Research at Your University: Green Polymers Using Supercritical Carbon Dioxide

Impact investigates the work currently being undertaken by scientists at your university.

What’s the Research About?

Silvio Curia and Steve Howdle from the Chemistry department have been researching ways to make polymers using supercritical carbon dioxide (scCO2). Behaving like both a liquid and a gas at the same time, scCO2 is a supercritical fluid at a particular temperature and pressure. What makes this so exciting is that you only need a temperature of 31°C to use it (not far off room temperature).

Using scCO2 negates the need for toxic solvents (a liquid the reaction takes place in), lowers the temperature required to make polymers and allows the use of enzymes to facilitate the reaction. The enzyme is acting as a catalyst, which speeds up the reaction and is a far greener alternative to the expensive metal catalysts currently used in the plastics industry. This is due to the fact that extremely high temperatures are required to manufacture these polymers and use non-renewable materials such as crude oil.

In this recent publication, scCO2 and the enzyme Novozym 435 were used to make polymers of the naturally occurring molecule azelaic acid, the active ingredient in some spot treatments. The acid is found on wheat, rye and barley.

Several polymers of various lengths were produced with good yields (this being the actual amount made compared to the theoretical amount that could be made) of between 82 and 91% demonstrating this is a viable production method. 

“Novozym 435 acts as a catalyst, which speeds reactions and is a far greener alternative to the expensive metal catalysts currently used in the plastics industry”

What are Polymers?

Polymers are a vital part of modern life as we know it. They are long chains of a large number of smaller blocks (monomers) connected by chemical bonds and are the major components in plastic bags (polyethylene), drinks bottles (PET or polyethylene terephthalate), Teflon coated pans (PTFE or polytetrafluoroethylene) and many other everyday items. They are also found in our own bodies (proteins and DNA) and some have even been found in outer space.  

What’s the Chemistry Involved?

c6py00066e-f2_hi-res Reaction mechanism for Novozym435 with Supercritical Carbon Dioxide 

Azelaic acid is a dicarboxylic acid. The carboxylic acid functional group (COOH) can react with an alcohol group (OH) on the 1,6-hexanediol in a condensation polymerisation reaction with water as the by-product. As both molecules contain functional groups on both ends of the molecule this reaction can repeat many times, with the polymer extending each time.

In order to limit the length the polymer reaches, Curia and Howdle use MPEG (methoxy poly(ethylene glycol)) molecules as end-cappers. By changing the amount of MPEG present, you can change the statistical likeliness of one capping the molecule, thus restricting its length.

Although scCO2 is not a good solvent, it can penetrate polymers and act as a plasticiser. This makes the growing polymer much less viscous and hence lowers the temperature needed for further polymerisation to occur.  As it has both liquid and gas properties it is able to support the reactants and products throughout the reaction. By putting the vessel back to atmospheric pressure the scCO2 changes back to a gas releasing the products. This has the added benefit of not contaminating the product which can sometimes happen using conventional liquid solvent systems.  

Why is this interesting?

The polymer has both water-loving (hydrophilic) and water-hating (hydrophobic) parts. This means when it is added in to water, the polymers self-assemble into tiny bubbles called micelles (below).  

Untitled The white circles in these images are micelles

The interesting thing about micelles is that they can be used to carry drug molecules around the body. By varying the length of the polymer produced, you can add smaller or bigger drug molecules to it. The micelles will then release the drug once they reach their destination.

Other Publications from the School of Chemistry

Metal organic frameworks (MOF) have a lot of potential for many applications in the future such as catalysts and sensors. Research by the School in collaboration with the School of Engineering has investigated the effect of microwaves on reactants within MOFs. A. Laybourn et al. Phys. Chem. Chem. Phys., 2016, 18, 5419-5431.

CO2 capture and storage will be a vital technology for a sustainable future. Recent research has shown that the effectiveness of the materials used in this technology is highly dependent on the size of micro-pores within in the material and not, as had been previously thought, on the presence of nitrogen. B. Adeniran and R. Mokaya, Chemistry of Materials, 2016, 28, 994-1001.

Biological compounds found in nature are of great interest as chemists try to find new ways of capturing the sun’s energy. Researchers at the School, in collaboration with the Department of Chemical and Environmental Engineering at the Nottingham’s Ningbo, China campus, have synthesised and analysed a compound containing an analogue of a natural enzyme. The electron transfer mechanisms were investigated and they have found details of problems future systems will have to overcome. P. Summers, et al., Inorganic Chemistry, 2016, 55, 527-536.  

Matthew Bird 

Header image by Matt Buck via Flickr

Further Images from Polymer Science Journal Publication

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Science editor for the University of Nottingham student magazine IMPACT
One Comment
  • Octapeptide
    17 March 2016 at 07:40
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    As the better materials to become more sustainable, green polymers are developing at a rapid speed. In terms of supercritical carbon dioxide, it is seen as a measurement of polymers, which can promote the chemical reaction medium in daily chemical industry.

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