Polyisoprene chemical shifts

In Figure 5 the 1r KMR spectrun of a partially cyclized gua3aile rubber (G-1) is shewn. IWR analysis of guayule rubbei has shewn that it is composed of cis-1,4-polyisoprene and that no 3,4-polyisoprene is detected. However, we detected a peak at 4.75 ppm, in the spectrun of G-1, almost idmtical with the 3,4-polyisoprene chemical shifts at 4.73 and 4.79 ppm. We assign this pe to the iso-1,4 structure ... 

There are only minimal chemical shift differences between cis and rrans isomers in the H NMR spectra of polymers like polybutadiene and polyisoprene, so this tool is less useful in characterizing structural isomers in diene polymers. 

The chemical shifts of the characteristic carbon signals in acyclic terpenes, polyprenols, and cis-trans isomerized poly-isoprenes are plotted in Fig. 3. Here, the chemical shifts are correlated using the w C-5 methyl carbon signal at 17.66 ppm as an internal standard (except for isomerized polyisoprenes) in order to compensate for the effect of solution concentration. It is clear that these chemical shifts are independent of the chain length of the compounds and can be used for the determination of the arrangement of isoprene units as well as the terminal units in various isoprenoid compounds (8). 

Chemical Shifts of the Olefinic Protcais in Cyclized Polyisoprene... 

The olefinic region foom the NMR spectra of three different cyclized polyisoprenes (H-1, L2-5, 12.-1) are shown in Figure 9. Amongst these spectra the various structures listed in Table III are observed. Assigpments for the a clic, exocyclic and iso-1,4 structures were based on peak intensi es and % results. A complete listing of all the olefinic chemical shift assignments is given in Table III. 

Gronski and co-workers [55], Beebe [56], Dolinskaya and co-workers [57] and Duch and Grant [58], used the chemical shift correction parameters for linear alkanes in the aliphatic region of C-NMR spectra to determine the relative amounts of 3,4 and c/s-1,4 units of polyisoprene. Microstructure studies have been carried out. 

The solid-state spectrum of diglycidyl ether of bisphenol-A (DGEBA) has been compared with the liquid-state spectrum [9], and the chemical shifts were examined using the steric-hindrance model. This model has also given consistent results in the analysis of chemical shifts of the a and conformers of tran5-l,4-polyisoprene in the solid state [10]. The sterie-hindrance model prediets shifts that are the same order of magnitude as those observed. 

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