Normally when I hear of olefin metathesis in connection with polymers, it is regarding polymerization (such as ring-opening metathesis polymerization (ROMP)). So I was surprised the other day to read a paper where metathesis was used to depolymerize a polymer, and not just any polymer, but a polymer lacking in olefin groups - polyethylene.
The report appears in Science Advances (open access) and shows off some clever tricks. The polyethylene is dissolved in a light alkane (naptha or similar) and then a dehydrogenation is carried out on both the polyethylene and the solvent. After that, the metathesis can happen.
A metathesis reaction is one where there a cross-exchange between two different chemicals. An example would be A-X + B-Y → A-Y + B-X. For olefin metathesis, the A and Y are on either side of a double bond, as are the B and X. And the same is true for the A-Y and the B-X, so the reaction is A=X + B=Y → A=Y + B=X. where A=X is the partially dehydrogenated polyethylene and B=Y is the partially hydrogenated light alkane. If the double bond is near the center of the PE molecule, you are able to pretty much cut the molecular weight in half in just one reaction. Since the molecules are dehydrogenated in multiple locations, the PE can quickly be reduced to very short chains by allowing the reaction to repeatedly occur. And all the while, the degradation products are completely soluble in the solvent.
The technique works for the whole spectrum of PE, from Mw = 3350 daltons to ultra-high molecular weight (Mv = 1.7 x 106 daltons), as well as LDPE and LLDPE (no surprise there, but glad they checked) as well as on PE that had antioxidants compounded into it. Would it work for polypropylene? Polystyrene? PVC? Inquiring minds want to know!
The statistics of this reaction are intriguing to think about. This degradation reaction is actually more akin to a condensation reaction (run in reverse, of course) than the addition reaction that created the PE, but there is so much more. Is there an optimal level of dehydrogenation (as a function of MW, MWD, branching...)? Is there is an optimal light alkane mix? Would having alkenes already in it help or hurt? Modeling this could be quite a bit of fun.
The use of the end product as a fuel is suggested by the authors, who strongly believe that it is an economically feasible route, much better than anaerobic pyrolysis. No numbers are provided however. I won't get into criticizing them at this point, since this is just a first discovery and with just 56% yield, there is a lot of work ahead for someone. But this process and its simplicity seems promising and I would encourage the researchers to push on.