This invention provides a method for preparing, in high purity and high yield, heterobifunctional derivatives of poly(ethylene glycol) or related polymers. A chromatographic purification step is not necessary in the method. In accordance with the method of the invention, an intermediate polymer having a formula of W-Poly-OH is provided bearing a removable group W at one terminus. The intermediate polymer W-Poly-OH is first altered by modifying the OH group to a first functional group X, followed by the removal of W to generate a second hydroxyl group. The latter hydroxyl group may then be further converted to a second functional group Y, thus providing the desired heterobifunctional derivative.