Spring 2008
Chap 9. Chain-growth Polymerization
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
Hanyang Univ.
Spring 2008
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.
Step-Growth Polymerization
1.
Molecular weight increases steadily.
2.
High molecular weight polymers are found at the end.
3.
Long reaction time needs to synthesize high conversion and high molecular weight.
Hanyang Univ.
Spring 2008
Chain Growth Polymerization
(1) Initiation
kd : Initiator decomposition rate constant
: 10-4 ~ 10-6 L/mole sec
Bond Energy = 46 kcal/mole
Heat (60ºC)
UV kd
AIBN
Primary radical
Unstable radical
Ri  ki M I 
Hanyang Univ.
Spring 2008
Chain Growth Polymerization
(2) Propagation
(Repetition of similar reaction)
Rp  k p M M 
kp : 102 ~ 104 L/mole sec (much faster than step-growth polymerization)
Hanyang Univ.
Spring 2008
Chain Growth Polymerization
(3) Termination
(a) Coupling or combination
Hanyang Univ.
Spring 2008
Chain Growth Polymerization
(3) Termination
(b) Disproportionation
ktd
ktd
kt=ktc+ktd
106 ~ 108 L/mole sec
Rt  kk M M   Rt M 
2
Hanyang Univ.
Spring 2008
Chain Growth Polymerization
(4) Chain Transfer
Physical chain length
Monomer, Polymer, Solvent or Chain transfer agent
Kinetic chain length
Hanyang Univ.
Spring 2008
Chain Growth Polymerization
Kinetic Chain Length :
kinetic chain length υ of a radical chain polymerization is defined as the
average number of monomer molecules consumed (polymerized) per
each radical, which initiates a polymer chain.
ex) Monomer # 4000
Radical
1,2,3,4
Disproportionation
υ =4,000/4 =1,000
Determined by steps 1, 2, 3. (Initiation, propagation, and termination)
(No chain transfer)
Physical Chain Length :
This condition contains Step 1, 2, 3, 4.
Hanyang Univ.
Spring 2008
Kinetic Chain Reaction
Non-Polymerization Reaction
Peroxide induced Bromination of Toluene
1) Initiation
Two types of reaction
 R-O-O-R
 R-O• + Br2
 R-O• + ФCH3
2RO•
ROBr + Br•
ROH + ФCH2•
(1)
(2)
(3)
Two radicals and two kinetic chains are formed by decomposition of
each ROOR molecules
Hanyang Univ.
Spring 2008
Kinetic Chain Reaction
2) Propagation
 Br• + ФCH3
HBr + ФCH2
(4)
 ФCH2• + Br2
ФCH2Br + Br •
(5)
Two special features
 The number of active species is fixed.
 Same reactions are repeated during the kinetic chain reaction.
Hanyang Univ.
Spring 2008
Kinetic Chain Reaction
3) Termination
 2 Br•
 2ФCH2•
 ФCH2• + Br •
Br2
ФCH2 CH2Ф
ФCH2Br + Br •
Net Effect of Kinetic Chain Reaction:
One ROOR molecule can cause formation of Br2, CH2CH2, CH2Br,HBr, ‥.
Hanyang Univ.
Spring 2008
Kinetic Chain Reaction
Comparison between Chain Polymerization & Chain Reaction
Chain reaction Ri = Rt
Reaction Rate
Steady state
Time
Induction period
In proportion to the O2 concentration
Hanyang Univ.
Spring 2008
Kinetic Chain Reaction
In the case of Chain reaction, there are induction periods, due to the
existence of inhibitor.
When an active center is formed, the reaction rate would be faster and
then go to steady state.
The whole reaction rate is reaching a plateau region.
After that, reaction rate decreases due to a loss of monomers or initiators.
Linear Chain-Growth:
Polymer of high DPn found easily in early reaction
Linear Step-Growth:
high extent of reaction value required to obtain high DPn
Hanyang Univ.
Spring 2008
Kinetic Chain Reaction
Comparison Free Radical Reaction & Ionic Reaction
