(XXIII) is a t 124 and is likely due to to the following cleavage with hydrogen rearrangement :
dH3
are due also to James Nelson who ran gas chromatographic analyses on most of these compounds.
CH3 I
C=O
HO
A/U
A 16p-methyl group also fosters this cleavage. D-ring cleavages lose their diagnostic value in certain types of compounds. A double bond between C-16 and C-17, for example, reduces the peak intensities to the extent that they may not be noticed. The C-13-C-17 bond is strengthened in these compounds by its vinylic placement. Similarly compounds with a 16,17-epoxy group do not show a clean D-ring cleavage, but do show a promi0
It
nent peak from loss of CH3C-and H20 from the D-ring. I n general, B- and C-ring cleavages in these compounds studied yielded peaks which were less valuable in characterization than the D-ring cleavages. An exception was found in progesterone and its derivatives. Thus, the strongest peak in progesterone
Similarly, the base peak in progesterone derivatives with a 6-methyl group (as XXIV and XXV) is a t 138. Progesterone and its derivatives also show the loss of a fragment of mass 42 which is believed to be C2H20from the A ring. Other characteristic fragments are identified by footnotes in the tables of mass spectra. ACKNOWLEDGMENT
The author is grateful to Robert Graber and other members of the Steroid Research Department of General Mills for providing samples and purity tests by thin layer chromatography. Thanks
LITERATURE CITED
(1) Bergstrom, S., Ryhage, R., Stenhagen, E., Acta Chem. Scand. 12,1349 (1958). (2) Bergstrom, S., Ryhage, R., Stenhagen, E., J. Lipid Res. 1,361 (1960). (3) Bergstrom, S., Ryhage, R., Stenhagen, E., Svensk Kem. Tidskr. 73, N o . 11, p. 566 (1961). (4) Beynon, 0. H., “Mass Spectrometry
and ita Application to Organic Chemistry,” p. 336, Elsevier, Amsterdam, 1960.
(5) Budeikiewice, H., Djeraasi, C., J. Am. Chem. SOC.84, 1430 (1962). (6) deMayo, P., Reed, R. I., Chem. and Ind. 1956,1481. (7) Friedel, R. A., Sharkey, A. G., Jr.,
ANAL.CHEM.28,940 (1956). (8) Friedland, S., Lane G., Longman, R., Ibid., 31, 169 Train, K., O’Neal, (1959). (9) Peterson, L. E., ANAL. CHEM. 34, 1850 (1962). (10) Peterson, L. E., Chem. & Ind. 1962, 264. (11) Reed, R. I., “Ion Production by Electron Im act,” pp. 195-9, Academic Press, New f o r k , 1962. (12) Reed, R. I., J. Chem. SOC.1958, 3432.
h.,
RECEIVEDfor review June 27, 1962. Accepted October 15, 1962.
Determination of cis-1,4 and frans-1,4 Contents of Polyisoprenes by High Resolution Nuclear M a gnetic Resonance HUNG YU CHEN Research Division, U. S. Industrial Chemicals Co., Division of National Distillers and Chemical Corp., Cincinnati 37, Ohio
b A method is described for the determination of the cis-1,4 and trans1,4 contents of 1 ,I-polyisoprene by nuclear magnetic resonance techniques. The error of the method is estimated to be less than 0.5%. The method can be extended to include the determination of the 1,2 and 3,4 contents of polyisoprenes. The error in these cases is estimated to b e between 2 and 3%. The results obtained with three synthetic polyisoprenes are presented.
