Topochemical Alkyne Nitrile Oxide Cycloaddition for Polymer Synthesis

Abstract Topochemical reactions, characterized by their solvent‐free, catalyst‐free, and lattice‐controlled nature, offer an attractive method for crystalline polymers. The limited number of reactions amenable to solid‐state execution necessitates a continued pursuit of novel topochemical transformations. Here, we report the first example of a topochemical alkyne nitrile oxide cycloaddition (TANOC) reaction, a spontaneous and regiospecific [3 + 2] cycloaddition that directly converts a monomer to an isoxazole‐linked polymer in a SCSC fashion at room temperature. We overcome the inherent reactivity of nitrile oxides through strategic steric protection, enabling the design of a stable monomer amenable to crystallization. Detailed structural analysis of both monomer and polymer by single‐crystal X‐ray diffraction, coupled with comprehensive spectroscopic and kinetic studies, confirms the formation of an isoxazole‐backbone polymer. Notably, despite an initially unfavorable monomer packing, significant voids within the crystal lattice facilitate a ∼118° rotation of the propargyl unit, achieving the precise geometry for spontaneous reaction. This unprecedented thermal topochemical transformation expands the toolkit for solid‐state polymer synthesis, offering a versatile platform for constructing functional materials where the isoxazole linkages can be readily transformed into diverse functionalities.