Natural rubber is obtained from the milky sap of the rubber tree, called latex. Its chains have the structure of 2-methylbuta-1,3-diene (isoprene) joined by addition polymerization.
Isoprene has two double bonds. When the molecules join, both double bonds open, and a new double bond forms between the second and third carbon atoms of each unit. This is called 1,4-addition, because the units are joined through carbon atoms 1 and 4.
The repeating unit is \mathrm{{-}CH_2{-}C(CH_3){=}CH{-}CH_2{-}}, so natural rubber is also called polyisoprene.
Each repeating unit contains a double bond, and the two parts of the chain on either side of it can be on the same side or on opposite sides. In natural rubber, almost all the units have the cis form.
In the cis form, the chain bends at every unit, so the chains cannot line up neatly. This is why natural rubber is soft and elastic.
Gutta-percha, obtained from another kind of tree, is the trans form of polyisoprene. Its straight chains pack closely and crystallize, so it is hard and not very elastic.
The chains in rubber are tangled and coiled, and they are always moving because of thermal motion.
When rubber is pulled, the coiled chains straighten out. When it is released, thermal motion makes the chains coil up again, so the rubber returns to its original shape.
If the chains are not linked to each other, however, they slowly slide past each other while the rubber is pulled. Raw rubber therefore does not spring back completely, and it becomes sticky when warm.
When rubber is heated with a few per cent of sulfur, sulfur atoms link neighbouring chains. This process was discovered by Charles Goodyear in 1839 and is called vulcanization.
The links stop the chains from sliding past each other, so vulcanized rubber springs back strongly, stays elastic over a wider range of temperatures and is much stronger. It is used for tyres and many other rubber goods.
When 30 to 40 per cent of sulfur is added, the chains are linked so tightly that the product is a hard material called ebonite.
Synthetic rubbers are made by addition polymerization of dienes, compounds with two double bonds. Butadiene rubber is made from buta-1,3-diene, \mathrm{CH_2{=}CH{-}CH{=}CH_2}, and chloroprene rubber, which resists weather and oil, from 2-chlorobuta-1,3-diene (chloroprene), \mathrm{CH_2{=}CCl{-}CH{=}CH_2}.
Polymerizing two kinds of monomer together is called copolymerization. Styrene-butadiene rubber (SBR), made from phenylethene (styrene) and buta-1,3-diene, is used mainly for tyres.
Acrylonitrile-butadiene rubber (NBR), made from propenenitrile (acrylonitrile) and buta-1,3-diene, resists oil, so it is used for fuel hoses and gloves.