J. Am. Chem. Soc. http://doi.org/5z5 (2015)
Ring-opening metathesis polymerization (ROMP) is a powerful technique during which chain growth occurs by alkene metathesis between the chain end (coordinated to a catalyst) and a cyclic olefin. However, as the polymerization is driven by the release of ring strain, using large macrocycles as monomers is usually ineffective — and this limits the length and complexity of repeat units in the resulting polymer chains. Recently, ROMP of an unstrained cyclohexene was induced by a 'relay' approach: an adjacent alkyne moiety first undergoes metathesis with the growing chain end, and this enables fast intramolecular metathesis with the normally unreactive cyclohexene alkene. Because of the relative positioning of the alkyne and alkene, this opens the cyclohexene ring despite little to no release of ring strain.
Now, Will Gutekunst and Craig Hawker at the University of California, Santa Barbara have taken advantage of this relay strategy to carry out ROMP of larger macrocycles, and, importantly, ones that contain multiple functionalities along the cycle. They designed and synthesized a universal 'polymerization trigger' molecule containing an enyne functionality to enable ring opening of the macrocycle by intramolecular metathesis. The macrocylic monomer features this enyne trigger and any sequence to be incorporated into the final polymer, and they are connected through carboxylic acid and alcohol moieties. Taking advantage of this, Gutekunst and Hawker used short oligopeptides connected to the trigger molecule. This gave some degree of sequence control in the resulting polymer, which contained repeated segments of tetra- or penta-peptides.
They found that amino-acid-containing monomers polymerized slower than simpler, alkane chain monomers, but still produced polymers with narrow dispersities and controlled molecular weights. Another notable feature is that the amino acids in the polymer backbone are susceptible to hydrolysis, in contrast to traditional ROMP-synthesized polymers, whose hydrocarbon backbones are very resistant to degradation. This method promises versatility in both polymer sequence and properties, and may prove to be a useful complement to more traditional ROMP methods.
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Hansell, C. Put a ring on it. Nature Chem 7, 614 (2015). https://doi.org/10.1038/nchem.2317