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Chapter 4: Polymer

Form 5 Chemistry Bab 4: Polymer

4.1 Polymer

  • A polymer is a long chain molecule made from many repeating basic units (monomers); the reaction is called polymerisation. (p.144)
  • Natural polymers: starch, protein, cotton, natural rubber. Synthetic polymers: nylon, polyethene, polystyrene, PVC. (p.144)
  • Monomers: starch and cellulose → glucose; protein → amino acid; natural rubber → isoprene; polystyrene → styrene; polypropene → propene; PVC → vinyl chloride. (p.145)
  • Thermoplastic polymers (polyethene, PVC, nylon) can be remoulded on heating and recycled. Thermosetting polymers (melamine, bakelite) cannot be remoulded. Elastomers (polyurethane, SBR) stretch and return to original shape. (p.145)
  • Addition polymerisation involves monomers with C=C double bonds. Examples: ethene → polyethene (bags, bottles); propene → polypropene (toys, textile); chloroethene → PVC (pipes, insulators); styrene → polystyrene (heat insulation, food packaging). (p.146)
  • Condensation polymerisation involves at least two different monomers each with two functional groups, forming a polymer plus a small molecule such as H2O or HCl. Terylene (polyester) and nylon (polyamide) are examples. (p.147)
  • Nylon is made from 1,6-diaminohexane and decanedioyl dichloride, with HCl as by-product. (p.148)
  • Synthetic polymers are lightweight, strong, hard, inert, resistant to heat and good insulators; used in medical apparatus, waterproofing, bags and textiles. (p.149)
  • Non-biodegradable polymers cause pollution; solutions include biodegradable and photodegradable plastics and recycling. (p.150)

4.2 Natural Rubber

  • Natural rubber in latex is polyisoprene; its monomer is isoprene (2-methylbut-1,3-diene). (p.151)
  • The negatively charged protein membrane makes rubber particles repel each other, preventing coagulation. Latex is a colloid. (p.151)
  • Natural rubber: soft, elastic, low heat resistance, electrical insulator, easily oxidised (C=C bonds), reactive to chemicals, waterproof. (p.152)
  • Acid coagulates latex; alkali (e.g. ammonia) prevents coagulation. Bacteria in the air produce lactic acid that coagulates latex naturally. (p.153)
  • Coagulation steps: H+ from acid neutralises the negative charge of the protein membrane; particles collide, membrane breaks, rubber polymers combine. (p.154)
  • Alkaline solutions contain OH− that neutralise acid produced by bacteria, so the membrane stays negatively charged. (p.155)
  • Vulcanisation produces sulphur cross-links between polymer chains by reacting C=C double bonds with sulphur. Discovered by Charles Goodyear in 1844. (p.156)
  • Alternative vulcanisation methods: peroxide, irradiation, metal oxide (sulphur-free, more environmentally friendly). (p.157)
  • Vulcanised rubber vs unvulcanised: more elastic, harder, stronger, more heat resistant and more resistant to oxidation. (p.158)

4.3 Synthetic Rubber

  • Synthetic rubber is an elastomer polymer, mostly made from petroleum by-products. Examples: neoprene, styrene-butadiene rubber (SBR), silicone rubber. (p.159)
  • Uses: neoprene (conveyor belts, hoses), SBR (tyres, shoe soles), silicone rubber (medical implants, utensils), thiokol (sealants), nitrile rubber (gloves). (p.160)
  • Synthetic rubber is mass-produced and not weather dependent but takes a long time to decompose; natural rubber decomposes faster but is less resistant to heat and solvents. (p.160)
  • Rubber waste can be recycled or reused to reduce disposal. (p.161)
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