Ionic Polymerisation

Video Available Study Notes

Video Lecture

Study Notes

Ionic Polymerisation — Note

Ionic Polymerisation

1. Introduction

Ionic polymerisation is a chain-growth polymerisation in which the active centre of the growing chain carries an ionic charge — either positive (cationic) or negative (anionic). Initiation, propagation and termination involve ionic species rather than free radicals. Ionic processes demand high purity (dry, oxygen-free) reagents and often give rapid, well-controlled growth (and in some cases living polymerisation).

  • Active centres: cations (cationic) or anions (anionic).
  • Requires exceptionally pure monomer and inert conditions.
  • Possible to obtain narrow molecular weight distributions (especially in living anionic systems).

2. Types of Ionic Polymerisation

A. Cationic Polymerisation

Initiated by electrophiles/protons or Lewis acids. Works best with monomers that stabilise a positive charge (electron-rich alkenes / vinyl monomers).

Typical initiators / catalysts: H2SO4, HClO4, BF3, AlCl3, TiCl4.
Example:
CH2=C(CH3)2 + H+ → [CH2-C+(CH3)2]
This cationic centre adds further isobutene units → polyisobutene.

B. Anionic Polymerisation

Initiated by nucleophiles/strong bases. Suitable monomers are those that stabilise a negative charge (electron-withdrawing groups). Many anionic systems can be living (no spontaneous termination).

Typical initiators: n-BuLi, NaNH2, alkali metals, sodium alkoxides.
Example:
CH2=CH-CN + :Nu- → [CH2-CH(-CN)]-
The anionic centre propagates by adding more monomer units; deliberate termination (e.g., H2O) gives an end-group.

3. General Mechanism (Initiation, Propagation, Termination)

A. Initiation

Formation of the ionic active centre by interaction of initiator and monomer (or by heterolytic cleavage).

Cationic example:
Monomer + H+ → Monomer+ (carbocation)
Anionic example:
Monomer + Nu- → Monomer- (carbanion)

B. Propagation

The ionic active end reacts with a new monomer unit, transferring the charge to the chain end and extending the chain.

Cationic propagation (schematic):
R–C+ + CH2=CHR' → R–CH–CH+–R'
Anionic propagation (schematic):
R–C- + CH2=CHR' → R–CH–CH-–R'

C. Termination

Termination routes depend on the type:

  • Cationic: Reaction with nucleophiles (water, solvents), counter-ion recombination, chain transfer to monomer or solvent.
  • Anionic: Often no spontaneous termination (living polymerisation). Deliberate termination by protonation (e.g., H2O) or electrophile.
Anionic termination example:
–[CH2–CH(CN)]- + H2O → –CH2–CH(CN)–H + OH-

4. Key Characteristics

  • Requires extremely dry, oxygen-free conditions to avoid quenching of ionic centres.
  • Reactions can be very fast; some systems polymerise at very low temperatures (e.g., –78 °C for anionic).
  • Anionic polymerisation can be living: the chains remain active until intentionally terminated ? precise control of molecular weight and architecture (block copolymers).
  • Cationic polymerisation can be highly sensitive to impurities and to the presence of nucleophiles (including solvent).
  • Typically gives narrow molecular weight distributions when well controlled.

5. Representative Systems & Examples

Type Monomer Initiator/Catalyst Product / Notes
Cationic Isobutene (CH2=C(CH3)2) BF3/H2O, HClO4 Polyisobutene (lubricant additives)
Cationic Vinyl ethers Lewis acids (e.g., TiCl4) Poly(vinyl ether) — adhesives, coatings
Anionic Styrene n-BuLi Polystyrene; living polymerisation enables block copolymers
Anionic Acrylonitrile (CH2=CH–CN) NaNH2, alkali metals Polyacrylonitrile (precursor for carbon fibers)

6. Advantages

  • High reaction rates (in many systems).
  • Controlled / living polymerisation possible (especially anionic) ? precise molecular weight control and architectures.
  • Capability for block copolymer synthesis and narrow polydispersity.

7. Limitations

  • Extreme sensitivity to moisture, oxygen and impurities; requires rigorous inert techniques (glovebox, dry solvents).
  • Limited monomer scope — monomers must stabilise ionic intermediates.
  • Equipment and operational complexity increase costs for industrial scale-up in some systems.

8. Applications

  • Production of specialty polymers (polyisobutene, poly(vinyl ethers)).
  • Manufacture of polystyrene and synthetic rubbers (via anionic routes).
  • Preparation of block copolymers and well-defined polymer architectures for advanced materials (thermoplastic elastomers, nanostructured polymers).
  • Precursor polymers for high-performance applications (e.g., carbon-fiber precursors from polyacrylonitrile).

9. Practical Notes for Laboratory & Industrial Work

  • Use rigorously dried glassware and anhydrous solvents; employ inert atmosphere (N2 or Ar).
  • Monitor temperature closely — many ionic polymerisations are temperature-sensitive and may require low-temperature baths.
  • Choose initiator/solvent systems to control counter-ion effects in cationic polymerisation (counter-ions influence stability/reactivity of carbocations).
  • For living anionic polymerisation, avoid proton sources until deliberate termination is desired.

Navigation

Previous: Addition Polymerisat... Back to Module Next: Coordination Polymer...