The synthesis, characterization, and crystal structures of poly(2,6-naphthalenebenzobisoxazole) and poly(1,5-naphthalenebenzobisoxazole)

The rigid-rod polymers, poly(2,6-naphthalenebenzobisoxazole) (Naph-2,6-PBO) and poly(1,5-naphthalenebenzobisoxazole) (Naph-1,5-PBO) were synthesized by high temperature polycondensation of isomeric naphthalene dicarboxylic acids with 4,6-diaminoresorcinol dihydrochloride in polyphosphoric acid. Expectedly, these polymers were found to have high thermal aa well as thermoozidative stabilities, similar to what has been reported for other polymers of this class. The chain conformations of Naph-2,6-PBO and Naph-1,5-PBO were trans and the crystal structures of Naph-2,6-PBO and Naph-1,5-PBO had the three-dimensional order, although the axial disorder existed for both Naph-2,6-PBO and Naph-1,5-PBO. Naph-2,6-PBO exhibited a more pronounced axial disorder than Naph-1,5-PBO because of its more linear shape. The repeat unit distance for Naph-2,6-PBO (14.15 Å) was found to be larger compared with that of Naph-1,5-PBO (12.45 A) because of the more kinked structure of the latter. The extents of staggering between the adjacent chains in the oc projection of the crystal structure were 0.25c and 0.23c for Naph-2,6-PBO and Naph-1.5-PBO, respectively. Naph-1,5-PBO has a more kinked and twisted chain structure relative to Naph-2,6-PBO. The kinked and twisted chain structure of Naph-1,5-PBO in the crystal seems to prevent slippage between adjacent chains in the crystal structure. The more perfect crystal structure of Naph-1,5-PBO may be due to this difficulty in the occurrence of the slippage.