This study presents a novel method for creating protein-polymer bioconjugates with tunable sieving properties. The researchers used a controlled radical branching polymerization (CRBP) technique with a water-soluble inibramer, sodium 2-bromoacrylate, to graft branched polymers directly onto the surface of chymotrypsin (CT) in open air. The resulting bioconjugates exhibited well-defined architecture, high branching density, and controlled molecular weight. Conformational analysis confirmed the controlled grafting of branched polymers, and enzymatic assays showed that densely grafted polymers prevented protein inhibitor penetration while retaining up to 90% of the enzyme's activity. The study demonstrates a promising strategy for designing protein-polymer bioconjugates with tunable sieving behavior, opening avenues for applications in drug delivery and biotechnology. The traditional "grafting-to" strategy, which involves attaching pre-synthesized polymers like PEG to proteins, has limitations in terms of polymer chain interactions with other biomolecules. In contrast, the "grafting-from" approach, which involves growing polymers directly from the protein surface, offers better control over grafting density and site-specific polymer growth. Atom transfer radical polymerization (ATRP) has been widely used for grafting-from proteins, enabling high grafting density, site-specific polymer growth, and the rational synthesis of protein-polymer conjugates with improved solubility, stability, and functionality. Despite these advances, simple protein-polymer conjugates do not fully retain their functionality because the polymer chains do not completely eliminate interactions between the protein surface and other biomacromolecules. In the context of therapeutic enzyme-polymer conjugates, it is desirable to repel protein-antibody interactions and protease-mediated hydrolysis while retaining their activity toward substrates and ligands. The study introduces a fully oxygen-tolerant CRBP technique in water using inibramer chemistry and dual photo redox/copper catalysis. The term "inibramer" refers to a monomer that can initiate the branching process only after it is incorporated into the polymer chain. A water-soluble inibramer, sodium 2-bromoacrylate (SBA), triggered branching during photoinduced ATRP of methacrylate monomers in one pot. As a result, well-defined branched polymers with controlled molecular weights, degrees of branching, and low dispersity values were obtained in water. The researchers developed a first straightforward approach to introduce branching into protein-polymer hybrids using inibramers. They prepared well-defined bioconjugates of proteins with branched polymers allowing for tunable degrees of branching in one-pot. Subsequently, they investigated and compared the sieving behaviors of synthesized bioconjugates. The study shows that the branched polymers interact intimately due to the higher degree of branching,This study presents a novel method for creating protein-polymer bioconjugates with tunable sieving properties. The researchers used a controlled radical branching polymerization (CRBP) technique with a water-soluble inibramer, sodium 2-bromoacrylate, to graft branched polymers directly onto the surface of chymotrypsin (CT) in open air. The resulting bioconjugates exhibited well-defined architecture, high branching density, and controlled molecular weight. Conformational analysis confirmed the controlled grafting of branched polymers, and enzymatic assays showed that densely grafted polymers prevented protein inhibitor penetration while retaining up to 90% of the enzyme's activity. The study demonstrates a promising strategy for designing protein-polymer bioconjugates with tunable sieving behavior, opening avenues for applications in drug delivery and biotechnology. The traditional "grafting-to" strategy, which involves attaching pre-synthesized polymers like PEG to proteins, has limitations in terms of polymer chain interactions with other biomolecules. In contrast, the "grafting-from" approach, which involves growing polymers directly from the protein surface, offers better control over grafting density and site-specific polymer growth. Atom transfer radical polymerization (ATRP) has been widely used for grafting-from proteins, enabling high grafting density, site-specific polymer growth, and the rational synthesis of protein-polymer conjugates with improved solubility, stability, and functionality. Despite these advances, simple protein-polymer conjugates do not fully retain their functionality because the polymer chains do not completely eliminate interactions between the protein surface and other biomacromolecules. In the context of therapeutic enzyme-polymer conjugates, it is desirable to repel protein-antibody interactions and protease-mediated hydrolysis while retaining their activity toward substrates and ligands. The study introduces a fully oxygen-tolerant CRBP technique in water using inibramer chemistry and dual photo redox/copper catalysis. The term "inibramer" refers to a monomer that can initiate the branching process only after it is incorporated into the polymer chain. A water-soluble inibramer, sodium 2-bromoacrylate (SBA), triggered branching during photoinduced ATRP of methacrylate monomers in one pot. As a result, well-defined branched polymers with controlled molecular weights, degrees of branching, and low dispersity values were obtained in water. The researchers developed a first straightforward approach to introduce branching into protein-polymer hybrids using inibramers. They prepared well-defined bioconjugates of proteins with branched polymers allowing for tunable degrees of branching in one-pot. Subsequently, they investigated and compared the sieving behaviors of synthesized bioconjugates. The study shows that the branched polymers interact intimately due to the higher degree of branching,