Structural studies of highly-condensed polyaromatics by solid state NMR

Two mesophase pitches with Phi = 1 anisotropic content and excellent spinnability were synthesized from two types of pure aromatics, i.e., 1, 2, 4, 5 - tetramethylbenzene (durene, Du) and tetralin (Te) by methylene-bridge cn,ss - linking reaction and subsequent heat - treatment. In our research, the structural features of pyridine insoluble fractions (PI) of these two synthetic mesophase pitches were comparatively investigated by solid state nuclear magnetic resonance (NMR). It is shown that the PI fraction from Du based mesophase pitch contains abundant methyls and some amount of naphthenic groups and exhibits lower aromaticity and condensation degree. The molecules in mesophase pitch appear to have kata - type aromatic skeleton. Nevertheless, the PI fraction from Te mesophase pitch contains only methyl groups, exhibiting very high aromaticity and condensation degree. The molecules in mesophase pitch appear to have peri - type aromatic skeleton. The following factors should account for the differences between the two PI fractions of synthetic mesophase pitches: the structures of oligomers and the condensation variables.