Chemists find a way to turn carbon dioxide into a 'green' polymer

The detrimental effect of plastic on the environment is a cause of global concern. The solution to the problem is multifaceted, but sustainability has a big part to play. It’s this that drives interest in the production of chemicals from carbon dioxide (CO₂).

Professor of Chemistry Andrew Dove, who leads the Dove Research Group, a multi-national body of researchers focused on challenges in polymer and materials science, is at the forefront of such study. Working in the field of organocatalysis (a form of catalysis whereby the rate of a chemical reaction is increased by an organic rather than metal catalyst) he has discovered a method to turn CO₂ into a polymer – only the second time this has ever been achieved with organocatalysts using a specific monomer.

The successful synthesis of poly(trimethylene carbonate), PTMC, is detailed in a paper entitled ‘Metal-free synthesis of poly(trimethylene carbonate) by efficient valorization of carbon dioxide’ – the result of a project funded by the EU. Written by Andrew and three fellow researchers, it was published in the journal Green Chemistry in January 2019 and subsequently won the College’s Paper of the Month award.

‘There’s a lot of interest in using CO₂ in this way because of issues such as global warming and plastic in the environment, but it’s is a very inert molecule – it’s not very reactive – and it costs too much energy to activate it,’ says Andrew, who joined Birmingham’s School of Chemistry in 2017.

Catalytic methods are the most efficient and organocatalysis, which doesn’t use metals, is kinder to the environment. Organic species of catalyst are also often easier to reuse than metal equivalents. The major hurdle is that organic versions are not as active as metals.

‘So what we have done is to find a way to make a fully organic catalyst for this transformation – and it works! As far as our process is concerned, there’s only one other organocatalyst ever to have done what we’ve done.

The researchers used organocatalysis for the copolymerisation of carbon dioxide and oxetane.

‘A lot of CO₂ activation work produces small molecule carbonates and ureas,’ explains Andrew. ‘These are useful, but there’s not enough demand for them, so turning them into polymers has advantages. We took oxetane, which when combined with carbon dioxide gives you PTMC, and this is used in polyurethane synthesis (the process by which foam seats are made). It’s also a polymer that’s used in biomedical devices.’

Although the group’s discovery opens up a green route to an important polymer, it constitutes only a first step towards greening up the plastics industry. The big problem is that as well as being a ‘very challenging process’, it is also a long way from being economically viable.

‘This is a sustainable way to make polymers,’ says Andrew. ‘It’s environmentally beneficial because it’s more circular in terms of what happens at the end of a plastic item’s life. If you could make it economically viable, then it could make a difference. This is the first step towards doing that.’