This post is contributed by John Spevacek, an industrial polymer chemist and the author of the blog “It’s the Rheo Thing”
While organic chemists are familiar with the elements, very seldom do we ever make use of them as a reactant. Sure, we add elemental magnesium to Grignard reactions and we can add halogens/hydrogen across double bonds, but for the most part, the pure elements are oxidized or reduced or ionized or otherwise modified before they take part in our reactions.
The situation is even that much clearer for my field of polymer chemistry. Pure elements of any sort are just not used at all. We certainly don’t use elemental carbon and hydrogen to make polyolefins, and silicon wafers are useless for making silicone polymers. In short, the refined elements have no place in polymer chemistry.
A recent paper in Nature Chemistry (pay-per-view/subscription) showed that elemental sulfur can be directly co-polymerized with an organic molecule. What was more surprising yet was that the polymerization occurred without the use of solvents or even initiators.
From my perspective as a polymer chemist, the uses of sulfur are limited and have historically fallen into three categories. First are the polymers that have the sulfur in the backbone, such as polyphenylene sulfide (PPS), polyethersulfone (PES), and all the countless thiol-ene polymers. Another class are the polymers where the sulfur is peripheral to the backbone, usually as a sulfonate group such as in polystyrene sulfate. And lastly, there are the elastomeric materials where sulfur compounds have been used to vulcanize (crosslink) the polymer chains.
What all three of these sulfur-containing polymers have in common, however, is that none of them are prepared from elemental sulfur. They all require either a reduced or an oxidized form of sulfur in order to form the polymers.
As implied above, this new reaction is very simple. The researchers merely melted the sulfur and added 1,3-diisopropenyl benzene (DIB) at ratios from 90/10 to 50/50 w/w. The S8 rings of sulfur opened up and copolymerized with the vinyl groups.
The reaction mechanism is not explicitly detailed, but I imagine it to be similar to what occurs in thiol-ene polymerizations. Since the organic comonomer is difunctional, the resulting product is crosslinked, not through the sulfur atoms, but instead through the organic monomer. The authors (with tongue-in-cheek) call this “inverse vulcanization”. However, despite the existence of this crosslinking, the polymer still flows as a thermoplastic. (Evidently the numerous sulfide bonds are breaking and reforming under the shear). This is fortunate as it allows the plastic to easily be shaped into a final product using conventional equipment.
While this is the only polymerization reaction I know of using a pure element, this discover by itself is interesting although somewhat limited. Working with molten sulfur imposes two big restraints on the choice of comonomers – that they first be soluble in the molten sulfur and more importantly, that they not volatilize upon exposure to the heat (185 C). In other words, this new reaction opens up only a small set of potential polymers.
But what properties this polymer is already showing!
Consider batteries. We are surrounded in our modern lives by lithium-ion batteries. They are in our cellphones and laptops, our cordless power tools, and even the Mars Curiosity Rover. A relative drawback of these batteries is that the anions are metallic and therefore heavy, reducing the energy density. It’s long been known that lithium-sulfur batteries have a high energy density and lower cost, but the degradation of the sulfur electrodes limits their long-term stability.
Preliminary testing of a lithium battery using this new sulfur-based polymer, however, shows that the performance is nearly identical to that of a standard lithium-sulfur battery but without the degradation. When this result is combined with the ease of processing this new polymer, the potential for lithium-sulfur batteries has suddenly become a lot sunnier.
Almost as sunny yellow as the color of elemental sulfur.