I
1,4, 1,2, and 3,4 additions are possible, leading to the following structures: CH, ‘C=C /H I -CH/ \CHzcis-1,4 N ISOPRENE,
trans-] ,4
CH,
I
CHz=CCH-CHz-
I
I11
3,4 CHS I
-cH~--C-CH=CH~
IV
I
122
For the past few years infrared methods have been widely used for the determination of the microstructure of polyisoprene ( 1 , S , .&@. These methods are satisfactory for the determination of 1,2 and 3,4 addition units, but as pointed out by Corish (3)they leave much to be desired in the determination of cis- and trans-1,4 repeating units. The reasons for their poor accuracy and precision have been discussed by Tobolsky and Rogers (6). The low molar absorptivities of the bands used for the determination of the cis- and trans-1,4 repeat-
ing units are one of the principal sources of error. Slight errors in absorbance measurements produce considerable variations in the final results. In the high resolution NMR method described here, well resolved bands of high intensity are used for the determination of the cis- and trans-1,4 contents of 1,4-polyisoprene. Because of the high sensitivity of the method, even relatively low concentrations can be determined with an error of less than 0.5%. EXPERIMENTAL
Spectra were obtained at a frequency of 60 Mc. per second with a Varian V-4302 DP-60 NMR spectrometer equipped wLth a 12-inch electromagnet and magnetic flux stabilizer. The temperature of the probe was maintained a$ 25’ f 1’ C. Polymer solutions were prepared by dissolving from 1 to 501, by weight of the samples in the reagent grade solvents used in the VOL. 34, NO. 13, DECEMBER 1962
1793
investigation. The solutions Fvere sealed in a standard 5-mm. tube for the spectrometer with tetramethylsilane as internal standard. Peak shifts and areas were measured as described previously ( 2 ) . Commercial grades of Hevea and balata rubber consisting mainly of cis-lJ4-polyisoprene and trans-1,4-polyisoprene, respectively, were purified as described (d), using benzene instead of carbon tetrachloride as a solvent. A polyisoprene high in 3,4 unit content was prepared in this laboratory by polymerization of isoprene in ether with sodium as a catalyst as described by Tobolsky and Rogers (6). 2,4-Dimethyl-lJ5-hexadiene was purchased from Benzol Products Go., Kewark,
N. J.
I
1
1
I
I
I
2AO 2.20 2.00 LBO 1.ea CHEMICAL SHIFT IN RRM. FROM TETRAMETWLSILANE
I
Figure 1. Proton magnetic resonance spectrum of cis- and trans- lI4-polyisoprene mixture 1.
2.
Peak due to cis-methyl proton Peak due to fmns-methyl proton
RESULTS AND DISCUSSION
The high resolution SRIR spectruni of a polyisoprene solution in carbon tetrachloride containing units I, 11. 111, and IT' consists of seven groups of peaks TI ith the peak shifts determined previously (2). One of the group of peaks located a t about 1.75 p.p.m. from tetramethyl-
Table I. Peak Shifts of Methyl Protons in Mixtures of cis- and trans-1 14Polyisoprene
Peak shift from tetramethylsilane, p.p.m. ~is-1~4trans-l,4- DifferSolvent methyl methyl ence Carbon tetrachloride 1.67 1.59 0.08 Carbon disulfide 1.62 1.54 0 08 Benzene 1.79 1.65 0.14
Table It. Calculated and Measured Composition of Synthetic 1,4-Polyisoprene Mixtures
Solution 1 2 3 4
cis-1,4 content, mole yoa Calcd. Found 22.62 39.47 58.58 67.05
22.81 39.71 58.44 66.48
a Mole per cent cis-1,4 plus mole per cent trans-1,4 = 100.
Table 111.
