Polymers
Easy Overview
Plastic bottles, nylon ropes, rubber tires — all polymers. A polymer is just a long chain of smaller repeating units called monomers. Think of it like a paperclip chain — one paperclip is boring, but a million of them linked together can make something useful.
Classification of polymers
Based on source: natural (cotton, silk, rubber), semi-synthetic (rayon), synthetic (nylon, polythene). Based on structure: linear (thermoplastics — melt on heating), cross-linked (thermosetting — burn on heating). Based on polymerization mechanism: addition polymers vs condensation polymers.
Addition polymerization
Monomers with double bonds just keep adding to each other. No byproduct, just chain growth. Needs an initiator to start. Free radical mechanism: initiator breaks the double bond, chain grows. Examples: polythene (from ethylene), PVC (from vinyl chloride), Teflon (from tetrafluoroethylene).
Condensation polymerization
Monomers join by eliminating small molecules like water or HCl. Usually involves two different monomers with functional groups. Nylon-66 is made from hexamethylenediamine + adipic acid (water is released). Dacron (terylene) is made from ethylene glycol + terephthalic acid. These are tougher and more temperature-resistant.
Important polymers and their uses
Polythene: plastic bags and bottles. PVC: pipes, cables, flooring. Teflon: non-stick pans. Nylon-6,6: textiles, ropes, parachutes. Bakelite: electrical switches (thermosetting). Rubber: natural is from latex; vulcanization adds sulfur cross-links to make it stronger. Neoprene: synthetic rubber resistant to oil.
Biodegradable polymers
Regular plastics don't decompose for hundreds of years. Biodegradable polymers (PHBV, PLA) break down naturally. PHBV is made from bacteria. PLA is from corn starch. These are used in medical implants (sutures that dissolve) and eco-friendly packaging. The future of plastics is probably in this direction.
Key Points
- •Monomer = repeating unit; Polymer = chain of monomers
- •Addition polymers: no byproduct (e.g., polythene, PVC)
- •Condensation polymers: byproduct eliminated (e.g., nylon, terylene)
- •Thermoplastics: soften on heating (linear chains)
- •Thermosetting plastics: harden permanently when heated (cross-linked)
- •Vulcanization: adding sulfur to rubber for strength and elasticity
- •Biodegradable polymers: PHBV, PLA — break down naturally
Practice Questions
- Differentiate between addition and condensation polymerization with examples.
- Explain the preparation, properties, and uses of Nylon-6,6.
- What is vulcanization? Why is it important?
- How are biodegradable polymers beneficial over synthetic polymers?