Natural rubber isomers

Figure 1.3 shows several repeat units of cis-l,4-polyisoprene and trans-1,4-polyisoprene. Natural rubber is the cis isomer of 1,4-polyisoprene, and gutta-percha is the trans isomer. 

Figure 1.3 1,4-polyisoprene with R=CH3 (a) cis isomer natural rubber (b) trans... 

Polymers of chloroprene (structure [XII]) are called neoprene and copolymers of butadiene and styrene are called SBR, an acronym for styrene-butadiene rubber. Both are used for many of the same applications as natural rubber. Chloroprene displays the same assortment of possible isomers as isoprene the extra combinations afforded by copolymer composition and structure in SBR offsets the fact that structures [XIIll and [XIV] are identical for butadiene. 

Although the conditions of the polymerization reaction may be chosen to optimize the formation of one specific isomer, it is typical in these systems to have at least some contribution of all possible isomers in the polymeric product, except in the case of polymers of biological origin, like natural rubber and gutta-percha. 

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. 

Polymers containing each of these configurations are known, the most common being the cis- A and the 1,4-isomers. The first of these, poly(c/ -l,4-isoprene), is the macromolecular constituent of natural rubber the second is the material known as gutta percha. The latter, unlike natural rubber, has no elastomeric properties, but has a leathery texture. It has been used for diverse applications such as golf-ball covers and as an insulating material for the trans-Atlantic cables of the late nineteenth century. 

Figure 13 Chemical structures of trans and cis isomers of 1,4 polyisoprene (gutta-percha and natural rubber, respectively).

The determination of the various types of geometric isomers associated with unsaturation in Polymer chains is of great importance, for example, in the study of the structure of modern synthetic rubbers. In table below are listed some of the important infrared absorption bands which arise from olefinic groups. In synthetic "natural" rubber, cis-1, 4-polyisoprene, relatively small amounts of 1, 2 and 3, 4-addition can easily be detected, though it is more difficult to distinguish between the cis and trans-configurations. Nuclear magnetic resonance spectroscopy is also useful for this analysis. 

Natural rubber and guttapercha consist essentially of polyisoprene in a s-1,4 and trans-1,4 isomers, respectively. Commercially produced synthetic polyisoprenes have more or less identical structures but reduced chain regularity, although... 

Natural rubber (NR) and guttapercha consist essentially of polyisoprene in cis-l, 4 and trans-1,4 isomers, respectively. Commercially produced synthetic polyisoprenes have more or less identical structure but reduced chain regularity, although some may contain certain proportions of 1,2- and 3,4-isomers. Microstructure differences not only cause the polymers to have different physical properties but also affect their response to radiation. The most apparent change in microstructure on irradiation is the decrease in unsaturation. It is further promoted by the addition of thiols and other compounds.130 On the other hand, antioxidants and sulfur were found to reduce the rate of decay of unsaturation.131 A significant loss in unsaturation was found, particularly in polyisoprenes composed primarily of 1,2- and 3,4-isomers.132,133... 

Natural rubber is a polymer of isoprene- most often cis-l,4-polyiso-prene - with a molecular weight of 100,000 to 1,000,000. Typically, a few percent of other materials, such as proteins, fatty acids, resins and inorganic materials is found in natural rubber. Polyisoprene is also created synthetically, producing what is sometimes referred to as "synthetic natural rubber". Owing to the presence of a double bond in each and every repeat unit, natural rubber is sensitive to ozone cracking. Some natural rubber sources called gutta percha are composed of trans-1,4-poly isoprene, a structural isomer which has similar, but not identical properties. Natural rubber is an elastomer and a thermoplastic. However, it should be noted that as the rubber is vulcanized it will turn into a thermoset. Most rubber in everyday use is vulcanized to a point where it shares properties of both, i.e., if it is heated and cooled, it is degraded but not destroyed. 

Gutta-percha, the trans 1,4-isomer of natural rubber, is hard and brittle at room temperature. The reason for the difference in properties between the cis and trans isomers readily can be seen by inspecting molecular models. The chains with trans double bonds are able to lie along side of each other, forming a semicrystalline array, as shown in Figure 13-2. This ordered arrangement cannot be deformed easily, hence the material is hard and brittle. However, when the double bonds are cis, steric hindrance prevents the chains from assuming a similar ordered structure and the bulk of the material exists in a... 

The coordination catalysts were quickly extended to dienes and found to produce the long-sought objective of a "synthetic natural rubber, i.e., cis-1,4-polyisoprene. cis-1,4-Polybutadiene was also quickly produced. These were, and still are, erroneously referred to as stereo rubbers. They are actually unique geometric isomers rather than stereoisomers, but the name stereo rubber became established probably because of the relationship in time and catalyst usage to stereo olefin polymerization. 

Thus, the reactions of diene rubber with sulfur produce a variety of sulfurised structures. A schematic representation of different types of sulfurised structures in natural rubber (NR) is shown in Figure 9.1. The type of sulfurisation has been designated as Al, A2, Bl, B2 and Cl-types depending on the positions of sulfide attachment as seen in Figure 9.1 [15]. The c and t indicate cis and trans isomers of the structures. The main chain, saturated... 

The polymer in natural rubber (from the Hevea bmsiliensis tree) is pure cis polyisoprene gutta percha and balata are composed of the trans isomer. 

The discovery of. stereospecific polymerization methods, which led to the production of practically pure cis-1,4 polyisoprene (natural rubber contains 85 per cent of this isomer), raises the problem of the economic production of isoprene monomer (bPi.013 = 340c, df =0.681 >). 

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. 

Aj5 noted in Natural Rubber at the end oT Chopter 7. rubber is a urally occurring polymer of iaoprene. The double bonds of rubber have stereochemistry, but gufJa-pt rcha, the iS isomer of rubber, also occurs urally Harder and more brittle than rubber, gutta>perdia has a variety minor applications, including occasional use as the covering on golf... 

It is obtained from latex extracted from the Hevea brasiliensis tree. There exists another structural isomer called gutta-percha formed from po y trans-1,4-isoprene), whose elastic properties differ from those of natural rubber. 

As occurs in natural rubber, only the 1,4-cis isomer exhibits elastomeric characteristics. The most important synthetic diene rubbers are polychlor-oprene (neoprene) and rubbers derived from butadiene such as styrene-butadiene and acrylonitrile-butadiene copolymers. 

The stability of latex is due to a thin layer of proteins on particles, which acts as a colloid stabilizer. Natural rubber is practically obtained by the precipitation and drying of the latex. The precipitation is done with acids (acetic acid is commonly used for this purpose) when the isoelectric point of the protecting protein is reached (pH 4.6). The macromolecules have a MW between 5 10 to 3 10 Dalton and contain between 600 to 50,000 units of isopentene. Due to the double bond, both cis and trans isomers are possible for the monomer units. It was determined that natural rubber is an isotactic polymer formed exclusively from cis units and has the following (idealized) structure (in reality the polymer is not perfectly planar) ... 

The complete hydrogenation of natural rubber (the addition of H2 to all double bonds) gives a product that is indistinguishable from the product of the complete hydrogenation of gutta-percha. Explain how this strengthens the conclusion that these two substances are isomers of each other. 

Note that in the cis isomer the two CH2 groups are on the same side of the C=C bond, whereas the same groups are across from each other in the trans isomer. Natural rubber is poly-CM-isoprene, which is extracted from the tree Hevea brasiliensis (Figure 25.4). 

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