# _molecules_
## 1 Introduction  
Since [PERSON] discovered the sulfur curing of Natural Rubber (NR) in 1839 [1, 2], many studies have been carried out to understand its mechanism. The broadly accepted mechanism was summarized by [PERSON] and [PERSON] (Figure 1) [3]. In general, sulfur attaches at the allyic position to the polymer. It is activated by the inclusion of an activated accelerator complex formed by Zinc oxide and stearic acid where accelerator fragments are attached as ligands. [PERSON] [5, 6] investigated this intermediate complex further and concluded that it is a dinuclear bridging bidentate Zinc/stearate complex with the following structure: (Zn\({}_{2}\)(\(\mu\)-O\({}_{2}\)CC\({}_{17}\)H\({}_{35}\))\({}_{2}\))\({}^{2+}\)(OH\({}^{-}\))\({}_{2}\)\(\cdot\)XY, where X and Y are water and/or a rubber segment (Figure 2). No information is given how the hydrogen atom is abstracted by this complex to enable the final polymer-polymer coupling. Figure 1: Sulfur curing mechanism by substitution reaction at the allyic position according to [3, 4]. The place where the complex is formed was further investigated by [PERSON]. She found out that its formation takes place only at the surface of the Zinc oxide (ZnO) crystal. At the end of the curing reaction where sulfur bridges between the polymer chains are formed, Zinc Sulfide (ZnS) as well as Zn(stearate)\({}_{2}\) are additionally formed as side products (Figure 3) [7]. The sulfur crosslink formation is not homogeneous but localized, resulting in highly crosslinked (ZnS- and Zn-stearate rich) as well as un-crosslinked (ZnS- and Zn-stearate deprived) regions [8]. These more general findings can be used to understand the reactions during the production of a tire tread compound in a better way. A modern green tire tread compound is based on a silica/silane-filled Solution Styrene Butadiene Rubber (SSBR)/Butadiene Rubber (BR) compound [9]. In addition to the coupling of the polymer chains via sulfur bridges, a new focus has to be put on the silica/silane/polymer bonding via the sulfur function of the silane in such a compound. It was assumed that the sulfur function of the silane, typically a bis(triethoxysilypropyl)tetrasulfide (TESPT), is also connected to accelerators, which are part of the active complex (Figure 4) [10]. Connecting the assumption of the coupling of the silane to the active complex to the latest findings of [PERSON] and [PERSON] would lead to the conclusion that the active Zinc complex does not contribute to the formation of the polymer-filler bonds in the assumed way. If the complex is bound to the Zinc oxide surface where it supports the coupling of the sulfur to the mobile polymer chains, it is unlikely to assume that the same supporting function takes place for the silane coupled to the non-mobile silica particle. Zinc oxide as well as  
Figure 3: Chemical reactions take place at the surface of the Zinc oxide (ZnO) crystal in the rubber matrix [7]. Figure 2: Dinuclear bridging bidentate Zinc/stearate complex according to [5]. silica particles are distributed in the polymer matrix; such a supporting function of the complex would have the pre-condition that both solid particles are in connection to each other, which is usually not the case [11]. This polymer-filler crosslink reaction between silica and polymer via silane was further investigated by introducing an alternative group of sulfur silanes, the mercaptoisilanes. The use of these mercaptoisilanes as an alternative to the traditional tetra or disulfide silanes led to an improved dispersion behavior of the silica, an improved rolling resistance, but also to a deteriorated processing behavior. [PERSON] investigated their behavior in depth and concluded that the observed effects were based on a combination of an addition reaction and a radical propagation reaction, where the sulfur radical of the mercaptoisilane couples directly to the vinyl group of the main polymer, the SSBR, following an Anti-Markovnikov mechanism.