Polymers are composed of a large number of repeating units (small molecules) called monomers. A polymer is, therefore, made up of thousands of monomers joined together to form a large molecule of colloidal dimension, called macromolecule. The properties of polymers are intricately related to the structural elements of the material. Most polymers are organic in origin and are based on hydrocarbons, i.e., they are composed of hydrogen and carbon. Moreover, the intramolecular bonds are covalent. Each carbon has four electrons that may participate in covalent bonding, whereas every hydrogen atom has only one bonding electron. A single covalent bond exist when each of the two bonding atoms contribute one electron, as shown in Fig. for methane. Double and triple bonds exist between two carbon atoms involve the sharing of two and three pairs of electrons, respectively. Figure shows the structural formula for ethylene (C2H4).
The process of linking together of monomers is called polymerisation. The need to start with the process of polymerisation lies on the necessity of breaking the double bond (C=C) of the monomer. It utilizes the valence of the partially filled outer shell of the carbon atom (carbon has a valence of 4) to join smaller units together to form larger chain of molecules. Oxygen, sulphur, silicon, or nitrogen can be used to replace the carbon atom. This requires considerable energy. Polymerisation mechanisms may be of the following two types:
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