Infrared Determilnation of Noncyclic Imides and Polyimides Shalaby W. Shalabyl and Edward L. McCaffery Chemistry Department, Lowell Technological Institute, Lowell, Mass.
INFRARED ANALYSIS has been useful in the characterization of various nitrogen-containing organic compounds and macromolecules (I-3). The position of the N-H and C==O stretching frequencies in amides and polyamides are well known as are the carbonyl stretching frequencies in cyclic imides and polyimides (I). Recently we have been able to
I
prepare linear, noncyclic polyimides for the first time. Because data from infrared analyses were needed to complement other analyses used in characterizing the polymers, we attempted to find information concerning the C=O stretching frequency in simple, noncyclic imides. Such data are unavailable. We will attempt in this paper to assign the imide e 0 stretching frequency in polyimides and to observe how the band behaves in polyimides of differing molecular weights where both the amide and the imide carbonyl bands are present. EXPERIMENTAL
Apparatus. The infrared spectra were taken with a PerkinElmer Model 21 and a Beckman IR-10 recording spectrophotometers. Solid samples were run as Nujol or hexachlorobutadiene mulls with sodium chloride plates. Soluble compounds were examined in chloroform solution in a sodium chloride liquid-cell with a thickness of 0.25 mm. The viscosity data were obtained with an Oswald viscometer on polymer solutions between 2 and 10 grams/liter. The intrinsic viscosity was calculated by extrapolating the inherent viscosity to zero concentration. The number-average molecular weight measurements were made on a Mechrolab Model 301-A vapor pressure osmometer in chloroform or methyl ethyl ketone solution at concentrations between 2 and 7.5 grams/liter. A Brice-Phoenix light-scattering photometer was used to evaluate weightaverage molecular weight. Reagents. The polyimides were prepared by two different procedures. METHODA. The reaction was carried out in a 250-ml three-neck ground-joint flask equipped with a magnetic stirrer, dropping funnel, reflux condenser and drying tube, and a nitrogen inlet. The system was flame-dried and flushed with nitrogen under reduced pressure. The dried apparatus was allowed to cool to room temperature under an atmosphere of
nitrogen. A solution of 0.03 mole of the amine in 100 ml of dried, freshly distilled N,N-dimethylacetamide along with 15 ml of dried, freshly distilled triethylamine were added to the reaction vessel. The mixture was cooled in an ice bath. A solution of 0.03 mole of purified diacid chloride in 20-mi of dried, freshly distilled, ethanol-free chloroform was added rapidly to the well-stirred, cooled amine solution. The reaction was continued for hr; the ice bath was removed, and the reaction mixture was allowed to come to room temperature. The reaction periods in different runs were increased from liZ to 191/2hr. The volatile components were distilled at reduced pressure; the residue was poured into 400 ml of ether, and the precipitate was separated by filtration. The precipitate was washed successively with 100 ml of water, 200 ml of ethanol, 100 ml of water, 200 ml of methanol, and 100 ml of ether. The residue was dried to constant weight in a vacuum oven at 50” C. METHODB. The apparatus was identical to Method A. The reaction flask was charged with 0.03 mole of purified diacid chloride and a solution of 0.03 mole of the purified amine dissolved in 120 ml of freshly distilled, dry pyridine. The mixture was refluxed for 2 hr. The reaction mixture was cooled to room temperature and the product was isolated as in Method A. Preparation of N,N-Dibenzoylaniline. Benzoyl chloride (8.44 grams, 0.06 mole) was reacted with 2.79 grams (0.03 mole) of aniline using Method A. After 2 hr the product was precipitated in ether, was filtered, and was washed several times with 100-ml portions of cold water. The product was air-dried and was recrystallized from methanol to give 4.5 grams (50%) of colorless needles melting at 162-4” C. After two further recrystallizations from methanol, the product melted at 163.5-5” C [lit ( 4 ) 160-1” C ; lit (5) 1634” C]. The melting point of an equimolar mixture of the product and an authentic sample of benzanilide (mp 1611.5” C) occurred at 140-4” C. The infrared spectrum of N,N-dibenzoylaniline exhibited a strong doublet at 1645 and 1680 cm-l, assigned to the imide carbonyl. A chloroform solution of the imide showed only a single band at 1670 cm-1. Hydrolysis of N,N-dibenzoylaniline gave benzoic acid and aniline in a 2/1 ratio. Molecular weight. Calculated for C20H1aN02: 301. Found: 297. Analysis. Calculated: C, 79.7%; N, 4.65%. Found: C, 80.1%; N,4.5%. RESULTS
The polyimides prepared in this study were made either from terephthaloyl chloride and various amines :
1 Present address, Department of Industrial Designs, College of Applied Arts, Giza, Egypt.
(1) L. J. Bellamy, “Infrared Spectra of Complex Molecules,” 2nd ed., Methuen and Co., London, 1958. (2) R. M. Silverstein and G . C. Bassler, “Spectrometric Identification of Organic Compounds,” Wiley, New York, 1963. (3) R. Zbindin, “Infrared Spectroscopy of High Polymers,” Academic Press, New York, 1964.
