Radical Polymerization - Applications

Applications

Free radical polymerization has found myriad applications including, but 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, chemical surfactants, 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. 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. 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 (Figure 27). 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 distribution 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).

The power of free radical polymerization in polymerization from surfaces has also been exemplified in the synthesis of nanocomposite hydrogels. These gels are made of water-swellable nano-scale clay (especially those classed as smectites) enveloped by some network polymer and are often biocompatible and have mechanical properties (such as flexibility and strength) that make them promising candidates for applications such as synthetic tissue. Synthesis of these materials is currently possible only by the use of free radical polymerization. The general synthesis procedure is depicted in Figure 28. The clay is first dispersed in water where it forms very small, porous plates. Subsequent addition of the organic monomer, generally an acrylamide or acrylamide derivative, is immediately proceeded by addition of the initiator and a catalyst. The initiator is chosen to have stronger interaction with the clay than the organic monomer, so it preferentially adsorbs to the surface of the clay. The entire mixture of clay, organic monomer, initiator, catalyst, and water solvent is heated to initiate polymerization. Polymers grow off of the initiators which are in turn bound to the clay. Due to recombination and disproportionation reactions, growing polymer chains bind to one another, forming a strong, cross-linked network polymer, with clay particles acting as branching points for multiple polymer chain segments. Free radical polymerization used in this context allows the synthesis of polymers from a wide variety of chemical substrates (the chemistries of suitable clays are quite varied), and the termination reactions unique to chain growth polymerization are taken advantage of for the actualization of a material with flexibility, mechanical strength, and biocompatibility.