Investigation of Molecular Weight and Terminal Group Effects on the Time-of-Flight Secondary Ion Mass Spectra of Polyglycols

A series of poly(ethylene glycol) (PEG) and poly(propylene glycol) (PPG) samples ranging in molecular weights from 400 to 100 000 Da were characterized by time-of-flight secondary ion mass spectrometry. Molecular weight values from the oligomer distributions in the spectra were determined and compared to those obtained by conventional methods. The fragmentation patterns of the polyglycols were examined and can be explained by mechanisms involving simple chain scissions. The predominant fragmentation mechanisms involve scission of the C-O bonds, similar to fragmentation pathways observed for aliphatic ethers in other types of mass spectrometry. The relative fragment intensities of the clusters in the PEG and PPG fragment patterns were examined as a function of sample molecular weight. Although no dramatic changes in relative intensity were observed for the PEGs, the PPG fragment patterns were found to change with the molecular weight of the polymer. A series of poly(ethylene glycols) containing different terminal groups were also examined to determine the effect of the terminal group on fragmentation. In general, the major peak series observed in the spectra of small diacrylate-, dimethacrylate-, dipalmitate-, and distearate-terminated PEGs correspond to either oligomers or single cleavage of the PEG chain. Peaks resulting from cleavage within the terminal group or cleavage of tbe terminal group from the PEG chain were generally not observed for MW < 4000. The terminal groups in high molecular weight PEGs can be qualitatively identified on the basis of the fine structure of clusters within the main chain fragment patterns. In addition, it was determined that PEG distearate was actually terminated with a mixture of palmitate and stearate groups.