Chemistry 367L/392N

Macromolecular ChemistryMacromolecular Chemistry

Step Growth Chain Growth

Lecture 5 Lecture 5

Chemistry 367L/392N

Paul Flory Clears Things UpPaul Flory Clears Things Up

Polymer Structure is distinct from polymerization processPolymer Structure is distinct from polymerization process

O

O O

OH

HO

O

OH

HO

AdditionPolymerization

CondensationPolymerization

"Condensation"Polymer

Chemistry 367L/392N

Basic Types of Polymerization MechanismsBasic Types of Polymerization Mechanisms

Chain-growth

Step-growth

Ring-openingO

OO-

O

RO-O

O

ROH+

CH2C

O

HO CH2 OH CH2C

O

O CH2 O HH

H2O

DimerTetramerOctamerHexamer

...

Chemistry 367L/392N

Chain growth systemChain growth system

The characteristic of a chain polymer is that The characteristic of a chain polymer is that polymer polymer growth takes place by monomer reacting only with the growth takes place by monomer reacting only with the reactive centersreactive centers.. Monomer does not react with Monomer does not react with monomer and the different-sized species such as dimer, monomer and the different-sized species such as dimer, trimer, and n-mer do not react with each other. The trimer, and n-mer do not react with each other. The polymerization ceases when the active center is polymerization ceases when the active center is destroyed by termination reaction(s). destroyed by termination reaction(s).

Chemistry 367L/392N

A condensation takes place between two A condensation takes place between two polyfunctional polyfunctional molecules to produce one larger molecules to produce one larger polyfunctional molecule with the polyfunctional molecule with the possiblepossible elimination of a small molecule such as water. elimination of a small molecule such as water. The reaction continues until one of the reagents The reaction continues until one of the reagents is used up. is used up.

Step Growth systemStep Growth system

Chemistry 367L/392N

The chain growth vs. step growthThe chain growth vs. step growth

Step

Chain

Chemistry 367L/392N

- Step-growth polymerization- Step-growth polymerization

Chemistry 367L/392N

- Chain-growth polymerization- Chain-growth polymerization

Chemistry 367L/392N

Chain Growth PolymerizationChain Growth Polymerization

DP = No/N = 12 / 7 = 1.7 (for 50%, b)

1 1 2 3 4 5 6 7

Chemistry 367L/392N

Step Growth PolymerizationStep Growth Polymerization

DP = No/N = 12 / 9 = 1.3 (for 50%, b)

Chemistry 367L/392N

The relationship between MThe relationship between Mwtwt and conversion and conversionWith no termination reactionsWith no termination reactions

The chain growth systemThe chain growth system

Chemistry 367L/392N

The relationship between MThe relationship between Mwtwt and conversion and conversionWith termination reactionsWith termination reactions

The chain growth systemThe chain growth system

Chemistry 367L/392N

The relationship between Mwt and conversion

Step growth systemStep growth system

Chemistry 367L/392N

Distinguishing features of chain- and Distinguishing features of chain- and step-polymerization mechanismsstep-polymerization mechanisms

Chemistry 367L/392N

Let’s look at this more closely….Let’s look at this more closely….

Consider a flask of monomer ….If there are No

molecules in the flask at time = 0 and N remaining at time t then the DP at time t is the average degree of polymerization… must just be N0/N!

Chemistry 367L/392N

The Carothers EquationThe Carothers Equation

If there are No molecules at time = 0 and N remaining at time t then the amount reacted is N0-N and we can define p as the “conversion” or fraction reacted then as

p= (No – N ) / No or

N = No( 1 – P)

If DP is the average degree of polymerization… N0/N ….substituting gives

DP = 1 / (1 – P)

and for P = 0.98 (98% conversion), DP = only 50!

High Molecular weights are hard to get this wayHigh Molecular weights are hard to get this way

Chemistry 367L/392N

It all happens at the end!!!It all happens at the end!!!

DP = 1 / (1 – P)

Chemistry 367L/392N

A-A, B-B vs A-B ???A-A, B-B vs A-B ???

Cl C

O

H2C

H2C C

O

O CH2-CH2-OH

Cl C

O

H2C

H2C C

O

Cl + HO-CH2-CH2-OH

A……………………………….BA……………………………….B

A ………………….AA ………………….A B..................BB..................B

Chemistry 367L/392N

Chain Growth PolymersChain Growth Polymers

Chain Chain polymer growth takes place by monomer reacting only with polymer growth takes place by monomer reacting only with the reactive centersthe reactive centers.. Monomer does not react with monomer and Monomer does not react with monomer and species such as dimer and trimer do not react with each other. The species such as dimer and trimer do not react with each other. The polymerization ceases when the active center is destroyed by polymerization ceases when the active center is destroyed by termination reactions. termination reactions.

Reactive intermediates in chain-growth polymerizations include

radicals, carbanions, carbocations, and organometallic complexes

Chemistry 367L/392N

Radical Chain-Growth PolymersRadical Chain-Growth Polymers Look at classical example of polymerizations of Look at classical example of polymerizations of

ethylene and substituted ethylenesethylene and substituted ethylenes

R

An alkene Rn

n

Chemistry 367L/392N

Radical Chain Growth PolymerizationRadical Chain Growth Polymerization Among the initiators used for radical chain-growth

polymerization are diacyl peroxides, which decompose as shown on mild heating

O

O

O

O

Dibenzoyl peroxide

O

O

2 + 2CO2

A phenyl radical

A benzoyloxy radical

2

Chemistry 367L/392N

Benzoyl peroxide is a popular Benzoyl peroxide is a popular pharmaceuticalpharmaceutical

Chemistry 367L/392N

Radical Chain Growth PolymerizationRadical Chain Growth Polymerization

Another common class of initiators are azo Another common class of initiators are azo compounds, which also decompose on mild compounds, which also decompose on mild heating or with absorption of UV lightheating or with absorption of UV light

Azoisobutyronitrile (AIBN)

or h N NNC

NC NN CN

+2

Alkyl radicals

•

Chemistry 367L/392N

InitiationInitiation

CHH2C CH2 CH

Chemistry 367L/392N

PropagationPropagation

CHH2CCH2 CH CH2 CHH2C CH

Chemistry 367L/392N

TerminationTermination

Ph*

RR

n2

Ph*

RR

n

R R

n Ph*

Ph*

RR

n2

R

n PhH3C

DisproportionationDisproportionation

CouplingCoupling

+