Termonomer Analysis in Ethylene Propylene Terpolymers A. G. Altenau, L. M. Headley, C. 0. Jones, and H. C. Ransaw The Firestone Tire & Rubber Company, Central Research Laboratories, Akron, Ohio 44317
POLYMERIZATION of ethylene and propylene results in a saturated copolymer. In order to vulcanize this rubber, some unsaturation has to be introduced. This is commonly done by adding a few per cent of a nonconjugated diene (termonomer) such as dicyclopentadiene, 1,Chexadiene, or ethylidene norbornene, during the polymerization. Since only one of the double bonds of the diene reacts during polymerization, the other is free for vulcanization. The amount of unsaturation left in the ethylene propylene terpolymer (EPDM) is of great interest because the vulcanization properties will be affected. The various ways of determining double bonds in EPDM have been reviewed (I). The iodine monochloride method has been used for a variety of polymers. These polymers include those which are highly unsaturated, such as polybutadiene and polyisoprene (2-4), and polymers having low unsaturation such as butyl rubber (5)and EPDM ( I ) . Considerable work has been done investigating the side reactions of iodine monochloride with different polymers (5). These side reactions are substitution and splitting out rather than the desired addition reaction. Infrared has been used for determination of unsaturation in EPDM ( I ) . Determination of extinction coefficients for the various termonomers is required if quantitative work is done. Pyrolysis gas chromatography has been used by many workers to determine the overall composition of EPDM (I). In attempting to determine the third component in EPDM, difficulties might be anticipated, since this component is normally present in amounts around 5 weight per cent. However, dicyclopentadiene was identifiable in EPDM, even when the amount incorporated was so small it was of no practical value ( I ) . Chen and Field (6) have shown that the structure of the small amount of isoprene incorporated into butyl rubber can be determined by time averaging NMR. Sewell and Skidmore (7) have identified the diene (termonomer) in ethylene propylene rubber by the characteristic chemical shifts of four different termonomers by time-averaged NMR spectra. This work introduces a time-averaging NMR method for determining the per cent termonomer in EPDM. Comparison of values obtained by this NMR method and the iodine monochloride method is made. The chemical shifts and splitting pattern of the olefinic response help identify the termonomer. Identification of the termonomer can also be made by infrared, which is briefly discussed in this work. The three termonomers studied are those which are most widely used in industry: 1,4-hexadiene, dicyclopentadiene, and ethylidene norbornene. (1) R. Hank, Rubber Chem. Technol., 40, 936 (1967). ANAL.ED., (2) A. R. Kemp and G . S. Miller, IND. ENG.CHEM., 6, 52 (1943). (3) A. R. Kemp and H. Peters, ibid., 15, 52 (1943). (4) J. Rehner, Ind. Eng. Chem., 36, 118 (1944). 22, (5) T. S. Lee, I. M. Kolthoff, and E. Johnson, ANAL.CHEM., 995 (1950). ( 6 ) H. Y . Chen and J. E. Field, J. Polym. Sci., Part B , 5 , 501 ( 1967). (7) P. R. Sewell and D. W . Skidmore, J . Polyrn. Sei., Part A - I , 6, 2425 (1968). 1280
0
EXPERIMENTAL
Reagents. Reagent grade iodine monochloride was obtained from Matheson, Coleman & Bell and used as received. Apparatus. All samples except those analyzed by infrared were extracted overnight with acetone, using a Soxhlet apparatus. A Varian HA-60-IL nuclear magnetic resonance spectrometer was used to record the spectra, and a Varian (2-1024 time-averaging computer was used for the time-averaging work. A Beckman IR-4 was used to record the infrared spectra. Procedure. IODINEMONOCHLORIDE METHOD. Unsaturation values on the different EPDM samples were determined using the procedure of Lee, Kolthoff, and Johnson (5). Although this method was developed for butyl rubber, it is also used with EPDM polymers because of the similarities in structure and unsaturation levels. Unsaturation values were also determined in some of the EPDM samples using Kemp and Peters method (2, 3). NMR METHOD.The purified sample is dissolved in carbon disulfide containing TMS. The concentration of the solution is about 5 %. Only signals from the ethylene and propylene are seen in the NMR spectrum. No signals from the termonomer are observed in the NMR spectrum under normal operating conditions because of its low concentration. Therefore, for NMR to detect protons of the termonomer, time-averaging is used. The olefinic proton signals of the termonomer are well separated from the ethylene and propylene signals and are therefore ideal protons to time-average for qualitative and quantitative purposes. The ethylene and propylene signals are time-averaged for 10 scans. Then with an offset of 190 Hz and a 250-Hz sweep width, the olefinic protons of the termonomer are timeaveraged for 100 to 200 scans. This number of scans was generally sufficient to obtain a large enough response for accurate area measurement of the olefinic signals. The output signal attenuation settings in the instrument were calibrated since different attenuations were used in the timeaverage