In Elastomer, the Polymer chains are held together by the weakest intermolecular forces. These weak forces permit the polymer to be stretched. A few cross-links are introduced in between the chains which help the polymer to released to its original position after the forces is released.
Buna-S: Buna-S is obtained by the polymerization of butadiene and styrene in presence of sodium metal.
Buna-N: Buna-N is obtained due to co-polymerization of two parts of butadiene with one part of acrylonitrile in pressure of sodium metal.