Researchers at Carnegie Mellon University hope to revolutionize large-scale atom transfer radical polymerization (ATRP) processes by using environmentally safe compounds including sugars and vitamin C. ATRP produces a diverse range of industrial plastics for applications ranging from drug delivery systems to resilient paints.
The "green" version of ATRP reportedly produces specialty polymers for coatings, adhesives, lubricants, cosmetics, electronics, and numerous other markets more efficiently. It will also reduce industrial purification costs before and after running a reaction and permits production of new, unprecedented materials, say the researchers.
During conventional ATRP, scientists produce a complex polymer structure using a special catalyst to add as many as a few monomer units at a time to a growing polymer chain. This requires a balance between two species of copper (Cu) catalyst, Cu(I) and Cu(II). But as the ATRP reaction progresses, Cu(II) builds up. Typically, more Cu(I) is added to compensate for this effect and maintain balance between the two copper species. But this approach ultimately generates materials with high overall levels of copper -- levels too costly to remove efficiently during large-scale production, says principal inventor Krzysztof Matyjaszewski, professor of Natural Sciences and director of the Center for Macromolecular Engineering in the Mellon College of Science at Carnegie Mellon.
The team's novel use of "excess reducing agents" reportedly lowers the amount of copper catalyst from 5,000 ppm to 10 ppm. Steadily adding environmentally benign "reducing" agents -- vitamin C, sugars, or standard free radicals -- chemically reduces Cu(II) to Cu(I). This continuously reduces Cu(II) to Cu(I) at the same rate Cu(II) forms while retaining the desired balance between the two states. Ultimately, the overall amount of Cu catalyst used drops by as much as a factor of 1,000 and makes the new process more efficient by a factor of 100 and thus more amenable to industrial production," Matyjaszewski says.
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. This significantly lowers removal costs, say the researchers.
The new ATRP technique can also produce chains of a higher molecular weight thereby extending the range of materials. For example, chemists could grow high molecular weight polymers with precise control. This would provide even larger templates for nanoscale carbon structures used in field emission displays for computer screens and in semiconductors. Additionally, it could also lead to the production of "smart" materials that respond intelligently to such altered conditions as changes in pressure, acidity, light exposure, and other variables.
The work appears in a special Oct. 17, 2006 issue of the Proceedings of the National Academy of Sciences (PNAS) devoted to materials science. The National Science Foundation and the Carnegie Mellon consortium of industrial partners supported this research.
Carnegie Mellon University
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