mPEG DBCO, also called methoxy PEG DBCO, is a pegylated DBCO (dibenzocyclooctyne) derivative that can react with azide group without need any metal catalyst. The strain-promoted 1,3-dipolar cycloaddition of cyclooctynes and azides, also termed as the Cu-free Click reaction, is a bioorthogaonal reaction that enables the conjugation of two molecules in aqueous solution. PEG DBCO derivatives have excellent water solubility, possess fast kinetics and stability in aqueous buffer. They are one of the best pegylation reagents to modify proteins, peptides, antibodies, biopolymers and particles via copper free Click Chemistry reaction.