NEET Chemistry Notes Polymers – Concept of Polymers

NEET Chemistry Notes Polymers – Concept of Polymers

Concept of Polymers

Concept of Polymers

Polymers are defined as very large molecules having high molecular mass (10³-10⁷u). These are formed by joining of repeating structural units, called monomers, on a large scale. The monomer units, are linked to each other by covalent bonds. The process of formation of polymer from respective monomers is called polymerisation.

Classification of Polymers
On the different basis the polymers are classified as follows:

On the Basis of Source

• Natural Polymers These polymers are found in plants and animals, e.g, proteins, cellulose, starch, resins and rubber, etc.
• Semi-synthetic Polymers These polymers are derived from natural polymers by chemical methods, e.g. cellulose acetate (rayon), cellulose nitrate, etc.
•  Synthetic Polymers These are man-made polymers, e.g. plastic (polythene), synthetic fibres, buna-S, etc.

On the Basis of Structure

• Linear Polymers These polymers consist of long and straight chains, e.g. high . density polythene, polyvinyl chloride, etc.
•  Branched Chain Polymers These polymers consist of linear chains with some branches, e.g. low density polythene.
• Cross-linked or Network Polymers, e.g. bakelite, melamine, etc.

On the Basis of Mode of Polymerisation

•  Addition Polymers or Chain Growth Polymers These are formed by addition reactions between monomer molecules having multiple bonds, e.g. polythene, SBR.
•  Condensation Polymers or Step Growth Polymers These are formed by condensation process with elimination of small molecules like NH₃,H₂0, etc., e.g. terylene, nylon-6 6, nylon-6, etc.

On the Basis of Monomers Involved

• Homopolymers These are formed by polymerisation of single monomeric species, e.g. polythene, polypropene, etc.
•  Copolymers These are formed by polymerisation of two or more different monomer units, e.g. buna-S, buna-N, etc.

On the Basis of Molecular Forces

•  Elastomers These are rubber-like solids with elastic properties. They have weak intermolecular forces.
  e.g. buna-S, buna-N, neoprene, etc.
• Fibres Fibres , are the thread forming solids and have intermolecular hydrogen bonding, e.g. nylon-66, polyesters (terylene), etc.
• Thermoplastic Polymers These polymers possess intermolecular forces of attraction, intermediate between elastomers and fibres, e.g. polythene, polystyrene, polyvinyls, etc. On long exposure to air and sunlight, thermoplastics become brittle due to the evaporation of plasticizer.
• Thermosetting Polymers These polymers are cross linked or heavily branched molecules, e.g. bakelite, urea-formaldehyde resins, etc.