Model "supramolecular IPNs" were developed via the formation of a hydrogen-bonded, supramolecular network of 2-ureido-4-[1H]-pyrimidinone (UPy) telechelic poly(ethylene-co-1-butene) (SPEB) in the presence of photopolymerizable, hydroxyl-terminated polybutadiene (HTPB). The role of a supramolecular elastomeric phase in mechanical toughening of IPNs was explored through (1) dynamic dissociation and reassociation of the noncovalent, UPy supramolecular associations, and (2) interphase formation. While an similar to 4X increase in tensile toughness of the HTPB matrix was observed through incorporation of 10 wt % ethylene-propylene rubber (EPR)-as a conventional elastomeric toughening agent into HTPB, it was shown that adding the same amount of supramolecular elastomer SPEB to HTPB led to similar to 600x enhancement in tensile toughness. Strain rate-dependent mechanical response and fractography studies revealed that this dramatic toughness enhancement was due to dissociation/reassociation of the dynamic UPy linkages in the elastomeric phase that facilitated dilatational yielding of the IPN. This toughness enhancement was only observed in combination with the existence of strong interfacial coupling between the matrix and supramolecular phase as revealed by transmission electron microscopy and dynamic mechanical analysis. By exploiting noncovalent dynamics and interfacial control in interpenetrating networks, pathways are envisioned toward a new class of tough materials.