Polymer Synthesis CHEM 421 Chapter 3.9 (Odian). Polymer Synthesis CHEM 421 Free Radically...

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Transcript of Polymer Synthesis CHEM 421 Chapter 3.9 (Odian). Polymer Synthesis CHEM 421 Free Radically...

  • Slide 1
  • Polymer Synthesis CHEM 421 Chapter 3.9 (Odian)
  • Slide 2
  • Polymer Synthesis CHEM 421 Free Radically Polymerized Monomers
  • Slide 3
  • Polymer Synthesis CHEM 421 Thermodynamics of Chain Polymerizations Relief of strain is the driving force Exothermic process
  • Slide 4
  • Polymer Synthesis CHEM 421 Thermodynamics G, H, and S G = H - T S Chain polymerizations: Enthalpy Exothermic (- H) Entropy Negative S Polymer favored from enthalpic considerations but un-favored from entropic considerations
  • Slide 5
  • Polymer Synthesis CHEM 421 Thermodynamics H polymerization 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
  • Slide 6
  • Polymer Synthesis CHEM 421 Thermodynamic Trends Ethylene Styrene -Methyl styrene or Ethylene methyl acrylate methyl methacrylate PTFE (!) Most exothermic polymerization known (kJ/mole) (J/K-mole)
  • Slide 7
  • Polymer Synthesis CHEM 421 Equilibrium Considerations For most polymerizations, there is a temperature where the reaction becomes reversible The position for the monomer / polymer equilibrium will be dependent on the temperature G = H - T S Polymerization:S = De-polymerization:S = With increasing temperature the equilibrium will shift? k p k dp
  • Slide 8
  • Polymer Synthesis CHEM 421 Equilibrium Considerations When R p = R dp Ceiling temperature
  • Slide 9
  • Polymer Synthesis CHEM 421 Thermodynamics The reaction isotherm: G = G + RTlnK is applicable. Where G is the G 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
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  • Polymer Synthesis CHEM 421 Thermodynamics At equilibrium, G = 0 by definition: G = - RTlnK Equilibrium constant is defined by K eq = k p / k dp K eq = = [M n+1] [M n][M] 1
  • Slide 11
  • Polymer Synthesis CHEM 421 Thermodynamics 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 T c = H S + Rln[M] c
  • Slide 12
  • Polymer Synthesis CHEM 421 Ceiling Temperature Poly(-methyl styrene) T g = 170 C T ceil = 61 C Processing temperature?
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  • Polymer Synthesis CHEM 421 Ceiling Temperature Poly(methyl methacrylate) T g = 125 C T ceil = 164 C Recycle PMMA 99% MMA monomer 300 C
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  • Polymer Synthesis CHEM 421 Heats of Polymerization and Ceiling Temperatures MonomerHeat of Polymerization (kcal/mol) Ceiling Temperature (C) Styrene-16235 -Methyl styrene-761 Methyl acrylate-20---- Methyl methacrylate-13164 Ethylene-26407 Propylene-21300 Isobutene-1750
  • Slide 15
  • Polymer Synthesis CHEM 421 Polymerization Processes Solution Polymerization Bulk (Mass) Polymerization Heterogeneous Polymerizations
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  • Polymer Synthesis CHEM 421 Solution Polymerizations Ingredients Monomer Solvent Initiator R p = k p [M] ( k d f [ I ] / k t ) 1/2 k p [M] 2 (k t k d f [I] ) 1/2 = k p [M] [M] 2 k t [M] 2 = R p R i R t = =
  • Slide 17
  • Polymer Synthesis CHEM 421 Solution Polymerization Solvent, monomer & initiator Polymer remains soluble in the solvent Easy temperature, viscosity, MW control Free radical kinetics apply Solvent M MM M M M M M M M I I I I I hh or Solvent M M M M I I I P P P P P P P P
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  • Polymer Synthesis CHEM 421 Solution Polymerization Considerations: Chain transfer to solvent Purity of polymer (difficulty in removing solvent) Used for: vinyl acetate, acrylonitrile, and esters of acrylic acid
  • Slide 19
  • Polymer Synthesis CHEM 421 Bulk (Mass) Polymerizations Ingredients: Monomer and Initiator only Kinetics follows solution polymerization kineticsR p and
  • Slide 20
  • Polymer Synthesis CHEM 421 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