Sequential Single-Crystal-to-Single-Crystal Transformations Generate Polymer Polymorphs.

Accessing structurally well-defined polymer polymorphs remains extremely challenging because obtaining polymer single crystals suitable for atomic-level structural characterization is rare. Here we report an unusual sequence of single-crystal transformations in which a peptide monomer designed for topochemical azide-alkyne cycloaddition (TAAC) undergoes spontaneous single-crystal-to-single-crystal (SCSC) polymerization to form a 1,4-triazolyl-linked polymer, despite a crystal packing seemingly predisposed for the formation of a 1,5-linked product. Structural and computational analyses reveal that large-amplitude rotations of the azide and alkyne groups enable the unexpected regioselectivity. Prolonged heating subsequently triggers a rare SCSC polymorphic transition of the polymer crystal, generating a second conformational polymorph with distinct chain packing and mechanical properties. These results demonstrate that sequential single-crystal transformations can provide controlled access to polymer polymorphs from a single monomer crystal.