Metal-free technique supports wider use of controlled radical polymerisation

Researchers in the US have developed a novel way to overcome one of the major hurdles preventing the widespread use of controlled radical polymerisation.

A technique called Atom Transfer Radical Polymerisation is emerging as a key process for creating well-defined polymers for materials including adhesives and electronics but current ATRP methods use metal catalysts, which are a hindrance to applications in which metal contamination is problematic.

This new method of radical polymerisation – developed by chemists and materials scientists from University of California, Santa Barbara and The Dow Chemical Company – doesn’t involve heavy metal catalysts like copper. Instead, the metal-free ATRP process uses an organic-based photocatalyst and light as the stimulus for the highly controlled chemical reaction.

In a statement, Craig Hawker, director of the Dow Materials Institute at UC Santa Barbara said: ‘The grand challenge in ATRP has been: how can we do this without any metals?

‘We looked toward developing an organic catalyst that is highly reducing in the excited state, and we found it in an easily prepared catalyst, phenothiazine.’

‘It’s ‘drop-in’ technology for industry,’ said Javier Read de Alaniz, principal investigator and professor of chemistry and biochemistry at UC Santa Barbara. ‘People are already used to the same starting materials for ATRP, but now we have the ability to do it without copper.’

Copper, even at trace levels, is a problem for microelectronics because it acts as a conductor, and for biological applications because of its toxicity to organisms and cells.

Read de Alaniz, Hawker, and postdoctoral research Brett Fors, now with Cornell University, led the study that was initially inspired by a photoreactive Iridium catalyst. Their study was recently detailed in a paper titled ‘Metal-Free Atom Transfer Radical Polymerisation,’ published in the Journal of the American Chemical Society. The research was made possible by support from Dow, a research partner of the UCSB College of Engineering.

According to UCSB, ATRP is already used widely across dozens of major industries, but the new metal-free rapid polymerisation process pushes controlled radical polymerisation into new areas and new applications.

‘Many processes in use today all start with ATRP. Now this method opens doors for a new class of organic-based photoredox catalysts,’ said Hawker.

The university added that controlling radical polymerisation processes is critical for the synthesis of functional block polymers. As a catalyst, phenothiazine builds block copolymers in a sequential manner, achieving high chain-end fidelity. This translates into a high degree of versatility in polymer structure, as well as an efficient process.

Hawker said: ‘Our process doesn’t need heat. You can do this at room temperature with simple LED lights. We’ve had success with a range of vinyl monomers, so this polymerisation strategy is useful on many levels.

‘The development of living radical processes, such as ATRP, is arguably one of the biggest things to happen in polymer chemistry in the past few decades. This new discovery will significantly further the whole field.’