Oxidation and Mechanochemistry

Whilst there was circumstantial evidence that these theories might be correct it was not until 1952 that Pike and Watson eventually provided experimental verification of the mechanochemical mechanism. The evidence for mechanical rupture may be summarized as follows. 

The negative temperature coefficient of mastication is consistent with the fact that a decrease in viscosity reduces the shearing stresses (at a given shear rate) available to rupture the chains. 

In the case of natural rubber it is difficult to reduce the average molecular weight below about 70000 for the same reason as above, i.e. the lower viscosity of the lower molecular weight material prevents the development of adequate shear stresses. 

Radical acceptors such as thiophenol, in the absence of oxygen, have an effect similar to that of oxygen with the molecular weight falling on mastication towards, once again, a limiting value of about 70 000. 

Final confirmation that the radicals were in fact broken polymer molecules was demonstrated by the use of highly sensitive radical acceptors which could be detected in trace amount when attached to a ruptured polyisoprene chain comprising several thousand isoprene units. The chemicals used were l,r-dinaph-thyl disulphide labelled with radioactive sulphur and 1,T-diphenyl-2-picryl hydrazyl (DPPH) (Ayrey et a/., 1956). 

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