Chapter 28- Polymers

Polymers Overview

  • Definition: Polymers are large organic molecules made of repeating units called monomers, covalently bonded together.
  • Types of Polymers: Can be naturally occurring (e.g., polysaccharides, proteins) or synthetic (e.g., plastics).
  • Polymerization Process: The joining of monomers can form two main types of polymers: homopolymers (from a single type of monomer) and copolymers (from two or more types of monomers).

Classification of Polymers

  • Chain-Growth (Addition) Polymers:
    • Formed by the addition of monomers via chain reactions.
    • Example: Conversion of vinyl chloride to poly(vinyl chloride).
  • Step-Growth (Condensation) Polymers:
    • Formed via joining monomers containing two functional groups with the loss of small molecules (e.g., H2O, HCl).
    • Used in polyamides and polyesters.

Common Synthetic Polymers

  • Polyethylene: Water bottles, plastic bags.
  • Nylon 6,6: Backpacks, clothing.
  • Lexan: Helmets, goggles.
  • Polystyrene: Foam products, containers.

Properties of Polymers

  • Molecular Weight: Polymers have high molecular weights (10,000 to 1,000,000 g/mol) and are mixtures of various chain lengths.
  • Polymer Structure: Written with the repeating unit in brackets (e.g., [X]) to simplify the structure.
  • Crystalline and Amorphous Regions:
    • Crystalline regions: Ordered segments providing strength and toughness, held together by intermolecular forces (e.g. van der Waals forces).
    • Amorphous regions: Randomly arranged segments that allow flexibility, making polymers softer.

Polymerization Mechanisms

  • Radical Polymerization:

    • Initiation: Generated radicals start the polymerization by adding to the alkene monomer.
    • Propagation: A new radical is formed, which continues to add more monomers to grow the chain.
    • Termination: Achieved when two radicals combine or by disproportionation (transfer of hydrogen from one radical to another).

  • Cationic Polymerization:

    • Involves carbocations and follows Markovnikov’s rule, stabilizing the carbocation by electron-donating substituents.

  • Anionic Polymerization:

    • Involves carbanions, used with electron-withdrawing groups to stabilize the negative charge.
    • Known as living polymerization because it can continue if more monomers are added while maintaining an active carbanion.

Synthetic Rubber and Vulcanization

  • Natural Rubber: Composed of isoprene, good for waterproofing but soft.
  • Gutta-Percha: A harder rubber than natural rubber with E-configured double bonds.
  • Vulcanization:
    • Mixing rubber with sulfur creates cross-links (disulfide bonds), improving elasticity and strength.

Types of Step-Growth Polymers

  • Polyamides: Examples include Nylon (e.g., Nylon 6,6, Nylon 6) formed by the reaction of diacids/diamines or lactams.
  • Polyesters: Formed via nucleophilic acyl substitutions (e.g., PET from terephthalic acid and ethylene glycol).
  • Polycarbonates: Contain carbonyl groups linked to OR groups, used in products like Lexan.

Environmental Impact of Polymers

  • Synthesis and Disposal: Issues include use of petroleum, energy consumption, and waste accumulation in landfills.
  • Green Polymer Synthesis: Focus on renewable materials, non-toxic reagents, and solvent-free processes.
  • Biodegradable Polymers: Such as PHAs can decompose by microbial action. Examples include polyhydroxybutyrate (PHB).

Recycling Polymers

  • Recycling Codes: Different types of plastics are recycled with varying ease (1-6).
  • Chemical Recycling: Processes can revert polymers to monomers for reuse (e.g., PET, Nylon 6).
  • Plasticizers: Added to improve flexibility in stiff polymers.