1. Introduction to Addition Polymerisation

Addition polymerisation is a type of chain-growth polymerisation in which unsaturated monomers (containing double or triple bonds) join together without the elimination of any small molecules. The process continues until the active chain ends are terminated.

• Requires monomers with C=C or C?C bonds (e.g., alkenes, vinyl monomers).
• No by-products are formed.
• Molecular weight increases rapidly in the early stages.
• Initiators or catalysts are often required.

2. Stages of Addition Polymerisation

1. Initiation — Formation of an active species (radical, cation, or anion).
2. Propagation — Sequential addition of monomers to the active chain end.
3. Termination — Deactivation of the active chain end.

3. Types of Addition Polymerisation

• Free Radical Polymerisation — Initiated by free radicals.
• Cationic Polymerisation — Initiated by proton acids/Lewis acids.
• Anionic Polymerisation — Initiated by nucleophiles.
• Coordination Polymerisation — Catalysed by Ziegler–Natta catalysts.

The most common method is free radical polymerisation, used to produce polyethylene, PVC, and polystyrene.

4. Principle of Free Radical Polymerisation

In free radical polymerisation, unpaired electron species (free radicals) initiate chain growth by breaking the ?-bond of monomers, producing new radicals that propagate the reaction.

5. Steps in Free Radical Polymerisation

A. Initiation

Free radicals are generated by decomposition of an initiator (thermal or photochemical).

Common initiators: Benzoyl peroxide, azo compounds, cumene hydroperoxide.

B. Propagation

The radical adds to another monomer, extending the chain:

C. Termination

Chain growth stops by:

• Combination — Two radicals combine:
  R-(CH2-CH2)n· + ·(CH2-CH2)m-R' → R-(CH2-CH2)n+m-R'
• Disproportionation — H transfer forms a saturated and an unsaturated chain:
  R-(CH2-CH2)n· + ·(CH2-CH2)m-R' → R-(CH2-CH2)n-H + R'-(CH2-CH=CH-CH2)m-1

6. Examples of Polymers Produced by Free Radical Polymerisation

• Polyethylene (PE) — from ethene
• Polypropylene (PP) — from propene
• Polyvinyl chloride (PVC) — from vinyl chloride
• Polystyrene (PS) — from styrene
• Polyacrylonitrile (PAN) — from acrylonitrile

7. Advantages

• Simple reaction setup.
• Wide range of monomers applicable.
• Can be carried out in bulk, solution, suspension, or emulsion.

8. Limitations

• Poor control over molecular weight distribution.
• Random termination; difficult to prepare block copolymers.
• Oxygen inhibits the reaction by trapping radicals.