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Thermodynamics of Chain Polymerizations: Key Considerations and Trends

Explore the thermodynamics of chain polymerizations, with a focus on factors such as enthalpy, entropy, and equilibrium considerations that influence the polymerization process. Learn about ceiling temperatures, polymerization processes like solution and bulk polymerization, and the distinctive characteristics of each method. Gain insights into the challenges and benefits associated with different polymerization techniques.

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Thermodynamics of Chain Polymerizations: Key Considerations and Trends

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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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