Derivatization of Low Molecular Weight Polymers for Characterization by Matrix-assisted Laser Desorption/Ionization Time-of-flight Mass Spectrometry

Abstract

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) is shown to give inaccurate results for the analysis of low molecular weight ethoxylate polymers. It is demonstrated that when the degree of ethoxylation is low (n <9), MALDI-TOFMS produces substantially higher values for the number-average molecular weight (Mn) than obtained with more classical methods such as NMR spectrometry and a wet chemical method to determine the hydroxyl number. Apparently, this is the result of discrimination of the former technique against lower molecular weight ethoxylates in a polymer distribution. The results presented in this paper demonstrate this discrimination through the analysis of a series of ethoxylates with progressively lower Mn values. It was also found that by derivatization of the polymer with phthalic anhydride, to produce the carboxylate derivative, the discrimination is reduced and data obtained with MALDI-TOFMS match more precisely the results obtained with the other methods. © 1997 John Wiley & Sons, Ltd.