chap 11. non-radical addition polymerization
DESCRIPTION
Chap 11. Non-Radical Addition Polymerization. Anionic Polymerization. -the growing chain end bears a negative charge. The mechanism of anionic polymerization is a kind of repetitive conjugate addition reaction . (the "Michael reaction" in organic chemistry). Cationic Polymerization. - PowerPoint PPT PresentationTRANSCRIPT
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Chap 11. Non-Radical Addition PolymerizationAnionic Polymerization-the growing chain end bears a negative charge
The mechanism of cationic polymerization is a kind of repetitive alkylation reaction.
The mechanism of anionic polymerization is a kind of repetitive conjugate addition reaction .(the "Michael reaction" in organic chemistry)
Cationic Polymerization-the growing chain end bears a positive charge
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B-Z + CH2=CHX B-CH2-CH- Z+
X
Initiation:
Propagation:
M- Z+ + M MM- Z+
Termination:
M- Z+ + HT MH + ZT
General Scheme
Anionic Polymerization
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CH LiCH3CH2
CH3
CH2 CH CHCH3CH2
CH3
CH2 CH Li
CH2 CH Li CH2 CH CH2 CH CH2CH Li
CH2 CH Li H OH CH2 CH2 Li OH
Initiation:
Propagation:
Termination:
+
+
+ +
Styrene Polymerization
Anionic Polymerization
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Anionic Polymerization
Characteristics of an Ideal Anionic Polymerization
Negative centers repel one another and thus termination by recombination is not possible. An ideal polymerization is “living”, which does not terminate until a terminator is added.
Initiation is normally very fast relative to propagation and all chains grow simultaneously. This leads to polymers with low polydispersity or monodispersity.
Theoretically:
nn
w
xM
M 11
• The rate of polymerization for methacrylates and styrenes is high even at -78 oC. This is partly for the high concentration of the anion centers.
• The degree of polymerization
0
0
I
MKxn
• K=1 or 2 depending on initiator used.
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Initiation by Electron Transfer
- .
- .
CH2 CH CH2 CH-.
CH2 CH-.
CH2 CHCH2CH--
+ KTHF
-78 oC
K +
K + + +
2
K +
K +K +K
+
0
02
I
Mxn
• Polymerization mostly done in THF and not nonpolar solvents like cyclohexane or benzene for the solubility the complex in THF.
• The degree of polymerization is given by
Anionic Polymerization
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Initiation by Nucleophilic Attack
CH2 CH CH3(CH2)4 CHCH3(CH2)2CH2 Li Li
COOCH3
CH2 C
CH3
N Li N
COOCH3
CH2 C
CH3
Li
THF
-78 oC+
++
THF
-78 oC+ +
0
0
I
Mxn
• Polymerization can be done in both polar and nonpolar solvents.
• The degree of polymerization is given by
Anionic Polymerization
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Initiation by Living Polymer
CH2 CH Li CH2 C
CH3
COOCH3
CH2 CH CH2C
CH3
COOCH3
LiTHF
-78 oC++ +
CH2 CHLiCH2 C
CH3
COOCH3
++
Because the starting anion has to be a stronger Lewis base than the resulting anion.
But not
Anionic Polymerization
0
0
I
Mxn
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Propagation
M Z M M MZ+-Z+-
Z+- +CovalentBond
Contact IonPair
Solvent SeparatedIon Pair
Free ions
Solvent polarity increases
kP increases
Polymer tacticity decreases
• Kp can vary by orders of magnitude.
• The polydispersity remains low because the rate of inter-conversion between the different forms is much faster than that of polymerization.
Anionic Polymerization
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Termination
M MH
M MCOO
MCH2Br CH2Br
MM
MCH2Br CH2Br
M Br
Z+-
By proton
H+
Z+ +
By CO2
Z+- CO2
Z+-
By using a limiting amount of 1,2-dibromoethane
Z+-2
K=2
By using a much excess of 1,2-dibromoethane
Z+-
Anionic Polymerization
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Anionic Polymerization (1) proton donor H2O or ethanol Strong base is not enough for initiation.
