Semiquantitative Approach for Polyester Characterization Using Matrix-Assisted Laser Desorption Ionization/Time-of-Flight Mass Spectrometry Approved by 1H NMR

Abstract

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 (R2 = 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.