Chemistry 4362

1
Radical Chain Polymerization Molecule Empire
Building by Radical Groups
Chain-Growth Polymerization (Addition) Processes
1. Free radical Initiation Processes
2. Cationically Initiated Processes
3. Anionically Initiated Processes
4. Group Transfer Polymerization
5. Coordination Polymerization
2
Characteristics of Chain-Growth Polymerization
1. Only growth reaction adds repeating units one
at a time to the chain
2. Monomer concentration decreases steadily
throughout the reaction
3. High Molecular weight polymer is formed at
once polymer molecular weight changes
little throughout the reaction.
4. Long reaction times give high yields but
affect molecular weight little.
5. Reaction mixture contains only monomer, high
polymer, and about 10-8 part of growing
chains.
3
The Chemistry of Free Radical Polymerization
Radical Generation
R
R
2 R
-
Initiator Radicals
Initiation
R
C
C
R
C
C

Monomers
Propagation
R
C
C
C
C
C
C
C
R

Termination
C
C
C
R
R
C
C

R
C
C
C
C
C
R
Polymer
4
Free Radical Polymerization Mechanisms
1. Overview Free radical polymerization
processes involve at least three
mechanistic steps.
A. Initiation
1. Radical Formation (Generation)
D
In
In
In
In

h
v
, etc.
2. Initiation
In
M
In
M

5
B. Propagation
In-M1 . M2 In-M1M2.
In-M1M2. M3 In-M1M2M3.
In-M1M2M3MX. MY In-M1M2M3MXMY.
6
C. Termination
1) Radical Coupling (Combination)
In-MX. .MY-In In-MX-MY-In
In
In
In
In

2) Disproportionation (?-hydrogen transfer)
H
H
H
H
In
M
In
M
C
C
C
C

y
x
H
H
H
H
M
In
CH
In
M
H
C
CH
CH

y
x
3
2
2
7
D. Chain Transfer (sometimes) An atom is
transferred to the growing chain,
terminating the chain growth and
starting a new chain.
Chain Transfer to Chain Transfer Agent
P
R
H
P
R

x

x
Chain Transfer to Monomer
Px. H2CCH-(CO)OR
Chain Transfer to Polymer
Causes Branching
H
P
P
P
P

x

y
x
y
8
E. Inhibition and Retardation a retarder is a
substance that can react with a radical to
form products incapable of reacting with
monomer. An inhibitor is a retarder which
completely stops or inhibits polymerization.
2. Monomers that are susceptible to free radical
addition
A. Vinyl Monomers
H
C
CHX
H
C
CH
Cl
2
2
Vinyl chloride
F
H
H
X
F
H
H
Y
Vinylidene fluoride
9
B. Allyl Monomers
X
Cl
Allyl Chloride
C. Ester Monomers
1) Acrylates
OH
OR
O
O
Acrylic Acid
Acrylate Esters
10
2) Methacrylates
O
O
OH
OR
Methacrylate Esters
Methacrylic Acid
3) Vinyl Esters
O
Vinyl Acetate
O
D. Amide Monomers
O
O
NH
NH
2
2
Acrylamide Methacrylamide
11
3. Monomers that are not susceptible to Free
Radical Addition
A. 1,2-a-olefins (Polymerize to oils only)
x
B. Vinyl ethers
R
O
O
methyl vinyl ether
C. 1,2-disubstituted Ethylenes
Cl
Cl
1,2-dichloroethylene
H
H
12
4. Initiation Getting the thing started!
A. Radical Generators (Initiators)
1. Benzoyl Peroxide
O
O
0
80-90
C
C
O
O
C
O
C
O
2 CO

2
2
(continued)
13

New Active Site
Initiator End-Group
Ph
Ph
2) t-Butyl Peroxide
CH
CH
CH
3
3
3
0
0
120
-140
C
H
C
C
H
C
C
O
O
C
CH
2
3
3
3
CH
CH
CH
3
3
3
(continued)
14
CH
3
O
H
C
C

