Thermal Degradation of Polyolefins and of Polymers from Conjugated Dienes
One early study of thermal degradation of polyethylene was carried out on low molecular weight polymers [453]. Later the work was repeated with high-density polyethylene [454]. The volatile products were identified by gas chromatography. The biggest portion of the volatiles was found to be propylene.
The remaining materials were methane, ethane, propane, and a number of unsaturated and saturated higher hydrocarbons. The quantity of ethylene increased slightly with an increase in the temperature of pyrolysis. Formation of propylene can occur in two ways. The first one is through intramolecular radical transfer to the second carbon, followed by a decomposition reaction. This, however, is not believed to contribute significantly to the quantity of propylene obtained, based on theoretical consideration [454]. The second one can occur as a result of scissions of C–C bonds located in the positions b to the terminal double bonds. Such double bonds probably form in large quantities during decomposition and this process is thought to be the main source of propylene [454]: It was observed that isotactic polypropylene decomposes thermally by a mechanism that varies at different temperatures and conditions [455]. Thus, at 340C the major volatile product is propane, while at 380Citisn-pentane, and at 420C it is propylene. The propane is believed to originate from some weak spot in the polymeric chain. Formation of n-pentane involves a radical abstraction and a six-membered ring formation in a backbiting process. Propylene may come from a free-radical depolymerization process or a cyclic six-membered ring formation involving a terminal double bond [455]. The thermal degradation of diene polymers was the subject of several studies [456, 457]. The scheme for polyisoprene and polybutadiene degradation was postulated in part by Golub and Garguila [458, 460]. It is based on infra-red spectra and NMR studies of the products:
In addition, spectroscopic evidence shows that cis–trans isomerizations as well as cyclizations occur in the process of thermal degradation of 1,4-polyisoprene. It is interesting that the cis–trans isomerizations were observed at temperatures as low as 200C[460].
