homogeneous , high melting , flexible copolyamides , adopted as molding compositions and for textile applications , are conveniently prepared by prepolymerizing either mixture of adipic acid and hexamethylenediamine , or admixture thereof with at least one other short - chain dicarboxylic acid , or mixture of a salt of adipic acid , or salt of admixture of adipic acid with at least one other short - chain dicarboxylic acid , and hexamethylenediamine , together with free hexamethylenediamine , said prepolymerization being carried out in the presence of a catalytically effective amount of a strong inorganic oxyacid , or strong organic oxyacid other than a carboxylic acid , or alkali or alkaline earth metal thereof , said resulting prepolymer having amino end groups , and thence polycondensing said prepolymer with a fatty acid dimer .