Poly(para-phenylene) (PPP) is a fully aromatic, rigid rod polymer with unique structural and conductive properties. As an engineering plastic, its attractiveness arises from its thermal stability (mp&gt;500.degree. C.), high strength, chemical inertness, and solvent resistance. When doped with either n- or p-type dopants, the polymer forms highly conducting charge transfer complexes with conductivities up to 500 S/cm. However, the structural properties which make PPP so attractive also make it a difficult polymer to synthesize. In addition, many of the observed properties of the polymer depend on the method of production.
Previous methods of producing PPP directly have met with only limited success. For example, oxidative cationic polymerization of benzene to produce PPP has been attempted. However, only short oligomers of ten to fifteen repeat units containing mixtures of linear 1,4- and non-linear 1,2-units were formed.
Polymerizations using nickel catalyzed aryl coupling of 1,4-dihalobenzenes were attempted. While this method produced a completely linear molecule, only short oligomers consisting of ten to twelve units were formed.
The problem with these direct synthetic methods is that the inherent insolubility of the polymer causes it to precipitate out of solution before high molecular weight materials can be formed. Electrochemical coupling of benzene has also been used, but the resulting film is insoluble and composed of a mixture of 1,4- and 1,2- units.
In order to circumvent the problem of the inherent insolubility of PPP in production and processing, soluble precursor methodologies have been developed. For example, polymers of 1,3-cyclohexadiene (CHD) have been used as a soluble precursor polymer. In particular, poly(cyclohexadiene) has been reacted with bromine and then pyrolyzed to eliminate HBr. Unfortunately, this polymerization route also produces a precursor polymer with a mixture of 1,4-and 1,2-linkages. In addition, the elimination reaction is not very efficient, since HBr readily reacts with unsaturated intermediates.
Recently, the efficient production of PPP has been reported, via the pyrolysis of a soluble precursor polymer prepared from the radical polymerization of the acetyl and methoxycarbonyl derivatives of 5,6-dihydroxy-1,3-cyclohexadiene (DHCD); see, e.g., D. G. Ballard et al., Macromolecules, Vol. 21, pp. 294-304 (1988) and D. R. McKean, Macromolecules, Vol. 20, pp. 1787-1792 (1987). The starting cis-diol is produced by the microbial oxidation of benzene. The precursor films are soluble and can be processed before pyrolyzing into the final polymer. However, the radical polymerization produces about 85% 1,4-units and 15% 1,2-units. The 1,2-units create "kinks" in the polymer, thereby reducing the elimination efficiency of the precursor and the mechanical properties of the final polymer.
In order for the good mechanical properties of PPP to be realized, an aspect ratio of at least 100 consecutive linear units per 1,2-unit must be obtained. All previous routes to PPP have either produced low molecular weight materials due to insolubility of the growing polymer, or have incorporated a significant amount of 1,2-linkages in the chains, or both. Hence, it is desirable to find an exclusively 1,4-polymerization method which can be used in combination with the efficient precursor method described above.