Polymers
Welcome, students of HSLC Guru! In this lesson we will study Chapter 15 — Polymers from your ASSEB Class 12 Chemistry syllabus. Polymers are giant molecules built from thousands of small repeating units called monomers. From the cotton in our clothes, the rubber in tyres, the plastic in our water bottles to the proteins, starch and DNA inside our bodies, polymers literally surround and sustain us. This chapter explains what polymers are, how they are classified, how they are made, the differences between common synthetic polymers, and the modern push toward biodegradable polymers that protect our environment.
Chapter Summary
Definition and basic concept. A polymer is a high molecular mass macromolecule formed by the repeated linking of a large number of simple low molecular mass molecules called monomers. The process of joining monomers to form a polymer is known as polymerisation. The word polymer comes from the Greek words “poly” meaning many and “meros” meaning parts. The repeating structural unit of a polymer is called the repeat unit. For example, polythene is made of repeating —CH₂—CH₂— units derived from the monomer ethene (CH₂=CH₂). The number of monomer units in a polymer chain is called the degree of polymerisation (DP), and it largely determines the strength, viscosity and physical properties of the polymer.
Classification of polymers. Polymers are classified on several bases. (i) Based on source: natural polymers like starch, cellulose, proteins, natural rubber and nucleic acids; semi-synthetic polymers like cellulose nitrate, cellulose acetate and vulcanised rubber; synthetic polymers like polythene, nylon, PVC, bakelite and polystyrene. (ii) Based on structure: linear polymers with long straight chains (HDPE, nylon, PVC), branched chain polymers with side chains on the main backbone (LDPE, glycogen, amylopectin), and cross-linked or network polymers in which chains are interconnected by covalent bonds (bakelite, melamine, vulcanised rubber). (iii) Based on mode of polymerisation: addition polymers formed by addition of unsaturated monomers (polythene, PVC, teflon) and condensation polymers formed with the elimination of small molecules like water, methanol or ammonia (nylon-66, terylene, bakelite). (iv) Based on molecular forces: elastomers with weak intermolecular forces and high elasticity (BUNA-S, neoprene, natural rubber), fibres with strong intermolecular hydrogen bonds and high tensile strength (nylon, terylene, silk), thermoplastics that soften on heating and harden on cooling without losing properties (polythene, polystyrene, PVC), and thermosetting polymers that become permanently hard on heating due to extensive cross-linking (bakelite, melamine, urea-formaldehyde resin).
Types of polymerisation. Addition polymerisation (chain-growth) involves the repeated addition of unsaturated monomers without loss of any small molecule. It proceeds by three mechanisms: (a) free-radical polymerisation, initiated by peroxides like benzoyl peroxide that generate free radicals; (b) cationic polymerisation, initiated by acid catalysts like H₂SO₄ or BF₃ which form a carbocation intermediate, suitable for monomers with electron-donating groups (isobutylene); (c) anionic polymerisation, initiated by bases or organometallic compounds like butyl lithium, suitable for monomers with electron-withdrawing groups (acrylonitrile, styrene); (d) Ziegler-Natta polymerisation, which uses a coordination catalyst of triethyl aluminium and titanium tetrachloride [Al(C₂H₅)₃ + TiCl₄] to produce stereoregular, high-density linear polymers like HDPE and isotactic polypropylene. Condensation polymerisation (step-growth) occurs between bifunctional or trifunctional monomers with elimination of simple molecules such as water, ammonia, methanol or HCl. Examples include nylon-66, terylene, bakelite and melamine. Polymers may also be classified as homopolymers (made from a single monomer, e.g., polythene) or copolymers (made from two or more different monomers, e.g., BUNA-S from butadiene and styrene).
