Polymers Class 12 Science Chapter Notes

Key Terminology in Polymers

Polymer

A high molecular mass substance (macromolecule) consisting of repeating structural units (monomers) linked by covalent bonds.

Monomer

A simple molecule that can chemically bond to other identical or different molecules to form a polymer.

Polymerization

The process by which monomers are converted into a polymer.

Homopolymer

A polymer formed from the polymerization of a single type of monomer. Example: Polythene from ethene.

Copolymer

A polymer formed from two or more different types of monomers. Example: Buna-S from 1,3-butadiene and styrene.

Elastomer

Polymers with rubber-like properties, characterized by weak intermolecular forces, allowing them to be stretched. Example: Natural rubber.

Thermoplastic Polymers

Linear or slightly branched polymers that soften on heating and harden on cooling, and can be remoulded. Example: PVC, Polythene.

Thermosetting Polymers

Cross-linked or heavily branched polymers that, on heating, undergo extensive cross-linking and become infusible and cannot be remoulded. Example: Bakelite, Urea-formaldehyde resin.

Biodegradable Polymer

Polymers that can be decomposed by the action of living organisms (bacteria, fungi). Example: PHBV.

Understanding Polymerization Mechanisms

Polymerization is the fundamental process of creating polymers from monomers. It is broadly classified into two main types: Addition Polymerization and Condensation Polymerization.

Addition Polymerization, also known as chain-growth polymerization, involves the repeated addition of monomer molecules to a growing polymer chain without the loss of any small molecules like water or ammonia. The empirical formula of the polymer is the same as that of the monomer. This process typically occurs with unsaturated monomers containing double or triple bonds. The mechanism can proceed through three main pathways:

1. Free Radical Mechanism: This is the most common method, initiated by a free radical (e.g., from benzoyl peroxide). It has three steps: initiation (creation of free radical), propagation (radical adds to a monomer, creating a new radical which adds to another monomer, and so on), and termination (two radicals combine to end the chain growth).
2. Cationic Polymerization: Initiated by an acid (a source of H+), it forms a carbocation which then propagates the chain.
3. Anionic Polymerization: Initiated by a strong base (like BuLi), it forms a carbanion that propagates the chain.

Examples include the formation of polythene from ethene and PVC from vinyl chloride.

Condensation Polymerization, or step-growth polymerization, involves a reaction between two bifunctional or polyfunctional monomers. In each step, a small molecule like water (H₂O), ammonia (NH₃), or hydrogen chloride (HCl) is eliminated. The empirical formula of the repeating unit is not the same as the monomers. This process continues until long polymer chains are formed. Examples include the formation of Nylon 6,6 from hexamethylenediamine and adipic acid, and Dacron (Terylene) from ethylene glycol and terephthalic acid.

Addition vs. Condensation Polymerization

Must Remember for Polymers

• Nylon 6,6 Monomers: Hexamethylenediamine (6 carbons) and Adipic acid (6 carbons). The name 6,6 indicates the number of carbon atoms in the two monomers.
• Nylon 6 Monomer: Caprolactam. It's a homopolymer formed by ring-opening polymerization.
• Bakelite Monomers: Phenol and Formaldehyde. It is a thermosetting plastic.
• Natural Rubber Monomer: Isoprene (2-methyl-1,3-butadiene). It is a polymer of cis-1,4-isoprene units.
• Vulcanization: The process of heating natural rubber with sulphur (3-5%) to improve its physical properties like elasticity, strength, and resistance to abrasion. Sulphur forms cross-links at the double bonds.
• Biodegradable Polymers: Know at least two examples: PHBV (Poly-β-hydroxybutyrate-co-β-hydroxyvalerate) and Nylon 2-Nylon 6.
• Monomers of PHBV: 3-hydroxybutanoic acid and 3-hydroxypentanoic acid.
• Teflon (PTFE): Chemically inert and resistant to heat. Its monomer is Tetrafluoroethene (CF₂=CF₂).
• Buna-S Monomers: 1,3-Butadiene and Styrene. 'Bu' for butadiene, 'na' for sodium (catalyst), 'S' for styrene.
• Buna-N Monomers: 1,3-Butadiene and Acrylonitrile. 'N' for nitrile group.

Thermoplastics vs. Thermosetting Plastics

Common Polymerization Reactions

• {"title":"Formation of Polyvinyl Chloride (PVC)","bodyMarkdown":"Vinyl chloride monomers undergo addition polymerization to form PVC.\nMonomer: Vinyl Chloride (CH₂=CHCl)\nReaction: n(CH₂=CHCl) --(Polymerization)--> [-CH₂-CHCl-]_n\nThis is a chain-growth (addition) process."}
• {"title":"Formation of Nylon 6,6","bodyMarkdown":"Hexamethylenediamine reacts with adipic acid in a condensation polymerization reaction, eliminating water molecules.\nMonomers: Hexamethylenediamine [H₂N-(CH₂)₆-NH₂] and Adipic acid [HOOC-(CH₂)₄-COOH]\nReaction: n H₂N(CH₂)₆NH₂ + n HOOC(CH₂)₄COOH → [-NH(CH₂)₆NHCO(CH₂)₄CO-]_n + 2n H₂O"}
• {"title":"Identifying Monomers","bodyMarkdown":"Problem: Identify the monomer(s) in the polymer: [-CF₂-CF₂-]_n\nSolution: This is an addition polymer. To find the monomer, identify the repeating unit and introduce a double bond between the carbons. The repeating unit is -CF₂-CF₂-. The monomer is Tetrafluoroethene (CF₂=CF₂). The polymer is Teflon (PTFE)."}

Exam Traps and Scoring Tips

For the Polymers chapter, marks are often lost in simple recall.

• Monomer Names & Structures: Be precise. Don't mix up the monomers for Nylon 6 (Caprolactam) and Nylon 6,6 (Hexamethylenediamine and Adipic acid). Drawing the structures is often required.
• Reaction Equations: When asked for a polymerization reaction, always show the 'n' for the number of units and write the full equation, including any by-products like H₂O in condensation reactions.
• Classification: Questions on classifying a polymer (e.g., Is Bakelite a thermoplastic or thermosetting plastic? Is natural rubber an elastomer or a fibre?) are very common. Remember the basis of classification (source, structure, forces).
• Specific Uses: Questions might ask for one important use of a polymer like Teflon (non-stick coating) or Bakelite (electrical switches). Memorize one key application for each major polymer.

Quick Revision Check

• What are the monomers of Bakelite? Phenol and Formaldehyde.
• What is the function of sulphur in the vulcanization of rubber? Sulphur forms cross-links between the poly-isoprene chains, which improves the rubber's strength, elasticity, and resistance to heat and chemicals.
• Give an example of a biodegradable polymer and name its monomers. PHBV (Poly-β-hydroxybutyrate-co-β-hydroxyvalerate). Its monomers are 3-hydroxybutanoic acid and 3-hydroxypentanoic acid.
• Differentiate between a homopolymer and a copolymer with one example each. A homopolymer is made from a single type of monomer (e.g., Polythene from ethene). A copolymer is made from two or more different monomers (e.g., Buna-S from 1,3-butadiene and styrene).

Frequently Asked Questions on Polymers

Frequently Asked Questions