Cyclic oligomers in the cationic polymerization of cis- and trans-2,3-dimethylthiirane

✍ Scribed by Wilfried M. Van Craeynest; Eric J. Goethals

• Publisher: Elsevier Science
• Year: 1976
• Tongue: English
• Weight: 452 KB
• Volume: 12
• Category: Article
• ISSN: 0014-3057
• DOI: 10.1016/0014-3057(76)90132-4

✦ Synopsis

Under the influence of triethyloxonium tetrafluoroborate in methylene chloride, cis-and trans-2,3-dimethylthiirane are transformed to their corresponding polymers almost instantaneously (at temperatures between 0 and 20 °) but within a few hours these polymers completely degrade to form a mixture of low molecular weight substances. The cis-monomer leads to a mixture of cyclic tetramer, cis-butene and two geometric isomers of 3,4,6,7-tetramethyl-l,2,5-trithiepane. By 1H-NMR it was shown that these two isomers are the trans-eis-trans and the trans-trans-trans forms. The trans-monomer leads to a mixture of trans-butene and 3,4,6,7-tetramethyl-l,2,5-trithiepane which is present as a mixture of the cis-cis-cis and the cis-trans-cis isomers. In both cases, the butene and the trithiepanes were formed in equimolar quantities. The formation of these oligomers is explained by assuming that polymerization occurs via an SN2 propagation reaction between the three-membered cyclic sulphonium ion (the active species) and monomer, followed by a degradation reaction occurring via a back-biting mechanism.