BCN PEG linkers is a class of PEG linkers is a class of labeling reagents containing a very reactive BCN (bicyclo[6.1.0]nonyne) group, BCN PEG is reactive toward azide without Cu catalyst. The reaction can be run either in organic solvents or in aqueous solution depending on the solubility and property of substrate molecules. The PEG space of the reagent increases the hydrophilicity and fine-tunes the properties of the PEGylated molecules.