Cis-l,4-Polyisoprene

Figure 1.3 shows several repeat units of cis-l,4-polyisoprene and trans-1,4-polyisoprene. Natural rubber is the cis isomer of 1,4-polyisoprene, and gutta-percha is the trans isomer. 

NBRin pLASTOMERS, SYNTHETIC - NITRILE RUBBER] (Vol 8) -cis-l,4-polyisoprene in pLASTOMERS SYNTHETIC - POLYISOPRENE] (Vol 9)... 

Zhou L.L. and Eisenberg A., lonomeric blends. II. Compatibility and dynamic mechanical properties of sulfonated cis-l,4-polyisoprenes and styrene/4-vinylpyridine copolymer blends, J. Polym. Sci., Polym. Phy., 21, 595, 1983. 

The synthesis of transtactic structures is based on catalysts in which the transition metal belongs to the 3d block (Ti, Cr, V, Ni). Particular emphasis is devoted to the synthesis of trans butadiene/piperylene copolymers and to their blends with synthetic cis-l,4-polyisoprene, with the aim of increasing the "green strength" of the latter. 

The copolymerization of butadiene in trans configuration with suitable comonomers represents a second route for obtaining a wide range of strain induced crystallizable elastomers, with melting point tailorable in a wide range of temperatures. These copolymers can be used, in particular, in blends with other crystallizable rubbers (e.g. synthetic cis-l,4-polyisoprene) in order to improve their "green strength". 

Fig. 16.4. SFC chromatogram of long-chain polyprenols in E. ulmoides leaves. The sample had Mn 3.99 x 103 (calibrated against cis-l,4-polyisoprene standards) and MJMn 1.41. The numbers in the chromatogram represent degrees of polymerization for polyprenol homologues [10].

Hoses -ofLLDPE [OLEFIN POLYMERS - POLYETHYLENE - LINEAR LOW DENSITY POLYETHYLENE] (Vol 17) -ozone protection [ANTIOZONANTS] (Vol 3) -cis-l,4-polyisoprene m [ELASTOMERS SYNTHETIC - POLYISOPRENE] (Vol 9) -precipitated silica m [SILICA - AMORPHOUS SILICA] (Vol 21)... 

Comparison of results from the polymerizations of butadiene and isoprene with an AlR3-TiCl4 catalyst system reveals some interesting features. The 1 to 1 Al/Ti ratio yields a cis-l,4-polyisoprene and a mns-1,4-polybutadiene. Kinetic studies have, in fact, indicated that in both cases, the rate of polymerization at this ratio is proportional to the first power of the monomer pressure 6, 21). At lower Al/Ti ratios, higher trans-l,A- content is observed in both polyisoprene and polybutadiene. At comparable Al/Ti ratios, lower temperatures increase the trans-l,A-structure in both polymers. Although essentially all-cis-1,4-poly isoprene and all-... 

If physical crosslinks are used in an IPN system, then we would have a thermoplastic IPN. Up to this time, few IPNs or SINs have been made where both components are elastomeric. One of these rare works involves the crosslinking of cis-l,4-polyisoprene (Elastollan) in thermoplastic polyurethanes (Morthane). ... 

Moore, C.G. Trego, B.R. Structural characterization of vulcanizates. Part IV. Use of triphenylpho-sphine and sodium di-n-butyl phosphite to determine the structures of sulfur linkages in natural rubber, cis-l,4-polyisoprene, and ethylene-propylene rubber vulcanizate networks. J. Appl. Polym. Sci. 1964, 8, 1957. 

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... 

This is an example of the preparation of ABA-type thermoplastic elastomer. Styrene is polymerized first since styryl initiation of isoprene is faster than the reverse reaction. The reaction is carried out in a nonpolar solvent with Li" " as the counterion to enable predominantly cis-l,4-polyisoprene to be formed in the second growth stage. The living polystyrene-6/ocfc-polyisoprene AB di-block copolymer resulting from the second stage is then coupled by a double nucleophilic displacement of Cl ions from a stoichiometric equivalent of dichloromethane to give a polystyrene-61ock-polyisoprene-/)/ock-polystyrene triblock copolymer. 

Rabek found in his investigations of the photo-oxidative degradation of cis-l,4-polyisoprene in solution that benzophenone [496, 502], Michler s ketone (4,4 -bis-(dimethylamino)-benzophenone) [508] and thiobenzo-phenone [494, 495] sensitize the reaction (Fig. 19). In the absence of oxygen benzophenone promotes the formation of microgel. 

Thus, a mechanism of cis-l,4-polyisoprene oxidation could be that shown in reactions (234)—(237)... 

This problem was also investigated by Rabekand Skorupa [505, 510]. Methylene blue, Rose Bengal, rhodamine 6G and eosin were used. These dyes are insoluble in benzene and hexane, but they dissolve in polar solvents such as water, alcohols and ketones. When these dyes are dissolved in methanol (concentration 10-2 —10-3 mole l-1) and added to benzene or to the solution of cis-l,4-polyisoprene in benzene, in the volumetric ratio 1 9, they are not precipitated. The presence of dyes in such solutions of polymers produces an appreciable decrease in viscosity when the solutions are exposed to visible light (Fig. 21). 

More detailed investigations by Rabek and Pajak [508] show that DPPH (concentration 10-3—10-4 mole l-1) added to a solution of cis-l,4-polyisoprene in benzene or n-heptane accelerates the photodegradation in the presence as well as in the absence of oxygen. It was discovered that the products of photochemical decomposition of DPPH have stronger sensitizing properties with respect to the photodegradation of diene polymers than has the undecomposed DPPH. 

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