Raman spectroscopic study of non-catalytic hydrogenation of unsaturated rubbers

The non-catalytic hydrogenation of natural rubber (NR) and two epoxidized NRs (ENRs) i.e. ENR-22 and ENR-40 containing 22 and 40 mol% of epoxide, respectively, was carried out using p-toluenesulfonylhydrazide (TSH) as a hydrogenating agent. A two-fold molar excess of TSH compared with unsaturated units of the rubber was used. The evidence of hydrogenation is a decrease in the intensity of the characteristic signal of the carbon-carbon double bond stretching vibration of the rubber in both the Raman and FT-IR spectra. The percentage hydrogenation was successfully determined by Raman spectroscopy since the vibrational mode of the carbon-carbon unsaturation is strongly Raman active. The progress of the hydrogenation could be monitored by means of the techniques mentioned above as a function of reaction time. The maximum degree of hydrogenation of NR is ∼89% whereas in the case of ENR-22 and ENR-40 it reaches 94 and 96%, respectively. Solid-state ¹³C NMR spectroscopy was also used to confirm the microstructure characteristics of the hydrogenated rubbers. ¹³C NMR analysis showed that cis-trans isomerization of carbon-carbon unsaturations occur during hydrogenation.