During 1948–1954, the field emphasized connecting solution behavior to molecular architecture, using intrinsic viscosity, diffusion, and temperature effects as keys to chain dimensions. A random coil picture with hydrodynamic interactions and hindered internal rotation linked microstructure to macroscopic rheology, guiding Debye- and Flory-inspired analyses. The era established methods for practical molecular weight estimation from viscosity data and laid groundwork for solvation and structure–property thinking. Historical Significance: This period solidified the central idea that microstructure dictates rheological—and processing—behavior, anchoring modern polymer solution theory and setting the stage for later kinetic and structural explorations in polymerization, including branching phenomena in polyethylene.