disclosed are methods of conjugating biologically active substances , particularly , alpha - interferon , with a hyaluronan or a mixture of a hyaluronan with at least one other hydrophilic polymer having a functional group capable of reacting with divinyl sulfone . also disclosed are stable intermediates formed by partially reacting a hyaluronan with divinyl sulfone and stopping the reaction before completion to leave free , or reactive vinyl groups on the hyaluronan molecule available for conjugation with the biologically active substance .