Some Comments on a Recent Article Dealing with Three Aliphatic-Aromatic Polyesters and Their Spectral Identification

This work discusses the results appearing in Wu et al.'s (2014) article dealing with three aliphatic-aromatic polyesters based on terephthalic acid and their spectral identification. Using an instrument equipped with a switch that allows ions with masses below 5kDa to be discarded (ion deflector), the authors report the MALDI spectrum of a commercial poly(ethylene terephthalate) with signals up to 80kDa. I plot the spectral intensities and fit them with a Gaussian curve. The result is carefully examined. Thereafter, attention is moved to the MALDI spectra obtained when the ion deflector is turned off. A computation that gives the number-average and the weight-average molar mass (Mn and Mw) was performed, but the result was Mn=1362g/mol and Mw=1416g/mol. These values are much too low and they cannot be used. In order to explain this failure, I modified a model developed for poly(pentylene adipate). This hypothesis is discarded and the correct explanation is found. I also discuss other types of instrumentation, for instance, Py-GC/MS combined with library-search and also NMR spectroscopy, with specific reference to the chemical shifts of the -CH2- protons in aliphatic-aromatic polyesters.