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.