Pyrolysis of polyolefin elastomers

Abstract

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‐ cyclisation of (particularly) methyl‐substituted n‐alkenes/alkanes of sufficiently high carbon number; and the reaction of intermediate molecules such as olefin + diene by a Diels‐Alder reaction. At the highest temperatures investigated, high hydrogen yiel...

Comparison of literature models for volatile product formation from the pyrolysis of polyisobutylene at mild conditions: Data analysis, free-radical mechanistic considerations, and simulation of initial product-forming pathways

“…For partial pyrolyses [6b,7g, 14,15], there is similar variability. Minor amounts of neopentane [3a,3b,3d, 4,15,19,20,34], isobutane [3b,7g, 15,19,34], and methane [3a,3b,15,18,19,20,34] have been reported. For partial pyrolyses, such H-rich products are of interest with respect to stoichiometrically balancing f t > 1 in the residue (see above); however, quantitative data are not abundant.…”

Sequencing of Sequence-Defined Oligourethanes via Controlled Self-Immolation

“…Controlled chemical degradations, , such as the classic Edman degradation, prevailed in the early days of polymer science but have fallen out of favor with the widespread adoption of NMR and mass spectrometry. , However, if the sequence-defined polymer is designed with degradative-sequencing in mind, controlled chain-end depolymerization still offers a powerful approach to primary structure determination. A limit to this approach is that copolymers seldom depolymerize in a controlled manner , as some homopolymers do . Triggered and complete depolymerizations, termed self-immolative polymers, are one solution to this problem. , In general, removal of a triggering group from the chain end results in a spontaneous cascading elimination or cyclization event that releases all monomers, the kinetics for which depend upon the structure of the polymer backbone and its breakdown mechanism.…”