Chemistry 367L/392N

Macromolecular ChemistryMacromolecular Chemistry

Lecture 14 Lecture 14

Michael Michael SzwarcSzwarc

Chemistry 367L/392N

Midterm ExamMidterm Exam IIII�� Where:Where: Right here….in the lecture room WEL 3.502

�� When:When: Tuesday 3/24/09 at 3:30 – 5PM

�� What:What: Covers lectures through today

�� Bring:Bring: Pencil, eraser, Calculator only…closed book!

�� Do:Do: Study lecture notes, homework, reading assignments, graduate presentations and test I

�� Do not: Do not: Memorize derivations…but do know the principles! Go over homework problems.

�� Please: Please: Do a good job!

Chemistry 367L/392N

F

f

Chemistry 367L/392N

Poly(styrene-alt-maleic anhydride)

OOO

+δ

δ

OOO+

OOO

[ ]n

Charge transfer complex

δ+δ-δ+δ-

Chemistry 367L/392N

Chemistry 367L/392N

I+IV or II+III →

alternating

I+III, II+IV and same →Statistical Copolymers

Χ

Χ I+II or III+IV → Poor Copolymerization

Chemistry 367L/392N

Q and eQ and e

� Generalizations:– This is a purely empirical relationship

– Q and e come from measurements of r1 and r2

– Ideal condition is same Q and e values

– Proceeds poorly if Q1 and Q2 are very different– Tends toward alternating if Q’s are the same and e’s are

large but of opposite sign.

– This assumes that Q and e are not dependent on solvent, temp, etc…..but we know that r1 and r2 are….

– Weak linkage to “linear free energy” relationships…and the Hammet ρ σ

Hammett Equation

� Hammett observed a linear free energy

relationship between the log of the relative rate constants for ester hydrolysis and the log of the relative acid ionization (equilibrium) constants for a series of substituted benzoic esters & acids.

log (kx/kH) = log (Kx/KH) = ρσρσρσρσ

� He arbitrarily assigned ρρρρ, the reaction constant, of the acid ionization of benzoic acid a value of 1.

Definition of Hammett ρρρρ

C

O

OH

X

C

O

O

X

+ H

These σσσσp values are obtained from the best fit line having a slope = 1

Hammett Plot

y = 0.9992x - 4.5305

R2 = 0.9907

-5.3

-5.1

-4.9

-4.7

-4.5

-4.3

-4.1

-3.9

-3.7

-1 -0.5 0 0.5 1

sigma pL

og

K

substituent σσσσp Eq. constant log K

-NH2 -0.66 0.00000554 -5.25649

-OCH3 -0.27 0.000015 -4.82391

-CH3 -0.17 0.000023 -4.63827

-H 0.00 0.000034 -4.46852

-Cl 0.23 0.000055 -4.25964

-COCH3 0.5 0.000088 -4.05552

-CN 0.66 0.000128 -3.89279

-NO2 0.78 0.000166 -3.77989

Hammett Plots

� Aryl substituent constants (σσσσ) were determined by measuring the effect of a substituent on a reaction rate (or Keq). These are listed in tables, and are constant in widely different reactions.

� Reaction constants (ρρρρ) for other reactions may also be determined by comparison of the relative rates (or Keq) of two differently substituted reactants, using the substituent constants described above.

� Some of these values (σσσσ and ρρρρ) are listed on the following slide.

Hammett Rho & Sigma Values

Substituent (Sigma) Values σσσσ (the electronic effect of the substituent;

negative values are electron donating)

p-NH2 -0.66 p-Cl 0.23

p-OCH3 -0.27 p-COCH3 0.50

p-CH3 -0.17 p-CN 0.66

m-CH3 -0.07 p-NO2 0.78

Reaction (Rho) Values ρρρρ

CH2COCH3

O

CH2CO + CH3OH

OOH

ρρρρ = + 2.4

X X

CH3OHC Cl

H

X

φ

C OCH3 + HCl

H

φ

X ρρρρ = - 5.0

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Conclusions-radical copolymers

� Co-polymers are important

– Properties depend on composition

� In general, these materials are

heterogeneous

� Mayo equation allows calculation of composition knowing r1 and r2

� Finemann Ross approach allows determination of r1 and r2

� Alfrey – Price allows estimate of r1 and r2

Chemistry 367L/392N

We discussed A B type step growth polymers -ABABABA-

If is the number of A molecules at the beginning of the

polymerization and is the number if B molecules,

We define r, the stoichiometric imbalance as

What about step growth copolymers??

