Determination of Structures of Branched Paraffins and Branched Olefins in the Thermal Degradation Products of Polypropylene by 13C NMR

Abstract

An isotactic polypropylene was thermally degradated under atmospheric pressure at 380°C. The obtained products were hydrocarbons being liquid at room temperatures. The analytical gas-liquid chromatogram (GLC) of these products including trace components (Fig. 3) showed more than forty peaks. Among the components, the eight major ones, which were excess of 3 wt % of the total weight of the products (Table 2) and corresponded to the GLC peak numbers 1 to 8 in Figure 3, were dealt with. The above eight components were obtained by the careful use of a precision fractionator. The chemical structure of each component was determined principally by 13C NMR spectra and Mass spectra. Ultimately, the eight components were identified in terms of the retention time of GLC as follows: pentane, 2-methyl-l-pentene (the dimer olefin of propylene), 4-methylheptane (the trimer paraffin), 2, 4-dimethyl-l-heptene (the trimer paraffin), 4, 6-dimethylnonane(the tetramer paraffin), 2, 4, 6-trimethyl-l-nonene(the tetramer olefin), 4, 6, 8-trimethylundecane (the pentamer paraffin), and 2, 4, 6, 8-tetramethyl-l-undecene (the pentamer olefin). In 13C NMR spectra of the tetramer paraffin (Fig. 4), the tetramer olefin (Fig. 5), the pentamer paraffin (Fig. 6) and the pentamer olefin (Fig. 7), the resonances due to the tacticity of dyad and triad of the propylene oligomers were observed. On the basis of the results of determination of chemical structures of the thermal degradation products and of assignment of resonances due to the tacticity of the oligomers, it was found that the thermal degradation products consisted of about 72 wt % of branched mono-olefins and about 21 wt % of branched paraffins, and that polypropylene having isotactic structure was thermally degradated into propylene oligomers having heterotactic structure. Finally, the thermal degradation mechanism of polypropylene was discussed. © 1976, The Chemical Society of Japan. All rights reserved.