Soluble polyacetylenes by the ring-opening methathesis polymerization of substituted cyclooctatetraenes

Citation

Ginsburg, Eric Jay (1991) Soluble polyacetylenes by the ring-opening methathesis polymerization of substituted cyclooctatetraenes. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/wqt5-ka47. https://resolver.caltech.edu/CaltechETD:etd-06212007-094300

Abstract

Ring-opening metathesis polymerization has been applied to the synthesis of substituted polyacetylenes. The set of Me3M-substituted cyclooctatetraenes, where M = C, Si, Ge and Sn, have been polymerized using a well-defined tungsten alkylidene as an active metathesis catalyst. The resulting high molecular weight polyacetylenes have a substituent located at every eighth carbon atom, on the average. This substitution pattern allows substantial electronic delocalization along the main chain, in contrast to more highly substituted polyacetylenes which have been studied in the past.

The properties of the four polymers depend on the size of the side chain. As the substituent is varied from t-Bu to Me3Si to Me3Ge or Me3Sn, the absorption maximum of the polymer decreases in energy. Also, whereas poly(t-butylcyclooctatetraene) and poly(trimethylsilylcyclooctatetraene) are soluble in organic solvents, including benzene, methylene chloride, and tetrahydrofuran, the trimethylgermyl- and trimethylstannyl-substituted polymers are not totally soluble.

As synthesized, the polymers contain a high percentage of cis double bonds. They may be isomerized to a form containing mostly trans double bonds either photochemically or thermally. The trans isomers absorb at lower energy than do the cis isomers, indicating that the trans polymers can assume a more conjugated conformation. A set of visible absorption spectra obtained over the course of a photochemical isomerization contain an isosbestic point, which indicates that a number of double bonds are isomerizing simultaneously, i.e., the isomerization does not proceed a bond at a time.

Molecular mechanics calculations have been used to explore the effect of the steric requirements of the side chain on the conformation of the polyene backbone. It is observed that those polyenes which are soluble are calculated to be significantly twisted because of nonbonded interactions between the substituent and the protons on adjacent carbon atoms. Substituted polyenes calculated to have a more planar conformation are found to be insoluble in the trans form.

trans-Poly(trimethylsilylcyclooctatetraene) films are greenish gold and appear metallic. After iodine doping, they have an electrical conductivity of ~ 10(-1) [omega](-1) cm(-1). The doped polymer has been used to investigate the properties of conducting polymer/silicon Schottky-type solar cells in a collaborative effort with Drs. Michael Sailor and Nathan Lewis. It was found that the polymer/semiconductor interface is not subject to Fermi-level pinning, and that the efficiency of the device is limited by the properties of the silicon, not those of the interface, in contrast to devices made with conventional metals on silicon. The third-order nonlinear optical coefficient of the undoped polymer has been measured by Dr. Joseph Perry and was found to be 2±1 x 10(-11) esu.

Preliminary investigations of the polymers by scanning tunneling microscopy have been initiated in collaboration with Drs. Reginald Penner and Lewis. Some images which may represent individual polymer strands have been obtained, although interpretation of the data is complicated by the appearance of defects in the underlying graphite substrate.

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