Single‐crystal x‐ray diffraction studies of trans, trans‐1, 4‐bis‐[&bgr;‐pyridyl‐(2)‐vinyl]‐benzene (P2VB) and its solid‐state addition photopolymerization to a high‐crystallinity nearly three‐dimensionally aligned linear polymer are presented. Both P2VB and poly‐P2VB are orthorhombic. The lattice parameters area=21.09±0.16,b=9.61±0.05, andc=7.29±0.03 Å for P2VB, anda=19.16±0.20,b=10.69±0.05, andc=7.45±0.15 Å for poly‐P2VB. For both monomer and polymer the unit cell contains four monomer molecules. From systematic absences, the probable space group of P2VB isPbca. The observed absences for poly‐P2VB are also consistent with this space group. The solid‐state photopolymerization of P2VB is controlled by the crystal structure of this monomer with the orthorhombic axes of monomer and polymer on the average aligned in the same direction. The slight change in unit cell dimensions, the probably identical unit cell symmetry, and the invariance of average unit cell orientation suggest that minimal molecular rearrangement occurs during polymerization. Models are proposed for the polymerization mechanism and are related to the configuration and lattice structure of the resulting polymer. To obtain more detailed knowledge of the structure of crystalline P2VB and hence of the polymerization mechanism, energy minimization packing calculations have been performed. This monomer was found to deviate appreciably from a planar molecular structure. The present study is compared with recent work on the solid‐state photopolymerization of an isomeric compound trans, trans‐2, 5‐distyrylpyrazine (DSP).