Roles of Dinuclear Bridging Bidentate Zinc/Stearate Complexes in Sulfur Cross-Linking of Isoprene Rubber

The roles of the intermediate [Zn2(μ-O2CC17H35)2]2+·4X (X; a hydroxyl group, water, and/or a rubber segment) in the sulfur cross-linking of isoprene rubber are clarified for the first time using in situ time-resolved zinc K-edge X-ray absorption fine structure spectroscopy and in situ time-resolved infrared spectroscopy along with density functional theory calculations. The combined experimental and computational investigation suggests that N-(1,3-benzothiazol-2-ylsulfanyl)cyclohexanamine (CBS) is most easily hydrolyzed on the dinuclear bridging bidentate zinc/stearate intermediate, when a water molecule coordinates to the zinc cation opposite the zinc cation which is coordinated by the nitrogen atom of the benzothiazole group in CBS. The newly produced intermediate with coordinated 1,3-benzothiazole-2-thiolate and cyclohexylamine (CHA) is also found to most readily induce a sulfur insertion among possible candidates to generate subsequent intermediates, when CHA is removed from the intermediate and a water molecule coordinates to the zinc cation which is coordinated by the nitrogen atom of benzothiazole group. The novel dinuclear bridging bidentate zinc/stearate complexes apparently accelerate the sulfur cross-linking of isoprene rubber. Despite the long history of rubber science and technology, these intermediates have been mysterious. The present work will clarify the vulcanization mechanism and will advance the rubber chemistry for a new paradigm of vulcanization technique in the 21st century.