Methyltetrazine-PEG12-t-butyl ester is PEG derivative containing methyltetrazine and t-butyl ester moieties. It is one of the most stable tetrazine compounds commercially available due to the electron donating properties of the methyl group on the methyltetrazine. The t-butyl protected carboxyl group can be deprotected under acidic conditions. The hydrophilic PEG spacer increases water-solubility.