Polyphosphoesters (PPEs), a versatile class of biodegradable and biocompatible polymers, have been proposed as alternatives to poly(ethylene glycol) (PEG), which is suspected to be responsible for anaphylactic reactions in some patients after the administration of PEGylated compounds, e.g., in the current Covid-19 vaccines. We present the synthesis and characterization of a novel set of protein-polymer conjugates using the model protein myoglobin and a set of PPEs with different hydrophilicity and molar mass. We report an extensive evaluation of the (bio)physical properties of the protein within the conjugates, studying its conformation, residual activity, and thermal stability by complementary techniques (UV-vis spectroscopy, nano-differential scanning calorimetry, and fluorometry). The data underline the systematic influence of polymer hydrophilicity on protein properties. The more hydrophobic polymers destabilize the protein, the more hydrophilic PPEs protect against thermally induced aggregation and proteolytic degradation. This basic study aims at guiding the design of future PPEylated drugs and protein conjugates.