Commercial polymers and biodegradable polymers. Common addition polymers: Polythene exists as LDPE (low density, branched, used in plastic bags and squeeze bottles, made by free-radical polymerisation under high pressure) and HDPE (high density, linear, used in buckets and pipes, made by Ziegler-Natta catalyst at low pressure). Polypropylene is used in ropes, packaging and bottles. PVC (polyvinyl chloride) is used in pipes, raincoats and cable insulation. Teflon from tetrafluoroethene is chemically inert and used as a non-stick coating. Polyacrylonitrile (PAN, Orlon) is used as wool substitute. Polystyrene is used in insulation and packaging. Neoprene from chloroprene is oil-resistant rubber. BUNA-S (styrene-butadiene rubber) is used in tyres; BUNA-N (acrylonitrile-butadiene) is used in oil seals. Common condensation polymers: Nylon-6 (from caprolactam) and Nylon-66 (from hexamethylenediamine and adipic acid) are used in textiles, ropes and tyre cords. Terylene/Dacron is a polyester of ethylene glycol and terephthalic acid used in fabrics and PET bottles. Bakelite (phenol + formaldehyde) is used in switches and handles. Melamine-formaldehyde resin is used in unbreakable crockery, and urea-formaldehyde resin in adhesives and laminates. Biodegradable polymers can be decomposed by microorganisms; examples are PHBV (poly-β-hydroxybutyrate-co-β-hydroxy valerate) used in controlled drug release, and Nylon-2-nylon-6, an alternating copolymer of glycine and aminocaproic acid. Molecular mass of polymers is expressed as number-average molecular mass (M̄n = ΣNᵢMᵢ / ΣNᵢ) and weight-average molecular mass (M̄w = ΣNᵢMᵢ² / ΣNᵢMᵢ); their ratio M̄w/M̄n is called the polydispersity index (PDI), which equals 1 for a monodisperse sample.
1-Mark Questions
Q1. Define a polymer.
Answer: A polymer is a high molecular mass macromolecule formed by the repeated linking of a large number of small molecules called monomers.
Q2. What is a monomer?
Answer: A monomer is the simple, low molecular mass repeating unit that combines to form a polymer (e.g., ethene is the monomer of polythene).
Q3. Name the monomer of natural rubber.
Answer: Isoprene (2-methyl-1,3-butadiene).
Q4. What is degree of polymerisation?
Answer: The number of repeating monomer units present in a polymer chain is called the degree of polymerisation (DP).
Q5. Give one example of a thermosetting polymer.
Answer: Bakelite (phenol-formaldehyde resin).
Q6. Name the catalyst used in Ziegler-Natta polymerisation.
Answer: Triethyl aluminium with titanium tetrachloride [Al(C₂H₅)₃ + TiCl₄].
Q7. What is a copolymer?
Answer: A polymer formed from the polymerisation of two or more different types of monomers is called a copolymer (e.g., BUNA-S).
Q8. Name one biodegradable polymer.
Answer: PHBV (poly-β-hydroxybutyrate-co-β-hydroxy valerate) or nylon-2-nylon-6.
Q9. What are elastomers?
Answer: Elastomers are polymers with weak intermolecular forces and a coiled structure that allows them to be stretched and to return to original shape (e.g., natural rubber, BUNA-S).
Q10. Write the full form of PVC.
Answer: Polyvinyl chloride.
2/3-Mark Questions
Q1. Differentiate between addition and condensation polymerisation.
Answer: (i) Addition polymerisation occurs by simple addition of unsaturated monomers without loss of any small molecule, while condensation polymerisation involves elimination of small molecules like water, ammonia or methanol. (ii) Addition polymers have only carbon backbone and same empirical formula as monomer (e.g., polythene from ethene), while condensation polymers have heteroatoms in the backbone and different formula (e.g., nylon-66, terylene). (iii) Addition is chain-growth; condensation is step-growth.
Q2. Distinguish between thermoplastic and thermosetting polymers with examples.
Answer: Thermoplastics soften on heating and harden on cooling and can be remoulded repeatedly without changing chemical nature; they have linear or slightly branched structures (e.g., polythene, PVC, polystyrene). Thermosetting polymers undergo extensive cross-linking on heating and become permanently hard and infusible; they cannot be remelted or reshaped (e.g., bakelite, melamine, urea-formaldehyde resin).
Q3. Distinguish between LDPE and HDPE.
Answer: LDPE (low-density polythene) is prepared by free-radical polymerisation at high pressure (1000–2000 atm) and high temperature; it is highly branched, soft and used in plastic bags and squeeze bottles. HDPE (high-density polythene) is prepared using Ziegler-Natta catalyst at low pressure (6–7 atm); it is linear, tough and rigid, used in pipes, buckets and bottles.
Q4. Write the monomers of (a) Nylon-66, (b) Terylene, (c) Bakelite.
