A method is provided for preparing water-soluble polymer derivatives bearing a terminal carboxylic acid or ester thereof. The method involves the hydrolyzing an ortho ester of a water-soluble polymer so as provide the corresponding acid. In addition, the invention provides water-soluble polymers bearing a terminal carboxylic acid or ester thereof, intermediates and reagents useful in carrying out the method, as well as gels, pharmaceutical formulations, conjugates related to the described water-soluble polymer derivatives.