Topochemical Alkyne Nitrile Oxide Cycloaddition for Polymer Synthesis

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.