3
O
CH
3
O
O
3) Azobisisobutyronitrile (AIBN)
CH3 CH3 H3C C NN C CH3
CN CN
60oC or hn
(continued)
15
CH
CH
3
3
Ph
H
C
C
H
C
C
C
CH
N

2
3
3
H
2
CN
CN
4) Cumyl Hydroperoxide
CH
3
C
O
OH
OH

Ph
O
CH
3
(continued)
16
(continued)
O

Ph
O
O
O
Ph
O
O
17
Hydroperoxides can generate radicals by induced
decomposition from growing polymer chains

P
H
O
O
R
PH

O
O
R
R
OO
2
2 RO
O

R-OO-OO-R
2
What effect does this have on the polymerization
process?
Acting as a chain-transfer agent, it reduces
the degree of polymerization and molecular mass.
18
5) Redox Initiator Systems
2
3
Fe
HO OH Fe
H
O
O
H

OR
2-
-
O
S
O
O
SO
SO
SO


3
3
3
4
2-
-
SO

S-SO
4
3
19
6) Photoinitiators (Photocleavage Norrish I)
O
O
h
v
C
HO

OH
benzoin
C
H
H
C

Ph
OH
OH
Ph
Ph
H
20
(continued)
OR
O
O
h
v
C
C
O
benzil
2
C
21
7) Photoinitiators (Photo-Abstraction)
O
Photosensitizer

O
h
v
Ph
Ph
benzophenone
excited state
R
R
R
R
C
H
N
OH

C
N
R
R
Coinitiator
Ph
Ph
R
R
22
5. Propagation - Keeping the thing going!
A. The addition of monomer to an active center
(free radical) to generate a new active
center.
H
X
R
C
C
CH
R
C
CH
C
2
H
H
H
2
2
2
X
X
X
H
X
X
R
C
C
C
CH
H
H
etc.
etc.
2
2
n
X
X
(continued)
23
Examples
Polystyrene
H
Ph
R
C
CH
R
C
C
C
CH
2
H
H
H
2
2
2
n
Ph
Ph
Ph
O
CH
R
C
C
CH
3
H
H
2
2
O
C
O
Polymethyl Acrylate
O
CH
3
H
R
C
C
C
C
CH
H
H
H
2
2
2
C
O
C
O
O
O
CH
CH
3
3
24
B. Configuration in Chain-Growth Polymerization
1) Configuration Possibilities
favored
CH
H
C
P
C
C
?-attack ?-attack
H
2
H
X
2
X
P
.
H
HC
CH
X
P
CH
C
2
2
X
X
sterically
and electronically unfavored
25
2) Radical Stability Considerations
Which possible new active center will have the
greatest stability?
.
P
C
CH
P
C
CH
2
H
H
2
2
P
C
CH
?-attack produces resonance stabilized free
radical
H
2
26
H
No resonance stabilization
P
CH
C
X
2
______________________________________________
CH2
O
CH
2
?
P
CH
HC
C
O
CH
X
3
O
C
O
CH
3
P
?
P
C
CH
H
C
C
C
O
CH
2
3
H
H
H
2
C
O
O
P
C
CH
H
H
Secondary radical is resonance stabilized
O
CH
2
3
C
O
O
CH
3
27
(more examples)
Cl
Cl
H
?
X
P
C
CH
2
Cl
H
Cl
P
H
Cl
Cl
?
P
C
C
H
2
H
Cl
Cl
Cl
Cl
P
C
C
P
C
C
H
H
2
2
Cl
Cl
Tertiary radical is resonance stabilized
28
3) Steric Hinderance Considerations
HC
CH
X
2
X
P
For large X, ?-substitution is sterically
favored
CH
H
C
2
X
4) Radical Stability
3o gt 2o gt 1o
29
5 ) Bottom Line
Resonance and steric hinderance considerations
lead to the conclusion that ?-substitution
(head-to-tail) is strongly preferred in
chain-growth polymerization.
H
H
H
H
C
C
C
C
C
C
C
C
H
H
H
H
2
2
2
2
X
X
X
X
Alternating configuration
30
6. Termination - Stopping the thing!
A. Coupling (most common)
H
H
P
C
P
C
C
C

y
x
H
H
2
2
X
X
H
H
P
C
C
P
C
C
y
x
H
H
2
2
X
X

• - occurs head-to-head
• produces two initiator fragments (end-groups)
  
• per chain.

