A Spring-Loaded Monomer

By incorporating a trans double bond into an eight-carbon annulene, chemists have made a strained ring structure that snaps open, paving the way for a ring-opening metathesis polymerization reaction that produces a new type of polymer (Angew. Chem. Int. Ed., DOI: 10.1002/anie.200704355). The researchers, led by Columbia University???s Colin Nuckolls, hope to render this polymer useful in electronic applications by oxidizing it into thin strips of graphene.

The unusual new annulene is a benzannulated version of cyclooctatetraene, in which one of the cyclooctatetraene moiety???s double bonds is in the trans configuration. This trans double bond introduces a kink in the annulene, giving it a helical shape. The p system of the trans double bond is nearly perpendicular to the p system of the rest of the molecule, isolating it from conjugation.

According to density functional theory, the compound???s strain energy is 18 kcal per mole higher than that of its all-cis isomer. This excess strain turns the molecule into a spring-loaded monomer for ring-opening metathesis polymerization. In the presence of a ruthenium-based catalyst, the annulene polymerizes into a new type of oligophenylene vinylene with all-ortho linkages.