mPEG-pALD (MW 5000) is a significant monofunctional PEG derivative with reactive groups that can interact with the N-terminus of amines, peptides, and proteins to form imines containing a C=N double bond, which can be further reduced. Polyethylene glycol aldehyde or ketone derivatives are capable of forming hydrolytically stable acyl hydrazone bonds through reaction with hydrazine or hydrazide for reversible PEGylation.