Polymers: Complete Guide & Formula Sheet with Free PDF Download (JEE & NEET)
Polymers — Competishun
Polymers: Complete Guide & Formula Sheet with Free PDF Download (JEE & NEET)
Polymers is one of the most application-oriented chapters in Class 12 chemistry. It carries moderate weightage in JEE and NEET, with 1-2 questions appearing every year. This chapter deals with the chemistry of macromolecules that form the basis of many materials we use daily, from plastics and fibres to rubbers and biopolymers.
This chapter covers the classification of polymers based on source, structure, molecular forces, and mode of polymerisation. It discusses important addition polymers like polythene, PVC, polystyrene, and Teflon, as well as condensation polymers like nylon-6,6, terylene, and bakelite. The chapter also covers natural and synthetic rubbers, vulcanisation, and biodegradable polymers.
This page gives you the complete guide to Polymers with all concepts explained in depth. You will find clear definitions, classification tables, reaction mechanisms, and practice questions. Download the free PDF below and keep it handy for quick revision before your JEE Main, JEE Advanced, or NEET exam.
Download the Polymers Complete Guide PDF
Get all Polymers concepts, classification, addition and condensation polymers, rubbers, and biodegradable polymers in one clean PDF, free. Perfect for JEE and NEET revision.
Download Free PDFWhat are Polymers?
Polymers are everywhere in our daily lives. They form the basis of plastics, fibres, rubbers, and many natural materials like proteins, cellulose, and starch. The word "polymer" comes from the Greek words "poly" (many) and "meros" (parts).
Monomer
A small molecule that combines with others to form a polymer. Examples: ethene, vinyl chloride, styrene, glucose, amino acids.
Degree of Polymerisation
The number of monomer units in a polymer chain. Denoted by n. Higher n means higher molecular mass.
Glossary of Polymers Terms — Complete A to Z
Before diving deep into each topic, let's understand the key terminology used in this chapter:
| Term | Definition |
|---|---|
| Polymer | A high molecular mass compound formed from many monomers. |
| Monomer | A small molecule that combines to form a polymer. |
| Polymerisation | The process of forming a polymer from monomers. |
| Addition Polymer | Formed from unsaturated monomers without loss of any small molecule. |
| Condensation Polymer | Formed from bi-functional monomers with loss of a small molecule (H₂O, NH₃, HCl). |
| Copolymer | A polymer formed from two or more different monomers. |
| Elastomer | A polymer with weak intermolecular forces that can be stretched and returns to original shape. |
| Thermoplastic | A polymer that softens on heating and can be reshaped. |
| Thermosetting | A polymer that becomes rigid and infusible on heating (cross-linked). |
| Vulcanisation | The process of heating rubber with sulphur to form cross-links. |
| Degree of Polymerisation | The number of monomer units in a polymer chain. |
| Mastering these terms is essential for understanding the chemistry of polymers. | |
Classification of Polymers
Polymers can be classified based on four different criteria:
1. Based on Source
- Natural Polymers: Found in nature. Examples: starch, cellulose, proteins, natural rubber.
- Semi-Synthetic Polymers: Chemically modified natural polymers. Examples: cellulose acetate (rayon), vulcanised rubber.
- Synthetic Polymers: Man-made polymers. Examples: polythene, nylon-6,6, bakelite, PVC.
2. Based on Structure
| Structure | Description | Examples |
|---|---|---|
| Linear | Long, straight chains with no branching | HDPE, PVC, nylon |
| Branched | Chains with branches at various points | LDPE, amylopectin |
| Cross-linked | Chains connected by covalent bonds | Bakelite, vulcanised rubber |
| Cross-linked polymers are rigid and insoluble, while linear polymers are thermoplastic and soluble. | ||
3. Based on Molecular Forces
| Type | Intermolecular Forces | Properties | Examples |
|---|---|---|---|
| Elastomers | Weakest (van der Waals) | Stretchable, elastic | Natural rubber, neoprene |
| Thermoplastics | Moderate | Softens on heating, reshapable | Polythene, PVC, nylon |
| Fibres | Strong (H-bonds) | High tensile strength | Nylon-6,6, terylene |
| Thermosetting | Very strong (cross-linked) | Rigid, infusible | Bakelite, melamine |
| Force order: Elastomer < Thermoplastic < Fibre. This means tensile strength increases while elasticity decreases. | |||
4. Based on Mode of Polymerisation
- Addition Polymers: Formed from unsaturated monomers without loss of any small molecule.
- Condensation Polymers: Formed from bi-functional or poly-functional monomers with loss of a small molecule (H₂O, NH₃, HCl).
Addition Polymers — Chain Growth Polymerisation
Free Radical Mechanism of Addition Polymerisation
The polymerisation of ethene to polythene is a classic example of addition polymerisation via a free radical mechanism.
- Initiation: The initiator (e.g., benzoyl peroxide) decomposes to form free radicals. The radical adds to the monomer to form a new radical.
- Propagation: The radical chain grows by successive additions of monomer units.
- Termination: Two radicals combine to form a stable polymer, ending the chain.
