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Applications of Free Radical Polymerization

Free radical polymerization has found myriad applications too diverse to provide an exhaustive list. These applications include, but are not limited to, the manufacture of polystyrene, thermoplastic block copolymer elastomers (which may be used for a wide variety of applications including adhesives, footwear, and toys), cardiovascular stents[2], chemical surfactants,[3] and lubricants.

Free radical polymerization has many uses in research as well. One novel and particularly interesting application, one that exemplifies the power of the technique, is its use in the functionalization of carbon nanotubes.[4] Carbon nanotubes, due to their intrinsic electronic properties, tend to form large aggregates in solution, precluding their use for useful applications. Adding small chemical groups to the walls of nanotubes can eliminate this propensity toward aggregation and can be used to tune the response of a nanotube to its surrounding environment; the use of polymers instead of smaller molecules can be used to drastically modify nanotube properties (and conversely, nanotubes can be used to modify polymer mechanical and electronic properties). For example, Lou et. al.[5] were able to demonstrate the coating of carbon nanotubes by polystyrene by first polymerizing polystyrene via chain radical polymerization and subsequently mixing it at 130 °C with carbon nanotubes to generate polystyrene radicals and graft them onto the walls of carbon nanotubes. The advantage of this approach lies in the order of chemical reaction – rather than growing a polymer off of a carbon nanotube (the “grafting from” approach), chain growth polymerization is used to first synthesize a polymer with predetermined properties. Purification of the polymer can be used to obtain a more uniform length distrubition before grafting onto the nanotubes. Conversely, the “grafting from” approach, performed with radical polymerization techniques such as atom transfer radical polymerization (ATRP) or nitroxide-mediated polymerization (NMP) allows rapid growth of high molecular weight polymers (as opposed to the aforementioned “grafting to” approach where large, bulky polymers prohibitively slow the ability for free radical chain ends to find and couple with the nanotubes).

Figure XXX: Grafting of a polystyrene free radical onto a carbon nanotube.[5]

[1] W. A. Braunecker, K. Matyjaszewski, Progress in Polymer Science 2007, 32, 93. [2] R. E. Richard, M. Schwarz, S. Ranade, A. K. Chan, K. Matyjaszewski, B. Sumerlin, Biomacromolecules 2005, 6, 3410. [3] C. Burguiere, S. Pascual, B. Coutin, A. Polton, M. Tardi, B. Charleux, K. Matyjaszewski, J. P. Vairon, Macromolecular Symposia 2000, 150, 39. [4] C. M. Homenick, G. Lawson, A. Adronov, Polymer Reviews 2007, 47, 265. [5] X. D. Lou, C. Detrembleur, V. Sciannamea, C. Pagnoulle, R. Jerome, Polymer 2004, 45, 6097.