Free radical inhibitors and antioxidants, such as hindered phenols, can operate by radical chain transfer to afford a stable radical or closed shell product. During our investigations of the Click modification of polybutadienes with tetrazines, we discovered that the resulting 1,4-dihydropyridazine cycloadducts are effective free radical inhibitors. In this study we used the delayed gelation of a crosslinking, free radical polymerization to gauge the potency of a three different dihydropyridazines and one phenolic free radical inhibitor. 5-Phenyl-1,4-dihydropyridazines were prepared by reacting styrene with either 3,6-dichlorotetrazine (DCP), 3,6-bis(3,5-dimethyl-1H-pyrazol-1-yl)tetrazine (DDPP), or dimethyl tetrazine-3,6-bis(carboxylate) (DMP). A solution of styrene-divinyl (3:1) with 1 mol% AIBN as initiator heated to 70 °C under argon, forms a glassy, hard gel in 8.93 minutes. With 1 mol% 2,6-di-<i>tert</i>-butyl-4-methylphenol added to the solution, the same reaction takes 13.0 minutes to form a gel. With 1 mol% DCP, gelation was observed in 14 minutes. With DDPP, gelation occurred after in 9 minutes, no different from the inhibitor-free reaction. With DMP, gelation occurred only after 115.7 minutes, or <u>26 times longer </u>than with BHT. Each molecule of dihydropyridazine can transfer up to two hydrogen atoms to free radical chain reactions driven by the formation of the aromatic pyridazine.

1,4-Dihydropyridazines as inhibitors of free radical polymerization and gelation of styrene-divinyl benzene

254th National Meeting (2017)

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