Pyrolysis mechanisms of natural rubber deduced from the dependence of product yields on sample size

✍ Scribed by Sally Groves; Roy Lehrle

• Publisher: Elsevier Science
• Year: 1992
• Tongue: English
• Weight: 460 KB
• Volume: 28
• Category: Article
• ISSN: 0014-3057
• DOI: 10.1016/0014-3057(92)90257-3

✦ Synopsis

The formation of oligomers by secondary reactions in polymer pyrolysis can be distinguished from their formation as primary products by studying the dependence of product yields and ratios on sample size. This study has been carried out for natural rubber (cis-l,4-polyisoprene), degraded on a thermocouple-controlled filament at 500 °. The sample sizes were in the range 0.2-1.8 #g, deposited from solution on a portion of the filament of area 3.0 mm 2. The results show that the monomer and the three principal dimers (DI, D2 and D3) are formed mainly as primary products, though each of the dimers is also formed by secondary processes involving monomer recombination. For sample sizes above 1/zg, there is evidence that D3 isomerizes to D2. In contrast to dimers, trimers are formed very largely by secondary reactions involving monomer and dimer, and all three dimers are reactive in this respect. Trimers may also be formed by a process involving three monomer molecules, but it is considered that this process is more likely to be reaction of monomer with a "transient" dimeric species. Mass spectrometric analysis suggests that probable identities of the dimers observed in the present study are DI = 1,7,7-trimethyl-bicyclo 2,2,l-hept-2-ene, D2 = 2,5,6-trimethyl 1,3,6-heptatriene and D3 = I-methyl-4-(1-methylethenyl)cyclohexene; (dipentene/limonene). A reasonance-stabilized biradical structure is proposed for the transient dimeric species.