Polyisoprene—Natural Rubber

Rubber includes a range of polymers that are all elastomers. The first rubber was extracted from the sap of hevea trees in Central America, and the most famous natural polymer is polyisoprene. 

Fermentation of cellulose has been recommended as the basis of chemicals such as ethanol and acetic acid, but there is no commercial achievement till date. The derivatives of cellulose are one or more of the three hydroxyl groups present in glucopyranoside reiterating unit, e.g., methyl cellulose and hydroxyl-ethyl cellulose. 

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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. 

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. 

Materials that are constructed from organic polymers such as polyethylene, polystyrene, polyisoprene (natural rubber and a synthetic elastomer) and poly(vinyl chloride) are common features of our daily lives. Most of these and related organic polymers are generated from acyclic precursors by free radical, anionic, cationic or organometallic polymerisation processes or by condensation reactions. Cyclic precursors are rarely used for the production of organic polymers. 

Emulsion SBR (free-radical initiator) Polyisoprene (natural rubber)... 

Cis-1,4 polyisoprene (natural rubber or synthetic isoprene rubber) and trans-1,4 polyisoprene (balata or guttah-percha) show strongly different properties. 

The common name of this compound is isoprene and its ICJPAC name is 2-methyl-l, 3-butadiene. Polymerization of isoprene gives polyisoprene, natural rubber. Polymerization may lead to cis or trans versions of polyisoprene. 

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 >). 

Figure 5.1. Molecular structures of the chemical repeat units for common polymers. Shown are (a) polyethylene (PE), (b) poly(vinyl chloride) (PVC), (c) polytetrafluoroethylene (PTFE), (d) polypropylene (PP), (e) polyisobutylene (PIB), (f) polybutadiene (PBD), (g) c/5-polyisoprene (natural rubber), (h) traw5-polychloroprene (Neoprene rubber), (i) polystyrene (PS), (j) poly(vinyl acetate) (PVAc), (k) poly(methyl methacrylate) (PMMA), ( ) polycaprolactam (polyamide - nylon 6), (m) nylon 6,6, (n) poly(ethylene teraphthalate), (o) poly(dimethyl siloxane) (PDMS).

On the other hand, if the cross-link density is low (the length of the chains between cross-links is large) and the mobility of the chains is high, the cross-linked material is called an elastomer. An example of a typical elastomer is cw-l,4-polyisoprene (natural rubber), which, by means of a cross-linking reaction with sulfur (vulcanization), gives rise to a network structure (see Fig. 1.4). 

Figure 1.4 (a) cis-1,4-Polyisoprene (natural rubber), (b) Cross-linking reaction with sulfur (vulcanization). 

Figure 1.4 Vulcanization of natural rubber with sulfur, (a) Linear polyisoprene (natural rubber), (b) An idealized structure produced by vulcanization with sulfur. The number (x) of sulfur atoms in sulfide cross-linkages is 1 or 2 in efficient vulcanization systems but may be as high as 8 under conditions where cyclic and other structures are also formed in the reaction, (c) The effect of cross-linking is to introduce points of linkage or anchor points between chain molecules, restricting their slippage.

The basic form of the model of P D used in this work, which will be described elsewhere in detail, [22] treats the diffusion of small amounts of "simple" spherical penetrants, such as gas molecules, in "smooth-chained" polymers, such as poly (ethylene terephthalate) (PET) and cis-polyisoprene (natural rubber). [12] Whenever necessary, generalized equations are being used, for example for simple nonspherical penetrants [13] and for polymers which possess closely spaced, bulky side groups such as poly(vinyl chloride) (PVC). [14]... 

Common polymers like polyethylene, nylon, polyisoprene (natural rubber), and polystyrene have these properties to very diverse extents. From this point of view, polystyrene seems to be, by far, the best one. Another but less known polymer, polydimethylsiloxane, is perhaps even better, but its systematic use is more recent. 

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