Synthesis and characterization of high performance polybenzoxazoles

Abstract

High molecular weight poly(hydroxy amide)s and polybenzoxazoles possessing a variety of backbone compositions have been synthesized via low temperature polycondensation and a novel catalytic cyclization reaction. Preliminary experiments involved the synthesis of poly(hydroxy amide)s and polybenzoxazoles based on a bisphenol-A aminophenol and isomeric acid chlorides. The polymers exhibited Tg's around 280°C, limited solubilities and only fair thermo-oxidative stabilities. In an effort to improve these properties. fluorinated monomers were investigated. A number of novel fluorinated monomers and commercial monomers were incorporated into poly(hydroxy amide)s and polybenzoxazoles using the above mentioned polymerization/cyclization techniques. Enhanced solubilities, glass transition temperatures and thermooxidative stabilities were realized. Difficulties encountered during the polymerization of the fluorinated monomers prompted the study of polymerization conditions. Results of these studies indicate that the electronic environment about the bis(o-aminophenol)s and acid chlorides dictates the ability of these monomers to participate in an ester forming side reaction. Physical and spectroscopic evidence was presented to support this conclusion. Investigations were also conducted to determine optimum cyclization conditions. It was shown that pyridine hydrochloride and p-toluenesulfonic acid monohydrate were active cyclization catalysts when used in conjunction with an azeotroping solvent, such as toluene or 0- dichlorobenzene. In general, complete cyclization of the oxazole rings could be achieved in as little as 6 hours at 175°C. Unfortunately, generic cyclization conditions were not found. Additionally, phenylethynyl terminated polybenzoxazoles were successfully synthesized using the above techniques. Molecular weight control and phenylethynyl incorporation were confirmed by spectroscopic, thermal and mechanical analyses.