901, 1970 (Translated in Polymer Sei. U.S.S.R. 12: 4, 1023, 1970) 11. T. E. LIPATOVA, L. A. BAKALO, A. L. SIROTINSKAYA and V. S. LOPATINA, Vysokomol. soyed. A12: 911, 1970 (Translated in Polymer Sci. U.S.S.R. 12: 4, 1036, 197U) 12. L. SUTTON (Ed.), Tables of Interatomic Distances and Configurations in Molecules and Ions, London, p. 129, 205, 1958 13. J. HINE and M. HINE, J. Amer. Chem. Soc. 74: 5266, 1952
ANALYSIS OF THE NORMAL VIBRATIONS OF MACROMOLECULES BY MEANS OF ELECTRONIC DIGITAL COMPUTERS* V. Z. KOMPAI~IETS, E. F. OLEINIK a n d N. S. YENTKOLOPYAN Chemical Physics Institute, U.S.S.R. Academy of Sciences (Received 6 August 1970)
INFRARED s p e c t r o s c o p y is one of the m o r e i m p o r t a n t physical m e t h o d s of a n a lysing t h e s t r u c t u r e a n d c o m p o s i t i o n of macromolecules. T h e v a r i o u s p r o b l e m s t h a t are solved b y I R analysis include d e t e r m i n a t i o n o f t h e energetically m o s t f a v o u r a b l e c o n f o r m a t i o n s o f p o l y m e r molecules, calculation of t h e r m o d y n a m i c functions, i n v e s t i g a t i o n of t h e effect o f different s u b s t i t u e n t s on t h e v i b r a t i o n a l s p e c t r a a n d hence on t h e physical p r o p e r t i e s of p o l y m e r s , etc. Spectral p a t t e r n s are u s u a l l y r a t h e r complex, a n d a huge n u m b e r of different f a c t o r s m a y influence t h e position o f t h e b a n d s a n d t h e f o r m s of n o r m a l v i b r a t i o n s , so t h e n e e d arises for t h e o r e t i c a l analysis o f t h e v i b r a t i o n s of s a m p l e s u n d e r i n v e s t i g a t i o n [1]. H o w e v e r , despite t h e obvious need for this sort o f analysis t h e a m o u n t of m a t h e m a t i c a l w o r k i n v o l v e d m a k e s it v e r y difficult. I t would t h e r e f o r e be v e r y desirable if p r o g r a m m e s could be d e v e l o p e d to facilitate t h e use o f c o m p u t e r s * Vysokomol. soyed. A14: No. 3, 669-679, 1972.