LDPE vs HDPE — The Same Monomer, Different Products
| Property | LDPE (Low-Density Polyethylene) | HDPE (High-Density Polyethylene) |
|---|---|---|
| Conditions | 1000-2000 atm, 350-570 K | 6-7 atm, 333-343 K |
| Catalyst | O₂/peroxide (free radical) | Ziegler-Natta (TiCl₄ + Al(C₂H₅)₃) |
| Structure | Branched | Linear, close packed |
| Density | 0.92 g cm⁻³ | 0.97 g cm⁻³ |
| Uses | Squeeze bottles, wire insulation | Buckets, pipes, bottles |
| The same monomer (ethene) gives two different polymers under different conditions. LDPE is branched and more flexible; HDPE is linear and stronger. | ||
Important Addition Polymers
| Polymer | Monomer | Structure | Uses |
|---|---|---|---|
| Polythene (LDPE/HDPE) | Ethene (CH₂=CH₂) | [-CH₂-CH₂-]ₙ | Bags, bottles, pipes |
| Polypropene (PP) | Propene (CH₂=CH-CH₃) | [-CH₂-CH(CH₃)-]ₙ | Ropes, toys, pipes |
| PVC (Polyvinyl Chloride) | Vinyl chloride (CH₂=CHCl) | [-CH₂-CHCl-]ₙ | Raincoats, water pipes, flooring |
| Polystyrene (PS) | Styrene (C₆H₅CH=CH₂) | [-CH₂-CH(C₆H₅)-]ₙ | Insulator, toys, TV cabinets |
| Teflon (PTFE) | Tetrafluoroethene (CF₂=CF₂) | [-CF₂-CF₂-]ₙ | Non-stick cookware, gaskets |
| Orlon (PAN) | Acrylonitrile (CH₂=CH-CN) | [-CH₂-CH(CN)-]ₙ | Substitute for wool, carpets |
| Teflon is highly stable due to strong C-F bonds. It is non-stick and chemically inert. | |||
Condensation Polymers — Step Growth Polymerisation
Polyamides and Polyesters
| Polymer | Monomers | Linkage | Uses |
|---|---|---|---|
| Nylon-6,6 | Hexamethylenediamine + Adipic acid | Amide (-CONH-) | Bristles, textiles |
| Nylon-6 | Caprolactam | Amide (-CONH-) | Tyre cords, ropes |
| Terylene (Dacron) | Ethylene glycol + Terephthalic acid | Ester (-COO-) | Crease-resistant fabric, safety belts |
| Glyptal | Ethylene glycol + Phthalic acid | Ester (-COO-) | Paints and lacquers |
| Nylon-6,6 and terylene are fibres with high tensile strength due to strong intermolecular forces (H-bonds in polyamides, dipole-dipole in polyesters). | |||
Equations for Important Condensation Polymers
Phenol-Formaldehyde Polymers — Novolac and Bakelite
Novolac
- Formation: Phenol + HCHO (acid catalyst) → Novolac (linear polymer)
- Structure: Linear, thermoplastic
- Uses: Paints, adhesives
Bakelite
- Formation: Phenol + HCHO (base catalyst) → Bakelite (cross-linked polymer)
- Structure: 3-D network, thermosetting
- Uses: Combs, phonograph records, electrical switches
- Property: Cannot be re-moulded once set
Natural and Synthetic Rubbers — Elastomers
Natural Rubber
Natural rubber is cis-1,4-polyisoprene. The monomer is isoprene (2-methyl-1,3-butadiene).
- Structure: cis double bonds prevent close packing, making the chains coiled and elastic.
- Properties: Weak van der Waals forces, elastic, useful range 283-335 K.
- Limitation: Soft and sticky at high temperatures; brittle at low temperatures.
Vulcanisation of Rubber
Vulcanisation is the process of heating raw rubber with sulphur at 373-415 K. This forms cross-links between the polymer chains through sulphur bridges (-S-S-).
- Effect: Increases tensile strength, widens the elastic range, reduces stickiness.
- Application: Tyre rubber contains about 5% sulphur.
- Discovery: Charles Goodyear.
| Property | Raw Rubber | Vulcanised Rubber |
|---|---|---|
| Tensile Strength | Low | High |
| Elastic Range | 283-335 K | Wide |
| Stickiness | High | None |
| Water Uptake | Large | Low |
| Vulcanised rubber is an elastomer with cross-links. It is stronger and more durable than raw rubber. | ||
Synthetic Rubbers
| Rubber | Monomer(s) | Key Property | Use |
|---|---|---|---|
| Neoprene | Chloroprene (homo) | Resists oils | Conveyor belts, gaskets |
| Buna-S | Butadiene + Styrene (3:1) | Very tough | Auto tyres, floor tiles |
| Buna-N | Butadiene + Acrylonitrile | Resists petrol and oil | Oil seals, tank lining |
| Buna = Bu(tadiene) + Na. Neither Buna-S nor Buna-N contains sulphur. Sulphur is added during vulcanisation. | |||
Biodegradable Polymers — The Future of Polymers
Important Biodegradable Polymers
| Polymer | Monomers | Uses |
|---|---|---|
| PHBV | 3-Hydroxybutanoic acid + 3-Hydroxypentanoic acid (copolymer) | Packaging, controlled drug release |
| Nylon-2-nylon-6 | Glycine + Amino caproic acid | Biodegradable polyamide |
| PHBV is a copolymer of 3-hydroxybutanoic acid and 3-hydroxypentanoic acid. It is biodegradable and used for packaging. | ||
Molecular Mass of Polymers — Number Average and Weight Average
Polymers are mixtures of chains of different lengths. Therefore, their molecular mass is expressed as an average.
