Hydrogen-bonded aggregates in bulk polymers

Hydrogen bonds (H-bonds) are central in Nature , forming the “glue” of many biological materials and processes. Clustering and cooperativity of those bonds are functional principles in natural polymers such as proteins, cellulose, starch, and DNA, as well as synthetic polymers, among them polyamides or polyurethanes. The absence of mobile low-molecular-weight solvents in bulk polymers, combined with intrinsic anisotropy and restricted segmental motion of the surrounding polymer matrix, limits the diffusion of the H-bond motifs when covalently tethered onto a polymer skeleton, so inducing aggregates and higher-order assemblies of H-bonds then decisive for the final material properties. This review addresses fundamental aspects of H-bond association and the formation of supramolecular aggregates within different bulk polymers, discussing the influences of molecular proximity of the H-bonds and novel technical methods to extract dynamic information on their association state. There is a focus on specific sets of H-bond motifs, starting from “simple H-bonds” such as alcohol dimers (OH OH), carboxylic acid dimers (CA-CA), and amide dimers (Am-Am), to more complex assemblies, including ureidopyrimidinone dimers (UPy-UPy), hydroxyl‑pyridine complexes (OH-Pyr), thymine-diaminotriazine/diaminopyridine complex (Thy-DAT/DAP), and Hamilton-wedge-barbiturate/cyanurate complexes (HW-Ba/Cy). By providing a concise summary of literature (335 references) to illustrate the roles of H-bonds in polymers, this review identifies future strategies in the design of functional H-bonded materials, such as in self-healing elastomers, 3D-printing, or as reversible adhesives.