Natural Poly isoprene

Over 2000 plants produce polyprenes, mostly in a mixture with terpenes and waxes. Today, almost all natural rubber is taken from trees of the Hevea brasiliensis species. One hectare of trees yields 500-2000 kg of latex per year. The trees are broached with angular cutters and the latex is 

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Mention may here be made of the fact that natural unvulcanised rubber is poly-isoprene (2-methylbutadiene) ... 

The elasticity of a polymer is its ability to return to its original shape after being stretched. Natural rubber has low elasticity and is easily softened by hearing. Flowever, the vulcanization of rubber increases its elasticity. In vulcanization, rubber is heated with sulfur. The sulfur atoms form cross-links between the poly-isoprene chains and produce a three-dimensional network of atoms (Fig. 19.17). Because the chains are covalently linked together, vulcanized rubber does not soften as much as natural rubber when the temperature is raised. Vulcanized rubber is also much more resistant to deformation when stretched, because the cross-... 

The polymer in natural rubber consists almost entirely of ci -poly(isoprene) (1.6). The molecules are linear, with relative molar mass typically lying between 300 000 and 500 000. The macromolecular nature of rubber was established mainly by Staudinger in 1922, when he hydrogenated the material and obtained a product that retained its colloidal character, rather than yielding fragments of low relative molar mass. 

Successive 1,4 units in the synthetic polyisoprene chain evidently are preponderantly arranged in head-to-tail sequence, although an appreciable proportion of head-to-head and tail-to-tail junctions appears to be present as well. Apparently the growing radical adds preferentially to one of the two ends of the monomer. Which of the reactions (6) or (7) is the preferred process cannot be decided from these results alone, however. Positive identification of both 1,2 and 3,4 units in the infrared spectrum shows that both addition reactions take place during the polymerization of isoprene. The relative contributions of the alternative addition processes cannot be ascertained from the proportions of these two units, however, inasmuch as the product radicals formed in reactions (6) and (7), may differ markedly in their preference for addition in one or the other of the two resonance forms available to each. We may conclude merely that structural evidence indicates a preference for oriented (i.e., head-to-tail) additions but that the 1,4 units of synthetic polyisoprene are by no means as consistently arranged in head-to-tail sequence as in the naturally occurring poly-isoprenes. 

EPR Ethylene-propylene rubber NR Natural rubber poly isoprene... 

A similar comparison can be made with cis-poly(isoprene), natural rubber, by taking advantage of the fact that the polymer is very slow to crystallize [164], Consequently, the comparison can be made between the supercooled, noncrystalline polymers at 0°C and the semi-crystalline polymer (31% crystalline) at the same temperature. The Tlc values for each of the five carbons involved were again found to be the same for the completely disordered polymer and the semicrystalline one, so that a similar conclusion can be made with regard to their chain structure. 

C. J. Carman Earlier in your talk you showed the carbon Ti data and NOEF for partially crystalline and amorphous poly-isoprenes. Was this a natural rubber which had been allowed to crystallize to different degrees or was this a synthetic rubber ... 

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. 

RUBBER (Synthetic). Any of a group of manufactured elastomers that approximate one or more of the properties of natural rubber. Some of these aie sodium polysulfide ( Thiokol ). polychloiopiene (neoprene), butadiene-styrene copolymers (SBR), acrylonitrilebutadiene copolymers (nitril rubber), ethvlenepropylene-diene (EPDM) rubbers, synthetic poly-isoprene ( Coral, Natsyn ), butyl rubber (copolymer of isobutylene and isoprene), polyacrylonitrile ( Hycar ). silicone (polysiloranei. epichlorohy-drin, polyurethane ( Vulkollan ). 

Fig. 4 Renewable platform chemicals used in olefin metathesis (a) plant oils and fatty acids, (b) terpenes and terpenoids, (c) phenylpropanoids, (d) natural rubber (cw-1,4-poly isoprene), (e) carbohydrates, (f) amino acids and peptides, and (g) furans...

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. 

Several other elastic materials may be made by copolymerising one of the above monomers with lesser amounts of one or more monomers. Notable amongst these are SBR, a copolymer of butadiene and styrene, and nitrile rubber (NBR), a copolymer of butadiene and acrylonitrile. The natural rubber molecule is structurally a c/i-1,4-poly isoprene so that it is convenient to consider natural rubber in this chapter. Some idea of the relative importance of these materials may be gauged from the data in Table 11.14. 

The best-known elastomer is natural rubber, poly-isoprene (Scheme 6.10). Isoprene (Scheme 6.10a) is a liquid at room temperature, which polymerises readily to give the elastomer polyisoprene (Scheme 6.10b). The polymerisation produces two main geometrical isomers (see Section S2.1.) Natural rubber is the aU-c form of polyisoprene (Scheme 6.10c), in which the methyl (—CH3) groups and hydrogen (H) atoms are on the same side of the carbon-carbon double bond. Rubber latex, a milky liquid, is a suspension of rubber in water. It is found in many plants (e.g. in dandelions) as well as mbber... 

Scheme 6.10 (a) The structure of an isoprene molecule. (b) Bond redistribution and subsequent polymerisation to form poly(isoprene). (c) The structure of natural rubber, all-cri-poly(isoprene). (d) The structure of guttapercha, all-rrani -poly(isoprene)... 

Natural rubber (poly-isoprene) Yellow-brown Green-violet-blue... 

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