Synthesis, structure and properties of a helical nylon‐3 derivative

A review of the polyamide poly(α‐isobutyl‐L‐aspartate), embracing synthesis, crystal structure and properties, is reported. The interest in this nylon‐3 derivative arises from its ability to adopt conformations similar to the characteristic α‐helix of polypeptides. Two different synthetic approaches, both of them starting from L‐aspartic acid, have been followed for the preparation of this polyamide. Poly‐condensation in solution of α‐isobutyl‐L‐aspartate pentachlorophenyl ester yielded polymers with molecular weights up to near 80 000. About six times higher values were obtained by anionic polymerization of β‐lactam of α‐isobutyl‐L‐aspartate. Highly regular chains were attained in both cases. Two crystalline forms, hexagonal and tetragonal, made of helices with 3.25 and 4 residues per turn, respectively, were characterized by X‐rays and electron microscopy. Transitions between these two forms in the solid state were induced by heating, whereas a helix‐coil conversion promoted by acids was found to take place in solution. Liquid crystal properties have been observed for solutions of this polymer in helicogenic solvents, and films with cholesteric structure were spontaneously generated from them by casting. On the contrary, uniaxially oriented films could be obtained when cast under the effect of an electric field. A shear piezoelectric response of about 12–13 pC/N was measured for these films.