(8) Duke, F. R., Quinney, P., Ibid., 76, 3800 (1954). (9) . , Edwards, J. D.. Chem. Revs. 50, 455 (1952). (10) Flagg, J. F., J . Am. Chem. SOC.63, 557 (1941). (11) Gryder, J. W., Dodson, R. W., Ibid., 71, 1894 (1949); 73,2890 (1951). (12) Halperin, Joseph, Taube, Henry, Zbid., 74, 375, 382 (1952). (13) Hershey, A. V., Bray, W. C., Ibid., 58, 1760 (1936). (14) Hoering, T. C., Butler, R. C., McDonald, H. O., Ibid., 78,4829 (1956). (15) Hornig, H. C., Libby, W. F., J. Phys. Chem. 56, 896 (1952).
(16'1 Marcus. R. A.. J . Chem. Phus. 867. ,--, 872 (1957): (17) Meier, D. J., Garner, C. S., J. Phys. Chem. 56, 853 (1952). (18) Meyers, 0. E., Kennedy, J. R., J . Am. Chem. Soc. 72, 897 (1950). (19) Rutenberg, A. C., Halperin, Joseph, Taube, Henry, Zbid., 73, 4487 (1951). (20) Sheppard, J. C., Wahl, A. C., Ibid., 75, 5133 (1953). (21) Skrabal, Anton, Weberitsch, S. R., Monatsh. Chem. 36, 237 (1913). (22) Swain, C. G.,Hedberg, K., J . Am. 72, 3373 (1950). Chem. SOC. (23) Taube, Henry, Chem. Revs. 50, 69 (1952).
(24) Taube, Hm-y, J. Ant. Chem. SOC. 70, 1216 (1948). (25) Ibid., 77, 4481 (1955). (26) Taube, Henry, Myers, Howard, Rich, R. L., Ibid., 75, 4118 (1953). (27) Wahl, A. C., Deck, C. F., Ibid., 76, 4054 (1954). (28) Westheimer, F. H., Chem. Revs. 45, 419 (1949). (29) Zwolinski, B. J., Marcus, R. J., Eyring, H., 1bid., 55,157 (1955).
RECEIVEDfoi, review July 31, 1958. Accepted Decomber 8, 1958.
Determination of Unsaturation Distribution in Polybutadienes by Infrared Spectrometry ROBERT S. SILAS, JOY YATES, and VERNON THORNTON Phillips Petroleum Co., Bartlesville, Okla. b The availability of polybutadienes with widely varying structures has led to a study of the absorption of cis-1,4 addition in the 12- to 16-micron region and the development of a method for unsaturation distribution using high cis-l,4, trans-l,4, and 1,2(vinyl) polymers as the only calibration standards. The cis-1,4 absorption band, at 13.5 microns in high cis polymers, varies in position and shape as the amount of cis-1,4 addition changes. An empirical function of the area of the absorption band between 12.0 and 15.75 microns was used to measure cis-1,4 addition. frans-l,4 and 1,2(vinyl) addition was measured a t 10.3 and 1 1 .O microns, respectively. The method was applied successfully to a number of polybutadienes with widely varying distributions. The results indicate that this method, derived solely from polybutadienes, can be applied with confidence to polybutadienes with any unsaturation distribution.
I
polymerization of butadiene, addition of the monomer may take phce in three ways, giving rise to the following configurations : N THE
-HzC
\
CH2- -H&
H
/
\ c=c/ / \
H CH2trans-1,4
H"."c\H
cis-1,4 -CH-CH-
I
The physical properties of a polybutadiene depend to a large extent on the distribution of the monomer units among these three configurations. Several methods for determining unsaturation distribution in polybutadienes have been published. Hampton (3) determined trans-l,4, I,2(vinyl), and cis-1,4 addition at 10.3, 11.0, and 13.8 microns, respectively. Binder (2) determined trans-1,4 and 1,2 (vinyl) addition a t the same wave lengths, but cis-1,4 addition a t 14.7 microns. Richardson (6) determined trans-l,4 and I,2(vinyl) addition only, obtaining cis-1,4 addition by difference. The disagreement about the proper method for measuring cis-1,4 addition was due to several factors: the unavailability of polybutadienes with large amounts of cis-1,4 addition and the variability and low intensity of bands characteristic of cis-1,4 addition. However, all three methods give similar results when applied to emulsion polymerized polybutadienes. The development of new catalyst systems in recent years (4, 6) has made possible the preparation of polybutadienes with almost any unsaturation distribution desired. Because such polymers were available, the present work was undertaken with two principal objectives: to find infrared absorption bands suitable for measuring ci+1,4 addition and to develop a method for determining unsaturation distribution in polybutadienes with any type of distribution.
