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10.569 Synthesis of Polymers Prof. Paula Hammond

Lecture 4: Common Processing Approaches, Near-equilibrium vs. Far from Equilibrium, Homogeneous Solution and Bulk Polymerizations

Acid catalyst [HA]

1

1 11 no

a kt p= − = −⎡ ⎤⎣ ⎦ − π

np linear ↑ with time For self-catalyzed case:

( )

2 2

2

12 1

1n o

p M kt= = +⎡ ⎤⎣ ⎦− π

correlates with

⇒ np ↑ with t Basic rate expression for acid catalyst: 3 12pR k K HA COOH OH= ⎡ ⎤⎡ ⎤⎡ ⎤⎦⎣ ⎦

R k K COOH OH= ⎡ ⎤ ⎡ ⎤⎣ ⎦ ⎣ ⎦

⎣ ⎦⎣decrease

Self-catalyzed: Slower reaction

2

3 12,p COOH

Gets extremely slow at the end of the reaction

COOHOH + C O

O

COOHOH + C O

O

Polyester (k ∼ 1)

COOHNH2 + C N

O H

COOHNH2 + C N

O H

Polyamide (k ∼ 10) In some cases k ∼ 100 (hungry polymer)

Typical process conditions for near equilibrium step growth: • High T

- to increase k (k↑ with T↑) - to aid in byproduct removal

• Bulk or mass polymerization (no solvent) - if the reactants are miscible (forming 1 uniform, mixed phase), get

highest concentrations possible - no need for separations step (remove solvent) - viscosity η low enough to process until high MW - product can be directly processed into final form

• Solvents - may be needed to solubilize two monomers (reactants)

Citation: Professor Paula Hammond, 10.569 Synthesis of Polymers Fall 2006 materials, MIT OpenCourseWare (http://ocw.mit.edu/index.html), Massachusetts Institute of Technology, Date.

- could allow higher T to be approached without scorching polymer - carrier (dilutant) for viscous media (exp for high MW)

Interchange Reactions in Near-Equilibrium Polymerization (k ∼ 1-10) Ex: polyamide (but same thing happens in polyester)

HHN RC

O

NHRCOH

O

m+ H

HN RC

OHN

xR C

OHN RC

O

OHy..

HO

H

HHN RC

OHN R C

OHN

mR NHRC

O

OHy

H NHRC

O

OHx

+C

O

Shuffling of chain

segments

m+y+1

Illustration of shuffling of chain segments (redrawn below):

HHN RC

O

NHRC

O

m HHN RC

OHN

xR C

OHN RC

O

OHy..

HO

H

OH2 +

HHN RC

O

NHRC

O

mOH

N C

R

H

O

RHN H

C

O

HN RC

O

OH

x

y

H O

HHHN RC

O

NHRC

O

mO

N C

R

O

RHN H

C

O

HN RC

O

OH

x

y

H

H

Continued on next page

10.569, Synthesis of Polymers, Fall 2006 Lecture 4 Prof. Paula Hammond of 6 Citation: Professor Paula Hammond, 10.569 Synthesis of Polymers Fall 2006 materials, MIT OpenCourseWare (http://ocw.mit.edu/index.html), Massachusetts Institute of Technology, Date.

See previous page

HHN RC

O

NHRC

O

mO

N C

R

O

RHN H

C

O

HN RC

O

OH

x

y

H

HH

HN RC

O

NHRC

O

m

N

C

R

O

RHN H

C

O

HN RC

O

OH

x

y

H O H

HHN RC

O

NHRC

O

mN

C

R

O

RHN H

C

O

HN RC

O

OH

x

y

OH

H

HHN RC

O

NHRC

O

mN C

R

O

RHN H

C

O

HN RC

O

OH

x

y

O H

H

HHN RC

OHN R C

OHN

mR NHRC

O

OHy

H NHRC

O

OHx

+C

O

• If no removal of byproduct (H2O) ⇒ No net change in MW • Only if H2O is removed during interchange will MW change (increase) ⇒ Can be used to increase MW in the final form of a product 10.569, Synthesis of Polymers, Fall 2006 Lecture 4 Prof. Paula Hammond of 6 Citation: Professor Paula Hammond, 10.569 Synthesis of Polymers Fall 2006 materials, MIT OpenCourseWare (http://ocw.mit.edu/index.html), Massachusetts Institute of Technology, Date.

Real Industrial Processes Diacid + Diol → low MW polymers

Crosslinked networks

Less need for high π

Making polyethylene terephthalate (PET) (polyester) Trade names: Mylar®, Dacron®, Terylene®


C

O

C

O

OCH2CH2 O

n

Tm = 270oC

High mechanical strength (from aromatic group rigidity)

Tough (flexibility in backbone)

Partially crystalline (adds to toughness) ⇒ $9.5 billion/year

Ideal material for carpet, clothing, photographic substrates, Boil-in-Bag meals, PET Coke bottles

Need high MW Very slow Very expensive HOC

O

COH

O

+ HOH2C

H2C OH Instead…

2-Step Process 1. dimethyl terephthalate

H3COC

O

COCH3

O

+ 2 HOH2C

H2C OH....

ethylene glycol (EG)easy to purify, cheap

HO CH2CH2 OC

O

CO

O

CH2CH2 OH + 2 H3C OH

Methanol (collect as it is dripped off)

T ∼ 150 – 210 oC P ∼ 1 atm Done in “solvent” of excess ethylene glycol (increase rate of forward rxn) Detailed mechanism for above reaction:

H3COC

O

COCH3

O

+ HO CH2CH2....

OH H3COC

O

COCH3

O

O CH2CH2OHHHO CH2CH2 ....OH

H3COC

O

C

O

O

CH2CH2OH

O CH2CH2

..OH

OCH3

HHH3COC

O

C

O

O

CH2CH2OH

OCH3

H

H3COC

O

C

O

O CH2CH2OH

+ HO CH3

repeatC

O

C

O

OCH2CH2OHHOH2CH2CO

2. T = 270oC – 280oC (increase T) Drip stops

P = 0.5 – 1 torr (create vacuum) Drive off EG

HO CH2CH2 OC

O

CO

O

CH2CH2 OH H OCH2CH2OC

O

C

O

OCH2CH2OH

+ n-1 HO CH2CH2 OH

n

n Ester

Interlinkage

Byproduct removed by distillation column

Advantages:

1. Stoichiometry balance not needed (only one monomer) 2. Dimethyl ester more easily purified 3. Ester interchange k (rate constant) is larger than k for acid + alcohol (better

kinetics) 4. removal of ethylene glycol (EG) is cleaner than H2O removal


Practical Example #2: Polyester anhydrides for Biodegradable polymer systems

1. diacid + diol → oligomeric polyester


HO

O

R

O

OR'

O

O

R

O

OHnHO

O

R

O

OH

+HO

R'HO

1

(excess) “macromer” 2. form ester macromer dianhydride

H3C

O

O

O

CH3

O

O

R

O

OR'

O

O

R

O

Onacetic anhydride

O

H3C

O

OH+

H3C

O

OH

2

1

acetic acid byproduct

3. melt condensation

add heat O

O

O

O

R

O

OR'

O

O

R

O

O mn

2

polyester

- ester groups: relatively hydrolysable - anhydride groups: can be even more hydrolytically susceptible ⇒ degradable polymer ⇒ alkyl esters, acids

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