Sonochemical polymerization

As mentioned earlier, polymerization techniques can also be used in the presence of nanotubes for preparation of polymer/CNT nanocomposite materials. In these, in-situ radical polymerization techniques of polymerization in the presence of CNT filler under or without applied ultrasound. Both new factors (presence of CNT and ultrasound) can affect reaction kinetics, stability of suspension or the size of prepared particles. For example, ultrasound waves can open C=C bond of monomer, which starts polymerization initiation. Thus vinyl monomers (styrene, methyl methacrylate or vinyl acetate) can be polymerized without addition of initiator, only by application of ultrasound. This is called sonochemical polymerization method (15,33,34). 

Similar effects on copolymer composition, total conversion, and RMM-control in the styrene-isoprene system were found, with the analogous traces for Fig. 19 shifted to slightly more anodic values, with a better total conversion at high potential imder ultrasound. In both systems, sonications were run in a bath at 25 kHz. The copolymerization of a-methyl- and 4-bromostyrene was also examined, 7 again with similar effect, using a small "Buehler-type probe (25 kHz). No sonochemical polymerization of the monomer occurs over a 24-h period in the absence of an electric potential. 

The polymerization can be carried out in a dispersion of the partieles in a monomer solution or in a monomer melt. Monomers are far less viseous and more soluble than polymers. Thus, this route is specially suited for the preparation of nanocomposites with a high density of particles, when the polymer is insoluble or when it does not melt. Polyimide-bonded magnets of rare earth alloys have been prepared in this way. In order to enhance cohesion, the polymer can be grafted to the particle surface by irradiation polymerization. High homogeneity is achieved by sonochemical polymerization. ... 

Sonochemistry is also proving to have important applications with polymeric materials. Substantial work has been accomplished in the sonochemical initiation of polymerisation and in the modification of polymers after synthesis (3,5). The use of sonolysis to create radicals which function as radical initiators has been well explored. Similarly the use of sonochemicaHy prepared radicals and other reactive species to modify the surface properties of polymers is being developed, particularly by G. Price. Other effects of ultrasound on long chain polymers tend to be mechanical cleavage, which produces relatively uniform size distributions of shorter chain lengths. 

Sonochemical homopolymerization of dichlorosilanes in the presence of sodium is successful at ambient temperatures in nonpolar aromatic solvents (toluene or xylenes) only for monomers with a-aryl substituents. Dialky 1-dichlorosilanes do not react with dispersed sodium under these conditions, but they can be copolymerized with phenylmethyldichlorosilane. Copolymers with a 30-45% content of dialkylsilanes were formed from equimolar mixtures of the corresponding comonomers. Copolymerization might indicate anionic intermediates. A chloroterminated chain end in the polymerization of phenylmethyldichlorosilane can participate in a two-electron-transfer process with sodium (or rather two subsequent steps separated by a low-energy barrier). The resulting silyl anion can react with both dichlorosilanes. The presence of a phenyl group in either a or P position in chloroterminated polysilane allows reductive coupling, in contrast to peralkyl species, which do not allow the reaction. Therefore, dialkyl monomers can copolymerize, but they cannot homopolymerize under sonochemical conditions. 

Metallic nanopartides were deposited on ceramic and polymeric partides using ultrasound radiation. A few papers report also on the deposition of nanomaterials produced sonochemically on flat surfaces. Our attention will be devoted to spheres. In a typical reaction, commerdally available spheres of ceramic materials or polymers were introduced into a sonication bath and sonicated with the precursor of the metallic nanopartides. In the first report Ramesh et al. [43] employed the Sto-ber method [44] for the preparation of 250 nm silica spheres. These spheres were introduced into a sonication bath containing a decalin solution of Ni(CO)4. The as-deposited amorphous clusters transform to polyciystalline, nanophasic, fee nickel on heating in an inert atmosphere of argon at a temperature of 400 °C. Nitrogen adsorption measurements showed that the amorphous nickel with a high surface area undergoes a loss in surface area on crystallization. 

The chemical methods for the preparation of nanomaterial could be categorized as either template-directed or template-free. The template synthesis methods commonly used for the production of one-dimensional nanostructured PANI are further subdivided into hard template (physical template) synthesis and soft template (chemical template) synthesis approach according to the solubility of the templates in the reaction media. Non-template routes for the synthesis of one-dimensional nanostructured PANI such as rapid-mixing reaction method, radiolytic synthesis, interfacial polymerization, and sonochemical synthesis have also been reported [56], Other approaches like combined soft and hard template synthesis are also known. An overview of hard-template, soft-template, and template-free procedures are presented in the following paragraphs. 

Also examined was the copolymerization of a-methylstyrene and 4-bromo-styrene [169], again with similar effect, here using a small Buehler-type probe of 25 kHz. This did not produce any sonochemically induced polymerization of the monomer over a 24-hour period without the application of an electric potential, representing an important control experiment since ultrasound is well known to produce radical species which could themselves influence polymerization. 

Qiu GH, Wang Q, Wang C, et al. (2007) Polystyrene/Fe304 magnetic emulsion and nanocomposite prepared by ultrasonicaUy initiated miniemulsion polymerization. Ultrason Sonochem 14 55-61... 

Recently, an ambient-temperature sonochemical reductive coupling process was developed. The reaction is carried out in the presence of an ultrasound and results in relatively high (Mn = 50,000-100,000) molecular weight materials with narrow molecular weight distributions. In addition, it was reported that polymers can also be formed by anionic ring-opening polymerization of... 

The sonochemical method has been found useful in many areas of material science, starting from the preparation of amorphous products - and insertion of nanomaterials into mesoporous materials, to deposition of nanoparticles on ceramic and polymeric surfaces. ... 

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