Despite recent advances in catalytic conversion of plastic waste into high-value chemicals, the interactions between polymers and catalysts, which are highly porous nanomaterials, are not well understood. Fundamental understanding of these interactions and the ability to modulate them would allow us to design effective catalysts for polymer upcycling reactions. To study the interactions between polymers and nanomaterials, quantifying them is essential. In this thesis, we employ a quantitative technique to directly measure the contact angle between polymers and nanoparticles, enabling us to investigate various aspects of polymer-catalyst interactions that are crucial for polymer upcycling reactions. We measure the contact angle of polyolefins with silica nanoparticles, a commonly used support in heterogeneous catalysis, and modify the surface chemistry of these nanoparticles by depositing catalytic metals and metal oxides via atomic layer deposition (ALD) and using silane chemistry. Our findings show that the polarizability of polyolefins plays a significant role in their interactions with catalytic support materials, and modifying the polarity of the support material could be an effective way to tune polymer-catalyst interactions. Further, we find that the polymer-catalyst interactions are dominated by interactions of the polymers with the support materials and not with the metal catalytic sites. We also probe the influence of common plastic additives, namely primary antioxidants (PAOs), benzophenones and hindered amine light stabilizers (HALS), on the polymer-catalyst interactions by adding them to purified high-density polyethylene (HDPE) and measuring the contact angle on silica nanoparticles. We observe that while the addition of PAOs and benzophenones to polyolefins does not affect their interactions with silica significantly, HALS strongly alter the polymer-silica interactions. Hence, the presence of different types of additives in plastics might necessitate different strategies to design catalysts for polymer upcycling reactions. Overall, this thesis sheds light on some key unanswered questions on polymer-catalyst interactions and paves the way for future innovations in polymer upcycling and engineering catalysts for other polymeric reactions.