Plastics are ubiquitous in society due to their diverse applications, but they persist in nature without degradation, creating environmental problems for living creatures. Unfortunately, most recycling techniques either downgrade the quality of virgin plastics or demand high energy inputs, consequently necessitating other efficient approaches. To address these problems, photo-mediated strategies were developed to upcycle or recycle plastic wastes to useful small molecule feedstocks as sustainable alternatives for the plastic valorization.The contents of this dissertation detail the developments of catalyst-controlled photooxidation and photothermal conversion for polymer deconstruction and chemical transformation. Chapter 1 presents the challenges of current plastic recycling techniques and introduces the concepts of photooxidative degradation and photothermal conversion. Chapter 2 describes the optimization of photooxidative degradation of polystyrene to benzoic acid using a chlorine radical generated by light irradiation to FeCl3. Chapter 3 expands upon this work and investigates the polystyrene degradation mechanism and product distribution using a site-selective bromine radical from a FeBr3 photocatalyst. Chapter 4 utilizes a similar bromine radical system to initiate the cumene-phenol process to transform cumene to phenol. Chapter 5 shows the optimization of photothermal depolymerization of polystyrene to styrene monomer using carbon black as a photothermal agent and white LED light. Chapter 6 explores the synthesis of copolymers with strained bicyclobutane comonomers and assesses their thermal properties and depolymerization behaviors through photothermal depolymerization.