Vulcanization of Butadiene Rubber by Means of Cyclic Disulfides. 1. A 2D NMR Study on the Cross-Link Structure of a BR Model Compound Vulcanizate

Abstract

The vulcanization of cis,cis,cis-1,5,9-cyclododecatriene (ccc-1,5,9-CDT) by means of 1-oxa-4,5-dithiacycloheptane, zinc dimethyl dithiocarbamate (ZDMC), and 1,12-diaminododecane was studied as a model compound for true vulcanizates using several 1D and 2D NMR techniques in order to determine cross-linking sequence and overall molecular structure. Because of the relatively high solubility of the products, typical solution 2D NMR pulse sequences could be applied offering a framework of reference for the solid state NMR studies on true vulcanizates to follow. 13C DEPT spectra yielded the multiplicity of the 13C resonances, followed by HETCOR techniques to identify the corresponding 1H resonances. COSY, DQF−COSY (double quantum filtered COSY) and TOCSY (total correlation spectroscopy) experiments provided the homonuclear (long-range) through-bond connectivities leading to the complete elucidation of the molecular framework. The experimentally found 13C chemical shifts were compared with the calculated shifts and appeared to be in good agreement. Using this “model compound” the applicability of several NMR techniques to determine structural characteristics proved to be possible. Moreover, the study allowed the comparison of the solution NMR techniques mentioned with the solid state NMR techniques used in the elucidation of the cross-link sequence and overall structure of true vulcanizates.

Vulcanization of Butadiene Rubber by Means of Cyclic Disulfides. 3. A 2D Solid State HRMAS NMR Study on Accelerated Sulfur Vulcanizates of BR Rubber

“…The use of 2D high-resolution magic angle spinning (HRMAS) solid state NMR techniques, allowed us to gain insight into both the structure and the underlying mechanism of cross-link formation on a molecular level. Previous results on the vulcanization of a model compound ( cis , cis , cis -1,5,9-cyclododecatriene, ccc -1,5,9-CDT) appeared to be in good agreement with 2D solution NMR data of a BR graft, indicating the formation of alkyl cross-links. NMR analysis led to the entire elucidation of the molecular structure and it was concluded that addition had occurred instead of α substitution.…”

Vulcanization of Butadiene Rubber by Means of Cyclic Disulfides. 2. A 2D Solid State HRMAS NMR Study on Cross-Link Structures in BR Vulcanizates

“…In a first attempt to gain insight into the structure of BR networks, a low molecular weight model for BR , i.e., cis , cis , cis -1,5,9-cyclododecatriene ( ccc -1,5,9 - CDT), 1 , was vulcanized using the cyclic disulfide 1-oxa-4,5-dithiacycloheptane. Solution NMR proved that bis(alkyl) cross-link structure 2 (Figure ) was formed due to addition of the cyclic disulfide to the carbon double bonds in the model compound . The other possible reaction mode leading to 3 , the formation of cross-links resulting from α substitution analogous to accelerated sulfur vulcanization of model compounds, was not observed in the present study.…”

Insights into the Vulcanization Mechanism through a Simple and Facile Approach to the Sulfur Cleavage Behavior

“…To deal with these problems mentioned above, countless researchers were focused on finding model compounds such as cyclohexene, , 1-octene, 2,6-dimethylocta-2,6-diene, cis -polyisoprene, and cis -1,4-polybutadiene to simulate the vulcanization process and analyze the as-prepared structures of these model compound/sulfur systems. In fact, a large number of resultant structures containing polysulfidic structure, cyclic sulfides, pendant side group structure, and isomerization structures of double bond are derived from the above model compound/sulfur systems, and a large number of modern characterization methods such as equilibrium swelling experiments, high-resolution solid-state NMR, ,− scattering techniques, low-field time-domain NMR experiments, and computer simulation , are used for the clarification of the vulcanization mechanism. However, these data could not end the debate between the ionic and radical theory because such approaches are indirect and cannot monitor the intrinsic chemical reactions occurred in the vulcanization process.…”