Natural Rubber and Gutta-Percha

Natural Rubber and Gutta-Percha.—These cis and trans forms of 1,4-polyisoprene may be similarly reduced to structures amenable to calculation. Assuming the usual bond dimensions and angles, 

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Although the conditions of the polymerization reaction may be chosen to optimize the formation of one specific isomer, it is typical in these systems to have at least some contribution of all possible isomers in the polymeric product, except in the case of polymers of biological origin, like natural rubber and gutta-percha. 

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. 

Exercise 29-19 Ozonizations of natural rubber and gutta-percha, which are both poly-2-methyl-1,3-butadienes, give high yields of CH3COCH2CH2CHO and no CH3-COCH2CH2COCH3. What are the structures of these polymers ... 

With the notable exceptions of natural rubber and gutta-percha, almost all naturally occurring polymers are optically active. Historically, interest in optically active synthetic polymers has focused on modeling natural polymers, interpreting the conformational properties of macromolecules in solution, and investi-... 

As described in Section 1.1, the first commercial polymers, which were naturally occurring, were polyisoprenes (natural rubber and gutta-percha) and subsequently cellulose derivatives. From the early twentieth century, various totally synthetic polymers were introduced. Farbenfabrrken Bayer introduced bulk polymerized totally synthetic elastomers in 1910. Poly(dimethyl butadiene) synthetic rubber was produced commercially by Bayer in Leverkusen during World War I. The 1920s saw the commercial development of polystyrene (PS) and poly(vinyl chloride) (PVC). In 1934, the IG Farbenindustrie (a combine of Bayer, BASF, Floechst, and other firms) began to commercially manufacture butadiene-acrylonitrile copolymer (N BR) as an oil resistant rubber and in 1937 butadiene-styrene copolymer (SBR) intended for pneumatic tires. 

Before the advent of infra-red analysis, natural rubber (and gutta percha) was identified by the Weber test which involved bromination. The rubber was reacted with bromine to form a dibromide which was then treated with a solution of phenol in carbon tetrachloride. A violet colouration of the residue developed after gentle boiling of the mixture. (This test also gives positive results for synthetic polyisoprenes and butyl rubber.)... 

Isoprene polymers, both of low molecular weight such as squalene, and of high molecular weight such as natural rubber and gutta percha also differ from mono-olefins in that intramolecular reactions can occur because of the multiple unsaturation and the short distances between the double bonds. For such materials it has been shown that a diperoxide-hydroperoxide structure is formed which Holland and Hughes (1949) proposed as ... 

A further complication of the chemical structure of polymers lies in the possibility of different chemical isomeric forms within a repeat unit or between a series of repeat units. Natural rubber and gutta percha are chemically both polyisoprene, but the former is the cis form and the latter is the trans form (see Figure 1.5). The characteristic properties of rubber are a consequence of the loose packing of molecules (i.e. large free volume) that arises from its stmcture. 

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