Electrically driven polymerisation
The developers of the atom transfer radical polymerisation (ATRP) process for making industrial plastics have conceived an electrically driven alternative that they say gives greater control and promises lower environmental impact.
In traditional ATRP, polymers are assembled from monomers using a copper catalyst. The process relies on paired redox reactions between two species of copper – the activator Cu(I) and the deactivator Cu(II). Occasionally though, one of the reactions will stop spontaneously, leading to an accumulation of Cu(II).
Previously, polymerisation was sustained by adding more Cu(I), but this generates materials with sometimes toxic levels of copper – up to 5,000ppm – which are hard to remove. Inventor of ATRP at Carnegie Mellon University, USA, Professor Krzysztof Matyjaszewski, and colleagues, then found that reducing agents such as sugars or vitamin C could decrease the amount of copper catalyst required for ATRP reactions.
The new approach, called electrochemically mediated ATRP (eATRP), goes a step further in balancing the reactions. Matyjaszewski and Visiting Assistant Professor of Chemistry Andrew Magenau have found that adding electricity capitalises on the redox reaction by moderating the transfer of electrons.
They have used a computer-controlled potentiostat to apply a programmable electrochemical potential across the reaction. By doing so, they have found they can slow down, speed up, or start and stop polymerisation by changing the current or voltage.
As a result, the amount of copper catalyst required has been cut to 50ppm, rivalling the previous method of applying reducing agents.
No other parameters have to be changed, claims the team. ‘In part, the elegance of this process is that ATRP can be manipulated as a function of electrochemical means instead of common stimuli, such as temperature, pH or chemical reducing agents,’ says Magenau. He adds that, theoretically and experimentally eARTP is as fast as ARTP, although no direct comparisons have yet been made.
On its eco-friendly credentials, Magenau says that while eARTP improves on ARTP, it does need supporting electrolytes to improve conductivity, but that ‘greener’ variations of eARTP that minimise or eliminate them are being explored.
No work has been done yet on the mechanical properties of eARTP polymers, and there are no details at this stage on possible products, although Magenau says hybrid materials are being made that may be suitable for biomedical applications.
Nor are there firm details yet on industrial trials or commercial practicality, although Magenau says, ‘at the moment, traditional ATRP methods are more pragmatic than eATRP, and further research is being conducted to optimise eATRP for industrial implementation.’
Reviewing the research, Brian Brooks, Emeritus Professor of Chemical Engineering at Loughborough University, UK, says, ‘In some circumstances it shows it can achieve good control of radical polymerisation, although process scale-up could be difficult because good agitation is required in separate reactor zones. Also, attention should be given to electrode design so that process scale-up can become feasible.’
He says that, although the use of environmentally unfriendly chemical reducing agents can be eliminated, the use of an ‘environmentally unfriendly solvent’ may be required.