13C n.m.r. determination of units formed by irregular addition in polyisoprene

¹³C n.m.r. spectroscopic data obtained for model compounds imitating regular and irregular addition of monomer units in linear polyisoprene are compared with the chemical shifts calculated using the empirical regularities found for the branched alkanes and alkenes and a good correlation is established. The validity of the results obtained was confirmed by investigation of the carbon spectra of hydrogenated and unhydrogenated polyisoprenes which contain chain fragments with irregular addition of units. Samples of hydrogenated polyisoprene are shown to give resonance lines that correspond to the methylene carbons of head-to-head and tail-to-tail addition and show chemical shifts at 34.62 ppm and 27.61 ppm, respectively. For the unhydrogenated polyisoprenes, the methylene carbons of trans- and cis-units in head-to-head addition were found to absorb at 38.6 ppm and 31.4 ppm, respectively, with those in the tail-to-tail addition of both isomers absorbing at 28.4–28.8 ppm. The latter findings offer a practical means of characterizing irregularities in polyisoprenes.