- Ionic Initiation – multiple bond addition, ring opening polymerization
- Radical Initiation – Ring-opening polymerization is not initiated.
CH3
CH2
For the cationic initiation, it will not be free radical.
CH3
isobutylene
Ex)
R
CH2
CH3
+
H3C
CH2
H
+
H2C
CH2
H3C
CH2
H3C
Because of resonance stability
Hanyang Univ.
Spring 2008
Kinetic Chain Reaction
Comparison between Free Radical Reaction & Ionic Reaction
Ionic
Anionic
Cationic
Ring opening
+
End-groups Generally, combine with counter-ion or gegenion
E.g. R+ AlCl4―
Degree of association depends on below
Solvent-system polarity
Stability of end group
Type & size of scounter-ion
Temp.
Degree of association influences:
Reaction rates
Termination rate
Stereospecificity
Termination Depend on monomer
unimolecular Combination or
consumption, Living polymer or
disproportionation reaction
Block copolymer
Free Radical
-
Truly Free Radical
(no association)
Bimolecular
combination or
disproportionation
Hanyang Univ.
Spring 2008
Kinetic Chain Reaction
Comparison between Free Radical Reaction & Termination Step of Ionic Reaction
A) Free Radical Termination
CH2
.CH
.CH CH
X
X
2
..H .
CH CH
.CH
X
X
ktd (disproportionation)
ktc (coupling
or combination)
CH2 CH CH CH2
X
X
Two molecules involved
= bimolecular reaction
CH2
H
CH
CH
+
X
CH CH2
X
unsaturated group.
Hanyang Univ.
Spring 2008
Kinetic Chain Reaction
B) Cationic Termination
+
CH2 CH
[XY]
-
R
EX)
AlCl4
X= Cl
X
CH2 CH
+
ÀÌ °æ¿ì
-
Y
R
Anionic capture is similar to combination of free radical reaction.
But, this reaction can’t include increasing of MW because of unimolecular reaction
Hanyang Univ.
Y=AlCl3
Spring 2008
Kinetic Chain Reaction
..H
+
CH + [XY]
+ HY + Y
proton release
R
R
The proton release is similar to disproportination of free radical..
But, one chain joins in the reaction
unimolecular reaction
C) Anionic
Termination
C) Anionic
Termination
Termination is rare but can occur by loss of a hydride ion
CH2
CH
+
M
R
-H
CH
CH
+ HM
R
Unimolecular type termination occurs
Hanyang Univ.
Spring 2008
Kinetic Chain Reaction
Reaction rate
E
Chain Transfer
Disadvantage
Polymer System
Anionic
Cationic
Termination
Termination
Unimolecular
High concentration of growing
chains 10-2~10-3molar Rates 104~105
times higher than free radical
Activation Energies Similar
Negligible
+
Rigorous Purity or precautions
Solution or Bulk
Free Radical
Termination
Bimolecular
Relativity low
concentration 10-8~10-9
molar occurs at high
Rates
+
Wide variety of
polymer systems,
Gas, Solid, Solution,
Bulk, Precipitation,
Suspension, Emulsion
Hanyang Univ.
Spring 2008
Kinetic Chain Reaction
Free Radical Initiated Polymerization of Unsaturated monomers
Kinetic Scheme
Initiation
Two step sequence-Both enter into overall rate
1. Initiator decomposition
I2
kd
2I
2. Addition of Initiator fragment to the monomer, Initiation of Chain growth.
I+M
ki
IM
Primary radical species
The efficiency of Initiator - Determined by competition of desired reaction and side reaction
Generally, 0.5 << f << 1
Hanyang Univ.
Spring 2008
Kinetic Chain Reaction
A.Cage Effect –primary recombination
Initiator fragments surrounded by restricting cage of solvent
Ex)
O
O
CH3
H3 C
C
C
O O
O
O
O
O
CH3
CH3
(acetyl peroxide)
Hanyang Univ.
Spring 2008
Kinetic Chain Reaction
I) Recombination possible
I2
2I
