Polyisobutylene scissions

Degradation is the predominant process in irradiation of polyisobutylene. Generally, the scission yield increases with temperature in accord with the temperature dependence of the formation of unsaturation. For... 

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. 

T. Sawaguchi and M. Seno, Thermal degradation of polyisobutylene effect of rotational motion around C-C- bond on the /1-scission leading to monomer formation, Polym. Degrad. Stab., 54(l) 23-32, October 1996. 

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. 

The polymer polyisobutylene (—C(CH3)2—C—) presents a case where G(cl) = 0. This polymer is entirely amorphous (except on stretching, when it crystallizes) and the linear increase of /My (D) with dose has been established over a wide range of initial molecular weights. From these results it has been concluded that the scissions take place at random positions. ... 

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... 

At each stage of degradation, the M /M for the polyisobutylene remained constant at 2.0 which is a classical example of random scission. (See Fig. 2.8, which is Fig. 5 of Scott [14a].) Figure 2.9, which is Fig. 2 of the same reference, illustrates the influence of the original molecular weight distribution on the random degradation process. 

Fig. 2.8 Ultrasonic irradiation of a concentrated polyisobutylene solution [39a] initial = 2. Relative changes of and are compared with the theoretical behavior for random scission of a most probable MWD [14a].

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