Low-molecular-weight poly(imide-amide)s obtained by copolycondensation of 4,4′-methylenedi(phenyl isocyanate), trimellitic anhydride and benzoic acid in N-methyl-2-pyrrolidone, 2. Structural defects observed by 1H and 13C NMR

The in depth examination (by ¹H and ¹³C NMR, DSC, SEC and TGA) of the low-molecular-weight poly(imide-amide)s synthesized with the mixture [4,4′-methylenedi(phenyl isocyanate) (M) + trimellitic acid anhydride (T) + benzoic acid (B)] in N-methyl-2-pyrrolidone (NMP) at temperatures lower than 180°C allowed the identification and quantitative determinations of all the unexpected and relatively stable structures. While amide functions are readily synthesized, the formation of imide from anhydride and isocyanate functions passes through 4-amido-1,2-benzenedicarboxylic acid (Tⁱ), 1,4-di-amido-2-benzenecarboxylic acid (T″) and disubstituted urea (u). At 180°C Tⁱ completely disappears but traces of T″ and u remain; thus, the formation of imide can never reach its theoretical quantity. Traces of moisture in NMP may play the role of catalyst in the formation of imide, and reaction mechanisms are proposed.