I
CH
AH2 1,2(vinyl)
EXPERIMENTAL
Selection of Analytical Bands. The bands a t 10.3 and 11.0 microns used
in the methods cited above for measuring trans-1,4 and 1,2(vinyl) addition, respectively, were also used in the present method. Theqe bands, which have been assigned to out-ofplane hydrogen deformations ( f ) , are strong and relatively free from interference, ma king them suitable for analytical hano s. Abporbance a t the band maximum was calculated directly from the transmittance indicated on the spectrum. Accordin < to Bellamy ( I ) , the out-ofplane hydrogen deformation in cisolefins is near 14.5 microns, but the exact posir,ion varies considerably in known coripounds. I n Figure 1 are shown spectra of three polymers with high cis-I, 1, high trans-1,4, and high 1,2(vinyl) addition, respectively. Examination of these and other polymers indicate that tmns-1,4 addition has only general aksorption in the 12- to 16micron region and that 1,2 (vinyl) addition has a weak band a t 14.8 microns and a wea'rer band a t 12.5 microns. In the high cis polymer, virtually all of the absorption in the 12- to 16-micron region is due to cis-1,4 addition. The absorption is complex. At Ieast three components can be detected a t 12.9, 13.5, and 14.5 microns. An attempted graphical resolution of the absorption indicated that several other bands may also be present. The abr;orption of &1,4 addition in the 12- t ) 16-micron region has been studied ir a large number of polymers with diff went unsaturation distributions. As the amount of cis-1,4 addition decreases, the band maximum shifts gradually to about 13.8 microns, the 12.9-micron band disappears rapidly, and the >*elativeintensity of the 14.5micron kland compared to the 13.5micron b m d increases. Other changes may also occur. I n the high trans-1,4 polymer shown in Figure 1, the weak VOL. 31, NO. 4, APRIL 1959
529
bands a t 13.5 and 14.5 microns can be traced to cis-l,4 addition. I n the high 1,2(viriyl) polymer, the bands a t 14.3 and possibly 14.6 microns can also be traced to cis-1,4 addition. A possible explanation of the behavior of the cis band can be found in the apparent sensitivity of the vibration to environment. That i t is sensitive to environment is shown by its variable position in known compounds, whereas the corresponding bands in trans and vinyl olefins are remarkably constant in position. The exact wave length of the vibration in a particular cis-1,4 unit may therefore depend on the configuration of adjacent units and possibly of units further removed. The recorded absorption band is the sum of the absorption of all cis units. When the cis concentration changes, the shape of the absorption band would change as the environment of the cis units changed. I n the above discussion it was tacitly assumed that the 12.9-micron band had the same origin as the other cis bands is this region. There is an alternative assignment for this band. It may be due to the -CH,-CH2group. The American Petroleum Institute spectrum of 1,5-hexadiene shows a band at this position. The spectra of carbon disulfide solutions of high trans-polybutadienes, which have this group, have no such band, but spectra of crystalline high trans-polybutadienes do show a band here. It is apparently active only in the crystalline state in trans polymers and disappears completely in solution. Tie behavior of the absorption band due to cis-1,4 addition indicated that no single wave length could be used for analytical purposes. Rather, a measure of the entire absorption band needed to be used. The most logical function would appear to be the apparent integrated absorption intensity. Preliminary work indicated this would be satisfactory, but the time involved in replotting the spectra on an absorbance us. frequency scale made this approach impractical for analyzing large numbers of samples. The function finally chosen is defined by the equation 01
=
log- SIodX f IdX
where IO and I are the intensities of the incident and transmitted radiation, respectively, a t wave length A. This function depends on both the shape and the intensity of the band measured. It is convenient, because both integrals can be determined directly from the recorded spectra. In the particular application described here, this function appeared to be linear with concentration, but no theoretical reason why this is necessarily true has yet been found. The choice of this function must therefore remain arbitrary a t this time. The limits for the integrals in 1 were taken a t 12.0 and 15.75 microns. At these wave lengths, the wings of the band appeared t o have merged with the general background absorption. The values of the integrals were deter-
530
0
ANALYTICAL CHEMISTRY
Figure 1. Spectra of polybutadienes A.
