The invention relates to materials and methods of conjugating a water soluble polymer to an oxidized carbohydrate moiety of a blood coagulation protein comprising contacting the oxidized carbohydrate moiety with an activated water soluble polymer under conditions that allow conjugation. More specifically, the present invention relates to the aforementioned materials and methods wherein the water soluble polymer contains an active aminooxy group and wherein an oxime linkage is formed between the oxidized carbohydrate moiety and the active aminooxy group on the water soluble polymer. In one embodiment of the invention the conjugation is carried out in the presence of the nucleophilic catalyst aniline. In addition the generated oxime linkage can be stabilized by reduction with NaCNBH3 to form an alkoxyamine linkage.
