The aromatic heterocyclic copolymer is produced by (a) reacting (i) an aromatic diaminodithiol compound, hydrogen atoms of thiol groups of which compound are substituted with substituted or unsubstituted alkyl groups, with (ii) an aromatic diamino compound and (iii) a dicarboxylic acid derivative in an organic solvent, to produce a precopolymer; and (b) heating the precopolymer to cause a thiazole ring closure reaction. This aromatic heterocyclic copolymer is contained as a reinforcing polymer in a molecular composite material.

Polybenzazole polymers can be advanced rapidly to high molecular weight at relatively high temperatures and high shear in a dehydrating solvent acid.

The invention is a condensation polymerization reaction in which functionally terminated polybenzazole ligomers are reacted in a solvent acid at a temperature of at least 150.degree. C. and a shear of at least about 10 sec..sup.-1 under conditions so that the desired molecular weight is reached in no more than about 4 hours. The oligomers may be, for example, derived from the reaction of 4,6-diaminoresorcinol bis(hydrochloride) and terephthalic acid. The concentration of oligomers is high enough so that the reaction mixture contains liquid crystalline domains.

Salts that contain AA-PBZ monomer ions and BB-PBZ monomer ions can be precipitated from an aqueous solution by contacting soluble salts of the monomers in an aqueous solution. The monomer salt can be polymerized by ordinary techniques to form polybenzazole polymers without the need for devolatilization and with very accurate stoichiometric control.

Polybenzazole polymers can be synthesized rapidly in large quantities at high molecular weight and high concentration by a two-step process of forming oligomers in a batch reactor and advancing the molecular weight of the polymer in an extruder.