Study of the Mechanism for Poly(p-phenylene)benzoxazole PolymerizationA Remarkable Reaction Pathway To Make Rigid-Rod Polymers

The mechanism of poly(p-phenylenebenzoxazole) formation from terephthalic acid (TA) and diaminodihydroxybenzene dihydrochloride (DADHB) in polyphosphoric acid (PPA) was studied. The solubility of TA in PPA with 86% P2O5 content was determined to be 0.02% at 100 °C and 0.06% at 140 °C. Dissolved TA existed as three species, TA, α-(4-carboxybenzoyl)-ω-hydroxypoly(oxyphosphinico), and α,α‘-[1,4-phenylenebis(carbonyl)]bis[ω-hydroxypoly(oxyphosphinico)]. DADHB also reacts with PPA, and DADHB, α-(2,4-diamino-5-hydroxyphenyl)-ω-hydroxypoly(oxyphosphinico), and α,α‘-(4,6-diamino-1,3-phenylene)bis[(ω-hydroxypoly(oxyphosphinico)] were found. During the course of polymerization, conversion of DADHB was higher than that of TA. NMR, IR, and mass spectroscopy characterization of PBO oligomer and its product with benzoic-carboxy-13C acid suggested both chain ends were capped with DADHB, which is different from oligomers of conventional AA plus BB step-growth polymers. An unprecedented polymerization mechanism is proposed to account for this unusual oligomer structure and the remarkable fact that high molecular weight PBO was obtained even when 5% excess of one of the monomers, TA, was used. The reactivities of the two functional groups in DADHB toward carboxylic acid to form benzoxazole in PPA were different.