Synthesis and properties of novel aromatic polyamides with non-conjugated bichromophoric units in the main chains

A dicarboxylic acid bearing two cinnamic acids units linked in their para positions by a methylene bridge was synthesized by the reaction of 4,4′-methylene-bis(benzaldehyde) and malonic acid, characterized by spectral and elemental analysis and used to prepare a series of novel photocrosslinkable aromatic polyamides through its reaction with various ether-bridged aromatic diamines. ¹H and ¹³C NMR, IR and UV spectroscopy confirmed the structure of the monomer and photopolymers. The resulting polymers with inherent viscosities of 0.65–1.14 dL/g were soluble in polar aprotic solvents, such as DMF, DMSO, DMAc and NMP, and showed good film-forming properties. Their weight-average and number-average molecular weights, determined by GPC (polystyrene standard), were in the range 57,400–84,300 and 24,000–40,900, respectively. The percent of water absorbed at 65% relative humidity varied between 1.57% and 2.35%. The polyamides exhibited glass transition temperatures between 202 and 239 °C, and they were stable up to a temperature of 350 °C in air. Wide-angle X-ray scattering diagrams did not show any crystalline reflection and no endothermic peak attributable to polymer melting was observed in DSC curves of the polymers. These photosensitive polymers exhibited strong UV absorption maxima both in solution and film state, and displayed three photochemical transformations under the UV exposure, viz. a trans-cis isomerization, favored in the early stage of UV irradiation, a bimolecular cyclodimerization, leading to the generation of a cyclobutane ring, and a photo-Fries rearrangement evidenced only in polymer solutions.