Understanding the relationship between a polymer’s molecular structure and its bulk properties (i.e., mechanical and morphological behaviors) is necessary for the design of next-generation materials with targeted properties. In this vein, we have developed methods to alter and probe the influence of polymer composition on the material properties of two high-value products—thermoplastic elastomers (TPEs) and polyurea (PU). By systematically varying the molecular weight distribution shape in TPEs, we were able to precisely determine the role of chain length composition on the mechanical and morphological properties. Furthermore, by developing an experimental protocol for in-situ structural characterization using X-ray scattering techniques, we were able to elucidate the influence of polymer composition on PU materials under high-pressure conditions. These experimental methods have clearly demonstrated the value of understanding structure-property relationships in TPEs and PU-based materials, particularly how polymer composition can be used as a handle to predictably tune polymer properties.