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 cy clic 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 hi gh-resolution magic angle spinning (HRMAS) solid state NMR spectroscopy in order to determine cross-linking sequence and overall molecular structure. DEFT-135 HRMAS spectra recorded at 60 degrees C provided the multiplicity o f the different C-13 resonances; interpretation of the HETCOR HRMAS spectra yielded the corresponding H-1 frequencies. Direct and long-range through b ond homonuclear connectivities were obtained from HRMAS COSY and TOCSY (rot al correlation spectroscopy) experiments. Comparison of these results with solution NMR data on hexyl disulfide grafted BR and previously described wo rk on a cis,cis,cis-1,5,9-cyclododecatriene (ccc-1,5,9-CDT) based model sys tem led to the elucidation of the entire network structures. The C-13 chemi cal shifts found experimentally appeared to be in good agreement with chemi cal shifts calculated for the proposed structures. It was shown that cross- linking proceeds by means of addition of the cyclic disulfides to the carbo n double bonds of BR instead of alpha substitution.