Vulcanization of Butadiene Rubber by Means of Cyclic Disulfides. 2. A 2D Solid State HRMAS NMR Study on Cross-Link Structures in BR 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 13 C resonances; interpretation of the HETCOR HRMAS spectra yielded the corresponding 1 H frequencies. Direct and long-range through bond homonuclear connectivities were obtained from HRMAS COSY and TOCSY ( to tal c orrelation s pectroscop y ) 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 13 C 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 .