5.2 Ionic polymerization: cationic and anionic mechanisms

Ionic polymerization involves charged active centers, either positive (cationic) or negative (anionic). These mechanisms offer precise control over polymer properties but require specific monomers and careful reaction conditions.

Compared to free radical methods, ionic polymerization allows for better molecular weight control and complex architectures. However, it's more sensitive to impurities and has a narrower range of suitable monomers.

Ionic Polymerization Mechanisms

Cationic vs anionic polymerization mechanisms

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• Cationic polymerization
  • Formation and propagation of positively charged active centers (carbocations) through addition of monomers
  • Initiated by strong acids (sulfuric acid, perchloric acid) or Lewis acids (aluminum chloride, boron trifluoride)
  • Termination occurs via chain transfer or combination with a counterion
• Anionic polymerization
  • Formation and propagation of negatively charged active centers (carbanions) through addition of monomers
  • Initiated by strong bases (sodium hydroxide, potassium hydroxide) or electron transfer agents (sodium naphthalene)
  • Termination occurs via chain transfer or reaction with impurities (water, carbon dioxide)

Monomers for ionic polymerization

• Monomers suitable for cationic polymerization
  • Contain electron-donating substituents that stabilize the carbocation (isobutylene, vinyl ethers, N-vinylcarbazole, styrene)
• Monomers suitable for anionic polymerization
  • Contain electron-withdrawing substituents that stabilize the carbanion (styrene, butadiene, isoprene, methyl methacrylate, acrylonitrile)

Components of ionic polymerization

• Initiators
  • Generate active centers (carbocations or carbanions) to start polymerization
  • Strong acids for cationic (hydrochloric acid, sulfuric acid), strong bases or electron transfer agents for anionic (n-butyllithium, sodium naphthalene)
• Co-initiators
  • Assist in formation of active centers and control polymerization rate
  • Water, alcohols, ethers for cationic; alkali metal alkoxides for anionic
• Solvents
  • Provide medium for polymerization reaction and influence stability of active centers and polymerization rate
  • Low polarity solvents for cationic (hexane, toluene), polar aprotic solvents for anionic (tetrahydrofuran, dioxane)

Ionic vs free radical polymerization

• Advantages of ionic polymerization
  • Better control over molecular weight and molecular weight distribution
  • Ability to produce block copolymers and other complex architectures
  • Lower polymerization temperatures and faster reaction rates
  • Absence of chain transfer and termination by combination
• Limitations of ionic polymerization
  • Sensitive to impurities (moisture, oxygen) and require stringent reaction conditions
  • Limited monomer scope compared to free radical polymerization
  • More expensive due to need for high-purity reagents and solvents
  • Difficulty controlling stereochemistry of resulting polymer