Mar. 24, 1999 ANAHEIM, CA--A fortuitous collaboration between two teams of chemists has led to the discovery of novel compounds that give unprecedented control over polymerization reactions. The technique, which enables chemists to control precisely the length and other characteristics of the molecular chains in polymer materials, promises to usher in a whole new generation of specialty plastics and advanced materials with properties tailored for specific applications.
Polymer chemists at the IBM Almaden Research Center in San Jose identified the new compounds while testing a library of novel chemical structures developed by researchers at the University of California, Santa Cruz. Craig Hawker, an IBM chemist affiliated with the NSF Center for Polymer Interfaces and Macromolecular Assemblies, said using the new compounds to initiate polymerization reactions makes it possible to design polymers with complex molecular architectures and very specific qualities.
"The possibilities are endless," said Hawker, who presented the findings Tuesday at the annual meeting of the American Chemical Society (ACS) in Anaheim.
It is hard to imagine modern life without the enormous variety of plastics and artificial fibers used to make everything from shampoo bottles and foam cushions to automobile parts and nylon fabrics. All these materials are made of polymers, spaghetti-like molecules constructed from smaller building blocks called monomers joined together into long chains.
Despite the obvious successes of existing procedures for synthesizing polymers, chemists are eager to develop new techniques that give them more control over the polymerization process. One of the main drawbacks of current techniques is the inability to control the lengths of the resulting polymer chains. Most polymerization reactions produce a mixture of polymer molecules with widely varying chain lengths.
Other researchers made a promising breakthrough in the early 1990s using a molecule called tetramethylpiperidine-N-oxide, or TEMPO, to initiate polymerization reactions. TEMPO somehow controls the polymerization reaction as it proceeds, resulting in polymer chains of similar lengths. TEMPO's main shortcoming is that it only works with styrene monomers to produce polystyrenes.
Rebecca Braslau, an associate professor of chemistry at UC Santa Cruz, was not even thinking about polymerization techniques when she began creating a library of nitroxide compounds similar to TEMPO and related compounds called alkoxyamines. Her research interests involved the interaction of optically active nitroxides with carbon radicals. Then she met Hawker at the 1997 ACS meeting and they both realized that Braslau's library of novel compounds was ideally suited for Hawker's polymer research.
Braslau, working with postdoctoral researcher Vladimir Chaplinski, prepared samples of more than 20 different alkoxyamine compounds from the library and sent them to Hawker for testing. Hawker and his colleagues, Didier Benoit and Felix Rivera Jr., sorted through the samples and found one compound in particular that showed extraordinary potential as a polymerization initiator.
"One product, which we are calling the Vladimir initiator, can be used with all types of monomers, not just styrenes, so we can make a wide range of polymers in a very controlled fashion," Braslau said.
This new technique can also be used to create complex polymers, called copolymers, which incorporate more than one type of monomer. For example, molecules called block copolymers consist of a chain of one monomer joined to a chain of a completely different monomer, resulting in a polymer with the combined properties of two different materials. Block copolymers are used in products such as the inks for inkjet printers, but they are expensive to make. With the Vladimir initiator, block copolymers can be made more cheaply and with a wider range of monomers than is currently possible, Hawker said.
The potential applications of these techniques range from drug delivery to water treatment, Hawker noted. "The commercial implications are potentially huge," he said.
For example, Hawker is interested in polymers used in manufacturing silicon chips. The photolithography process used to create the microscopic features on a computer chip involves the use of a polymer that coats the silicon wafer and delineates the pattern to be etched into its surface. The sharpness of that pattern depends on the uniformity of the chain lengths in the polymer. The new polymerization techniques may ultimately lead to much faster computer chips, Hawker said.
Hawker estimates that it may be another three years or so before the new techniques are ready for commercial applications. But eventually he expects to see a wealth of new materials made possible by this research.
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