A multi-vinylsulfone containing molecule is described herein. The multi-vinylsulfone containing molecule can be formed by dissolving a water soluble polymer containing a hydroxyl group in an aqueous solution to form a polymer solution; adding a molecule containing two vinylsulfone groups to the polymer solution; and forming a modified polymer by controlling a number of the vinylsulfone groups that are added to the polymer. A hydrogel is also described herein that can include the multi-vinylsulfone containing molecule and a multi-thiol containing molecule. The hydrogel can be formed from an aqueous solution that includes the multi-vinylsulfone containing molecule and the multi-thiol containing molecule by undergoing gelatation upon delivery to a site in the body. Also described is a drug delivery system that employs the hydrogel.