Chemistry 367L/392N

Macromolecular ChemistryMacromolecular Chemistry

Lecture 11 Lecture 11 B

HT

Chemistry 367L/392N

Light Scattering ExperimentLight Scattering Experiment

MeasureI/I0 = f(θ)

Chemistry 367L/392N

Standard ApproachStandard 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 ottemperatures and wavelengths. Ratio is tabulated in many reference books.

CH3

Chemistry 367L/392N

IA = lntensity of analyte (sample I – solvent I)no = Solvent RIIS = Intensity of standard (Toluene?)nS = RI of Standard RS = Rayleigh ratio of standard

22

0044

00

2222

==dcdcdndn

nnNN

KKAA

λλππ

λo = laser wavelengthNA = Avogadros numberno = Solvent RIdn/dc = differential RI increment

SS

SSSS

AA RRnnIInnII

RR22

22

00==θθ

KCKC22

1122

θθ

ccAAMMRR

++==

Some Messy but accessible Constants Some Messy but accessible Constants

Chemistry 367L/392N

Molecular Weight ExampleMolecular Weight Example

Chemistry 367L/392N

Light ScatteringLight Scattering

Therefore a Therefore a ““ Debye plotDebye plot”” of of KC/KC/RRθθ

versusversuscc should give a straight should give a straight line whose intercept at zero concentration will be line whose intercept at zero concentration will be 1/Mw1/Mw and and whose slope will be 2whose slope will be 2AA22!!

For Rayleigh For Rayleigh scatterersscatterers, ,

KCKC22

1122

θθ

ccAAMMRR

++== (y = b + (y = b + mxmx))

Note that the moment of the distribution is Mw !Note that the moment of the distribution is Mw !

Chemistry 367L/392N

The Debye PlotThe Debye PlotK

C/Rө

(1/M

w)

Conc (g/ml)

xx

xx

x

1/Mw1/Mw KCKC22

1122

θθ

ccAAMMRR

++==

Chemistry 367L/392N

Chain Transfer in Free Radical Chain Transfer in Free Radical PolymerizationPolymerization

� A termination and re-initiation reaction

Chemistry 367L/392N

Chain Transfer Agents (Chain Transfer Agents (CTAsCTAs))

� Thiols are efficient examples

Chemistry 367L/392N

Chain Transfer to PolymerChain Transfer to Polymer� Creation of branches� “ Back biting”

Chemistry 367L/392N

Branching in PolyethyleneBranching in Polyethylene� Common Branch length is 4 or 5….why ???

H

H3C

H3C

Chemistry 367L/392N

Chain Transfer to InitiatorChain Transfer to Initiator� Example for acrylonitrile and BPO

� What effect does this process have on DP, PDI??

Chemistry 367L/392N

Chain Transfer KineticsChain Transfer Kinetics

The chain transfer constant, C is defined as the ratioThe chain transfer constant, C is defined as the ratioof the chain transfer and propagation rate constants;of the chain transfer and propagation rate constants;

C = kC = ktrtr/k/kpp

There is some C, which includes a term for transfer to a There is some C, which includes a term for transfer to a CTA, and one to monomer, solvent, polymer, etc.CTA, and one to monomer, solvent, polymer, etc.

The higher the value of C the smaller the amount The higher the value of C the smaller the amount required to lower the molecular weightrequired to lower the molecular weight

Chemistry 367L/392N

Chain transfer constant

B uSHI n H

BuS*I n

* + +

ptr kk=

ptrI

kk=CI

ptrS

kk=CS p

trMk

k=CM

Chemistry 367L/392N

DPDPnn = degree of = degree of polymerisationpolymerisationWITH transfer agentWITH transfer agent

DPDPoo = degree of = degree of polymerisationpolymerisationWITHOUT transfer agentWITHOUT transfer agent

Measurement of Chain Transfer ConstantsMeasurement of Chain Transfer Constants

+=monomer

gentTransfer aCs

DPDPn 0

11

CCSS = Chain Transfer = Chain Transfer ““ constantconstant”” or or ““ coefficientcoefficient””

The Mayo Equation:The Mayo Equation:

Chemistry 367L/392N

1/DP0

Cs

1/D

P

[S]/[M]

Generic Mayo plot

Chemistry 367L/392N

CHAIN TRANSFER AGENTSCHAIN TRANSFER AGENTS

Chain transfer activity dependent upon the monomer being Chain transfer activity dependent upon the monomer being polymerisedpolymerisedandandthe structure of the CTAthe structure of the CTA

For Styrene @ 60oC

CTA Cs x 104 Comment

Benzene 0.02 Addition to propagating radicalToluene 0.1 Resonance stabilizedEthyl Benzene 0.7 Weakening of C-H BondAcetone 4.1CCl4 110 Weak C-C1 BondCH3(CH2)3SH 210,000 Weak S-H Bond

Chemistry 367L/392N

Chain Transfer constants at 60 Chain Transfer constants at 60 °°CC

480,000210,000n-Butyl Mercaptan

10,700110CCl4

7.0n-Butyl amine

1503.4CHCl3

201.6n-Butanol

21.60.125Toluene

1.20.023Benzene

Vinyl acetateSryreneCTA

Chemistry 367L/392N

no CTA

Benzene(CS = 0.02)

CCl4 (CS = 110)

n-Butyl mercaptan (CS = 210,000)

[Transfer Agent][Styrene]

1/D

P ~

1/(

mol

.wt)

Effect of CTA on DP of Styrene @ 60Effect of CTA on DP of Styrene @ 60OOCC

Chemistry 367L/392N

Transfer to PolymerTransfer to Polymer

IntraIntramolecularmolecularreaction/backbiting reaction/backbiting →→

-- short chain branchesshort chain branches

InterIntermolecular reaction molecular reaction →→

-- long chain brancheslong chain branches

C(Psty) = 10 * 104

C(PMMA)= 0.1 - 360 * 104

Chemistry 367L/392N

Trapping RadicalsTrapping Radicals

OH

BHT

O

RHR

+

O

O

R

R

Chemistry 367L/392N

Trapping carbon centered radicalsTrapping carbon centered radicals� Carbon centered radicals stopped by addition to

oxygen or carbon

O O OR

O

Benzoquinone

R.

O OR

O O

R HR O O

R

R H+

HO O

R

Tautomerize

Chemistry 367L/392N

Stable Radical InhibitorsStable Radical Inhibitors

N N

O2N

NO2

O2N

Diphenylpicrylhydrazyl, DPPH

N

O

TEMPO

O

O

Galvanoxyl

NN

NN

Triphenylverdazyl

Chemistry 367L/392N

Some Familiar CTASome Familiar CTA’’ss

αα--tocopheroltocopherol Vit E

Ascorbic acid Ascorbic acid VitVit CC Butylated HydroxyAnisole BHA

Chemistry 367L/392N

BButylatedutylated HHydroxyydroxyTTolueneoluene� BHT radical will not initiate new chains

Chemistry 367L/392N

Interesting CTAInteresting CTA’’ss

N

O

N

O

PhenylPhenyl--aa--tt--butylnitronebutylnitrone

PBNPBN

TEMPOTEMPO

Chemistry 367L/392N

Chemistry 367L/392N

Radical Theory of AgingRadical Theory of Aging

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

BH

T