Answer: (a) Nylon-66: Hexamethylenediamine [H₂N(CH₂)₆NH₂] and adipic acid [HOOC(CH₂)₄COOH]. (b) Terylene/Dacron: Ethylene glycol (HOCH₂CH₂OH) and terephthalic acid. (c) Bakelite: Phenol (C₆H₅OH) and formaldehyde (HCHO).
Q5. Explain the term “polydispersity index” (PDI).
Answer: Polymers contain chains of varying lengths and hence varying molecular masses. The ratio of weight-average molecular mass to number-average molecular mass, PDI = M̄w / M̄n, is called the polydispersity index. For natural polymers PDI ≈ 1 (monodisperse). For synthetic polymers PDI > 1 (polydisperse). A higher PDI indicates wider variation in chain length.
Q6. What is vulcanisation? Why is it carried out?
Answer: Vulcanisation is the process of heating natural rubber with sulphur (3–5%) at 373–415 K in the presence of accelerators such as zinc oxide. Sulphur creates cross-links between polyisoprene chains. This makes rubber harder, stronger, more elastic, more resistant to abrasion, organic solvents and temperature changes. Vulcanised rubber is used in automobile tyres.
5/7-Mark Questions
Q1. Classify polymers on the basis of (a) source, (b) structure, (c) mode of polymerisation, and (d) molecular forces, with one example of each.
Answer: (a) Based on source: Natural polymers obtained from plants or animals (starch, cellulose, proteins, natural rubber); Semi-synthetic polymers derived from natural polymers by chemical modification (cellulose nitrate, cellulose acetate, vulcanised rubber); Synthetic polymers made entirely in the laboratory (polythene, nylon-66, PVC, bakelite). (b) Based on structure: Linear polymers with long, straight chains packing closely (HDPE, nylon, PVC); Branched chain polymers with side branches on the main chain (LDPE, glycogen, amylopectin); Cross-linked or network polymers with extensive covalent links between chains (bakelite, melamine, vulcanised rubber). (c) Based on mode of polymerisation: Addition polymers formed by addition of unsaturated monomers without loss of any molecule (polythene from ethene, PVC from vinyl chloride); Condensation polymers formed with elimination of small molecules (nylon-66, terylene, bakelite). (d) Based on molecular forces: Elastomers have weak van der Waals forces and elastic stretchable nature (BUNA-S, neoprene); Fibres have strong hydrogen bonds and dipole-dipole forces giving high tensile strength (nylon-66, terylene); Thermoplastics have moderate forces and soften on heating (polythene, PVC); Thermosetting polymers have heavy cross-linking and become hard on heating (bakelite, melamine).
Q2. Describe the mechanism of free-radical addition polymerisation of ethene.
Answer: Free-radical polymerisation of ethene (CH₂=CH₂) to form polythene proceeds through three steps. Step 1 — Initiation: An initiator like benzoyl peroxide (C₆H₅COO)₂ decomposes on heating to give phenyl free radicals (C₆H₅•). The radical attacks ethene to form a new larger radical: C₆H₅• + CH₂=CH₂ → C₆H₅—CH₂—CH₂•. Step 2 — Propagation: The growing radical adds successively to many ethene molecules, building a long chain: C₆H₅—CH₂—CH₂• + n CH₂=CH₂ → C₆H₅—(CH₂—CH₂)ₙ—CH₂—CH₂•. Step 3 — Termination: Two radicals combine (or disproportionate) ending the chain: 2 R• → R—R. The product is low-density polythene (LDPE) with branched chains because of chain transfer reactions during propagation. The reaction requires high pressure (1000–2000 atm) and 350–570 K.
Q3. Write the preparation, properties and uses of (a) Teflon, (b) Bakelite, (c) BUNA-S.
