High-Field Nuclear Magnetic Resonance Studies Reveal New Structural Landscape of Sulfur-Vulcanized Natural Rubber.

Despite intensive research efforts over the past 3 decades, the structural analysis of sulfur-vulcanized natural rubber (NR) remains challenging owing to the complexity and low population of its sulfur moieties. Herein, solid vulcanized NR samples and NR samples reacted with sulfur and other reactants in an organic solvent were analyzed by solid-state NMR with fast magic-angle spinning and solution NMR, respectively. The present high-field two-dimensional NMR analysis revealed six novel sulfur moieties in these samples, including cyclic sulfides, cyclic di/polysulfides, and crosslinked structures with a vinylidene group. While previous studies reported a variety of sulfur-crosslinked structures in NR, our analysis identified only two dominant types of crosslinked structures that matched those reported previously. Our NMR assignments for the crosslinked structures were inconsistent to a large extent with those presented in the previous studies; thus, in the current work, the crosslinked structures were reassigned using the new data. Based on quantitative NMR analysis, this study also provides the first tangible evidence that cyclic rather than crosslinked sulfides can be the dominant sulfur moieties in vulcanized NR. These results may drastically alter the previously established structural landscape of sulfur-vulcanized NR.

33S nuclear quadrupole resonance study of dibenzyl disulfide toward understanding of cross-linked structures in rubber.

Experimental and theoretical investigations of a sulfur-33 electric-field-gradient (EFG) tensor of disulfide bonds in 33S-labled dibenzyl disulfide have been presented. Temperature dependence of quadrupolar frequencies, νQ, is observed in the temperature range between 80 and 280 K, in which single peaks appear in all the 33S nuclear quadrupole resonance (NQR) spectra. Analysis of nutation echo 33S NQR spectra at 200 K yields the quadrupolar coupling constant, CQ value, of 46.8(6) MHz and the asymmetry parameter, ηQ, of 0.48(7). The orientation of the 33S EFG tensor of the disulfide is obtained by quantum chemical calculations: the largest EFG tensor component, VZZ, is approximately perpendicular to the molecular plane (C-S-S), and the smallest component, VXX, is approximately 41° off the C-S bond. Extensive quantum chemical calculations are systematically performed to investigate dependences of 33S EFG tensors on changes of torsion angles in disulfide and trisulfide bonds, indicating that analysis of νQ and CQ values potentially makes it possible to assign the secondary structures of cross-linking in a rubber.