4 . Natural Rubber Natural rubber is a coiled linear 1 , 4-polymer of isoprene .
In the polymer chain of natural rubber , the residual double bonds are located between C2 and C3 of the isoprene unit . All these double bonds have cis configuration , and thus natural rubber is cis-l , 4-polyisoprene .
In the natural rubber , there is no polar substituent . The only intermolecular forces are van der Waals ’ type . The cis-configuration gives the polymeric chain of natural rubber a coiled structure . As a result , it can be stretched by the application of a force . When the force is removed , the chain returns back to its original coiled shape .
Natural rubber is soft and sticky . It can be used only in the temperature range 10 ° C-50 ° C . At higher temperature , it becomes soft and at low temperature , it becomes brittle . It has higb water absorption capacity . It is attacked by oxidising agents and organic solvents . As such , it cannot be used very extensively for commercial puposes .
Vulcanisation of Rubber
The properties of natural rubber can be modified by introducing -S-Spolysulphide crosslinks in its structure . This process of introducing -S-Scrosslnks in the structure of natural rubber by heating with sulphur at 11O ° C is called vulcanlsation of rubber .