1.4- c/s-polyisoprene

Table 3. Isoprene Specification for the Preparation of High c/s-Polyisoprene ...

The latex of the Sapota achras yields a thermoplastic material, chicle, consisting of about 17.4% hydrocarbon, 40% acetone soluble resin and 35% occluded water. The hydrocarbon appears to contain both trans- and c/s-polyisoprene. Although originally introduced as gutta pereha and natural rubber substitutes, deresinated chicle has become important as the base for chewing gum. Like other polyisoprenes, it is meeting competition from synthetic polymers. 

The standard polymers used for rubber linings consist of materials that are cross-linkable macromolecules which, on mixing with suitable reactants that form strong chemical bonds, change from a soft deformable substance into an elastic material. These polymers include natural rubber and its corresponding synthetic, c/s-polyisoprene, styrene-butadiene rubber, polychloroprene, butyl rubber, halogenated butyl rubbers, acrylonitrile-... 

Note 2 A classic example of vulcanization is the crosslinking of c/s-polyisoprene through sulfide bridges in the thermal treatment of natural rubber with sulfur or a sulfur-containing compound. 

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. 

Because of the geometric regularity present, gutta-percha (/ra/w-polyi-soprene) and stretched NR from Hevea brasitiensis (c/s-polyisoprene) are crystalline. Random copolymers are usually amorphous, but some may be crystalline if the comonomers, such as ethylene and tetrafluoroethylene, are similar in size. Block copolymers may have crystalline domains if either of the com-... 

A pseudo solid-like behavior of the T2 relaxation is also observed in i) high Mn fractionated linear polydimethylsiloxanes (PDMS), ii) crosslinked PDMS networks, with a single FID and the line shape follows the Weibull function (p = 1.5)88> and iii) in uncrosslinked c/.s-polyisoprenes with Mn > 30000, when the presence of entanglements produces a transient network structure. Irradiation crosslinking of polyisoprenes having smaller Mn leads to a similar effect91 . The non-Lorentzian free-induction decay can be a consequence of a) anisotropic molecular motion or b) residual dipolar interactions in the viscoelastic state. 

In a second paper Brown and Tinker [101] examined the effects of a number of parameters, such as the accelerator used in the sulfur curing, peroxide versus sulfur curing, and swelling ratio at constant crosslink density. The results for c/s-polyisoprene show that the value of H% is independent of accelerator. However, lower values of H% were seen for peroxide-cured materials. The results for BR were independent of curant the reasons... 

About one out of every 150 chain carbon atoms is cross-linked in a typical natural rubber (c/s-polyisoprene) compound with good properties. The density of such a vulcanizate is 0.97 gcm at 25°C. The gas constant / = 8.3 X 10 ergs mol K = 1.987 cal mol K. Estimate the modulus of the sample at low extensions. 

Fig. 21. Yield, crazing and fracture behaviour of c/s-polyisoprene (cross-linked by suli tu) below T . Pre-orientation 100% (after Ref.

Synthetic polyisoprene displays during pyrolysis an identical behavior with natural rubber. Pyrolysis of natural rubber, which is pure c/s-polyisoprene, is reported frequently in literature [29-35]. A review of the subject can be found in [36]. 

The pyrolysis products of synthetic c/s-polyisoprene in an oxidative pyrolysis are very likely identical with those shown in Table 7.1.5. 

The development of the Ziegler-Natta catalysts has affected rubber production as well. Eirst, it facilitated the synthesis of all-c/s polyisoprene and the demonstration that its properties were nearly identical to those of natural rubber. (A small amount of synthetic natural rubber is produced today.) Second, a new kind of synthetic rubber was developed all-c/s polybutadiene. It now ranks second in production after styrene-butadiene rubber. 

Figure 6. Aliphatic carbon signals in c s-polyisoprene from Hevea brasiliensis (natural rubber) ( denotes signals due to impurities) (14).

The polymer of isoprene is called polyisoprene. It exists in two forms, cis- and frons-polyisoprene. The two forms are called geometric isomers. They have the same kind and number of atoms, but the atoms are arranged differently in the two forms. Natural rubber consists of trans-polyisoprene, while another product found in rubber plants, gutta percha, is made of c/s-polyisoprene. 

The positive inductive effect of the methyl group in polyisoprene enhances the rate of ozone addition to the double bonds from 6 10" for SKD to 4 10 M s for Carom IR 2200. The infrared spectra of ozonized, 4-tram-polyisoprene (Z-IR) show two intense bands at 1,100 and 1,725 cm, characteristic of ozonide and keto groups, respectively [19, 35], These spectra are identical with the well-known spectra of l,4-c/s -polyisoprenes (E-IR), as far as ozonide and carbonyl bands are concerned [22, 36], It was found that the integrated intensity of the peak at 1,100 cm in the E-IR and Z-IR spectra is equal for one and the same amount of reacted ozone. By analogy with the peak at 1110 cm, the integral intensity of the peak at 1,725 cm is also one and the same. The latter show that, according to the infrared spectra, the degradation efficiencies of E-IR and Z-IR with respect to the amount of consumed ozone do not practically differ. 

The first patent on PAB was granted to Parkes in 1846 for two natural polymers co-vulcanized during blending in the presence of CS2, i.e., a natural rubber (NR = amorphous c/s-polyisoprene, IR) with gutta-percha (GP = semicrystalline trans-polyisoprene, IR). Thus, mbber PAB predates that of synthetic polymers by ca. 80 years (PMA/PVAc 1929). Notably, while the early plastics were bio-based, their usage fell to <5 wt% nowadays slowly recovering from the absolute dominance of synthetic, petroleum-based plastics. 

HON Hong, J.-S. and Lee, D C., Phase equilibrium in poly(a-methylstyrene)/c/s-polyisoprene/cyclohexane system, Pollimo, 16, 470, 1992. 

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