The set of 1,3,4,6-tetraphenylhexa-l,5-dienes (1) represents a perturbation of Cope's rearrangement by four radical-stabilizing phenyl groups all positioned to drive the transition region toward the homolytic-colligative end of the mechanistic spectrum. The appearance of (Z)-isomers being suppressed thermodynamically by a steric interaction of +2.6 kcal mol(-1) per cia double bond, an equilibration that is stereochemically not of any Cope type, emerges as the predominant reaction. It is an interconversion of rac-(E,E)-1 and meso-(E,E)-1 (48:52; 77.3-115.3 degrees C) with the following values of the enthalpy, entropy, and volume of activation: Delta H-double dagger = 30.7 +/- 0.2 kcal mol(-1), Delta S-double dagger = +2.1 +/- 0.4 cal mol(-1) K-1, and Delta V-double dagger = +13.5 +/- 0.1 cm(-3) mol(-1), respectively. Structures have been established by X-ray crystallographic analysis; a possible relationship between dihedral angle and bond lengths in the styrene portions is proposed. The entropy of activation is incompatible with a chair or boat Cope rearrangement; the volume of activation is neither low enough for a pericyclic Cope ("concerted") mechanism nor high enough for a homolytic-colligative mechanism involving full dissociation as the rate-determining step. Trapping and a crossover experiment give some but only partial support to the intermediacy of free radicals. At higher temperatures, however, electron spin resonance experiments demonstrate an equilibrium with kinetically free (E,E)-1,3-diphenylallyl radicals. These observations are rationalized in terms of geometric reorganization within the confines of a 'cage'. Resolution by chiral chromatography of rac-(E,E)-1 allows recognition of a fast racemization (40-65 OC), of which Delta H-double dagger (21.3 +/- 0.1 kcal mol(-1)). Delta S-double dagger (-13.2 +/-0.3 cal mol(-1) K-1), and Delta V-double dagger (-7.4 +/- 0.4 cm(-3) mol(-1)) are consistent with a pericyclic Cope rearrangement. Enriched (Z)-isomers undergo Cope rearrangements in accord with the known influence of axiality and the chair/boat alternative on the energy of the transition region.