ionic and group transfer polymerizations - kmutt ionic... · side reaction in anionic...
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Ionic and Group Transfer Polymerizations
Dr. Jatuphorn Wootthikanokkhan
School of Energy, Environment and Materials, KMUTT
Introduction
Fewer polymers are produced industrially via ionic
polymerizations,compared to free radical polymerization
Monomers used in ionic polymerization must contain
substituted groups capable of stabilizing carbocations or carbanions.
Commercial polymers prepared by ionic polymerizations
Cationic Polymerization (CP.) Polyisobutylene
Butyl rubber (copolymer of isobutylene and isoprene),
Anionic Polymerization (AP.) cis-1,4-Polyisoprene
cis-1,4-Polybutadiene
SBS(block copolymer)
Anionic polymerization
Monomers having substituent groups capable of stabilizing a carbanion (through either resonance or
inductive effects) are suitable for anionic polymerization Examples of the substituent groups are
- CN - C=C - COOR -
Anionic polymerization
Two EWGs are so effective in stabilizing anions that even water can initiate cyanoacrylate ("Super Glue"). Weak bases
(such as those on the proteins in skin) work even better.
Anionic Initiation
If the propagating anion is not very strongly stabilized, a powerful nucleophile is required as initiator.
Anionic initiators
Most widely used inititors for anionic polymerization include Organometallic compounds e.g. BuLi Alkaline metal in ammonia (from KNH2) Complex of alkali metals and aromatic compounds (electron
transfer precess)
Na + Na+ +
-Radical anion
Na+
-
CH2 CH
+
CH2 CH-
Na+
+
CH2CH CH2 CHNa+ Na+--
+
CH2CH CH2 CHNa+ Na+--
d i - a n i o n
s t y r y l r a d i c a l a n i o n
Dimerization
Interactions between ions
UV U+V- U+// V- U+ + V-
Covalent bonding
Contact ion pair
Solvent separated ion pair
Solvated free ions
Increasing strength of interaction
The tighter the ion pair, the slower the rate of propagation
Effects of solvent types on reaction rates
Solvent Relative Rate
Benzene 1
Dioxane 2.5
THF 225
1,2-Dimethoxyethane 1900
http://chem.chem.rochester.edu/~chem421/anionic.htm
This is related to polarity of the solvent.
Effects of counter ion types on reaction rates
Counterion Relative Rate
Li+ 1
Na+ 3.6
K+ 21.1
Rb+ 22.9
Cs+ 26.1
Larger counter-ions usually form looser ion pairs, so there is
an increase in rate
http://chem.chem.rochester.edu/~chem421/anionic.htm
Side reaction in anionic polymerization
Acrylates have problems in anionic propagation because of chain transfer to polymer. The hydrogen atoms adjacent to the ester groups are slightly
acidic, and can be pulled off by the propagating anion. The new anion thus
created can reinitiate, leading to branched polymers.
Termination
When carried out under the appropriate conditions, termination reactions do not occur in anionic polymerization. One usually adds
purposefully a compound such as water or alcohol to terminate the process. The new anionic species is too weak to reinitiate.
Functionalize the chain end
Initiation and termination in the
anionic polymerization with KNH2
KNH2 K+NH2
-+
NH2
-CHCH2+ NH2
CHCH2
-
NH3CHCH2
-+ NH2CH2CH2 +
-
Kinetic of anionic polymerization
Consider potassium amide-initiated polymerization in ammonia
Initiation KNH2 K
+ + NH2-
NH2- + M H2NM -
Ri = ki[NH2-][M]
Kinetic of anionic polymerization
Propagation Rp = kp[M][M-]
Termination Termination is resulted primarily by transfer
of anion to the solvent
Rtr = ktr[M- ][NH3]
Kinetic of anionic polymerization
Using the Steady State Assumption (SSA) Ri = Rtr
ki[NH2-][M] = ktr[M
-][NH3] and [M-] = ki[NH2
-][M] ktr[NH3] substituting [M-] in Rp we obtain
Rp = kpki[M]2[NH2-]
ktr[NH3]
Kinetic of anionic polymerization
Kinetic chain length ( v ) v = Rp/Rtr = kp[M][M-] ktr[M-][NH3]
v = kp[M] ktr[NH3]
Kinetic chain length and DP
In the case of a simple anionic initiator (e.g. BuLi) v = DP
In the case of electron transfer initiator such as Na/naphthalene
2v = DP (because polymer grow simultaneously from each end of a di-anion, a growing
specie)
“Living” anionic polymerization
In the absence of impurities or any proton sources, anionic polymerization could proceeds via the “Living” mechanism
Consequences of the living polymerization - low polydispersity GPC standards can be prepared - quenching with CO2 leads to carboxyl-terminated polymer - addition of a second monomer leads to block copolymer
Living anionic polymerization
Chains are initiated all at once (fast initiation)
Little or no termination (except purposeful).
