Sulfur vulcanizates of high cis-butadiene rubber (BR) obtained by using different curing systems such as conventional (CBS) and efficient sulfur (sulfur donor, TMTD) vulcanizing systems were investigated in detail using high-resolution magic angle spinning (HRMAS) solid state NMR spectroscopy in order to identify the type of cross-linking sequence and overall molecular structure. DEPT-135 HRMAS spectra recorded at 60 °C provided the different 13C-resonances; interpretation of the HETCOR HRMAS spectra yielded the corresponding 1H frequencies. Direct and long-range through bond homonuclear connectivities were obtained from COSY and TOCSY HRMAS experiments, whereas NOESY HRMAS experiments provided the dipolar through space interactions. Comparison of these results with solution NMR data on a BR model compound vulcanizate led to the elucidation of the entire network structure. The 13C chemical shifts found experimentally appeared to be in good agreement with chemical shifts calculated for the proposed structure. Using the described vulcanizing systems, it was shown that cross-linking proceeds by means of α substitution rather than addition in contrast with our studies on cyclic disulfides based vulcanization reactions of BR. By using CBS or TMTD as vulcanizing agents, differentiation in sulfur cross-link length could be established. Moreover, the presence of several cyclic sulfides in the vulcanizates was observed and a mechanism for their formation is suggested.

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.

Vulcanizates of high cis-butadiene rubber (BR) obtained by using several cyclic disulfides such as 1,2-dithiacyclooctane, 1-oxa-4,5-dithiacycloheptane, and 2,3,12,13-tetrathia-[4,4]-metacyclophane, were studied by means of high-resolution magic angle spinning (HRMAS) solid state NMR spectroscopy in order to determine cross-linking sequence and overall molecular structure. DEPT-135 HRMAS spectra recorded at 60 °C provided the multiplicity of the different 13C resonances; interpretation of the HETCOR HRMAS spectra yielded the corresponding 1H frequencies. Direct and long-range through bond homonuclear connectivities were obtained from HRMAS COSY and TOCSY (total correlation spectroscopy) experiments. Comparison of these results with solution NMR data on hexyl disulfide grafted BR and previously described work on a cis,cis,cis-1,5,9-cyclododecatriene (ccc-1,5,9-CDT) based model system led to the elucidation of the entire network structures. The 13C chemical shifts found experimentally appeared to be in good agreement with chemical shifts calculated for the proposed structures. It was shown that cross-linking proceeds by means of addition of the cyclic disulfides to the carbon double bonds of BR instead of α substitution.