Gutta percha, 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. 

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. 

Natural mbber (Hevea) is 100% i7j -l,4-polyisoprene, whereas another natural product, gutta-percha, a plastic, consists of the trans-1,4 isomer. Up until the mid-1900s, all attempts to polymerize isoprene led to polymers of mixed-chain stmcture. 

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. 

As noted in the Chapter 7 Focus On, rubber is a naturally occurring polymer of isoprene, or 2-methyl-l,3-butadiene. The double bonds of rubber have Z stereochemistry, but gutta-percha, the E isomer of rubber, also occurs naturally. Harder and more brittle than rubber, gutta-percha has a variety of minor applications, including occasional use as the covering on golf balls. 

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).

Which is a trans isomer (a) gutta percha or (b) hevea rubber ... 

This polymer is a hard plastic that occurs naturally as gutta-percha or balata. Since the trans isomer packs better than the cis isomer, it has a higher specific gravity, a higher degree of crystallinity, and a higher melting point (67 C) than the ds isomer of polyisoprene. The chemical and the solvent resistance of the trans polymer are simitar to those of the ds polymer. 

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... 

There is another product called gutta-percha, which is a geometric isomer of rubber, but is useless as an elastomer. Draw its structure and identify which is cis and which is trans. 

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

Chemically, chicle is a transpolyisoprene isomer of gutta percha. When heated above 60 °C it softens. Chicle was the original gum in chewing gum. It is claimed that it was originally brought to the United States of America by General Di Santa Anna who led the Mexicans at the Alamo. 

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. 

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. 

Gutta-Percha, The purified, coagulated, milky exudate of various trees of the genus Palaquium, Sapotaceae. Habit. Malayan Archipelago. Extensive review Williams, Earn. Hot. 18, 3-26 (1964). Defined as a trans isomer of rubber. Rubber has a repeat period of 8.2 A, whereas a-gutta-percha has 8.7 A and 0-gut la-pare ha has 4.8 A. The short period of 0-gutta-percha identifies it almost uniquely as an all -traits polyisoprene. 

The polymer industry traces its beginning to the early modifications of shellac, natural rubber (NR — an amorphous c -l,4-polyisoprene), gutta-percha (GP — a semi-crystalline trfl i-l,4-polyisoprene), and cellulose. In 1846, Parkes patented the first polymer blend NR with GP partially co-dissolved in CSj. Blending these two isomers resulted in partially crosslinked (co-vulcanized) materials whose rigidity was controllable by composition. The blends had many apphcations ranging from picture frames, table-ware, ear-trumpets, to sheathing the first submarine cables. 

The polymer of isoprene is called polyisoprene. It exists in two forms, cis- and frons-polyisoprene. The two forms are called geometric isomers. They have the same kind and number of atoms, but the atoms are arranged differently in the two forms. Natural rubber consists of trans-polyisoprene, while another product found in rubber plants, gutta percha, is made of c/s-polyisoprene. 

Since ozonolysis of Gutta percha gives the same product as natural rubber, the only structural formula for Gutta percha (from our list of possibles) would be the trans isomer of natural rubber. Hence, Gutta percha is also known as poly-trans-isoprene ... 

Gutta percha consists primarily of the polymer of trans-l,4-isoprene (Fig. 10.2). It is a structural isomer of natural rubber, which is CM-l,4-polyisoprene, and the structure of gutta percha means that it has greater linearity and crystallizes more readily than natural rubber. These features mean that gutta percha is harder, less tough and more brittle than its structural isomer [11],... 

Chapter 8 deals with various reactions of polymers including those of natural and synthetic rubber. That includes vulcanization of rubber. While there are very many commercial applications of the cis isomer, gutta-percha utilization is limited to wire coatings, impregnation of textile belting, and as a component of some varnishes. Its use is limited, because it is considerably harder than natural rubber. 

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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