TRANS-ISOPRENE UNITS IN NATURAL-RUBBER

Small amounts of trans isoprene units in natural rubber that originated from the initiation reaction during biosynthesis of the polymer are differentiated by C-13-NMR spectroscopy from those of cis-trans isomerization origin. Examinations on various commercial natural rubbers and rubbers from latexes by C-13-NMR reveal that the level of trans isoprene units that arises from isomerization of cis isoprene unit is insignificant as compared to those of biosynthesis origin. H-1-NMR signals of methyl protons in dimethylallyl-trans-trans or trans-trans-trans, trans-trans-cis and cis-trans-cis are assigned on the basis of the spectra of model compounds and synthetic polyisoprene. Purified natural rubber of low molecular weight obtained by fractionation shows distinctive H-1-NMR signals due to resonances of methyl protons in dimethylallyl-trans-trans and trans-trans-cis arrangements whereas the characteristic signal that arises from cis-trans-cis is not detected. This confirms that the natural rubber molecule is a high molecular weight homologue of trans-cis polyprenol having the two-trans and poly-cis structural arrangement.