CMU team's work could lead to a new plastic that breaks down
Monday, July 21, 2003
By Byron Spice, Post-Gazette Science Editor
Few things made by humans are quite so long-lived as plastic. Sure, some items end up in the recycling bin, but the vast majority of plastic cups, bags, auto body parts, computer housings and DVDs will be around for the long haul.
Chemists at Carnegie Mellon University, however, are exploring a new method of making polymers that promise to be rapidly degradable by light, by acids and bases in water and, presumably, by bacteria. It's a method applicable to virtually anything made by the process known as radical polymerization, which accounts for about half of all polymers in use today, including plastic foam, vinyl, latex paints, grocery bags, Plexiglas, and automobile coatings.
Studies are just beginning, however, on questions about how easily degradable these polymers might be and the cost of making them, at least initially, may limit them to specialized applications.
All polymers are basically long, long chains of repeating chemical units, called monomers. The CMU team, headed by Krzysztof Matyjaszewski, essentially has found a way to introduce weak links into the chains and to space those links evenly through the chains.
These added links are susceptible to degradation by both light and by water-based chemical processes, Matyjaszewski said. And if the spacing is done properly, the remaining bits of chain should be small enough to be seen as food by bacteria, making the polymer biodegradable.
"This is rather clever," said Eric Beckman, chairman of chemical engineering at the University of Pittsburgh. Questions remain about how expensive and how stable the resulting polymers might be, he added, but the use of a well-developed process like radical polymerization and familiar polymers could make it an attractive technology.
Though it might be possible to make a plastic foam cup that, when tossed on a roadside, would degrade within weeks instead of remaining as litter, Matyjaszewski anticipates the more likely use, at least initially, will be for biomedical applications, such as drug delivery. The polymers might allow for precisely timed release of medications, or might be designed to target specific tissues.
It should be possible to adjust the speed at which different products degrade, he said, with some items disintegrating within minutes or hours, while others might be designed to last a decade or more. Some products, of course, are not intended to degrade at all, such as protective auto-body coatings.
Producing biodegradable polymers has proven to be a complex problem, with few successes, noted Richard Gross, a distinguished polymer chemist at Polytechnic University in Brooklyn, N.Y.
The most commercially notable to date is polylactic acid, or PLA, a polymer derived from corn. A joint venture of Dow Chemical and agricultural giant Cargill called Cargill Dow is now producing plastics made from PLA. DuPont also is working on a corn-derived plastic.
The work at CMU has been under way for about a year now. Matyjaszewski, who directs the Center for Macromolecular Engineering, and Im Sik Chung, a postdoctoral associate, reported on one such degradable polymer this spring in the journal Macromolecules. They modified a clear, hard plastic polymer known as poly(methyl methacrylate), or PMMA, to chemists, but best known to consumers as Plexiglas or Lucite.
Adding a ring
A key concept underlying the technique was developed 20 years ago by the late William Bailey, a research chemist at the University of Maryland. He found that polymers could be made degradable by adding a special monomer that included some atoms bonded together in the form of a ring; when this monomer is "polymerized," or incorporated into a chain of monomers, the ring opens up and incorporates vulnerable chemical bonds into the chain.
The process is called, not surprisingly, ring-opening polymerization.
Matyjaszewski and his colleagues fine-tuned the process by developing a way of altering the reactivity of the ring-opening monomer, making it easier to incorporate into different polymers.
When that monomer's ring opens, it reveals one bond that can be broken by ultraviolet light and a second bond that can by broken by processes involving water.
They also married the ring-opening polymerization to a technique Matyjaszewski invented, called atom transfer radical polymerization, or ATRP. Normally, radical polymerization is controlled chaos, generating chains of monomers of wildly varying lengths. But ATRP makes it possible to carefully control the growth of the chains, synthesizing chains of uniform length and with particular shapes or structures.
ATRP also makes it possible to combine different types of monomers into the same hybrid polymer, or copolymer, and to arrange the monomers in almost any order or pattern. In this case, the CMU team was able to use ATRP to space the "weak link" monomers as needed to ensure that it would eventually fall apart into bite-size pieces for bacteria, making it biodegradable.
Polytechnic's Gross said the method should prove useful, particularly for producing plastics for special uses. But the use of the exacting ATRP process might limit broader use in everyday polymers, he said. James Spanswick, associate director of the CMU macromolecular engineering group, said it's possible that these degradable copolymers could be produced by more conventional means. By carefully tailoring the reactivity of the ring-opening monomer and by feeding it at designated intervals as polymerization takes place, it would be possible to distribute the ring-opening monomers randomly along the length of the resulting copolymer.
This might make it practical to produce more "commodity" polymer, such as plastic sheeting for agricultural use, said Spanswick, who spent 25 years with Amoco Chemicals before joining CMU.
Degradable agriculture sheeting, an idea examined and abandoned in the 1970s, could be used to cover the ground around crops, preventing weeds and eliminating the need for chemical herbicides, Spanswick said. Following the growing season, the sheeting could be tilled into the soil, where water would break it apart and bacteria would eat it.
First, however, scientists are trying to figure out just how easily the polymers degrade. Alan Russell, director of the University of Pittsburgh's McGowan Institute for Regenerative Medicine and a chemist who has collaborated with Matyzaszewski in the past, is now running to tests to see whether the polymers break down with water under normal conditions, or if they require enzymes, or extremes of heat, acidity or alkalinity to fall apart properly.
Other testing will then be necessary to determine the degree to which bacteria could digest the remaining bits, he noted.
It's possible that the technology will be driven as much by recyclability as biodegradability, Russell suggested. The same qualities that help these plastics degrade also would make them easier to reprocess into new plastic products, he explained.