End group and irregular structure analysis in thermally prepared sodium polyaspartate by 1H and 13C NMR spectroscopy

Sodium polyaspartate (SPA) was synthesized by the hydrolysis of poly(succinimide) (PSI) prepared by the thermal polycondensation of L-aspartic acid at 260 degrees C for 6 h, and the microstructures of the polymer were analyzed in detail using H-1 and C-13 NMR spectroscopy Several end groups and irregular structures which had most likely been produced by deamination and decarboxylation were identified and quantified. The presence of branched units was demonstrated by the detailed analysis of the methine proton region using the specially prepared model polymers. The number of the branched units was 3 per 100 monomer units, and the average number of monomer units between two adjacent branch points and between a branch point and an end group was estimated to be 13. The comparison of the quantities of the minor structures in SPA with those of the corresponding ones in PSI revealed that side reactions scarcely occur during the hydrolysis of PSI to SPA.