31
B. Disproportionation
H
H
H
H
In
M
In
M
C
C
C
C

y
x
H
H
H
H
M
In
CH
In
M
H
C
CH
CH

y
x
3
2
2
- Production of saturated chain and 1
unsaturated chain per termination
- Produce one initiator fragment (end-group) per
chain
32
C. Factors affecting the type of termination
that will take place.
1) Steric factors - large, bulky groups attached
directly to the active center will hinder
coupling
2) Availability of labile ?-hydrogens
3) Examples PS and PMMA
H
H
P
C
C
C
C
P

x
y
H
H
2
2
Combination (coupling)
Polystyrene
(continued)
33
H
H
P
P
C
C
C
C
y
x
H
H
2
2
Ph
Ph
Ph
CH3 H3C PX CH2-C. .
C-CH2- PY CO OC O
O CH3 CH3
PMMA

• Sterically
• hindered
• 5 b-Hydrogens
• Disproportion-
• ation dominates

(continued)
34
CH3 H3C PX CH2C HC-CH2-
PY CO OC O O
CH3 CH3

• Electrostatic Repulsion Between Polar Groups
• Esters, Amides, etc.

35
Polyacrylonitrile (PAN)
PX CH2-CH. . HC-CH2- PY
d C?N d- d- N?C d
One might assume electrostatic repulsion in this
case. BUT, how about electrostatic attraction
from the nitrogen to the carbon? Also, steric
hindrance is limited. At 60oC, this terminates
almost exclusively by coupling!
36
D. Primary Radical Termination
PX CH2-CH. . In X
PX CH2-CH-In X
More Likely at High In.
So molecular mass can be controlled using
chain-transfer agents, hydroperoxide initiators,
OR higher levels of initiator!
37
7. Chain-Transfer - Rerouting the thing!

• Definition The transfer of reactivity from the
• growing polymer chain to another species. An
• atom is transferred to the growing chain,
• terminating the chain and starting a new one.

PX CH2-CH. X-R ? PX CH2-CHX
R. Y Y
B. Chain-transfer to solvent
PX CH2-CH. CCl4 ? PX CH2-CHCl
Cl3C. Y Y
38
C. Chain-transfer to monomer
PX CH2-CH. H2C CH
PX CH2-CH2 H2C C.
OR
39
H H PX CH - C. H2C CH
PX CH2CH. H3C - C.
40
Propylene Why wont it polymerize with Free
Radicals?
PX CH2-CH. HCHCH CH3 CH3
PX CH2-CH2-CH3 CH2CH-CH2.
H2C-CH-CH2
?
Chain-transfer occurs so readily that propylene
wont polymerize with free radicals.
41
D. Chain-transfer to polymer
PX CH2-CH2-CH2.
CH2-CH2-CH2
PX CH2-CH2-CH3 CH2-CH-CH2
?
Increases branching and broadens MWD!
E. Chain-transfer to Initiator (Primary
Radical Termination)
PX CH2. R-O-O-R ? PX CH2-OR
. OR
42
F. Chain-transfer to Chain-transfer Agent
Definition The transfer of reactivity from
the growing polymer chain to another species.
An atom is transferred to the growing
chain, terminating the chain and starting a new
one.
Examples R-OH R-SH R-Cl R-Br
PX CH2-CH2. HS-(CH2)7CH3
PX CH2-CH3 . S-(CH2)7CH3
. CXH-CH2- S-(CH2)7CH3
etc., etc., etc.
43

• Inhibition and Retardation - Preventing the
  thing
• or slowing it down!