0

AN0

BN

0

0

B

A

N

Nr =

Chemistry 367L/392N

DP is the number of monomer units divided by number of chains

−+

+

==

pr

N

r

rN

N

NDP

A

A

r

21

12

1

20

0

Which luckily reduces to…rpr

rDP

21

1

−+

+=

Chemistry 367L/392N

rpr

rDP

21

1

−+

+=

Note that if there is no stoichiometric imbalance, r =1 and we get…..

This is nice!!!

pDP

−=

1

1 The Carothers Equation!!

When A is totally consumed (p =1) then….

r

rDP

−

+=

1

1

Chemistry 367L/392N

rpr

rDP

21

1

−+

+= “A” is A-□-A and and B is really B-□-B…..

B’-□A-□-B

B’-□Monofunctional

A-□-A + B-□-BEquimolar

B-□-BA-□-B

B-□-BA-□-A + B-□-B0

0

B

A

N

Nr =

0

0

B

A

N

Nr =

0

'

0

0

2 BA

A

NN

Nr

+=

0

'

0

0

2 BA

A

NN

Nr

+=

Chemistry 367L/392N

This illustrates the high precision required to

achieve high molecular weights

!!!

DP

DP

r

rDP

−

+=

1

1

Chemistry 367L/392N

Chemistry 367L/392N

Capping TrickOften wise to use a capping agent than to

use excess of one monomer

CH2CH2OH + CH2CH2O C

O

CH2CH2O C

O

C

O

OH

CH2CH2O C

O

Ph

Ph

+~

~ ~ ~

~ ~

“molecular weight stabilization”

Chemistry 367L/392N

Conclusions

� A tiny amount of monofunctional monomer has a big effect�You will see this in the homework

�We can now calculate the copolymer composition for both radical chain growth and step growth polymerizations.

�We need to know

� the Copolymer equations� Mayo Plot,

� Finemann Ross Plot

� Implications of r1 and r2

� Implications of Q and e

Anionic polymerization

1914, Schlenk reacts Na with butadiene and styrene

1929, Ziegler proposes a mechanism

1952 Higginson, styrene, KNH2, kinetic study

1956 Szwarc, sodium naphthalene, Styrene,

living polymerization conception

60's, commercial products were available

90‘s, study on the living polymerization of polar

monomers

History

Monomers

Alkenes with polar substituents

CH2

C H

C N

CH2

C H

NO O

CH2

C

C O

CH3

O CH3

Acrylonitrile Nitroethene Methyl methacrylate

For example

Conjugated dienes

CH2

CHCH

2C

CH3

CH2

CH CH CH2

CH2

CH C CH2

CH3

styrene αααα-Me-styrene

butadiene isoprene

Reactivity of monomers

Group A:

Group B:

Group C:

Group D:CH

2CH CH CH

2

CH2

C(CN)2 CH 2 C

CN

COOC 2H 5CH

2CH

NO 2

> >

CH 2 CHCN CH 2 C

CN

CH 3 CH 2 CH C

O

CH 3> >

CH 2 CH

CO OCH 3

CH 2 C CH 3

COO CH 3

>

CH2 CH CH2 C

CH3

Group A

Group B and group C

Group D

R

R R ,

R R R+

, ,

>>

>>

Initiator should be

stronger

Stronger or

weaker

only

Reactivity Type of polymerization

Initiator and initiation

(1) alkali metal Lithium (Li)

Sodium (Na)

Potassium (K)

-Na CH

2CH

electron

transferCHCH

2Na ++ .

e

-Na

-CH CH

2Na CHCH2

++dimerize

radical couple

Szwarc’s

favorite

K: soluble in ethers

Li and Na dispersion:

insoluble in hydrocarbon

Solubility of alkali metals

homogeneous or heterogeneous

Organic reaction is on the surface

adsorbtion

reaction

desorption

Alkali metal complex

-Na Na +

+ [ ]

complex (greenish blue color)e

.

preparation

initiator

a. Species

b. Initiation

c. Solubility

Aromatic radical-anions:

Sodium naphthalene

Lithium naphthalene

Alkali metal Aromatic compound

-Na CH

2CH

+[ ].

+Initiation:

-electron

transferCHCH

2Na +. +

-Na

-CH CH

2Na CHCH

2++

dimerize

radical couple

soluble in polar solvents only

( such as: ether)

Solubility:

Living test

Szwarc’s Experiment