Revision: Std XII >> Introduction to Polymer Chemistry MAH-MHT CET (PCM/PCB) Introduction to Polymer Chemistry

Small molecules that are interlinked together to form polymers are called as monomers.

Thermosetting polymers are cross-linked or heavily branched polymers which get hardened during the molding process. These plastics cannot be softened again on heating. Examples of thermosetting plastics include bakelite, urea-formaldehyde resins.

Polymeric solids which form threads are called fibres.

Polymers having the property of elasticity in which a substance gets stretched by external force and restores its original shape on the release of that force are called elastomers.

The man-made fibres prepared by polymerization of one monomer or copolymerization of two or more monomers are called synthetic fibres.

A polymer is a large molecule of very high molecular mass formed by the repeated combination of a very large number of one or more types of small molecules called monomers.

The process by which a network of cross-links is introduced into an elastomer is called vulcanization.

Polymers which are elastic in nature are called rubber.

The process by which a network of cross links is introduced into an elastomers is called vulcanisation.

• Polymers are very large molecules formed by joining many small molecules, having a high molecular mass.
• The small repeating units that make up polymers are called monomers.
• Monomers are linked together by covalent bonds to form long chains.
• The process of joining monomers to form a polymer is called polymerisation.

• Polymers are classified based on their source or origin into three types: natural, semi-synthetic, and synthetic polymers.
• Natural polymers are obtained from plants and animals, such as cellulose, starch, proteins, cotton, and silk.
• Semi-synthetic polymers are chemically modified forms of natural polymers, e.g., cellulose acetate and cellulose nitrate (rayon).
• Synthetic polymers are man-made and prepared in laboratories by polymerisation, such as nylon, polythene, Teflon, and Bakelite.
• Synthetic polymers are further used to make fibres, rubbers, and plastics, which have wide industrial applications.

• Polymers can be classified based on structure into linear, branched, and cross-linked (network) polymers.
• Linear polymers have long, straight chains of monomers joined in a line; e.g., HDPE, PVC, polystyrene.
• Branched polymers have a main chain with side branches of different lengths; e.g., LDPE, polypropylene.
• Cross-linked (network) polymers have chains connected by strong covalent bonds, forming a 3D network structure.
• Cross-linked polymers are hard and rigid due to strong bonding; common examples include Bakelite and melamine.

• Polymers are classified based on the mode of polymerisation into addition (chain growth), condensation (step growth), and ring-opening polymerisation.
• Addition polymerisation involves repeated addition of monomers (with double/triple bonds) without loss of small molecules; e.g., Teflon, PAN.
• Condensation polymerisation occurs between bi-functional or polyfunctional monomers with elimination of small molecules like water or alcohol; e.g., Nylon, Terylene.
• Ring-opening polymerisation involves polymerisation of cyclic monomers to form long chains, usually without elimination of small molecules; e.g., caprolactam.

• Polymers can be classified based on the type of monomers into homopolymers and copolymers (heteropolymers).
• Homopolymers are formed from only one type of monomer repeated many times; e.g., polythene, Nylon-6, PAN.
• Copolymers (heteropolymers) are formed from two or more different types of monomers.
• Copolymers show combined properties of different monomers; e.g., Buna-S (from butadiene and styrene), Buna-N.

Polymers are classified as biodegradable or non-biodegradable based on their biodegradability.

1. Biodegradable Polymers

• Meaning: Polymers that decompose by microorganisms or enzymes.
• Nature: Environment-friendly, as they break down naturally.
• Examples: PHBV, Nylon-2–Nylon-6.
• Special point: PHBV is a copolymer used in packaging, medical devices, and drug delivery.
• Use: Helps reduce environmental pollution.

2. Non-Biodegradable Polymers

• Meaning: Polymers that do not decompose by microorganisms.
• Nature: Persist in the environment for a long time.
• Examples: Polythene (LDPE, HDPE), Teflon, Nylon-6,6, PAN.
• Use: Widely used in plastics, fibres, containers, insulation, etc.
• Disadvantage: Causes environmental pollution due to slow decomposition.

• Vulcanisation of rubber is the process of introducing cross-links between polymer chains in an elastomer.
• It is done by heating natural rubber with sulphur and additives at a temperature of 373 K to 415 K.
• Sulphur forms cross-links at the double bonds of polyisoprene chains.
• Vulcanisation makes rubber stronger, less sticky, and more resistant to swelling by organic liquids.
• It also improves the elasticity and durability of rubber for practical use.