The molecular weight and structural regularity of the polyphenylene produced from thermal conversion of two precursor polymers based on the acetyl derivative of 5,6-dihydroxy-1,3-cyclohexadiene depend on the stereochemistry of the precursor and the presence of aromatization catalysts. Two competing reactions occur during the bulk thermal conversion of these precursors : (1) thermal chain fracturing of the polymer and (2) thermally-induced acid elimination (aromatization) resulting in polyphenylene formation. The relative rates of these two processes ultimately determine the molecular weight of the final product and depend heavily upon the stereochemistry of the polymer backbone. For a 1,4-linked stereoregular precursor polymer made by nickel-catalyzed polymerization, the onset of chain degradation occurs before the onset of aromatization during heating. Consequently, this precursor only affords low-quality polyphenylene oligomers, despite having a regular stereochemistry that is ideal for facile cis pyrolytic acid elimination. On the other hand, the reverse relationships are true for its radically polymerized analog containing 10% 1,2-linkages. Although chain degradation still occurs during the pyrolysis of this atactic precursor, the relative amount of backbone fracturing is less than that of aromatization. The problems associated with the limited thermal stability of the precursor polymers can be overcome through the use of Bronsted and Lewis acid catalysts during bulk pyrolysis. Acids lower the onset temperature of aromatization to a regime well below that at which thermal chain scission can occur by selectively catalyzing the acid elimination reaction in both precursor polymers. However, characterization of the resulting polyphenylenes made from both polymers indicates that the structural regularity of the polyphenylene produced by the acid-catalyzed aromatization process depends entirely on the regiochemistry of the initial precursor. High molecular weight, structurally regular poly(p-phenylene) is produced only by the acid-catalyzed bulk aromatization of the 1,4-linked stereoregular polymer. Acid-catalyzed bulk pyrolysis of the radically polymerized analog only affords polyphenylene containing substantial amounts of 1,2-linkages.