Catalytic Effect of Potassium Xanthates and Related Compounds on Disulfide Bond Enrichment of Polyalkylene Sulfides Synthesized in the Course of Episulfide Polymerization

Abstract

The original method for the preparation of high-molecular-weight polyalkylene sulfides was reported. Assuming anomalous peculiarities of the reaction (high polymerization rates, high degrees of polymerization, and huge discrepancy between the expected M_n values and the experimentally obtained values), the priority task was set to study the mechanism underlying the observed new type of polymerization. Thus, it was demonstrated that xanthate and related molecules could act as pure catalysts, facilitating both the chain-growth polymerization (ring-opening of episulfides) realized via an anionic route and the direct attack of the sulfur atom of one episulfide molecule on the methylene carbon atom of the second (neighbor) episulfide molecule, accompanied by the subsequent formation of a stable thiiranium-based zwitterionic adduct. The role of xanthate and related compounds as catalysts and stabilizing particles was further supplemented by modeling the attack of thiolate on the sulfur atom of a thiiranium-based adduct. The xanthate molecule acting as a catalyst was found to be involved in all stages of the process discussed by sharing the potassium atom with the sulfur atoms of active components of the system (the initial episulfide molecule, thiolate, and the zwitterionic intermediate). The subsequent analysis revealed the exceptional transparency of the materials obtained, which was found to exceed 99%. The pronounced self-healing ability was also found to be a distinctive feature of the synthesized high-molecular-weight polyalkylene sulfides enriched with disulfide bonds.