Polymers often have the ability to undergo large deformations before failure, setting them apart from other materials. Considerable effort in current polymer research is dedicated to predicting and tailoring mechanical properties. Notably, there is an emphasis on understanding structure-to-property relationships. This dissertation focuses on three specific polymeric materials—glassy polycarbonates, biopolymer gels, and phase-segregated polyurea elastomers—to explore how nanoscale features influence bulk mechanical properties during large deformations. The first part of my dissertation describes the covalent incorporation of a force-sensitive spiropyran in glassy polycarbonate. When sufficient force is applied, the spirocyclic C-O bond breaks and the molecule isomerizes with a distinct color and fluorescence change. Monitoring the emission change during deformation allows the use of spiropyran as a molecular probe to understand how stress accumulates within a polymer glass during plastic deformation. I found that spiropyran activation is strain rate dependent, revealing that polymer glass chains become dynamic during large deformations. The second part describes using metal-ligand crosslinking to create environmentally responsive biopolymer gels made from Nereis virens marine worm jaw protein (NVJP-1). I investigated how the choice of metal cation and the environmental pH influences these non-covalent crosslinks, thus modifying the swelling and stiffness of the gels. This adaptability offers a pathway for creating functional polymers with dynamic mechanical properties. The third part of my dissertation involves modeling the heterogeneous nanostructure of polyurea using finite element representative volume element simulations. Polyurea has chains with both glassy and elastomeric segments, resulting in heterogeneity. I successfully predicted key rate-dependent mechanical behaviors by incorporating models that account for the glassy and elastomeric phases. I validated my model with mechanical and microstructural characterization experiments. My finite element simulations capture microstructure transformations that closely mirror those observed in small-angle X-ray scattering experiments.