A method for forming linkages by cycloaddition reactions, wherein the method comprises reacting a first tetrazine with a first dienophile followed by reacting a second tetrazine with a second dienophile, wherein the reaction of the first tetrazine with the first dienophile proceeds in the presence of the second dienophile, wherein (i) the first tetrazine comprises a group of the formula: wherein R 3 is C 1 -C 3 -alkyl; (ii) the first dienophile comprises a trans-cyclooctenyl group of the formula: wherein R 1 is hydrogen, halogen, C 1 -C 4 -alkyl, (R a O) 2 P O-C 1 -C 4 -alkyl, (R b O) 2 P -C 1 -C 4 -alkyl, CF 3, CN, hydroxyl, C 1 -C 4 -alkoxy, -O-CF 3, C 2 -C 5 -alkenoxy, C 2 -C 5 -alkanoyloxy, C 1 -C 4 -alkylaminocarbonyloxy or C 1 -C 4 -alkylthio, C 1 -C 4 -alkylamino, Di-(C 1 -C 4 -alkyl)amino, C 2 -C 5 -alkenylamino, C 2 -C 5 -alkenyl-C 1 -C 4 -alkyl-amino or Di-(C 2 -C 5 -alkenyl)amino; and R a, R b independently are hydrogen or C 2 -C 5 -alkanoyloxymethyl; (iii) the second tetrazine comprises a group of the formula: and (iv) the second dienophile comprises a cyclooctynyl group of the formula: wherein R 2 is hydrogen, halogen, C 1 -C 4 -alkyl, (R c O) 2 P O-C 1 -C 4 -alkyl, (R d O) 2 P -C 1 -C 4 -alkyl, CF 3, CN, hydroxyl, C 1 -C 4 -alkoxy, -O-CF 3, C 2 -C 5 -alkenoxy, C 2 -C 5 -alkanoyloxy, C 1 -C 4 -alkylaminocarbonyloxy or C 1 -C 4 -alkylthio, C 1 -C 4 -alkylamino, Di-(C 1 -C 4 -alkyl)amino, C 2 -C 5 -alkenylamino, C 2 -C 5 -alkenyl-C 1 -C 4 -alkyl-amino or Di-(C 2 -C 5 -alkenyl)amino; and R c, R d independently are hydrogen or C 2 -C 5 -alkanoyloxymethyl. (i) the first tetrazine comprises a group of the formula: wherein R 3 is C 1 -C 3 -alkyl; R 3 is C 1 -C 3 -alkyl; (ii) the first dienophile comprises a trans-cyclooctenyl group of the formula: wherein R 1 is hydrogen, halogen, C 1 -C 4 -alkyl, (R a O) 2 P O-C 1 -C 4 -alkyl, (R b O) 2 P -C 1 -C 4 -alkyl, CF 3, CN, hydroxyl, C 1 -C 4 -alkoxy, -O-CF 3, C 2 -C 5 -alkenoxy, C 2 -C 5 -alkanoyloxy, C 1 -C 4 -alkylaminocarbonyloxy or C 1 -C 4 -alkylthio, C 1 -C 4 -alkylamino, Di-(C 1 -C 4 -alkyl)amino, C 2 -C 5 -alkenylamino, C 2 -C 5 -alkenyl-C 1 -C 4 -alkyl-amino or Di-(C 2 -C 5 -alkenyl)amino; and R a, R b independently are hydrogen or C 2 -C 5 -alkanoyloxymethyl; R 1 is hydrogen, halogen, C 1 -C 4 -alkyl, (R a O) 2 P O-C 1 -C 4 -alkyl, (R b O) 2 P -C 1 -C 4 -alkyl, CF 3, CN, hydroxyl, C 1 -C 4 -alkoxy, -O-CF 3, C 2 -C 5 -alkenoxy, C 2 -C 5 -alkanoyloxy, C 1 -C 4 -alkylaminocarbonyloxy or C 1 -C 4 -alkylthio, C 1 -C 4 -alkylamino, Di-(C 1 -C 4 -alkyl)amino, C 2 -C 5 -alkenylamino, C 2 -C 5 -alkenyl-C 1 -C 4 -alkyl-amino or Di-(C 2 -C 5 -alkenyl)amino; and R a, R b independently are hydrogen or C 2 -C 5 -alkanoyloxymethyl; (iii) the second tetrazine comprises a group of the formula: and (iv) the second dienophile comprises a cyclooctynyl group of the formula: wherein R 2 is hydrogen, halogen, C 1 -C 4 -alkyl, (R c O) 2 P O-C 1 -C 4 -alkyl, (R d O) 2 P -C 1 -C 4 -alkyl, CF 3, CN, hydroxyl, C 1 -C 4 -alkoxy, -O-CF 3, C 2 -C 5 -alkenoxy, C 2 -C 5 -alkanoyloxy, C 1 -C 4 -alkylaminocarbonyloxy or C 1 -C 4 -alkylthio, C 1 -C 4 -alkylamino, Di-(C 1 -C 4 -alkyl)amino, C 2 -C 5 -alkenylamino, C 2 -C 5 -alkenyl-C 1 -C 4 -alkyl-amino or Di-(C 2 -C 5 -alkenyl)amino; and R c, R d independently are hydrogen or C 2 -C 5 -alkanoyloxymethyl. R 2 is hydrogen, halogen, C 1 -C 4 -alkyl, (R c O) 2 P O-C 1 -C 4 -alkyl, (R d O) 2 P -C 1 -C 4 -alkyl, CF 3, CN, hydroxyl, C 1 -C 4 -alkoxy, -O-CF 3, C 2 -C 5 -alkenoxy, C 2 -C 5 -alkanoyloxy, C 1 -C 4 -alkylaminocarbonyloxy or C 1 -C 4 -alkylthio, C 1 -C 4 -alkylamino, Di-(C 1 -C 4 -alkyl)amino, C 2 -C 5 -alkenylamino, C 2 -C 5 -alkenyl-C 1 -C 4 -alkyl-amino or Di-(C 2 -C 5 -alkenyl)amino; and R c, R d independently are hydrogen or C 2 -C 5 -alkanoyloxymethyl.