Pyrolysis field ionization mass spectrometry of hydrocarbon polymers

Abstract

In earlier work from our laboratory it was shown that pyrolysis-field ionization-mass spectrometry (Py-FIMS) has the ability to generate mass spectra for long, recognizable sequences from a number of saturated hydrocarbon polymers (polyolefins). In this study it is shown that two diene rubbers, polybutadiene (PBd) and polyisoprene (PIso), also produce long sequences via Py-FIMS. These diene rubbers yielded only one series of intense oligomers at lower masses (up to ~ 800 Da); these pyrolyzates are multiples of the monomer unit. The higher-mass pyrolysis products are only observable during the initial stage of decomposition (~ 310–325 °C). The major features of the pyrolysis spectra for the two polymers can be interpreted in terms of free radical degradation mechanisms. The production of long sequences of pyrolyzate oligomers may be explained in terms of main-chain scission followed by intramolecular radical transfer (backbiting) reactions and β-scission to produce volatile pyrolyzates. Polynorbornene (PNb) was also studied as an example of a polymer with a cyclic hydrocarbon monomer unit. The thermal degradation of PNb does not appear to have been studied previously. The Py-FI mass spectrum is rather complex, but the most prominent pyrolyzates are norbornene oligomers. As with PBd and PIso, the thermal decomposition of PNb can be explained by free radical degradation chemistry.