Single-Crystal-to-Single-Crystal Synthesis of a Rope-Ladder Polymer.

Here, we report a single-crystal-to-single-crystal (SCSC) synthesis of a fully organic ladder polymer via a topochemical azide-alkyne cycloaddition (TAAC) reaction. A designed monomer featuring a rigid benzene core with flexible peptide-linked azide and alkyne groups crystallized with three conformers in the asymmetric unit, as evidenced by the single-crystal X-ray diffraction (SCXRD) analysis. Hydrogen bonding between the amino acid residues facilitates the organization of the monomer in a reactive geometry with inter-conformer azide-alkyne end-to-end distances within Schmidt's criteria. Upon thermal activation, two of the three monomer-conformers reacted to form a dimer, leading to an intermediate 1:1 cocrystal of monomer and dimer. The intermediate cocrystal reacted further, forming a structurally perfect ladder polymer in an SCSC manner. SCXRD analysis confirmed the regiospecificity of the reaction yielding 1,5-triazolyl linkages in both the intermediate dimer and the polymer. The polymerization process was also studied by powder X-ray diffraction (PXRD), differential scanning calorimetry (DSC), and Fourier-transform infrared spectroscopy (FT-IR). This study demonstrates the scope of topochemical polymerization strategies to synthesize architecturally attractive crystalline polymers that enable precise structural elucidation.