Comparative study of p-phenylenediamine antioxidant effectiveness in styrene-butadiene and polyisoprene rubber through NMR calculations * Ingrid Jelemensk  a , Martin Breza * Department of Physical Chemistry, Faculty of Chemical and Food Technology, Slovak Technical University, SK-81237, Bratislava, Slovakia article info Article history: Received 23 March 2020 Accepted 16 April 2020 Available online 21 April 2020 Keywords: DFT geometry optimization NMR shifts Antioxidant effectiveness Linear regression analysis abstract Structures of six p-phenylenediamine (PPD) antioxidants were optimized and their NMR shifts were evaluated by B3LYP calculations. The dependences of experimental values of Antioxidant Effectiveness (AEX) and Molar Antioxidant Effectiveness (AEM) in styrene-butadiene (SBR) and polyisoprene (PIR) rubber on the calculated NMR chemical shifts of nitrogens, tertiary carbons and hydrogens bonded to them were investigated by means of a multiple linear regression analysis. Based on regression parameter values, the significance of the N centers between aromatic rings (A) seems to be higher than of the N centers between aromatic rings and aliphatic chains (B) except AEM data in PIR. The extent of shielding the A center activity by the cumyl group in ortho-position was estimated. Regression parameters ob- tained for SBR are significantly higher than for PIR. © 2020 Elsevier Ltd. All rights reserved. 1. Introduction The rate of the oxidative degradation of organic materials which are exposed to oxygen can be reduced using antioxidants (Aox). Aromatic secondary amines, especially N-phenyl-N 0 -alkyl-p-phe- nylenediamines (PPD) are the most important commercial antioxi- dants used prevailingly in rubber industry [1,2]. The most important reactive radical intermediates formed during degradative reactions are hydroxyl (HO), alkoxyl (RO) and peroxyl (ROO) radicals which can readily abstract hydrogen atoms from polymer molecular back- bones, ultimately breaking down the polymer molecules. The sup- posed mechanism of inhibition by PPD antioxidants consists of several steps. Reaction products of primarily formed amine radicals are benzoquinonediimines and nitroxyl radicals, which have anti- oxidative effects as well. The benzoquinonediimine-type products may finally undergo hydrolytic or condensation reactions by forming benzoquinonemonoimine-type structures or N-heterocyclic com- pounds [1,2]. Simon et al. investigated the relation between the structure of the PPD antioxidants and their efficiency in polyisoprene rubber (PIR) by non-isothermal DSC (Differential Scanning Calorimetry) measurements [3e9]. They have found that the effectiveness (AEX or AEM e see Appendix) of PPD antioxidants under study is indi- rectly proportional to the dissociation energy of the CeH bond at the tertiary carbon atom in the neighbourhood of the nitrogen atom. PPD antioxidants were investigated theoretically by Density Functional Theory (DFT) model studies [10e23]. Pusk arov a et al. [20,21] have found a nearly linear relation between AEM values of PPD antioxidants and NMR shifts of amine hydrogens, of amine nitrogens and probably of the tertiary carbon atom neighboring to the nitrogen between aromatic ring and the side aliphatic chain. Cibulkova et al. [6] performed a comparative DSC study of six PPD antioxidants in styrene e butadiene (SBR) and in polyisoprene rubber (PIR). The values of their antioxidant effectiveness at 180  C in both rubber matrices were determined. The aim of our study is to test the linear dependence of experimental AEX and AEM values in these matrices on the calculated NMR shifts of the active centers of the studied PPD antioxidants. 2. Method Standard B3LYP/6-311G* geometry optimizations of six neutral molecules of PPD antioxidants (see Figs. 1e6) in singlet ground spin states were performed using GAUSSIAN09 program package [24]. Stability of the optimized structures was confirmed by vibrational * In the memory of Prof. Stanislav Biskupic (1949e2016). * Corresponding author. E-mail address: martin.breza@stuba.sk (M. Breza). Contents lists available at ScienceDirect Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab https://doi.org/10.1016/j.polymdegradstab.2020.109196 0141-3910/© 2020 Elsevier Ltd. All rights reserved. Polymer Degradation and Stability 177 (2020) 109196