Synthesis poly

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Synthesis poly

  1. 1. Polymer Synthesis CHEM 421 Odian Book: Chapter 4
  2. 2. Polymer Synthesis CHEM 421 Emulsion Polymerizations • Economically important • Western countries 108 tons/year • 30% of all polymers made by free radical methods –emulsion polymers accounts for 40-50% of this • First employed during WWII for production of synthetic rubber • Today: MMA, VC, vinylidene chloride, styrene, fluoropolymers, vinyl acetate, EVA, SA, SBR, chloroprene, etc
  3. 3. Polymer Synthesis CHEM 421 Emulsion Polymerization Recipe • Water (continuous phase) • Water-insoluble monomer • Water-soluble initiator • Surfactant (detergent)
  4. 4. Polymer Synthesis CHEM 421 Surfactants H2O Hydrophobic / Lipophilic core Surfactant Concentration Unimers Micelles Critical Micelle Concentration (CMC)
  5. 5. Polymer Synthesis CHEM 421 Surfactants Types - Anionic - Cationic - Amphoterics - Non-ionics
  6. 6. Polymer Synthesis CHEM 421 Emulsion Polymerization Recipe
  7. 7. Polymer Synthesis CHEM 421 Emulsion Polymerizations Polym’z Rate Surfactant Concentration Critical Micelle Concentration
  8. 8. Polymer Synthesis CHEM 421 Kinetics of Emulsion Polymerization Percent Conversion Time I II III
  9. 9. Polymer Synthesis CHEM 421 Kinetics of Emulsion Polymerization Rate % Conversion I II III
  10. 10. Polymer Synthesis CHEM 421 Before Initiation I I I I I I I I I M M M M M M M M M M MM M M M M M M M Monomer Droplet ca. 1 micron diameter conc = 1011 /mL stabilized by soap Micelle Containing Monomer ca. 75 Å diameter conc = 1018 /mL Relative surface area 1 : 560 Initiation of micelles statistically favored
  11. 11. Polymer Synthesis CHEM 421 Interval One: 0 – 15 % Conversion I • I I I I I I • I I M M M M M M M M M M MM M PP•• PP•• M M Micelles Containing Monomer Active latex particle Micelles Containing Monomer Micelles Containing Monomer Active latex particles Inactive latex particles Inactive latex particles
  12. 12. Polymer Synthesis CHEM 421 Qualitative Details Conversion Micelles Monomer Droplets Particle Number Particle Size Comments I 0 – 15% present present increases increases Nucleation period, Increasing Rp II III
  13. 13. Polymer Synthesis CHEM 421 Interval Two: 15 – 80% Conversion I I I I I • I I M M M M M M MM M PP•• PP•• I • I M M PP•• Inactive latex particles Inactive latex particles Inactive latex particles Active latex particles Active latex particles I • I M PP•• Active latex particles No micelles Number of particles constant, therefore Rp = constant
  14. 14. Polymer Synthesis CHEM 421 Kinetics of Emulsion Polymerization Number of Micelles Time I II III Number of Polymer Particles 1018 0 1015 0
  15. 15. Polymer Synthesis CHEM 421 Qualitative Details Conversion Micelles Monomer Droplets Particle Number Particle Size Comments I 0 – 15% present present increases increases Nucleation period, Increasing Rp II 15 – 80% absent present constant increases Constant # of particles, Cp = constant III
  16. 16. Polymer Synthesis CHEM 421 Interval Three: 80 – 100% Conversion I I I I I • I M M M M M M M PP•• PP•• I M M PP•• I • M PP•• M PP•• M PP•• I • No monomer droplets No micelles
  17. 17. Polymer Synthesis CHEM 421 Qualitative Details Conversion Micelles Monomer Droplets Particle Number Particle Size Comments I 0 – 15% present present increases increases Nucleation period, Increasing Rp II 15 – 80% absent present constant increases Constant # of particles, Cp = constant III 80 – 100% absent absent constant roughly constant Constant # of particles, Cp = decreasing
  18. 18. Polymer Synthesis CHEM 421 Emulsion Polymerization Kinetics • Once inside a particle, radical propagates as rp = kp[M] • Overall rate: Rp = kp[M][P. ] • [P. ] = N’ñ (where N’ = the sum of micelle and particle concentrations and ñ = average # of radicals per particle) • Therefore, –Increase N’ to increase rate! ][' MknNR pp =
  19. 19. Polymer Synthesis CHEM 421 Emulsion Kinetics, cont. • Smith-Ewart Kinetics: –Case 2: ñ = 0.5 (MOST CASES!) » 1 radical per particle » Half of the particles active, half not active –Case 1: ñ<0.5 » Radical can diffuse out of the particle » Monomer with higher water solubility –Case 3: ñ>0.5 » Termination constant is low » High viscosity, initiator; large particles
  20. 20. Polymer Synthesis CHEM 421 Emulsion Polymerization Kinetics • How to increase Rp? –Increase N’ to increase rate »Increase surfactant concentration to increase N’ ][' MknNR pp =
  21. 21. Polymer Synthesis CHEM 421 Molecular Weight in Emulsion Polymerizations • Molecular weight determined by rate of growth of a chain divided by rate of radical entry (ri) –How to increase molecular weight? DP rp = ——ri Ri = —— N ri = kp[M]rp N kp [M] Ri = ———DP
  22. 22. Polymer Synthesis CHEM 421 Free Radical Solution Polymerizations • Recall – To increase molecular weight… » Increase monomer concentration » Decrease initiator concentration – To increase Rate of Polymerization » Increase monomer concentration » Increase initiator concentration ٧ = kp [M] 2 (kt kd f [I])1/2 = ————— Can’t do both! Rp = kp [M] (kd f [I] / kt)1/2

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