Highly conjugated, substituted polyacetylenes via the ring-opening metathesis polymerization of monosubstituted cyclooctatetraenes

Citation

Gorman, Christopher B. (1992) Highly conjugated, substituted polyacetylenes via the ring-opening metathesis polymerization of monosubstituted cyclooctatetraenes. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/2y3b-8q23. https://resolver.caltech.edu/CaltechTHESIS:08312011-085045975

Abstract

A number of monosubstituted cyclooctatetraenes were polymerized via ring-opening metathesis polymerization, producing derivatives of polyacetylene that were highly conjugated. Most of these polymers were soluble in the nascent, high-cis form, and some remained soluble upon isomerization to the trans form. The polymers had a high molecular weight (as determined by gel permeation chromatography) and were composed of polyacetylene backbones (as determined by Raman spectroscopy). Not all of the polymers were completely soluble after isomerization to the trans form. Observations relating the steric bulk of the side group with solubility behavior indicate a tradeoff between solubility and conjugation. Structural variation permitted the synthesis of very highly conjugated, yet soluble polyacetylenes. For example, trans-poly-s-butyICOT, had a low-energy optical absorption (560 nm in tetrahydrofuran, 586 nm in CS_2) corresponding to extended conjugation. The polymer was formed as an amorphous film, yet it had a conductivity after iodine doping of 0.03 S/cm. THG measurements at 1064 nm gave I χ (3) I = 2 ± 1 x 10^(-11) esu for polytrimethylsilylCOT. Activation parameters for cis/trans isomerization of trans-poly-s-butyICOT in solution were obtained. NMR spectroscopy revealed that two isomers exist in the so-called "trans" form, suggesting the existence of cis/trans and trans/trans diads in the polymer. The trisubstituted olefin at which the side-group is attached can isomerize between the cis and the trans forms with a very low barrier (ΔG‡ =18.9± 0.4 kcal/mol was measured at 50°C by magnetization transfer in benzene). Both semi-empirical (AM1) and empirical (MM2) calculations on model compounds were used to provide a model relating side group sterics with twisting in the polymer main chain. Molecular dynamics was used to probe the magnitude of motion in the polymer main chain. AMI heat-of-formation calculations were used to explore the cis/trans preference of a trisubstituted double bond in a polyene chain. Polyacetylenes bearing chiral side groups were synthesized and explored. Strong Cotton effects were observed for the backbone π → π* transition. The data suggest that the side group geometrically perturbs the main chain, possibly twisting it in predominantly one screw sense.

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