Polymers are a class of chemical compounds with high molecular weight made of smaller, identical molecules (monomers), linked together. The process by which the molecules are linked together to form a polymer is called polymerization. Polymer molecules can be straight-chains, branched chains or cross-linked chains. Requirement are multible double bonds on the monomers. Hereby one distinguishes between a polycondensation and polyaddition.
Polycondensation: Requirements are the functional groups on one monomer. This results in a polycondensate and a side product (which is often water). The peptide bonds, forming peptides and proteins are formed by polycondensation. Most natural polymers, such as proteins and DNA, are condensation polymers.
Addition polymerisation: involves the linking of monomers with double bonds. The double bond of one monomer breaks and links onto the neighbouring monomer.This process continues to form polymers, some of which can be many thousands of monomers long. One example for this type of polymerisation is the reaction to form polyethene which is initiated by a free radical. A free radical contains an unpaired electron and is very reactive. This free radical attacks the double bond of ethene and bonds to the carbon atom of ethene. In the process it creates another free radical which attacks the double bond of a neighbouring ethene molecule.