Mechano-radicals

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... 

One of foe advantages of ESR technique is that a radical species can be identified by an analysis of foe ESR spectrum. Such an identification of a mechano-radical is in effect a means of locating the point of the fracture induced by the maraoscopic destruction of polymeric materials, provided that the radical is prinutrily produced by the mechanical actions. Thus, identification of a mechano-radical gives basic information for elucidating the molecular mechanism of fracture. Although identifications of the mechano-radicals of various polymers were reported (9—17, 23-26, 27),... 

And also reactivity of the mechano-radicals will be mentioned in the later part of this review. 

The review is not intended to constitute an exhaustive report but rather a survey of our efforts and findings in ESR studies on the mechano-radicals of several polymers. 

No ESR spectrum, even observed immediatdy after the milling at 77 K, is attributed to the primary mechano-radical. 

If homolytic scission of polymer main-chain is induced by mechanical fracture, primary mecbano-radical must be formed in a pair. However, for homogeneous polymers, like PE, PTFE and PB, it is hard to demonstrate the pair formation of mechano-radical. Simplest heterogeneous polymer, polypropylene, is probably the best sample for the purpose of experimental verification of pair formation of the mechano-radical. 

ESR spectrum from PP mechano-radic was reported by Radtsig and Butyagfn 34) and they identified the radical... 

The simulated spectrum is shown as b in Fig. 7 and the similarity of the spectra is satisfactory. This analysis of the observed spectrum convinces us that the mechano-radical is formed in a pair by breaking the main-chain of polypropylene. 

Polymethylmethacrylate is one of the heterogeneous polymer and ESR studies on this polymer have been done from the early stage of ESR spectroscopy ( 5). Russian researchers reported ESR ctra observed from the PMMA mechano-radicals (28,46-48). No ir formation of the n hano radicals was mentioned by these authors, although PMMA is one of the suitable polymers on wludi Ihe ptdr formation of mechano-radicals is s rched. An ESR spectrum (38, 49) observed at 77 K from the PMMA milled for 24 hrs. is shown as a in Fig. 8. It is well known (50,51) that PMMA radical produced by the main chain scission shows the characteristic quintet-quartet spectrum when PMMA is irradiated by y-rays. It is reasonable to assume that one partner of the pair formation of the PMMA mei no-radical is the species. 

Production of mechano-radical in Nylons by either larger deformations or fractures were extensively investigated by DeVries and his collaborators (17,18,53), Peterlin and his coworkers (54, 55) and Becht and Fischer (22). ESR studies on mechano-radicals of other pol3rmers, including polystyrene (49, 56, 57), pol)ndnylacetate (49,57—59) poly o-methylstyrene (11), polyisobutyrene (11), Natural Rubber (22), other elasttxners (60-62), polysaccharides (6ii), and cellulose (63, 64). 

At first, it should be noted that mechano-radicals are not produced by fracture in organic solids consisting of the low molecular compounds, sudi as solid benzene... 

It is also interesting to note that species of the mechano-radicals, viiich were described in the Section III, are generally different from those produced by ionizing radiations the primary mechano-radical are ail of the scission type, while the latter ones are of non-scission type,... 

V. Anomalous Decay of Mechano-radical Effect of Triboeiectricity (78)... 

Decay curves of PP radicals are shown in Fig. 12. The solid curve in Fig. 12 is a decay curve of the If mechano-radical produced by the sawing and tl dotted line is that of the PP radical produced by 7-iiradiation. The radicals i oduced by 7-irradiation decay step-wise with increasing temperature of the heat treatments, and no increase of the ESR intensity is observed as a result of the heat treatments. This is common and normal behavior in the dec y of polymer radicals (79-81). Contrastly the decay curve of the mechano-radical increases to a maximum at 173 K and decreases more rapidly than that by 7-irradiation in the higher temperature region. It was also found that the ESR spectra changed in shape during this anomalous increase of the intensity, as shown in Fig. 13. The ESR spectrum observed before the heat treatment was... 

Decay curves of the PP mechano-radicals were obtained under the same experimental conditions except for the electrical circumstance. One decay was obtained under the condititm of electrical insulation, in which the entire apparatus of sawing, including the experimenter, was electrically insulated from the ground. The other was done under the condition, in whidh the sawdust of the sample was not insulated but electrically grounded to the earth at 77 K before being transfened to an ESR... 

The conclusive explanation for the anomalous behavior in the decay proc of the mechano-radical Is ven by the follomng reaction schemes ... 

In this review reactmties of mechano-radicals will be classifted into four categories radic conversion, reaction with oxygen, photoreaction, and copolymerization initiated by mechano-radicals. 

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