Answer: (a) Teflon is prepared by free-radical polymerisation of tetrafluoroethene (CF₂=CF₂) under pressure with persulphate catalyst: n CF₂=CF₂ → —(CF₂—CF₂)ₙ—. It is chemically inert, has high thermal stability, and a non-stick surface. Used in cookware coatings, gaskets, seals and lab equipment. (b) Bakelite is a thermosetting polymer prepared by condensation of phenol with formaldehyde in presence of acid or base catalyst. The initial product is novolac (linear) which on heating with formaldehyde forms cross-linked bakelite. It is hard, infusible, electrically insulating. Used in electrical switches, plugs, handles of utensils and combs. (c) BUNA-S (Styrene-Butadiene Rubber, SBR) is a copolymer of 1,3-butadiene (CH₂=CH—CH=CH₂) and styrene (C₆H₅—CH=CH₂) prepared by free-radical polymerisation in presence of sodium (“BU” for butadiene, “NA” for sodium, “S” for styrene). It is tough, abrasion-resistant and used in automobile tyres, rubber soles, hoses and floor tiles.
Q4. Define number-average and weight-average molecular masses of a polymer. How do they differ? Why are both required?
Answer: Polymer samples contain chains of different lengths so molecular mass is expressed as average values. Number-average molecular mass M̄n is the total mass of all molecules divided by the total number of molecules: M̄n = ΣNᵢMᵢ / ΣNᵢ, where Nᵢ is the number of molecules of mass Mᵢ. It is determined by colligative properties such as osmotic pressure. Weight-average molecular mass M̄w is defined as M̄w = ΣNᵢMᵢ² / ΣNᵢMᵢ. It is determined by techniques like light scattering and ultracentrifugation, and gives more weightage to heavier chains. They differ because heavier molecules contribute disproportionately more to mass than to number. The ratio M̄w/M̄n is the polydispersity index (PDI). For a sample of identical chains M̄w = M̄n and PDI = 1 (monodisperse, e.g., natural polymers like proteins). For most synthetic polymers PDI > 1 (polydisperse). Both averages together describe the molecular mass distribution of a polymer, which controls its mechanical strength, viscosity and processability.
Q5. What are biodegradable polymers? Discuss PHBV and Nylon-2-Nylon-6 with their importance.
Answer: Biodegradable polymers are polymeric materials that can be broken down by enzymes secreted by microorganisms (bacteria, fungi) into simple harmless products such as CO₂, water and biomass. They reduce the menace of plastic waste pollution. (i) PHBV (poly-β-hydroxybutyrate-co-β-hydroxy valerate) is a copolymer of 3-hydroxybutyric acid and 3-hydroxypentanoic acid (valeric acid), in which the monomers are joined by ester linkages. It is degraded by bacteria. PHBV is used in specialty packaging, orthopaedic devices and in controlled drug release. (ii) Nylon-2-Nylon-6 is an alternating polyamide copolymer of glycine (H₂N—CH₂—COOH) and ε-aminocaproic acid (H₂N—(CH₂)₅—COOH). It is biodegradable. Importance: Biodegradable polymers (a) reduce non-degradable plastic waste, (b) are eco-friendly, (c) save fossil resources and (d) find use in surgical sutures, bone implants and drug-delivery systems where the polymer slowly degrades inside the body.
Multiple Choice Questions (MCQs)
Q1. The monomer of natural rubber is —
(a) Chloroprene (b) Isoprene (c) Butadiene (d) Styrene
Answer: (b) Isoprene.
Q2. Which of the following is a thermosetting polymer?
(a) Polythene (b) PVC (c) Bakelite (d) Nylon-66
Answer: (c) Bakelite.
Q3. Nylon-66 is obtained from —
(a) Caprolactam (b) Adipic acid + hexamethylenediamine (c) Phenol + formaldehyde (d) Ethylene glycol + terephthalic acid
Answer: (b) Adipic acid + hexamethylenediamine.
Q4. Ziegler-Natta catalyst is —
(a) Pd/BaSO₄ (b) Ni/H₂ (c) Al(C₂H₅)₃ + TiCl₄ (d) V₂O₅
Answer: (c) Al(C₂H₅)₃ + TiCl₄.
Q5. Teflon is the polymer of —
(a) Vinyl chloride (b) Tetrafluoroethene (c) Acrylonitrile (d) Styrene
Answer: (b) Tetrafluoroethene.
Q6. BUNA-S is a copolymer of —
(a) Butadiene + styrene (b) Butadiene + acrylonitrile (c) Isoprene + styrene (d) Chloroprene + styrene
Answer: (a) Butadiene + styrene.
Q7. Which of the following is a biodegradable polymer?
(a) Bakelite (b) Polythene (c) PHBV (d) Polystyrene
Answer: (c) PHBV.
