Soluble, highly conjugated derivatives of polyacetylene from the ring-opening metathesis polymerization of monosubstituted cyclooctatetraenes: Synthesis and the relationship between polymer structure and physical properties

Article abstract of DOI:10.1021/ja00057a024

Using well-defined tungsten-based olefin metathesis catalysts, a family of partially substituted polyacetylenes have been synthesized via the ring-opening metathesis polymerization (ROMP) of monosubstituted cyclooctatetraenes (RCOT). These polymers are highly conjugated, as evidenced by their visible absorption maxima. They are of high molecular weight, as evidenced by gel permeation chromatography, and most members of the family are soluble in the as-synthesized, predominantly cis form. The polymers can be isomerized to the predominantly trans form using heat or light. The rate of thermal isomerization was monitored by visible absorption spectroscopy. Polymers containing, in general, secondary or tertiary groups immediately adjacent to the main chain remain soluble in the trans form and are, in most cases, still highly conjugated. Overall, there is a connection between the steric bulk of the side group in polymers of monosubstituted COTs, their effective conjugation length, and their solubility. The side group twists the main chain of the polymer and also induces a preference for cis units in the chain. The tradeoff between conjugation and solubility has been explored, and highly conjugated polyacetylenes that are still soluble have been discovered. In the solid state, these polymers are observed to be amorphous by wide-angle X-ray scattering and near-infrared scattering. The amorphous nature of these samples correlates with the relatively low temperature cis-trans isomerization in the solid state. Upon iodine doping, these polymers become electrically conductive, although their conductivities are smaller than those of unsubstituted polyacetylene. Both empirical and semiempirical computational methods indicate an increased preference for cis linkages in partially substituted polyacetylene chains and show twists around the single bonds adjacent to the side groups in the polymer chain. The relative magnitude of these twists can be used to rationalize the differences in solubilities of the various polyacetylene derivatives, and these models provide a means of visualizing the conformation of the polymer, at least on its smallest size regime. The computations have also been useful in the rational design of new soluble polyacetylene derivatives with high effective conjugation lengths. By modeling and then synthesizing chains containing sec-butyl and other secondary groups, these properties have been realized.