POLYMER LETTERS VOL. 5, PP. 501-503 (1967)

CONFIGURATION OF ISOPRENE STRUCTURE IN BUTYL RUBBER BY TIME-AVERAGING HIGH RESOLUTION

NUCLEAR MAGNETIC RESONANCE

In making butyl rubber, a small amount of isoprene is usually copoly- merized with the isobutylene to facilitate vulcanization. The isoprene component thus incorporated in the butyl rubber can be present in three possible configurations, i.e., 1,4-, 1,2-, and 3,4-. The question concern- ing which of these configurations was the prevalent one in butyl rubber was investigated by Rehner in 1944 (1). His chemical method was based on the fact that only butyl rubber with isoprene units in 1,2- or/ and 3,4- configurations would yield formaldehyde and formic acid on ozonization. Since these compounds were not detected in this reaction product, he concluded, indirectly, that the isoprene segment in the butyl rubber was essentially in the l,.I-configuration.

In this communication, we would like to report that, by means of a time-averaging high resolution nuclear magnetic resonance method, we are able to observe the absorption peak due to the 1,4-isoprene unit in a typical commercial grade butyl rubber. Since the IR has failed to de- tect such low concentrations of isoprene in the butyl rubber ( 6 ) ) the pre- sent NMR result seems to be the only direct observation of this struc- tural component. W e would also like to point out that such measure- ments may be useful in determining quantitatively the isoprene unit in butyl rubber a s well a s other polymers with minor constituents such a s the third monomer unit in ethylene propylene terpolymer (4).

The sample used for this measurement was Enjay's commercial grade butyl rubber #325 with nominal isoprene content 2.2 mole-% (2). It was purified twice by precipitation in methyl alcohol and dried under high vacuum a t room temperature. B & A Instrument grade carbon tetrachlo- ride (Allied Chemical Co.) was used for preparing the sample solution. The concentration of the solution was approximately 3 moles of mono- mer unit per liter of solution or approximately 12% by weight. The time- averaged spectra were obtained in a manner described previously (3,4). The spectra are given in Figure 1. The bottom traces in the figure are single scan recordings. The peak a t approximately 1.1 ppm from the reference peak of tetramethylsilane (TMS) is assigned to the two methyl groups, and that at approximately 1.4 ppm to the methylene group of the isobutylene unit in the butyl rubber. No detectable olefinic peak was observed in the single scan recordings because of the extremely low concentration of the isoprene component in the sample. By using the time-averaging technique, the signal strength was enhanced about ten t imes. The weak peak appearing at approx 5.1 ppm in the top trace of

50 1

502 POLYMER LETTERS

I 1 I I . . , _ _ I . I . . . I I 6.0 5.0 U M W 4.0 1.0 2.0 1.0 0

Fig. 1. NMR spectra of the butyl rubber (Enjay 325). Two top traces are time-averaged NMR spectra of the butyl rubber solutions in carbon tetrachloride a t different gain settings. Two bottom traces are single scan spectra of the same solution; the upper trace was made with the same instrumental conditions a s used for the upper trace of the time- averaged spectra.

the time-averaged spectra in the figure is due to the olefinic hydrogen of the isoprene unit in the butyl rubber. From its peak shift, as well as its shape, it shows that practically all of the isoprene unit in the butyl

POLYMER LETTERS 503

rubber sample is in 1,4-configuration ( 5 ) . It is also possible to estimate the intensity ratio of the peaks from the time-averaged spectra. From the peak ratio, it was determined that the concentration of the isoprene unit was 2 mole-% which s e e m s to be in reasonable agreement with the manufacturer’s specification (2.2 mole-%).

References

(1) J. Rehner, Jr., Ind. Eng. Chem , 24, 46 (1944). (2) The Vanderbilt Rubber Handbook, R. T. Vanderbilt Company, Inc.,

(3) H. Y. Chen, J. Polymer Sci. B, 4, 891 (1966). (4) H. Y. Chen and J. E. Field, Pittsburgh Conference on Analytical

Chemistry and Applied Spectroscopy, March 5 , 1967. ( 5 ) H. Y. Chen, Anal. Chem., 2, 1134 (1962). (6) L. R. McNall and L. T. Eby, Anal. Chem., 22, 951 (1957).

New York, 1958, 92 pp.

Hung Yu Chen J. E. Field

Research Division The Goodyear Tire & Rubber Company Akron, Ohio 44316

Received March 6, 1967