Macromolecular Chemistry
Lecture 5
Step Growth
Chain Growth
Chemistry 367L/392N
Paul Flory Clears Things Up
Polymer Structure is distinct from polymerization process
Addition
Polymerization
O
O
HO
Condensation
Polymerization
O
O
O
H
HO
O
H
"Condensation"
Polymer
Chemistry 367L/392N
Basic Types of Polymerization Mechanisms
Step-growth
O
HO C CH2 CH2 OH
Dimer
Tetramer
Octamer
Hexamer
...
O
H O C CH2 CH2 O H
H2O
Chain-growth
O
Ring-opening
O
RO-
+
O
O-
O
RO
O H
Chemistry 367L/392N
Chain growth system
The characteristic of a chain polymer is that
polymer growth takes place by monomer reacting
only with the reactive centers. Monomer does not
react with monomer and the different-sized species
such as dimer, trimer, and n-mer do not react with
each other. The polymerization ceases when the
active center is destroyed by termination
reaction(s).
Chemistry 367L/392N
Step Growth system
A condensation takes place between two
polyfunctional molecules to produce one
larger polyfunctional molecule with the
possible elimination of a small molecule such
as water. The reaction continues until one of
the reagents is used up.
Chemistry 367L/392N
The chain growth vs. step growth
Step
Chain
Chemistry 367L/392N
- Step-growth polymerization
Chemistry 367L/392N
- Chain-growth polymerization
Chemistry 367L/392N
Chain Growth Polymerization
11
2
3
4
5
6
7
DP = No/N = 12 / 7 = 1.7 (for 50%, b)
Chemistry 367L/392N
Step Growth Polymerization
DP = No/N = 12 / 9 = 1.3 (for 50%, b)
Chemistry 367L/392N
The chain growth system
The relationship between Mwt and conversion
With no termination reactions
Chemistry 367L/392N
The chain growth system
The relationship between Mwt and conversion
With termination reactions
Chemistry 367L/392N
Step growth system
The relationship between Mwt and conversion
Chemistry 367L/392N
Distinguishing features of chain- and
step-polymerization mechanisms
Chemistry 367L/392N
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 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 way
Chemistry 367L/392N
It all happens at the end!!!
DP = 1 / (1 – P)
Chemistry 367L/392N
A-A, B-B vs A-B ???
O
O
H2
C
H2
C
Cl
+ HO-CH2-CH2-OH
A ………………….A
B..................B
Cl
C
O
Cl
C
C
O
H2
C
H2
C
C
O
CH2-CH2-OH
A……………………………….B
Chemistry 367L/392N
Chain Growth Polymers

Chain polymer growth takes place by monomer
reacting only with the reactive centers. Monomer
does not react with monomer and species such as
dimer and trimer do not react with each other. The
polymerization ceases when the active center is
destroyed by termination reactions.

Reactive intermediates in chain-growth
polymerizations include radicals, carbanions,
carbocations, and organometallic complexes
Chemistry 367L/392N
Radical Chain-Growth Polymers

Look at classical example of polymerizations of
ethylene and substituted ethylenes
R
An alkene
n
R
n
Chemistry 367L/392N
Radical 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
Di be n z oyl
pe roxide
2
O
O
A be n z oyl oxy
radi cal
+ 2 CO 2
2
A ph e n yl
radi cal
Chemistry 367L/392N
Benzoyl peroxide is a popular
pharmaceutical
Chemistry 367L/392N
Radical Chain Growth Polymerization

Another common class of initiators are azo
compounds, which also decompose on mild
heating or with absorption of UV light
N N
N C
C N
Azoisobutyronitrile (AIBN)
 or hn
+
2
•
N C
Alkyl radicals
N N
Chemistry 367L/392N
Initiation
H2C
CH
CH2
CH
Chemistry 367L/392N
Propagation
CH2
CH
H2C
CH
CH2
CH
H2
C
CH
Chemistry 367L/392N
Termination
Coupling
Ph*
n Ph*
n
R
2 Ph*
R
R
R
n
R
R
2 Ph*
+
n
R
R
n Ph
H3C
R
Disproportionation
Chemistry 367L/392N