Little or no depolymerization.
All chains grow under identical conditions.
MW of polymer prepared via living anionic polymerization
Narrow MW distribution (PD approaches 1.0, typically 1.05 - 1.2).
The MW is predictable (unlike other polymerizations) by using data from
Conversion Formula weight of the repeating units Mole ratio between monomer and initiator
Kinetic of the LIVING anionic polymerization
-d[M]/dt = kp [I]o[M]
d[M]/[M] = -kp [I]odt
ln [M] - ln [M]o = - kp [I]o t
ln [M]/[M]o = - kp [I]ot
[M]/[M]o = e- kp [ I ]o t
Kinetic chain length of the LIVING anionic polymerization
v = ([M]o - [M]) / [ I ]o
v = ( [M]o / [ I ]o ) – ( [M] / [ I ]o)
= ( [M]o / [ I ]o) - [M]oe-kp [I]ot / [ I ]o
v = [M]o (1 - e-kp [I]ot ) / [ I ]o
Group Transfer Polymerization (GTP)
Initiators used are organosilicon compounds. These
include methyl trimethylsilyl acetal of dimethylketene
R OR C=C R OSiR3
(bifluoride ion is used as a catalyst)
Group Transfer Polymerization
The mechanism of
GTP is not known with certainty. In the case of
anionic catalysis, the
mechanism has been
porposed by Webster (J. Am. Chem. Soc, 105 (1983) 5703) as below
R
C
R
C
OR
OSiR3
H2C
CH3
O
OCH3
C
C
R
C
R
C
OR
O
H2C
CH3
OSiR3
OCH3
C
C
Group Transfer Polymerization
Advantages of GTP include the following Wider temperature range (-100 - 100 degree C) Polydispersity close to 1 is obtainable
It does not polymerize other monomers except (meth)acrylate and acrylonitrile.
Monomers reactivity in CP.
For aliphatic monomers, the more stable the carbocation intermediates, the greater the rate of polymerization.
CCH 2 CHCH 2 CH 2CH 2
CH 3
CH 3 CH 3
> >
Monomers reactivity in CP.
For para-substituted styrenes OCH3 > CH3 > H > Cl For ortho-substituted styrenes,
substitutents retard the propagation regardless of the type of substituents due to the “steric effect”
CH 2CHCH 2CH CH 2CH CH 2CH
OCH 3 CH 3 Cl
> > >
Others cationic polymerizable monomers
CH2 CH OR CH2 CH
OR
+
CH2 CH
OR+
N
H2C CH
N
H2C CH
+
N
H2C CH
+
Initiating systems for CP.
Acids such as H2SO4, H3PO4
Lewis acids in combination with proton sources (or cation sources), for example,
BF3 / H2O AlCl3 / RCl
Initiation in CP
Initiation in CP
RCl + AlEt 2Cl R+ AlEt2Cl2 -
M
RM+ AlEt2Cl -
Initiation in CP
BF3OH-
H+ CH2 C
R
R
+ CH3 C+
R
R
+ BF3OH-
BF3 H2O+ BF3OH-
H+
CH2 C
R
R
CH3 C+
R
R
+ BF3OH-
H2O
BF3OH-
H3O+
Propagation in CP
Cationic polymerization is normally a non-living polymerization
Chain transfer reactions are common in CP.
Transfer to polymer chains (Hydride abstraction)
Transfer to monomers (Proton transfer)
Transfer to solvents (electrophilic substitution)
Chain transfer to polymer
CH 2 CH +
CH 3
CH 2 C H
CH 3
X-
CH 3+
CH 2 CH
CH 3
CH 2 C
CH 3
X-
CH 3+
H
+
Transfer to monomer
Chain transfer to water
CH2 C+
R
R
+ H2O CH2 COH
R
R
H+X
-+x-
Types of Terminations in Cat. Polymn.
1. Reacting with counter-ion
Proton transfer from chain end to the counter-ion
Chain end chlorination
2. Sometime, the above chain transfer reactions are
considered to be the termination.
Kinetic in cationic polymerization
Initiation Ri = ki[I][M] Propagation Rp = kp[M][M+] Termination with counter ion Rt = kt[M+] Transfer reaction to monomer
Rtr = ktr[M][M+]
Kinetic in cationic polymerization
Steady State Assumption (SSA)
Ri = Rt ki[I][M] = kt[M+] [M+] = ki[I][M]/kt
Substituting [M+] in Rp
Rp = kpki[I][M]2
kt
Kinetic in cationic polymerization
In the absence of chain transfer, the kinetic chain length, v, is equal to DP
v = DP = Rp/Ri = Rp/Rt
= kp[M][M+] kt[M+]
v = kp[M]/kt
(Note that v is independent with [I]) Why?