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

Abstract

A poly(tetrahydrofuran) (PTHF) sample of average molecular weight 2000 was subjected to low temperature pyrolysis. The polymer was pyrolyzed in Pyrex tubes (closed 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 150-350°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). Eleven 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 poly(tetrahydrofuran) ensues about 175°C in inert atmosphere. The initial pyrolysis products all have at least one hydroxyl end group, which is retained from the low molecular weight starting polymer. The other end group is ‘ethyl ether’, ‘propyl ether’, ‘butyl ether’, or ‘aldehyde’. At higher temperatures (250–350°C) there is an increasing tendency to form products with a combination of ‘alkyl ether’ and/or ‘aldehyde’ end groups. The proportion of products containing the hydroxyl end group diminishes at the higher temperatures. There is little tendency to form ‘alkenyl ether’ end groups during PTHF pyrolysis. Only the ‘butenyl ether’ is observed to any appreciable extent. This is evidently formed mainly via dehydration of oligomers containing a hydroxyl end group. A free radical mechanism is proposed to explain the main degradation products.