Polyisobutylene 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. 

The mechanism of y-ray irradiation-induced scission of polyisobutylene was studied, based on the structural characterisation of end-groups by 13C-NMR as well as GC, GC/ MS, and SEC [77], The assignments of signals were made by comparison with those from model compounds and predictions based on empirical rules. Quantitative 13C-NMR measurements of chain-ends allowed the determination of radiation yield of products and of chain scission. 

Simultaneously with Charlesby s findings, work along similar lines was carried out in G. E. s Research laboratories in Schenectady (22) and also in Research Institutes in the Soviet Union, although the latter only became known several years later (23). The results of this research demonstrated that in addition to polyethylene, many other polymers could be cross-linked by radiation. These include silicones, rubber, poly (vinyl chloride), polyacrylates and, to a lesser extent, polystyrene. In contrast, polymers such as polymethacrylates, polyisobutylene, polytetrafluoroethylene and cellulose underwent "degradation" by main-chain scission. These early findings were confirmed and extended to other compounds by numerous studies. 

Hexahydropyrene sensitized chain scission of polypropylene and polyisobutylene during light irradiation Q20).Polycyclio hydrocarbons have a important role in sensitized photooxidation of polyisoprene (122).polys tyrene (123) poly(methyl methacrylate) (123-126). It is quite probable that these reactions can also occur with participation of singlet oxygen. 

High energy irradiation (electron beam or gamma) of polymeric materials results in a multitude of chemical reactions. The two main reactions are crosslinking, as observed in polyethylene and chain scission, for example polyisobutylene. In polypropylene with its structure in between polyethylene and polyisobutylene, both reactions are observed. The relative importance of chain scission (degradation) over crosslinking depends on the physical state, the irradiation parameters, presence of... 

Polymers containing a completely substituted carbon in the chain often undergo chain scission on oxidation or exposure to UV or gamma radiation. This is because rearrangements are formed when an unpaired electron is left on a structure as in polyisobutylene ... 

Because the industrially important process of cross-linking rubber often is carried out under oxidative conditions, and because chain scission generally is undesirable, other agencies must be employed for cross-linking when the chain contains such unstable groups. A case in point is polyisobutylene that is easily scissioned by peroxides. However, the corresponding commercial rubber is a copolymer with a small amount of isoprene so that sulfur can be used to cross-link via the resulting residual unsaturation in the main chain. 

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