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Team:Leicester/Chemistry
From 2012.igem.org
Chemistry
The breakdown of polystyrene is a quest many scientists have faced in recent years. Now more than ever before, there is an urgency to find a biological route to degrade polystyrene in an economical and environmentally safe way: the inability to do this is really the only downside of polystyrene as a useful material. We, the organic chemists, are responsible for coming up with reactions to breakdown or convert polystyrene into useful products. We are working alongside the biochemists and geneticists of the iGEM team to find a safe and useful way to break down polystyrene.
Our research into current chemical techniques did not show anything useful, this suggested that we were entering new territory which added to the excitement. Equipped with the knowledge from our first two years of studying chemistry and our problem solving skills gained along the way, we devised chemical syntheses to the breakdown and/or conversion of polystyrene. Below is our syntheses and we hope that they will help any researchers who wish to take the project further. Our aim was to use readily available chemicals and in abundance, this would make the whole process as cheap as possible. One of our sponsors, Styropack, converted polymers to biodegradable polylactic acids; we took this idea as our starting point for our research.
Mechanism Insight
Mechanism 1: This was our initial idea using knowledge of our sponsors work. We thought this decomposition would be easy. Then we realised our mistake! We were researching the wrong route. (Decomposing styrene, not polystyrene!)
Mechanism 2 and 3: Once we had conferred, we realised that these mechanisms would not be possible as a heterogeneous catalyst Palladium would not work in this reaction so we looked for a suitable homogeneous catalyst.
Mechanism 4: Out of all the mechanisms we studied, this is by far the most promising. Despite this, there is still an urgent need to find an enzyme that can generate a reactive oxygen species. This must then react further to oxidise the benzyl carbon as well as degrade the remaining aliphatic chain.
Mechanism 5: This was an attempt at using transition metal chemistry to break the aliphatic chain of the hydrocarbon. It is a theoretical idea but one that paves the way for more creative methods.
Mechanism 1
The first mechanism shows the conversion of styrene (monomer of polystyrene) to lactic acid, which can be converted to polylactic acid (a biodegradable alternative). This mechanism involves the use of harsh and dangerous chemicals such as Pyridinium chlorochromate and Chromic acid. The first step shows how benzyne is formed, which is also a carcinogen. This can be reduced to benzene and disposed of easily or stablised by a transition metal.
This route, though it may work for styrene, will not work for the breakdown of the polymer itself.
Mechanism 2
The second mechanism shows that as polystyrene is heated up, the intermolecular forces weaken. Because benzene can undergo electrophilic aromatic substitution, it can be treated with Nitric Acid to afford a new compound that contains a new Nitrogen dioxide side group. The final product of this reaction is a benzene ring with an alcohol functional group. We are still researching practical uses and possible further reactions on this line.
Mechanism 3
This is an alternative mechanism to that proposed in mechanism 2. Instead of adding an alcohol functional group, a Bromine is added. Ultimately this results in the formation of an acid anhydride chain, which is degradable in the environment. The aliphatic chain would need to be both weakened and then broken to afford pure anhydride monomers.
The mechanism above, in combination with mechanism 5 below, may work to break down polystyrene into smaller parts.
Mechanism 4
This final mechanism draws on peroxyl product formation. Organic peroxyls are used in chemical reactions in industry so it may be possible that this organic peroxyl will have a use in the near future.
This seems like a viable chemical route. Conversion of the product above to compounds such as Benzoyl Peroxide has pharmaceutical uses such as acne creams but the product will be of very low yield if not impure. To add to the mechanism above, the use of a cytochrome P450 or a peroxidase may be able to generate a reactive oxygen species that will oxidise the benzyl carbon. Degradation of the aliphatic hydrocarbon chain may also be possible via mechanism 5 to yield more simple stablised products.
Mechanism 5
The fifth mechanism shows how we might be able to degrade polystyrene by pursuing an inorganic synthesis. This is an interesting concept as most chemical research today is influenced by inorganic chemistry. We believe the route below could serve as a template for future efforts to breakdown polystyrene. However key reagents are notably missing so we have left this open to further research.
Hazards and Safety
| Reagents | Reagents/Formula | Melting Point<.th> | Main Hazard |
|---|---|---|---|
| Sodium Hydroxide | NaOH | 318 oC, 591 K, 604 oF | Corrosive |
| Sodium Amide | NaNH2 | 210 oC, 483 K, 410 oF | Not Listed |
| Ammonia | NH3 | -77.73oC, 195 K, -108 oF | Oxidising, Toxic, Flammable, Irritant |
| Platinum | Pt | 1768.3 oC, 2041.4 K, 3214.9 oF | Not Listed |
| Pyridinium chlorochromate (PCC) | C5H5NHClCrO3 | 205 oC, 478 K, 401 oF | Oxidising, Toxic, Flammable, Carcinogenic, Irritant |
| Chromic Acid | H2CrO4 | - | Powerful oxidising agent, further reactions produce toxic and corrosive products |
| Nitric Acid | HNO3 | -42 oC, 231 K, -44 oF | Toxic, Flammable, Irritant |
| Sulphuric Acid | H2SO4 | 10 oC, 283 K, 50 oF | Oxidising, Toxic, Flammable, Carcinogenic, Irritant |
| Palladium | Pd | 1554.9 oC, 1828.05 K, 2830.82 oF | Oxidising, Toxic, Flammable, Carcinogenic, Irritant |
| Copper(I) bromide | CuBr | 492 oC, 765 K, 918 oF | Not Listed |
| Magnesium | Mg | 650 oC, 923 K, 1202 oF | Not Listed |
| Tetrahydrofuran | THF | -108.4 oC, 165 K, -163 oF | Flammable, Irritant |
| Carbon Dioxide | CO2 | -78 oC, 194.7 K, -109 oF | Not Listed |
| Hydronium | H3O+ | Not Listed | Not Listed |
To conclude, this research has shown us that although there may not be a definite chemical route, we have researched and pitched our ideas to show how chemical approaches may work if a biological aspect is taken into account. By finding useful enzymes or bacteria which can carry out certain steps of the syntheses in appropriate conditions it will no doubt be possible to break down polystyrene. We are confident that in the near future that there will definetly be a cheap way to break down polystyrene that is both economical and safe. We have learnt much about the application of chemistry in synthesis and gained invaluable skills. We believe that we are steps closer to solving the worlds problem of polystyrene.
References
Vincent N. Cavaliere, Marco G. Crestani, Balazs Pinter, Maren Pink, Chun-Hsing Chen, Mu-Hyun Baik, and Daniel J. Mindiola. Room Temperature Dehydrogenation of Ethane to Ethylene Journal of the American Chemical Society
