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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 with the biochemists and geneticists of the iGEM team to find a convenient and useful way to break down polystyrene.
Research into current chemical techniques did not show anything immediately useful - suggesting that we were entering new territory, which was something that was exciting to know. Equipped with knowledge from our two years of studying chemistry and our problem solving skills, we have devised synthetic chemical routes to breakdown and/or the conversion of polystyrene. The iGEM team have received excellent support froml a major EPS production company, Styropack. Styropack's parent company (Synbra) is developing replacements for polystyrene based on polylactic acid (PLA) which are much more easily biodegradable. We thought that it would be useful to explore the chemistry of degrading polystyrene into lactic acid, so it could be the feedstock for its own replacement.... This was our starting point for our research.
Mechanism Insight
Mechanism 1: This was our initial idea. We thought this decomposition would be easy. Then we realised our mistake! We were researching the wrong route. (Decomposing styrene, not polystyrene!)
Mechanism 2: Once we had conferred we realised that this mechanism would not be possible as a heterogeneous catalyst Pd would not work in this reaction.
Mechanism 3: Out of 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 regenerate a reactive oxygen species. This must then react further to oxidise the benzyl carbon as well as degrade the remaining aliphatic chain. A good example are peroxidise enzymes which could ne investigated further.
Mechanism 1
The first mechanism shows the conversion of styrene (monomer of polystyrene) to lactic acid, which can be converted to polylactic acid, an more biodegradeable product. This mechanism involves the use of harsh and dangerous chemicals [INSERT CHEMICAL NAMES]. The first step shows how benzyne is formed, which is 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. But the conversion of the monomer unit into lactic acid is an important downstream step for making waste polystyrene useful.
Mechanism 2
The second mechanism shows that as polystyrene is heated up, the intermolecular forces weaken. Because benzene can undergo electrophilic aromatic substitution, it is treated with nitric acid to afford a new compound that contains a new NO2 side group. The final product of this reaction is a benzene ring with an alcohol side 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 OH side group, a bromine group is added. Ultimately this results in the formation of an acid anhydride chain, which is degradable in the environment. The hydrocarbon chain would need to be both weakened and then broken so as to afford pure anhydride monomers which can be easily broken down.
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 many chemical reactions in industry so it may be possible that this organic peroxyl will have a use in the near future.
This seems likel to be a viable biological route. Conversion of the product to a smaller unit of benzoyl peroxide could have pharmaceutical uses. 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.
Mechanism 5
The fifth mechanism shows how we might be able to degrade polystyrene by pursuing an inorganic method. 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 still 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 the team organic chemists that although there may not be a definite chemical route, we have researched widely and pitched in our ideas to inform how biological approaches could work. 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 set out to break down polystyrene completely and we have learnt much about the application of chemistry in synthesis.
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
