Methoxy poly(ethylene glycol) KAT 10K (mPEG KAT 10K) is a monofunctional, methyl ether PEG featuring a terminal potassium acyltrifluoroborate for facile, rapid functionalization. Potassium acyltrifluoroborates (KATs) are stable functional groups that undergo rapid amide-forming ligations with hydroxylamines in aqueous media, in the presence of unprotected functional groups. In addition to its compatibility, these reactions proceed relatively quickly, lending to their use with sensitive biological reagents. This conjugation reaction offers a new approach to the synthesis of complex molecules without the complication of side reactions, such protein-polymer conjugates. KATs also undergo amide or imide-forming ligations in acidic conditions when reacted with primary amines or amides, respectively, as an alternative to classical acylation chemistry.