Crystal structure of an aliphatic polyoxamide containing methyl side-groups: Poly(2-methyl-1,8-octamethyleneoxamide)

Poly (2-methyl-1,8-octametyleneoxamide) (nylon-MOMD-2) was prepared from dibutyl oxalate and 2-methyl-1,8-octamethylenediamine. The chemical structure was investigated by ¹³C nuclear magnetic resonance (¹³C NMR). The ¹³C NMR spectrum revealed that the methyl groups distribute among the four β-sites at random. The crystal structure and its temperature dependence were also investigated by molecular mechanics calculations, X-ray diffraction, and differential scanning calorimetry. The structure was determined to be monoclinic and a = 6.26 Å, b = 8.80 Å, c = 14.7 Å, and β = 50.7°. The torsion angle of the NH–CH₂ bonds has a skew conformation and there are the two distinct torsion angles in the crystal structure. The crystal structure of nylon-MOMD-2 has a disordered and statistical structure. The crystal structure shows no phase transition on heating until its melting point, although the unit cell expands in the a and b axis directions.