Rubber cis-1 ,4-Polyisoprene

Natural rubber, cis-1,4-polyisoprene, cross-linked with sulfur. This reaction was discovered by Goodyear in 1839, making it both historically and commercially the most important process of this type. This reaction in particular and crosslinking in general are also called vulcanization. 

The determination of the various types of geometric isomers associated with unsaturation in Polymer chains is of great importance, for example, in the study of the structure of modern synthetic rubbers. In table below are listed some of the important infrared absorption bands which arise from olefinic groups. In synthetic "natural" rubber, cis-1, 4-polyisoprene, relatively small amounts of 1, 2 and 3, 4-addition can easily be detected, though it is more difficult to distinguish between the cis and trans-configurations. Nuclear magnetic resonance spectroscopy is also useful for this analysis. 

Chain orientation influences the melting point, which is understandable from the equation Tm - AH/AS. When the polymer is stretched in the rubbery phase, its entropy is already so much decreased (see Figure 4.3 and, later on, in 5.1), that AS has decreased. For natural rubber (cis-1.4-polyisoprene) a relation between Tm and the strain has been established experimentally as shown schematically in Figure 4.4. 

Alternatively, thermolysis yields the terminal alkene RCH=CH2- Note that, if propene or higher alkenes are used instead of ethene, then only single insertion into Al-C occurs. This has been commercially exploited in the catalytic dimerization of propene to 2-methylpentene-I, which can then be cracked to isoprene for the production of synthetic rubber (cis-1,4-polyisoprene) ... 

Figure 9.5 Representation of two primary chains of natural rubber (cis-1,4 polyisoprene) linked by vulcanization with sulfur.

Natural rubber CIS-1,4-polyisoprene (>99%) Sulfur Good Good... 

During the last two decades, a number of diene homopolymers and copolymers have been developed to fill the diverse elastomer needs in the production of tires. The earliest developments were mainly concerned with the preparation of stereoregular cis-1,4-polyisoprene, as a substitute for natural rubber, using... 

A convenient term for any material possessing the properties of a rubber but produced from other than natural sources. A synthetic version of natural rubber has been available for many years with the same chemical formula, i.e., cis-1,4-polyisoprene, but it has not displaced the natural form. See also Butyl Rubber, Chloroprene Rubber, Ethylene-Propylene Rubber, Nitrile Rubber, Silicone Rubber and Styrene-Butadiene Rubber. 

Natural rubber may be considered as a linear polymer of isoprene (2-methyl-1, 3-butadiene) and is also called as cis -1,4-polyisoprene. 

The structural uniformity of synthetic polymers is in general not as perfect as in the case of their natural counterparts. However, using special initiators and optimized polymerization conditions, it is possible to prepare quite homogeneous CIS-1,4-polyisoprene ( synthetic natural rubber ). 

Bristow, G.M. Relation between stress-strain behavior and equilibrium volume swelling for peroxide vulcanizates of natural rubber and cis-1,4-polyisoprene. J. Appl. Polymer Sci. 9, 1571-1578 (1965). 

Synthetic Polyisoprene Rubbers (IR). IR is a cis-1,4 polyisoprene synthetic natural rubber. However, it does not contain the non-rubber substances that are present in NR. One can differentiate between two basic types of synthetic polyisoprene by the polymerization catalyst system used. They are commonly referred to as high cis and low cis types. The high cis grades contain approximately 96-97% cis-1,4 polyisoprene. 

The coordination catalysts were quickly extended to dienes and found to produce the long-sought objective of a "synthetic natural rubber, i.e., cis-1,4-polyisoprene. cis-1,4-Polybutadiene was also quickly produced. These were, and still are, erroneously referred to as stereo rubbers. They are actually unique geometric isomers rather than stereoisomers, but the name stereo rubber became established probably because of the relationship in time and catalyst usage to stereo olefin polymerization. 

Rubber is a natural polymer that is obtained from the rubber tree and has the all cis-1, 4-polyisoprene structure. This structure has been duplicated in the laboratory and is called synthetic rubber, made with the use of Ziegler-Natta catalysis. 

Scheme 22 Synthesis of telechelic acetoxy-terminated cis- 1,4-polyisoprene by olefin metathesis depolymerization of natural rubber [144]...

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

By contrast, cis- 1,4-polyisoprene is produced in limited amounts, since it is not price competitive with natural rubber (owing to the relatively high costs of manufacturing the isoprene monomer). The same applies to trans- 1,4-polyisoprene, which is more expensive than its natural counterparts gutta percha and balata. 

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. 

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