Analysis of Synthetic Polyisoprenes
(Mole per cent) cistrans1,2 1,4 1,4 Con3,4 Sample Content Content tent Content
1794
ANALYTICAL CHEMISTRY
4 a n e was bhon 11to be due t o the methyl protons in I, I1 and 111. The spectrum obtained nith a synthetic mixture of cis- and trans-1,4-natural rubber using a s101r magnetic field sxeep rate showed that the two methyl groups in I and 11 n cre not completely equivalent. Under conditions of high resolution two well resolved pcahs wcre 0bser.r ed. The degree of separation oi these t v o peak> corresponding to the cis-l,4 and trans-1,4 contents was solvent dependent, presumably because of slight differences in solvent effectq on the methyl groups. The peak shifts of the methyl protons in mixtures of Heyea and balata rubber in different solvents are shown in Table I. Benzene vas the solvent of choice because it gave the greatest separation of the tIvo peaks and n as the best sohent of the three for polyisoprene. The peak separation n a s not dependent on the solvent concentration. .i sixteenfold dilution of a 57, solution of cis- and trans-1,4-polyisoprene n ith benzene had no significant effect on the difference in peak shift. A part of the high resolution S l I R spectrum of an approximately 37, mixture of Hevea and balata rubber in benzene using a slow magnetic field sneep rate is shonn in Figure 1. The cisand trans-l,4-niethyl peaks are clearly resolved. The methyl peaks are relatively narrow conipared t o most proton peaks obtained with polymers. The line nidth measured from half the maximum height was 3 cycles per second, which is equivalent to 0.05 p.p.m. nith less than 5% overlapping of the peaks. K i t h a little experience, the overlapping portions of the peaks can be sepalatecl very accurately by viqual judgment. Since the concentrations of the cis- and trans-1,4 repeating units are direct!) proportiona! t o the areas of the correy~ondingpeaks, the concentrations of thcse repeating units can be determined by accurate measurement
of the t n o peak areas. The results of analysis of four synthetic mixtures of He\.ea and balata rubber in benzene are shown in Table 11. Under the conditions used, the average error in determining the cis- and trans-1,4 content of 1.4-polyisoprene was less than 0.5%. The accurate peak shift of the methyl group of I11 nas determined by using 2,5-diniethy1-1,5-hexadiene and a polyisopienc high in 3,4- content in benzene as reference compounds. Within the limits of experiments1 crror, this methyl peak TV:E found t o have the same peak shift value as the methyl peak of the trans-1,4 unit. This was confirmed from the spectra of mixtures of these references and balata rubber. K i t h the tletmnination of this peak shift, the method can be extended to the analysis of polyisoprer es containing all four types of addition units. The 1,4, 1.2, and 3,4 unit contents are determined by the previously described procedure (2). The ~ i s - 1 ~and 4 trans-1,4 plus 3,4 unit contents are then determined by the present method. From these data, the total composition can easily be calculated. The results obtained by application of the two procedures to three polyisoprenes synthesized JT ith alfin and alkali metal catalyst systems are shonn in Table 111. The d u e s are averaged froin several spcctra obtained n ith the sanip sample. The error is eqtiniated to be 2 to 3% ( 2 ) . The S l I R method described here has. several adyantages over existing infrared procedures for the determination of cis- and trans-l,4 contents of 1.4-polyisoprene>. The sensitivity and accuracy of the method arc consideraldy better, because of the high i n t e n d y obtained froin the methyl group protons nhich account for almost 407. of t h e total proton content of the polymer. The peaks are n ell resolved, so that peak areas can be measured accurately. The problem of radio-frequency pon er snt-
uration described hy Chen (8) is not critical, as the relaxation constants of these methyl groups should be very close, because of the similarity of their molecular environments. Therefore, high radio-frequency power can be used t o get high signal to noise ratios for the determination of low concentrations. For polyisoprenes containing all Sour structural units, the present method can be comhincd with the NMR method described previously (2) or one of the infrared methods, which are satis-
factory for the determination of the 1,2 and 3,4 Unit contents of polyisoprenes (1,S, 4 , 5 ) . Thus polyisoprenes contnining a wide range of 1,4, 1,2, and 3,4 repeating units can he analyzed with an estimated over-all error of about 2 to
3y0.
LITERATURE CITED
(1) Binder, J. L., Ransaw, H. C., ANAL.
CHEM.29,503 (1957). (2) Chen, H. Y.,lbid., 34, 1134(1962). (3)33, Corish, 975 (1960), P. J., Rubber Chem. & Technol. (4) Cunneen, J. I., Higgins, G. M. C., Watson, W. F., J . Polymer Sci. 40, 1
(1959).
