ROMPing around the Carnival

Here is my contribution to Chemical and Engineering News’ “my favorite chemical reaction” blog carnival! I call it the “popcorn-stringing” reaction. Because, of course, one needs popcorn when at the Carnival!ROMP stands for Ring-Opening Metathesis Polymerization. The popcorn stringing analogy comes from the cyclic momomer being popped open and then, through bond metathesis, the molecules are strung together to make a polymer. The release of the ring strain is the driving force for the polymerization.

Kernels are the monomers that are popped open into popcorn that can be stringed together! The catalyst drawn is Grubbs' 2nd generation catalyst.

ROMP is a fascinating reaction for many reasons, one of which is of the range of metal complexes that can be used to carry out the reaction; including tungsten, molybdenum, rhenium, ruthenium and titanium carbenes. The most interesting developments in ROMP catalysts are those that can carry out living polymerizations*. These types of polymerizations can lead to polymers with a variety of sequences (copolymers) without sacrificing precise control of the material’s dimensions that, in turn, lead to a wider range of properties and applications.  Also, in terms of sustainable chemistry, these catalyst can lead to greener processing routesby improving efficiency and saving on atom economy (having every reagent used count).

The polymers that can be synthesized through ROMP have a lot of industrial applications. One of the most famous is Norsorex, or polynorbornene, which is a shock-absorbing materialused in protective equipment, sound insulation, and vibrational damping. Through a single monomer, ROMP can access structures normally difficult copolymerize with individual monomers.  One example is a perfectly alternating copolymer of 1,4-butadiene and isoprene.

perfectly alternating copolymer of 1,4-butadiene and isoprene from an octadiene

As with any other chemical reaction, there are limitations.  ROMP’s limitation is that the monomers must be cyclic and possess sufficient ring strain.  Although it might be difficult to synthesize momomers with these desired attributes, it is possible to derivatize naturally existing materials. Last year, the Larock group developed biorenewable-based thermosets from the ring-opening metathesis polymerization (ROMP) of fatty alcohols derived from soybean oil and castor oil.#  Of course, this work illustrated the robustness and versatility of Grubbs’ catalyst (like so many papers), but what is key is that it also highlighted the substantial role that metathesis catalysts can have in the development of polymers from biorenewable feedstocks as the pressures of traditional petroleum-derived feedstocks grows.

Well, that is a great place to end.  The thought about using renewable feedstocks not just for energy but also for carrying out chemistry is an important direction to consider.  Actually, a reaction very similar to ROMP that I have not expanded is, ROP (Ring-Opening Polymerization) and that polymerization is responsible for the material in biodegradable “corn pens”(polylactic acid).

lactic acid (monomer) can be derived from corn

ROP reactions are carried out by different types of catalyst than mentioned above, but same general idea of ring strain (lactic acid) as the driving force for the reaction.

Now, when you think of corn or popcorn, in addition to potential sources of renewable energy, I hope you will also think about ring-opening polymerizations.

Definitely time for me to go make some popcorn.

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*Living Polymerizations are polymerizations that satisfy the following three criteria:
1. Polymers have a narrow mass distribution. Meaning that the Gaussian distribution curve of polymer lengths is narrow indicating that a majority of the polymers are similar in length.
2. No premature chain termination. This helps to achieve the first point. If the growth of the polymer chains are cut off at different times then there will be a wider distribution of polymer lengths. This point also allows us to build copolymers. Once all of the first monomer is consumed and a second monomer is added, the polymerization continues adding in the second monomer.
3. All the chains start fast and at the same time.

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Image credits!