Supramolecular polymers are attractive materials due to their dynamic, reversible interactions, offering stimuli-responsive and self-healing properties. However, achieving these characteristics within mechanically robust, end-functionalized polymers remains challenging, as the high molar masses required for entanglement dilute the concentration of end groups, limiting supramolecular aggregation. Recently, it has been shown that this limitation could be mitigated by blending a high molar mass end-modified polyester with a low molar mass additive, resulting in drastic improvement in the materialsâ melt properties. To generalize this approach to a broader range of polymers, we investigated this co-assembly strategy and established clear structureâ property relationships as a function of polymer molar mass, additive concentration, and temperature. We employed a soft, amorphous and hydrophobic polymer matrix based on hydrogenated poly(isoprene) (HPI), end-modified with short, ditopic, β-sheet forming oligopeptide ligands. First, we developed reliable synthetic protocols for preparing well-defined poly(isoprene) (PI) telechelics spanning a wide molar mass range, far exceeding the HPI entanglement threshold. This was achieved with a difunctional anionic initiator following a detailed study on its capability to show bidirectional growth. The telechelic PI were then quantitatively hydrogenated and functionalized with oligopeptide ligands. Secondly, we investigated the phase behavior of the blends of modified polymer and the additive and demonstrated that, below a threshold additive concentration, the co-assembly between polymer end-groups and the additive resulted in the formation of antiparallel β-sheet nanofibrils, whereas reference blends of the additive and unmodified HPI only led to the solidification of the additive into bulk precipitates. The nanofibrils exhibited higher dissociation temperatures compared to aggregates formed solely by polymer end-groups, and we found that the stability of this phase originated from incorporation of the additive into micelles formed by end-groups at elevated temperatures. These micelles stabilized roughly equimolar amounts of the additive and end-groups regardless of polymer molar mass. However, the threshold concentration decreased as molar mass increased due to a lower end-group fraction involved in micellization. Finally, we showed that the supramolecular aggregates exhibited a hierarchical organization, comprising uniformly dispersed, high-aspect-ratio nanofibrils. These nanofibrils had uniform diameters corresponding to a stack of 4-5 β-sheets, arranged locally in a hexagonal packing. Notably, the addition of additive below the threshold content systematically led to longer, better defined nanofibrils, which was particularly noticeable for the highest molar mass polymer where aggregation was otherwise almost suppressed. In contrast to the viscous behavior of unmodified HPI, the modified polymers showed pronounced rubber-like properties, originating from the presence of polymer-bridged supramolecular aggregates and an entanglement network. Notably, the addition of the additive led to enhanced elastic responses and increased plateau moduli, promoting polymer bridging and filler reinforcement.