PITTSBURGH--Using environmentally safe compounds like sugars and vitamin C, scientists at Carnegie Mellon University have vastly improved a popular technology used to generate a diverse range of industrial plastics for applications ranging from targeted drug delivery systems to resilient paint coatings.
The revolutionary improvement in atom transfer radical polymerization (ATRP) now enables large-scale production of many specialty plastics, according to the scientists, whose work appears in a special issue of the Proceedings of the National Academy of Sciences (PNAS) devoted to materials science. This edition will be published Oct. 17.
The new "green" version of ATRP will allow existing materials to be made more efficiently, reducing industrial purification costs before and after running a reaction and permitting the production of new, unprecedented materials.
"By reducing the level of the copper catalyst used in ATRP, we have made this process at least 100 times more efficient and much more amenable to industrial processes," said Krzysztof Matyjaszewski, J.C. Warner Professor of Natural Sciences and director of the Center for Macromolecular Engineering in the Mellon College of Science at Carnegie Mellon.
Developed by Matyjaszewski, ATRP is a broadly adopted process that allows the production of specialty polymers for coatings, adhesives, lubricants, cosmetics, electronics and numerous other markets. ATRP's strength lies in its ability to combine chemically diverse subunits (monomers) into multiple arrangements that create specialized polymers. This technology enables production of "smart" materials that can respond intelligently to altered environments, such as changes in pressure, acidity, light exposure and other variables.
ATRP is being licensed to several companies that have already begun commercial production in the United States, Europe and Japan. But Matyjaszewski says large-scale production of polymers by ATRP has been limited because ATRP previously required a high concentration of copper catalyst that had to be removed from finished products.
"Our new ATRP processes significantly reduce the cost of recycling the catalyst and also decrease the release of hazardous reaction byproducts found in industrial waste," Matyjaszewski added.
During ATRP, scientists produce a complex polymer structure using a special catalyst to add one or a few monomer units at a time to a growing polymer chain. ATRP requires a balance between two species of copper (Cu) catalyst, CuI and CuII. But as an ATRP reaction progresses, CuII builds up. Typically, researchers add more CuI to compensate for this effect and maintain the balance between the two copper species. But this approach ultimately generates materials with high overall levels of copper -- levels that are too costly to remove efficiently on a large-scale industrial basis.
The PNAS report highlights the team's novel use of "excess reducing agents" to lower the amount of copper catalyst from 5,000 parts per million (ppm) to 10 ppm. The team showed that you can steadily add environmentally benign "reducing" agents -- vitamin C, sugars or standard free radicals -- to chemically reduce CuII to CuI. This unprecedented approach continuously reduces CuII to CuI at the same rate CuII forms while retaining the desired balance between the two states. Ultimately, this technique dramatically lowers the overall amount of Cu catalyst used in ATRP by as much as 1,000 times.
The team's new technology virtually eliminates the need to remove miniscule amounts of catalyst remaining in a product. For example, many ATRP-generated plastics for medical implants would be acceptable from a health perspective because they contain so little copper. However, if the target application -- such as a coating for a biomedical stent -- absolutely requires the removal of residual catalyst, companies will now have much less of it to take out, significantly lowering removal costs, according to the authors.
The new ATRP technique also allows for production of higher molecular weight chains, thereby extending the range of accessible materials that could be made using this method. For example, chemists could grow high molecular weight polymers with precise control, providing even larger templates for nanoscale carbon structures used in computer screen field emission displays and semi-conductors that regulate the flow of electricity in sensors, some only a fraction of the width of a hair.
ATRP differs significantly from conventional polymer manufacturing methods. This "living," synthetic process can be shut down or restarted at will, depending on how the temperature and other conditions of the reaction are varied. ATRP is an exceptionally robust way to uniformly and precisely control the chemical composition and architecture of polymers as well as the growth of every polymer chain, all while employing a broad range of monomers.
Much of the research progress and commercial success related to ATRP is due to two research consortia Matyjaszewski has initiated and led. These successful consortia have allowed many companies to incorporate ATRP methodologies into the development of new products for their specific markets. Companies from around the world send their employees to train in Matyjaszewski's laboratory. For more information, visit www.chem.cmu.edu/groups/maty/center/.
This research was supported by the National Science Foundation and the Carnegie Mellon consortium of industrial partners.