Mechanochemical yield

One of the main problems in investigation of mechanochemical transformations consists in the relations between product yield and mechanical energy consumed by the process. Butyagin and Pavlychev [8] proposed to characterize mechanochemical yield by the ratio of the moles of product to the amount of the energy consumed (mol/MJ), similarly as in radiation chemistry. In reality, the researches most often record the dependence of the transformation degree a versus the time of mechanical treatment of powder mixture in mills. If the power of apparatus is known,... 

The ealeulation of mechanochemical yields allows to compare the efficiencies of different devices. Besides, the comparison between data obtained at different amounts of consumed energy allows to conclude that mechanochemical yield per unit energy is a constant [7]. 

We obtained the data on mechanochemical yields for the reaction BaCOj + WO3 = BaW04 + CO21 depending on power input. The power of a planetary mill was controlled by a frequency of rotation and determined experimentally from the consumption of electrical energy used only for balls operation (blank consumption being subtracted). 

The results are shown in Fig. 3.5. According to Fig. 3.5b, mechanochemical yield per unit of consumed energy appeared to be a constant only starting from some threshold power per 1 g of the mixture under treatment. This threshold is about 5 W/g. The existence of threshold effect in solids is connected with fragile-viscous transition the latter occurs only when a definite level of energy input to a solid is exceeded. 

Unusual reactivities of mechano-radicals have been reported in a few instances. To explain the pseudo first-order kinetics and the high yield of linear block copolymers formed during the mechanochemical degradation of a mixture of... 

In the third method called mechanochemical activation synthesis (MCAS), a mixtnre of metal componnd, viz. metal chloride, is ball-milled to induce a reaction to yield a high-hydrogen capacity hydride. 

Mechanochemically activated reactions between molecular solids to yield a co-crystalline adduct. 

Scheme 3 The solid yielded by mechanical mixing of the reactants can be used to seed crystal growth from solution to obtain crystals for single-crystal X-ray diffraction experiments. This procedure allows one to compare the X-ray powder diffraction pattern measured on the mechanochemical sample with that calculated on the basis of the single-crystal experiment for the solids obtained by crystallization via seeding of a solution of the ground powder of the adduct...

On the other hand, the mechanochemical solid-state reaction was found to be the most suitable for this purpose. Thus, when the solid-state reaction was conducted for Cgo in the presence of one equivalent or less of KCN under the HSVM conditions for 30 min, a clean reaction took place to give the [2-1-2] fullerene dimer C120 (3) in 30% yield while 70% of Cgo was recovered unchanged (Scheme 2) [20]. It is to be noted that no cyanated fullerene such as 4 was obtained in comparison to the result of a liquid-phase reaction in o-dichloroben-zene (ODCB)-DMF [21]. This is apparently ascribed to the difference in reactivity of the initially formed cyanated Cgo anion with or without solvation. 

Recently, a solvent-free variation was found to be equally effective. The inorganic base Na2C03 was utilized to replace the organic base DBU in a mechanochemical reaction under high-speed vibration milling conditions [37]. The yields with diethyl bromomalonate are in the range 40-50%. 

The smallest subimit of these fullerene polymers is the C j2o dimer [368], Synthesis and examination of the properties of this molecule are, therefore, of considerable interest. Beside the solid-state mechanochemical reaction of with KCN -described in Section 3.2.3 -the dimer Cj2o can be synthesized at 5 GPa and 200 °C in good yields from a bis(ethylene-difhio)TTF-C5Q complex [369]. Cj2o is a dark brown solid that is almost insoluble in most organic solvents. 1 ts solubility decreases with increasing purity of the sample this behavior is also known for pristine C q. 

Certain solid phases, on the other hand, cannot be obtained (even as microcrystalline powders) by crystallization experiments, but instead can be generated only by other types of preparation procedure. Some types of preparation processes commonly (or in some cases inherently) yield microcrystaUine products, including (1) preparation of materials directly from solid-state chemical reactions (see Sect. 6.6), (2) preparation of materials by solid-state desolvation processes (see Sect. 6.4), (3) preparation of materials by solid-state grinding (mechanochemical) processes (see Sect. 6.2), and (4) preparation of materials directly by rapid precipitation from solution (as opposed to crystallization) (see Sect. 6.7). Again, structure determination from powder XRD data may represent the only opportunity for determining the structural properties of new solid phases obtained by such processes. 

Temperature Mechanochemical degradation of polyamide by vibromilling exhibited a negative temperature coefficient. The grafting yield should thus increase on decreasing temperature. In fact, the reaction is temperature insensitive. From 10 to 40° C, only a modest decrease of grafting yield was observed. Homopolymerization is almost unaffected by temperature (see Fig. 6). 

In summary, modern pulping processes may be considered to be chiefly mechanical, mechanochemical (semichemical), and chemical processes. The classification in terms of pulp yield is depicted in Figure 1. 

Butyagin P.Yu., Pavlichev J.K. Energetic yields of mechanochemical reactions. Reactivity of Solids 1986 1, 345-61. 

It was confirmed that mechanochemically formed radicals are active enough to initiate polymerization of vinyl monomers [254]. Due to the quick reactivity of the formed carbon centered radicals P with oxygen, an inert atmosphere must be strictly observed during the process to avoid any competition with the generation of thiyls [243]. Moreover, the presence of hydroperoxides formed in polymers due to the reactivity with oxygen diminishes the yield of binding of thiyls to the polymer chain as a result of the HD AO activity of ASH (ASSH). 

The yield of the adduct formation is dependent on the efficiency of the mechanochemical formation of radicals, on the viscosity of the mechanochemically treated polymer and the temperature of the process due to the reversibility in the binding of thiyls [243]. 

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