A Study of Poly(benzo[1,2-d:5,4-d‘]bisoxazole-2,6-diyl-1,4-phenylene) Reactions at Elevated Temperatures

The reaction mechanism of poly(benzo[1,2-d:5,4-d‘]bisoxazole-2,6-diyl-1,4-phenylene) (PBO) at elevated temperatures was studied by means of model compounds and a 13C-labeled PBO polymer. Products from model compound thermal degradation suggest that the dominating reactions are homolytic scission of a single bond and decomposition of a heterocyclic ring to produce an aromatic nitrile. The major volatile products from PBO high-temperature degradation were CO2, benzonitrile, and dicyanobenzene. Benzene was also a product. Correlation of 13C in the polymer and the volatile products suggests that benzonitrile and dicyanobenzene result from decomposition of the heterocyclic ring(s) but not from substitution of cyano radicals onto aromatic rings. Scrambling of carbons was not an important reaction in the PBO pyrolysis. A reaction mechanism for PBO at elevated temperatures is proposed.