Source Year: 1996

To determine the structure of the initiating species for rubber formation, the structure of the terminal group of rubber (isolated from 1-month-old seedlings of H. brasiliensis) was determined using 13C NMR and 1H NMR spectroscopy. The presence of trans-isoprene units in a trans-trans linkage was confirmed. The number of trans-isoprene units was estimated to be about 2. Two methyl-proton signals in the trans-trans sequence were detected by 1H NMR spectroscopy. This spectral evidence showed that the trans-isoprene units were 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 either regularly tapped trees or from seedlings. Thus, the direct initiating species of rubber formation is either a C15 allylic diphosphate consisting of two trans-isoprene units as in the case of farnesly diphosphate (FDP), but which is modified at the dimethylallyl group, or it is FDP, the dimethylallyl group of which is modified after polymerization.