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A novel extension of the Yamazaki reaction is used to prepare block copolymers having rigid blocks of poly-(p-benzamide) (PBA) and semiflexible blocks of polyamide-hydrazide. A PBA prepolymer having M ≅ 10,000 was synthesized by the usual Yamazaki reaction using triphenylphosphite. As previously reported, higher-molecular-weight PBA could be obtained using 4-N-(4′-aminobenzamido)benzoic acid containing a preformed amide linkage. Addition of p-aminobenzhydrazide and terephthalic acid then led to formation of the polyamide-hydrazide blocks using as the active reactant the diphenylphosphite formed as a by-product in the first polymerization. Evidence that a block copolymer is produced includes an increase in inherent viscosity during the second step, differences in the solubility of the copolymer compared to the homopolymers, and comparison of the phase diagram of the block copolymer in N-methylpyrrolidone having 4% added LiCl with those of a random copolymer, and of mixtures of the two homopolymers. The critical concentration required to form a nematic phase in solutions of the block copolymers is correlated with the length (or axial ratio) of the rigid block, and with its proportion in the copolymer.

4,4′-Diaminobenzanilide (DAB) was reacted with diacids via the Yamazaki phosphorylation reaction to yield simple polyamides and random copolyamides. The use of DAB was found to promote polycondensation because DAB contains a preformed amide linkage which minimizes the amounts of by-products formed. It also maximizes polymer solubility since the monomer is unsymmetrical. The order of the inherent viscosity values of the polymers obtained from DAB by reaction with different diacids, isophthalic > aliphatic > terephthalic, was found to parallel polymer solubility in the reaction medium. The inherent viscosity of the polyamide based on terephthalic acid, DAB-T, was increased greatly by copolymerization, isophthalic acid being more effective than an aliphatic diacid in this regard. None of the polymers formed a thermotropic nematic phase, but the copolymer having an equimolar ratio of terephthalic and adipic acid formed anisotropic solutions in 100% sulfuric acid at polymer concentrations exceeding 40%. Strong films were cast from such solutions.