Cw-1,4-Polyisoprene

Some of the applications of the organometallic compounds of lanthanides are as catalysts for (i) stereo specific polymerization of diolefins and in particular to obtain high yields of 1,4-ci.v-polybutadiene and 1,4-cw-polyisoprene and copolymer of the two monomers. The order of effectiveness of the rare earths as catalysts is Nd > Ce, Pr < Sm, Eu. The nature of halogen of the Lewis acid affecting the catalytic activity is in the order Br > Cl > I > F. Detailed work on the activity of cerium octanoate-AlR3-halide showed stereo specificity with cerium as the primary regulator. Cerium is thought to form jr-allyl or 7r-crotyl complexes with butadiene. 

Commercial samples of l,4-c/5-polybutadiene (SKD E-BR) polybutadiene (Diene 35 NFA BR) 1,4-cw-polyisoprene (Carom IR 2200 E-IR) and polychloroprene (Denka M 40 PCh) were used in the experiments (Table 1). The 1, 4-trans -polyisopiene samples were supplied by Prof. A. A. Popov, Institute of Chemical Physics, Russian Academy of Sciences. All mbbers were purified by threefold precipitation from CCl solutions in excess of methanol. The above mentioned elastomer stmctures were confirmed by means of H-NMR spectroscopy. Ozone was prepared by passing oxygen flow through a 4-9 kV electric discharge. 

The possibility of obtaining spin-labelled rubbers by interaction of their solutions in the inert solvents with a mixture of nitrogen dioxide and oxygen has been demonstrated in the work [34]. Such rubbers can be prepared simply and rapidly by reactions of block polymeric samples with gaseous NOj [35], The experiments were carried on 1,4-cw-polyisoprene (PI) and copolymer of ethylene, propylene and dicyclopentadiene. The samples had the form of cylinders of 1.5 cm height and 0.4 cm in diameter. On exposure these polymers to NO2 (10 -2.3xl0" molxk ) at 293 K, identical EPR spectra were registered. The spectra represent an anisotropic triplet... 

TABLE 10.5 Intermediates and products of the ozonolysis of 1,4-cw-, and 1,4-tmns-polyisoprenes... 

The polymerization of butadiene monomer proceeds with chain propagation via 1,2-,, A-trans- or 1,4-cw-additions. If the polymerization is controlled to form mostly the 1,2-addition product, the polymer has a — CH2— chain with a terminal vinyl, — CH=CH2, substituent, at alternating carbon atoms. However, if 1,4-addition dominates the polymerization proceeds to form a polymer chain with a molecular structure of — (CH2 —CH=CH—CH2) —, normally with a trans configuration at the double bond. 2-Chloro-1,3-butadiene (CH2=CC1—CH=CH2 chloroprene) and 2-methyl-1,3-butadiene (isoprene) are polymerized in a similar manner. With these compounds, the asymmetry of the carbon atoms at positions 1 and 4 produces a variety of addition products with 1,2-, 1,4-cw,, A-trans, and 3,4-configurations. In the case of polyisoprene, which in nature occurs as natural rubber, the 1,4-cis configuration is the dominant structure. A summary of the polymerization products of butadiene, isoprene, and chloroprene is provided in Fig. 31. 

One infrared spectral method used for the determination the monomer units of SBR is known as Hampton s method [63]. This method utilizes the characteristic monosubstituted aromatic band intensity at 699 cm (A ) for the determination of styrene unit composition, and the scheme defined as Morero s method (see seetion on polybutadiene and polyisoprene) is used to determine the butadiene moiety. As noted in the section on polybutadiene, the 1,4-cw, 1,4-trans, and 1,2-addition components appear in the IR spectrum at 724, 965, and 910 cm . ... 

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. 

On the other hand, if the cross-link density is low (the length of the chains between cross-links is large) and the mobility of the chains is high, the cross-linked material is called an elastomer. An example of a typical elastomer is cw-l,4-polyisoprene (natural rubber), which, by means of a cross-linking reaction with sulfur (vulcanization), gives rise to a network structure (see Fig. 1.4). 

In 1955 investigators from the Firestone Tire and Rubber Company and the B. F. Goodrich Company announced the synthesis of polyisoprene with over 90%i cw-1,4- structure. The work at Firestone was based on lithium metal catalysts, whereas the work at Goodrich was the result of using Ziegler-Natta-type coordination catalysts. ° ... 

The sulfonation of low molecular weight model olefins was undertaken to determine the feasibility of this approach. Competitive sulfonations using acetyl sulfate were carried out on the model compounds below, representing the repeat structures of cis-l,4-polyisoprene (PIP), cw-l,4-polybutadiene (c-PBD), and trans-l,4-polybutadiene (Z-PBD), respectively. It was necessary to model both the cis and trans isomeric forms of 1,4-polybutadiene, since ttey have a nearly equal probability of occurrence when the anionic polymerization (Ii counterion) is conducted in a nonpolar hydrocarbon medium... 