II) If elimination reaction occurs while the free radical in-cage,
Formation of stable molecules due to Radical combination.
And formation of Inactive Species.
Hanyang Univ.
Spring 2008
Kinetic Chain Reaction
B. Induced Decomposition –Secondary combination
I) Through Radical attack on peroxide molecules
R + R-O-O-R
Finally,
RH + ROOR
R + ROOR
R=O + RO
ROR+ RO
Total number of radical does not change, but among them half molecules were wasted.
II) Chain Transfer to Solvent
(In this case, since just one radical was obtained half molecules were wasted.)
Hanyang Univ.
Spring 2008
Kinetic Chain Reaction
III) Reaction with Chain Radical
I. + M
IMn . +
IMn .
I2
IMnI
+
I.
Since not all Molecules participate in the initiation → Efficiency factor
f: Initiator Efficiency
= mole fraction of initiator fragments that actually initiate polymer chains.
0.5 < f < 1.0
Hanyang Univ.
Spring 2008
Kinetic Chain Reaction
C. Reaction Rate
d [ M ]
Ri 
dt
 ki [ I ][ M ]
by convention, two radical formation.
 2 fk d [ I 2 ]
If [M] is representative for the concentration of chain radical,
That is , M = IM
or = I
[M]
f  1 Ri is unrelated with [M]
f=[M]
f < 1 Ri is related with [M]
[M] , f
[I2] , f
due to induced decomposition
Hanyang Univ.
Spring 2008
Kinetic Chain Reaction
D. Initiator
N N - containing compounds.
S S
O O
O
O
CH3 C O O C CH3
O
O
C O O C
CH3
C
CH3
Acetyl peroxide 80~100C
Benzoyl peroxide 80~100C
CH3
O O C
Cumyl peroxide 120~140C
CH3
Hanyang Univ.
Spring 2008
Kinetic Chain Reaction
CH3
H3C
CH3
O O
CH3
CH3
t-butyl peroxide
CH3
CH3
H3C
Hydroperoxides, cumyl or t-butyl
80~100C
O OH
CH3
CH3
CH3
CH3 C
CN
N
N C
CH3
CH3
CN
2 CH3 C .
+
50~70C
CN
AIBN 2,2 azobisisobutyronitrile
Hanyang Univ.
N2
Spring 2008
Kinetic Chain Reaction
Propagation
d[M ]
Rp  
 k p [ M ][ M ]
dt
kp
I   M  IM 2 
kp
             IM n 
Termination
M.
ktc
.M
ktd
d [ M ]
Rt  
 2( ktc  ktd )[ M ]2
dt
By convention
Since 2 radical elimination
Hanyang Univ.
Spring 2008
Kinetic Chain Reaction
Overall Rate of Polymerzation
R o  k i [I][M]  k p [M][M]
(# of propagation step >>> # of initiation step)
R o ~ R p  k p [M][M]
Radical concentration
• Difficulty of measurement, low concentration. (~10-8molar)
• Thus, it is impractical using this therm.
• [M] elimination is desirable.
Hanyang Univ.
Spring 2008
Kinetic Chain Reaction
[M] elimination methods
Steady-State Assumption
Radical concentration increases at the start, comes to steady state simultaneously
and then reaction rate change becomes 0. (active centers created and destroyed at the same time)
Ri = Rt
2 fk d [ I 2 ]  2(ktc  ktd )[M ]2
1
1
fk d
[ M ]  (
) 2 [ I 2 ]2
ktc  ktd
1
or
R
 [ i ]2
2 Rt
d[M ]
 k  [ M ][M ]
dt
1
1
fk
d
 k  [ M ][ I 2]2 (
)2
ktc  ktd
R  
Hanyang Univ.
Spring 2008
Kinetic Chain Reaction
Mostly in case of f<1 system
→
[I2]1/2 (Square Root Dependence of [I2])
※ Odian Fig. 3-4
MMA using BPO
Rp
Vinyl Acetate
using AIBN
[I2]1/2
H
C
O
O
2
C
O
O
300C
BPO
C
CH3
O
N
N
C
.
-CO2
O
CN
CN
CH3
C
CN
CH3
CH3
Azobisisobutyronitrile
2 CH3
C.
+ N2
CH3
Hanyang Univ.
Spring 2008
Kinetic Chain Reaction
+
BPO +
system
1
2
Rp  [ I 2 ] [M ]
In case f < 1, but SRD is not applicable,
+
BPO +
CH3
1
2
Rp  [ I 2 ] [M ]
Because f is ‘dependent’ on [M]
Why?
Due to induced decomposition of toluene + [I2]
Hanyang Univ.
3
2
Spring 2008
Kinetic Chain Length (KCL)
At S-S assumption
(1) Disproportionation
KCL 

