Hydrogen atom transfer (HAT) chemistry has emerged as a powerful tool for selective molecular functionalization, finding increasing applications in polymer chemistry to control polymer properties and enable degradation. This study explores the versatility of HAT in two distinct areas of polymer science. First, we investigate the use of photocatalyzed HAT for the synthesis of reversible addition–fragmentation chain transfer (RAFT) agents (CTAs) by modifying various substrates, and evaluate the resulting CTAs in both thermal and photoinduced electron transfer (PET)-RAFT polymerization for controlled polymerization and copolymer synthesis. This approach is then extended to functionalize polycaprolactone (PCL) and polyvinyl acetate (PVAc), enabling the synthesis of graft copolymers. Second, we present a novel strategy to enhance the depolymerization of non-functionalized poly(methyl methacrylate) (PMMA) by enabling in situ activation of the polymer backbone using photoinduced HAT. We demonstrate that disulfide-based RAFT agents, particularly bis(dodecylsulfanylthiocarbonyl) disulfide, can effectively promote depolymerization under mild conditions to generate monomers. In both applications, photocatalysts, including iron(III) chloride (FeCl3), are investigated to promote HAT, leveraging the advantages of mild and efficient radical generation under light irradiation compared to conventional thermal HAT systems. This work highlights the broad potential of HAT chemistry in developing advanced polymer synthesis and degradation strategies