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

Abstract

The discovery that the commercial rubber antidegradant 6PPD reacts with ozone (O 3 ) 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 highlevel 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 oneelectron charge transfer initiates ozonation in these systems, as is sometimes argued. Instead, we find direct interaction between O 3 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 O 3 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.

Reactive Oxygen Species and Chromophoric Dissolved Organic Matter Drive the Aquatic Photochemical Pathways and Photoproducts of 6PPD-quinone under Simulated High-Latitude Conditions

“…The majority of transformation products were consistent with highly oxidized quinone ring-opening products, which is consistent with previous observations of 6PPDQ transformation in the presence of ozone . Considering previous mechanistic insight from Rossomme et al, who assessed 6PPD transformation to 6PPDQ, we believe the central quinone ring system of 6PPDQ is rapidly oxidized by pathways exclusive of ozone, such as aqueous reactive oxygen species. Interestingly, an unknown C 21 H 26 N 2 O 5 transformation product was detected in both positive and negative mode.…”

Reactive oxygen species and chromophoric dissolved organic matter drive the aquatic photochemical pathways and photoproducts of 6PPD-Quinone under simulated high-latitude conditions

“…The majority of transformation products were consistent with highly oxidized quinone ring opening products, which is consistent with previous observations of 6PPDQ transformation in the presence of ozone. 29 Considering previous mechanistic insight from Rossomme et al, who assessed 6PPD transformation to 6PPDQ 30 , we believe the central quinone ring system of 6PPDQ is rapidly oxidized by pathways exclusive of ozone, such as aqueous reactive oxygen species. Interestingly, an unknown C21H26N2O5 transformation product was https://doi.org/10.26434/chemrxiv-2023-0mnh8 ORCID: https://orcid.org/0000-0002-4158-524X Content not peer-reviewed by ChemRxiv.…”

Occurrence and Oxidation Kinetics of Antioxidant p-Phenylenediamines and Their Quinones in Recycled Rubber Particles from Artificial Turf

“…N,N′ -Substituted p -phenylenediamines (PPDs) are a broad range of anthropogenic chemicals that have been extensively used as antioxidants in the rubber industry due to their superior capacity in protecting rubber products. − Among these, N -(1,3-dimethylbutyl)- N′ -phenyl- p -phenylenediamine (6PPD), as the most frequently used antioxidant, has been listed as a high production volume (HPV) chemical . The annual production of 6PPD alone in China and the U.S. exceeded 20 and 50 million tons in 2020. , Other PPDs, such as N -isopropyl- N′ -phenyl-1,4-phenylenediamine (IPPD), N,N′ -diphenyl- p -phenylenediamine (DPPD), and N,N′ -ditolyl- p -phenylenediamine (DTPD), are also regarded as HPV chemicals with substantial annual productions. − However, massive production and consumption have resulted in incredible emissions of PPDs into the environment. The growing evidence indicated that PPDs and related degradation products were ubiquitously detected in urban runoff, − sediments, , airborne particles, , and even in human specimens (urine and blood) including sensitive populations (children and pregnant women), , attracting considerable concern regarding their potential risks to the environment and human health.…”