(2)
Ctr,s=10-3 (small chain transfer constant)
EtOH high MW product ethoxide no longer living.
C H 2 C
H
: - + H 2 O C H 2 C H 2 + O H
-
H2O Ctr,s=10H2O low MW polymer No living polymer
CH2 C
H
:- + C2H5OHCH2 CH2
+ C2H5O-
Strong base is not enough for initiation.
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Anionic Polymerization (3) Termination can occurred by hydride elimination without impurities.
a)
b) anionic species(active center) react with chain endsto form inactive allylic anion.
C H 2 C H C H 2 C H : - N a +
C H 2 C H C H C H
+ N a + H - :
C H C H C H 2 C H + C H 2 C H . . -
C H 2 C H 2 + C H C H C H 2 C H . . -
1,3 diphenylallyl anion very unreactive, highly resonance stabilized
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Anionic Polymerization Termination of polar monomer
In this case, although initiator or active center attack monomer, that means non-polymerization.
C H 2 C
C H 3
C O C H 3
O
+ R - L i + C H 2 C
C H 3
C R
O
+ C H 3 O - L i +
+ CH2 C
CH3
COOCH3
CH2 C:-CH3
COOCH3
Li+
CH2 C
CH3
COOCH3
C
O
C
CH3
CH2 + Li+CH3O-
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Anionic Polymerization Backbiting or intramolecular rexn
4) Hugginson-Wooding System
J.Chem. Soc. 1952
Polymerization of styrene conducted in liq. NH3 at bp -33C
(1) reaction rate ↑ as [I] and [M]2
I=K+NH2- rate ↑ as [NH2
-] ↑ but as [K+] ↓
(2) MW [K+] and [NH2-]
(3) Polymer is formed without unsaturation.
Cyclic trimer at the end of chain
C H 2 C C
O C H 3 O
C C H 3
C O O C H 3
C H 3 C O O C H 3
C H 2 C C
O
C C H 3
C O O C H 3
C H 3 C O O C H 3
+ C H 3 O - - ̈
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Anionic Polymerization
][
]][[]][[ 2
2
K
KNHMKkMHNkR i
ii
N H 2 - C H C H 2 +
ki H 2 N C H 2 C : -
H
][
]][[
2
2
KNH
NHKk
KNH2 K NH2
k
If [K+] , then Ri
Dissociation of initiator
Initiation step
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Propagation
Anionic Polymerization
]][[ MMkR pp
H 2 N M n - + M H 2 N M n . M
- kp
Termination
][
][][
]][[
][][
3,
22
3,
22
NHk
NHMkk
NHKk
KNHMkkKR
str
pi
str
pip
][
][][
]][[
3,
2
1
222
1
2
1
22
1
NHk
KNHMkKkR
KNHMKkR
str
pip
ii
H 2 N C H 2 C H C H 2 C : - H
+ N H 3 H 2 N C H 2 C H C H 2 C
H
H
: N H 2 - +
ktr,s
n n
Occurs by chain transfer
Rtr=ktr,s[M-][NH3+]Overall Rate using Steady state assumption. (RiRt).
Add KCl Rp slow down
[K+]=[NH2-]
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Anionic Polymerization
][
][
][
][
33, NHC
M
NHk
MkX
sstr
pn
In dehydrate state,
Chain transfer constant for solvent
molekcalEEEE
rateoverall
RateDPtemp
molekcalEEE
trpiR
n
trpnx
/9
/4
Activation energy for Xn
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Anionic Polymerization
1 1
1 n
wn
nn
w
M
MX
XM
M
RONa + nCH2 CH2
O
CH2CH2O-Na+RO(CH2CH2O)n- 1
In Flory
If there is no termination rxn, we can obtain narrow MW distribution.
5) Base Initiated Polymerization - a powerful nucleophile is required as initiator
NO2 C O> > SO2 > CO2
>>> CH3CH CH2
C N > S O > C 2 H 5
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Anionic Polymerization
6) Practical Comments
If we use metal as an initiator, propagation rate is fast.
purity import!