spectra of the ethylene and propylene protons and the olefinic protons of the termonomer. The termonomer protons were time-averaged at an output sensitivity ten times greater than the time-average spectra of the ethylene and propylene protons. An example of the calculation of per cent termonomer from time-averaged spectra follows. The time-averaged response of the ethylene and propylene signals is 25.5 and the time-averaged response of the olefinic signals of the termonomer is 0.044. These responses were measured by a planimeter. Both of these responses have been adjusted for the number of scans and any attenuation differences, and the termonomer response has been divided by the number of olefinic protons. The total number of ethylene and propylene protons per termonomer proton is 580 which is the quotient of 25.5 divided by 0.044. This ratio is independent of the ethylene to propylene ratio because each monomer has the same carbon to hydrogen ratio, and therefore all combinations of ethylene and propylene have the same carbon to hydrogen ratio, There are two hydrogens for each carbon atom. Since there are 580 protons from ethylene and propylene, it follows that there are 580 divided by 2 or 290 carbon atoms per proton of termonomer. The sum of the products from multiplying the number of carbon
ANALYTICAL CHEMISTRY, VOL. 42, NO. 11, SEPTEMBER 1970
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Figure 1. Time-averaged NMR spectra of E P D M containing one of the following termonomers: Upper spectrum: 1,eHexadiene Middle spectrum: Dicyclopentadiene Lower spectrum: Ethylidene norbornene
and hydrogen atoms by their respective atomic weights gives the number of grams of ethylene and propylene per proton or mole of termonomer. 290 x 12 = 3480 grams of carbon per mole of termonomer 580 X 1 = 580 grams of hydrogen per mole of termonomer 3480 580 = 4060 grams of ethylene and propylene per mole of termonomer Therefore the weight per cent of termonomer is equal to mol wt termonomer x 100 % termonomer = 4060 mol wt termonomer
+
+
It was previously mentioned that there are two hydrogens per carbon atom. Multiplying the weight of two hydrogen atoms by seven gives the weight of a methylene group. It follows then that multiplying 25 which is the total num'
0.044'
ber of ethylene and propylene protons per proton of termonomer by seven gives the total grams of ethylene and propylene per mole of termonomer. The above formula reduces to mol wt termonomer x 100 % termonomer = 25.5 (7) mol wt termonomer 0.044 The response of the aliphatic protons of the termonomer was not subtracted from the response of the ethylene and propylene protons because it did not contribute significantly to the overall aliphatic proton response.
+
RESULTS AND DISCUSSION
This study began with the intention of developing a method of identifying the termonomer in EPDM by the chemical shift and splitting pattern of its olefinic protons. Since the olefinic response was found to be sizeable with a reasonable
,
1
1) i
-
number of time averaged scans, it appeared that this technique could be useful for both identification and quantitative determination of the termonomer. Sewell and Skidmore (7) have previously shown that the termonomer can be identified by time-averaged NMR spectroscopy. They noted that very accurate chemical shifts cannot be determined in time-averaged NMR spectra because of the possibility of magnetic field drift during the scanning time. In order to increase the accuracy of the measurement of chemical shifts, we added trichloroethylene and p-dioxane to the carbon disulfide solution of the EPDM. The chemical shifts of the trichloroethylene and p-dioxane are 6.4 and 3.5 ppm, respectively. The signals of the olefinic protons of the termonomers are between these reference signals. The spectrum is time-averaged from the trichloroethylene to the p-dioxane signals for 100 to 200 times. This number of scans is necessary to obtain an adequate response of the olefinic protons of the termonomer. The chemical shifts of the termonomer peaks are accurately measured using the known chemical shifts of the reference compounds. Figure 1 shows the timeaveraged spectra of EPDM's containing either 1,Chexadiene, dicyclopentadiene, or ethylidene norbornene. Infrared. Infrared can also be used to identify the termonomer. A thin film, approximately 0.005 inch, of the unextracted EPDM is prepared in a laboratory press at 150 "C. The film is mounted on a film holder and the infrared spectrum recorded. Figure 2 shows infrared spectra of EPDM
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Table I. Determination of Termonomer in EPDM-Comparison of Methods (Data Shown as Weight Per cent Termonomer) Lee, Kolthoff, Kemp and Identification of termonomer NMR and Johnson Peters 7.3 5.0 Dicyclopentadiene 3.8 4.7 Dicyclopentadiene 1.1 1.6 , . . 1,4-Hexadiene 4.8 6.6 ... Dicyclopentadiene 4.1 4.3 ... Dicyclopentadiene 1.9 2.9 ... 1,4-Hexadiene 2.0 1.8 ... Ethylidene norbornene 9.6 7.7 ... Ethylidene norbornene 9.0 10.4 ... Ethylidene norbornene 4.6 4.8 ... Ethylidene norbornene 5.7 5.9 9.0 Ethylidene norbornene 2.8 3.6 4.5 Ethylidene norbornene 1.7 2.3 4.8 Ethylidene norbornene 4.6 5.4 6.0 Ethylidene norbornene I
.
