NMR studies of irregular structures in polyisoprene

Abstract

Irregular structures of synthetic polyisoprenes and natural rubber were studied by 1H and 13C NMR spectroscopy. In synthetic polyisoprene, only the (3, 4) unit was observed besides the regular structure, but in natural rubber, it was not observed. The (3, 4) unit content, which has an important effect upon the properties of the polymer, decreased as the molecular weight increased. In the 1H NMR spectrum of synthetic polyisoprene dissolved in CCl4, the = CH2 proton signal of the (3, 4) unit was separated into two peaks. The higher-field peak was assigned to a terminal (3, 4) unit and the lower-field peak to an inner (3, 4) unit. The terminal (3, 4) unit content of the polymer suggested that there are branches in the polymer chain. The 13C NMR spectrum of hydrogenated isoprene oligomer indicated that the polymerization of isoprene propagated by the (4, 1) addition and there is a considerable amount of the (3, 4) unit at the chain end in the (4, 3) structure. © 1980 The Society of Polymer Science, Japan.