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

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.