Characterization of brominated natural rubber by solution-state 2D NMR spectroscopy

Bromination of natural rubber was analyzed through nuclear magnetic resonance (NMR) spectroscopy, in which small 1H and 13C NMR signals, appearing after bromination of natural rubber, were positively assigned by 1D and 2D NMR measurements. The brominated natural rubber (BrDPNR) was prepared by bromination of deproteinized natural rubber (DPNR) with N-bromosuccinimide (NBS) in dichloromethane at 303 K for 3 h under a nitrogen atmosphere. The 13C NMR signals at 20.5, 22, 29, 29.8, 35.8, 40 and 112.4 ppm were assigned to the primary and the secondary carbons of the brominated trans-1,4-isoprene units, whereas the signals at 24.5 and 32.6 ppm were assigned to the primary and the secondary carbons of the brominated cis-1,4-isoprene units. During the bromination of DPNR with NBS, the isoprene units underwent cis-trans isomerization.

The bromination of natural rubber with NBS occurred after the hydrogen abstraction of cis-1,4-isoprene units. The reaction between the allylic carbon radicals and bromine radicals generate the brominated trans-1,4-isopren units.43