Abstract
Inverse vulcanization is a potential route to the use of the large excesses of elemental sulfur, creating high-sulfur-content polymers with many potential applications. The addition of a metal diethyldithiocarbamate catalyst was previously found to bring several benefits to inverse vulcanization, making the process more attractive industrially. Herein is reported the establishment and exploration of a library of catalysts for inverse vulcanization. Three ranges of catalysts and up to 32 compounds and their combinations have been investigated. By trialing these alternative catalysts, several tentative deductions about the mechanism have been made. It has been found that stronger nucleophiles give a greater rate enhancement, but with the tradeoff that harder bases may promote hydrogen sulfide byproduct formation. Monomer binding by the cation may be a crucial mechanistic step, and it is possible that the catalysts act as phase transfer agents between the immiscible sulfur and organic phases. Additionally, the versatility of catalytic inverse vulcanization has been demonstrated with several different comonomer families.
Highlights
Catalytic inverse vulcanization permits lower reaction temperatures, which may be explained by the weakening of sulfur−sulfur bonds by coordination to the metal center, as the high temperatures of uncatalyzed inverse vulcanization are theorized to be necessary for homolytic sulfur−sulfur bond cleavage
It was proposed that the DEDC organic comonomer because it is inexpensive, can undergo ligand may assist in the cleavage of sulfur−sulfur bonds and inverse vulcanization without a catalyst
Catalytic inverse vulcanization has been demonstrated with a variety of representative crosslinkers and catalysts
Summary
In order to mitigate acid rain, petrochemical feedstocks are purified of sulfurous compounds by means of hydrodesulfurization and the Claus process, yielding large quantities of elemental sulfur as a byproduct (more than 60 million tons per annum).[1−4] While a small portion of this sulfur is used to create fertilizers and sulfuric acid, among other applications, the supply of sulfur greatly outweighs the demand, leading to megaton quantities of sulfur being stored in open-air stockpiles with unexplored environmental consequences (Figure 1a).[4,5] These stockpiles of sulfur are expected to expand more rapidly in coming years, as the depletion of fossil fuels drives the use of previously avoided petrochemical resources that contain greater levels of sulfur contamination.As such, elemental sulfur is a cheap, abundant, and underutilized resource, with wide availability for use in new applications.[4,5] In terms of materials chemistry, pure elemental sulfur can be self-polymerized, but the resulting homopolymer is unstable and depolymerizes back to the monomer, S8 rings, upon cooling.[3]. These inverse vulcanized polymers show promising applicability in the fields of electrochemistry, where they may act as cheap and effective cathode materials in lithium sulfur batteries; remediation of water, due to their ability to take up heavy metals such as mercury, and optics, where they may act as highly refractive and infraredtransparent components.[7,12−15]
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