A self-healing crystal that repairs multiple cracks

We report self-healing in crystals during topochemical polymerization. Pursuing our interest in chiral crystalline polymers, we synthesized a squaramide-based monomer for topochemical azide-alkyne cycloaddition polymerization. The squaramide unit biased the molecular packing in the crystal and aligned the molecules in head-to-tail fashion as designed, positioning the azide and alkyne units of adjacent molecules at proximity in an arrangement suitable for their topochemical click reaction. This monomer undergoes single-crystal-to-single-crystal (SCSC) polymerization, spontaneously at room temperature, via regiospecific 1,3-dipolar cycloaddition, yielding the corresponding triazole-linked polymer, in a few days. When heated at 60 oC, the polymerization completes in 24 h, in a SCSC manner. Interestingly, upon continuous heating from rt to 110 oC, the monomer crystals develop multiple cracks and they self-heal immediately. Increase in heating rate leads to the formation of longer and wider cracks, which also heal instantaneously. The self-healed crystals retained their integrity and for the first time, crystal structure of a self-healed crystal could be analyzed by single-crystal X-ray diffraction. The quality of the self-healed crystal and its diffraction data are as good as the fully reacted crystals, at rt or 60 oC, without developing cracks. Rational analyses for cracking and self-healing are presented.