Mechanisms cleavage

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. 

Linford and coworkers have shown that the attachment of alkenes to H-terminated silicon surfaces can also be initiated by direct mechanical scribing, in a process termed chemomechanical functionalization [145-147]. The reaction of 1-alkenes (as well as 1-alkynes) leads to attachment of the molecule to the surface through two new Si—C bonds. The proposed mechanism is the mechanical cleavage of Si—H and Si—Si bonds, leading to silicon radicals that then react with the reactive liquid. Interestingly, Linford and coworkers have also extended this work to show that chemomechanical functionalization can be carried out not only on H-terminated Si, but also on sihcon covered with oxide, and have shown that the process works with a variety of halides, alcohols, and epoxides in both the liquid and gas phase [146]. 

This is shown by a repetition of the experiment shown in Fig. 37 which is impossible on the once-reacted surface. Mechanical cleavage of the sample is required to repeat the experiment. 

In the case of the tricyclic epoxides la-c the outcome of the reaction depends on the ring size. The formation of 2 and 3 was explained by the following mechanism. Cleavage of the epoxide ring with boron trifluoride diethyl ether complex gives the zwitterionic intermediate 4. Subsequent transfer of fluoride anion to the cationic center C2 leads to fluoroborate 5 (path a), which is hydrolyzed to yield 2. Spiro ketone 3 is obtained by a 1,2-alkyl shift in 4 (path b). 

The conditions for this reaction are favorable for the 8 1 mechanism. Cleavage of the ether occurs so as to form the more stable carbocation intermediate. 

The purpose of surface preparation is to remove contamination and weak surface layers, to change the substrate surface geometry, and/or introduce new chemical groups to provide, at least in the case of metals, an oxide layer more receptive to the adhesive. An appreciation of the effects of pretreatments may be gained from surface analytical or mechanical test techniques. Experimental assessments of the effects of surface pretreatment, even when using appropriate mechanical tests, are of limited value unless environmental exposure is included. Self-stressed fracture mechanical cleavage specimens, as discussed in Chapter 4 and in the texts edited by Kinloch(2,5) for example, are therefore referred to wherever possible. 

Fig. 27. Evidence for one-dimcnsional chain growth in layer perovskites Fibrous structure of polymerized compound 99a, obtained upon mechanical cleavage of crystals shown in Fig. 25. Insert Electron diffraction pattern of a polymeric fiber...

However, during the last 10 years, it was found that mechanical cleavages of macromolecules in a number of cases play a very important role, since the free macroradicals formed in this case promote the beginning of complex chemical chain processes, leading ultimately to a profound change in the structure of the macromolecules and the properties of the polymer substances formed from them. 

Since the primary event in these chemical processes is the mechanical cleavage of macromolecules, this new field of chemistry has received the name of mechanochemistry. ... 

Thus, a mechanical field is capable of initiating chemical reactions by direct mechanical cleavage of chemical bonds, as well as of changing the rate of purely chemical reactions already begun. 

It is easy to understand that the intensity of mechanical cleavages of the macromolecules should depend sharply on the temperature [27,... 

Mechanical cleavages of chemical bonds in certain cases prove to be the cause of the appearance of the long since detected [30-32] phenomenon, encountered only in polymers, which has received the name... 

All this requires a systematic study of the mechanism of the mechanical cleavage of macromolecules, the chemical properties of free macroradicals, and their reactions with the inhibitor molecules and other macromolecules. A theory of mechanochemical reactions in the reprocessing of polymers and the use of objects made from them should be created on the basis of these investigations. 

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