Thermally crosslinked rigid…rod aramids, 1. Synthesis of a new monomer and its polymerization

With 2,2,6,6‐tetraoxo‐1,3,5,7‐tetrahydro‐2,6‐dithia‐s‐indacene‐4,8‐diamine (DSDA) (8), a new monomer for the synthesis of thermally crosslinkable rigid‐rod aramids was developed. DSDA was copolymerized with p‐phenylenediamine and terephthaloyl dichloride by a low‐temperature solution polycondensation to give high‐molecular‐weight aramids (inherent viscosity ηinh > 2,2 dL/g). The fraction of DSDA could be varied from 0 to 100 mol‐% of the diamine used in the condensation. All polymers showed a weight loss due to thermal elimination of SO2. Thermal crosslinking could be effected by curing the samples at high temperatures (280–320°C). The crosslinking reaction was investigated by means of thermogravimetry (TG), thermogravimetry/mass spectrometry (TG‐MS), Fourier transform infrared (FT‐IR), 13C cross polarization‐magic angle spinning nuclear magnetic resonance (13C CP‐MAS NMR) and electron paramagnetic resonance (EPR). The activation energy for this reaction was found to be between 150 and 170 kJ/mol. Curing of the polymer samples led to the generation of stable, matrix‐isolated radicals.