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High polymers, also known as macromolecules, are large molecules of colloidal dimensions
(10–3 to 10–6 mm in diameter) having high molecular weight (10,000 to millions). Small molecules
called monomers undergo polymerisation reaction and form macromolecules. The examples
of polymeric materials are resins, plastics and rubbers.
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Polymerisation may be defined as the union of two smaller molecules of similar or different
types with or without elimination of water resulting in the formation of new C-C linkages. The
mechanism by which polymerisation takes place may be addition or condensation.
A monomer for polymerisation should be bi-, or polyfunctional, i.e., it must contain two or
more functional groups. Some of the other functional groups are hydroxyl acid, amino acid,
di-amino acid, di-acids, di- or polyalcohols.
The structures depend on the functionability of monomers. In case of a bi-functional monomer
there will be two reactive groups at its ends. These groups may align side by side to form a
straight chain like molecule as shown in Fig. 16.1. The monomer units are linked by primary
covalent bonds and the different chains are held together by secondary force of molecular
attraction. However, during the chain growth side chains may also develop leading to branched
chain molecules as shown in Fig. 16.2.
In case of poly-functional groups the monomer molecules are connected to each other by
covalent bonds and form a three-dimensional network (Fig. 16.3).
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