Chapter 15 — Polymers — covers the science of macromolecules that form plastics, rubber, fibres, and many biological materials. From PVC pipes to nylon clothing to DNA in your cells, polymers are everywhere! This chapter carries 4-5 marks in Board exams and is mostly factual — learn the monomers, types of polymerisation, and properties.
Key Concepts
Classification of Polymers
Based on Source
| Type | Examples |
|---|---|
| Natural | Starch, cellulose, proteins, natural rubber, DNA |
| Semi-synthetic | Cellulose acetate (rayon), cellulose nitrate, vulcanised rubber |
| Synthetic | Polythene, nylon, PVC, Bakelite, Teflon |
Based on Structure
| Type | Structure | Properties | Examples |
|---|---|---|---|
| Linear | Long straight chains | High density, high m.p. | HDPE, nylon, polyester |
| Branched | Linear with side branches | Low density, low m.p. | LDPE, amylopectin, glycogen |
| Cross-linked (Network) | 3D network | Hard, rigid, infusible | Bakelite, melamine, vulcanised rubber |
Based on Polymerisation Type
| Addition Polymerisation | Condensation Polymerisation |
|---|---|
| Monomers with C=C double bond | Monomers with two functional groups |
| No by-product | Small molecule lost (H₂O, HCl, NH₃) |
| Homopolymer: same monomer | Usually copolymer: 2 different monomers |
| Examples: PE, PP, PVC, PS, Teflon | Examples: Nylon, Polyester, Bakelite |
Based on Molecular Forces (Mechanical Properties)
| Type | Forces | Properties | Examples |
|---|---|---|---|
| Elastomers | Weak van der Waals | Elastic, can be stretched | Natural rubber, Buna-S, Neoprene |
| Fibres | Strong H-bonds | High tensile strength, crystalline | Nylon-66, Terylene, silk |
| Thermoplastics | Intermediate forces | Soften on heating, can be remoulded | Polythene, PVC, polystyrene |
| Thermosetting | Covalent cross-links | Hard, rigid, cannot be remoulded | Bakelite, melamine, urea-formaldehyde |
Important Polymers — Monomers and Reactions
Addition Polymers
LDPE: high pressure (1000-2000 atm) + peroxide catalyst → branched, flexible
HDPE: low pressure + Ziegler-Natta catalyst (TiCl₄ + Al(C₂H₅)₃) → linear, tough
Polypropylene (PP): nCH₂=CHCH₃ → (−CH₂−CH(CH₃)−)ₙ
PVC: nCH₂=CHCl → (−CH₂−CHCl−)ₙ (pipes, cable insulation)
Polystyrene (PS): nCH₂=CHC₆H₅ → (−CH₂−CHC₆H₅−)ₙ (thermocol, toys)
Teflon (PTFE): nCF₂=CF₂ → (−CF₂−CF₂−)ₙ (non-stick cookware)
Polyacrylonitrile (PAN/Orlon): nCH₂=CHCN → (−CH₂−CHCN−)ₙ (synthetic wool)
Rubber
cis-polyisoprene → elastic
Vulcanisation: Heating rubber with sulphur (3-5%) → cross-links form → harder, more elastic, less sticky. Charles Goodyear discovered this.
