Turning crystalline materials into polymers is tricky business. Polymerizing agents can’t slip into the crystal’s tightly packed matrix of molecules, so scientists have to use heat, pressure, or light and hope the crystal’s molecules are in the right orientation for polymerization to proceed. A new type of crystalline polymer has been discovered by researchers in California (Science 2014, DOI: 10.1126/science.1245875). A team led by Fred Wudl of the University of California, Santa Barbara, and Yang Yang of UCLA found that crystals made from two derivatives of the dye molecule bis(indenedione) polymerize in the presence of visible light—a rarity in solid-state polymerization. Before polymerizing, the crystals appear orange. After polymerization begins, the crystals become colorless, allowing visible light to penetrate further and prompt more polymerization. The polymerization process is reversible at elevated temperatures, and because the resulting polymer is strong along its polymer axis, it may find use as a strengthening component in composites. Should the polymer break, a color change would occur, so weakened areas could be spotted easily.