materials chemistry

Polymers from Elemental Sulfur

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.

Until now.

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.

Molecular Mechanics

Synthesising small molecular machines has been somewhat limited to making molecules that can walk or spin round cogwheels. Mind you that is still pretty impressive. Now things are set to change with a recent publication in Science by Dr Leigh from the University of Manchester in the UK.

The Manchester group synthesised a rotaxane (a molecular ring) threaded through an axle that consists of peptides. The rotaxane has a thiolate moiety that removes the amino acids in sequence and transfers them to the site of the new growing peptide chain. There is a wonderful summary in C&E News, with an interesting video of how it all works.

The group used 1018 machines in parallel to generate milligram quantities of a single sequence peptide. This mimics the ribosome in its valuable function in the generation of peptides. The “arm” picks up the amino acid by a transacylation reaction and delivers them to a different place on the ring.

There are still some problems to be solved, for example the rather slow reaction rate as the ring needs about 12 hours to make the amide bonds. Compare this to the 15-20 bonds per second produced by the ribosome itself. There are a few other problems, and no doubt they are being addressed at this moment. However this paper and the technology are impressive and will probably have a great future.

Imagine what several moles of these could do once things become optimised, producing peptides of any sequence one desired, natural or unnatural. This is a fascinating concept and I look forward to seeing lots more appearing on this system.

Life in Chemical Development, Part 2.

In the first part of this little series I recounted my experience with two steps of a four-step sequence, now I would like to move on to the last two steps: The preparation of a benzyl chloride and it’s conversion to a benzyl azide.

If you remember I had to convert 7098 kg of the benzyl alcohol ultimately to the azide. According to the plan:

Now benzyl halides are well known for their lachrymatory properties and this one made me cry just thinking about it. All that was required was to walk past the building, where it was being produced, to burst into tears and I had to run 46 batches (1.02 kMol) to make this stuff plus 9 for use tests. In fact we made the chloride then almost immediately concerted it to the azide.

As part of the safety checks in the pre-reaction control of the equipment the conductance of the enamelled stainless steel reactor was checked to make sure there were no cracks in the enamelling, it was deemed to be ok so we carried on. The alcohol was placed in a 630 L reactor and 312.8 kg of 37% hydrochloric acid was pumped in. The solution was heated slowly to an internal temperature of 90-93°C ( to avoid loosing too much HCl) and held there for 5 hours. During the reaction a two-phase system formed and we all cried. The product was on the bottom and it was separated from the acid after cooling to 40°- 45°C because the compound solidified at 37°C. It was then filtered and the pH adjusted to 9-12 with 30% NaOH solution and stored at 40°C as a two-phase system with water with minimal stirring and constant pH adjustment maintaining the 9-12 range. In the meantime we got things going for the conversion to the benzyl azide, more about that later.

When we examined the filter from the very last reaction we observed bits of blue glass. I hear you say “not again”. I don’t seen to have much luck with enamelled reactors. Well this time we were really lucky, and I mean really. Have a look at these two pictures.

The hole was a hairline crack in the enamel. Now this did not show up in the conductivity tests as it was right up at the top of the reactor where the stirrer joined together with the motor and could not be reached with the equipment we had, a pathetic excuse really. Maybe we should have used, you know that beer that reaches places that other beers can’t.  Remember under the enamel is stainless steel and we were using almost boiling 37% hydrochloric acid. So the acid seeped through during the course of the 46 batches and started munching away at the steel. The metal was so thin that if you pinched it between thumb and forefinger you could move the bottom part back and forward. I would say that one more reaction and the stirrer would have broken off at 100 rpm making God only knows what kind of mess. Furthermore it is well known that the presence of iron (rust) benzyl halides decompose exothermically at quite low temperatures. I can’t remember the exact temperatures but it moves the decomposition point (where the exotherm begins in DSC measurements) down about 50 or so degrees and increases the size of the exotherm markedly. So I guess we were lucky on two points, we stopped just in time and we were using steel with a very low iron content. After I saw this and realised the implications I my knees started knocking together and I staggered across the road to a pub and had a few stiff drinks and went home where I continued the treatment.

