mPEG-pALD (MW 10000) is a significant mono-functional PEG derivative, featuring reactive groups that can engage with the N-terminus of amines, peptides, and proteins to form an imine containing a C=N double bond, with the potential for further reduction. Polyethylene glycol aldehyde or ketone derivatives can produce hydrolytically cleavable acylhydrazone bonds through reaction with hydrazine or hydrazides for reversible PEGylation.