Synthesis of Polyimides from 3,3'-dihydroxybenzidine and Conversion to Polybenzoxazoles

Abstract

The polyimides containing the hydroxy group were synthesized from 3,3 '-dihydroxybenzidine (I) and previously prepared tetracarboxylic dianhydrides, 3,3''',4,4'''- p-quaterphenyltetracarboxylic dianhydride (IId), 3,3'''',4,4''''- p-quinquephenyltetracarboxylic dianhydride (IIe) and 3,3''''',4,4'''''- p-sexiphenyltetracarboxylic dianhydride (IIf), and the properties were compared with those of corresponding polyimides from commercially available dianhydrides. The polyimides were synthesized by the conventional two-step procedure of ring-opening polyaddition in N-methyl-2-pyrrolidinone (NMP) and subsequent thermal cyclic dehydration. All the polyimides were soluble in NMP on heating. The glass transition temperature ( Tg) of the polyimides ranged from 336—360°C, and were not dependent on the structure of tetracarboxylic dianhydride. The storage modulus of some polyimides did not decrease at high temperature. All the polyimides underwent thermal conversion to polybenzoxazoles upon heating to 500°C for 1 h under vacuum. The thermal conversion of the polyimides to polybenzoxazoles was confirmed by Fourier transform infrared spectrometry. The 10% weight loss temperature ( T10) of the polybenzoxazoles was 530—700°C in nitrogen, and the polybenzoxazoles were thermally stable. In particular, the T10 values of the polybenzoxazoles from IId, IIe, and IIf were above 630°C.