The present invention is related to compositions which can undergo targeted degradation under certain physiological reducing conditions. The compositions may, for example, be hydrogels in which maleimide-functionalized low molecular weight heparin is cross-linked with various thiol-functionalized polyethylene glycol multi-arm star polymers. Both the gelation and degradation of the hydrogels can be modified by careful selection of the thiol. For example, hydrogels prepared from aryl thiol-maleimide adducts can undergo a retro Michael addition-type reaction, leading to degradation of the composition and release of the bioactive molecule.