Transition-metal-catalyzed polymerization of heteroatom-functionalized cyclohexadienes: stereoregular precursors to poly(p-phenylene)

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 difficult to synthesize and process. Direct synthetic approaches have given either ill-defined material with a mixture of para, meta, and ortho linkages and crosslink or insoluble oligomers. Precursor strategies to PPP have been devised in which the synthetic and processing difficulties of the direct methods have been overcome through the use of a soluble intermediate polymer. The most successful of the precursor strategies was developed at ICI by Ballard et al. This process involves the radical polymerization of the bis(acetyl) or bis(methoxycarbonyl) derivatives of cis-5,ddihydroxy- 1,3-cyclohexadiene (l), a microbial oxidation product of benzene. The resulting polymers are subsequently aromatized to yield PPP by thermally induced acid elimination. This process, however, yields only phenylene oligomers due to fracturing of the precursors during pyrolysis. Fracturing is believed to arise from the 90% 1,4-linkages, 10% 1,2-linkages, and random stereochemistry along the precursor backbones which result from the nonstereospecific nature of the radical polymerization. A route to 100% 1,4-linked PPP precursors with the correct stereochemistry for facile cis-pyrolytic elimination of the pendant groups has been developed which combines the efficiency and processability of the ICI process with the regio- and stereochemical control possible through transition-metal catalysts (Scheme I).