Supramolecular block copolymers composed of discrete blocks have promising properties for nanotechnology resulting from their ability to combine well-defined morphologies with good bulk material properties. Here, we present the impact of a well-defined siloxane block in either the main-chain or present as pendant grafts on the properties of supramolecular block copolymers that form ordered nanostructures with sub-5 nm domains. For this, two types of supramolecular block copolymers were synthesized based on the ureidopyrimidinone-urethane (UPy-UT) motif. In the first, oligodimethylsiloxanes (oDMS) of discrete length were end-capped with the UPy-UT motif, affording main-chain UPy-UT-Sin. In the second, the UPy-UT motif was grafted with discrete oDMS affording grafted UPy-UT-g-Si7. For the two systems, the compositions are similar; only the molecular architecture differs. In both cases, crystallization of the UPy-UT block is in synergy with phase segregation of the oDMS, resulting in the formation of lamellar morphologies. The grafted UPy-UT-g-Si7 can form long-range ordered lamellae, resulting in the formation of micrometer-sized 2D sheets of supramolecular polymers which show brittle properties. In contrast, UPy-UT-Sin forms a ductile material. As the compositions of both BCOs are similar, the differences in morphology and mechanical properties are a direct consequence of the molecular architecture. These results showcase how molecular design of the building block capable of forming block copolymers translates into controlled nanostructures and material properties as a result of the supramolecular nature of the interactions.