`=>`The use of polymers in the manufacture of plastic buckets, cups and saucers, children’s toys, packaging bags, synthetic clothing materials, automobile tyres, gears and seals, electrical insulating materials and machine parts has completely revolutionised the daily life as well as the industrial scenario.

`=>` The polymers are the backbone of four major industries viz. plastics, elastomers, fibres and paints and varnishes.

`=>` The word ‘polymer’ is coined from two Greek words : poly means many and mer means unit or part.

`color{green}("Polymer ")` : The term polymer is defined as very large molecules having high molecular mass `(10^3-10^7u)`. These are also referred to as `color{green}("macromolecules")`, which are formed by joining of repeating structural units on a large scale.

`color{green}("Monomer ")` : The repeating structural units are derived from some simple and reactive molecules known as monomers and are linked to each other by covalent bonds.

`color{green}("Polymerisation ")` : The process of formation of polymers from respective monomers is called polymerisation. The transformation of ethene to polythene and interaction of hexamethylene diamine and adipic acid leading to the formation of Nylon 6, 6 are examples of two different types of polymerisation reactions.

(i) `color{red}(undersettext(Ethane)(n CH_2 = CH_2) oversettext(Polymerisation)→ undersettext(Repeating unit) (n- [-CH_2 - CH_2 - ]-) → undersettext(Polythene polymer)(-[-CH_2-CH_2 - ]_n))`

(ii) `color{red}(undersettext(Hexamethylene diamine)(n NH_2 (CH_2)_6 NH_2) + undersettext(Adipic acid)(nHCOOC (CH_2)_4 COOH) oversettext(Polymerisation)→ [overset(overset(H)(|))N- (CH_2)_6 - undersettext(Nylon 6, 6)( overset( overset(H)(|))N- overset(overset(O)(||))C)- (CH_2)_4 - overset(overset(O)(||))C-]_n)`

There are several ways of classification of polymers based on some special considerations. The following are some of the common classifications of polymers :

Under this type of classification, there are three sub categories.

(i) `color{green}("Natural Polymers ")` : These polymers are found in plants and animals. Examples are proteins, cellulose, starch, resins and rubber.

(ii) `color{green}("Semi-synthetic Polymers ")` : Cellulose derivatives as cellulose acetate (rayon) and cellulose nitrate, etc. are the usual examples of this sub category.

(iii) `color{green}("Synthetic Polymers ")` A variety of synthetic polymers as plastic (polythene), synthetic fibres (nylon 6, 6) and synthetic rubbers (Buna-S) are examples of man made polymers extensively used in daily life as well as in industry.

There are three different types based on the structure of the polymers.

(i) `color{green}("Linear Polymers ")` : These polymers consist of long and straight chains. The examples are high density polythene, polyvinyl chloride, etc. These are represented as shown in fig.1.

(ii) `color{green}("Branched Chain Polymers ")` : These polymers contain linear chains having some branches, e.g. low density polythene. These are depicted as shown in fig.2.

(iii) `color{green}("Cross Linked or Network Polymers ")` : These are usually formed from bi-functional and tri-functional monomers and contain strong covalent bonds between various linear polymer chains, e.g. bakelite, melamine, etc. These polymers are depicted as shown in fig.3.

Polymers can also be classified on the basis of mode of polymerisation into two sub groups.

(i) `color{green}("Addition Polymers ")`: The addition polymers are formed by the repeated addition of monomer molecules possessing double or triple bonds.

`color{green}("Example ")`: The formation of polythene from ethene and polypropene from propene.

`color{red}("Note ")`

`color{green}("Homopolymer ")`: The addition polymers formed by the polymerisation of a single monomeric species are known as homopolymers, e.g., polythene.

`color{red}(undersettext(Ethane)(nCH_2= CH_2) → undersettext(Polythene)(-(CH_2-CH_2)_n) text(Homopolymer))`

`color{green}("Copolymer ")` : The polymers made by addition polymerisation from two different monomers are termed as copolymers, e.g., Buna-S, Buna-N, etc.

`color{red}(undersettext(1 , 3 - Butadiene )(nCH_2 = CH =CH_2) undersettext(Styrene)(n C_6H_5CH=CH_2) → undersettext{Butadiene styrene copolymer (Buna- S)}(-(CH_2-CH=CH-CH_2-CH_2-overset(overset(C_6H_5)(|))CH)_n))`

(ii) `color{green}("Condensation Polymers ")` : The condensation polymers are formed by repeated condensation reaction between two different bi-functional or tri-functional monomeric units.

● In these polymerisation reactions, the elimination of small molecules such as water, alcohol, hydrogen chloride, etc. take place.

● The examples are terylene (dacron), nylon 6, 6, nylon 6, etc.

`color{red}("Example ")` Nylon 6, 6 is formed by the condensation of hexamethylene diamine with adipic acid.

`color{red}(n H_2 N (CH_2)_6 NH_2+n HOOC (CH_2)_4 COOH → undersettext(Nylon 6 , 6)(-[-NH(CH_2)_6
NHCO(CH_2)_4CO-]_n) + n H_2O)`

`=>` A large number of polymer applications in different fields depend on their unique mechanical properties like tensile strength, elasticity, toughness, etc.

`=>` These mechanical properties are governed by intermolecular forces, e.g., van der Waals forces and hydrogen bonds, present in the polymer. These forces also bind the polymer chains.

`=>` Under this category, the polymers are classified into the following four sub groups on the basis of magnitude of intermolecular forces present in them.

(i) `color{green}("Elastomers ")` : These are rubber-like solids with elastic properties.

● In these elastomeric polymers, the polymer chains are held together by the weakest intermolecular forces.

● These weak binding forces permit the polymer to be stretched.

● A few ‘crosslinks’ are introduced in between the chains, which help the polymer to retract to its original position after the force is released as in vulcanised rubber.

● The examples are buna-S, buna-N, neoprene(fig.1), etc.

(ii) `color{green}("Fibres ")` : Fibres are the thread forming solids which possess high tensile strength and high modulus.

● These characteristics can be attributed to the strong intermolecular forces like hydrogen bonding.

● These strong forces also lead to close packing of chains and thus impart crystalline nature.

● The examples are polyamides (nylon 6, 6)(fig.2), polyesters (terylene), etc.

(iii) `color{green}("Thermoplastic Polymers ")` : These are the linear or slightly branched long chain molecules capable of repeatedly softening on heating and hardening on cooling.

● These polymers possess intermolecular forces of attraction intermediate between elastomers and fibres.

● Some common thermoplastics are polythene, polystyrene, polyvinyls(fig.3.), etc.

(iv) `color{green}("Thermosetting Polymers ")` : These polymers are cross linked or heavily branched molecules, which on heating undergo extensive cross linking in moulds and again become infusible.

● These cannot be reused.

● Some common examples are bakelite, urea-formaldelyde resins, etc.

The addition and condensation polymers are nowadays also referred as chain growth polymers and step growth polymers depending on the type of polymerisation mechanism they undergo during their formation.