Polymers: Formation and Characteristics

Polymerization

• Polymers are macromolecules formed by joining of repeating structural units on a large scale.
• The repeating structural units are called Monomers.

Monomers and Polymers

• Monomers are small molecules that combine with each other to form polymer molecules.
• Macromolecules refer to large molecules built from smaller subunits.
• When all the subunits are of the same type, the macromolecules are called polymers, and the subunits are monomers.
• Examples of polymers: DNA, proteins, carbohydrates.
• Example of macromolecule: triglycerides (fat).

Classification of Polymers

Based on Source

• Natural polymers: occur in nature (e.g. protein, nucleic acid, cellulose, starch).
• Semi-synthetic polymers: derived from natural sources, but undergo chemical treatment (e.g. cellulose nitrate, cellulose acetate).
• Synthetic polymers: man-made (e.g. plastics, Nylon, Buna-s, PVC, polyethylene).

Based on Structure

• Linear polymers: long chains (e.g. high-density polythene, polyvinyl chloride).
• Branched chain polymers: chain with branches (e.g. low-density polythene).
• Cross-linked or network polymers: string covalent bond between chains (e.g. Bakelite, Melamine).

Based on Type of Monomer

• Homo polymer: made from only one monomer (e.g. A-A-A-A-A-A-A-A).
• Co-polymer: made from two or more different monomers.
  • Types of co-polymers:
    • Random copolymer: -A-B-B-A-B-A-A-B-
    • Alternating copolymer: -A-B-A-B-A-B-A-B-
    • Block copolymer: -A-A-A-A-B-B-B-B-
    • Graft copolymer:

Based on Molecular Forces

• Elastomers: weakest intermolecular forces (e.g. rubber, buna-S).
• Fibers: strongest intermolecular forces (e.g. Nylon, Polyesters, Terylene).
• Thermoplastic: can be remolded by heating and cooling (e.g. Polythene, PVC).
• Thermosetting Plastic: cannot be reused (e.g. Bakelite, urea formaldehyde).

Based on Mode of Polymerization

• Addition polymerization: monomer contains one double bond.
• Condensation polymerization: monomer contains bi-functional group.

Addition Polymerization

• Examples of addition polymers:
  • Polythene (two types)
  • Teflon (Polytetrafluoroethene)
  • Polyacrylonitrile
  • Buna S
  • Natural Rubber (cis-1,4-poly isoprene)
  • Synthetic Rubber (Neoprene, Buna-N)
  • Poly Vinyl Chloride (PVC)
  • Poly styrene

Vulcanization of Rubber

• Heating with sulphur and an additive to improve physical properties of natural rubber.

Mechanism of Addition Polymerization

• Step 1: Chain initiating step
• Step 2: Chain propagation
• Step 3: Chain termination

Condensation Polymerization

• Involves repetitive condensation between two bi-functional monomers, with loss of simple molecules (e.g. water, alcohol).
• Examples:
  • Polyamides (Nylon 6,6, Nylon 6)
  • Polyesters (Terelene, Glyptal)
  • Phenol Formaldehyde Polymer (Bakelite)
  • Melamine Formaldehyde Polymer