7) Propagation Kinetics
]][[ MMkR pp
propagation is not so fast compared to radical polymerization
About many living polymers
[M:-] = [I][M] = is about 10-9 to 10-7 molar[M:-] = 10-3 to 10-2 molar
kp for free radical case is 5103 l/molesec Kp : depends on solvent and counter ionCounter ion and active center can be separated by changing solvent then rxn rate increase
conc. of anion = conc. of initiaor
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(1) Evaluatation of Individual Propagation Rate Constants
Propagation rate constant for free ion and ion pair. [P-]:conc. of free ion [P-(C+)]: conc. of ion pair
])][([]][[ MCPkMPkR ppp
Anionic Polymerization
2
1
)])([(][
][][,
]][[
)]([1
K)( .
]][[][
)]([][
CPKP
CPionaddif
CP
CP
K
CPCPEqat
MMkRM
CPkPkk app
pppapp
p
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Anionic Polymerization* How to measure kp, kp, K ?
][
log 0
MC
C
.apppkslope
appk
2
1
][ M
p
pp
kintercept
Kkkslope 2
1
)(
t
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Anionic Polymerization
][
][][
C
MKP
][][
][CZ
MKP
Conc. of living and conc. of free ion
][][
][)]([CZ
MKMCP
A salt that must be soluble in THF with common ion to gegen ion is added to reaction mixture.
[C+][CZ]
At high added salt conc.
Conc. of added salt is [CZ]
][
)(
CZ
Kkkkk pp
pappp
][
)]([][
M
CPkPkk
originally
pappp
apppk
][CZ
p
pp
k
Kkkslope
int
)(
Now
Then able to get kp-, kp
, K from two graphs.
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Anionic PolymerizationEffect of gegen ion on Anionic Polymerization of Styrene
THF Dioxane
kp K107 kp
- kp
Li+ 160 2.2 6.5104 0.94
Na+ 80 1.5 3.4
K+ 60~80 0.8 19.8
Rb+ 50~80 0.1 21.5
CS+ 22 0.02 24.5
- Why kp- is the same value?; kp- is much more larger than kp Thus we can say that reactivity of free ion is much greater than that of ion pairs.
- In the case of dioxane?;In dioxane which is not tend to be solvating it has reverse tendency compared to the case of THF. Solvation is not important in dioxane. Cs is too big that there is no difference. Explanation is that there is not so solvating power of Cs
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Anionic Polymerization
]][[
]][:[
MRLikR
MLiMkR
ii
pp
Look at difference.Unassociated species
Li+ genenion in aromatic hydrocarbon
Let’s say we are using BuLi initiator.
solvation as well as is important!
Although 1,2 diethoxyethane reduce , kp varies 1~1000 fold because of highly solvating ether . Reactivity of free ion < Reacitivity of ion pair
In aromatic hydrocarbon, unassociated species dominate rate.Depend on unassociated species in very low conc.
(nC4H9Li)6 C4H9Li6
K1
(R Mn-Li+)2 2C4H9 Mn-Li+K2
CH2 C
H
RLi
LiCH
RCH2 2 CH2 CLi
H
RCovalent character
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Anionic Polymerization
6
1
6946
1
1 ])[(][ LiHCKRLi
rateinitiationinorder
Rtheinorder
LiMKLiM
R i
6
12
1
2
1
22
1
2
6
1
]):[(]:[
][
Evidence — viscosity measurement before and after term we find that living
polymer is associated after termination, viscosity drops.
Because initiators and ion pairs are reduced,
Polymerization rexn in Aliphatic HC is lower than inaromatic HC.
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Effect of solvent and gengenion on Copolymerization of Styrene and isoprene at 25 C
Solvent% Styrene in copolymer
Na+ counter ion Li+ counter ion
Nonsolvent 66 15
Benzene 66 15
Triethyl ether 77 59
Ethyl ether 75 68
THF(highly saturating solvent)
80 80
Generally sodium is more ionic than lithium
Lenz P.437 Table 13-9
Anionic Polymerization
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Cationic PolymerizationThe growing chain bears a positive charge. The active sites are either carbenium ions or oxonium ions.
