The present invention is directed compositions of the formula: (I) wherein: D is a biologically active moiety X is an electron withdrawing group Y and Y are independently O or S R1 is selected from the group consisting of H, C1-6 alkyls, aryls, substituted aryls, aralkyls, heteroalkyls, substituted heteroalkyls and substituted C1-6 alkyls (n) is an integer from 1 to about 12 and R2 is a polyalkylene oxide. In preferred embodiments, the prodrugs contain a polyethylene glycol having a molecular weight of at least about 20,000.
