Approaches to novel AB heteroaromatic polyamides based on cyanoimidazoles

Abstract

We report the synthesis of several AB monomers based on 2-amino-4,5-dicyanoimidazole. Polymerization of the monomers to poly(imidazoleamide)s, by several methods is described. Amino acids 2-amino-4-cyano-1-methyl-5-imidazolecarboxylic acid and 2-amino-5-cyano-1-methyl-4-imidazolecarboxylic acid were prepared by mono-ethanolysis of 2-amino-4,5-dicyanoimidazole followed by methylation of the 1-nitrogen. The resulting regioisomeric mixture of ethyl esters was separated by fractional crystallization and hydrolyzed to the desired amino acids. The regioisomeric structure was determined by NOE studies of the decarboxylated amino acids. The corresponding acid chlorides were prepared with thionyl chloride and isolated as the HCl salts. Activated esters were prepared by reacting the acid chlorides with alcohols such as 1,1,1-trifluoroethanol and 2-chlorophenol. Model compounds were prepared by the acylation of aminocyanoimidazoles but yields were low. The low nucleophilicity of the aminocyanoimidazolecarboxylic acids was partially overcome by the use of the acylation catalyst 4-dimethylaminopyridine (DMAP) and the activating agent silicon tetrachloride. The acid chlorides were polymerized in amide solvents such as hexamethyl phosphoramide with pyridine and DMAP. A copolyamide consisting of both regioisomers was prepared from a regioisomeric mixture of the acid chlorides. In addition, aminolysis of 2-chlorophenyl 2-amino-4-cyano-1-methyl-5-imidazolecarboxylate resulted in a low yield of poly(imidazoleamide). Poly(imidazoleamide)s were red to tan which suggest conjugation along the polymer backbone. The solubility of the polyamides varied from amide solvents to sulfuric acid depending on the regioisomeric structure and molecular weight. The molecular weight of the heteroaramids was in the range from 1500 to 4000 based on viscosity measurements in sulfuric acid. The poly(imidazoleamide)s were thermally stable in excess of 300°C under air and nitrogen atmosphere. © 1993 John Wiley & Sons, Inc.