Tetrazine PEG derivatives are used to react with TCO (transcyclooctene) groups. The reaction of tetrazine and TCO is a bioorthogaonal reaction that enables the conjugation of two molecules in aqueous solution fast and efficiently. Tetrazine PEG 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.