Isomerized natural rubber

FIGURE 26.56 Log Abrasion loss by a blade (solid lines) and log cut growth rate (dashed hnes) of noncrystallizing rubber compounds as function of log frictional and log tearing energy, respectively isomerized natural rubber (NR), 2 styrene-butadiene rubber (SBR), and 3 acrylate-butadiene rubber (ABR). (From Champ, D.H., Southern, E., and Thomas, A.G., Advances in Polymer Friction and Wear, Lieng Huang Lee (ed.), Plenum, New York/London, 1974, p. 134.)... 

Gutta-percha. The name, derived from Malayan ge-tah pertcha=latex of the percha tree, for a natural rubber (structure, see there) from the gutta-percha trees Palaquium gutta and P. oblongifolia, Sapotaceae) with properties similar to those of balata. In Sumatra, Java, and south east India, the rapidly coagulating latex of incised trees is collected, rapidly kneaded, and marketed as raw G. Pure G. is the all-trans-isorntr of polyisoprene, related to balata molecular mass ca. 100000. In contrast to the cis-isomeric natural rubber, G. is hard and less elastic but not brittle, it softens at 25-30°C, becomes plastic at 60 °C, and melts at >100°C with decomposition and formation of a sticky mass. For uses, see literature. 

Partially isomerized natural rubber has been of some interest as a non-crystallizing material (Cunneen and Higgins, 1%3) with better rubberiness at low temperatures than usually experienced with natural rubber (comparisons being made in the vulcanized state). Commercially, however, the material has not been able to compete with oil-extended or plasticized rubber blends, in particular those containing polybutadiene or with synthetic polyisoprenes. 

It appears that the glass transition temperature of an amorphous polymer is an important characteristic. Grosch obtained the isothermal friction-speed curves of four different rubber compounds on a glass countersurface. The rubber compounds used were styrene-butadiene, aery Ionitrile butadiene, butyl and isomerized natural rubber. The curves were all reduced to 20 C, making use of the W.L.F. transform. The peak values of friction were found to occur at different speeds. In this series, acrylonitrile butadiene rubber has the highest Tg. It attains its peak value of friction at the lowest of the four speeds. At the other extreme, isomerized natural rubber having a low Tg, attains its peak friction at the highest of the four speeds. The other two compounds lie between the extremes in the expected order. 

Fig. 5b. Comparison of abrasion and crack growth data. Isomerized natural rubber o abrasion crack growth...

Related to stereoregularity is the possibility of cis, trans isomerism. The molecule of natural rubber is a c/s-1,4-polyisoprene whilst that of gutta percha is the trans isomer. 

An isomeric derivative of natural rubber produced by a variety of chemical treatments. The products are resinous and find application as reinforcing agents in natural rubber compounds (particularly in shoe soles) and as rubber-to-metal adhesives. 

Such differences in structure can have a profound effect on the physical properties of a polymer. Thus natural rubber, which comprises cis-1,4-poly(isoprene), is a soft rubbery material at room temperature, whereas guttapercha, which comprises the corresponding ftms-isomer, is semi-crystalline and hard. The method of polymerisation determines the isomeric form of the polymer. 

Irradiation of purified natural rubber films results in gas evolution with a quantum yield [53] of about 10 3 and insolubilization of the polymer. However, the major photochemical processes in vacuo are cis—trans isomerization, loss of unsaturation and formation of cyclopropyl groups [49] with quantum yields estimated at 0.041, 0.083 and 0.018, respectively [47]. An over all mechanism identical to that already... 

Hydrocarbon Solvents One of the most important synthetic and commercial aspects of anionic polymerization is the ability to prepare polydienes [poly(l,3-dienes)] with high 1,4-microstructure using lithium as the counterion in hydrocarbon solutions [3, 156]. The key discovery was reported in 1956 by scientists at the Firestone Tire and Rubber Company that polyisoprene produced by lithium metal-initiated anionic polymerization had a high (>90%) cm-1,4-microstructure similar to natural rubber [47], In general, conjugated 1,3-dienes [CH2=C(R)-CH=CH2] can polymerize to form four constimtional isomeric microstructures as shown below. The stereochemistry of the anionic polymerization of isoprene and... 

FIGURE 3.1 Magnified portion of NMR spectrum of natural rubber vulcanized to half its maximum torque. The peak at 16 ppm arises due to cis-to-trans isomerization (Mori and Koenig, 1998). 

Halogenation reactions of unsaturated polymers follow two simultaneous paths, ionic and free radical. Ionic mechanisms give soluble products from chlorination reactions of polybutadiene." The free-radical mechanisms, on the other hand, cause crosslinking, isomerization, and addition products. If the free-radical reactions are suppressed, soluble materials form. Natural rubber can be chlorinated in benzene with addition of as much as 30% by weight of chlorine without cycliza-tion. " Also, chlorination of polyalkenamers, both cis and trans, yields soluble polymers. X-rays show that the products are partly crystalline. The crystalline segments obtained from 1,4-trans-polyisoprene are diisotactic poly( 0 /rw-dichlorobutamer)s while those obtained from the 1,4-cis isomer are diisotactic polyOAfieo-l,2-dichlorobutamer)s. ... 

Polyisoprenes and polybutadienes can also be modified by reactions with carbenes. Dichloro-carbene adds to natural rubber dissolved in chloiofoim in a phase transfer reaction with aqueous NaOH and a phase transfer reagent. Solid sodium hydroxide can be used without a phase transfer reagent. There is no evidence of cis-trans isomerization and the distribution of the substituents is random. ... 

Polymers may be further classified as cis-isomer and tram-isomer, based on the geometrical isomerism of the repeating units. Examples are cis-, 4-polyisoprene (natural rubber) and tram-1,4-polyisoprene (Gutta percha, plastic). There are also three different classifications of polymers based on the chemical constituents present in the structures. 

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