Polymer Synthesis CHEM 421 Reading (Odian Book): Chapter 2-1, 2-2, 2-4.
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Transcript of Polymer Synthesis CHEM 421 Reading (Odian Book): Chapter 2-1, 2-2, 2-4.
Polymer SynthesisCHEM 421
Reading (Odian Book):
Chapter 2-1, 2-2, 2-4
Polymer SynthesisCHEM 421
Step Growth Polymerizations
• Bifunctional monomers
A B
a bn
A A B B+
a a b bn
• AB monomers
Polymer SynthesisCHEM 421
Example Polymers via Step Growth Reactions
C
O
OH
+H
+
H2O
C
O
HO
HO CH2CH2 OH
C
O
C
O
O CH2CH2 On
DacronTM, MylarTM
N CH2 N C
O
CH2 C
O
HH nx y
Polymer SynthesisCHEM 421
Importance of Equal Reactivity
FA° = # of A groups at the beginning
FA = # of A groups at any given time
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—B A—ba—B +A—ba—B +A—ba—B +
p = conversion = 1 – FA / FA°
Xn = average degree of polymerization
= 1 – (6 / 10)
Xn = ———————————————— total # of molecules present initially
total # of molecules present at time t
Polymer SynthesisCHEM 421
Importance of Equal Reactivity
p = 0.0 Xn = 1
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—B A—ba—B +A—ba—B +A—ba—B +
A—ba—ba—ba—B + A—B A—ba—ba—B +A—ba—B +
A—ba—ba—ba—B + A—B A—ba—ba—ba—ba—B +
A—ba—ba—ba—B + A—ba—ba—ba—ba—ba—B
p = 0.4 Xn = 1.67
p = 0.6 Xn = 2.5
p = 0.7 Xn = 3.33
p = 0.8 Xn = 5
Polymer SynthesisCHEM 421
MW and Conversion
Given that:
[M] = [M]0 - [M]0 p
Rewriting:
[M] = [M]0 (1 – p)
1
(1 – p)
=
Rewriting:
1 [M]0
(1 – p) [M]
=
Knowing:
[M]0
[M]
Xn
=Xn
Polymer SynthesisCHEM 421
Implications of Carothers Equation
• Conversion Xn
– 50% 2
– 80% 5
– 90% 10
– 95% 20
– 99% 100
– 99.5% 200
= 1
(1 – p)
Xn
Polymer SynthesisCHEM 421
Weight Average and Number Average Molar Masses
Polymer SynthesisCHEM 421
Factors Involved in the Synthesis of High MW Linear Polymers via Step-Growth Reactions
• 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
Polymer SynthesisCHEM 421
Methods for Polyester Synthesis
• Direct reaction
• Acid halide / hydroxyl
• Transesterification
• Melt acidolysis
Polymer SynthesisCHEM 421
Direct Reaction
C
O
OH + HO
C
O
O + H2O
• “Le Chatelier’s Principle”
Polymer SynthesisCHEM 421
Mechanism
C
O
OH +R H C
O
OHR
H
Polymer SynthesisCHEM 421
Overall Reaction
C
O
OH +RH
C
O
ORR' OH R' + H2O
• Self-catalyzed
Rp [OH] [COOH] [COOH]
• Catalyzed by added acid ( [H+] = constant)
Polymer SynthesisCHEM 421
Equilibrium Considerations:Closed System
C
O
OH +RH
C
O
ORR' OH R' + H2O
• 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
[ester] [H2O]
[RCOOH] [ROH]Keq = ————————
( p [M]0)2
( [M]0 – p [M]0)2Keq = ————————
p2 [M]02
[M]02 ( 1– p )2
= ————————p2
( 1– p )2 = ————
Polymer SynthesisCHEM 421
Equilibrium Considerations:Closed System
C
O
OH +RH
C
O
ORR' OH R' + H2O
• Solve for p yields:
• Knowing that:
[ester] [H2O]
[RCOOH] [ROH]Keq = ————————
K½
1+ K½ p = ————
= 1
(1 – p)
Xn
1 + K½
=Xn
Polymer SynthesisCHEM 421
Effect of Equilibrium Constant on “p” and Degree of Polymerization:
Closed System
Polymer SynthesisCHEM 421
Effect of Equilibrium Constant on “p” and Degree of Polymerization:
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…
• Therefore must drive the equilibrium
Polymer K
Polyesters 1 - 10
Polycarbonates 15
Transesterifications 0.1 – 1.0
Polyamidation 102 - 103
Polymer SynthesisCHEM 421
Open System• 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
– Solve for [H2O]
( p [M]0)2
( [M]0 – p [M]0)2Keq = ————————
p [H2O]
[M]0 ( 1– p )2 = ——————
[H2O] = p [M]0= 1
(1 – p)
Xn
p [H2O] (Xn)2
[M]0
= ——————
K [M]0
Xn ( Xn – 1)[H2O] = —————
1
Xn2
[H2O] ——
Polymer SynthesisCHEM 421
Drive the Equilibrium
• 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
Polymer SynthesisCHEM 421
Effect of Water Concentration on Degree of Polymerization:
Open, Driven System
Polymer SynthesisCHEM 421
How best to drive the Equilibrium?
• 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