Chapter 3.9 (Odian)

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• Chapter 3.9 (Odian)

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Free Radically Polymerized Monomers
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Thermodynamics of Chain Polymerizations

• Relief of strain is the driving force
• Exothermic process

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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

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Thermodynamics

• ?Hpolymerization 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

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Thermodynamic Trends

• EthyleneStyrene a-Methyl styrene or
• Ethylene methyl acrylate methyl methacrylate
• PTFE (!)
• Most exothermic polymerization known

(kJ/mole)
(J/K-mole)
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Equilibrium Considerations

• For most polymerizations, there is a temperature
  where the reaction becomes reversible
• The position for the monomer / polymer
  equilibriumwill be dependent on the
  temperature
• ?G ?H - T ?S
• Polymerization ?S
• De-polymerization ?S
• With increasing temperature the equilibrium will
  shift?

kp kdp
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Equilibrium Considerations

• When Rp Rdp
• Ceiling temperature

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Thermodynamics

• The reaction isotherm ?G ?G RTlnKis
  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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Thermodynamics

• At equilibrium, ?G 0 by definition
  ?G - RTlnKEquilibrium constant is defined by
  Keq kp / kdp

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Thermodynamics

• CombineMc is the equilibrium monomer
  concentration as a function of reaction
  temperature
• The monomer concentration in equilibrium with the
  polymer increases as the temperature increases

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Ceiling Temperature

• Poly(a-methyl styrene)
• Tg 170 C
• Tceil 61 C
• Processing temperature?

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Ceiling Temperature

• Poly(methyl methacrylate)
• Tg 125 C
• Tceil 164 C
• Recycle

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Heats of Polymerization and Ceiling Temperatures
Monomer Heat of Polymerization(kcal/mol) Ceiling Temperature(C)
Styrene -16 235
a-Methyl styrene -7 61

Methyl acrylate -20 ----
Methyl methacrylate -13 164

Ethylene -26 407
Propylene -21 300
Isobutene -17 50

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Polymerization Processes

• Solution Polymerization
• Bulk (Mass) Polymerization
• Heterogeneous Polymerizations

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Solution Polymerizations

• Ingredients
• Monomer
• Solvent
• Initiator

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Solution Polymerization

• Solvent, monomer initiator
• Polymer remains soluble in the solvent
• Easy temperature, viscosity, MW control
• Free radical kinetics apply

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Solution Polymerization

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

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Bulk (Mass) Polymerizations

• Ingredients Monomer and Initiator only
• Kinetics follows solution polymerization
  kineticsRp and ?

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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