Polyisoprene, high molecular weight

A number of high molecular weight polyisoprenes occur in nature which differ from natural rubber in that they are essentially non-elastic. As with natural rubber they are obtained from the latex of certain plants but they differ in that they are either frani-l,4-polyisoprenes and/or are associated with large quantities of resinous matter. 

Many of the synthetic elastomers now made are still polymerized by a free radical mechanism. Polychloroprene, polybutadiene, polyisoprene, and styrene-butadiene copolymer are made this way. Initiation by peroxides is common. Many propagation steps create high molecular weight products. Review the mechanism of free radical polymerization of dienes given in Chapter 14, Section 2.2. 

Rubber is synthesized and sequestered on cytsolic vesicles known as rubber particles. Rubber transferase is localized to the surface of the rubber particles, and biosynthesis is initiated through the binding of an allylic pyrophosphate (APP, a pyrophosphate, produced by soluble trans- rtnyl transferases) primer. Progressive additions of IPP molecules ultimately result in the formation of high molecular weight cjT-l,4-polyisoprene. The rubber transferase also requires a divalent cation, such as Mg + or Mn +, as cofactor. 

ELASTOMERS. Of natural or synthetic origin, an elastomer is a polymer possessing clastic (rubbery) properties. A polymer is a substance consisting of molecules which are. in the most part, multiples of low-molecular-weight units, or monomers. As an example, isoprcnc (2-methylbutadiene-1,3) is C 2 and normally is from 1,000 to 10.000 for rubbers. Although they differ in composition from natural rubber, many of these high-molecular-weight materials are termed. synthetic lubbers. See also Rubber (Natural). 

The reaction of AIBN with different diols (1,4-butanediol, 1,3-butanediol, 1,2-propylene glycol, ethylene glycol) leads to di(x-hydroxyalkyl)-2,2 -azobisisobutyrate 34). The di(4-hydroxybutyl)-2,2 -azobisisobutyrate has many advantages compared to 4,4 -azobis (4-cyano-n-pentanol) an easy synthesis, a good solubility in hydrocarbons, no transfer reactions. This initiator yields functionalized polyisoprenes with a high molecular weight 26). Its catalytic efficiency is similar to that of the other initiators. 

All spectra were obtained with the Varian HR-300 NMR Spectrometer, using H in normal mode, with occasional use of Fourier transform for very high molecular weight samples. Hexachlorobuta-diene was used as solvent, with 1%> hexamethyldisiloxane as reference. The temperatures used were 110 C. for polyisoprene and 125 C. for polybutadiene. The polymer samples were prepared with sec-butyllithium as initiator, using the high vacuum techniques described elsewhere (13). 

Problem 2.9 It has been determined that r YJ for high molecular weight cis-polyisoprene is (2.01v m, where n is the total number of bonds in the... 

In summary, cationic polymerization gives polymers of structure P8a Ziegler-Natta complexes mainly lead to blocks of structure P8b. This study gives further evidence for cyclization reactions of high-molecular-weight polyisoprenes. This point of view has been confirmed by the study of the cyclization of model polyisoprene molecules with two, three, or four monomer units toward the catalysts able to initiate such a reaction (24). 

Polymer of high molecular weight are in the main incompatible, but it is often advantageous to compound two polymers in order to obtain a combination of new physical properties which either polymer alone cannot have. For example, EPDM rubber is incompatible with diene rubbers such as polyisoprene, polybutadiene,... 

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