# _molecules_
## 7 Curing Behavior of High-Vinyl BR Versus Low-Vinyl BR
Such an internal crosslink reaction is already known from curing reactions where triallyl cyanurate (TAC) or triallyl isocyanate (TAIC) are used as co-agents [23]. This means the assumed direct sulfur coupling can take place, but the formation of a radical along the polymeric chain triggers a further coupling reaction to the neighbored vinyl group. In the end, no crosslinks between different polymer chains are formed, which leads to the observed but not pronounced increasing torque. Figure 16: Curing behavior of high-vinyl BR. Figure 14: Coupling of TESPT via a sulfur radical to a double bond in a polymeric chain (according to [20]). Figure 15: Structure of the used high-vinyl BR type (according to [21; 22]). This leads to the conclusion that an effective direct sulfur coupling needs a \"spacer\" such as the styrene group in SSBR or a cis or trans BR unit in the low-vinyl BR to hinder such a coupling reaction to adjacent vinyl groups. By comparing the achieved final torque in Figures 13 and 16 for the BR-compounds with activators, another conclusion can be drawn. The high-vinyl BR shows that even with activators, there is an almost 30% lower final torque than the high-cis BR with activators. This indicates that even in the high-vinyl BR with activators, such an adjacent crosslink reaction might take place, which is ineffective for the increase in crosslink density and therefore also ineffective in causing an increase in torque. This explanation is also true for the observed lower torque for the high-vinyl SSBR in comparison to the low-vinyl SSBR (Figure 9). Additionally, high-vinyl BR or SSBR have much less allylic hydrogen atoms where the \"traditional\" sulfur coupling reaction via the activated Zn complex can take place [24]. For this reason, the addition reaction towards the double bond is much more likely. Curing Behavior of High-Vinyl SSBR/High-Cis BR Filled with Silica/Silane Versus Filled with Carbon Black  
Up to now, all investigated compounds did not include any filler. In a silica/silane-filled compound, many interactions are reported between the activators and silica as well as silane (Figure 18) [25]. Furthermore, the coupling of the silane-modified silica to the polymer was also reported to follow the \"traditional\" sulfur coupling at the allylic position (Figure 4). This coupling was assumed to be supported from the active Zinc complex. As discussed before, this support was considered to be unlikely due to the formation of the Zinc complex at the surface of the non-mobile Zinc oxide. To find out which influence the silica/silane system has on the direct coupling reaction between a sulfur radical and a vinyl group, a rubber compound with 80 phr high-vinyl SSBR and 20 phr high-cis BR filled with silica/silane (Bis(triethoxysilpropyl) disulfide (TESPD) was investigated. Two compounds were evaluated: one with curing activators ZnO and stearic acid, and one without these activators. Furthermore, these rubber compounds were based on a model green tire tread compound with 1.5 phr DPG, 2.0 phr TBBS and 1.4 phr sulfur. The compounds were compared with carbon black (CB)-filled high-vinyl  
Figure 17: Radical coupling of adjacent vinyl groups. Figure 18: Manifold interactions between ZnO, stearic acid and silica, as well as silane (according to [25]). SSBR/high-cis BR compounds where such manifold interactions between the different ingredients as described in Figure 18 were not expected. Figure 19 shows the curing behavior of the silica/silane on the left side as well as that of the CB-filled compounds on the right side. The used materials and methods are described in more detail in Appendix A. The curing of high-vinyl SSBR/high-cis BR without activators (ZnO/stearic acid) also starts in CB-filled SSBR/BR slightly earlier and leads to the same final torque. This means that the direct coupling of the sulfur radical to the vinyl group also does occur in a CB-filled system and the presence of CB influences the curing behavior only slightly.