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 vulcan izates using several 1D and 2D NMR techniques in order to determine cross-l inking sequence and overall molecular structure. Because of the relatively high solubility of the products, typical solution 2D NMR pulse sequences co uld be applied offering a framework of reference for the solid state NMR st udies on true vulcanizates to follow. C-13 DEFT spectra yielded the multipl icity of the C-13 resonances, followed by HETCOR techniques to identify the corresponding H-1 resonances. COSY, DQF-COSY (double quantum filtered COSY ) and TOCSY (total correlation spectroscopy) experiments provided the homon uclear (long-range) through-bond connectivities leading to the complete elu cidation of the molecular framework. The experimentally found C-13 chemical shifts were compared with the calculated shifts and appeared to be in good agreement. Using this "model compound" the applicability of several NMR te chniques to determine structural characteristics proved to be possible. Mor eover, the study allowed the comparison of the solution NMR techniques ment ioned with the solid state NMR techniques used in the elucidation of the cr oss-link sequence and overall structure of true vulcanizates.