Single-crystal-to-single-crystal (SCSC) polymerization offers an effective protocol for the environmentally friendly preparation of polymer single crystals (PSCs) with extremely high crystallinity and very large molecular weights. Single-crystal X-ray diffraction (SCXRD) serves as a powerful technique for the in-depth characterization of their structures at a molecular level. Hence, a fundamental understanding of the structure-property relationships of PSCs is within our reach. Most of the reported PSCs, however, suffer from poor solubility, a property which hampers their post-functionalization and solution processability when it comes to practical applications. Here, we report soluble and processable PSCs with rigid polycationic backbones by taking advantage of an ultraviolet-induced topochemical polymerization from an elaborately designed monomer that results in a multitude of photoinduced [2 + 2] cycloadditions. The high crystallinity and excellent solubility of the resulting polymeric crystals enable their characterization both in the solid state by X-ray crystallography and electron microscopy and in the solution phase by NMR spectroscopy. The topochemical polymerization follows first-order reaction kinetics to a first approximation. Post-functionalization of the PSCs by anion exchange renders them super-hydrophobic materials for water purification. Solution processability endows PSCs with excellent gel-like rheological properties. This research represents a major step towards the controlled synthesis and full characterization of soluble single-crystalline polymers, which may find application in the fabrication of PSCs with many different functions.