# _molecules_
## 7 Curing Behavior of High-Vinyl BR Versus Low-Vinyl BR  
If the assumed direct sulfur coupling occurs in high-vinyl SSBR, it can be expected that the same mechanism will also occur in BR. To prove this, a low-vinyl (CB24) as well a high-vinyl BR (Europrene HV80) were selected. Their cure behavior was tested in an unfilled compound with 2.0 phr TBBS and 1.4 phr sulfur with and without the addition of 2.5 phr ZnO and 2.5 phr stearic acid.. Figure 12 shows the chemical structure of the used high-cis BR, and Figure 13 displays the curing behavior of the low-vinyl BR with 96 wt% of cis double bonds. Additional information about the used materials and methods are described in Appendix A. It shows that the compound without any activator shows a significant increase in torque but results in a still lower final torque in comparison with the compound with activators. CB24 does contain 96 wt% of cis double bonds; therefore, this behavior was unexpected. An answer for this can be found in the literature. [PERSON] [12] reported that an alkene (cis-3-hexene) can react directly with mercaptosilane in a radical reaction and forms a variety of different reaction products (Figure 6). The direct coupling product between mercaptosilane and the alkene was formed with a lower yield in comparison with that of the reaction between mercaptosilane and a vinyl-alkene. It clearly indicates that also the cis double bond in SSBR can couple to sulfur via a direct mechanism. The comparison with the observed NR which consists nearly 100% of cis double bonds needs to consider one big difference. The cis double bond of NR has additionally a methyl group as a side group along the polymer chain. This methyl group adjacent to the double-bonded carbon atom in NR has an electropositive effect which enhances the activity of the unsaturated double bonds in the backbone of the polymer chains towards ionic reactions. A radical coupling might be hindered by this electron positive effect. Additionally, a steric hindrance might affect the reaction. Nevertheless, a small amount of a direct coupling to the cis double bond might still occur, supporting the formation of the observed maximum in the cure curve (Figure 11). Another hint can be found in a study from [PERSON] [20]. Here, the following mechanism for the reaction of TESPT with a double bond was proposed (Figure 14). It shows that TESPT is first cleaved into fragments carrying a sulfur radical. These radicals can further couple chemically to the double bond of a polymeric chain. Both examples from the literature show that a direct addition reaction from a sulfur radical (formed from a silane) is possible in a cis double bond in the main chain of an SSBR. For this reason, it can again be considered that the direct sulfur coupling in the case of the curing reaction of BR with sulfur is possible. As [PERSON] [12] already reported, this shows that the reaction of a cis-alkene with a sulfur radical imparts a lower yield than the vinyl-alkene. This can explain the observed behavior in Figure 13: the high-cis BR compound without activators shows a significant increase but still has a lower final torque than the one with activators, meaning that a direct coupling via an S-radical is also possible to cis double bonds but with lower yields. It was now expected that the BR with a high-vinyl content (Figure 15) would also show, in the absence of activators, the same final torques that were observed before in the high as well as the low-vinyl SSBR. Surprisingly, a completely different behavior was observed (Figure 16). Figure 13: Curing behavior of unfilled low-vinyl/high cis-BR compounds. The high-vinyl BR compound without activators ends up in a very low final torque. How can this unexpected behavior be explained? Radicals are very reactive; they couple to the closest reaction partner. This means that because they have a polymeric structure with adjacent vinyl groups, they react preferably with each other (Figure 17). The presence of such adjacent vinyl groups with a vinyl content of 77% is highly likely.