Polymer Synthesis CHEM 421

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Polymer Synthesis
CHEM 421
• Chapter 3.9 (Odian)
Free Radically Polymerized
Monomers
Polymer Synthesis
CHEM 421
Thermodynamics of Chain
Polymerizations
• Relief of “strain” is the driving force
–Exothermic process
Polymer Synthesis
CHEM 421
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
Polymer Synthesis
CHEM 421
Thermodynamics
Polymer Synthesis
CHEM 421
• 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
Thermodynamic Trends
(kJ/mole)
(J/°K-mole)
Polymer Synthesis
CHEM 421
• Ethylene
Styrene
α-Methyl styrene
or
• Ethylene
methyl acrylate
methyl methacrylate
• PTFE (!)
– Most exothermic
polymerization known
Equilibrium Considerations
Polymer Synthesis
CHEM 421
• For most polymerizations, there is a temperature where
the reaction becomes reversible
• The position for the monomer / polymer equilibrium
kp
kdp
will be dependent on the temperature
• ΔG = ΔH - T ΔS
– Polymerization:
– De-polymerization:
ΔS =
ΔS =
• With increasing temperature the equilibrium will shift?
Equilibrium Considerations
• When Rp = Rdp
– Ceiling temperature
Polymer Synthesis
CHEM 421
Thermodynamics
Polymer Synthesis
CHEM 421
• The reaction isotherm:
ΔG = ΔG° + RTlnK
is 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
Thermodynamics
Polymer Synthesis
CHEM 421
• At equilibrium, ΔG = 0 by definition:
ΔG° = - RTlnK
Equilibrium constant is defined by
Keq = kp / kdp
[Mn+1•]
1
Keq = ————— = ———
[Mn•] [M]
[M]
Thermodynamics
Polymer Synthesis
CHEM 421
• Combine:
ΔH°
Tc = —————
ΔS° + Rln[M]c
[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
Ceiling Temperature
• Poly(α-methyl styrene)
–Tg = 170 °C
–Tceil = 61 °C
• Processing temperature?
Polymer Synthesis
CHEM 421
Ceiling Temperature
Polymer Synthesis
CHEM 421
• Poly(methyl methacrylate)
–Tg = 125 °C
–Tceil = 164 °C
• Recycle…
PMMA
300 °C
99% MMA monomer
Heats of Polymerization and
Ceiling Temperatures
Monomer
Polymer Synthesis
CHEM 421
Heat of Polymerization
(kcal/mol)
-16
-7
Ceiling Temperature
(°C)
235
61
Methyl acrylate
Methyl methacrylate
-20
-13
---164
Ethylene
Propylene
Isobutene
-26
-21
-17
407
300
50
Styrene
α-Methyl styrene
Polymerization Processes
Polymer Synthesis
CHEM 421
• Solution Polymerization
• Bulk (Mass) Polymerization
• Heterogeneous Polymerizations
Solution Polymerizations
Polymer Synthesis
CHEM 421
• Ingredients
–Monomer
–Solvent
–Initiator
Rp = kp [M] (kd f [I] / kt)1/2
Rp
Rp
kp [M] [M•]
kp [M]
٧ = — = — = ———2 = —————
Ri
Rt
2 (kt kd f [I])1/2
2 kt [M•]
Solution Polymerization
M M I M
M I M M I
M I
M
M
I
Solvent M
•
•
•
•
hν
or
Δ
Polymer Synthesis
CHEM 421
P M P
M
I
I
P
P M
M P P I
P Solvent P
Solvent, monomer & initiator
Polymer remains soluble in the solvent
Easy temperature, viscosity, MW control
Free radical kinetics apply
Solution Polymerization
Polymer Synthesis
CHEM 421
• Considerations:
–Chain transfer to solvent
–Purity of polymer (difficulty in removing
solvent)
• Used for: vinyl acetate, acrylonitrile,
and esters of acrylic acid
Bulk (Mass) Polymerizations
Polymer Synthesis
CHEM 421
• Ingredients: Monomer and Initiator only
• Kinetics follows solution polymerization
kinetics…Rp and ٧
Bulk (Mass) Polymerization
Polymer Synthesis
CHEM 421
• 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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