Preferred aryl imidate activated polyalkylene oxides are represented by the structure of Formula I, wherein R is a water-soluble polyalkylene oxide, L is a moiety forming a hydrolytically stable, covalently bonded linkage between the polyalkylene oxide and the phenyl ring of the aryl imidate, R1 is a moiety selected from alkyl, phenyl, phenylalkyl, cycloalkyl, and the like, and X is a terminal moi