Matrix-assisted laser desorption
ionization/time-of-flight (MALDI/ToF)
mass spectrometry and 1H NMR were used for the structural
investigation of isophthalic and maleic acid copolyesters with neopentyl
glycol. Since both methods provided information on the ratio of incorporated
acid components and terminating groups, results were compared and
linear correlations (R
2 = 0.96–0.98)
could be found. This suggests that MALDI/ToF MS is a suitable tool
for the semiquantitative characterization of polyester systems. For
the isophthalic/maleic acid ratio, MALDI results yielded constantly
lower values than 1H NMR, which was attributed to varying
ionization efficiencies of homo- and copolyesters. Ratios of carboxylic
and hydroxylic terminating groups, which are conventionally still
measured by time consuming complex titrations, were measured with
MALDI and 1H NMR and were in good agreement. Both methods
either excluded or distinguished unreacted monomers in the polyester
bulk in contrast to acid–base titrations where those monomers
severely distort the results. Additional structural information could
be gained including the observation of cyclic structures (MALDI), E/Z isomerism from maleic to fumaric acid,
and the statistical distribution of the acid components within the
polyester chain (1H NMR). While 1H NMR peak
assignments have to be verified by 13C NMR and multidimensional
techniques, MALDI/ToF MS provides a straightforward technique that
can be applied to other polyester systems without major alterations.