current applications, highlighting the importance of LRP as a synthetic technique77, 78. Nanotechnology is one major field where polymers synthesized using LRP methods are proving to be extremely useful. One area that is especially important is the synthesis of metal nanoparticles. Interest in nanoparticles lies in their unique properties79-83 as well as the ability to create catalysts with large active surface area and low metal quantity84-87. While there are many methods currently available little is known about the synthetic mechanism and many of the techniques utilize very harsh reaction conditions79, 80, 84. There are two techniques that are most commonly used: application of amphiphilic copolymers to create nanoreactor micelles83, 88-91 and high temperature reflux of metal precursors in the presence of polymers and reducing agents, polyol synthesis80, 87, 92.
1.6.1 Nanoparticle Synthesis Utilizing Polymer Micelles
Synthesis of metal nanoparticles using polymer micelles requires the use of block copolymers where the blocks differ have different properties, polarity or acidity are the most common. In current research, the metal species are isolated in the core of the micelles by creating a more favorable environment for the polar complexes79. Use of amphiphilic block copolymers in a fairly nonpolar solvent will create a core where the metal complexes will migrate to the polar core77, 89. Another system where solvents with higher polarity are used require a polymers with a polar-polar coordinating formation. In this case, both blocks are polar so the formation of the micelle is entirely dependent on the polar coordinating block’s ability to coordinate to the metal species90. The important feature of this type of system is that the reactions can be performed in water90, 93 which is important for the development of more environmentally friendly techniques. Control in particle size and shape are determined by the micelles ability to separate the metal species from the bulk eliminating the possibility of metal reduction outside of the micelles.
In some reactions, typically those using polymers with block of different polarities, the micelle is first created and then a polar solution of the metal species is added and allowed to equilibrate until all of the metal complexes have migrated to the polar core89. While in syntheses where both blocks are the same polarity, the addition of the solution containing the metal species
allows for the formation of the micelles since the coordination to the metal is what provides the linkages between the different polymer chains90. In either case, once the micelles are formed with the metal complexes in the core, polar reducing agents such as sodium borohydride88, 90, 91, 93 are added to the reaction mixture. Upon the introduction of the reducing agent, nanoparticle formation will begin within the micellar core. By restricting the mobility, especially in cases where the micelle has been crosslinked88, of the metal complexes, the size and shape of the particle formed is determined by the micelle90. If all of the metal is incorporated within the micelles, a narrow distribution of particle size should be observed. When the synthesis results in a wide size distribution, the metal complex was not sufficiently isolated in the micelle cores and reduction is occurring both in the core and in the reaction bulk.91, 93
This type of synthetic technique has proven extremely useful for formation of nanoparticles of various metals in different size ranges94. Increasing the concentration of metals, changing the identity of the polymer, and different reduction speed account for the versatility found in this method. Micelles prove to be ideal nanoreactors for nanoparticles since they are immediately stabilized through the steric bulk of the polymer surrounding the particles preventing agglomeration93. The introduction of polar-polar micelles creates a unique opportunity in the field of medicine for the metal particles synthesized in this manner. If the micelles are produced in an aqueous solution, they can be used as delivery agents53, 88 where once the micelle reaches an area of differing polarity the contents are released due to destruction of the micellar structure. 1.6.2 Polyol Particle Synthesis
Metal particle synthesis using homopolymers as stabilizers is considered a self-assembly method where the various components are added to a solution and give the desired results80, 95. Alcohols are most commonly used as the reducing agent in these techniques and are used in large quantities to increase the solubility of the polymer as well. In these reactions, a metal precursor complex is added to a water-alcohol solution with the desired polymer82, 92. This solution is then refluxed for an hour or more until the reaction is seen to go to completion. Uv-vis spectroscopy is the most common method92, 95, 96 utilized to monitor the progress of the reaction since a majority of the metal complexes are active in this region92. As the particles form, the
polymers self-assemble around the particles through interaction with the surface. This self assembly is responsible for the control of the particle size since the particle ceases growth when they are completely encapsulated by the polymer.
After the reaction mixture is cooled the stabilized particles are separated and redispersed96. Ultimately the polymer plays several roles in these preparations: they influence the size and shape of the particles, act as stabilizers, and affect the catalytic behavior. Size and shape are influenced by the polymer identity due to differing interaction strengths given the different functional groups on the polymer chains83. The stronger the polymer-metal interaction the more quickly the capping occurs resulting in smaller particles. These interactions can also inhibit the growth of different faces of the metal crystal while allowing growth of other faces changing the shape of the particle that is ultimately formed.
One of the most important functions, and what is typically considered the primary function of the polymer, is the stabilization of the particles94. As in 1.6.1, the polymers are utilized as stabilizers because of the steric bulk82, 83, 87 that they add to the particle inhibiting the ability of the metal centers to combine with each other. If the polymer is chosen properly agglomeration is prohibited and the nanoparticles remain small with a narrow size distribution for an extended period87. This is an important property since the desired behavior of the particles is often exclusively due to their size. Therefore, in order for the particles to be useful industrially the properties must remain consistent for long periods so the stabilization is essential.
Lastly, the catalyst behavior of the metal nanoparticles can be strongly influenced by the chosen polymer. In order for the particles to remain catalytically active, the substrates must be able to reach the particle within the protective polymer coating. This means that the environment created by the polymer must be designed so that the desired compounds are able to migrate to the particle87. Changing the identity of the polymer can alter the polarity, electrostatic properties, or structural formation around the nanoparticle creating particles with catalytic specificity87. The metal-polymer surface interaction can also affect the catalytic properties of the nanoparticle. When polymers coordinate or interact with the surface of the particle the behavior of the metal is changed since this will change the metal characteristics from those of a pure metal surface87.
This method has been utilized in order to synthesize metal nanoparticle of various metals80, 85, 87 in a wide variety of sizes87, 92, 96, however, very little is known about the mechanism which drives the nanoparticle assembly.