Natural rubber isoprene

A similar comparison can be made with cis-poly(isoprene), natural rubber, by taking advantage of the fact that the polymer is very slow to crystallize [164], Consequently, the comparison can be made between the supercooled, noncrystalline polymers at 0°C and the semi-crystalline polymer (31% crystalline) at the same temperature. The Tlc values for each of the five carbons involved were again found to be the same for the completely disordered polymer and the semicrystalline one, so that a similar conclusion can be made with regard to their chain structure. 

Natural polymerization of isoprene (natural rubber) produces the less symmetrical cis isomer as can be seen in the up- and down-orientation of the methyl group. This will result in the amorphous namral rubber. The synthetic more symmetrical trans isomer is a crystalline hard and rigid solid and will be obtained upon polymerization at low temperature. This transoid geometry is more stable than the cisoid due to the existence of the methyl group which facilitates the addition upon trans orientation. 

Poly(isoprene) occurs naturally as cis-l,4-poly(isoprene) (natural rubber) and as fraws-l,4-poly(isoprene) (gutta percha, balata). Both isomers can also be prepared synthetically. 

Poly(isoprene) (natural rubber) OCH3 3 /H /C— -H2C CH2- -70 36 Elastomer... 

The remaining five polymers have extremely low values of ASc. These values can be related to some element of disorder within the crystal structure. The low value of ASc per bond found for poly(tetrafluoroethylene) can be attributed to the room temperature polymorphic transition and the associated entropy change.(233,234) On the other hand, poly(cis-l,4-isoprene), natural rubber, is not known to undergo any polymorphic transitions at atmospheric pressure. Yet ASc per bond is only 0.43 e.u. There is, however, some disagreement in interpreting the x-ray diffraction pattern of this polymer. Nyburg has concluded that the crystal structure is statistically... 

Fig. 13. C-13 spectrum of poly-c/s-l,4-isoprene (natural rubber) the sample is a piece ot vulcanized vacuum tubing (without carbon black) inserted into a 5 mm O. D. NMR tube...

Poly(l,4-isoprene) (natural rubber) Hydroxyl-terminated natural rubber Polyisoprene vulcanizate... 

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