Transcript Polymer Synthesis CHEM 421 Living Polymerization

Polymer Synthesis
CHEM 421
• Odian Book
Chapter 3-15, 5-3
Polymer Synthesis
CHEM 421
Living Polymerization
Living Polymerization
Neutral and Highly Reactive
or
Polymer Synthesis
CHEM 421
Living Polymerizations
Polymer Synthesis
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• Living polymerizations are chain growth polymerizations which
proceed in the absence of irreversible chain transfer and
termination steps.*
• Diagnostic Characteristic of Living Polymerizations
– The reaction proceeds until all monomer is consumed. If more
monomer is introduced then the polymerization will continue
– The number average molecular weight, Mn, is a linear function of
conversion.
– The number of propagating chains (active centers) is constant and
independent of conversion.
– Mn can be controlled by the reaction stoichiometry.
– Sequential monomer addition results in the preparation of block
copolymers.
– Resulting polymers will exhibit a narrow molecular weight distribution
and the polymer must exhibit a Poisson distribution in molecular
weight.**
* Szwarc, M., Nature, 1956, 178, 1168.
** Flory, P. J., J. Am. Chem. Soc., 1940, 62, 1561.
Living Polymerization
Polymer Synthesis
CHEM 421
• No termination or chain-transfer side reaction
during polymerization
–Control of molecular weight
grams of monomer
» M n=
Moles of initiator
–Narrow molecular weight distribution
–Synthesis of block copolymers by sequential
monomer addition
–Control of polymer chain microstructure
–End-group functionalization
Initiation
Polymer Synthesis
CHEM 421
• Nucleophilic Initiation of Vinyl Monomers
CH3
CH3CH2CH
Li
H2C
CH
CH Li
Bu CH2 CH
CH Li
Li
CH Li
Anionic Polymerization of
Styrene
Polymer Synthesis
CHEM 421
Anionic Polymerization of
Styrene
Polymer Synthesis
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Anionic Polymerization of
Styrene
Polymer Synthesis
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Anionic Polymerization of
Styrene
Polymer Synthesis
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Initiators: Organolithiums
• Soluble in hydrocarbons
• Direct nucleophilic attack
• No electron transfer
(RLi)N
N= 6,4,2
MW=
g of monomer
moles of initiator
Polymer Synthesis
CHEM 421
The Poisson Distribution in
Molecular Weight
Polymer Synthesis
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• The constraints imposed on a living polymer require
that the polymer exhibit a Poisson distribution in
molecular weight.*
Conversion
PDI
Time
Mw/Mn
DPw
1
1 
DPn
DPn
Conversion
Conversion & Mw/Mn as a Function of
Time
Scale of “Livingness”
Polymer Synthesis
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• Conversion should be linear with time in semi-logrithmic coordinates
• Deceleration indicates termination or deactivation of catalyst.
ln[M]o/[M]t
Ln[M]o/[M] vs. Time
kp/kt = 1.05
kp/kt = 10
kp/kt = 100
Living
• When chain termination cannot
be completely suppressed
(kt > 0):
- deviation from living behavior
becomes more pronounced
with time
Time
- The degree to which a
reaction deviates at time, t, is
proportional to the ratio of the
rate of propagation to
termination.
End Group Functionalization
Carboxylic acid terminated:
Primary alcohol terminated:
Polymer Synthesis
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End Group Functionalization
Polymer Synthesis
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Monomers
Polymer Synthesis
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• Generally olefin w/ EWG or delocalizing groups
Methyl acrylate (MA)
Methyl methacrylate (MMA)
t-Butyl methacrylate (tBuMA)
Ion Pair Aggregation
• Need to stabilize propagating anions
Polymer Synthesis
CHEM 421
Ion Pair Aggregation
• Need to stabilize propagating anions
Polymer Synthesis
CHEM 421