Synthesis and Recycling

1. "C-C Activation": Polymer Waste Degradation, Recycling, and Upcycling

Essentially, chemistry is about bond making and bond breaking. Much of chemists’ attention focuses on bond making, i.e., to synthesize molecules for better use and higher value. Bond breaking is sometimes perceived as detrimental because it results in molecular deconstruction and property degradation. However, bond breaking proceeds bond making. If well-controlled, bond breaking can lead to the synthesis of high-value products.

Some bonds are extremely difficult to break, e.g., breaking the triple bond in N2 in the presence of H2 to synthesize NH3. Controllably breaking other bonds, e.g., C-C and C-H bond (often referred to as ‘C-C and C-H activation’), is as challenging as, if not more difficult than, the breakage of N2 triple bond. Besides the high activation energies, the key lies in the “controllably breaking” because of the ubiquity of C-H and C-C bonds in organic molecules—one can hardly differentiate one from another unless with judiciously controlled chemistries.

We are interested in controllably breaking C-C bonds. The C-C bond breaking parallels with the ‘C-H’ bond activation, an endeavor by many chemists (e.g., Hartwig). Together, C-C and C-H bond activation are the two keys to organic molecular synthesis and functionalization.

Controllable C-C bond breaking is also relevant in plastics recycling, especially polyolefins. In our early attempts, we achieved controllable C-C bond breaking in polystyrene (PS), polyethylene (PE), and polypropylene (PP). For PS, the relatively bulky phenyl group attached to the -CH-CH2- backbone allows us to selectively break the -C-C- bond and recover benzene with a Lewis acid catalyst (see Xu et al., PNAS 2022). For PE, all the C-C bonds are identical, so one cannot differentiate which bond to cleave selectively. Thus, we take a controlled strategy to break the C-C bonds mildly and produce relatively short chains (but not overly short chains to yield gaseous products). The mild reaction condition for C-C bond cleavage is achieved through a thermal gradient reactor: the reactor bottom is heated to a high temperature to induce radical chain scission, and the reactor top is cooled to a low enough temperature to quench the wild “radical reactions.” This way, we produce “PE segments” that are further upcycled to produce surfactants (e.g., fatty acids, detergents, etc.). A Similar strategy applies to PP to break C-C bonds and achieve backbone chain scission (see Xu et al., Science 2023).

PNAS, 2022, 119, 34, e2203346119. 

Angewandte Chemie International Edition, 2023, e202307042. 

Science, 2023, 381, 666–671.     Free link to download here.

A practical problem:

Based on the law of entropy, the vast bulk of polymer wastes deposited to the natural environment will eventually reach every corner of the planet, threatening the environment and inhabitants of the earth. Because the natural degradation of polymers is extremely slow, to mitigate the challenge, significant efforts have been dedicated to design and synthesize biodegradable polymers. The over 5000 million tons of commodity polymer wastes that humankind have accumulated to date, as well as the over 400 million tons being added annually, however, cannot wait for hundreds of years to natural degrade but urgently demand a solution to minimize the ramifications. 

In our lab, we aim to degrade, recycle, and upcycle polymer wastes into high-value chemicals.