Computational Studies of Rubber Ozonation Explain the Effectiveness of 6PPD as an Antidegradant and the Mechanism of Its Quinone Formation

The discovery that
the commercial rubber antidegradant
6PPD reacts
with ozone (O3) to produce a highly toxic quinone (6PPDQ)
spurred a significant research effort into nontoxic alternatives.
This work has been hampered by lack of a detailed understanding of
the mechanism of protection that 6PPD affords rubber compounds against
ozone. Herein, we report high-level density functional theory studies
into early steps of rubber and PPD (p-phenylenediamine)
ozonation, identifying key steps that contribute to the antiozonant
activity of PPDs. In this, we establish that our density functional
theory approach can achieve chemical accuracy for many ozonation reactions,
which are notoriously difficult to model. Using adiabatic energy decomposition
analysis, we examine and dispel the notion that one-electron charge
transfer initiates ozonation in these systems, as is sometimes argued.
Instead, we find direct interaction between O3 and the
PPD aromatic ring is kinetically accessible and that this motif is
more significant than interactions with PPD nitrogens. The former
pathway results in a hydroxylated PPD intermediate, which reacts further
with O3 to afford 6PPD hydroquinone and, ultimately, 6PPDQ.
This mechanism directly links the toxicity of 6PPDQ to the antiozonant
function of 6PPD. These results have significant implications for
development of alternative antiozonants, which are discussed.