| Quantity | Formula | Meaning |
|---|---|---|
| Number Average (M̄ₙ) | M̄ₙ = Σ NᵢMᵢ / Σ Nᵢ | Average based on the number of molecules |
| Weight Average (M̄w) | M̄w = Σ NᵢMᵢ² / Σ NᵢMᵢ | Average based on the weight of molecules |
| Polydispersity Index (PDI) | PDI = M̄w / M̄ₙ | Measure of the spread of molecular masses |
| M̄w ≥ M̄ₙ ⇒ PDI ≥ 1. PDI = 1 only for natural (monodisperse) polymers. Synthetic polymers usually have PDI > 1. | ||
Thermoplastic vs Thermosetting Polymers
| Property | Thermoplastic | Thermosetting |
|---|---|---|
| Structure | Linear or branched | Cross-linked |
| On Heating | Softens | Infusible |
| Reusable | Yes (can be reshaped) | No (cannot be reshaped) |
| Examples | Polythene, PVC, nylon | Bakelite, melamine |
| Thermoplastics soften on heating and can be remoulded. Thermosetting polymers become rigid and infusible on heating due to cross-linking. | ||
Practice Questions — From JEE and NEET
| Question | Answer |
|---|---|
| Q1: What is the monomer of natural rubber? | Isoprene (2-methyl-1,3-butadiene). |
| Q2: What is the difference between addition and condensation polymers? | Addition polymers are formed from unsaturated monomers without loss of any small molecule. Condensation polymers are formed from bi-functional monomers with loss of a small molecule like H₂O or NH₃. |
| Q3: What is the monomer of Teflon? | Tetrafluoroethene (CF₂=CF₂). |
| Q4: What is vulcanisation of rubber? | Heating raw rubber with sulphur to form cross-links, improving tensile strength and elasticity. |
| Q5: What is the polymer obtained from phenol and formaldehyde? | Bakelite (cross-linked thermosetting polymer). |
| Q6: What is the PDI of a monodisperse polymer? | 1 (M̄w = M̄ₙ). |
| Q7: Which catalyst is used for the polymerisation of ethene to HDPE? | Ziegler-Natta catalyst (TiCl₄ + Al(C₂H₅)₃). |
| Q8: Give an example of a biodegradable polymer. | PHBV (poly-β-hydroxybutyrate-co-β-hydroxyvalerate). |
| Practise these types of questions to become comfortable with applying Polymers concepts in exam scenarios. | |
All Polymers Formulas at a Glance
| Formula/Concept | What It Means |
|---|---|
| M̄ₙ = Σ NᵢMᵢ / Σ Nᵢ | Number average molecular mass |
| M̄w = Σ NᵢMᵢ² / Σ NᵢMᵢ | Weight average molecular mass |
| PDI = M̄w / M̄ₙ | Polydispersity index (≥1) |
| n CH₂=CH₂ → [-CH₂-CH₂-]ₙ | Polymerisation of ethene |
| n H₂N-(CH₂)₆-NH₂ + n HOOC-(CH₂)₄-COOH → Nylon-6,6 + 2n H₂O | Nylon-6,6 formation |
| n HO-CH₂-CH₂-OH + n HOOC-C₆H₄-COOH → Terylene + 2n H₂O | Terylene formation |
| Memorise these formulas for Polymers. They are the key to scoring full marks in this chapter. | |
Common Mistakes in Polymers
- Confusing addition and condensation polymers: Addition polymers have no loss of small molecules; condensation polymers lose H₂O, NH₃, etc.
- Forgetting the difference between LDPE and HDPE: LDPE is branched (low density), HDPE is linear (high density).
- Misidentifying the monomer of natural rubber: Natural rubber is cis-1,4-polyisoprene (monomer: isoprene).
- Confusing thermoplastic and thermosetting: Thermoplastic softens on heating; thermosetting becomes rigid and infusible.
- Forgetting the vulcanisation process: Vulcanisation involves heating rubber with sulphur to form cross-links.
- Not checking the difference between Nylon-6 and Nylon-6,6: Nylon-6 is made from caprolactam; Nylon-6,6 is made from hexamethylenediamine and adipic acid.
Why Polymers Matters for JEE and NEET
- Moderate weightage: Polymers appears in 1-2 questions in every JEE Main, JEE Advanced, and NEET chemistry paper.
- Foundation for materials chemistry: Understanding polymers is essential for understanding plastics, fibres, rubbers, and composites.
- Direct scoring: Many questions are direct, especially on classification, monomers, and applications.
- Conceptual clarity: This chapter rewards students who understand the classification and properties rather than just memorizing names.
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Frequently Asked Questions — Polymers
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