Polymer Synthesis CHEM 421 •Chapter 3.9 (Odian)
Feb 13, 2016
Polymer SynthesisCHEM 421
• Chapter 3.9 (Odian)
Polymer SynthesisCHEM 421
Free Radically Polymerized Monomers
Polymer SynthesisCHEM 421
Thermodynamics of Chain Polymerizations
• Relief of “strain” is the driving force–Exothermic process
CH2 CHn
CH3
CH
CH3
H2C
C CHH
XH
H
C
C
H
H
X
Polymer SynthesisCHEM 421Thermodynamics
• ΔG, ΔH, and ΔSΔG = ΔH - T ΔS
• Chain polymerizations:» Enthalpy
»Exothermic (- ΔH)
» Entropy
»Negative ΔS
• Polymer favored from enthalpic considerations but un-favored from entropic considerations
Polymer SynthesisCHEM 421Thermodynamics
• 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
Polymer SynthesisCHEM 421Thermodynamic Trends
• EthyleneStyrene α-Methyl styrene
or• Ethylene
methyl acrylate methyl methacrylate
• PTFE (!)– Most exothermic
polymerization known
(kJ/mole) (J/°K-mole)
Polymer SynthesisCHEM 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• ΔG = ΔH - T ΔS
– Polymerization: ΔS =– De-polymerization: ΔS =
• With increasing temperature the equilibrium will shift?
M + M Mn+1 + Heatkp
kdp
Polymer SynthesisCHEM 421
Equilibrium Considerations
• When Rp = Rdp
– Ceiling temperature
Polymer SynthesisCHEM 421Thermodynamics
• 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
Polymer SynthesisCHEM 421Thermodynamics
• At equilibrium, ΔG = 0 by definition: ΔG° = - RTlnK
Equilibrium constant is defined by Keq = kp / kdp
Keq = ————— = ———[Mn+1•]
[Mn•] [M] [M]1
Polymer SynthesisCHEM 421Thermodynamics
• 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
Tc = —————ΔH°
ΔS° + Rln[M]c
Polymer SynthesisCHEM 421Ceiling Temperature
• Poly(α-methyl styrene)
–Tg = 170 °C
–Tceil = 61 °C
• Processing temperature?
CH2 Cn
CH3
Polymer SynthesisCHEM 421Ceiling Temperature
• Poly(methyl methacrylate)
–Tg = 125 °C
–Tceil = 164 °C
• Recycle…
CH2 Cn
C
CH3
O
O
CH3
PMMA 99% MMA monomer300 °C
Polymer SynthesisCHEM 421
Heats of Polymerization and Ceiling Temperatures
Monomer Heat of Polymerization(kcal/mol)
Ceiling Temperature(°C)
Styrene -16 235α-Methyl styrene -7 61
Methyl acrylate -20 ----Methyl methacrylate -13 164
Ethylene -26 407Propylene -21 300Isobutene -17 50
Polymer SynthesisCHEM 421
Polymerization Processes
• Solution Polymerization
• Bulk (Mass) Polymerization
• Heterogeneous Polymerizations
Polymer SynthesisCHEM 421
Solution Polymerizations
• Ingredients–Monomer
–Solvent
–Initiator
Rp = kp [M] (kd f [I] / kt)1/2
kp [M]
2 (kt kd f [I])1/2 = —————kp [M] [M•]
2 kt [M•]2 = ———
Rp Rp
Ri Rt
٧ = — = —
Polymer SynthesisCHEM 421Solution Polymerization
• Solvent, monomer & initiator• Polymer remains soluble in the solvent• Easy temperature, viscosity, MW control• Free radical kinetics apply
Solvent
MM M
MM
MM
MM
M
I
I
II
I
hνorΔ
Solvent
M M
MM
I
I
I
PP
P
P
P
P
P P
Polymer SynthesisCHEM 421Solution Polymerization
• Considerations:–Chain transfer to solvent
–Purity of polymer (difficulty in removing solvent)
• Used for: vinyl acetate, acrylonitrile, and esters of acrylic acid
Polymer SynthesisCHEM 421
Bulk (Mass) Polymerizations
• Ingredients: Monomer and Initiator only
• Kinetics follows solution polymerization kinetics…Rp and ٧
Polymer SynthesisCHEM 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