Definition Compounds that slow down or stop
poly- merization by forming radicals that are
either too stable or too sterically hindered to
initiate poly- merization OR they prefer coupling
(termination) reactions to initiation reactions.
PX CH2-CH. O O
para-Benzoquinone
Will Not Propagate
PX CH2-CH2-O- -O.
PX CH2-CH-O-O .
PX CH2-CH. OO
44
Kinetics of Free Radical Polymerization
1. Initiation
kd
(RDS)
I 2 R. Radical Generation
ki
R. M M1. Initiation
Assuming that ki gtgtkd and accounting for the fact
that two Radicals are formed during every
initiator decomposition, The rate of initiation,
Ri, is given by Ri dMi
2fkdI dt f efficiency of the
initiator and is usually 0.3lt f gt0.8
45
2. Propagation
kp
M1. M M2.
kp
We assume that the reactivity of the
growing chain is independent of the length of the
chain.
M2. M M3.
kp
M3. M M4.
. . .
kp
Mx. M Mx1.
Rp - dM kpM .M dt
46
3. Termination
ktc
Mx. . My Mx-My (Combination)
ktd
Mx. . My Mx My (Disproportionation)
Since two radicals are consumed in every
termination, then
Rt 2kt M .2
4. Steady State Assumption
Very early in the polymerization, the
concentration of radicals becomes constant
because Ri Rt
? 2fkdI 2kt M .2
47
2fkd I 2kt M .2
Solve this equation for M .
M . (fkd I/kt)1/2
Substituting this into the propagation expression
Rp kpM .M kp M(fkd I/kt)1/2
Since the rate of propagation, Rp, is essentially
the rate of polymerization, the rate of
polymerization is proportional to I1/2 and M.
48
5. Kinetic Chain Length, n
Definition The average number of monomer
units polymerized per chain initiated. This is
equal to the Rate of polymerization per rate of
initiation
n Rp/Ri Rp/Rt under steady state
conditions.

• kpMM. kpM
• 2ktM.2 2ktM.

n will decrease with increases in initiator
concentration or efficiency.
__kpM___ 2(f ktkdI)1/2
n
DP n if termination is exclusively by
disproportionation.
DP 2n if termination is exclusively by
coupling.
49
6. When Chain-transfer is Involved
When chain-transfer in involved, the kinetic
chain length must be redefined.
Bottom Line
1/ntr 1/n CmM CsS CII
M Where Cx ktr, x /kp
50
7. Qualitative Effects a Summary
Factor Rate of Rxn MW M Increases Increa
ses I Increases Decreases kp Increases I
ncreases kd Increases Decreases kt Decreases
Decreases CT agent No Effect Decreases Inhib
itor Decreases (stops!) Decreases CT to
Poly No Effect Increases Temperature Increases
Decreases
51
Thermodynamics of Free Radical Polymerization
DGp DHp - TDSp
DHp is favorable for all polymerizations and
DSp is not! However, at normal temperatures,
DHp more than compensates for the negative DSp
term.
The Ceiling Temperature, Tc, is the temperature
above which the polymer depolymerizes. At Tc ,
DGp 0. ? DHp - Tc DSp 0 DHp Tc DSp
? Tc DHp/ DSp
52
Thiol-ene Polymerization A Brief Introduction
Thiols (mercaptans) can react with any -ene
any double bond. After all, they ARE
chain-transfer agents! They serve as a bridge
between step-growth and chain-growth
polymerization processes because they use free
radicals in a step-growth polymerization process.
HS-R-SH H2CCH-R-CHCH2 HS-R-S-CH2-
CH-R-CHCH2
UV
53
If either thiol or ene is only monofunctional,
no polymerizations will take place. The thiol
will serve as a chain-transfer agent and a
standard free radical polymerization of the ene
will take place. If the If the mole ratio of
thiol to ene is close to one, no Effective
polymerization will take place. If both are
difunctional and in stoichiometric balance, a
linear polymer will form. In order to get a
crosslinked thiol-ene polymer, the thiol must be
at least trifunctional.
54
The process begins with a hydrogen abstraction
from the thiol a very rapid process to form a
thiyl radical
(HS)2-R-SH . In ? (HS)2-R-S . H-In
The thiyl radical attacks a double bond
(HS)2-R-S . H2CCX R ?
(HS)2-R-S-CH2-CX R

This radical then abstracts a hydrogen atom

(HS)2-R-SH
(HS)2-R-S-CH2-CX R ? etc.