Polyphenylquinoxaline/Poly-p-Tolylquinoxaline Copolymers: The Search for a Cross-Linkable PPQ

As part of a continuing effort to reduce or eliminate the thermoplasticity long associated with polyphenylquinoxalines, a series of phenylquinoxaline/p-tolylquinoxaline copolymers has been prepared and their thermal stability and cross-linkability evaluated. Previous studies have indicated poly-p-tolyquinoxaline (PTQ) to be significantly more cross-linkable but also markedly less thermo-oxidatively stable than polyphenylquinoxaline (PPQ). Thus, an optimum copolymer composition combining the cross-linkability of PTQ and the stability of PPQ was sought. In fact, no such composition exists for this system. Whereas a small percentage of PTQ leads to substantial loss in stability, it has virtually no effect on cross-linkability. For example, the half-life at 400 C in air for 10% PPQ has been reduced by 55% relative to PPQ while its cross-linkability is almost unchanged. Apparently, the cross-linking reaction is inefficient such that only high concentrations of the cross-linkable species leads to a useful enhancement of the thermal cross-linkability. Conversely, the thermal oxidation reaction appears to be quite efficient. It is concluded that this copolymer approach can be successful in eliminating thermoplasticity if a system can be found in which the relative efficiency of the competing reactions is reversed.