Abstract
The initiating molecular species for rubber formation in Hevea brasiliensis has been investigated by structural characterization of the terminal group in the rubber by means of 13C NMR and 1H NMR spectroscopy. The presence of trans-isoprene units in trans-trans linkage was confirmed by the characteristic C-1-methylene-carbon signal of the trans-isoprene unit. The number of trans-isoprene units was estimated to be about two from the relative intensity of this signal, and the degree of polymerization of various fractions from transesterified deproteinized natural rubber. Two methyl-proton signals in the trans-trans sequence were detected by 1H NMR spectroscopy. This spectral evidence shows that the trans-isoprene units are in the trans-trans-cis sequence at the initiating terminal. However, the expected methyl- and olefinic-carbon signals and methyl-proton signal of the dimethylallyl group were not detected in natural rubber from regularly tapped trees and from a Hevea seedling. Thus, the direct initiating species of rubber formation is either a C 15 allylic diphosphate consisting of two trans-isoprene unites as in the case of farnesyl diphosphate (FDP), but which is modified at the dimethylallyl group, or FDP, the dimethylallyl group of which is modified after polymerization.
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