Oxidation of polyisoprene

In the paper by Ptschelintsev et al. [28] on the oxidation of polyisoprene, it was reported that one recombination event of free radicals leads to scissions of... 

The comparison of measurements for oxidation of polyisoprene indicates that the increase of chemiluminescence precedes that of oxygen uptake (Figure 12) and that a maximum of chemiluminescence is situated around an inflexion point of the oxygen absorption curve. 

Figure 12 Chemiluminescence intensity and absorption of oxygen [43] scans for oxidation of polyisoprene in oxygen, temperatures 90°C and 100°C.

Mayo et al. [422] found that methylene blue, 3-ethoxy-5-methoxy-xanthone, crystal violet and acridine red sensitize the photo-oxidation of polyisoprene in chlorobenzene on exposure to UV light. The reaction rate is highest when the concentration of the dye is 10-4 mole 1 1. The addition of 2,6-di-tert-butyl-4-methylphenol to the solution does not stop the photo-oxidation reaction but merely reduces the quantity of oxygen absorbed. It may be concluded that the oxidation cannot occur by the free radical mechanism. 

Photoactive Additives.—Ferric compounds, in particular, the chloride, continue to attract much interest as photosensitizers for thermoplastics. " From e.s.r. work the mechanism appears to involve a redox reaction resulting in the formation of active hydroxy-radicals. Photodegradable polyethylene film has been developed by doping it with radiation-modified atactic polypropylene and hydroxyethyl-ferrocene. Several workers have studied the dye-sensitized photo-oxidation of polyisoprene and di-n-butyl sulphide embedded in Augustyniak and... 

Photo-oxidation of cis-polybutadiene was also shown to involve singlet oxygen [570]. Attacks by singlet oxygen on double bonds with formation of allylic hydroperoxides and shifts of these double bonds according to the ene reaction were confirmed by several other studies [571]. On the other hand, a study of model compounds for photo-oxidation of polyisoprene failed to show formation of endo peroxides by 1,4-cycloaddition [572]. 

Polyisoprene.—Several papers have been published on the kinetics and mechanism of the photo-oxidation of polyisoprene induced by singlet oxygen/ " The trans >C=C< groups were found to be more reactive than the cis groups/ ... 

Kuz minskii [8] demonstrated the general features by a direct method. He observed an extensive oxidation of polyisoprene during mastication at temperatures above 80°C. He investigated also the influence of a mechanical activation on oxidation. He followed the appearance of infrared (IR) absorption bands for the carbonyl (1750-1720 cm" ) and hydroxyl groups... 

Mill, T., K. C. Irwin, and F. R. Mayo Accelerated oxidations of polyisoprene. III. Photosentitized oxidations in solution. Rubber Chem. Technol. 41. 296 (1968). 

X. Cohn, L. Audouin, J. Verdu, M. Le Huy. Kinetic modelling of the thermal oxidation of polyisoprene elastomers. Part 2 Effect of sulfur vulcanization on mass changes and thickness distribution of oxidation products during thermal oxidation. Polymer Degradation and Stability 92(5), 898-905, May (2007). 

Elastomers. Elastomers are polymers or copolymers of hydrocarbons (see Elastomers, synthetic Rubber, natural). Natural mbber is essentially polyisoprene, whereas the most common synthetic mbber is a styrene—butadiene copolymer. Moreover, nearly all synthetic mbber is reinforced with carbon black, itself produced by partial oxidation of heavy hydrocarbons. Table 10 gives U.S. elastomer production for 1991. The two most important elastomers, styrene—butadiene mbber (qv) and polybutadiene mbber, are used primarily in automobile tires. 

Figure 3 shows how the elementary structure of the polymer affects the observed patterns of chemiluminescence response in oxygen at 120°C. As expected, the oxidizability decreases in the order polyisoprene < polybutadiene < polypropylene < polyethylene. However, it is difficult to understand why the maximum light emission is almost 2x lower for polybutadiene than for PP. The oxidation of polybutadiene occurs via secondary peroxyl radicals, while in... 

Suppression of the Thermo-Oxidative Crosslinking of Polyisoprene by Addition of an Antioxidant... 

Because of the content of double bonds and a tertiary C-atom, polyisoprene undergoes numerous chemical transformations by the action of oxygen, light, or heat, like chain scission and crosslinking.The addition of suitable stabilizers can suppress these reactions even over a period of years. In the case of the thermo-oxidative crosslinking of polyisoprene this effect can be shown with the following experiment. 

Pyrolysis of synthetic polyisoprene in the presence of oxygen also is expected to be identical to that of natural rubber. Thermal oxidation of natural rubber is assumed always to be associated with scission, although photo-oxidation at low temperature may involve peroxide formation without scission. The effect of oxygen is to increase the reaction rate of scission and therefore to decrease the temperature where the scission starts. The oxidation may take place after the initial formation of a free radical that reacts with oxygen ... 

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. 

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]. 

Uncontrolled oxidation of rubber is detrimental to its physical properties. Oxidation reactions take place readily at unsaturated groups in polymers and are often referred to collectively as epoxidation however, oxidation under controlled conditions can lead to useful products such as the epoxidized natural rubber introduced by the Malaysian Rubber Producers Association (Schults etal., 1983 Cunneen and Porter, 1965 Ceresa, 1965 Avery and Watson, 1956). Natural rubber in the latex form is treated with hydrogen peroxide dissolved in acetic acid. This gives 50% epoxidized natural rubber. This rubber shows very interesting physical properties and excellent carbon black dispersion. Similarly, nonaqueous epoxidizations of synthetic polyisoprene can be achieved... 

Figure 8 Specific viscosity r] p at 25 C of microemulsion containing AOT, isooctane, and water (mass water/AOT ratio of 2.5) as a function of the amount of added triblock copolymer PEO-PI-PEO [PEO = poly(ethylene oxide) PI =polyisoprene], (From Ref 135.)...

In a continuation of the studies reported so far triblock terpolymers of polyisoprene (PI)—poly(2-vinyl pyridine) (P2VP)—polyethylene oxide (PEO) was synthesized using benzyl potassium as the initiator and sequential addition of monomers [44]. The polymerization of isoprene and 2-vinylpyridine was carried out in THF at - 78 C. Ethylene was distilled into the solution and polymerized at 50 °C for 5d. Termination was effected by... 

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