Natural polyisoprenes

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. 

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Diene polymers refer to polymers synthesized from monomers that contain two carbon-carbon double bonds (i.e., diene monomers). Butadiene and isoprene are typical diene monomers (see Scheme 19.1). Butadiene monomers can link to each other in three ways to produce ds-1,4-polybutadiene, trans-l,4-polybutadi-ene and 1,2-polybutadiene, while isoprene monomers can link to each other in four ways. These dienes are the fundamental monomers which are used to synthesize most synthetic rubbers. Typical diene polymers include polyisoprene, polybutadiene and polychloroprene. Diene-based polymers usually refer to diene polymers as well as to those copolymers of which at least one monomer is a diene. They include various copolymers of diene monomers with other monomers, such as poly(butadiene-styrene) and nitrile butadiene rubbers. Except for natural polyisoprene, which is derived from the sap of the rubber tree, Hevea brasiliensis, all other diene-based polymers are prepared synthetically by polymerization methods. 

At least one natural polyisoprene consists of both cis and trans isomers. Chicle, found in Central America, is approximately 75% trans, the remainder being ds. It was used as the base for chewing gum for many years, until it was replaced in the midtwentieth century by the synthetic polymer poly (vinyl acetate). 

Rubber tyres are by far the most visible of rubber products. Identification is trivial and collection is well organized. Recycling and disposal, however, are less evident. A major route for tyres is their use as a supplemental fuel in cement kilns. Major compounds in tyres are styrene-butadiene rubber (SBR), synthetic and natural polyisoprene rubber, steel cord, carbon black, zinc oxide, sulphur and vulcanization-controlling chemicals. Tyres can be retreaded, which is economic for large sizes (truck tyres), or ground to crumb or powder (cryogenic grinding). Such materials have some limited market potential as an additive in asphalt, and in surfaces for tennis courts or athletics. 

Cyclohexane as the mobile phase requires the use of polyisoprene standards, as polystyrene standards are not soluble in this solvent. It should be noted that calibration by polystyrenes results in an overestimation of molar masses by a factor of around 2, compared to the use of polyisoprene standards [10]. It is, therefore, necessary to carry out universal calibration or to convert molar masses using the Mark-Houvink coefficients relative to synthetic or natural polyisoprenes [4,5,8,11,12]. 

According to this mechanism, natural rubber chains are expected to have one dimethylallyl terminal unit and one isoprenyl pyrophosphate terminal unit the latter may give rise to a hydroxyl group by hydrolysis. From this point of view, acyclic terpenes in the generalized structure (II) may be appropriate models for the structural characterization of natural polyisoprenes by 13C NMR spectroscopy. 

This (synthetical) elastomer is analogous to the (natural) polyisoprene and can also be vulcanized to make rubber (see Chapter 1, Section 4.4). 

Tanaka Y (2001) Structural characterization of natural polyisoprenes Solve the mystery of natural rubber based on structural study. Rubber Chemistry and Technology 74 355-375. 

Tanaka, Y. (1989). Structure and biosynthesis mechanism of natural polyisoprene. Prog. Polym. Set 14(3), 339-371. 

The above discussion relates to the "drying of oils by oxygen. Materials like natural rubber (see Natural Polyisoprenes section) are vulcanized with sulfur. The unsaturated oils can be made to undergo a similar set of reactions (48). Brown factice is made by first blowing a drying oil, then reacting it with 5-30% of sulfur at 250-350°F for 1-2 hours. 

They are also natural polyisoprenes and are isomers of natural rubber. They are extracted from sheets of plants in the family of sapotaceae (Southeast Asia and Equatorial America). The polymer chains are constructed from repeating units of 1,4-trans isoprene structure. Contrary to natural rubber, they partially crystallize spontaneously according to two conformations one is 2i helical (a-form), the other one is totally transplanar and is equivalent to a li helix ( -form). The mechanical properties of these materials are determined by the values of the transition temperatures. Amorphous zones undergo a glass transition at —60 C and the two crystalline forms (a and P) melt at 64°C. The corresponding polymers are thus thermoplastics and acquire (after vulcanization) a highly elastic behavior beyond 70°C. 

Natural polyisoprene Natural rubber (NR) 500-760 -60 to 120 (-75 to 250) Excellent physical properties good resistance to cutting, gouging, and abrasion low heat, ozone, and oil resistance good electrical properties Pneumatic tires and tubes heels and soles gaskets extruded hose... 

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