the invention provides a sterically hindered polymers and conjugates formed therefrom that comprise a water - soluble and non - peptidic polymer backbone having at least one terminus covalently bonded to an alkanoic acid or alkanoic acid derivative prior to conjugation , wherein the carbon adjacent to the carbonyl group of the acid or acid derivative group has an alkyl or aryl group pendent thereto . the steric effects of the alkyl or aryl group allow greater control of the hydrolytic stability of polymer derivatives . the polymer backbone may be poly .