Polyisoprene structures

The hard thermoplastic hydrocarbon obtained from the latex of Mimusops globosa. Balata has the same molecular formula as natural rubber (C5H8)n but has the tram polyisoprene structure whereas natural rubber has c/ s-structure. 

Finally, one last type of natural polymer is natural rubber, obtained from the rubber tree and having the all cw-l,4-polyisoprene structure. This structure has been duplicated in the laboratory and is called synthetic rubber, made with the use of Ziegler-Natta catalysis. 

Synthetic cA-l,4-polyisoprene (structure 5.42) is produced at an annual rate of about 100,000 t by the anionic polymerization of isoprene when a low dielectric solvent, such as hexane, and K-butyllithium are used. But, when a stronger dielectric solvent, such as diethy-lether, is used along with w-butyllithium, equal molar amount of tra i -l,4-polyisoprene and cA-3,4-polyisoprene units is produced. It is believed that an intermediate cisoid conformation assures the formation of a cis product. An outline describing the formation of cA-1,4-polyisoprene is given in structure 5.42. 

Behavior of Carbethoxynitrene toward Polyisoprene Structures. After studying the reaction of a number of divalent carbon derivatives with polyenes (17, 18), attention was directed to monovalent nitrogen derivatives. Of these, carbethoxynitrene, N—C02Et, gives addition reactions with olefins (12, 13). It can be produced either by photolysis of ethyl azidoformate (12), or by a-elimination (13) (Figure 5). 

In the formation of elastomers from diolefin monomers such as butadiene or isoprene, there are a number of possible structures. Since the control of these structures is critical in obtaining optimum properties, this area has received great attention from the synthesis chemist. The possible polyisoprene structures are ... 

Balata. A gutta-percha-like tough, hard, poorly elastic polyisoprene (structure see under natural rubber) obtained from the resin-rich milk of wild-growing tropical trees (Mimusops balata, Sapotaceae) in Venezuela and Brazil. 

These results suggest that only polyisoprenes having long sequences of cis-1,4 or trans-1,4 units have absorption bands at 1130 cm and 1150 cm (8.85 pm and 8.69 pm), respectively. It is also evident that the analysis of synthetic polyisoprenes using these absorption bands leads to distorted results. Kossler and Vodchnal [47] obtained better results using the absorption bands at 572, 980 and 888 cm (17.48, 10.20 and 11.26 pm) for cis-1,4 and trans-1,4 and 3,4 polyisoprene structural units, respectively. Various combinations of different absorption bands permits one to conclude if a polymer is more of the block copolymer type or a mixture of stereoregular polymers. 

We have discussed above the uncured properties of a synthetic rubber which are similar to those of NR. A major difference between the rubbers is that the synthetic SBR has better thermal oxidative resistance than NR. Under oxidative ageing conditions, 1,4-polybutadiene structure tends to crosslink to a greater extent relative to undergoing chain scission. The reverse is the case for NR (cw-l,4-polyisoprene structure). The greater resistance to oxidative degradation of HTSBR vulcanizates is indicated by a comparison of plots of flex life (cycles to failure) of HTSBR and NR versus strain amplitude, shown in Fig. 34. 

Raman spectroscopy can differentiate between internal and external bonds as well as cis and trans isomerism and conjugation in compounds with ethylenic linkages. The type of unsaturation in butadiene and isoprene rubbers can be determined from the intense Raman scattering of the C=C stretching modes [55]. The trans- and cw-l,4-polybutadiene structures scatter at 1664 and 1650 cm , respectively. The 1,2-vinyl structure of polybutadiene scatters at 1639 cm and this scattering is well-resolved from that of the 1,4-polybutadiene structures. For poly-isoprene, a slightly different situation prevails. The cis- and ran -l,4-polyisoprene structures are riot resolved, and they scatter at 1662 cm , but the 3,4-polyisoprene structure scatters at 1641 cm , and the 1,2-vinyl stracture scatters at 1639 cm [56,57]. 

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