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 copolymerized 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 concerning which of these configurations was the prevalent one in butyl rubber was investigated by Rehner in 1944. 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 1,4-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 detect such low concentrations of isoprene in the butyl rubber, the present NMR result seems to be the only direct observation of this structural component. We would also like to point out that such measurements may be useful in determining quantitatively the isoprene unit in butyl rubber as well as other polymers with minor constituents such as the third monomer unit in ethylene propylene terpolymer.