Buna-S (SBR): Copolymer of 1,3-butadiene + styrene (synthetic rubber for tyres)
Buna-N (NBR): Copolymer of 1,3-butadiene + acrylonitrile (oil-resistant rubber)
Neoprene: Polymer of chloroprene (2-chloro-1,3-butadiene) — oil/solvent resistant
Condensation Polymers
(66 = 6 C in diamine + 6 C in diacid) — used in ropes, fabrics, gears
Nylon-6: Caprolactam (ring-opening polymerisation) → same as polycaprolactam
(6 = 6 C in monomer) — used in tyre cords, textiles
Terylene (PET/Dacron): Ethylene glycol + Terephthalic acid → polyester + H₂O
Used in: PET bottles, polyester fabric, magnetic tapes
Bakelite: Phenol + Formaldehyde (→ Novolac → Bakelite with cross-links)
Thermosetting — used in electrical switches, handles
Melamine-formaldehyde: Melamine + formaldehyde → cross-linked polymer
Used in unbreakable crockery, laminates (Formica)
Biodegradable Polymers
| Polymer | Composition | Use |
|---|---|---|
| PHBV | 3-hydroxybutyrate + 3-hydroxyvalerate | Packaging, orthopaedic implants |
| Nylon-2-nylon-6 | Glycine + aminocaproic acid | Biodegradable alternative |
Important Definitions
| Term | Definition |
|---|---|
| Polymer | High molecular mass substance formed by repetition of monomer units |
| Monomer | Simple molecule that combines to form a polymer |
| Homopolymer | Polymer made from one type of monomer |
| Copolymer | Polymer made from two or more different monomers |
| Vulcanisation | Cross-linking rubber with sulphur to improve properties |
| Thermoplastic | Polymer that softens on heating and can be remoulded |
| Thermosetting | Polymer that hardens permanently on heating (cannot be remoulded) |
Solved Examples — NCERT Based
Example 1: Identifying Monomers
Q: Write the monomers of: (a) Nylon-66 (b) Terylene (c) Buna-S
Solution:
(a) Nylon-66: Hexamethylenediamine (H₂N−(CH₂)₆−NH₂) + Adipic acid (HOOC−(CH₂)₄−COOH)
(b) Terylene: Ethylene glycol (HOCH₂CH₂OH) + Terephthalic acid (HOOC−C₆H₄−COOH)
(c) Buna-S: 1,3-Butadiene (CH₂=CH−CH=CH₂) + Styrene (C₆H₅CH=CH₂)
Example 2: Addition vs Condensation
Q: Classify the following as addition or condensation polymers: PVC, Bakelite, Polythene, Nylon-6.
Solution:
Addition: PVC (CH₂=CHCl), Polythene (CH₂=CH₂)
Condensation: Bakelite (phenol + formaldehyde, loses H₂O), Nylon-6 (ring-opening of caprolactam — technically addition, but classified as condensation due to amide linkage)
Example 3: Thermoplastic vs Thermosetting
Q: Why can Bakelite not be remoulded but polythene can?
Solution: Bakelite has extensive cross-links (covalent bonds between chains) forming a rigid 3D network. Once set, these cross-links cannot be broken by heating → thermosetting. Polythene has only weak intermolecular forces between linear chains → softens on heating → can be remoulded → thermoplastic.
Example 4: LDPE vs HDPE
Q: How do LDPE and HDPE differ? What causes the difference?
Solution:
LDPE: Made at high pressure with peroxide catalyst → branched chains → low packing → low density (0.92 g/cm³) → flexible, used in packaging films.
HDPE: Made at low pressure with Ziegler-Natta catalyst → linear chains → tight packing → high density (0.97 g/cm³) → tough, rigid, used in pipes and bottles.
Important Questions for Board Exams
1 Mark Questions
- What is the monomer of Teflon?
- Name the polymer used in non-stick cookware.
- What is vulcanisation?
- What is the difference between a homopolymer and a copolymer?
2 Mark Questions
- Distinguish between thermoplastic and thermosetting polymers with examples.
- Write the monomers and one use each of Nylon-66 and Terylene.
- What is LDPE and HDPE? How do they differ?
- Name one natural and one synthetic biodegradable polymer.
3 Mark Questions
- Classify polymers based on structure. Give one example of each type.
- Explain the preparation and uses of: (a) Bakelite (b) Nylon-66 (c) Buna-S.
- What is natural rubber? How is it vulcanised? What properties change after vulcanisation?
5 Mark Questions
- What are polymers? How are they classified based on (a) source (b) polymerisation type (c) molecular forces? Give examples.
- Write the monomers, polymerisation reactions, and uses of: PVC, Teflon, Nylon-66, Terylene, and Bakelite.
Quick Revision Points
- Addition: C=C monomers, no by-product (PE, PVC, PS, Teflon, PAN)
- Condensation: bifunctional monomers, loses H₂O (Nylon, Polyester, Bakelite)
- Thermoplastic: remould (PE, PVC, PS) | Thermosetting: cannot remould (Bakelite, melamine)
- Elastomers: rubber | Fibres: nylon, terylene (strong H-bonds)
- Natural rubber: cis-polyisoprene | Vulcanisation: + S → cross-linked
- Nylon-66: hexamethylenediamine + adipic acid | Nylon-6: caprolactam
- Terylene (PET): ethylene glycol + terephthalic acid
- LDPE: branched, flexible | HDPE: linear, rigid (Ziegler-Natta catalyst)
- Bakelite: phenol + formaldehyde → cross-linked thermosetting
- Teflon: PTFE → (−CF₂−CF₂−)ₙ → non-stick, chemically inert
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