The mechanical properties of poly-p-phenylene and its simplest oligomer, biphenyl, have been explored by ab initio and semiempirical molecular orbital methods. The contribution of zero-point energy to the torsional barrier is negligible, but the variation of vibrational entropy with torsion increases the height of the free energy barriers to rotation through the D-2h and D-2d conformations. The computed Hessians argue for a backbone that becomes 14% stiffer when strained to planarity. Cluster calculations of the polymer using periodic boundary conditions illuminate the role of bowing in compression and the importance of intermolecular forces to compressive mechanical performance of rigid-rod polymers.