Degradation of polyisoprene

Figure 14.27 Degradation of polyisoprene by photolysis (a), and ozonol-ysis (6).

Mayo et al. [404] found that phenylhydrazine catalysed markedly the degradation of polyisoprene in solution with oxygen. The effect increases with the amount of phenylhydrazine. Considering the above results it seems possible that hydrazines formed during the photolysis of DPPH catalyse the polyisoprene oxidation [508]. It was also found that the addition of hydrazine hydrate accelerated the thermal oxidation of polyimides [187]. 

Comparative studies on the thermal degradation of polyisoprene, natural rubber (cis-polyisoprene) and gutta percha (t r tns-polyisoprene) all of which have the same chemical composition have shown that they differ in their thermal degradation characteristics. Studies under vacuum at 290-380 "C have shown that the decomposition of natural rubber (NR) is initiated at comparatively low temperatures at a considerable rate, whereas its decomposition rate at higher temperatures (above 330 "C) is to some extent slower than that of gutta percha and polyisoprene (Figure 2.1). The half-life temperature for synthetic polyisoprene is 320 C [1, 2]. 

Oxidative and UV degradation. Polymers that contain sites of unsaturation, such as polyisoprene and the polybutadienes, are most susceptible to oxygen and ozone oxidation. Figure 14.27 illustrates a typical oxidative degradation of a common elastomer. The figure shows the combined effect of light and oxygen (photolysis) and the action of ozone (ozonolysis). 

This reaction was reported for the benzophenone photosensitized degradation of polypropylene (92),polystyrene (95), poly(vinyl alcohol) (94),polyisoprene (95,96),polyurethane (97) and polyadenylic acid (98) In solid state benzophenone also produce an extensive crosslinking of polyethylene (99-105) It has also been found that benzophenone and its derivatives caused an initial rapid oxidation,increasing with ketone concentration (70) Currently the relative importance of singlet oxygen formation in energy transfer reaction between excited benzophenone and molecular oxygen is discussed (92,104) ... 

Rabek found in his investigations of the photo-oxidative degradation of cis-l,4-polyisoprene in solution that benzophenone [496, 502], Michler s ketone (4,4 -bis-(dimethylamino)-benzophenone) [508] and thiobenzo-phenone [494, 495] sensitize the reaction (Fig. 19). In the absence of oxygen benzophenone promotes the formation of microgel. 

Rabek [498, 505] examined the rapid degradation of cis-1,4-polyisoprene on exposure to light in the presence of p-quinones p-quinone, chloranil, 1,4-naphthoquinone, anthraquinone, phenanthrene-quinone and 1,2-benzanthraquinone (Fig. 20). Later, Rabek [504] also found that p-quinone sensitizes the photo-oxidation of polystyrene in benzene in the presence of UV radiation. 

Following Ashby s first observation (7) of chemiluminescence from the oxidative degradation of polymers, a number of papers have appeared dealing with oxidative chemiluminescence from a variety of polymers (8-16). In this chapter we continue the 1,4-polyisoprene work with a study of the low-temperature chemiluminescence emitted in the autoxidation of three additional elastomers, cis-1,4-polybutadiene, amorphous 1,2-polybutadiene, and fmns-polypentenamer. We also report the chemiluminescence obtained from singlet-oxygenated samples of cis-1,4-polybutadiene and trans-polypentenamer, as well as rate data for singlet oxygen reactions with the 1,4-polyisoprene, 1,4-polybutadiene, and model compounds in solution. 

Early attempts to bring about the degradation of c -polyisoprene by metathesis were unsuccessful, probably because of the short life of the catalyst (Ast 1970 Pampus 1970). The system WCl4[OCH(CH2Cl)2]2/Et2AlCl/anisole (1/5/5) is longer-lived and under favourable conditions can reduce the MW of polyisoprene dissolved in benzene at 25°C from 10 to 2 x lO" in 10 days, without any marked loss of unsaturation or cis double-bond content (Korshak 1982). 

During high-energy irradiation in vacuo, for example, from a Co source, some main-chain degradation of natural rubber and other polyisoprenes occurs ... 

Figure 2.1 Dependence of the degree of thermal degradation of natural rubber (cis-polyisoprene) (1), gutta percha (tr jns-polyisoprene) (2), and polyisoprene (3) on temperature. Source Author s own files...

The thermal degradation of diene polymers was the subject of several studies [456, 457]. The scheme for polyisoprene and polybutadiene degradation was postulated in part by Golub and Garguila [458, 460]. It is based on infra-red spectra and NMR studies of the products ... 

In addition, spectroscopic evidence shows that cis-trans isonaerizations as well as cyclizations occur in the process of thermal degradation of 1,4-polyisoprene. It is interesting that the cis-trans isomerizations were observed at temperatures as low as 200 C [460]. 

Sadaka, F., Campistron, I., Laguetre, A., Pilard, J.F. Telechelic oligomers obtained by metathetic degradation of both polyisoprene and styrene-butadiene rubbers. Appl. Recycl. Waste Tire Rubber 98, 736-742 (2013)... 

Thermo-oxidative studies have also been reported on polyisoprenes and on natural rubber. In the latter case, the effect of relative molecular weight on thermo-oxidation as evidenced by carbonyl group formation was investigated. A mechanism for formation of isoprene during thermal degradation of natural rubber has been proposed. It involves a cyclic intermediate. 

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