(4) P. Kay, Ber., 26,2855 (1893). (5) F. Beilstein, “Handbuch der Organischen Chemie,” 1st ed., Vol. XII, 1929, p 274. VOL 40, NO. 4, APRIL 1968
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or from aniline and adipoyl or sebacoyl chlorides: 0
CgH,jNHz
0
II
+
0
II
*
Cl-CfCHzkC-Cl
3C, x = 8
2c, x = 4
The pertinent data concerning the polymers are summarized in Tables I and 11. The reference standard, N,N-dibenzoylaniline, had been prepared previously but under such stringent conditions and with such poor characterization that it was desirable to attempt the preparation with the technique of the polymer formation. The polymers shown in Table I vary in molecular weight all the way from simple oligomers to high polymers. Since most of the polymers appeared to be amine-terminated, it seemed reasonable that one could derive at least a qualitative
Table I. Physical Properties of the Noncyclic Polyimides Reaction Yield, time, [VI, Polymer hr‘ mpo, C ml/g 73 295 13 ... 60 20 1A 350 dec ... 80 11 1B 2b 350 dec 46 IC 20 6oooc 70 10 350 dec ... 75 ID 20 310 dec 12 ... 76 1E 20 185-220 5 50 598 2c 4 17CL210 5 3c 4 605 64
an
a
Method A.
* Method B. zw.
Table 11. Carbon-Nitrogen Analyses for the Noncyclic Polyimides Found Calculated
73
z
z
z
Polymer
Carbona
Nitrogenb
Carbon
Nitrogen
1A
70.7 71.9 75.6 63.0 62.2
7.1 6.1 6.2 9.6 9.5
70.9 52.7 75.3 62.7 62.7
6.9 6.1 6.3 10.4 10.4
1B
IC 1D
1E a
Schoniger oxygen-flask method.
Micro-Kjeldahl.
Table 111. Infrared Spectral Assignments for Noncyclic Imides
Compound Benzanilide N,N-Dibenzoylaniline (Nujol) N,N-Dibenzoylaniline (CHC13) 1A 1B IC 1D 1E 2c 3c
824
Amide c--O Amide N-H stretch, stretch, cm-1 cm-’ 1648 3315
...
...
1648 1648 1648 1642 1645 1640 1640
ANALYTICAL CHEMISTRY
Imide C=O stretch, cml
...
...
1645, 1680
...
1670 1670 1672 1670 1660, 1680 1660, 1680 1685 1685
3315 3315 3310 3300 3 300 3330 3330
insight into the extent of polymerization by comparing the relative intensities of the amide and imide absorption bands in the infrared. In view of the proximity of the imide and amide absorption bands, the amide C=O stretching band often appeared as a shoulder to the imide band. In these cases, it was more useful to estimate the amide end-group concentration from the N-H stretch rather than from the C=O stretch. The band assignments are summarized in Table 111. As the molecular weight of the polymers increased, the contribution of the amide end groups decreased and that of the imide groups increased, proportionately. Both amide bands actually disappeared in the higher molecular weight samples. DISCUSSION
The data show clearly that the C=O stretching frequency in noncyclic polyimides appears at about 1670 cm-1. Furthermore, it is obvious also that the peak height of either amide end-group band (N-H of -0) can be used to estimate the degree of polymerization for polymers prepared by the methods described in this paper. The singlet at 1670 cm-l which appears in the spectra of polyimides IA, IB, and 1C is attributable to the similarity of the two carbonyls bonded to the nitrogen atom. Apparently, the substituents C&, C6H13, and C6H6 are not polar enough to induce a sufficient dissimilarity between the two carbonyls; therefore, the carbonyls appear as a singlet. It would appear that chloroform also tends to equalize the carbonyl groups in N,N-dibenzoylaniline because of the polarity of the solvent which reduces the vibrational split to a minimum. Conversely, the nitro group in the aromatic side chain of polymers ID and 1E imparts an internal polarization which brings about a partial discrimination between the adjacent carbonyl groups, thereby inducing a split in the carbonyl stretching frequency. It was quite surprising to find the pronounced splitting which occurred in the Nujol spectrum of N,N-dibenzoylaniline. Several of the polymers prepared in this study exhibited absorption bands at 1710 and 1775 cm-’, the bands characteristic of an anhydride. The appearance of these bands suggested an interesting possibility. It has been shown that, under the conditions employed in the conduct of the experiments reported here, an acid chloride can react with water (6) to form an anhydride; therefore, a diacid chloride should be capable of reacting with water to form a polyanhydride. Such, indeed is the case (7). To verify both that water had been present in some of the reaction mixtures and that the reagent did not have an adverse effect upon the polymerization, we decided to prepare
(6) Organic Chemistry.” . , R. B. Wagner and H. D. Zook,. “Synthetic .. Wiley, New York, 1953. (7) E. L. McCaffery, Lowell Technological Institute, unpublished results, 1957.
polyanhydride I1 by reaction of terephthaloyl chloride with water using the polyimide Method A.
