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