Interactions between excipient polymer and the encased active pharmaceutical ingredient (API) govern a myriad of properties of the final drug formulation. Important aspects such as stability of the API, drug release profile, biological uptake and drug activity can be controlled by controlling the excipient-API interactions. Thus, understanding the specific drug-polymer interactions within the supramolecular assemblies is crucial for designing drug-delivery systems with engineered properties. Here we demonstrate investigations into the architecture of two systems: a) Where the excipient polymer and API are physically mixed together; and b) Where API is chemically conjugated to the polymer. In the first case, the mixing is enabled by anti-solvent directed millisecond co-precipitation of the drug molecule within excipient polymer matrix which allows for kinetic trapping of drug within the polymer matrix. We show that by choosing an appropriate polymer matrix, the drug molecule can be directed to adopt either an amorphous or a crystalline morphology. In the second case, chemical conjugation of the drug to the polymer, results in self-assembly of polymer encapsulated drug micelles. In this case, the micellar structure and concomitant drug release is shown to be governed by the amount of drug conjugated to the polymer. Detailed solid-state NMR spectroscopy investigations into these systems enable elucidation of the specific and resolvable interactions between the drug-and the polymer which guide the supramolecular assembly and the resultant pharmacological properties of the materials. We also demonstrate how the use of complimentary techniques such as XRD and EM, enable the development the all-important structure-property relationship in these systems.