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Chapter 3.9 (Odian)

Chapter 3.9 (Odian). Free Radically Polymerized Monomers. Thermodynamics of Chain Polymerizations. Relief of “strain” is the driving force Exothermic process. Thermodynamics. Δ G, Δ H, and Δ S Δ G = Δ H - T Δ S Chain polymerizations: Enthalpy Exothermic (- Δ H) Entropy

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Chapter 3.9 (Odian)

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  1. Chapter 3.9 (Odian)

  2. Free Radically Polymerized Monomers

  3. Thermodynamics of Chain Polymerizations • Relief of “strain” is the driving force • Exothermic process

  4. Thermodynamics • ΔG, ΔH, and ΔS ΔG = ΔH - T ΔS • Chain polymerizations: • Enthalpy • Exothermic (- ΔH) • Entropy • Negative ΔS • Polymer favored from enthalpic considerations but un-favored from entropic considerations

  5. Thermodynamics • ΔHpolymerization depends upon differences in: • Resonance stabilization of polymer versus the monomer • Steric strain in monomer versus the polymer • Hydrogen bonding or dipolar interaction in monomer versus the polymer

  6. Thermodynamic Trends • EthyleneStyrene α-Methyl styrene or • Ethylene methyl acrylate methyl methacrylate • PTFE (!) • Most exothermic polymerization known (kJ/mole) (J/°K-mole)

  7. Equilibrium Considerations kp kdp • For most polymerizations, there is a temperature where the reaction becomes reversible • The position for the monomer / polymer equilibriumwill be dependent on the temperature • ΔG = ΔH - T ΔS • Polymerization: ΔS = • De-polymerization: ΔS = • With increasing temperature the equilibrium will shift?

  8. Equilibrium Considerations • When Rp = Rdp • Ceiling temperature

  9. Thermodynamics • The reaction isotherm:ΔG = ΔG° + RTlnKis applicable. Where ΔG° is the ΔG of polymerization for the monomer and the polymer in the appropriate standard states • Monomer Std. State: pure liquid • Polymer Std. State: crystalline state if possible, otherwise amorphous state

  10. Thermodynamics [Mn+1•] 1 Keq = ————— = ——— [Mn•] [M] [M] • At equilibrium, ΔG = 0 by definition:ΔG° = - RTlnKEquilibrium constant is defined by Keq = kp / kdp

  11. Thermodynamics ΔH° Tc = ————— ΔS° + Rln[M]c • Combine:[M]c is the equilibrium monomer concentration as a function of reaction temperature • The monomer concentration in equilibrium with the polymer increases as the temperature increases

  12. Ceiling Temperature • Poly(α-methyl styrene) • Tg = 170 °C • Tceil = 61 °C • Processing temperature?

  13. Ceiling Temperature 300 °C PMMA 99% MMA monomer • Poly(methyl methacrylate) • Tg = 125 °C • Tceil = 164 °C • Recycle…

  14. Heats of Polymerization and Ceiling Temperatures

  15. Polymerization Processes • Solution Polymerization • Bulk (Mass) Polymerization • Heterogeneous Polymerizations

  16. Solution Polymerizations Rp Rp ٧ = — = — Ri Rt Rp = kp[M] (kd f[I]/ kt)1/2 kp [M] kp [M][M•] = ————— = ——— 2 (ktkd f [I])1/2 2 kt [M•]2 • Ingredients • Monomer • Solvent • Initiator

  17. Solution Polymerization M P P M M M I M hν I I M I M I M M or Δ P P M M I M M I P P I M P Solvent P Solvent • Solvent, monomer & initiator • Polymer remains soluble in the solvent • Easy temperature, viscosity, MW control • Free radical kinetics apply

  18. Solution Polymerization • Considerations: • Chain transfer to solvent • Purity of polymer (difficulty in removing solvent) • Used for: vinyl acetate, acrylonitrile, and esters of acrylic acid

  19. Bulk (Mass) Polymerizations • Ingredients: Monomer and Initiator only • Kinetics follows solution polymerization kinetics…Rp and ٧

  20. Bulk (Mass) Polymerization • Considerations: • Hard to control: high activation energies, gel effect • Equipment: elaborate, strong stirring due to viscosity increase • Temperature Control: local hot spots • Can lead to degradation, discoloration, and broad MW distribution • “Runaway” reactions • Used for styrene and methyl methacrylate (Chain Growth) • Low conversion and separation/recycling of un-reacted monomer

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