Polymers

Polymers – Introduction

• Monomer: Small molecules that combine to form macromolecules
• Polymer: Large macromolecule formed by repetition of monomer units

Classification of Polymers

1. Based on Source of Availability

• Natural polymers: Proteins, nucleic acids, cellulose
• Semi-synthetic polymers: Cellulose diacetate
• Synthetic polymers: Fibres, plastics, synthetic rubber

2. Based on Mode of Polymerisation

(a) Homopolymers & Copolymers

• Homopolymer: Formed from one type of monomer
  Example: Polythene from ethene
• Copolymer: Formed from two or more different monomers
  Example: Styrene–butadiene rubber (SBR)

(b) Addition Polymers

• Formed from unsaturated monomers (double/triple bond)
• No loss of small molecules
• Example: Polythene, PVC

(c) Condensation Polymers

• Formed by condensation with loss of small molecules (H₂O, NH₃, alcohol)
• Example: Nylon-66, Bakelite

Classification Based on Intermolecular Forces

1. Elastomers

• Held together by weakest intermolecular forces
• Can stretch and return to original shape
• Example: Vulcanised rubber

2. Fibres

• High tensile strength
• Strong intermolecular forces (H-bonding)
• Crystalline in nature; sharp melting point

3. Thermoplastics

• Intermediate intermolecular forces
• No cross-linking
• Example: Polythene, polystyrene

4. Thermosetting Polymers

• Low molecular mass semi-fluids
• On heating, form hard infusible mass
• Extensive cross-linking; 3-D network
• Example: Bakelite

General Methods of Polymerisation

1. Addition (Chain Growth) Polymerisation

• Monomers: alkenes, alkadienes or derivatives
• Occurs in stages with reactive intermediates
• Types based on intermediates:
  • Free radical
  • Carbocationic
  • Carbanionic

Free Radical Polymerisation (Steps)

• Initiation: Initiator → free radical
• Propagation: Growth of polymer chain
• Termination: Combination or disproportionation

Chain Transfer Agents

• Compounds that stop normal chain growth
• Example: CCl₄

Chain Inhibitors

• Form highly unreactive radicals
• Stop polymerisation
• Examples: Phenol, amines, quinoline, benzoquinone

Polymerisation of Conjugated Dienes

• 1,2-polymerisation
• 1,4-polymerisation (gives cis/trans forms)

Cationic Addition Polymerisation

• Initiated by acids (H⁺)
• Chain grows via carbocation
• Termination by anions

Some Commercially Important Polymers