THE MOST THOROUGH WAY to recycle plastics is to depolymerize them into their monomeric building blocks, which then can be used to make new polymers. Most depolymerization methods developed so far require high temperatures, careful solvent selection, or specialized high-pressure equipment. Now, using ionic liquids as solvents and standard, ambient-pressure laboratory equipment, Akio Kamimura and Shigehiro Yamamoto of Yamaguchi University, in Japan, have demonstrated a unique way of converting plastics back to the stuff they were originally made of (Org. Lett. 2007, 9, 2533).
Their depolymerization process still relies on high temperatures, but that's not a problem for the ionic liquids they use, Kamimura says. Compared with other solvents used for depolymerizations, he notes, "ionic liquids are less volatile under such high temperatures."
With an eye on recycling nylon and other polyamide plastics, the researchers experimented with a number of solvents and different reaction conditions. In a procedure they describe as "quite simple," they combined a sample of nylon 6, an ionic liquid, and a catalyst in a vessel and stirred the mixture under nitrogen at 300 °C and ambient pressure for about 1 hour. The ensuing depolymerization yielded the nylon's caprolactam monomer, which the researchers collected by distillation. The best yield of caprolactam,86%, was obtained with N-methyl-N-propylpiperidinium bis(trifluoro-methylsulfonyl)imide as the solvent and N,N-dimethyl-aminopyridine as the catalyst.
"This looks like very interesting and potentially useful work," comments Ted Aulich of the Energy & Environmental Research Center at the University of North Dakota, Grand Forks, where he has worked on waste plastics recycling.
Besides recovering reusable monomers, the researchers found they could recycle their reaction solvent five times without substantial loss of monomer yield. This is an advantage with regard to green chemistry, Kamimura says. The researchers continue to examine solvent recycling limits and application of this depolymerization method to other plastics.