Electron donating groups are needed as the R groups because these can stabilize the propagating species by resonance.
Ex)
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Cationic InitiatorsProton acids with unreactive
counterions
Lewis acid + other reactive compound:
* To use Lewis acid effectively as initiators, use coinitiator.
B
F
F
F
C2H5Cl C2H5 [BF3Cl]+. .
. .: +
cationogen
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Cationic Polymerization
OH
BF3OH H++ C C
C
C
CH3 C+
C
C
B F3OH
isobutylene
BF3 + OH2 BF3OH H+k e
CH3 C C C C+BF3O H + C C
C
C
kp
Typical Initiator Systems Co-initiator Initiator SnCl4 H2O AlCl3 HCl H2SO4 H2SO4
Order of reactivity AlCl3 > AlRCl2 > AlR2Cl >AlR3
HCl > CH3COOH > C6H5NO2 > > H2O >> CH3OH > CH3COCH3
Ex)
More acidic initiators are most effective in initiating polymerization
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Termination
Kinetics
Cationic Polymerization
2 *
3
*
2t
eipp M BFOH
k
KkkR
C C C
C
B F3OH
C
C
C
+ HB-F3OH
H
32ei3ii BF OH MKkHOFBH MkR
OHBF MkR 3pp
S SRR ti 32ei3tt BF OH MKkOHBF kR
32t
ei3 BFOHM
k
KkOHBF
Problem : temination reactions occur randomly.
[ * ] can control rxn
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Cationic Polymerization
MXk
constXk
ntr
nt
0
,. 0
Mkk
MkX
trt
pn
C1
C3
C 6
C7
C8
C15
C16
B17
F3OHk tr
C21
C22
C27
C30 C 32
C33
C34
B F3OH+
M k k
M k
RR
RX
trt
p
trt
pn
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Chain Transfer Reactions - Cationic vinyl polymerization is plagued by numerous side reactions,
most of which lead to chain transfer
• Difficult to achieve high MW because each initiator (*initiator can give rise to many separate chains because of chain transfer)
• These side reactions can be minimized But ! not eliminated by running the reaction at low temperature
Ex)
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Cationic Polymerization1) Ring opening polymerization
(1) Mechanism
carbon type polymzn.
Example of ROR : cyclic amides, sulfides, acetals, esters, lactam, alkanes, …
(2) Polymerizability
- unstable ring or ring which be cyclized easily are very reactive * 3,4 and 7-11 membered ring is the reactive ring
5,6 membered rings are stable and polymerize slowly, but, possible to polymerize
**3-membered ring most easily polymerize
O R CH2 OCH2
R
O
CH2
ROORCH2ORCH2
CH2
R
++
+. .. .
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Cationic Polymerization(3) polymerization of THF(Polytetrahydrofuran)
PF52 PF4 (PF6)+ -
PF4 (PF6)+ - + O PF4 O
+PF6
-
gegenion
O(CH2)4O(CH2)4 O(CH2)4
(CH2)4
+-
AO
O(CH2)4O(CH2)4O(CH2)4
+O (CH2)4
+
-A
H2O which is role of cocatalyst increase polyerization rateIt is possible to occur living polymerization, but occur termination or transfer together.
O (CH2)4 O+
-A+ O
(CH2)4
(CH2)4
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Cationic Polymerization(4)Kinetics
I + ZY K Y+(IZ)
-
initiator coinitiator
Y+(IZ)
-+ M
ki YM+(IZ)
-
nM O+
+ O Mn O (CH2)4O
CH2 C+
H
R
[SnCl4OH]- + CH2 CH
Rstrong initiator
CH2CHCH2
R
]][][[]][)([ MZYIKkMIZYkR iii
]][[ MMkR pp
Initiation
ex) styrene, stannic-chloride-H2O System [SnCl4OH-]H+
Propagation – can have a low activation energy and be rapid
or
Simple propagation reaction
Overall rate of polymerization may actually increase w/ decreasing temperature, means that termination has a high activation energy.