Ionic Polymerization

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Ionic Polymerization
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Anionic Polymerization
General Formula
B-
Anionic active center
Counter-ion
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What monomer?
Electron withdrawing groups (ester, cyano) or
groups with double bonds (phenyl, vinyl) are
needed as the R groups because these can
stabilize the propagating species by resonance.
Question What are the characteristic Q-e values
for such monomers? (q1)
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Anionic Initiation
For initiation to be successful, the free energy
of the initiation step must be favorable.
Therefore, it is necessary to match the monomer
with the appropriate strength of initiator so
that the first addition is "downhill."
Bottomline Nucleophile agents
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Concept of Living Polymerization

• There are only
• Initiation Chains are initiated all at once
• (fast initiation)
• Propagation All chains grow under
• identical conditions fast
  generally
• needs low temperature (why?
  q2).
• BUT
• Little or no termination (except purposeful)
• Little or no chain transfer

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Living Polymerization

• Consequently,

Reaction rate
Degree of polymerization
M- concentration of all anions in the
system M concentration of the monomer C
concentration of the initiator n the number of
anions that an initiator molecule produces
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MW in Living Anionic Polymerization
Poisson Distribution
Narrow distribution
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Typical Products

• CASE 1

Mono-disperse Polystyrene Gel Permeation
Chromatography (GPC) standard
CASE 2
unlike conventional rubbers that are covalently
crosslinked (vulcanized), these block copolymers
can be processed thermally and molded.
One or more blocks of flexible, low Tg polymer,
with other blocks of rigid, high Tg polymer. The
polymer actually exhibits two glass transitions,
one at high temperature and a second one below
room temperature.
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Answer

• q2 ionic polymerizations necessarily carry
  along a counterion, and their rates are much more
  sensitive to reaction conditions (e.g., solvent
  polarity, temperature).
• q1. monomers for anionic polymerization generally
  have
• High Q or high e values

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Cationic Polymerization
Initiation and Propagation
The mechanism of cationic polymerization is a
kind of repetitive alkylation reaction

• Propagation is usually very fast must
  often be run at low
• temperatures cooling large reactors is
  difficult and expensive.
• Trace of water kills reaction Careful
  drying of ingredients is
• necessary

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Cationic Polymerization Monomer
Electron donating groups are needed as the R
groups because these can stabilize the
propagating species by resonance
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Cationic Polymerization Initiators
Proton acids with unreactive counterions
Lewis acid other reactive compound
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Cationic Polymerization side reactions
Cationic vinyl polymerization is plagued by
numerous side reactions, most of which lead to
chain transfer. It is difficult to achieve high
MW because each 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.
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Review Vinyl Polymer Tacticity
Vinyl polymers that have single substituents
(e.g., propylene, the example below) or two
unsymmetrical substituents (e.g., methyl
methacrylate) have pseudoasymmetric carbon atoms
in the backbone.

• atactic polypropylene is a useless, gummy
  solid,
• the isotactic version is a highly crystalline,
  tough plastic that can even be
• made into fibers.

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Coordination (Ziegler-Natta) Polymerization
Important discovery R3Al Lewis acids
Another important discovery tacticity control

• Results
• Nobel Prize in Chemistry for Zeigler and Natta
  (1963)
• Multibillion industry

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Overall Scheme of Coordination Polymerization

• Limited to ethylene and other a-olefins like
  propylene.
• (Actually, it is the only good way to
  polymerize these monomers.)
• Produces linear polymer, with very few branches
  (e.g., high density
• polyethylene, HDPE).
• Capable of producing homo-tactic polymers.
• Most commercial initiators are insoluble
  complexes or supported on
• insoluble carriers.
• .

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The mechanism is poorly understood because it
takes place on the surface of an insoluble
particle, a difficult situation to probe
experimentally.
An approximation
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Concept Check Questions

• Styrene is almost a unique monomer, in that it
  can be polymerized by practically all methods of
  chain polymerization.
• A. Free radical
• B. Anionic
• C. Cationic
• D. Co-ordination (i.e., with a catalyst)
• Which of these methods would you use to make
  isotactic polystyrene?

1. If you needed to synthesize a set of narrow
   molecular weight standards (i.e., with
   poly-dispersities close to 1), which of the above
   methods would you use?

• Commercial atactic polystyrene is synthesized by
  which of the
• above methods?

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Concept Check Questions
4. What method would you use to synthesize a
triblock copolymer? A) Free radical
polymerization B) Anionic polymerization C) Using
a Ziegler Natta catalyst D) By putting it into a
bloody great pot and spitting on it to initiate
polymerization E) Condensation polymerization 5.
Suspension free radical polymerization of styrene
would be preferred over bulk polymerization to
overcome the problem of A) Branching B)
Cross-linking C) Stereo-isomerism D) Polymeric
impurities E) Temperature control during
polymerization
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Concept Check Questions
6. In emulsion polymerization, the principal
place where the monomer polymerizes is A) Monomer
droplets B) Aqueous phase C) Surfactant
micelles D) Surface of reactor E) Air-liquid
interface 7. Polypropylene produced
commercially using a Ziegler-Natta catalyst is
predominantly A) Atactic B) Isotactic C)
Syndiotactic