.
. I .
containing each of the above mentioned termonomers. Ethylidene norbornene has a broad band at 12.35 p ; dicyclopentadiene has a broad band at 14.3 p and a weak band at 10.55 p ; and 1,Chexadiene has a strong sharp band at 10.35 p and a weak band at 11.25 p. Although these termonomers have absorptions at other wavelengths, we have found the above wavelengths to be the most useful for identification. Quantitative Analysis of Termonomer. The Kemp and Peters method is a n accurate way of determining unsaturation in highly unsatuated polymers. The method, however, generally gives high results on rubbers with low unsaturation, such as butyl rubber. Lee, Kolthoff, and Johnson ( 5 ) introduced a n unsaturation
procedure for butyl rubber which minimized the effect of branching of the polymer during the reaction of IC1 with the polymer. Side reactions due to branching of the polymer were the explanation for the high results obtained on butyl rubber using the Kemp and Peters method. Table I compares the amount of termonomer found by this N M R method to the Kemp and Peters and Lee, Kolthoff, and Johnson methods. Actually the per cent unsaturation is found by the latter two methods and the weight per cent of termonomer is calculated from the unsaturation data. The first three samples in Table I were prepared in our laboratory and the remaining ones were commercial samples from different manufacturers. The termonomers were identified by N M R and infrared. Table I shows that the data obtained by the N M R method agree more closely with the Lee, Kolthoff, and Johnson method than with the method of Kemp and Peters. The difference between the latter two methods is best explained on the basis of side reactions occurring between the IC1 and polymer because of branching near the double bond ( 5 ) . The reason for the difference between the N M R and Lee, Kolthoff, and Johnson methods is not clear. The reproducibility of the N M R method was *lo t o 1 5 z . Although side reactions were minimized with butyl rubber by the Lee, Kolthoff, and Johnson method, this may not be the case with EPDM. Reaction of IC1 with EPDM's containing different terrnonomers could differ and consequently lead to varying side reactions. Different termonomers should not effect the NMR results. RECEIVED for review March 13, 1970. Accepted June 15, 1970. The authors thank the Firestone Tire and Rubber Company for permission to publish this work.
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Spectrophotometric DeterminationI of Sulfur Dioxide Amir Attari,l T. P. Igielski, and Bruno Jaselskis Department of' Chemistry, Loyola University, Chicago, Ill.
MICROAMOUNTS of sulfur dioxide are commonly determined by the West-Gaeke and modified colorimetric methods (1-7). These methods are primarily based on the use of acid bleached pararosanaline in the presence of formaldehyde with sulfur dioxide. All of these methods are quite sensitive and specific, but require very close controls t o achieve satisfactory precision. Stephens and Lindstrom (8) recommended an alternative method for the determination of sulfur dioxide based on the 1
Present address, Institute of Gas Technology, Chicago, Ill.
(1) P. W. West and G. C. Gaeke, ANAL.CHEM., 28, 1816 (1956). (2) P. W. West and F. Ordovea, ibid., 34, 1324 (1962). (3) A. J. Steigmann, J. SOC.Chern. Znd., 61-18 (1950). (4) S. Atkin, ANAL.CHEM., 22, 947 (1950). (5) P. F. Urone and W. E. Boggs, ibid., 23, 1517 (1951). (6) R. V. Nauman, P. W. West, F. Tron, and G. C. Gaeke, ibid., 32, 1307 (1960). (7) F. P. Scaringelli,B. E. Saltzman, and S. A. Frey, ibid., 39, 1709 (1967). (8) B. G. Stephens and F. Lindstrom, ibid., 36, 1308 (1964). 1282
reduction of iron(II1) by sulfur dioxide in the presence of 1, 10-phenanthroline and the subsequent formation of tris-1 , 10-phenanthroline iron(II), Fe(II)phen8*+. This method yields satisfactory results when the reduction is carried out at 50 "C. In the Stephens-Lindstrom method the amount of Fe(I1)phen32+ produced in the reaction of the Fe(II1) varies not only with the amount of sulfur dioxide but also with the temperature. In our laboratory we have modified the StephensLindstrom method by introducing acetate ion for the formation of a labile iron(II1) acetate phenanthroline species and, furthermore, we elucidated the stoichiometry of the reaction at various temperatures. EXPERIMENTAL Apparatus. A Cary Model 14 and Beckman DB Spectrophotometers were used for the absorbance measurements. The p H of the solutions was determined by a Corning Model 12 research p H meter. Known amounts of sulfur dioxide were introduced into the absorption flask either from a standard aqueous solution or from a gas flow system shown in Figure 1. The absorption train for the standard sulfur
ANALYTICAL CHEMISTRY, VOL. 42, NO. 11, SEPTEMBER 1970
