What is Polymer? How they are classified?

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 Introduction:

  • Polymers are macromolecules having high molecular weights and are composed of smaller repeating units called monomers. 
  • The term polymer is derived from the Greek root 'poly' means 'many' and 'meros' means 'parts'.
  •  Monomers are small and simple molecules and have the capacity of forming two chemical bonds with two other monomers.
  • The functionality of monomers depends upon the number of bonding sites in monomers. 
  • DNA in a human body, proteins, muscles, nerve fibers, nails, hairs, etc. are all polymers.
  • Carbohydrates have important functions in biological systems.
  • The toys of our children are plastic polymers.

Classification:

There are various ways to classify polymers based on source, structure, mode of forming, molecular forces, etc.

classification based on source:

Polymers are classified into three subclasses as given below,
  1. Natural polymers: These polymers are obtained either from plants or animals and are named plant polymers and animal polymers e.g. cellulose, jute, silk, wool, linen, etc.
  2. Semisynthetic polymers: The properties of natural polymers are modified by some chemical treatment. The properties which are modified are appearance, tensile strength, luster, etc. 
  3. Synthetic polymers: These are manmade, prepared in laboratories or industries from chemical substances. e.g. nylon, terylene, polythene, etc.

Classification based on the structure: 

The polymers are further classified into three types.
A) Linear polymers: 
They are made up of long continuous chains without any excess attachments (branches). 
The repeating units are joined together to form a long chain e.g. polyethylene, PVC.
 
       CH2
  n    ||   → ―CH2―CH2―  → ―[CH2―CH2―]n
       CH2
     ethene       repeating          polyethene
                         unit  


B) Branched polymers: 
These polymers consist of chain structures having one main chain of molecules with smaller chains as branches of the main chain. They have low density. 
e.g. polypropylene having methyl groups as branches 
                                        CH3              CH3
                                         |                  |
  n  CH3―CH=CH2 → ―CH―CH2―  → [― CH―CH2― ]n

      propylene                         repeating unit     polypropylene


C)Network or cross-linked polymers:
The linking of chain polymers by strong covalent bonds leads to a network-like structure. They are also called cross-linked structures due to cross-linking.
e.g. Melamine, bakelite, vulcanization of rubber, etc. 
Source: Internet


Classification based on the polymerization process:

These are of two types as given below:
A) Addition polymers:
The monomers are alkene and the polymers are formed by chain-growth polymerization. The addition polymers contain all the atoms of monomers.
e.g. PVC, Polyethene.

                     CN                    CN
                      |                      |  
          n CH2=CH  → ―CH2―CH―   → [―CH―CH―]n

             vinyl cyanide   repeating unit        orlon
                   (monomer)

B) Condensation polymer:
These polymers are formed by the combination of two monomers by elimination e, g. polyamide (nylons), polyesters, polyurethanes, etc.
The reaction between two or more monomers is stepwise. 
polymerization to form dimers, tetramers..........and so on.
(e.g. nylon, polyester, etc)
                        

Classification based on molecular forces:

The properties of polymers depend on molecular forces bind polymer chains either by hydrogen bonds or by Vander Waal's forces. These forces are called intermolecular forces. Polymers are further classified as follows:
a) Elastomer:
When polymer is stressed,the polymer chain stretches, but due to cross linking tearing is prevented. When the strain is relieved the chain returns to its original position. Thus polymers show elasticity and are called elastomers. They are soft and stretchy and are used in making rubber bands. e.g. neoprene,vulcanized rubber.

b) Fiber: 
Strong intermolecular forces in fibers of polymers are due to hydrogen bonding.These polymers possess high tensile strength and are used in textile industries, strong ropes, tyre cords, etc. Polymer chains form fibers by hydrogen bonding.
e.g. nylon, terylene.
 
c) Thermoplastic polymer: 
Their properties are between elastomers and fibers. These polymers are called Thermoplastic as on heating they become soft and on cooling become hard. Hence these polymers can be remoulded and recycled. They are either linear or branched chain polymers. e.g. polyehene, PVC, etc 

d) Thermosetting polymers: 
These do not become soft on heating under pressure, hence can not be remoulded and recycled. These are cross linked polymersand can not be reused. E.g. Bakelite.

 












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