Pyrolysis of polyolefin elastomers

Commercial samples of two ethylene—propylene terpolymers (EPDM), two isobutene— isoprene copolymers (IIR), one polyisobutene (PIB) and one chlorosulphonated polyethylene (CSPE) were pyrolysed at temperatures between 770 and 1370 K in a quartz micro-furnace. Volatile products were analysed by gas chromatography using wide-boiling-range and high-boiling-point columns with flame ionisation detectors and/or online quadrupole mass spectrometry using helium as carrier. Low molecular weight gases were separated on a crosslinked polystyrene bead column and detected with a katharo-meter using argon as carrier. Mechanisms are put forward to account for the temperature-dependence of pyrolysis-product yields as follows. In the primary pyrolysis (ca 770–870 K) EPDM products arise principally from 1 : 5 : 9(: 13 : 17) intramolecular hydrogen transfer accompanied by some unzipping. There is also evidence for a small amount of transfer to the third carbon atom and of some β-scission. IIR and PIB yielded isobutene presumably by stepwise cyclic unimolecular elimination. However, a number of other products including telomers, methane, iso-butane and neopentane are explained by telomerisation of isobutene and intramolecular transfer reactions following random scission; at 870 K, there is already evidence for some aromatisation. CSPE pyrolysis is complicated by preliminary loss of SO₂Cl groups and dehydrochlorination to form a polyene. Concomitant crosslinking may involve inter-molecular elimination of HCl and/or a Diels-Alder type reaction between two polymer molecules which have already undergone dehydrochlorination. Subsequent thermal degradation of the main chain proceeds principally by 1 : 5 hydrogen transfer. At intermediate temperatures (above 900 K) secondary pyrolysis occurs in which higher alk-l-enes formed by primary pyrolysis break down to lower molecular weight products probably by a modified Rice mechanism and associated intramolecular cyclic dissociation. Similar mechanisms are proposed to explain the secondary products from IIR and CSPE. At higher temperatures (above 1000 K) further fragmentation, cyclisation and aromatisation. occur. For all the polymers, hydrogen yields increase sharply and yields of C₃/C₄ products diminish; C₂ yields show maxima at temperatures about 1100 K. Below 1200 K, benzene is formed in largest yield. Toluene yields fall at temperatures above 1100 K, but naphthalene is formed in increasing yields up to a maximum at about 1220 K. At these higher temperatures, the product spectrum is simplified by the disappearance of molecules of intermediate size leaving only small fragmentation products and rather large polynuclear aromatics (anthracene, etc.). Two main routes are suggested for the formation of cyclic products from EPDM and IIR-

• a cyclisation of (particularly) methyl-substituted n-alkenes/alkanes of sufficiently high carbon number; and
• b the reaction of intermediate molecules such as olefin + diene by a Diels-Alder reaction.

At the highest temperatures investigated, high hydrogen yields are accompanied by a substantial drop in total yield of volatilisable products and this is attributed to further condensation of aromatics in coking reactions.