A hydroxyl group dictates handedness, pitch, and mechanics in a crystalline covalent helical polymer.

Abstract

Helical polymers are fascinating materials, yet retaining their helicity in the solid state remains a key challenge for practical applications. Here, we present a regiospecific topochemical azide-alkyne cycloaddition for synthesizing crystalline covalent helical polymers via a single-crystal-to-single-crystal transformation. Building on our prior glycine-proline-based systems, we investigated the effect of subtle modification: introducing a hydroxyl group to the proline unit. This minor change induced a helicity reversal (left- to right-handed) and a substantial reduction in pitch (8.9 to 4.9 Å). Single-crystal x-ray diffraction revealed that this profound reordering is driven by intra- and intermolecular hydrogen bonds (C─H⋯O and O─H⋯N), acting as supramolecular bridges between helices, enabling denser packing. Consequently, the glycine-hydroxyproline-based polymer exhibited markedly enhanced mechanics, with a Young's modulus of 18.57 ± 2.01 GPa and hardness of 1.09 ± 0.13 GPa, surpassing natural collagen. This study provides insights into controlling polymer topology and mechanics through subtle modifications, establishing a powerful paradigm for the design of next-generation helical materials with tailored functionalities.