Computational studies of rubber ozonation explain the effectiveness of 6PPD as an antidegradant and the mechanism of its quinone formation

Authors: Rossomme, Elliot; Hart-Cooper, William; Orts, William; McMahan, Colleen; HEAD-GORDON, MARTIN - University Of California Berkeley

Submitted to: Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/10/2023
Publication Date: 3/24/2023
Citation: Rossomme, E.C., Hart-Cooper, W.M., Orts, W.J., McMahan, C.M., Head-Gordon, M. 2023. Computational studies of rubber ozonation explain the effectiveness of 6PPD as an antidegradant and the mechanism of its quinone formation. Environmental Science and Technology. 57(13):5216-5230. https://doi.org/10.1021/acs.est.2c08717.
DOI: https://doi.org/10.1021/acs.est.2c08717

Interpretive Summary: Agricultural solutions such as 1) crops to produce domestic natural rubber, and 2) the availability of biobased fillers, has advanced the biobased content of products including vehicle tires. Biobased antidegradants for tires are urgently needed, especially since the 2021 discovery that the commercial rubber antidegradant chemical (6PPD) reacts with atmospheric ozone to produce a highly toxic by-product (6PPDQ), contaminating fisheries in the NW US. Identification of alternative, safer, chemicals has been hampered by lack of a detailed understanding of the chemical mechanisms involved. In this study, computational quantum mechanics was used to create a computer model of the relevant chemistry, successfully identifying a chemical mechanism that explains why PPD works so well and why it forms the 6PPDQ byproduct. These results are being used to identify safer alternatives, including biobased antiozonants.

Technical Abstract: The discovery that the commercial rubber antidegradants 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.