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 CH

nCH3

CH

CH3

H2C

C CHH

XH

H

C

C

H

H

X

Polymer SynthesisCHEM 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 SynthesisCHEM 421

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

Polymer SynthesisCHEM 421

Thermodynamic 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 421

Thermodynamics

• 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 421

Thermodynamics

• 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 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

Tc = —————ΔH°

ΔS° + Rln[M]c

Polymer SynthesisCHEM 421

Ceiling Temperature

• Poly(α-methyl styrene)

–Tg = 170 °C

–Tceil = 61 °C

• Processing temperature?

CH2 C

n

CH3

Polymer SynthesisCHEM 421

Ceiling 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 -16 235

α-Methyl styrene -7 61

Methyl acrylate -20 ----

Methyl methacrylate -13 164

Ethylene -26 407

Propylene -21 300

Isobutene -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 421

Solution Polymerization

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

Solvent

M

M M

MM

MM

MM

M

I

I

II

I

hνorΔ

Solvent

M M

M

MI

I

I

P

P

P

P

P

P

P P

Polymer SynthesisCHEM 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

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