The thermal breakdown mechanism of polybenzoxazoles and polybenzothiazoles

Abstract

Polybenzoxazoles, when heated up to 660°C in vacuum, lose most of their oxygen as carbon monoxide and carbon dioxide and possibly form Schiff base‐type structures. The carbon required for the formation of carbon monoxide is predominantly supplied by the benzene rings. About one out of every six CN linkages is eliminated as hydrogen cyanide, and some free nitrogen from this group is reduced to ammonia. The elimination of about 1 hydrogen atom per polymer unit (in the form of free hydrogen, hydrogen cyanide, ammonia, and methane) results in free radical formation and crosslinking. Carbon monoxide and carbon dioxide are also some of the major decomposition products of the benzoxazole in poly‐2,2′‐(1,8‐perfluorooctane)‐6,6′‐bibenzoxazole. In addition, fluorine from the perfluoroalkyl chain combines with ring hydrogen to form hydrogen fluoride, which in turn attacks the quartz of the crucible and reaction vessel to form silicon tetrafluoride. A polybenzothiazole, on heating to 625°C, crosslinks with elimination of some hydrogen. One out of every ten sulfur atoms is removed, essentially as hydrogen sulfide, leaving a Schiff‐base linkage; very few of the CN‐groups are eliminated as hydrogen cyanide. Two less stable polybenzothiazole systems, heated to 660 and 670°C, lose about 50% of their sulfur, mainly as hydrogen sulfide, and 20% of their nitrogen as hydrogen cyanide. Flash pyrolysis of model benzoxazoles and benzothiazoles was tried as a means to provide supporting data.

Thermal Degradation and Fire Behavior of High Performance Polymers

“…It was concluded that the main route was that of the cleavage of aromatic C-CO of the amide linkages, which led to the formation of carbodiimide bonds [142]. Temperature dependent mechanisms involving iminol tautomerization, condensation of iminols, hydrolysis and decarboxylation was also reported [143].…”

Quantum chemical PM‐3 study of the thermal stability of heterocyclic fragments of heteropolymers, 1. Five‐membered heterocycles: Azoles and imides

“…However, oxazole decomposition with acetylene formation is estimated to be more energy consuming than decomposition (4) and is, therefore, not considered. Maleimide decomposition with acetylene formation is close in ED to decompositions ( 5 ) and (6). However, in aromatic polyimides the maleimide ring is condensed with a benzene ring, and maleimide decomposition with formation of acetylene corresponds to decomposition of the condensed system with formation of the unstable biradical-like didehydrobenzene.…”

“…As in the case of oxazole, symmetry properties do not favor concerted mechanisms of maleimide decompositions ( 5 ) and (6). For this reason only two-step mechanisms were considered.…”