theaveragenumber of monomermoleculesconsumed(polymerized)
each radical (initiatesa polymerchain)
Rp
Rt ( Ri )
Knowing that
R t  R td  R tc
d[M]

dt
(2) Coupling or combination
t he rat econst .
Rtc  
1
k tc
2
( 1 polymerproduct from 2 polymerchains)
d [ M ]
dt term
Hanyang Univ.
Spring 2008
Kinetic Chain Length (KCL)
(3) Both (1)+(2)
Rp
Rp
2R p
2R p
KCL 



R td
Rt R  1 R
R td  R tc  R td
R t (1 
)
td
tc
2
Rt
define y 
Rtd
Rt
:
in thiscase, KCL 
fraction of term ination caused by disproportionation
2R p
Rt (1  y )
Hanyang Univ.
Spring 2008
Kinetic Chain Length (KCL)
Degree of Polymerization
The more concentration of monomer, DPn
The less concentration of initiator, DPn
DPn 
Monomer consumption rate
polymer formation rate
 d [ M ] / dt

d [ Polym er] / dt
(1) Dispropotionation
DPn  
(2) Coupling
DPn  2
Hanyang Univ.
Spring 2008
Kinetic Chain Length (KCL)
(3) Dispropotionation & Coupling
Polymer
생성속도
Polymer formation
rate  R td 
1
R tc
2
Monomer
consumption
rate  R p
모노머
소비속도
DPn 
DPn 
R td
Rp
2R p

 (1)
1
R
(
1

y
)
t
 R tc
2
wher e y 
2
1 y
knowing that
1
2
k
R p  k p  d f[I 2 ] [M]
kt

R t  R i  2fk d [I 2 ]
 ( 2)
 (3)
Hanyang Univ.
R td
Rt
Spring 2008
Kinetic Chain Length (KCL)
From (1),(2),(3)
In case of no Chain transfer, and valid S-S assumption
DPn 
k p [M ]
 kd

(1  y )kt 
f [ I 2 ]
 kt

or DPn 
k p 2 [M]2
(1  y)k t R p
1
2
 (4)
(5)
Hanyang Univ.
Spring 2008
Chain Transfer
M + XY
MX + Y
Chain transfer agent
If Chain transfer occurs Rp is unchangable but has an effect on DPn
(∵ Since [Y] instead of Rp=kp[M][M] )
ex)
R
H
H
C
C. +
H
X
C lC 4
R
H
H
C
C
H
X
Cl
+
.
CCl3
(1) Chain transfer occurs by solvents or additives
In this case, High chain transfer coefficient.
(2) Transfer occurs by monomer or polymer
Hanyang Univ.
Spring 2008
Chain Transfer
Inhibitor and Retarder
 Inhibitor
When Y take part in chain opening reaction, polymer moves from one site to another.
In this case, hydroquinone etc. are used as inhibitor.
 Retarder
When the reactivity of Y is low, controlling the MW of the monomer including
these two materials, Mercaptan etc. are used as Retarder.
Like this, when chain transfer condition arises
d [ Po lym er]
1
 Rtd 
Rtc   Rtr
dt
2
Rp
2Rp
 DPn 

d [ Po lym er]
Rt (1  y )  2 Rtr
dt
2k p [ M ][M ]

2k t [ M ]2 (1  y )  2 ktr [ M ][ XY ]

k p [M ]
k t [ M ](1  y )   k tr [ XY ]
Kn o win g th a t
1
kd f
[ M ]  {
[ I 2 ]}2
kt
Hanyang Univ.
Spring 2008
Chain Transfer
DPn 
k p [M ]
k

k t (1  y )  d f  [ I 2 ]
 kt

1
2
  k tr [ XY ]
1
2
k tr [ XY ]

k (1  y )  k d
1

 t
f

[
I
]


