Polyisoprene isomers

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. 

Figure 1.3 1,4-polyisoprene with R=CH3 (a) cis isomer natural rubber (b) trans... 

In the two polyisoprene isomers, the length of the repeat unit and the steric hindrance factor vary oppositely for the two isomers. The greater end-to-end distance in the trans isomer is the dominant influence on the order of 1q values. 

Polyisoprene (Synthetic). Polyisoprene has four possible chain unit geometric isomers cis- and /n j -l,4-polyisoprene, 1,2-vinyl, and 3,4-vinyl. 

Natural mbber (Hevea) is 100% i7j -l,4-polyisoprene, whereas another natural product, gutta-percha, a plastic, consists of the trans-1,4 isomer. Up until the mid-1900s, all attempts to polymerize isoprene led to polymers of mixed-chain stmcture. 

For the 1,2- and 3,4-addition, a chiral carbon (marked by an asterisk) is formed which has an R or 3 configuration, but there is no net optical activity, because equal amounts of the R and S configurations are formed. The R and S configurations along the polymer chains lead to diastereomeric isomers called isotactic, syndiotactic, and atactic. In isotactic polyisoprene all monomer units have the same configuration as illustrated for isotactic... 

Related to stereoregularity is the possibility of cis, trans isomerism. The molecule of natural rubber is a c/s-1,4-polyisoprene whilst that of gutta percha is the trans isomer. 

Fig. 2. To and We for isomers of polybutadiene and polyisoprene, and for typical average isomeric compositions in styrenic block copolymers.

Figure 13 Chemical structures of trans and cis isomers of 1,4 polyisoprene (gutta-percha and natural rubber, respectively).

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. 

Polyisoprene is composed of four structures as shown in Equation 5.48. As in the case of polybutadiene, it is the cis-, A structure that is emphasized commercially. The cA-1,4-polyisoprene is similar to the cw-l,4-polybutadiene material except it is lighter in color, more uniform, and less expensive to process. Polyisoprene is composition-wise analogous to NR. The complete cw-1,4 product has a Tg of about —71°C. Interestingly, isomer mixtures generally have higher Tg values. Thus, an equal molar product containing cA-1,4 trans-, A, and 3,4 units has a Tg of about —40°C. 

Tacticity and geometric isomerism affect the tendency toward crystallization the tendency increases as the tacticity (stereoregularity) is increased and when the geometric isomers are predominantly trans. Thus isotactic PS is crystalline, whereas atactic PS is largely amorphous and c/s-polyisoprene is amorphous, whereas the more easily packed trans isomer is crystalline. 

This polymer is a hard plastic that occurs naturally as gutta-percha or balata. Since the trans isomer packs better than the cis isomer, it has a higher specific gravity, a higher degree of crystallinity, and a higher melting point (67 C) than the ds isomer of polyisoprene. The chemical and the solvent resistance of the trans polymer are simitar to those of the ds polymer. 

Natural rubber and guttapercha consist essentially of polyisoprene in a s-1,4 and trans-1,4 isomers, respectively. Commercially produced synthetic polyisoprenes have more or less identical structures but reduced chain regularity, although... 

Natural rubber (NR) and guttapercha consist essentially of polyisoprene in cis-l, 4 and trans-1,4 isomers, respectively. Commercially produced synthetic polyisoprenes have more or less identical structure but reduced chain regularity, although some may contain certain proportions of 1,2- and 3,4-isomers. Microstructure differences not only cause the polymers to have different physical properties but also affect their response to radiation. The most apparent change in microstructure on irradiation is the decrease in unsaturation. It is further promoted by the addition of thiols and other compounds.130 On the other hand, antioxidants and sulfur were found to reduce the rate of decay of unsaturation.131 A significant loss in unsaturation was found, particularly in polyisoprenes composed primarily of 1,2- and 3,4-isomers.132,133... 

Spectra have been recorded of the following polymers Poly isobutylene. Schaufele has reported a spectrum of a non-turbid specimen of this material complete with depolarization data (18). Bands at Av - 2919 and 720 cm 1 are clearly polarized and must result from symmetrical modes. Schaufele was also able to show that the short chain hydrocarbon CH3(CH2)12CH3 could be examined satisfactorily but the results were very much sharpened by cooling to liquid air temperatures. Polyisoprene and Polybutadiene. Both of these in the cis form have also been examined at Southampton very recently. The trans isomers gave no spectra due to fluorescence but no interpretation has been attempted as yet. Spectra are shown in Fig. 7 and 8. 

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. 

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. 

The stereochemistry in a polymer such as polyisoprene arises because of the rigid nature of the carbon-carbon double bond. Each double bond in the polymer chain can exist in one of two possible stereoisomers as show below. Stereoisomers are compounds that differ from each other only in the way their atoms are oriented in space. If two similar groups are on the same side of the double bond (here the similar groups are the carbon atoms in the backbone of the polymer), the stereoisomer is called the ds isomer. If the two similar groups are on opposite sides of the double bond, the stereoisomer is called trans. 

At least one natural polyisoprene consists of both cis and trans isomers. Chicle, found in Central America, is approximately 75% trans, the remainder being ds. It was used as the base for chewing gum for many years, until it was replaced in the midtwentieth century by the synthetic polymer poly (vinyl acetate). 

The polymer in natural rubber (from the Hevea bmsiliensis tree) is pure cis polyisoprene gutta percha and balata are composed of the trans isomer. 

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 >). 

There are only minimal chemical shift differences between cis and rrans isomers in the H NMR spectra of polymers like polybutadiene and polyisoprene, so this tool is less useful in characterizing structural isomers in diene polymers. 

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