Abstract: The Kalow lab interrogates and exploits the relationship between molecular reactivity and macroscopic properties to discover polymeric materials relevant to human health and sustainability. We approach “reactivity-property” relationships bidirectionally: in reactivity-directed materials discovery, we use synthesis and physical organic chemistry to tune reactions occurring within polymer networks. In polymer networks composed of reversible covalent bonds, we translate changes in reactivity into macroscopic responses, ranging from repair in self-healing elastomers to photocontrolled stiffness in adaptable hydrogels. In properties-directed reaction discovery, we design photochemical reaction mechanisms that target desirable photophysical properties. Based on this principle, we have discovered a catalyst-free photopolymerization to produce n-type π-conjugated polymers, and a selective photoinduced cross-coupling of polyhalogenated dyes. Across these projects, light provides precise, tunable, and noninvasive spatiotemporal control over molecular reactivity.