Polyisoprene naturally occurring

Isoprene may be the naturally occurring alkene with the greatest economic impact. This compound, a major component of the sap of the rubber tree, is used to make the long-chain molecules of natural rubber (polyisoprene). As we describe in Chapter 13. the synthetic rubbers that make up most of today s tires are made from other alkenes. 

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. 

We might well expect this differing stereochemistry to have a marked effect on the properties of the polymer, and this is borne out by the two naturally occurring polyisoprenes, natural rubber and gutta percha. The former, which before vulcanisation is soft and tacky, has all cis junctions in its chains while the latter, which is hard and brittle, has all trans junctions. 

Polymerised isoprene. Naturally-occurring polyisoprenes are natural rubber (cA-form) and gutta percha (trans- form). The use of stereo specific catalysts has made possible the manufacture of synthetic cA-polyisoprene and fraws-polyisoprene both of which are now available commercially. 

The hardness of some of the polydisperse naturally occurring crystalline polymers is the result of a trans configuration in 1,4-polyisoprene (Figures 9.4 and 9.5). This geometry facilitates good fit between chains and results in inflexibility in the chains. The hevea or NR is, by comparison, soft because it is a cis configuration (Figures 9.4 and 9.5) that does not allow a... 

The most fundamental classification of polymers is whether they are naturally occurring or synthetic. Common natural polymers (often referred to as biopolymers) include macromolecules such as polysaccharides e.g., starches, sugars, cellulose, gums, etc.), proteins e.g., enzymes), fibers e.g., wool, silk, cotton), polyisoprenes e.g., natural rubber), and nucleic acids e.g., RNA, DNA). The synthesis of biodegradable polymers from natural biopolymer sources is an area of increasing interest, due to dwindling world petroleum supplies and disposal concerns. 

The chemical structure of naturally occurring ais polyisoprenes was determined by 13C NMR spectroscopy using acyclic terpenes and polyprenols as model compounds. The arrangement of the isoprene units along the polymer chain was estimated to be in the order dimethylallyl terminal unit, three trans units, a long block of ais units, and ais isoprenyl terminal unit. This result demonstrates that the biosynthesis of cis-polyisoprenes in higher plants starts from trans,trans,trarcs-geranylgeranyl pyrophosphate. ... 

This paper describes the structural analysis of naturally occurring ois-1,4 polyisoprenes using 13C NMR spectroscopy. First, the structural characterization of polyprenols, which are linear isoprenoid compounds containing 30 to 100 carbons, was carried out on the basis of information obtained from acyclic terpenes having various ois and trans isoprene units as model compounds. This method was also applied to the structural analysis of polyisoprenes. The elucidation of the structure of the end groups and the arrangement of isoprene units provides information on the mechanism of the biosynthesis of polyprenyl compounds in nature. 

Naturally occurring polymers include proteins, nucleic acids, cellulose (polysaccharides), and rubber (polyisoprene). Most synthetic polymers are organic compounds. Familiar examples are nylon, poly(hexamethylene adipamide) Dacron, poly (ethylene terephthalate) and Lucite or Plexiglas, poly(methyl methacrylate). 

Polymers are mainly divided into two groups, natural polymers, such as proteins, cellulose, silk and synthetic polymers, such as polystyrene, polyethylene, and nylon. In some cases, naturally occurring polymers can also be produced synthetically. An important example is natural rubber which is known as polyisoprene... 

Polyisoprenes occur in nature. They are also prepared synthetically. Most commercial processes try to duplicate the naturally occurring material. 

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