The molecular sieving properties of protein surface-attached polymers are the central features in how polymers extend therapeutic protein lifetimes in vivo. Yet, even after 30 years of research, permeation rates of molecules through polymer surrounded protein surfaces are largely unknown. As a result, the generation of protein polymer conjugates remains a stochastic process, unfacilitated by knowledge of structure-function-polymer architecture relationships. In this work, polymers are grown from the surface of avidin using atom transfer radical polymerization (ATRP) and used to determine how polymer length and density influence the binding kinetics of ligands as a function of ligand size and shape. The rate of binding is strongly dependent on the grafting density of polymers and the size of the ligand but interestingly, far less dependent on the length of the polymer. This study unveils a deeper understanding of relationship between polymer characteristics and binding kinetics, discovering important steps in rational design of protein polymer conjugates.