Polymer Synthesis
CHEM 421
Reading (Odian Book):
Chapter 2-1, 2-2, 2-4
Step Growth Polymerizations
• Bifunctional
monomers
Polymer Synthesis
CHEM 421
• AB monomers
Example Polymers via Step
Growth Reactions
DacronTM, MylarTM
Polymer Synthesis
CHEM 421
Importance of Equal Reactivity
Polymer Synthesis
CHEM 421
A—B + A—B + A—B + A—B + A—B + A—B + A—B + A—B + A—B + A—B
A—ba—B +
A—B +
A—ba—B +
A—ba—B +
A—ba—B +
FA° = # of A groups at the beginning
FA = # of A groups at any given time
p = conversion = 1 – FA / FA° = 1 – (6 / 10)
Xn = average degree of polymerization
total # of molecules present initially
Xn = ————————————————
total # of molecules present at time t
A—B
Importance of Equal Reactivity
Polymer Synthesis
CHEM 421
A—B + A—B + A—B + A—B + A—B + A—B + A—B + A—B + A—B + A—B
p = 0.0
A—ba—B +
A—B +
A—ba—B +
A—ba—B +
A—ba—B +
p = 0.4
A—ba—ba—ba—B
A—ba—ba—ba—B
A—ba—ba—ba—B
+
+
+
A—ba—B
+
A—ba—ba—B
A—ba—ba—ba—ba—B
+
+
Xn = 1
A—B
Xn = 1.67
A—B
p = 0.6
Xn = 2.5
p = 0.7
Xn = 3.33
p = 0.8
Xn = 5
A—B
A—ba—ba—ba—ba—ba—B
MW and Conversion
Polymer Synthesis
CHEM 421
Given that:
[M] = [M]0 - [M]0 p
Rewriting:
[M] = [M]0 (1 – p)
Rewriting:
1
(1 – p)
=
Knowing:
[M]0
Xn =
[M]
Xn =
1
(1 – p)
[M]0
[M]
Implications of Carothers
Equation
• Conversion
Xn
– 50%
2
– 80%
5
– 90%
10
– 95%
20
– 99%
100
– 99.5%
200
Xn =
Polymer Synthesis
CHEM 421
1
(1 – p)
Weight Average and Number AveragePolymer Synthesis
CHEM 421
Molar Masses
Factors Involved in the Synthesis of High MW
Linear Polymers via Step-Growth Reactions
Polymer Synthesis
CHEM 421
• High purity monomers
• Di-functionality
• Proper stoichiometry
• Very high conversions
• No side reactions
• Accessibility of mutually reacting groups
• In general:
– Suitable for bulk reactions
– Moderate viscosity during much of the reaction
– Incredible effort ($$) goes to pushing the reaction forward in last
stages
Methods for Polyester Synthesis
• Direct reaction
• Acid halide / hydroxyl
• Transesterification
• Melt acidolysis
Polymer Synthesis
CHEM 421
Direct Reaction
• “Le Chatelier’s Principle”
Polymer Synthesis
CHEM 421
Mechanism
Polymer Synthesis
CHEM 421
Overall Reaction
Polymer Synthesis
CHEM 421
• Self-catalyzed
Rp
[OH] [COOH] [COOH]
• Catalyzed by added acid ( [H+] = constant)
Equilibrium Considerations:
Closed System
Keq
Polymer Synthesis
CHEM 421
[ester] [H2O]
= ————————
[RCOOH] [ROH]
• Initial hydroxyl and carboxyl concentrations are [M]0
• Concentration of ester groups @ equilibrium is p [M]0
where p = extent of reaction @ equilibrium
• The concentrations of hydroxyl and carboxyl groups @ equilibrium must
therefore be:
( [M]0 – p [M]0)
• Therefore
Keq
( p [M]0)2
p2 [M]02
p2
= ———————— = ———————— = ————
( [M]0 – p [M]0)2
[M]02 ( 1– p )2
( 1– p )2
Equilibrium Considerations:
Closed System
Keq
Polymer Synthesis
CHEM 421
[ester] [H2O]
= ————————
[RCOOH] [ROH]
• Solve for p yields:
K½
p = ————
1+ K½
Xn = 1 + K½
• Knowing that:
Xn =
1
(1 – p)
Effect of Equilibrium Constant on “p”
Synthesis
and Degree of Polymerization: Polymer
CHEM 421
Closed System
Effect of Equilibrium Constant on “p”
Synthesis
and Degree of Polymerization: Polymer
CHEM 421
Closed System
• Indicates the limitation imposed by equilibrium on the synthesis
of high MW polymer
– @ Xn = 100 (corresponding to Mn ≈ 10k) can be obtained in
a closed system only if K is 104!!!
– Can not be done in a closed system for most polymers…
Polymer
Polyesters
Polycarbonates
Transesterifications
K
1 - 10
15
0.1 – 1.0
Polyamidation
102 - 103
• Therefore must drive the equilibrium
Open System
Polymer Synthesis
CHEM 421
• Extent to which one must drive the system in the
forward reaction
– can be seen by calculating the lowering of the small molecule
condensate concentration that is necessary to achieve a particular MW
– Knowing that
[H2O] = p [M]0
Keq
Xn =
1
(1 – p)
( p [M]0)2
p [H2O]
p [H2O] (Xn)2
= ———————— = —————— = ——————
( [M]0 – p [M]0)2
[M]0 ( 1– p )2
[M]0
– Solve for [H2O]
K [M]0
[H2O] = —————
Xn ( Xn – 1)
[H2O]
1
——
Xn2
Drive the Equilibrium
Polymer Synthesis
CHEM 421
• Need to remove the small molecule
condensate
–H2O
–HCl
• Small molecule condensate needs to
diffuse through and eventually out of the
reaction mixture
–Not easy because of high viscosity
–Can lead to reactions becoming diffusion
controlled
Effect of Water Concentration on
Degree of Polymerization:
Open, Driven System
Polymer Synthesis
CHEM 421
How best to drive the
Equilibrium?
Polymer Synthesis
CHEM 421
• Mixing is energy and capital intensive
–Wiped film reactors to increase surface area
• Increase diffusivity of the condensate
–Raise the temperature to lower the viscosity
of the melt
» Potential for side reactions
–Swell the melt with solvents
» Supercritical CO2