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Polymer SynthesisCHEM 421
• Odian Book
Chapter 3-15, 5-3
Polymer SynthesisCHEM 421
Living Polymerization
Polymer SynthesisCHEM 421
Co Co
Living Polymerization
Neutral and Highly Reactive
or
Polymer SynthesisCHEM 421
Living Polymerizations
• 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.
Polymer SynthesisCHEM 421
Living Polymerization
• No termination or chain-transfer side reaction during polymerization–Control of molecular weight
» Mn=
–Narrow molecular weight distribution
–Synthesis of block copolymers by sequential monomer addition
–Control of polymer chain microstructure
–End-group functionalization
grams of monomer
Moles of initiator
Polymer SynthesisCHEM 421
Initiation
• Nucleophilic Initiation of Vinyl Monomers
CH3CH2CH
CH3
Li H2C CH Bu CH2 CH
CH CH CH
Li
Li LiLi
Polymer SynthesisCHEM 421
Anionic Polymerization of Styrene
CH3CH2CH
CH3
Li H2C CH
cyclohexane, 25 oC
tight ion pair, red-orange solution
sec-BuLi styrene
Polymer SynthesisCHEM 421
Anionic Polymerization of Styrene
CH3CH2CH
CH3
Li H2C CH
cyclohexane, 25 oC
tight ion pair, red-orange solution
sec-BuLi styrene
CH2 CH Li
Bu CH2 CH CH2 CH Lin-1
Bu
Polymer SynthesisCHEM 421
Anionic Polymerization of Styrene
CH3CH2CH
CH3
Li H2C CH
cyclohexane, 25 oC
tight ion pair, red-orange solution
sec-BuLi styrene
Bu CH2 CH CH2 CH Li
THF
loose ion pair and f ree ions,blood-red solution
n-1
Bu CH2 CH CH2 CH Lin-1
Polymer SynthesisCHEM 421
Anionic Polymerization of Styrene
CH3CH2CH
CH3
Li H2C CH
cyclohexane, 25 oC
tight ion pair, red-orange solution
sec-BuLi styrene
Bu CH2 CH CH2 CH Li
THF
loose ion pair and f ree ions,blood-red solution
n-1
CH3OHLiCH3O
Bu CH2 CH CH2 CH Lin-1
Bu CH2 CH CH2 CHn-1
H
Polymer SynthesisCHEM 421
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 SynthesisCHEM 421
The Poisson Distribution in Molecular Weight
• The constraints imposed on a living polymer require that the polymer exhibit a Poisson distribution in molecular weight.*
Conversion & Mw/Mn as a Function of Time
Time
Co
nve
rsio
n
Mw
/Mn
ConversionPDI
nn
w
DP
11
DP
DP
Polymer SynthesisCHEM 421
• Conversion should be linear with time in semi-logrithmic coordinates
• Deceleration indicates termination or deactivation of catalyst.
Ln[M]o/[M] vs. Time
Time
ln[M
] o/[M
] t
kp/kt = 1.05kp/kt = 10kp/kt = 100Living
Scale of “Livingness”
• When chain termination cannot be completely suppressed (kt > 0):
- deviation from living behavior becomes more pronounced with time
- The degree to which a reaction deviates at time, t, is proportional to the ratio of the rate of propagation to termination.
Polymer SynthesisCHEM 421
End Group Functionalization
Carboxylic acid terminated:
Primary alcohol terminated:
CH2 CH Li
CH2 CH Li
CH2 CH Li
MeOH
[H ]
CO2
1)
2) HCl
O
CH2 CH
CH2 CH2
CH2 CH
C
O
O
MeO Li
Li
CH2 CH2 OH
Polymer SynthesisCHEM 421
End Group Functionalization
CH2 CH Li
SiCl
Me
Cl
Me
Si
Me
Me
PS PS
CH2 CH LiSiCl
Me
Me
Me CH2 CH Si Me
Me
Me
CH2 CH Li
SiCl
Cl
Si
Me
Cl
Cl
Me
PSPS
PSPS
Polymer SynthesisCHEM 421
Monomers
• Generally olefin w/ EWG or delocalizing groups
Methyl acrylate (MA)Methyl methacrylate (MMA)t-Butyl methacrylate (tBuMA)
R
C
OR'
O acrylates (acrylic esters)
R
H CH3CH3 CH3CH3
R'
1,3-butadiene
isoprene
styrenes & their derivatives
C N
acrylonitrile
NO2
Polymer SynthesisCHEM 421
Ion Pair Aggregation
• Need to stabilize propagating anions
R R RRM M M M
covalent bond tight "contact", "intimate"ion pairs
loose"solvent separated"
ion pairs
f ree ionsvery reactive
unhindered
nonpolar solvents (cyclohexane) polar solvents (THF)
less reactive, more controlled lewis base solvated Li+ ions
Polymer SynthesisCHEM 421
Ion Pair Aggregation
R R RRM M M M
covalent bond tight "contact", "intimate"ion pairs
loose"solvent separated"
ion pairs
free ionsvery reactive
unhindered
• Need to stabilize propagating anions
R H R H
Ka=[H ][R ]
[RH]
pKa = - log Ka
As Ka gets larger (stronger acid)
pKa gets smaller