ACKNOWLEDGMENT
The author thanks E. G . Pritchett and William Hoffman for synthesizing some of the polyisoprene samples used in this work.
( 5 ) Richardson, W. S., Sscher, A. J., Ibid., 10,353 (1953).
(6) Tobolsky, A. V., Rogers, C. E,, Rubber Chem. & Techml. 33,655 (1960). R~~~~~~ for review 25, 1962. Accepted October 8, 1962.
Determination of the Surface Population of Copper Oxide Whiskers by Electron Microscopy Techniques WILLIAM R. LASKO and WARREN K. TlCE Research Laborotories, United Aircraft Carp., East Hartford, Cann.
b Two electron microscope techniques are discussed for the characterization of unusual oxide whisker growths formed on copper. The direct transmission technique has been employed b y many investigators to determine the size, shape, and population of the growths. However, this technique is somewhat limited in affording an accurate population and shape estimation. The inability to make a true whisker count can be attributed in part to the masking error introduced b y the whiskers appearing in the foreground of the specimen surface and the inability to make a calculation of an exact areo. Other factors such as surface roughness and grain orientation, which also influence growth, cannat be observed b y this technique. In addition, the shape of the whisker is represented b y a silhouette which provides information in only two dimensions. On the other hand, the indirect or carbon replication technique employing selective etchantr permits a more quantitative estimation of the whisker population and reveals the true whisker shape.
ground of the specimen and because of the errors introduced by grain orientation (7) and surface roughness. In the proposed carbon replication technique employing selective etchants, a more accurate method of determining the surface population of whiskers is afforded. The technique also permits
/
,.. . ?
T
(8, 6) in the field of whisker technology, particularly in t.he attempts (S, 4) made t o postulate the mechanism of growth and the relation of this growth process t o the dislocation concept, has indicated the need for a new approach t o the measurement of whisker population. In the past, these population counts have been made using the direct-transmission technique from whiskers emanating from wires, holes in disks, slotted disks, etc. These measurements are subject to error because of thr masking effect introduced by the whiskers originating in the foreHE RLXEWED ACTIVITY
lhl
Figure 1. Oxide whiskers formed on copper in air at 400' C. for 30 minutes
( 15,000 X )
(I. b.
Direct tranrmirrion Replication
the observation of the true whisker shape as well as providing basic informs, tion about the nature of the growth site.
SPECIMEN PREPARATION TECHNIQUES
In preparing the specimens for this study extreme care was exercised t o ensure that the specimen surfaces examined by the transmission and replication techniques were similar. However, the area available for study using the replication technique was considerably larger than that afforded by the transmission method. Details of each of the techniques follows: Direct Transmission. I n the directtransmission techniaue disks 0.3 cm. in diameter were dnnched out of a 0.0152-cm. sheet of high purity (99.999'%j copper and a slot was prepared with a No. 6 jewelers' saw. T h c disks were degreased and annealed in argon at 4.50' C. for 4 hours t o eliminate the stresses induced during the preparation of the slotted disk for both techniques. In, addition, the disks were cleaned by etching with a 507, solution of HNOl followed by reduction in 507, HC1 and subsequent rinsing in demineralized water. This step was undertaken to ensure that all impurities and asperities introduced during cutting and annealing mere removed as well as t o make the surfaces for whisker growth homogenous. Whiskers were grown in the area of the slot by containing the disks in a porcelain boat and exposing to air for 30 minutes at a temperature of 400' C. A Hitachi HU-11 electron microscope was used to examine the growth products, and normnl bright fieldimages, using the 75-kv. beam, wcre taken a t 3000X and enlarged optically t o 15,000X. I n Figure IQ an clcctron micrograph of typical oxide growth products formed VOL. 34, N O , 1 3 , DECEMBER 1962
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