Disclosed are water soluble polymeric conjugates comprising the structure POLY-[Y—S—S-A]x, where POLY is a water soluble polymer; Y is a hydrocarbon-based spacer group, x is 1 to 25, S—S is a disulfide group attached to an sp3 hybridized carbon of Y; and A is a covalently linked residue of a pharmacologically active molecule. Preferably, the water soluble polymer is a PEG polymer. Also disclosed are polymeric reagents useful to prepare such conjugates, and methods of their formation and use.