Reactive chemistries for protein chemical modification play an instrumental role in chemical biology, proteomics, and therapeutics. Depending on the application, the selectivity of these modifications can range from precise modification of an amino acid sequence by genetic manipulation of protein expression machinery to a stochastic modification of lysine residues on the protein surface. Ligand-Directed (LD) chemistry is one of the few methods for targeted modification of endogenous proteins without genetic engineering. However, current LD strategies are limited by stringent amino acid selectivity. To bridge this gap, this thesis focuses on the development of highly reactive LD Triggerable Michael Acceptors (LD-TMAcs) that feature rapid protein labeling. Unlike previous LD approaches, the unique reactivity of LD-TMAcs enables multiple modifications on a single target protein. This capability is attributed to the tunable reactivity of TMAcs that enable the labeling of several amino acid functionalities via a binding-induced increase in local concentration while remaining fully dormant in the absence of protein binding. We demonstrate the utility of this method by selectively labeling membrane-bound carbonic anhydrase XII in live cells. Then, by using our LD-TMAc platform, we have developed target selective covalent Hydrophobic Tagging (HyT) probes. By target-selectively labeling the membrane CAXII with multiple hydrophobic adamantane tags, we demonstrate ~75% CAXII degradation in MCF7 cells in 30 minutes. We envision that the unique features of LD-TMAcs will find use in a range of applications from target identification, investigation of binding/allosteric sites, studies of membrane proteins, and targeted protein degradation. Finally, this thesis also involves the development of reactive polymers for covalent encapsulation and photo-responsive release of proteins and small molecule therapeutics in hydrogel drug depots. We envision the ability of our polymers to directly conjugate natural proteins without genetic or enzymatic engineering and release the encapsulated cargo in a photo-responsive and traceless manner that will find use in drug-encapsulated hydrogel systems.