A polyphenylquinoxaline which consists of 1 to 100 mole percent of structural elements of the formula (IV) ##STR1## and of 0-99 mole percent of the following repeat unit of formula (V) ##STR2## where R is selected from the group consisting of H, an alkyl group, a carbocyclic aromatic group, a heterocyclic aromatic group, or an alkoxy group, Ar.sub.1 is a carbocyclic aromatic or heterocyclic aromatic group, and B is selected from the representative group consisting of:Ar.sub.2 --Z--Ar.sub.3 (i)where Ar.sub.2 and Ar.sub.3 are the same or different carbocyclic aromatic or heterocyclic aromatic group and Z is selected from the representative group consisting of CO, SO or SO.sub.2 ;Z'--Ar.sub.4 --Z' (ii)where Z' is an activated carbocyclic aromatic or heterocyclic aromatic group and Ar.sub.4 is an aliphatic group, a carbocyclic aromatic or heterocyclic aromatic group. A process for manufacture a self polymerizable phenylquinoxaline subject to polymerization by aromatic nucleophilic substitution. The use of these polyphenylquinoxalines and there copolymers as thermally stable thermal plastics for use in aerospace, high temperature adhesive, microelectronic and membrane (gas and molecular separation membrane) application.