Mechanically induced bond scission

Fig. 5JS2. Shock-induced polarization of polymers has been studied by many investigators, with data as summarized. The typical behavior indicates a threshold compression of about 10%-15% followed by a rapid increase in value. The polarizations shown vary over three orders of magnitude. The author has proposed a mechanically induced bond-scission model to describe the effects. (See Graham [79G01].)...

These observations were the basis for the proposal that polymers, like ionic crystals, exhibit shock-induced polarization due to mechanically induced defects which are forced into polar configurations with the large acceleration forces within the loading portion of the shock pulse. Such a process was termed a mechanically induced, bond-scission model [79G01] and is somewhat supported by independent observations of the propensity of polymers to be damaged by more conventional mechanical deformation processes. As in the ionic crystals, the mechanically induced, bond-scission model is an example of a catastrophic shock compression model. 

Straining the macromolecules result in generation of stresses that may activate some bonds. Mechanically induced chain scission has been explored for grafting polymers and rubbers. 

But it needs to be emphasized that caution must be exercised in drawing too many strict (and/or inflexible) conclusions from activation energy data. This, of course, must also throw some doubt on, and add caution to, the inconsiderate use of model compounds as materials from which to project coal behavior during coal pyrolysis. However, on a positive note, the use of model compounds does offer valuable information about pyrolysis mechanism it is the means by which the conclusion is drawn with respect to coal that can hurt the effort. Finally, the concept of induced bond scission (McMillen et ah, 1989) also opens up the area of coal pyrolysis to the additional concept of selective bond breaking by addition of suitable reagents. 

Well before the advent of modern analytical instruments, it was demonstrated by chemical techniques that shear-induced polymer degradation occurred by homoly-tic bond scission. The presence of free radicals was detected photometrically after chemical reaction with a strong UV-absorbing radical scavenger like DPPH, or by analysis of the stable products formed from subsequent reactions of the generated radicals. The apparition of time-resolved ESR spectroscopy in the 1950s permitted identification of the structure of the macroradicals and elucidation of the kinetics and mechanisms of its formation and decay [15]. 

An inner-sphere electron reduction has been proposed as a possible mechanism for the Fe(II)-induced decomposition of 1,2,4-trioxolanes (ozonides) (75) and (76). Benzoic acid was found to be the major product. The nucleophilic Ee(II) species attack the ozonide from the less hindered side of the electrophilic 0-0 a orbital to generate exclusively the Ee(III) oxy-complexed radical (inner-sphere electron transfer). After selective scission of the C-C bond, the resulting carbon-centred radical produced the observed product. The substituent effect determine the regioselective generation of one of the two possible Fe(III)-complexed oxy radicals. The bond scission shown will occur if R is bulkier than R. ... 

Figure 27. Mechanism of radiation induced chain scission in PMMA. Homolysis of the mainchain-carbonyl carbon bond is indicated as the initial step. Acylcarbon-oxygen, sigma bond homolysis also occurs but rapid decarbonylation ultimately leads to the same indicated products.

The EE and phE mechanisms for neat polymers proposed by ourselves and others all involve the consequences of breaking bonds during fracture. Zakresvskii et al. (24) have attributed EE from the deformation of polymers to free radical formation, arising from bond scission. We (1) as well as Bondareva et al. (251 hypothesized that the EE produced by the electron bombardment of polymers is due to the formation of reactive species (e.g., free radicals) which recombine and eject a nearby trapped electron, via a non-radiative process. In addition, during the most intense part of the emissions (during fracture), there are likely shorter-lived excitations (e.g., excitons) which decay in a first order fashion with submicrosecond lifetimes. The detailed mechanisms of how bond scissions create these various states during fracture and the physics of subsequent reaction-induced electron ejection need additional insight. 

It was experimentally proved by ESR that all the polymers so far investigated generated mechanoradicals by various mechanical agitations. Based on the these experimental facts it can be concluded that macroscopic fractures of solid polymers induce microscopic destruction, Le, scission of chemical bonds, although molecular mechanisms for the bond scissions caused by mechanical forces still remain unknown. Most ESR experiments for detecting the mechanoradicals were done at 77 K. The low temperature is required only for stabilization of unstable radicals. It is reasonable, therefore, to assume that mechanoradicals are formed even at room temperature in the polymers in which mechanoradical production is observed at 77 K. 

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