m-PEG-NH2 (m-PEG-Amine) is a terminal amine PEGylation reagent that reacts with activated NHS esters to form stable amide bonds. It is a key intermediate in the preparation of specific functionalized activated MPEG. The use of MPEG-amines instead of MPEG-alcohols improved the physiological stability of MPEG-drug conjugates. The HCl salt form provides stability to M-PEG-NH2 in solid form.