A promising marriage between polymorphism and topochemistry: Synthesis of a polymer in two different secondary structures

We designed and synthesized a squaramide-based monomer decorated with azide and alkyne for topochemical azide-alkyne cycloaddition (TAAC) polymerization. The monomer crystallizes as two polymorphs, M1 and M2, both having crystal packing suitable for their topochemical polymerization. The squaramide bias the hydrogen-bonded molecular organization in both the polymorphs; H-bonded stacks of monomer are packed in the 3D forming head-to-tail organization of monomers with azide and alkyne units of adjacent monomer molecules juxtaposed in a transition-state-like arrangement for their regiospecific cycloaddition reaction. Along the polymerization direction, the monomers are arranged as sheets in polymorph M1 and helices in polymorph M2. Both the polymorphs undergo slow and spontaneous regiospecific TAAC polymerization at room temperature but react quickly at higher temperatures resulting in polymer linked by triazole through its 1,4-positions, with distinct mechanical responses; single crystals of polymorph M1 show expansion followed by contraction without any permanent dimensional change, whereas crystals of polymorph M2 undergo splitting upon heating. At moderate temperatures, both the polymorphs undergo single-crystal-to-single-crystal (SCSC) polymerization, resulting in two polymer-polymorphs with distinct secondary structures 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 stacked in a sheet-like fashion, both having the same chemical structure.