Constitutional Isomerism: Synthesis and Characterization of Ordered Polyamides from Isophthalic Acid and N-Phenylethylenediamine by Direct Polymerization

Abstract

An ordered (head-to-head (H–H) or tail-to-tail (T–T)) polyamide was prepared by direct polycondensation of a symmetric monomer, isophthalic acid (1) with N-phenylethylenediamine (2) as a non-symmetric monomer using the condensing agent diphenyl(2,3-dihydro-2-thioxo-3-benzoxazolyl)phosphonate. The polycondensation was carried out by slow addition of (1) to (2), and gave polyamide (3d) with an inherent viscosity of 0.29 dl g⁻¹. On the other hand, authentic H–H or T–T ordered polyamide (3a) and H–T ordered polyamide (3b) were obtained by a multi-step route. Random polyamide (3c) was synthesized by mixing isophthaloyl chloride and 2 all at once in N-methyl-2-pyrrolidinone. The microstructure of the polymers was investigated by ¹H and ¹³C NMR spectroscopy. It was found that their spectra were different due to their different constitutions, while polymer 3d obtained by direct polycondensation had the expected H–H or T–T sequence. Furthermore, the model reactions were studied in detail to demonstrate the feasibility of selective amidation and polymer formation.