I1
bands but exhibited the typical 0-H stretching frequency at 3405 cm-l. It is clear that the infrared analysis described here is not only an excellent analytical technique for the characterization of low molecular weight polyimides, but it can also be used to monitor the dichotomous role of water in the polyimidation reaction.
The polyanhydride exhibited two strong infrared bands at 1710 and 1775 cm-1. The polyimides prepared from the aliphatic diacid chlorides did not show the polyanhydride
RECEIVED for review November 17,1967. Accepted February 7, 1968.
-Epoq
n
Infrared Difference Analysis of Cyclic Units in Polyisoprene I. Kossler and J. Vodehnal Znstitute of Physical Chemistry, Czechoslovak Academy of Sciences, Prague
THEUSE OF difference spectra for quantitative analysis is well known [see e.g.(l)]. This paper describes the determination by the difference method of two components, similar in structure, which exhibit only minor shifts of absorption maxima. Martin ( 2 ) pointed out that in these cases the difference spectrum shows only vague maxima and minima. These maxima and minima can however be effectively used for quantitative analysis of cyclic units in polyisoprene. THEORETICAL BACKGROUND For overlapping absorption bands of components A and B , it holds, under the assumption of equal thickness of the sample cell (d) and the reference cell (d’) and equal concentrations in both sample cell (c) and reference cell (c’), that
where A (v) is absorbance at a wavenumber v, kA and kB are adsorptivities, x A and xB molar fractions of components A and B in the sample cell, xA’ and xB’ are their molar fractions in the reference cell, respectively. If the absorption maxima differ only slightly in position, a sharp maximum and minimum appear on the difference spectrum (Figure 10). For wavenumbers of the maximum (VI) and the minimum ( v 2 ) Equation 1 holds. By subtracting XB = 1 under the condition, that xB’ = 0, x A ‘ = I , x A and I, ( v 2 ) = I, (v4) we obtain the relationship
+
where K1 = kA(vl) - kA (v2),KZ = ke (VI)- kB (YZ) and C = KI - K2. When using the potassium bromide pellet technique, we obtain a similar equation, where the term l/cd is replaced by Flu, F being the square area of the pellet and a the weight of the sample in the pellet. For two components and unequal products of total con(1) W. J. Potts, “Chemical Infrared Spectroscopy,” Vol. 1, Wiley, New York, 1963. (2) A. E. Martin et al., Nature, 180,231 (1957).
centration and cell thickness cd and c’d’ it holds (if xB’ = 0, x A ’ = I , and X A X B = l), that
+
(3)
where D = KZ - Klcldl/cd. ANALYSIS OF CYCLOPOLYISOPRENES The infrared spectra of cyclopolyisoprenes (3, 4) do not show any specific absorption band which could be analytically valuable for determination of units with cyclic structure in a polymer containing both cyclic and linear units. The spectrum of the cyclic polymer shows a slight shift of the absorption maximum of asymmetric deformation vibration of the CH3 group and deformation vibration of the CH2 group near 1450-1470 cm-’ by about 10 cm-l as compared with the same vibration in linear cis-1,4-polyisoprene (3). This shift has been utilized for determination of cyclic units in polymers which contain 10-90z of these units by the above described method. Figure 2C shows the difference spectrum of cyclopolyisoprene-hevea (cis-1,4-polyisoprene) whose analytically applicable part is presented in Figure 1D. From Figure 2 it can be seen that the difference spectrum balata (trans-l,4po1yisoprene)-hevea ( 2 E ) has a shape similar to that of the difference spectrum cyclopolyisoprene-hevea but differs in the range of 800-900 cm-1 and 500-650 cm-l. The spectrum of a mixture of 80z of cyclopolyisoprene with 2 0 z of balata us. hevea (2F‘) shows that by examining the given ranges one can identify the trans-1,4 form. The band at 890 cm-’ indicates the presence of 3,4 structure. If the preliminary examination of the spectrum in the range of 800-900 cm-’, 500-600 cm-1 and near 890 cm-1 shows that the system contains only cis-1,4 form and cyclic units a calibration curve can be used in analysis. This is a common case in most cyclopolyisoprenes. The result of the analysis can be considered as a measure of a “degree of cyclicity.” At the present state of knowledge we cannot say if the cyclopolymer with a great number of short fused cyclic units has the same position of (3) I. Kossler, J. Vodehnal, and M. holka, J. Polymer Sci., A3,
2081 (1965). (4) J. L. Binder, J. Polymer Sci., EM,19 (1966). VOL 40, NO. 4, APRIL 1968
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