Synthesis and polymerization of novel formamidinium salts: A new pathway to polyamides derived from aromatic diamines

Abstract

A new polycondensation pathway has been developed for the preparation of polyamides at high temperatures. p‐Phenylenediamine was converted to N,N‐p‐phenylene bis(N′,N′‐dimethylformamidine) (I), which formed 1–1 and 2–1 salts with terephthalic and adipic acids, respectively: magnified imageDicarboxylate salts were polymerizable by heating in bulk or suspension. Low‐molecular‐weight poly(p‐phenyleneterephthalamide) was obtained from N,N‐p‐phenylene bis(N',N'‐dimethylformamidinium) terephthalate above 225°C. The low degree of polymerization was due to terephthalic acid sublimation as well as to the well‐known intractability of poly(p‐phenyleneterephthalamide). High‐viscosity poly(p‐phenyleneadipamide) was obtained from N,N‐p‐phenylene bis(N′,N′‐dimethylformamidinum hydrogen adipate) above 200°C. Both salts liberated dimethylformamide (DMF) during polymerization. The adipate salt also released 1 mole of adipic acid during the high‐temperature vacuum stage of polymerization. A polycondensation mechanism was proposed for each salt, based on thermal gravimetric analysis (TGA‐MS) and infrared (IR) analyses.The hydrolysis of N,N‐p‐phenylene bis(N',N'‐dimethylformamidine), N,N‐p‐phenylene bis(N',N'‐dimethylformamidinium chloride), and the two dicarboxylate salts of (I) was monitored by nuclear magnetic resonance (NMR) at room temperature. The dihydrochloride salt was most resistant to hydrolysis (kH 6.9 × 10−9 sec−1; relative rate 1.0) followed by (I) 7.1, terephthalate salt, 14.9, and adipate salt, 27.2. Both dicarboxylate salts possessed sufficient hydrolytic stability for use as polycondensation monomers