Transforming Polybutadiene with Tetrazine Click Chemistry into Antioxidant Foams That Fluoresce with Oxidation

The extent to which oxidative degradation of macromolecules can be delayed is generally limited by the low solubility of antioxidants in most polymers. This can be surmounted by synthesizing macromolecules with covalently attached antioxidant functionalities, but these are frequently expensive. Here, we demonstrate a simple click modification of polybutadienes (PDB) with 3,6-dichloro-1,2,4,5-tetrazine (DCT) that, in addition to modifying and stiffening the polymer chains, releases nitrogen gas to foam the solidifying polymers and generates dihydropyridazine groups that transform them into macromolecular antioxidants. Tetrazines react by a cycloaddition/cycloreversion reaction (Carboni-Lindsey reaction) with the C=C bonds to install 1,4-dihydropyridazine groups that increase the mass and rigidity of the butadiene macromolecules. The 1,4-dihydropyridazine group is an effective antioxidant that donates two hydrogen atoms per ring to combine with radicals and forms an aromatic pyridazine ring whose white fluorescence under UV permits visual monitoring of oxidation. Foams made by reacting liquid hydroxyl-terminated polybutadienes with DCT stabilize with thermoset formation through substitution reactions between the hydroxyl and dichlorodihydropyridazine groups.