Polymeric reagents comprising a polymer attached, either directly or through one or more atoms to a ketone or a related functional group such as ketone hydrate, thione, monothiohydrate, dithiohydrate, hemiketal, monothiohemiketal, dithiohemiketal, ketal, or dithioketal are provided. The polymeric reagents are useful for, among other things, forming polymer-active agent conjugates. Related methods, compositions, preparations, and so forth are also provided.