Q8. Terylene is a polymer of —
(a) Glycerol + phthalic acid (b) Ethylene glycol + terephthalic acid (c) Phenol + formaldehyde (d) Urea + formaldehyde
Answer: (b) Ethylene glycol + terephthalic acid.
Q9. Polymer used in non-stick cookware is —
(a) PVC (b) PAN (c) Teflon (d) Polystyrene
Answer: (c) Teflon.
Q10. Polydispersity index of a monodisperse polymer is —
(a) Zero (b) Less than 1 (c) Equal to 1 (d) Much greater than 1
Answer: (c) Equal to 1.
Fill in the Blanks
Q1. The monomer used to prepare PVC is __________.
Answer: Vinyl chloride (CH₂=CHCl).
Q2. Nylon-6 is prepared from __________.
Answer: Caprolactam.
Q3. The process of cross-linking natural rubber with sulphur is called __________.
Answer: Vulcanisation.
Q4. The ratio M̄w/M̄n is known as __________.
Answer: Polydispersity index (PDI).
Q5. Neoprene is the addition polymer of __________.
Answer: Chloroprene (2-chloro-1,3-butadiene).
True / False
Q1. Bakelite is a thermoplastic polymer.
Answer: False — bakelite is a thermosetting polymer.
Q2. HDPE is prepared using a Ziegler-Natta catalyst.
Answer: True.
Q3. Polythene is a condensation polymer.
Answer: False — polythene is an addition polymer.
Q4. Nylon-2-Nylon-6 is a biodegradable polymer.
Answer: True.
Q5. Elastomers have very strong hydrogen bonds between chains.
Answer: False — elastomers have weak intermolecular forces; fibres have strong hydrogen bonds.
Glossary
| Term | Meaning |
|---|---|
| Polymer | Large molecule formed by repeated linking of monomers. |
| Monomer | Small molecule that combines to form a polymer. |
| Polymerisation | Process of joining monomers to form a polymer. |
| Degree of polymerisation | Number of monomer units in a polymer chain. |
| Homopolymer | Polymer made of one type of monomer (polythene). |
| Copolymer | Polymer made of two or more types of monomers (BUNA-S). |
| Addition polymer | Polymer formed without elimination of any small molecule. |
| Condensation polymer | Polymer formed with loss of small molecules like water. |
| Elastomer | Polymer with weak forces and elastic, stretchable nature. |
| Fibre | Polymer with strong H-bonds and high tensile strength. |
| Thermoplastic | Polymer that softens on heating, can be remoulded. |
| Thermosetting | Polymer that becomes permanently hard on heating. |
| Ziegler-Natta catalyst | Al(C₂H₅)₃ + TiCl₄, used for stereoregular polymers. |
| Vulcanisation | Cross-linking of rubber with sulphur to improve strength. |
| Biodegradable polymer | Polymer broken down by microorganisms (PHBV). |
| PDI | Polydispersity index = M̄w / M̄n. |
Quick Revision Notes
1. Repeat unit and DP: Polymer = (monomer)ₙ where n is the degree of polymerisation. The repeat unit is the smallest part whose repetition forms the chain.
2. Memory aid for monomers: Polythene → ethene; Polypropylene → propene; PVC → vinyl chloride; Teflon → tetrafluoroethene; PAN → acrylonitrile; Polystyrene → styrene; Neoprene → chloroprene; Natural rubber → isoprene.
3. Memory aid for condensation polymers: Nylon-66 → hexamethylenediamine + adipic acid; Nylon-6 → caprolactam; Terylene → ethylene glycol + terephthalic acid; Bakelite → phenol + formaldehyde; Melamine plastic → melamine + formaldehyde.
4. Common copolymers: BUNA-S = butadiene + styrene (tyres); BUNA-N = butadiene + acrylonitrile (oil seals).
5. Biodegradable polymers: PHBV (drug release, packaging), Nylon-2-Nylon-6 (alternating glycine-aminocaproic acid), polyglycolic acid, polylactic acid (sutures).
Stay with HSLC Guru for more Class 12 ASSEB Chemistry chapters in clear, exam-ready English. Practise these polymer questions thoroughly — board examiners love asking about commercial polymers, monomers and biodegradable plastics. Best of luck!