Mechanochemical initiation

Grafting of MA on both isotactic and atactic polypropylene has been studied under a variety of conditions.Maleic anhydride, maleic acid, and monomethyl maleate have also been grafted to polypropylene by a thermal process at 325°C. Mechanochemical modification of both atactic and isotactic polypropylene may be accomplished by blending polymer-anhydride mixtures in laboratory mills with peroxide initiators. Mechanochemical grafting can also be brought about by using peroxidized polypropylene. Extraction studies clearly showed grafting was achieved.An additional reaction of MA with isotactic or atactic polypro-... 

Mechanical treatment alone may be sufficient to induce significant decomposition such processes are termed mechanochemical or tribo-chemical reactions and the topic has been reviewed [385,386]. In some brittle crystalline solids, for example sodium and lead azides [387], fracture can result in some chemical change of the substance. An extreme case of such behaviour is detonation by impact [232,388]. Fox [389] has provided evidence of a fracture initiation mechanism in the explosions of lead and thallium azide crystals, rather than the participation of a liquid or gas phase intermediate. The processes occurring in solids during the action of powerful shock waves have been reviewed by Dremin and Breusov [390]. 

Another approach was developed by Scott in the 1970 s (7.8) which utilises the same mechanochemistry used previously by Watson to initiate the Kharacsh-type addition of substituted alkyl mercaptans and disulphides to olefinic double bonds in unsaturated polymers. More recently, this approach was used to react a variety of additives (both antioxidants and modifiers) other than sulphur-containing compounds with saturated hydrocarbon polymers in the melt. In this method, mechanochemically formed alkyl radicals during the processing operation are utilised to produce polymer-bound functions which can either improve the additive performance and/or modify polymer properties (Al-Malaika, S., Quinn, N., and Scott, 6 Al-Malaika, S., Ibrahim, A., and Scott, 6., Aston University, Birmingham, unpublished work). This has provided a potential solution to the problem of loss of antioxidants by volatilisation or extraction since such antioxidants can only be removed by breaking chemical bonds. It can also provide substantial improvement to polymer properties, for example, in composites, under aggresive environments. 

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. 

Polyamides. As already described, polyamide degradation takes place in the dry state at the C—C bonds, whereas in the wet state an activated mechanochemical hydrolysis predominates. However, only the macroradicals obtained from —C—C— bonds are able to initiate graft reactions. 

Experiments were also performed with the aim of polymerizing a mixture of two monomers (69). The reaction rate and the composition of the graft copolymer conform to the rules of mechanochemical synthesis and radical copolymerization. If the two monomers have almost equal reactivities, the composition of the copolymer is approximately that of the initial monomer mixture (Fig. 20). The... 

G proteins are another family of membrane proteins believed to modulate mechanochemical transduction pathways. Mechanical stimulation changes the conformation of G protein that leads to growth-factorlike changes that initiate secondary messenger cascades leading to cell growth. It has been reported that cyclic strain of smooth muscle cells significantly decreased steady-state levels of G protein and adenylate cyclase activity. Muscular stimulation also appears to be coupled with G protein activation in small arteries. 

The goal of the present monograph is to generalize the works carried out in this research direction. The subject of investigations is the synthesis of complex oxides of the elements ofl-Vin groups of the Periodic Table from ordinary hydroxides. For numerous hydroxides, substantial differences in their properties are observed, in particular, the differences in acid-base characteristics due to the structure of electron shells of atoms and the nature of chemical bonds. This allows one to search the definite laws governing the formation of complex oxides, and to look for correlation between the structure of initial hydroxides, their thermodynamic parameters and kinetics of mechanochemical synthesis. 

Water is one of the products formed via mechanochemical interaction between hydroxides. Its presence can substantially effect on the mechanism of transport processes and, thus, on the kinetics of mechanochemical processes, since some reaction products (initial or final) can be dissolved in it. Hence, the reaction could proceed with the participation of the dissolved forms of components. 

In [27-29], hydrothermal, mechanochemical and solid-phase syntheses of calcium silicate from anhydrous and hydrated oxides were compared. Initial components were taken at Ca/Si ratio equal to 0.8 1.0 1.2. According to X-ray analysis, the interaction in the mixtures of anhydrous oxides under mechanical activation is not completed. The product being formed is X-ray amorphous. When heated at 600-800 C, it is crystallized in the form of a -Ca2Si04 (Fig. 3.6a). At higher temperatures, Ca3Si207 is formed P-CaSiOj is crystallized at 860 C. The observed sequence of stages is similar to those observed in solid-phase synthesis of wollastonite. The amount of p-CaSiOj at 900°C does not exhibit any substantial dependence on the initial fractions of the reagents. 

The syntheses of double, triple and more complicated compounds can be carried out on the basis of hydroxides and hydrated oxides. In order to achieve the most efficient interaction, one should select initial components taking into account their acid-base properties, solubility and possible hydrolysis in the presence of small amounts of water. The compounds or groups of compounds should be selected to provide a maximum difference in acid-base properties. A quantitative characterization of these properties was described in the previous chapters. By present, a substantial experience in soft mechanochemical synthesis of inorganic compounds has been accumulated. The description of experimental results presented below is divided into two parts. One of them deals with the synthesis of double compounds and an another with the synthesis of more complicated compounds. 

Lithium aluminates. The compounds Li0H-2Al(0H)3-mH20 and LiCl-2Al(0H)3-mH20 (here m = 0.5 1.0 2.0) are easily synthesized under very low mechanical loading (blade mixer) in stoichiometric Al(0H)3+Li0H H20 and Al(0H)3+LiCl H20 mixtures [1,2]. It was stated that the dispersion of the initial aluminium hydroxide strongly influenced on the kinetics of mechanochemical interaction. The interaction rate increases linearly with the specific surface of the initial hydroxide. Fig. 6.1 shows the data on reactivity of initial hydroxide with different specific surface area (6 and 2 mVg, respectively). 

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