Structural characterization of terminal groups in natural rubber: Origin of nitrogenous groups

The nitrogenous groups in natural rubber (NR) from Hevea brasiliensis have been regarded as an important factor governing cured rubber properties, although there is no structural evidence. The nitrogenous compounds in highly purified NR were characterized to disclose the origin of nitrogenous groups. The rubber particles in fresh latex were purified by washing, i.e., successive dilution of the cream fraction and concentration by centrifugation in the presence of surfactant. The nitrogen content of natural rubber from the washed rubber particles decreased from 0.281% to 0.015% level after washing 3-5 times, the level of which was similar to that of deproteinized natural rubber with a proteolytic enzyme. The presence of 1-2 nitrogen atoms per rubber chain was presumed based on the nitrogen content and K value. The nitrogen content of fractionated rubber fractions from rubber particles washed 5 times increased with increasing molecular weight. The nitrogenous group in the rubber chain is postulated to originate from mostly phospholipids associating with branch-points and from functional groups formed by oxidation and successive reactions in latex.