Branched PEG structures are particularly useful for the pegylation of different biomaterials. In this regard, the branched structure bearing a lysine core and two mPEG chains linked by urethane bonds (mPEG(2)lys) has been used for the pegylation of several important biomolecules. In this work we describe the preparation of a new functionalized mPEG featuring an alkoxy-carbonylimidazolium iodide reactive group that can be used for the urethane bonds formation required in mPEG(2)lys (20 kDa). The procedure for the preparation of the mPEG-alkoxy-carbonylimidazolium iodide involves the initial reaction of mPEG with N,N'-carbonylbismidazole (CDI) as phosgene equivalent, followed by alkylation with methyl iodide. The functionalized mPEG was reacted with a silylated lysine derivative affording mPEG(2)lys in a single step. Therefore, harmful reagents (like phosgene derivatives) are not employed, making the process safe and straightforward. A 2-step purification procedure of mPEG(2)lys is also presented. It is noteworthy that by following this method pure mPEG(2)lys was obtained with 41% isolated yield. The mPEG(2)lys was further converted to the N-hydroxysuccinimidyl ester by diimide activation and successfully used in the pegylation of interferon alpha-2a. (C) 2011 Elsevier Ltd. All rights reserved.