Cw-l,4-polyisoprene

Maleimides Alkyl and aryl maleimides in small concentrations, e.g., 5-10 wt% significantly enhance yield of cross-link for y-irradiated (in vacuo) NR, cw-l,4-polyisoprene, poly(styrene-co-butadiene) rubber, and polychloroprene rubber. A-phenyhnaleimide and m-phenylene dimaleimide have been found to be most effective. The solubihty of the maleimides in the polymer matrix, reactivity of the double bond and the influence of substituent groups also affect the cross-fink promoting ability of these promoters [82]. The mechanism for the cross-link promotion of maleimides is considered to be the copolymerization of the rubber via its unsaturations with the maleimide molecules initiated by radicals and, in particular, by allyfic radicals produced during the radiolysis of the elastomer. Maleimides have also been found to increase the rate of cross-linking in saturated polymers like PE and poly vinylacetate [33]. 

Fig. 4. Different possible conformations of cw-l,4-polyisoprene (poly[(Z)-2-methylbut-2-ene-l,4-diyl]) in the crystalline state, as viewed sideways along two orthogonal axes [17].

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. 

In 1910 scientists concluded that natural rubber was cw-l,4-polyisoprene. 

FIGURE 9.5 Ball-and-stick models of soft hevea rubber (cw-l,4-polyisoprene) (top) and hard gutta-percha (trfl i-l,4-polyisoprene) (bottom). 

Rubber molecules are synthesized from one APP molecule, which initiates the reaction, and the rubber polymer (cw-l,4-polyisoprene) is then polymerized by sequential condensations of the non-allylic IPP (magnesium cations are a required cofactor) with release of a diphosphate at each condensation. After initiation and elongation, a termination event occurs in which the rubber molecule is released from the enzyme. Despite the similar process, remarkable differences exist between plant species with respect to enzymatic reaction mechanisms and product molecular weight. 

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). 

The biodegradation of the cw-l,4-polyisoprene chain was achieved by Tsuchi, Suzuki, and Takeda. They used bacterium that belonged to the genus Nacardia and led to considerable weight loss of different soft-type NR-vulcanizates. The microbial... 

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. 

TABLE 3.2 Molecular-Mass Characteristics of Cw-l,4-Polyisoprene 1 and 2 Are the Methods of Catalyst Preparation... 

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. 

It is presumed that NR is a pure cw-l,4-polyisoprene, containing no detectable amounts of tram-l,A and 3,4 units as indicated by H-NMR at... 

Fig. 12.12 Dielectric loss versus temperature at fixed frequency of 10 Hz for cw-l,4-polyisoprene. The molecular weights are as follows circles,...

Rubber blending predates that of thermoplastics by nearly a century. In 1846, Parkes introduced the first blends of trans- and cw-l,4-polyisoprene, i.e., natural rubber (NR) with gutta-percha (GP). By varying the composition and/or adding fillers, the blends were formed into a variety of flexible or rigid articles. 

The chemical structure of CR is analogous to that of natural rubber (see Natural rubber-based adhesives) with the methyl group of cw-l,4-polyisoprene replaced by a chlorine atom, see Fig. 1. 

The beginnings of the polymer industry date back to 1846 when the first polymer blend was patented by Parkes which co-dissolved natural rubber (amorphous cw-l,4-polyisoprene) and gutta-percha (semi-crystalline tranr-1,4-polyisoprene) in CS2- The blending process yielded a partially crosslinked material. The process was known as co-vulcanization and the interesting aspect of the final blend was that its rigidity could be conttolled by composition. Due to this aspect, the blends were used in multiple application domains. What followed was the obtaining of synthetic... 

Figure 6.9. Two polymers of 2-methyl-l,3-butadiene [isoprene, CH2=C(CH3)CH=CH2]. The first, all cw-l,4-polyisoprene, is close to natural rubber (wrenched from the naturally occurring latex obtained by tapping Hevea brasiliensis, the rubber tree ) and can be obtained...

Chemically the NR produced by the Hevea brasiliensis species is almost pure cw-l,4-polyisoprene. So far none of the manufacturers of synthetic cis-, A-polyisoprene is able to achieve more than 95% of the cis isomer in their commercial products. Jitladda and coworkers have conducted extensive studies on the molecular structures of the NR molecules found in Hevea and correlated these to the biosynthesis of this in the trees. This is intrinsically linked to the end groups of the rubber molecules involving the phospholipids and proteins that are linked to the charging mechanism at the latex particle surface and hence has a direct impact on the stability of the latex. 

NR, cw-l,4-polyisoprene, is an elastomer. Commercially, NR is almost exclusively obtained from Hevea brasiliensis It exists in two isomeric forms, the cis-, and trans-, because of the presence of double bond in its chains. The NR... 

The successful synthesis of high cw-l,4-polyisoprene in 1954 aroused interest in the manufacture of large quantities of isoprene. Prior to this date only relatively small amounts of isoprene, which were used almost exclusively for the manufacture of butyl rubber (Section 2.10), were available from naphtha cracking operations. Several isoprene processes have been developed the more important are described in this section. 

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