B. C.
mined with a planirrieter. The value of Q! is used as an absoi.bance in this paper. Thus in any subs( quent reference to absorbance in the cis region, the function Q! is to be understood. Selection and Preparation of Samples. Seven samplft; of polybutadiene mere selected for intensive study. Three of these (polymers A, B, and C) were calibration san ples and were high trans, vinyl, and cis polymers. Spectra of carbon disulfide solutions of these three samples betwen 9 and 15.5 microns are given in F yure l . The other four (polymers D, I:, F, and G) were selected to give a reasonable spread of distribution in ordw to check the method. The samples were ,jurified by dissolr= ing them in benzene, filtering, and precipitating them with isopropyl alcohol. The samales were dried under vacuum to remoie any residiial solvent. Solutions were made by wighing the polymer, placing it in a voliimetric flask, and adding carbon disulfide (Merck reagent grade) to a point below the mark on the flask. When the p o l y m r had dissolved,
High frons-1,4 addition High 1,2(vinyl) addition High cis-1,4 addition
carbon disulfide was added to bring the level up to the mark and the flask vias shaken to mix the solution thoroughly. The infrared spectra were obtained immediately in order to minimize oxidation. All concentrations were 25 grams per liter except for polymer B, the high vinyl sample. Solutions of 10 grams per liter were used for this sample, because inore concentrated samples weie too viscous. A numher of samples were prepared by a simpler and less time-consuming procedure. No purification mas attempted. About 1.25 grams of sample mas placed in a bottle and 50 ml. of carbon disulfide was added from a graduated cylinder. When the sample cells were filled, 10 ml. of solution mas removed and placed in a tared vial. The carbon disulfide was evaporated and the concentration m s determined from the weight of the residue. Instrumental Techniques. All spectra were taken on a Perkin-Elmer hIodel21 spectrophotometer equipped with a sodium chloride prism. A slit schedule programmed for constant en-
100
78 0
;fj
2 w 60
5 z
Q
E 40
3
Z
Q
E 20 1 :
I I
I I
,
-01 9
I
\ l i t
U A I
I l l I
b l l
l
10 11 WAVE LENGTH (MICRONS)
+
Figure 2. Estimation of base line for 1,2 addition in a high trans-1,4polybutadiene
crgy was used for all spectra. With the conditions used, the noise level was 0.5% or less and the 100% line was constant within 1% of the full scale. Compensation for the carbon disulfide solvent was obtained by placing in the reference beam a cell of appropriate thickness filled with carbon disulfide. Three sample cells were found necessary. The sample cell thicknesses were 104.6, 520, and 1496 microns. The reference and sample cells were matched to 0.004 absorbance unit.
Calculations. The method described here is based on the assuniption that polybutadiene can be regarded as a simple three-component mixture of trans-1,4, 1,2(vinyl), and cis-1,4 units. The analysis then involves solving a set of three simultaneOLE equations of the form a' =
cia:
+ c& + cas:
(2)
which expresses the absorptivity at each analytical wave length in terms of the fractional concentration and absorptivity of each component. Absorptivity is defined as A/bc where A is absorbance, b is path length, and c is concentration. This set of equations can be inverted by reciprocal matrix methods and after suitable manipulation put in the form cI = (M:a1
+ Ill?az + M2aJ)-a!1
were chosen so that the measured component absorbed strongly and the other components absorbed weakly, relatively large errors can be tolerated in determining the ratios without affecting the final results appreciably. The determination of the term a:/a: can be used as an example of the procedure followed for obtaining values for the six ratios. Here a: and ai are the absorptivities of trans-1,4 addition a t 11.0 and 10.3 microns, respectively. In the spectrum of the high trans-1,4 polymer (polymer A), the background a t 11.0 microns was carefully estimated as shown in Figure 2. I n this way, the absorbance a t 11.0 microns was divided into two parts; A;, due to 1,2(vinyl) addition, and A: A$ due to f r ~ n s - 1 ~and 4 cis-1,4 addition. Similar operations were performed a t the cis position in this polymer, the trans and vinyl positions in the high cis-1,4 polymer and the trans and cis positions in the high 1,2 polymer. It can be shown that
(3)
where the ternis M i involve only the following six ratios
The advantage of using these ratios is that the concentrations of the components cancel. These ratios are used to correct for interference from the remaining components at each analytical band. Bccause the analytical bands
Table 1. Molar Absorptivities in Polybutadienes
Molar Absorptivity, Liters Moles-' Cm.-' Compoient
10.3 p
11.0 p
12.015.75 p
trans-1.4
addition
133
2.4
0.86
1,2(vinyl)
addition
6.7
addition
4.4
184
4.7
n's-l,4
Table 11.
