Mass spectral analysis of low-temperature pyrolysis products from poly(ethylene glycol)

A poly(ethylene glycol) sample of average molecular weight 2000 was subjected to low temperature pyrolysis. The polymer was pyrolyzed in Pyrex tubes (sealed at one end) placed at the end of the carrier gas inlet in a gas chromatograph. Pyrolysis was carried out under argon flow in the temperature range 150–325°C. After pyrolysis, the residue in the pyrolysis tube was analyzed by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) and by direct probe chemical ionization (isobutane CI-MS). Eight series of oligomeric pyrolyzates were characterized by MALDI-MS and CI-MS. Assignment of chemical structures was aided by tandem mass spectrometry (CI-MS/MS) and by deuteration of hydroxyl end groups in the pyrolyzate. Pyrolysis of PEG ensues as low as 150°C in inert atmosphere. At the lowest temperatures the dominant oligomeric products have hydroxyl and 'ethyl ether' end groups. At higher temperatures, 'methyl ether' and 'vinyl ether' end groups become more abundant in the pyrolyzates. The pyrolysis experiments indicate that the decomposition scheme is free radical in nature. Cleavage of CO bonds is preferred at the onset of pyrolysis; hydrogen abstraction following CO homolysis produces hydroxyl and 'ethyl ether' end groups. At higher temperatures, the abundances of the 'methyl ether' and 'ethyl ether' end groups become more balanced as relatively more CC cleavage occurs. The increasing abundance of 'vinyl ether' end groups at higher pyrolysis temperatures is primarily due to dehydration of hydroxyl end groups.