Nuclear magnetic resonance study of the hydrobromination reaction and the microstructure of hydrobrominated natural rubber

Abstract

The hydrobromination of natural rubber was studied by 1H and 13C nuclear magnetic resonance spectroscopy and resonances of partially hydrobrominated rubber were assigned on the basis of triads and dyads. Reaction of natural rubber with HBr in a non-polar solvent to low conversions did not hydrobrominate adjacent double bonds and caused an appreciable amount of cis-trans isomerization due to steric hindrance. Reaction in the presence of a Lewis acid (AlBr3) occurred more rapidly and caused cyclization rather than isomerization. In dichloromethane hydrobrominated rubber underwent E2 elimination to produce exomethylene groups. Contrary to expectations, addition of peroxides did not cause substitution by bromine at tertiary carbon atoms but produced the normal substitution at the quaternary position. Show more