Techniques for radical polymerization

Techniques for radical polymerization

1. Bulk polymerization

   • CHARACTERISTICS
     • It requires only monomer and monomer –soluble initiator.
     • Monomer can be heated in the presence of an initiator.
   • ADVANTAGES
     • High polymer yield per volume of reaction
     • Easy polymer recovery.
     • Volume of monomers is generally larger than the final polymers.
     • Density of polymers is greater than the monomer.
   • DISADVANTAGES
     • Difficulty of removing unreacted monomer.
     • Heat control.
   • EXAMPLES
     • Poly(methylmethacrylate)
     • Polystyrene

2. Solution polymerization

   • CHARACTERISTICS
     • Monomer and initiator must be soluble in the liquid
     • Boiling point is took into account
     • Solvent
       • must have the desired chain transfer characteristics.
       • Solvents with low-chain transfer constants are used to minimize reduction in chain length.
   • ADVATAGES
     • The presence of the solvent assists in heat removal and control.
     • Many free radical and ionic polymerizations are carried out.
   • DISADVANTAGES
     • Solvent recovery and removal (from the polymer) is necessary
     • Polymer yield per reaction volume is lower than for bulk reactions.
   • EXAMPLES
     • Poly(vinyl chloride)
     • Polybutadiene

3. Suspension polymerization

   • CHARACTERISTICS
     • A water-insoluble monomer and initiator are used.
     • Droplets of monomer-containing initiator and chain transfer agent are formed.
     • Chain transfer agent may be used to control chain size.
     • A protective colloidal agent is added to prevent coalescence of the droplets.
   • ADVENTAGES
     • good heat control
     • ease of removal of the discrete polymer particles
     • Solvent recovery and treatment problems are minimal.
   • DISADVENTAGES
     • The products may contain a number of impurities
       • including any of the agents added to assist in the polymerization process.
   • EXAMPLES
     • Poly(styrene-co-acrynonitrile)
     • SAN

4. Emulsion polymerization

   • CHARACTERISTICS
     • The system usually contains a water-soluble initiator, chain transfer agent, and a surfactant.
     • POLYMERIZATION
       • 1
         • Begins when the water-soluble initiator enters into the monomer-containing micelle.
       • 2
         • During it, the monomer migrates from the monomer droplets through the water and into micelles.
       • 3
         • As polymerization continues, monomer is transferred to the growing micelles.
     • The hydrophobic monomer forms large droplets that are stabilized by the surfactant.
   • ADVENTAGES
     • It is more rapid than bulk or solution polymerization at the same temperature
       • The conversion is essentially 100 percent.
     • Many water-soluble vinyl monomers may be polymerized by this technique.
     • The particles ranges from 0.05 to 5 nm in diameter.
     • Since relatively stable macroradicals are produced the termination rate decreases and high molecular weight product is rapidly produced.
     • Since propagation occurs in the micelles, the rate of polymerizations is proportional to the number of micelles present.
   • EXAMPLES
     • Acrylonitrile-butadiene-styrene (ABS)
     • Copolymerization of styrene and butadiene to form SBR rubber.
     • Polymerization of chloroprene to produce neoprene rubbers
   • DISADVENTAGES
     • Charged particles cannot be recovered by ordinary separation process.

5. Reference

   • Carraher Jr, C. E. (2017). Introduction to polymer chemistry. CRC press.