Rubber

Rubber is a natural polymer formed by the polymerisation of a monomer called isoprene. The structure of an isoprene molecule is shown below.

The formation of the polymer can be represented like this:

Why a double bond moves to the middle

Isoprene has two double bonds, one at each end of its four-carbon chain. When it polymerises, both of them open, and a new double bond forms between the two middle carbons. The end carbons are left with one free bond each, which join to the neighbouring units. So each repeating unit of rubber keeps one double bond, in the middle, unlike polythene, whose repeating unit has none. Those remaining double bonds are important: they are where sulphur can later join the chains together when rubber is vulcanised. Count the bonds at each carbon in the repeating unit and you will still find four at every one.

Natural rubber comes from latex, the milky sap tapped from the rubber tree — a crop grown widely in Sri Lanka's wet zone. The latex is a suspension of these long polyisoprene chains in water.