Back to the chemistry: Working with azides is particularly dangerous because of potential explosion and health hazards. Sodium azide is a very nasty compound. It is a CNS depressant and breathing the dust causes almost immediate breathing problems amongst others, see this page for more information, azides. Furthermore it also contains traces of hydrogen azide, which has similar biological behaviour to sodium azide but has the pleasant habit of being shock sensitive and hence explosive. The stirrer episode was bad enough; and we were using 70 kg of sodium azide per batch, my poor knees (never mind the liver). Even at pH 9 or above one can still detect HN3 in the gas phase. For the reaction we had an extensive gas washing system with 4 washers filled with 30% sodium hydroxide solution through which the exhaust went. At the end of this chain we periodically monitored for the presence of HN3 using ferric chloride spot tests, which are very sensitive for this compound. I’m happy to say that at the end of the chain we never detected any HN3. The reactor was specially made out of high quality tantalum steel, where the heavy metal content was minimised so we hopefully avoided the formation of heavy metal azides, I do not know if tantalum azide exists (perhaps someone who reads this may know) and heated glass tubing was employed for the transfers.

We threw the following into a 630 L reactor; 200 kg water, 2.6 mL of 30%NaOH solution, 700g tetra-n-butylammonium bromide, 70 kg sodium azide and a pH electrode. After heating this mixture to 90-95°C internal temperature and added the alkaline mixture of the benzyl chloride to it within 60 minutes. The pH drifted during this reaction and it was constantly monitored and kept between 9 and 12. The reaction is exothermic and the temperature control was also monitored closely during the 2 hour stirring at 90-95°C.

We then cooled to room temperature and filtered the lower organic phase (this time no glass was observed!) and removed the aqueous layer. This time everything went ok and from 55 batches we obtained a total of 9284.64 kg with an average purity of 94% and an average yield of 97.9%. All of the batches were released for the next step by QA. At last I was almost finished, I still had to dispose of all the azide containing waste from all the gas washers and all the water layers and reactor cleaning! This was really funny. We disposed of it by treatment of the waste with 37% hydrochloric acid and sodium nitrite, generating nitrogen, laughing gas and various other oxides or nitrogen that were washed out by the exhaust treatment. This was another foaming reaction, but by this time I was immune to foaming, didn’t worry me anymore. The aqueous phases went down to the water treatment plant.

There it was finished at last, with enough material for my colleague to play with. There is still more to tell about this chemistry but that will be part 3.

I hope you enjoyed my ramblings and look forward to many comments!

LeBron James Promotes Sheet-y Science

It’s been quite a year for the NBA All-Star: claiming his first NBA Championship, winning gold in the 2012 London Olympics, and now…promoting dietary supplements?

The product in question, Sheets®, offers variations on the “breath strips” made popular roughly a decade ago. Each strip contains different GRAS additives, such as melatonin to aid sleep, or caffeine in the Energy Sheets®. Despite the fecundity of the exclamation points in the FAQs, or even the curious swath of ‘beautiful people’ who promote this product, I’d be willing to give it a pass, if it weren’t for one teeny, tiny detail: the “Science page.”

Here’s the full scientific statement:

“It’s simple…Sheets® solve problems! Sheets® are paper-thin, individually wrapped pocket-sized strips. No cans. No bottles. Simply place on tongue and your problem dissolves. How? Sheets® are packed with nutrients/vitamins and other active ingredients that, when placed on tongue, will begin to dissolve allowing for easy digestion.

Hang on a second….AAAAAUGH!

OK, all better now. Let’s see if we can break that down further for our discerning audience. Apparently, the science of Sheets® involves dissolution (“place on tongue”) followed by digestion of nutrients/vitamins. Did everyone get/understand that, or should I repeat/rehash it again? Never mind those goofy pictures with the colorful stamped film, which looks uncomfortably like another orally administered molecule

Source: sheetsbrand.com

#EpicScienceFAIL

Let’s go to our good friend Google patents to find some real science on this sheet-y product. I dug up two documents in short order: US patent 4,713,243 (Johnson & Johnson, 1987) and US 6,419,903 B1 (Colgate, 2002). Both patents describe various technologies for impregnating thin, extruded films of soluble polymer with medicaments for oral administration. Translation – edible drug strips.

The base polymer of choice, even 25 years ago, seemed to be hydroxyalkyl cellulose, one form of which we call pullulan. Various swell-able filler polymers, such as gelatin, corn starch, or PEG (polyethylene glycol) mix with the pullulan to regulate its toughness and stiffness, as well as to serve as a carrier for the active ingredients. For the Colgate breath strips, these include zinc compounds or alpha-ionone, which help to fight volatile sulfur compounds (VSCs) in your mouth. The J&J patent reaches even further, engineering strips to fight bacteria (sulfadiazine), pain (potassium nitrate), or to reduce swelling (hydrocortisone).

Honestly? I was most surprised by the level of formulation science that goes into each strip: viscosity tests, dispersion, dissolution, adherence, blending, and extrusion. Sounds like the perfect job for a p-chemist.

Just don’t get LeBron involved. Please.