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Classification and mechanism of polymerisation (Free radical & Ionic)

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Classification and Mechanism of Polymerisation (Free Radical & Ionic)

Definition

Polymerisation is the chemical reaction in which many monomer molecules chemically combine to form a polymer, either by chain-growth or step-growth processes. In free radical polymerisation, the active centre of the growing chain is a free radical, whereas in ionic polymerisation, the active centre is an ion, either a carbocation in cationic polymerisation or a carbanion in anionic polymerisation.

Main Content

1. Classification of Polymerisation

Chain-growth polymerisation

  • The polymer chain grows by successive addition of monomer units to an active centre. It includes free radical, cationic, and anionic polymerisation. The molecular weight becomes high very early in the reaction. Examples: polyethylene, polystyrene, PVC, PMMA.

Step-growth polymerisation

  • Any two reacting species, whether monomer, dimer, or oligomer, can react with each other. High molecular weight is obtained only at a very high extent of reaction. Examples: nylon-6,6, terylene, bakelite.

2. Free Radical Polymerisation

Nature of active species

  • The chain reaction is initiated by free radicals, which are atoms or groups having an unpaired electron. These radicals are highly reactive and can add to unsaturated monomers such as alkenes. Examples of initiators: benzoyl peroxide, azobisisobutyronitrile (AIBN), hydrogen peroxide.

Suitable monomers and features

  • Free radical polymerisation is widely used for vinyl monomers such as ethene, propene, styrene, acrylonitrile, and methyl methacrylate. It is tolerant of many impurities and functional groups, so it is industrially very useful. Merits: simple operation, broad monomer scope, easy control of reaction conditions.

3. Ionic Polymerisation

Nature of active species

  • The growing polymer chain carries a positive or negative charge. In cationic polymerisation, the active centre is a carbocation; in anionic polymerisation, it is a carbanion. These mechanisms are highly sensitive to reaction conditions and require very pure monomers and solvents. Catalysts/initiators: Lewis acids such as AlCl₃, BF₃ for cationic; organolithium compounds such as n-butyllithium for anionic.

Suitable monomers and features

  • Ionic polymerisation is generally used for monomers that can stabilise the charged intermediate through resonance or electron-releasing groups. Examples: isobutylene, vinyl ethers, styrene, butadiene, acrylonitrile.
    It often gives polymers with controlled molecular weight and narrow molecular weight distribution, especially in anionic polymerisation.

Working / Process

1. Initiation

  • In free radical polymerisation, an initiator first decomposes to produce free radicals. These radicals attack the double bond of a monomer and create a new radical at the end of the chain.
  • In ionic polymerisation, the initiator or catalyst generates either a carbocation or a carbanion. This charged species adds to the monomer and creates a new active ionic chain end.
  • Example: In styrene polymerisation, a radical or ionic species opens the π-bond of the vinyl group and starts chain growth.

2. Propagation

  • The active centre repeatedly adds more monomer molecules one by one.
  • In free radical polymerisation, the unpaired electron is transferred to the terminal carbon after each monomer addition.
  • In ionic polymerisation, the positive or negative charge remains at the chain end and continues to react with additional monomer units.
  • This stage is responsible for rapid increase in chain length and polymer formation.

3. Termination

  • In free radical polymerisation, chains stop growing by combination, disproportionation, or reaction with inhibitors.
  • In cationic polymerisation, termination may occur by reaction with nucleophiles, loss of a proton, or transfer to solvent or monomer.
  • In anionic polymerisation, true termination may be absent in ideal conditions; the chain can remain active for a long time, making it a “living” polymerisation. Growth stops only when a terminating agent such as water, alcohol, or acid is added.
  • Chain transfer may also occur, reducing molecular weight by transferring the active centre to another molecule.

Advantages / Applications

Free radical polymerisation is versatile and industrially important

  • It can polymerise a wide variety of monomers and is used in the large-scale production of polyethylene, polystyrene, PVC, and PMMA.

Ionic polymerisation gives better control in suitable systems

  • Anionic polymerisation especially allows preparation of polymers with predictable molecular weight, narrow distribution, and block copolymers.

Wide range of applications in everyday materials

  • These mechanisms are used to manufacture plastics, rubber, coatings, adhesives, textile fibres, biomedical materials, sealants, and packaging materials.

Summary

  • Polymerisation is the process of forming polymers from monomers.
  • Free radical polymerisation uses radical intermediates, while ionic polymerisation uses charged intermediates.
  • Polymerisation mechanisms determine polymer structure, molecular weight, and properties.

  • Important terms to remember

  • Monomer

  • Polymer
  • Initiator
  • Free radical
  • Carbocation
  • Carbanion
  • Initiation
  • Propagation
  • Termination
  • Chain-growth polymerisation
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