Spectroscopic Characterization of Solid-Solid Transitions in Model Compounds for Copolymers Containing Sequential Thioethylene Units

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Polymers and High Performance Materials


In this paper, we describe the thermal and spectroscopic analysis of oligo(thioethylene) model compounds for copolymers containing sequential thioethylene units. Many thioethylene copolymers and model compounds show multiple thermal transitions by DSC, although the molecular-level behavior associated with such transitions is not known. The model compounds studied here consist of CH3(CH2)(9)S(CH2CH2S)(n)(CH2)(9)CH3, with n = 1-5. For n = 1, a crystal structure was identified by WAXD, C-13 CP/MAS, and FT-LR analysis that was different than the others. For all other model compounds (with n = 2-5), the crystal structures were similar with increased interactions between molecules as n increased, caused by increasing numbers of thioethylene segments resulting in more favorable gauche conformations at the internal C-S bonds. This leads to packing similar to that of poly(thioethylene) and causes the melting points of higher oligo(thioethylene) analogs to be higher than those of linear alkanes of similar lengths. From C-13 CP/MAS results, carbons adjacent to sulfur experience a shielding of ca. 0.6 ppm for every gamma-gauche interaction in the solid state and a 0.8 ppm upfield shift for each beta thioether in the trans conformation. In compounds with a mixture of trans and gauche conformers at the C-S bonds, librational motion increased the trans isomer content at temperatures above ambient but below melting transitions observed by DSC. This conformational motion was identified by C-13 CP/MAS, C-13 HP/MAS, Raman, and LR analysis. For two of the models, a distinct solid-solid transition (below the melting point) occurred to accommodate changes in thioethylene segment conformations. At this transition, an all-trans conformation was observed with increased intermolecular spacing similar to what is seen by WAXD for the lowest oligo(thioethylene) analog (it = 1) at room temperature. This thermal lattice expansion was observed for the model compound with five thioether moieties as discontinuous changes in solid-state C-13 NMR chemical shifts and C-D bond splitting and changes in the intensities and frequencies of LR vibrational modes. These crystal and conformational changes were completely reversible as monitored by C-13 and H-2 NMR, IR, Raman, and DSC. z

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