The present invention relates to methods and compositions for modifying peptides, polypeptides and proteins with polymers, especially glyco-mimetic polymers, so as to improve their biological activity or pharmacokinetic properties. The invention further provides methods and uses for such polymer-modified peptides, polypeptides and proteins. The invention is particularly suitable for use in the synthesis of polymer-modified synthetic bioactive proteins (FIG. 1D), and of pharmaceutical compositions that contain such proteins.