Radical approach gets polymerization just right

Advances in polymerization have enabled their use in cosmetics, drugs, biomedical devices, paints, coatings, adhesives, and microelectronics

Original source: Materials Today

Advances in the processes that create long chain polymers from small organic molecules – or monomers – have enabled their ubiquity in everything from cosmetics, drugs, and biomedical devices to paints, coatings, adhesives, and microelectronics. But the conditions for polymerization have to be just right.

The most common process, called radical polymerization (RP), uses radical chemistry to join monomers into a polymer chain. Over the last 25 years, the process has been refined and adapted to give better control over the final product. One particularly useful extension of the process is atom transfer radical polymerization (ATRP), developed by Krzysztof Matyjaszewski and his team at Carnegie Mellon University in the 1990s, which is simple to set up and can produce a wide range of functional materials.

“ATRP has become an everyday, rather than a specialty, polymerization method as a result of the breadth of available techniques and their robustness, conjoined with the simplicity of the reaction set up,” says Matyjaszewski.

In a comprehensive review, he and co-author Pawel Krys explain how ATRP uses Cu complexes to drive polymerization in a rather surprising way [European Polymer Journal 89 (2017) 482–523]. In conventional RP, the reaction proceeds very quickly, giving no time to tailor the chemical structure of the polymers produced. ATRP, by contrast, switches the growing polymer chains between a dormant ‘sleeping’ state and brief periods of activity. Extending the reaction time from a few seconds up to many hours provides a window of opportunity for manipulation of the polymers’ chemical structure.

“All the polymer chains start growing at the same time and grow synchronously, which allows polymers with narrow molecular weight distribution, desired molecular weight, and complex architectures to be obtained easily,” explains Matyjaszewski.

ATRP comes in two flavors: original (or ‘normal’) and ‘activator regeneration’. In the normal form, equivalent amounts of an initiator – usually an alkyl halide containing a halogen atom such as chlorine or bromine – and a catalyst in the lower oxidation state are used. A catalyst in this form, however, is unstable and difficult to handle. To get around this, and reduce the amount of catalyst required, activator regeneration ATRP uses an oxidized catalyst and a reducing agent to regenerate the metal in the lower oxidation state continuously and drive the polymerization. Lower levels of catalyst are desirable from both economic and environmental points of view.

More recently, interest has turned to metal-free catalysts and new ways of controlling the polymerization reaction externally.

“Light is an external stimulus, so polymerization can be stopped and restarted by turning on or off, or tuned by adjusting the irradiation wavelength, source intensity, and the distance from the reaction vessel,” points out Matyjaszewski. “Other stimuli include electrical current or mechanical forces that can provide spatiotemporal control and turn on/off polymerization.”

Substantial progress has been made in ATRP over the last 20 years and the future promises to be no less exciting. ATRP offers a simple setup, uses a wide range of commercially available reaction components, and can be conducted under different conditions, including ones that are biologically relevant. Better understanding of ATRP is paving the way for new advances in process optimization and commercialization of new products.

Ultimately, further refinement of ATRP could enable more sustainable, efficient, and ‘greener’ polymerization with substantially improved control, suggest Maciek Kopec and G. Julius Vancso of the University of Twente and senior editor of European Polymer Journal.

“Thanks to the deep mechanistic understanding provided by Matyjaszewski and co-workers, ATRP has become the technique of choice for the easy preparation of well-defined polymers and will continue to establish its enabling role in materials chemistry, with an increasing number of ATRP-made commercial products,” they say. “In the future, we anticipate a growing number of studies using the ATRP toolbox to synthesize sophisticated, complex polymer architectures such as block copolymers, bottlebrushes or (bio)hybrids with applications in medicine, energy conversion/storage, and other areas.”