and aldehydes also interfered, and, to a lesser extent, alcohols. The band a t 2.07 microns was also characteristic of aliphatic formates. For ethyl formate, absorbance of this band followed Beer's law. The concentration range studied was 0.12 mole per liter to 0.72 mole per liter, with an absorptivity of 0.10 liter per molecm. This is a rather weak band, and much larger relative errors were ob-
served-cg., 2.47% for a mixture of gram Of formate and 0.5 gram of ethyl acetate in 25 ml. of carbon tetrachloride. ACKNOWLEDGMENT
The authors thank C. H. DePuy for gas cbomatOgraPhic separations and
NMR analyses of deuterated formates.
LITERATURE CITED
(1) Goddu, R. F., ".2dvances in dnalytical Chemistry and instrumentation,^^ c. N. Reilly, ed., Interscience Publishers, 1960, pp. 374-5. (2) Goddu, R . F., Delker, D. -1., ANAL. CHEM.32, 140 (1960). (3) Pon-ers, R. M., Harper, J. L., Tai,
Han, A N A L . CHEM. 32, 1287 (1960). for review February 16, 1962. RECEIVED Accepted May 14, 1962.
Nuclear Magnetic Resonance Study of Butadiene-Isoprene Copolymers HUNG YU CHEN Research Division,
U. S. Industrial Chemicals Co.,
b Butadiene-isoprenecopolymers in the molecular weight range of 100,000 to 200,000 with varying ratios of butadiene to isoprene were studied by high resolution nuclear magnetic resonance techniques. Well resolved spectra of these copolymers were obtained in carbon tetrachloride solution at room temperature, despite the high molecular weight and the high viscosities of the solutions. Absorption peak assignments were made on the basis of recorded chemical shifts. By measuring the area under each individual peak, the relative concentration of the protons in different molecular environments may be determined. The relative concentrations of the various units (with about 2 to 3% maximum error) for an experimental 1 to 1 butadiene-isoprenecopolymer are presented. The method described is also applicable to polyisoprene and polybutadiene.
S
its discovery, high resolution nuclear magnetic resonance has proved to be an important tool in determining the chemical structure of molecules. However, up to the present time, relatively few articles have appeared in the literature on the application of high resolution NMR to the study of polymers. This situation may stem from the fact that very few polymers have a high enough solubility and a t the same time exhibit narrow enough line width to permit a detailed analysis of their spectra. A general discussion of the application of high-resolution nuclear magnetic resonance to polymers has been given by Bovey, Tiers, and Filipovich (1). I n butadiene-isoprene copolymers, INCE
1 134
e
ANALYTICAL CHEMISTRY
Division o f National Distillers and Chemical Corp., Cincinnati 37, Ohio
magnet and magnetic flux stabilizer. The temperature of the probe was maintained a t 25" 1" C. Polymer soluCHs tions were prepared by dissolving about I 5% by weight of the sample in reagent -CHe-C-CH=CH, (-4) grade carbon tetrachloride. The soluI tions were sealed in a standard 5-mm. tube for the spectrometer with tetramethylsilane as internal standard. The (B) frequency of the audio-oscillator employed was first calibrated a t intervals of 60 cycles per second against the 60CH, C.P.S. power line frequency. The area I under each peak may be measured -CHz-C'=CH-CH2(cis and trans) either by a Varian V-3521 NMR integra(C) tor or by cutting out peak areas from the chart and weighing with an ana-cH~-~x-cH=cH~ 0) lytical balance to the nearest 0.1 mg. Peak shifts with respect to the reference -CH%-CH=CH-CH*(cis and trans) peak were measured by the usual side (E) band technique. Because of some overlapping in the peaks, the weight method The assumption is made that secondwas used for most of the polymers under ary polymerization through double study for best accuracy. Although the bonds in the polymer is negligible. spectrometer was adjusted to avoid radio-frequency (rf) field saturation or Because of the type of polymerization reduce it to a minimum, the signal used, it is unlikely that this reaction strength for the peaks was still great takes place to any appreciable extent. enough for accurate area measurements. A slight change in the relative conCommercial grades of synthetic polycentrations of these units in a copolymer butadienes and polyisoprenes were used affects its physical properties. Hence, as reference compounds without further a reliable method of determining these purification. The natural rubber referrelative concentrations is highly deence was purified by solution in carbon sirable for quality control of the polytetrachloride and centrifuging for onehalf hour a t 2500 r.p.m. to separate the mer. I n this work, a rapid method clear solution from the undissolved gel. for the analysis of our experimental The pure rubber was then precipitated butadiene-isoprene copolymers by high by adding a large volume of acetone to resolution nuclear magnetic resonance the clear solution. The precipitated is described. After solution, 2 to 3 white gum was collected and dried under hours are required for the analysis of a vacuum. Other pure hydrocarbons group of six samples. used as references are listed in Table I.
butadiene and isoprene units are found in the following forms :
*
EXPERIMENTAL
RESULTS A N D DISCUSSION
Spectra were obtained a t a frequency of 60 megacycles per second with a Varian V-4302 DP-60 NMR spectrometer equipped with a 12-inch electro-
The spectra of butadiene-isoprene copolymers with a butadiene-isoprene ratio of 10 to 1, 4 to 1, and 1 to 1 are
Table 1.
Peak NO.
1
2
Peak Assignments for 1 to 1 Butadiene-Isoprene Copolvmer
Measured Peak Shift, P.P.M. from Tetramethylsilane 5.30
5 03
Table II. Composition of 1 to 1 Butadiene-Isoprene Copolymer
Av. Chem.
Type of Proton -CH=CHz
-CH=CH-CH=CHz
Reference Compound 1,5-Hexadiene 1-Octene 1-Hexene 1-Pentene cis- and trans-polybutadiene l15-Hexadiene 1-Octene 1-Hexene 1-Pentene
Shift of Ref. Compound, P.P.M. from Tetramethylsilane 5.56
Concentration Found, Mole
I
-CHz-C-CH=
I
CHz
I
8, 10, 9
CHa
I
5.31 4.86
CH- C-CH-CHz-
14, 16, 15
I
iH3
-CHz=CH-CHz-CHz-CH-CH=CHz
34, 30, 32
-CHZ-CH=CH-CHz-
40, 43 , 43
4, 1, 1
I
CH3 -CH=C-
72
Group CIIa
Natural rubber
5.05
2-Ethyl-I-butene 2,4,4-Trimethyl-l-pentene cis- and trans-polybutadiene Natural rubber ..
4.63
...
...
CHJ
I
3
4.65
CHFC-
4 5 6 7
1.98 1.58 1.26 0.92
CH-C=C CHpC=C CH-C-C CHa-C-C
shown in Figure 1. For the 1 to 1 copolymer, there are seven well resolved peaks in two distinct groups. Peaks 1, 2, and 3 on the low field side are due to various protons directly attached to olefinic carbons, while peaks 4, 5, 6, and 7 on the high field side are due to the remaining protons in the polymer
l0:l BUTADIENE-ISOPRENE COPOLYMER
1.98 1.68 ...
molecule. Detailed assignment of these peaks was made by comparing the measured peak shifts to the chemical shifts of the reference compounds of known structure described above, These assignment,s are listed in Table I. The area under an absorption peak is directly proportional to the concentrations of protons contributing to it, provided y2H21T1Tz