1.9
10.1
Unsaturation Distribution in Some Polybutadienes
Normalized Distribution % % % trans vinyl cis 84.0 2 . 4 8.6
Polymer A B 3 . 3 84.8 6 . 9 C 1 . 2 3 . 3 95.5 D 8 . 2 4 . 6 87.2 E 67.6 3.0 29.4 F 24.5 1 . 6 73.9 G 7 3 . 0 17.8 9.2 Assumed.
Total Found,
%
100.05 100.0" 1oo.oa 101.2 99.4 100.0 100.2
Q
There are similar equations for the other ratios. As a first approximation, terms other than the first were neglected in the numerator and denominator. This is thought justified because A: and Ai are small and the absorptivity ratios are less than unity. First approximations to each ratio were obtained in a similar manner. In further approximations, none of the terms mere neglected, but were calculated using previous approximations for the ratios. Three successive approximations were sufficient to obtain constant values. These values were then substituted in Equations 3. It was assumed that the three calibration polymers were lOOyo unsaturated. This results in an equation for each polymer of the form
where N i is the coefficient of l/d in Equations 3. Equations 5 were solved simultaneously for a:, u;, and a:, Substitution of these values back into Equations 3 gave the final set of equations for determining each type of addition. These equations were then applied to the other polymers studied. DISCUSSION OF RESULTS
In Table I are listed the molar absorptivities determined by the method. Coniparison of these data with those given by H.ampton (S),Binder ( 2 ) , and Richardson (6) shows fairly good agreement for both the major and minor absorptivities a t 10.3 and 11.0 microns. A more valid comparison of the methods
can be made by comparing the unsaturation distribution in Table I1 for polymer G, a 41' F. emulsion polybutadiene, with similar polymers reported by Hampton and Binder. The present method appears to give slightly lower cis-1,4 and slightly higher trans-1,4 addition, Eut agreement is still good. Table I.' lists the unsaturation distribution and total unsaturation found for the purified polymers A to G. It can be seen that the total unsaturation found for polymers D to G was close to 100%. I n subsequent application of this method t o a large number of samples, a tota of 100 =t1% was found in almost all cases. Chemical unsaturation of these polymers was about 95 t o 99%, determined on the unpurified polymer. E ecause the unpurified polymers mere known to contain some nonrubber components such as catalyst and antioxidant, it is felt that the chemical unmturation is somewhat low and the truc unsaturation is probably 100%. If this is true, then the function used t o measure cis-1,4 addition is essentially linear with concentration, because othe wise large deviations from a total of l C O ~ owould be expected in regions of greatest deviation from linearity. Four sets of data were obtained for each polymei, consisting of determinations by tn-o operators on each of two samples. II'ith the exception of vinyl and cis content in polymer B, spreads of 0.0 to 0.5C,G and average deviations of 0.0 to 0.2% were obtained. In general, agree nent of two operators on a single samp e was better than agreement for onts operator on duplicate samples. VOL. 3 1 , NO. 4, APRIL 1959
531
For vinyl and cis content in polymer
B, a spread of 1.470 and average deviation of O.5Y0 were obtained. These large deviations were probably caused by two factors. First, the solution concentrations were only 10 grams per liter as compared with 25 grams per liter for the other polymers. A proportionately larger error in concentration may be expected. Second, the correction for overlap of vinyl addition in the cis region is large, so that errors in measuring the vinyl concentration will affect the cis determination to an appreciable extent. It is difficult to determine the accuracy of this method, because of the assumptions which were made. If these assumptions are valid, however, then the accuracy will depend to a great extent on the accuracy with which the overlap coefficients were determined. It is felt that, with the single exception of the overlap coefficient for 1,2 addition of the cis band, the possible inaccuracy in the overlap coefficients will change the distributions by only 1 to 2%. For polymers containing small amounts of 1,2 addition (
