B BA A
Chain growth polymerization
I + M I[M]n
Step Growth polymerization
+ A-A-B-B A-AB-BA-AB-BA-A etc
ie individual monomers extend the chain
ie first monomers then oligomers extend the chain
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Polymer Chemistry
Useful books
I. Cowie Chemistry and physics of modern materials
G. Odian Principles of Polymerization
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Step growth polymerization
Chain growth polymerization
ARA + BR’B A-(RABR’) -Bn
conversion
Mol. Wgt.
conversion
Mol. Wgt.
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Chain growth and Polymerizability
ANIONIC, CATIONIC, RADICAL plus insertion and GTP
Anionic polymerization
Propagation and initiation
I- + =XX
-
Two aspects 1) Reactivity of the alkene to the anion (a nucleophile)2) Stability of the anion
For an alkene this reaction is essentially Michael addition
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Common monomers
O
O
ON
1) Monomers that can stabilize negative charge
2) Usually vinyl but cyclics too
3) Note alpha-substituants are too hindered
O
O
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The propagating centres
OO
OO
_
O-O
OO
ON
ON
_
O-N
ON
__
_ _
So most anionic polymerized VINYL monomers are Michael-type acceptors
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-
X
Propagation involves perturbations of the π*-alkene LUMO by the n-bonding HOMO of the nucleophile
Nu
HOMOLUMO π*
Important conclusions are
1) Propagation and initiation only occur if HOMO and LUMO are close enough
2) Propagation of a DIFFERENT comonomer occurs only if the HOMO can perturb the π* LUMO of the comonomer
E1
Reactivity of the alkene to the anion
Reactivity of the anion to the alkene is best dealt with using Frontier orbitals-It is an aspect that occurs early in the reaction-energy profile before the TS
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2) Stability of the anion
X
Eg new homo too stable, too low in energy
HOMO
LUMO π*
Too stable
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2) Stability of the anion
X
or new homo too instable, too high in energy
HOMO
LUMO π*
In stable does not perturb LUMO
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The consequences
require strong nucleophiles
eg nBuLi
Not all nucleophiles are initiators and not all monomers can be copolymerized
Weak electron withdrawing groups decrease energy of the anion HOMO less than CN and MeCOO in -MeCA
CN
OO
Initiated by weak nucleophileseg water
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CN
OO
_+ no propagation
CN
OO+
_ will propagate but side reactions a problem
OO
_+ no propagation
OO+
_ will propagate but side reactions a problem
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Side reactions and problems
XX H2O XX
H
O2
XX
OO
O
OO
OO
KILLS INITIATOR AND USES UP MONOMER
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O
O
O
O
O
O
Does not occur to a significant degree
Steric hindurance is important In Micheal additions
So us bulky initiators eg
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Cationic polymerization
Propagation and initiation
I+ + =X
X
+
Two aspects 1) Reactivity of the alkene to the cation2) Stability of the cation
X is electron donatingO
NOT electron deficient monomers
NOT methacrylate, acrylates, Vinyl ketones, acrylamides
NOT basic monomers
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Radical polymerizationPropagation and initiation
Two aspects 1) Reactivity of the alkene to the radical2) Stability of the radical
Initiation
Propagation
I-I 2 I.
I.
+ M IM .
Termination IM.2 IMMI or IM IM’
Rp = fkd
kt( )
1/2
[M][I]1/2
k
p
rate constantfor propagation
rate constant
for termination
Monomer concentration
initiator concentration
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R.
R=R
SOMO HOMO
σ-bondingHOMO
n-bondingSOMORRR .
Radical polymerization
LUMOπ -anti bonding
π -bonding
Can be either SOMO-HOMO
ie if SOMO is low energySOMO-HOMO
LUMO
1) Reactivity of the radical to alkene
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R.
R=R
HOMO
HOMO
n-bondingSOMORRR
.
LUMOπ -anti bonding
Or SOMO-LUMO
ie if SOMO is high energySOMO-LUMO
LUMO
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BUT BE CAREFUL!
THE NEW SOMO NEEDS TO PERTURB THE π* LUMO
R.
SOMO HOMO
σ-bondingHOMO
n-bondingSOMO
π -bonding
Very high EnergyNon-stabilized
O
R
O
So homo polymerization isVery difficult
Can be trapped by lowEnergy LUMO of Electron deficient monomer
LUMO of MVE
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Also,
OO
O1,2 substitution-sterics prevent propagation
OO
OOO
O
R
V. Electron deficient-low energy π*
R.
R=R
HOMOHOMO
LUMOπ -anti bonding
LUMO
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So can get alternating MVE-MA polymers
1) MVE is initiated R
O
But unstable and e-richDoes not perturb HOMO of MVEMVE π* too high energy
2) MA does not propagate due to sterics but π* is low energy
R
O OO
O New SOMO low energyStabilized radical
3) MVE HOMO is perturbed and addition occurs
R
O OO
O
O
Also Vinyl acetate
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R.
R=R
HOMO
HOMO
In general both SOMO-HOMO and SOMO-LUMO perturbation occurs
LUMOπ -anti bonding
LUMO
SOMO
SOMO
So monomers add randomly into the chain with a TENDENCY for
e-rich radicals (high energy SOMO) to add to e-deficient monomers (low energy LUMO)
e-deficient radicals (low energy SOMO) to add to HOMOs
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