Polymers are very large molecules, which are long chains of repeated smaller units joined together. These smaller units are known as monomers. The process of formation of polymers from the simple starting materials (monomers) is called as polymerization. It is also defined as the inter–molecular combination of large number of monomers to make a large molecule called a macro molecule with or without elimination of simpler molecules like water, HCl, NH₃ etc. Molecules are generally regarded as polymers rather than just very large molecules when there are around 50 or more monomer units in the chain.

Classification of polymers:
On the basis of their occurrence or mode of origin:
Natural polymers: The polymers obtained from nature (plants and animals) are called natural polymers. These polymers are very essential for life.
Eg: Starch,Cellulose, Proteins, Nucleic acids etc.

Synthetic polymers: The polymers which are prepared in the laboratories are called synthetic polymers.These are also called man made polymers.
Eg: Polyethene, PVC, nylon, teflon, bakelite, terylene, synthetic rubber etc.

Semi–synthetic polymers: These polymers are mostly derived from naturally occurring polymers by their chemical modifications.
Eg: Rayon and other cellulose derivatives like cellulose nitrate, cellulose acetate etc. are semi–synthetic polymers. Gun cotton which is cellulose nitrate used in making explosives.

Based on types of monomers:
On the basis of monomers, polymers are classified into two groups:
Homopolymer: It is formed by the polymerization of one type of monomers.
Eg: Polyethene and PVC

Copolymer: When two different types of monomers are joined in the same polymeric chain, the polymer is called a copolymer.
Eg: Nylon–66 and Buna–S rubber are copolymers.

On basis of structure:
Polymers are classified as follow based on their structure.
Linear Polymers: In these monomeric units are joined in the form of long straight chains, such polymers have high densities, high tensile strength and high melting point.
Eg: Polyethylene, nylons and polyesters.

Branched chain Polymers: In these, the polymers are mainly linear in nature but also possess some branches along the main chain.
Eg: low density polyethene (LDPE), Amylopectin and glycogen.

Cross linked polymers: Monomeric units are linked together to constitute a three dimensional network. They are hard, rigid, and brittle.
Eg: Bakelite, Melamine formaldehyde resin, etc.

Based on inter particle forces/molecular forces:
The practical utility of polymers is based on their mechanical properties such as tensile strength, elasticity, toughness etc. These mechanical properties depend upon the nature and strength of forces acting between the polymeric chains. Therefore, polymers are classified into following:

Elastomers: Polymer chain in elastomers are held up together by weakest attractive forces/inter molecular forces. Elastomers become soft on heating and can be moulded into desired shape. They are amorphous polymers having high degree of elasticity.
Eg: Synthetic rubber

Fibrous polymers: These are the polymers which have quite strong interparticle forces such as hydrogen bond. They have high tensile strength, low elasticity and crystalline nature. These polymers are thin, thread like and can be woven.
Eg: Nylon 6,6, Terylene, Dacron etc.

Thermoplastic polymer: These are linear, long chain polymers, which can be softened on heating and hardened on cooling reversibly. The chains in thermoplastic polymers associates through intermolecular forces; as a result the thermoplastics can be remolded because the intermolecular interactions reform deliberately upon cooling.
Eg: Polyethene(PE), Polypropylene(PP), Polyvinyl Chloride (PVC), Nylons, Polytetrafluoro ethylene(PTFE or Teflon) etc.

Thermosetting polymers: These are the polymers which become hard and infusible on heating. Once they have solidified, they cannot be softened. Heating results excessive cross linking between the chains forming three dimensional network. They are permanent setting polymers.
Eg: Phenol–formaldehyde resins and melamine–formaldehyde resins.

Based on the mode of synthesis:
Addition polymerization: Addition polymerization is the simple addition of the molecules. Polyethene is an addition polymer.

Condensation polymerization: These are formed by the polymerization of monomers with the elimination of small molecules such as NH₃, H₂O, CH₃ and OH.
Eg: Formation of Terylene.

Based on the sequence of synthesis:

Chain growth polymerization: Chain growth polymerization is the linking of molecules having multiple bonds. These unsaturated monomers have extra internal bonds which are able to break and link up with other monomers to form repeating chain. All the addition polymers are chain growth polymers, but few examples of condensation polymers are also present those mechanism proceed through chain growth polymerization.
Eg: Free radicals, Carbocation and Carbanion are three different types of reactive particles have been found to be formed during the chain polymerization.

Step growth polymerization: Step growth polymerization is defined as the polymerization in which polymers are formed by the stepwise reaction between functional groups of monomers. Step growth polymers increase in molecular weight at a very slow rate. Mostly condensation polymers are formed by step growth polymerization but few exceptions also exist.
Eg: Polymerization of polyurethane (addition polymer) proceeds through step growth mechanism.

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