Comparative study of p-phenylenediamine antioxidant effectiveness in styrene-butadiene and polyisoprene rubber through NMR calculations

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 obtained for SBR are significantly higher than for PIR. •DFT geometry optimization of six PPD antioxidants.•GIAO calculations of NMR shifts.•Linear regression of antioxidant effectiveness vs. NMR shifts.•Sterical shielding estimation for the o-cumyl derivative.•Different results for both rubber matrices.