Synthesis of polyaramids in γ-valerolactone-based organic electrolyte solutions

Abstract

The current synthetic procedures for polyaramids mainly involve the use of amide solvents such as N-methylpyrrolidone and N,N-dimethylacetamide. However, these solvents are suspected to be teratogenic and are considered ‘Substances of Very High Concern’ by the European Commission. Here we propose a benign alternative solvent system: an Organic Electrolyte Solution (OES) consisting of γ-valerolactone (GVL) and a small amount of the ionic liquid 1-methyl-3-octylimidazolium chloride, [C₈MIm][Cl]. Three commercially relevant polyaramids were synthesized: poly-p-phenylene terephthalamide (PPTA), poly-m-phenylene isophthalamide (PMIA) and copoly(p-phenylene/3,4′-diphenylether terephthalamide) (ODA/PPTA). PMIA was successfully synthesized in the OES containing [C₈MIm][Cl] in a molar fraction of x_IL = 0.043, achieving an inherent viscosity of η_inh = 1.94 ± 0.064 dL g⁻¹, which is on par with the current industrial standard and the benchmark lab scale synthesis. The reaction mixture could also be directly used for the wet spinning of polyaramid fibers, and all components of the solvent could be recycled in good yields by a series of evaporation and distillation steps. ODA/PPTA could be synthesized, but only rather low inherent viscosities were achieved. The reaction mixture was too viscoelastic to be spun by our small-scale spinning setup. PPTA always instantly precipitated and could not be synthesized from a [C₈MIm][Cl]/GVL OES. α-Picoline, the organic base which was added to capture the released HCl during the reaction, was found to play a pivotal role in the polymerization reaction. By undergoing an acid–base reaction with HCl, it forms a protic ionic liquid in situ which increases the solubility of the polymer.