A team of scientists from Princeton University found a molecule that connects in a very unusual way: as a repeating sequence of squares. Because of the block-patterned structure, the process can reverse under certain conditions — an intriguing idea if you want to recycle plastics.

In other words, the molecule can be “zipped up." Disconnnecting — or depolymerizing — the blocks in the structure can bring the polymer back to its pristine state, ready to be used again.

With zip-up potential, this kind of plastic could be used repeatedly, says the team, unlike most of today’s plastics that can only be recycled once, if at all.

The formal name of the molecule is oligocyclobutane, but the researchers in Princeton Professor Paul Chirik's lab casually call the wonder-material “the polymer of squares.”

The iron catalyst joins butadiene on each end to make a square. The same iron catalyst takes the polymer it makes and returns it to butadiene. "Chemical recycling!" Chirik told Tech Briefs.

The Chirik lab  explores sustainable chemistry by investigating the use of iron to build new molecules. With the polymer of squares, the iron does a kind of chemical persuasion.

Normally, individual monomers connect to form a spaghetti-like, S-shaped polymer. Acting as a catalyst, the iron gets the monomers to click together and create polymer squares that, in effect, zip up.

To "unzip" the monomers, the molecule is exposed to a vacuum in the presence of the iron catalyst, which reverses the process and recovers the original material. The transformation is a rare example of closed-loop chemical recycling, says Chirik.

The team, led by Chirik, the Edwards S. Sanford Professor of Chemistry; Megan Mohadjer Beromi of the Chirik Lab; and C. Rose Kennedy, formerly of the Chirik Lab reported their results in Nature Chemistry .

Watch a 2014 video interview with Prof. Chirik (Courtesy of Princeton University Department of Chemistry):

Previous ways of turning a material back to its pristine state have required many steps and expensive, specialized polymers. The polymer-of-squares molecule, by contrast, is a common one.

The form of polybutadiene, which has been known for over a hundred years, is used to make rubber and plastic products like tires and shoe soles. Butadiene, its parent material, is an abundant organic compound and a major byproduct of fossil fuel development.

The Princeton researchers must still thoroughly study the performance attributes of the oligocyclobutane, but the lab has provided a conceptual precedent for a chemical transformation that previously seemed impossible.

The oligocyclobutane molecule can be “zipped up” to make a polymer of squares for use in plastics, and then unzipped — depolymerized — back to its pristine state, ready to be used again.
The oligocyclobutane molecule can be “zipped up” to make a polymer of squares for use in plastics, and then unzipped — depolymerized — back to its pristine state, ready to be used again. (Courtesy of Jon Darmon, Princeton University Department of Chemistry)

In a short Q&A with Tech Briefs below, Chirik explains why the polymer of squares could change how we use plastics.

Tech Briefs: In a news release from Princeton University , you said: "“I honestly think this work is one of the most important things to ever come out of my lab.” Why is this study so important?

Prof. Paul Chirik: This work opens a new frontier to new materials and how we recycle them. Plastics are a key component of modern society and have enabled our quality of life.

Almost all are prepared from abundant hydrocarbon building blocks: ethylene, propylene and butadiene. These materials are responsible for everything from plastic bags to medical implants tires. Historically these plastics were made for their applications, properties, and low cost, with little consideration was given to their ultimate disposal and fate. The accumulation of plastic in the environment has heightened concerns over the fate of plastics.

Physical recycling has been inadequate — less than 15% of the materials are recycled and most degrade after one cycle. Our manuscript demonstrates an important new concept: the ability to chemically recycle a plastic, meaning return it to its consistent monomer. We do this with butadiene: one of the big hydrocarbon building blocks. It is a big discovery because it demonstrates a new material from butadiene — the first in nearly a 100 years, and it offers the ability to return it to monomer.

Tech Briefs: How did this work with polybutadiene begin, and when did you notice that its properties could solve the plastic problem?

Prof. Paul Chirik: We were studying iron catalysts and how they can be used to make new products from abundant feedstocks — particularly ethylene and butadiene. We had demonstrated that the iron could promote a new type of cycloaddition reaction, meaning it could combine ethylene and butadiene to make vinylcyclobutane, a square with an alkene on the end.

Rose Kennedy, a postdoc at the time reasoned that the remaining C=C [carbon-carbon] bond could undergo another cycloaddition — form another square — upon addition of butadiene. She was right!

Megan Mohadjer Beromi came along and showed you didn’t need the ethylene at all. Butadiene itself would react to make a chain of squares. Importantly all of the chemistry used to make these new C=C bonds can go in reverse. The same catalyst that was used to make the rings could make them go back to where they started: chemical recycling.