A new route to poly(p-phenylene): stereoregular precursors via transition-metal-catalyzed polymerization

Citation

Gin, Douglas L. (1993) A new route to poly(p-phenylene): stereoregular precursors via transition-metal-catalyzed polymerization. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/kxta-zd48. https://resolver.caltech.edu/CaltechTHESIS:12112012-102613746

Abstract

Poly(p-phenylene)(PPP) is an insoluble rigid-rod polymer that possesses remarkable thermal stability, chemical resistance, and electrical conductivity when doped. The structural properties that make PPP such an attractive engineering material also make it very difficult to synthesize and process. Although many direct and precursor routes to PPP have been developed they have generally afforded low molecular weight material containing a substantial amount of structural defects that are detrimental to the properties of the final product. We have developed a new precursor route to PPP which offers processability as well as a high molecular weight, high quality polymer.

1,4-Linked, stereoregular precursors to PPP were synthesized by transition-metal­ catalyzed polymerization of heteroatom-functionalized 1,3-cyclohexadienes. cis-5,6- Bis(trimethylsiloxy)-1,3-cyclohexadiene (TMS-CHD),a derivative of a microbial oxidation product of benzene, is polymerized stereospecifically by bis[(η ^(3-)allyl)trifluoroacetatonickel(II)] with yields up to 96%. Not only does this polymerization system afford a highly 1,4-linked, stereoregular polymer, but it also has the potential for molecular weight control. The resulting polymer, [1,4-poly(TMS-CHD)], is a soluble, processable, semicrystalline material. Although 1,4-poly(TMS-CHD) cannot be pyrolyzed to yield PPP directly, the trimethylsilyl ethers on the polymer can be transformed to better leaving groups such as acetates to give the corresponding stereoregular acetoxy polymer (100% acetylation; 93% overall yield). Due to the low thermal stability of the stereroregular backbone, aromatization of this acetoxy polymer to PPP requires Lewis or Brønsted acid catalysts. Acids lower the onset temperature of the acid elimination process to a temperature regime well below that at which depolymerization can occur. The high quality PPP produced by the acid-catalyzed aromatization of the stereoregular acetoxy polymer exhibits properties comparable to those of PPP samples in the literature. However, it is completely amorphous whereas PPP samples made by other routes are almost always semicrystalline. Since the physical and chemical properties of PPP and many other conjugated polymers depend highly on sample morphology, processing techniques for this material will have to be developed before its optimum properties can be realized.

Thesis Files