Single-Crystal-to-Single-Crystal Synthesis of a Polymer in Two Distinct Topologies.

A squaramide-based monomer, designed for topochemical azide-alkyne cycloaddition (TAAC) polymerization, crystallizes as two polymorphs, M1 and M2, both having crystal packing suitable for topochemical polymerization. The hydrogen-bonding between squaramide units  bias the molecular organization in both the polymorphs. 3D packing of H-bonded stacks of monomer lead to juxtaposition of  azide and alkyne units of adjacent molecules in a transition-state-like arrangement for their regiospecific cycloaddition reaction. The monomers are arranged as supramolecular sheets and supramolecular helices in polymorph M1 and M2 respectively. Both the polymorphs undergo slow and spontaneous regiospecific TAAC polymerization at room temperature, but react quickly at higher temperatures, resulting in 1,4-traizolyl-linked polymer, with distinct mechanical responses. Upon heating, single crystals of polymorph M1 show expansion followed by contraction without any permanent dimensional change, whereas crystals of polymorph M2 undergo splitting. At moderate temperatures, both the polymorphs undergo single-crystal-to-single-crystal (SCSC) polymerization, resulting in two polymer-polymorphs with distinct topologies that can be studied at atomic resolution by single-crystal X-ray crystallography. The polymorph M1 reacts to polymer P1 with β-sheet-like topology, and polymorph M2 reacts to polymer P2 having polymer chains of helical conformation. Nanoindentation experiments with crystals of these polymers revealed their distinct mechanical properties.