2 
k p [ M ]  kt
k p [M ]
DPn

k
[S ]
k [T ]
1
1

 tr , s
 tr ,t
 
k
[
M
]
k
[
M
]
DPn
DPn , 0
p
p
1
1
[S ]
[T ]

 CS
 CT
 
[M ]
[M ]
DPn
DPn , 0
1
See
Odian
P.235
From the slope of a graph
Chain transfer coefficient ‘Cs’
DPn
[5]/[M]
Hanyang Univ.
Spring 2008
Temperature Dependence of Rp and DPn
Assume : no chain transfer

 k d
R p  k p 

 k t
1
2

f  [I 2 ]
 [M]





  k d
ln R p  ln[M]  lnk p 
k
  t

k p  k p e
E  / RT

1

   ln[f[I 2 ]] 2  (1)

 

1
2
Ep
5 ~ 8 kc al/ mole
k d  k  d e E d / RT
Ed
30 kc al/ mole
k t  k  t e E t / RT
Et
2 ~ 5 kc al/ mole
Hanyang Univ.
Spring 2008
Temperature Dependence of Rp and DPn
Fr om Eq(1), and assume [I 2 ], [M]  c onst, f independent of T.
1
2
d ln R p
dT
 kd
 ]
d ln[k p 
kt 


dT
E t Ed
Ep 

2
2  7  2  15  0

RT 2
RT 2
 slope of lnRp/T is ( + )
 as T 
lnRp 
but Rate of Increase  as d lnRp/dT 
Hanyang Univ.
Spring 2008
Temperature Dependence of Rp and DPn
DPn 
2R p
R t (1  y )
d ln 

dT
, 
Rp
Rt

Rp
Ri
E t Ed

2
2  7  2  15  0
RT 2
RT 2
Ep 
 as T , ln   DPn 
[I] 
Rp 
T

but DPn 
DPn 
Hanyang Univ.
Spring 2008
Ceiling Temperature Polymer-Depolymerization Equilibria
ㆍCeiling Temperature
Polymerization and Depolymerization are in equilibrium
ΔGp = ΔHp – TΔSp
ΔHp : Heat of polymerization
ΔSp : Molecular arrangement changes between monomer and polymer
At eq. State ΔGp=0
Monomers can no longer be persuaded to form polymers by chain
polymerization above a certain temperature.
ceiling Temperature(Tc)
Hanyang Univ.
Spring 2008
Rate Eq. of Polymerization Reactions
at Depolymerization prominent Temperature
If,
M
Tc
Hanyang Univ.
Spring 2008
Ceiling Temperature Polymer-Depolymerization Equilibria
k
sec-1
kdp
kp[M]
kp[M]- kdp
Tc
: No reaction above Tc
300
400
500
Tc
Stable blow Tc
Hanyang Univ.
Spring 2008
Ceiling Temperature Polymer-Depolymerization Equilibria
※Odian Fig 3-18
Monomer
-H kcal/g mole
TcC (bulk)
St
16
235
MMA
13
164
Ethylene
26
407
Propylene
21
300
-methyl St.
7
6
Entropy changes for all polymers are not so different.
Sp= Sp- Sm
Hp= Hp- Hm
(–) value of Sp is higher
if (–) , exothermic.
Hanyang Univ.
Spring 2008
Trommsdorff Effect or Gel Effect
The increasing viscosity limits the rate of termination because of
diffusional limitations
restricted mobility of polymer radical
kp
( ∵ kp
( relative to [M]
)
const. in reaction progress, kt
drop off in reaction progress )
→ Autoaccerelation effect
Hanyang Univ.
Spring 2008
Trommsdorff Effect or Gel Effect
  1  e kt
 one would expect ξ  as t  But ξ  as [[M0] 
80%

60%
40%
10%
autoacceleratioan
as [M0] drastic
t
Hanyang Univ.
in .