Epoxidation of natural rubber studied by nmr spectroscopy

Epoxidation of natural rubber with peroxyacids in homogeneous solution (CHCl3, benzene) and in latex occurs without main chain fission or isomerization of the cis configuration. There are two possible modes of addition of oxygen to the double bond, which gives rise to diastereoisomers in dyad structures, as shown by the occurrence of splitting of 13C NMR resonances in 100% epoxidized rubber. In partially epoxidized rubber the 13C NMR resonances of the methylene carbon atoms have been fully assigned using pulse sequences and lanthanide shift reagents, in terms of triad sequences. Resonances in the olefinic region of the spectrum require assignment in terms of pentad sequences whereas those from the oxirane region are much more closely spaced and are assigned using triads. Intensity measurements on the methylene carbon NMR resonances show that the epoxidation reaction is a random process in both homogeneous solution and in latex particles.