Structure of Natural Rubber

Reproduced with permission from Y. Tanaka, A.H. Eng, N. Ohya, N. Nishiyama, J. Tangpakdee, S. Kawahara and R. Wititsuwannakul, Phytochemistry, 1996, 41, 1501. 

The absence of dimethylallyl-group in NR indicates that the initiating species for rubber formation in Hevea tree is not FDP, but FDP modified at the dimethylallyl-group, which is abbreviated here as (o [103,109,110]. This was confirmed by 13C-NMR analysis of in vitro polymerised rubber by incubation of the bottom fraction of fresh latex and isopentenyl diphosphate (IDP) [111]. The newly synthesised in vitro rubber formed in the presence of FDP and IDP showed the dimethylallyl group derived from FDP. On the other hand, no dimethylallyl group was detected in the in vivo rubber prepared without the addition of FDP [112]. 

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This discussion of the structures of diene polymers would be incomplete without reference to the important contributions which have accrued from applications of the ozone degradation method. An important feature of the structure which lies beyond the province of spectral measurements, namely, the orientation of successive units in the chain, is amenable to elucidation by identification of the products of ozone cleavage. The early experiments of Harries on the determination of the structures of natural rubber, gutta-percha, and synthetic diene polymers through the use of this method are classics in polymer structure determination. On hydrolysis of the ozonide of natural rubber, perferably in the presence of hydrogen peroxide, carbon atoms which were doubly bonded prior to formation of the ozonide... 

The synthesis of the exact molecular structure of natural rubber using a simple alkali metal focused increase attention on the mechanism of anionic polymerisations. 

Fig. 12. Crystal structure of natural rubber. According to Nyburg, molecules described by coordinates (x, y, 2) (A, B, C) may be statistically replaced by the isoclined molecules having coordinates (x, Vs— y, ) (A, B, C, D )...

From the time that isoprene was isolated from the pyrolysis products of natural mbber (1), scientific researchers have been attempting to reverse the process. In 1879, Bouchardat prepared a synthetic rubbery product by treating isoprene with hydrochloric acid (2). It was not until 1954—1955 that methods were found to prepare a high ar-polyisoprene which duplicates the structure of natural rubber. In one method (3,4) a Ziegler-type catalyst of trialkylaluminum and titanium tetrachloride was used to polymerize isoprene in an air-free, moisture-free hydrocarbon solvent to an all t /s- 1,4-polyisoprene. A polyisoprene with 90% 1,4-units was synthesized with lithium catalysts as early as 1949 (5). 

Structure of Natural Rubber Like many other plant products, natural rubber is a terpene composed of isoprene units (Section 25-8). If we imagine lining up many molecules of isoprene in the. v-cis conformation, and moving pairs of electrons as shown in the following figure, we would produce a structure similar to natural rubber. This polymer results from 1,4-addition to each isoprene molecule, with all the double bonds in the cis configuration. Another name for natural rubber is cis-1,4-polyisoprene. 

The all-cis structure of natural rubber is vita) to its elasticity. The all-trans compound is known and it is hard and brittle. Though dienes such as isoprene can easily be polymerized by cationic methods, the resulting rubber is not all-cis and has poor elasticity and durability. However, polymerization of isoprene in the Ziegler-Natta way gives an all-cis (90-95% at least) polyisoprene very similar to natural rubber. 

The usefulness of analytical pyrolysis in polymer characterization, identification, or quantitation has long been demonstrated. The first application of analytical pyrolysis can be considered the discovery in 1860 of the structure of natural rubber as being polyisoprene [10]. This was done by the identification of isoprene as the main pyrolysis product of rubber. Natural organic polymers and their composite materials such as wood, peat, soils, bacteria, animal cells, etc. are good candidates for analysis using a pyrolytic step. 

These particles are made from aggregates of 10 to 10 macromolecules of polyisoprene. The presence of the isoprene molecule in the structure of natural rubber makes it part of the polyterpenes family of compounds [1]. 

Due to the structure of natural rubber, with its conjugated system of double bonds and the ability to reduce the unwanted dynamic motions associated with a mounting system, isolation is its most important use. However, natural rubber use in seals and gaskets is a natural fit. The automotive chemist chooses natural rubber for several reasons. The elastic behavior of rubber can be attributed to electrostatic strain... 

Figure 22. Proposed cross-linked structure of natural rubber vulcanized with dlcumyl peroxide.

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)... 

Fig. 6.9 The chemical structure of natural rubber, m = methyl group. (Hydrogen atoms are not shown.)...

Rubber hydrocarbon is the principle component of raw rubber. The subject is discussed in greater detail in Chapter 7. Natural rubber is 97% cw-l,4-polyisoprene. It is obtained by tapping the bark of rubber trees (Hevea brasiliensis) and collecting the exudate, a latex consisting of about 32-35% rubber. A similar material can also be found in the sap of many other plants and shrubs. The structure of natural rubber has been investigated over 100 years, but it was only after 1920, however, that the chemical structure was elucidated. It was shown to be a linear polymer consisting of head-to-tail links of isoprene units, 98% bonded 1,4. 

J. T. Sakdapipanich and P. Rojruthai, Molecular Structure of Natural Rubber and Its Characteristics Based on Recent Evidence, Biotechnology -Molecular Studies and Novel Applications for Improved Quality of Human Life, ed. R. Sammour, InTech, 2012. 

The earliest emulsion polymers are those found in nature. Natural rubber (NR) latexes have been extracted from the rubber tree (Hevea brasiliensis) (286) for hundreds of years. The latexes are comprised of dispersions of high molecular weight, linear cis-1,4-polyisoprene (210) particles ranging in diameter from 10 nm to several microns. Because the natural mbber particles are in a colloidal form, they must first be separated from the aqueous phase by coagulation before processing. Since the molecular structure of natural rubber is stereoregular, it has excellent mechanical properties that have not been duplicated by modem synthetic mbbers. 

As already stated, the cis form is the structure of natural rubber. The trans form occurs naturally as gutta percha. Both forms can be manufactured synthetically. 

Figure A5.1.1 The spiral structure of natural rubber proposed to explain long-range elasticity.

Figure 5.1.35 Molecular structure of natural rubber (c/s-1,4-polyisoprene) and of gutta-percha (trans-1,4-polyisoprene), which has no importance for technical applications.

Whilst Faraday had shown in 1826 that rubber was a hydrocarbon of empirical formula CsHg few further important developments occurred until the closing years of the 19th century when the structure of natural rubber began to be progressively revealed, a process which only became substantially complete about 1930 and which is considered in the next chapter. 

In earlier chapters the chemical structure of natural rubber, the molecular nature of high elasticity and, in outline, the general dependence of properties on structure were considered. 

The 1,4- polydiene rubbers may have a variety of cis-ltrans- ratios ranging from the 100% cis- structure of natural rubber to polybutadienes with a trans- content in excess of 99%. In the late 1950s it was found that these polymers could be chemically treated in such a way that the cis-ltrans- ratio of an already formed polymer was altered. This process is known as cis-ltrans- isomerization and in the case of natural rubber leads to products with interesting properties. 

Fig. 11.9 Structures of natural rubber cis, natural, herea and trans gutta, percha, balata...

Trans-1,4-polyisoprene Figure 8.1 Structure of natural rubber. 

Syn. Ozonization G. Ozonisierung R ozonolyse O. is the splitting of a double bond by ozone. The reaction was and is used in the laboratory to identify structures of chemical compounds (e.g., structure of natural - rubber by Harries, 1905). 

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