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Brett Helms
Polymers have been broadly applied in synthesis applications for decades in part due to their macromolecular nature, which facilitates product recovery, and their multivalency, which enables high loading. Both heterogeneous and homogeneous polymer supported reagents, catalysts, scavengers, etc. are known and have widespread use in industry for the production of fine chemicals. It was clear, however, from the onset of their implementation in chemical synthesis that functional polymers displayed unique behavior in solution that was not observed for their small molecule counterparts. In fact, the reactivity of moieties bound to the support was regulated by steric accessibility, mircoenvironment, proximal confinement and other properties that were associated with the polymer. As these effects became enumerated, it was thought that they could be controlled through more precisely constructed materials. However, only recently have such methods for controlled polymer synthesis become available. Particularly useful have been the families of branched polymers: for example, dendrimers, dendronized polymers, star polymers, hyperbranched polymers, and shell crosslinked nanoparticles. These macromolecules have proven effectiveness in advanced applications, including light harvesting, vaccine and drug delivery, bioimaging, molecular transport, etc. In the realm of catalysis, they offer unprecedented opportunity for further development. Their globular nature is reminiscent of enzymes, whose function and efficiency are only now being approached by these systems.
Dendritic and other highly branched architectures have proven effective in the site isolation of reactive groups located at their cores. This phenomenon has been exploited in various applications including light harvesting, catalysis and molecular transport and is reminiscent of behavior exhibited by heterogeneous crosslinked polymeric materials. With respect to the latter, it had been shown that the site isolation afforded by solid supports for various reagents immobilized within the pores compatabilizes otherwise opposing species in solution: for example, oxidizing and reducing agents, or acids and bases. Thus multi-step reaction cascades employing incompatible reagents bound to spatially segregated heterogeneous polymers in the same reactor can be used to build up molecular complexity. Until our work, however, it was not clear whether this opportunity would extend to soluble polymers in solution. To that end, we prepared high MW multi-arm star polymers with opposing reagents covalently bound and thus sterically confined to their cores. The site isolation afforded by these materials was unprecedented and allowed for dual catalysis employing an acid and a base in the same reaction mixture.
Selected Publications
- Helms, B.; Guillaudeu, S. J.; Xie, Y.; McMurdo, M.; Hawker, C. J.; Fréchet, J. M. J. Angew. Chem. Int. Ed. 2005, 44, 6384-6387.
- Helms, B.; Liang, C. O. ; Hawker, C. J.; Fréchet, J. M. J. Macromolecules 2005, 38, 5411-5415.
- Liang, C. O.; Helms, B.; Hawker, C. J.; Fréchet, J. M. J. Chem. Commun. 2003, 2524-2525.
- Hecht, S.; Fréchet, J. M. J. Angew. Chem. Int. Ed. 2001, 40, 74-91.
- Hecht, S.; Fréchet, J. M. J. J. Am. Chem. Soc. 2001 , 123 , 6959-60.
- Piotti, M. E.; Rivera, F., Jr.; Bond, R.; Hawker, C. J.; Fréchet, J. M. J. J. Am. Chem. Soc. 1999 , 121 , 9471-2.
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