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Anionic
Polymerization
1. Introduction
2. Stereoregular Polymers
3. Anionic Polymerization
- Initiators, monomers and mechanism
- “Living” polymers, copolymerization
4. Coordination Polymerization
- Ziegler-Natta polymerization
5. Cationic Polymerization
1. Introduction
An ionic polymerization is an addition polymerization in which the
growing chain ends bear a negative or a positive charge.
These two alternative mechanisms are associated with different
catalyst systems and different reaction conditions.
Anionic
polymerization
Cationic
polymerization
2. Stereoregular Polymers
Ionic and coordination polymerizations can provide access to
polymers that have a stereoregular structure.
Any monomer molecule that possesses an symmetric center at a
skeletal atom has the capacity to form stereoregular polymers.
2.1 Isotactic polymers
Isotactic polymers are characterized by the presence of only one
symmetry type of monomer residue in the chain.
Ability to crystallize readily
This is a consequence of the fact that the regular disposition of
substituent groups along the chain permits the molecule to assume a
regular helical conformation and allows adjacent chains to pack
together in an ordered manner.
H
H
H
H
H
H
H
R
R
R
R
2.2 Syndiotactic polymers
Alternating configuration of residues
Ability to crystallize readily because of the opportunities that exist for
the formation of helices and for efficient chain packing
Polyvinyl alcohol
H
R
R
H
H
R
R
H
2.3 Atactic or heterotactic polymers
Contain no regular sequence of monomer residues along the chain
Because of this structure, low tendency for crystallization
Polyvinyl alcohol
H
R
H
R
H
R
R
H
3. Anionic and Coordination Polymerization
3.1 Initiators for Anionic Polymerization
Radical anions (sodium naphthalenide) must be prepared from
naphthalene and a sodium mirror in an ether type solvent.
3.2 Monomers for anionic polymerization
Typical monomers, styrene, methyl methacrylate, and acrylonitrile
The important thing to remember is that monomers which are
suitable for anionic polymerization generally contain electronwithdrawing substituent groups.
3.3 A typical experimental procedure
Figure. Apparatus for the anionic polymerization of styrene with the
use of n-butyllithium as an initiator.
3.4 The Mechanism of anionic polymerization
- Initiation
- Propagation
- Chain transfer or chain branching
does not occur to any appreciable extent with anionic systems, and this
is especially true if the reaction is carried out at low temperatures.
- Termination
Occurs either accidentally or deliberately when the active chain end
reacts with a molecule of carbon dioxide or with water, alcohols or other
protonic reagents.
Example
Styrene Polymerization_Initiator (n-butyllithium)
Initiation:
CH3CH2 CH
Li
+
CH2 CH
CH3CH2 CH CH2 CH Li
CH3
CH3
Propagation:
CH2 CH Li
+
CH2 CH
CH2 CH CH2 CH Li
Termination:
CH2 CH Li
+
H OH
CH2 CH2
+
Li OH
Characterization of Anionic Polymerization
1) In general, anion exist as not ion, but ion pair (gegen ion or counter ion or zwitter
ion). More complex than radical polymerization
2) The rate of polymerization is fast at low temperature.
3) Termination not exist. (It occurs by accident.)
When termination take places, it is occurred by proton of solvent or transfer agent,
which is transfer of segment.
Termination
1) Proton donor (H2O or ethanol)
H
CH2 C :- + C2H5OH
CH2 CH2 + C2H5O
2) Hydride elimination, if there is no impurities
3) Polar monomer
CH3
CH3
CH2 C:- Li+
+
CH2 C
COOCH3
COOCH3
CH3 O CH3
CH2 C
C C CH2
COOCH3
+
+ Li CH3O
4) Backbiting or intramolecular reaction
Cyclic trimer at the end of chain
3.5 “Living” Polymers
The term “living” polymer is applied to ionic polymerizations that are
not terminated.
1) Initiation occurs when an electron is transferred from sodium to the
monomer to generate a radical anion
2) Dimerization of the radical anion then takes place to form a dianion
3) Propagation can then occur at both ends of the dimer by the insertion
of monomer molecules into the ionic bonds.
Sodium naphthalenide functions as a catalyst in a very similar way.
The naphthalene radical anion either transfer an electron to the
monomer to form a monomer radical anion, or the naphthalene radical
anion itself may initiate monomer polymerization directly.
- Electron transfer agents
Initiation is very fast!
Resonance hybrid
styryl radical-anion
Dianion
One of the characteristic of “living” polymer systems is that they yield
polymers with very narrow-weight distributions.
Number of
molecules
The narrow molecular-weight distribution is largely a consequence of
the rapidity of the initiation step.
Chain length
Extremely narrow molecular-weight distribution that is characteristic of anionic
and radical-anionic polymerization products.
3.6 Copolymerization
Because many anionic chain ends remain “alive,” particularly at low
temperatures, even through the monomer has been consumed, chain
growth can be restarted by the addition of more of the same monomer
or addition of a different monomer.
If a different monomer is added, a copolymer is formed.
BBBBBAAAAACH2
2HCAAAAABBBBB
Styrene-Butadiene-Styrene (SBS) Triblock copolymers
Butadiene
Living polystyrene
Living styrene-butadiene
block copolymer
Dichlorodimethylsilane
The end copolymer is capped with a dichlorosilane
Dichlorodimethylsilane
Another chain of living polystyrene
Styrene-butadiene-styrene triblock copolymer
Summary
Anionic polymerization
- Initiators (alkyl metal)
- Initiation and propagation
- Living polymers