Single‐Crystal‐to‐Single‐Crystal Synthesis of a Rope‐Ladder Polymer

Abstract

Abstract 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.