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

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. 13 C DEPT spectra yielded the multiplicity of the 13 C resonances, followed by HETCOR techniques to identify the corresponding 1 H resonances. COSY, DQF − COSY ( d ouble q uantum f iltered COSY) and TOCSY ( to tal c orrelation s pectroscop y ) experiments provided the homonuclear (long-range) through-bond connectivities leading to the complete elucidation of the molecular framework. The experimentally found 13 C 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.