High density polyethylene (HOPE) Linear low density polyethylene (LLDPE) Isotactic polypropylene (iPP) Syndiotactic polypropylene (sPP) tram-1,4-Polyisoprene Syndiotactic polystyrene (sPS) Cyclooleflns Ethylene-propylene copolymers Styrene-ethylene copolymers cw -1,4-polybutadiene rrarw -1,4-Poly isoprene Random ethylene-a-olefin copolymers Ethylene-propylene rubber (EPR) Ethylene-propylene-diene copolymers (EPDM)... 

Silicone, natural, and synthetic rubbers have been used for the fabrication of implants. Natural rubber is made mostly from the latex of the Hevea brasiliensis tree and the chemical formula is the same as that of cw-1,4 polyisoprene. Natural rubber was found to be compatible with blood in its pure form. Also, cross-linking by x-ray and organic peroxides produces rubber with superior blood compatibility compared with rubbers made by the conventional sulfur vulcanization. 

FIGURE 14.9 Percentage rebound recovery measured at 298 K, with balls made from (1) cis-polybutadiene, (2) synthetic cw-polyisoprene, (3) natural cw-polyisoprene, (4) ethylene-propylene copolymer, (5) styrene-butadiene (SBR), (6) /ran -polyisoprene, and (7) butyl rubber. 

Conventional prefixes indicating cis and trans isomers are placed in front of the polymer name. An example is cw-1,4-polybutadiene, or in rran5-1,4-polyisoprene. 

Use of hydrocarbon solvents has an advantage in polymerizations of conjugated dienes, because they yield some steric control over monomer placement. This is true of both tacticity and geometric isomerism. As stated earlier, the insertions can be 1,2 3,4 or 1,4. Furthermore, the 1,4-placements can be cis or trans. Lithium and organolithium initiators in hydrocarbon solvents can yield polyisoprene, for instance, which is 90% cw-1,4 in structure. The same reaction in polar solvents, however, yields polymers that are mostly 1,2 and 3,4, or trans-lA in structure. There is still no mechanism that fully explains steric control in polymerization of dienes. 

Rubber hydrocarbon is the principle component of raw rubber. The subject is discussed in greater detail in Chapter 7. Natural rubber is 97% cw-l,4-polyisoprene. It is obtained by tapping the bark of rubber trees (Hevea brasiliensis) and collecting the exudate, a latex consisting of about 32-35% rubber. A similar material can also be found in the sap of many other plants and shrubs. The structure of natural rubber has been investigated over 100 years, but it was only after 1920, however, that the chemical structure was elucidated. It was shown to be a linear polymer consisting of head-to-tail links of isoprene units, 98% bonded 1,4. 

Use of Ziegler-Natta catalysts, as seen from Table 5.8, can yield an almost all-cw-l,4-polyiso-prene or an almost all-franj- 1,4-polyisoprene. The microstructure depends upon the ratio of titanium to aluminum. Ratios of Ti A1 between 0.5 1 and 1.5 1 yield the cis isomer. A1 1 ratio is the optimum. At the same time, ratios of fi Al between 1.5 1 to 3 1 yield the trans structures. The titanium-to-aluminum ratios affect the yields of the polymers as well as the microstructures. There also is some influence on the molecular weight of the product. Variations in catalyst compositions, however, do not affect the relative amounts of 1,4 to 3,4 or to 1,2 placements. Only cis and trans arrangements are affected. In addition, the molecular weights of the polymers and the microstructures are relatively insensitive to the catalyst concentrations. The temperatures of the reactions, however, do affect the rates, the molecular weights, and the microstructures. 

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 j-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 cw-I,4-polyisoprene. 

Isoprene rubber (IR). cw-1,4-Polyisoprene is a synthetic rubber with properties highly similar to but not quite identical with the properties of natural rubber. Trade names Cariflex (UK), Europrene (I), Natsyn (USA). 

Natural rubber. Its main ingredient is cw-1,4-polyisoprene. In spite of the increasing use of synthetic rubbers, it is still an important industrial basic material. 

Polyisoprene, The main ingredient of natmal rubber is cw-1,4-polyisoprene. The synthetic cw-l,4-polyisoprene is isoprene rubber. Natural guttapercha is tm/ty-1,4-polyisoprene. 

Figure 5.28 shows the unit cell of cw-1,4-poly (2-methyl butadiene), also known as c -polyisoprene or natural rubber. This second isomer of polyisoprene (see... 

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