Evaluate advantages and disadvantages of atom transfer radical polymerization in comparison with other methods of controlled radical polymerization. Give example.
Free radical polymerizations are of significant importance in the industrial sector for a variety of reasons. First, many monomers capable of undergoing chain reactions are available in large quantities from the petrochemical sector. In addition, free radical mechanisms are well understood and extension of the concepts to new monomers is generally straightforward. A third advantage of free radical routes is that the polymerization proceeds in a relatively facile manner: rigorous removal of moisture is generally unnecessary while polymerization can be carried out in either the bulk phase or in solution.Despite the advantage of molecular control of living systems, the experimental rigor involved in ionic polymerization is often too costly for industrial use and free radical routes are preferred.
The obvious disadvantage of deposition is the need to continuously dip and rinse the substrate to build up the surface in nanoscopic segments. In contrast, grafting of “living” polymers from various surfaces allows one to predictably and reproducibly control the attached polymer while affording the ability to grow macroscopic segments in a one step process. Living polymers also bear the added luxury of being able to produce block copolymers at any chain length. While the varieties of polymeric materials capable of being produced are limited only to one’s imagination, recent advances have shied away from the relatively expensive design of new materials and towards the modification of existent macromolecules. Changing the method of polymer processing can readily alter mechanical properties. For example, perhaps best known for its widespread use as a foam insulator polystyrene was initially used as a component in synthetic elastomers during the second world war and has since also been used as a rigid plastic
Evaluate advantages and disadvantages of nitroxide-mediated radical polymerization in comparison with other methods of controlled radical polymerization. Give example.
Nitroxide-mediated radical polymerization is a method of radical polymerization that makes use of an alkoxyamine initiator to generate polymers with well controlled stereochemistry and a very low polydispersity index. It is a type of reversible-deactivation radical polymerization.
The initiating materials for nitroxide-mediated radical polymerization (NMP) are a family of compounds referred to as alkoxyamines. An alkoxyamine can essentially be viewed as an alcohol bound to a secondary amine by an N-O single bond. The utility of this functional group is that under certain conditions, homolysis of the C-O bond can occur, yielding a stable radical in the form of a 2-center 3-electron N-O system and a carbon radical which serves as an initiator for radical polymerization.For the purposes of NMP, the R groups attached to the nitrogen are always bulky, sterically hindering groups and the R group in the O- position forms a stable radical, generally is benzylic for polymerization to occur successfully. NMP allows for excellent control of chain length and structure, as well as a relative lack of true termination that allows polymerization to continue as long as there is available monomer. Because of this it is said to be “living".
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In the case of a nitroxide-mediated polymerization reaction, the persistent radical is the nitroxide species and the transient radical is always the carbon radical. This leads to repeated coupling of the nitroxide to the growing end of the polymer chain, which would ordinarily be considered a termination step, but is in this case reversible. Because of the high rate of coupling of the nitroxide to the growing chain end, there is little coupling of two active growing chains, which would be an irreversible terminating step limiting the chain length. The nitroxide binds and unbinds to the growing chain, protecting it from termination steps. This ensures that any available monomer can be easily scavenged by active chains.
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