Salt Formation as a Regioselectivity Tuning Strategy in Topochemical Polymerization: Generating Two Structurally...

Precise regiochemical control in solid-state reactions is a major challenge, as conventional solution-based tuning methods are unavailable. Topochemical azide-alkyne cycloaddition (TAAC) offers a reliable path to triazolyl-linked polymers; however, the resulting triazole linkage is sensitive to crystal packing, yielding either a 1,4-triazolyl (trans) or 1,5-triazolyl (cis) geometry. We report an unprecedented strategy to tune this solid-state regiochemistry by employing crystalline salt formation. By designing an amine-functionalized monomer, M, we demonstrate that merely changing the counteranion (e.g., from the neutral monomer M1 to its salts M2-M4) fundamentally reorganizes the crystal lattice. This reorganization switches the outcome of the reaction from an exclusively trans-triazole-linked polymer (P1) to an exclusively cis-triazole-linked polymer (P3) in a topotactic fashion. Interestingly, the free amine M1 upon polymerization yields a stereocomplex containing alternating layers of enantiomeric P and M helical trans-triazole-linked polymer chains. Furthermore, we show that when a chiral sulfonate salt (M4) is used, it polymerizes to a yet another stereocomplex wherein the chiral anions template a novel PMMP helical repeat of cis-triazole-linked polymer chains. This stereocomplex formation involved a significant rotational rearrangement of the propargyl group (110°) while maintaining crystal integrity. This work establishes crystalline salt formation as a powerful tool for directing chemical reactivity, tuning the regiochemistry of topochemical reactions, and controlling chain packing modalities in the solid state.