Lecture 11

willson.cm.utexas.edu

Lecture 11

Macromolecular Chemistry

BHT

Lecture 11

Chemistry 367L/392N


Light Scattering Experiment

Measure

I/I 0 = f(θ)

Chemistry 367L/392N


Standard Approach



Measure scattering of an analyte relative to a well

characterized very pure liquid

Toluene is often used due to good scattering signal

and values well characterized for a range ot

temperatures and wavelengths. Ratio is tabulated in

many reference books.

CH 3

Chemistry 367L/392N


Some Messy but accessible Constants

K

=

2


4

λ N

0

A


⎜⎝

n

0

dn


dc

⎟⎠

λ o = laser wavelength

N A = Avogadros number

n o = Solvent RI

dn/dc = differential RI increment

2

KC

R

θ

=

1

M

+

2A c

2

=

I

R

A 0

θ

2

I

S

n

n

2

S

R

S

I A = lntensity of analyte (sample I – solvent I)

n o = Solvent RI

I S = Intensity of standard (Toluene)

n S = RI of Standard

R S = Rayleigh ratio of standard

Chemistry 367L/392N


Molecular Weight Example

Chemistry 367L/392N


Light Scattering

For Rayleigh scatterers,

KC

R

θ

1

= + 2A2c

(y = b +

M

(y = b + mx)

Therefore a “Debye plot” of KC/R θ versus c should give a straight

line whose intercept at zero concentration will be 1/Mw and

whose slope will be 2A2

2 !

Note that the moment of the distribution is Mw !

Chemistry 367L/392N


The Debye Plot

KC/Rө (1/Mw)

x

1/Mw

x

x

KC

R

x

θ

=

x

1

M

+

2A c

2

Conc (g/ml)

Chemistry 367L/392N


Chain Transfer in Free Radical

Polymerization

A termination and re-initiation reaction

Chemistry 367L/392N


Chain Transfer Agents (CTAs(

CTAs)

Thiols are efficient examples

Chemistry 367L/392N


Chain Transfer to Polymer

Creation of branches

“ Back biting”

Chemistry 367L/392N


Branching in Polyethylene

Common Branch length is 4 or 5….why

H

H 3 C

H 3 C

Chemistry 367L/392N


Chain Transfer to Initiator

Example for acrylonitrile and BPO

What effect does this process have on DP, PDI

Chemistry 367L/392N


Chain Transfer Kinetics

The chain transfer constant, C is defined as the ratio

of the chain transfer and propagation rate constants;

C = k tr /k p

There is some C, which includes a term for transfer to a

CTA, and one to monomer, solvent, polymer, etc.

The higher the value of C the smaller the amount

required to lower the molecular weight

Chemistry 367L/392N


Chain transfer constant

=

ktr k p

C I

=

k

trI

k

p

C S

=

k

trS

k

p

C M

=

k

trM

k

p

B uSH

I * + +

n I n H

B uS*

Chemistry 367L/392N


Measurement of Chain Transfer Constants

The Mayo Equation:

1 1

=

DP n DP 0

⎡Transfer agent

+ Cs⎢

⎣ monomer




DP n

DP o

C S

= degree of polymerisation WITH transfer agent

= degree of polymerisation WITHOUT transfer agent

= Chain Transfer “constant” or “coefficient”

Chemistry 367L/392N


Generic Mayo plot

1/DP

Cs

1/DP 0

[S]/[M]

Chemistry 367L/392N


CHAIN TRANSFER AGENTS

Chain transfer activity dependent upon the monomer being

polymerised and the structure of the CTA

For Styrene @ 60 o C

CTA C s x 10 4 Comment

Benzene 0.02 Addition to propagating radical

Toluene 0.1 Resonance stabilized

Ethyl Benzene 0.7 Weakening of C-H Bond

Acetone 4.1

CCl 4 110 Weak C-C1 Bond

CH 3 (CH 2 ) 3 SH 210,000 Weak S-H Bond

Chemistry 367L/392N


Chain Transfer constants at 60 °C

CTA

Benzene

Toluene

Sryrene

0.023

0.125

Vinyl acetate

1.2

21.6

n-Butanol

1.6 20

CHCl 3

3.4 150

n-Butyl amine 7.0

CCl 4

110

10,700

n-Butyl Mercaptan

210,000

480,000

Chemistry 367L/392N


Effect of CTA on DP of Styrene @ 60 O C

1/DP ~ 1/(mol.wt)

no CTA

n-Butyl mercaptan (C S = 210,000)

[Transfer Agent]

[Styrene]

CCl 4 (C S = 110)

Benzene

(C S = 0.02)

Chemistry 367L/392N


Transfer to Polymer

Intramolecular

reaction/backbiting →

- short chain branches

Intermolecular reaction →

- long chain branches

C(Psty) = 10 * 10 4

C(PMMA)= 0.1 - 360 * 10 4

Chemistry 367L/392N


Trapping Radicals

OH

O

R

+

RH

BHT

O

R

O

R

Chemistry 367L/392N


Trapping carbon centered radicals

Carbon centered radicals stopped by addition to

oxygen or carbon

R. R

O O O

O

Benzoquinone

O O R O O

Tautomerize

R

H

R O O

R

H

+

HO

O

R

R

Chemistry 367L/392N


Stable Radical Inhibitors

O 2 N

N N

NO 2

O 2 N

Diphenylpicrylhydrazyl, DPPH

N

O

TEMPO

O

N

N

N N

O

Triphenylverdazyl

Galvanoxyl

Chemistry 367L/392N


Some Familiar CTA’s

Ascorbic acid Vit C

Butylated HydroxyAnisole BHA

α-tocopherol

Vit E

Chemistry 367L/392N


Butylated

HydroxyTolueneoluene

BHT radical will not initiate new chains

Chemistry 367L/392N


Interesting CTA’s

N

O

N

O

Phenyl-a-t-butylnitrone

PBN

TEMPO

Chemistry 367L/392N


Chemistry 367L/392N


Radical Theory of Aging

BHT

Recent Review: Mechanisms of Ageing and Development Volume 125, Issues 10-11, October-November 2004

Chemistry 367L/392N

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