Free from main chain scission

Although it is well established that polyisobutylene degrades under radiation, the main-chain scission radicals were never observed as primary radicals in any of these ESR studies. A possible explanation is that the two free radicals formed by chain scission are unable to migrate from the reaction site. The two end-group radicals are then likely to react with each other by either recombination or disproportionation. 

Vehicle tire scrap containing polyisoprene rubber, SBR, and butadiene rubber was devulcanized by low-temperature phase-transfer catalyst. Both the devulcanizing agent composition and the process were patented. The novelty of this process lies in the use of low-temperature phase-transfer catalyst and a process temperature lower than 150" C. The devulcanized rubber of this invention is distinguishable from conventional reclaimed rubber in that the devulcanized rubber is substantially free from polysulfide crosslinks, which are selectively broken during the process with negligible main chain scission. 

Butyl rubber is a copolymer of isobutylene and I -2% isoprene. As a result the polymer chains contain internal double bonds which are expected to participate in cross-linking reactions. However, the overall molecular mass is expected to fall on irradiation due to the predominance of main-chain scission through the isobutylene units. Thus the radiation chemistry of the isoprene units within butyl rubber is accessible to study via solution NMR. In a comprehensive study Hill identified the primary free radical species by electron spin resonance spectroscopy at low temperatures, and the products of their subsequent reaction by C solution-state NMR. A number of new cross-link structures were identified and the mechanisms of cross-linking determined. Initial reaction involves addition of radicals either directly to the isoprene double bonds or to allyl radicals. Further addition of hydrogen atoms results in a mixture of fully-saturated and unsaturated cross-link structures. Cross-links of both H- and Y-type were identified and the yields of products agreed closely with the yields determined from measurement of changes in molecular weight on irradiation. 

Since destruction of polymer materials is very important for practical purposes, a large number of investigations on fracture phenomena in polymers have been carried out from both the experimental and theoretical points of view. Several reports provide indirect evidence for main chain scissions, for example decreases in molecular weight or initiations of the graft or block copolymerization after mastication. Direct evidence for chemical bond scission can be obtained from ESR measurements on fractured polymer materials [21]. The high reactivity and high mobility of free radicals produced by mechanical fracture (mechano-radicals) can also be followed. The ESR application to mechanical destruction of polymer materials is presented below. Temperature-dependent ESR spectra of polymer radicals produced... 

Campbell and Peterlin and Peterlin concluded from e.s.r. measurements on isotropic and highly drawn nylon 6 and 6.6 fibres that no detectable free radicals were formed in the isotropic state, whereas approximately 1 chain in 250 was fractured in a fibre under high axial tension at failure. These fractured chains were later identified with the tie molecules linking adjacent crystallites together in the fibre direction. Quantitative theories have since been developed by Kausch et and more recently by DeVries et alP which attempt to correlate creep, creep-rupture, and stress-relaxation in fibres in terms of the measured main chain scission. 

Irradiation effects in organic polymers result from the cleavage of chemical bonds and the subsequent reactions of intermediates generated thereby. These reactions lead to significant alterations in the physical properties of polymeric materials, and this topic has been covered in numerous books and articles [12,38-54]. As far as linear chain polymers are concerned, any changes in physical properties are due mainly to the formation of permanent main-chain scissions and intermolecular crosslinks. A general free-radical-based mechanism related to these processes is presented in Scheme 5.11. 

Scheme 5.15. Main-chain-scission-type radicals are also formed when free hydrogen atoms abstract hydrogen from polymer side groups (see upper part of Scheme 5.15).

Homolytic bond cleavage from excited states in irradiated polymers [30] can lead to a pair of free radicals via bond scission, involving main chain or side-chain substituents. 

It is well known that ESR is a sensitive eiqierimental technique to detect selectively free radicals having broken bonds or unpaired electrons. Therefore, one obtains direct experimental evidence for chemical bond scission, whenever an ESR spectrum was observed from a fractured polymer material. ESR studies on mechanical destruction give us a decisive answer to the question whether the macroscopic fracture of polymers results in breakage of main chains of the polymer molecules. And also ESR may provide interesting information which is helpful for crarelating phenomraiolopcal results with breaking of chemical bonds. 

Deterioration of polymers, PO in particular, resulting from y- or other high-energy irradiation, takes place in diverse areas of their application. This includes controlled modification of polymers for industrial purposes, use of polymers in some radiation exposed parts of nuclear reactors and radiation sterilization of food packaging materials or of equipment and materials for medical uses. Radiation-induced primary free radicals result from either the scission of the main chain... 

The temperature dependence of the G-values G, and G for scissitm and crosslinking of polystyrene under y irradiation is given in Table 7 The ratio GJG, increases from 0.02 at 30 °C to 2.8 at 150 C. This increase is due mainly to a tenfold increase in Gg with G,j decreasing slightly. These results are compatible with the increased disproportion of chain scission radicals idative to their combination, and analogous to the temperature dependence of mutual termination in free radical polymerizaticms. 

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