Polymers with Mechanophores
November 17, 2007 2 Comments
Stimulus-responsive polymers are all the rage at the moment. pH, temperature, electrical stimuli etc.. have all been used to get a polymer to respond to its environment. And now, well, now there are mechanical stimuli too.
In a recent JACS communication (DOI)Moore et al. describe the synthesis of mechanophore-linked addition polymers. Mechanophores are stress-sensitive units and through application of a stress, a chemical reaction is accelerated. Typical mechanophores, for example are Benzocyclobutene (InChI=1/C8H8/c1-2-4-8-6-5-7(8)3-1/h1-4H,5-6H) (BCB) or spiropyrans, which under go 4-pi or 6-pi stress-induced ring opening reactions.
Moore et al prepared addition polymers of the type PMA-BCB-PMA, starting from 1,2-bis(alpha-bromopropionyloxy)-1,2-dihydrobenzocyclobutene, using single electron transfer living radical polymerisation (SET-LRP):
Low (18 kDa), medium (91 kDa) and high (287 kDa) molecular weight polymers were synthesized, with PDIs of around 1.3. The ring-opening of the of the polymer was subsequently investigated by trapping the intermediate with N-(1-pyrene)maleimide as shown in step b of the above scheme (the pyrene moiety acts as a UV reporter).
To investigate the ring-opening reaction, the authors dissolved the polymer in acetonitrile together with the maleimide trap and a radical trap (2,6-di-tert-butyl-4-methylphenol, BHT) and subjected the solutions to pulsed sonication for 45 min under inert conditions. Analysis of the solution by UV spectroscopy showed, that no reaction had taken place in the low molecular weight polymer, but that both the medium and high-molecular weight polymers gave rise to significant UV signals, indicating that the maleimide is indeed incorporated via an electrocyclic ring opening process. Similar results were obtained for a high-molecular weight (170 kDa) PMA-spyropyran polymer:
When a solution of the colourless polymer was subjected to pulsed sonication, the solution changed colour to a pink hue with a new UV band at 550 nm. When the solution was exposed to ambient light at room temperature, the colour disappeared, which was found to be consistent with a known photolytic reversion to the closed form.
So there you have it, folks…..polymers responding to mechanical stimuli. Exciting work.
 Potisek, S. L. et al., J. Am. Chem. Soc., 129(45), 13808-13809 (2007)
Update (more InChIs by request)