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Free Radical Polymerization
Addition Polymerization
• Free radical polymerization is a chain reaction
process.
• Polymerization is occur at the reactive end of
the growing chain
– High Mw is obtained
• Free radical is a specific initiator that change
polymer structure and therefore will be treated
separately.
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Vinyl Polymers
• Polymer that derived from monomer
containing a vinyl group C C
H H
HX
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Free Radical Initiators
• Certain monomers (styrene, methyl
methactylate) undergo polymerization upon
heating in the absence of radical.
– However most polymerization processes requires
free radical
• 4 major types of initiators:
– Peroxides (ROOR) & Hydroperoxides (ROOH)
– Azo compounds
– Redox initiators
– Photo initiators
Peroxides & Hydroperoxides
• ROOR and ROOH decomposed at elevated
temperature. The rate of decomposition is
depends on the structure
– i.e. benzoyl peroxide
(BPO)
• The peroxide not necessarily reacts with
monomer but it may also undergo other
“waste” reactions
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Choosing Peroxides
• The extent of which side reaction occur
depends on the structure of peroxide, radical
stability and reactivity of the monomer.
BPO: t½ = 30 min at T = 100 °C ; tend to react better with monomers. Few side reactions.
Diacetyl peroxide
Radicals not too stable.
Tend to be wasted.
Very stable! React with monomers.
Almost no side reactions. (but, long t½ and high Temp)
Di-t-butyl peroxide (half-life:10hours at 120℃)
Low temperature Peroxide
• Initiation can occur at low temperature by
adding promoters.
– For example the use of N,N-dimethylaniline
Ph C
O
O O C
O
Ph + Ph N(CH3)2
Ph N O C
CH3
CH3 O
Ph + Ph CO2
Ph N
CH3
O
CH3
C
O
Ph Ph N(CH3)2 + Ph C
O
O
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Azo Compounds
• Azo compound having cyano groups on the
azo linkage will decomposed at low
temperature to give nitrogen and radical
– α,α'-Azobis(isobutyronitrile) (AIBN). Stable by resonance.
– Can also go through side reaction (waste)
Half-life of : 1.3 hours at 80℃.
Redox Initiators
• Low temperature initiation can be introduce by
electron transfer to form free radical.
– Useful for emulsion polymerization as with cumyl
hydroperoxide
• Rates can be easily control by changing
component concentration
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Photo Initiators
• Reaction is independent of temperature.
• Easy to control rate (wavelength) and easy to
stop.
Initiators Summary
PhotoRedoxAzoPeroxidesInitiator Type
Independent
on
temperature
LowModerate to
low
VersatileWorking
Temperature
VersatileVersatileModerateVersatileReactivity
Low
temperature
application.
Very convinent
Low
temperature
for emulsion
polymerization
VersatileAdvantage
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Thermal Polymerization
• Some monomers (styrene) can polymerize in
the absence of initiators.
– Generation of free radical moieties.
CH
CH
Polymerization Techniques
• Bulk polymerization
– Only monomer and monomer soluble initiator
• No contaminations
– The viscosity is increase as reaction proceed
• Problem with exothermic reaction due to heat transfer
• Difficult to remove non reacted monomer
• Polymer may precipitate and viscosity does not changed
much
– Use in the polymerization of PMMA, PS and low
Mw vinyl polymer
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Polymerization Techniques
• Solution Polymerization
– Solution of monomer, initiator and solvent
• May be use directly for solvent casting or spraying
– Maintain low viscosity
• Easy to control heat transfer
– Difficult to remove solvent
Polymerization Techniques
• Suspension Polymerization
– Monomer (+initiator) is break into droplet in a
non soluble liquid (water) by continuous mixing
• Form Granular beads
• Efficient heat transfer
• May cause agglomeration (problem with tacky polymers)
• Contamination
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Polymerization Techniques
• Emulsion Polymerization
– Monomer is dispersed by an emulsifier (soap)
• Good heat transfer
• Used directly as latex
• Contamination
– Only one initiator diffused into
micelle
• High Mw
• Maybe too high…
Polymerization Techniques
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Free Radical Mechanism
• Initiation
– Involve two steps:
• Formation by decomposition (rate controlling step)
• Addition of radical to monomer
Initiator 2R.
Free Radical Mechanism
• Propagation
– Addition of monomer radical to monomer
molecule follow by addition of macro radical to
monomer.
– Head-to-tail addition due to steric and electronic
effects is predominant.
• But not occur exclusive
(13-17% of head to head) (5-6%) (19%)
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Free Radical Mechanism
• Termination
– Two principles routes of termination
• Coupling (also known as combination)
• Disproportion – Transfer of atom (usually hydrogen)
from one chain to another
– Both reaction are diffusion control
Free Radical Mechanism
• The mechanism in which termination occur
depends on the structure of the monomer.
– Coupling of stericaly hindered is less favorable
– Electrostatic repulsion of polar group may raise activation
energy for coupling
– Availability of α-hydrogen for hydrogen transfer
2 CH2C
CH3
CO 2CH3
CH 2CH
CH 3
CO 2CH 3
+ CH=C
CH 3
CO 2CH 3
or CH 2 C
CH 2
CO 2CH 3
.
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Free Radical Mechanism
• Another termination mechanism is primary
radical termination
– Significant at
• High initiator concentration
• High viscosity limit diffusion of high Mw moieties
• The termination mechanism will effect the end
group nature of polymer chains
– Double bond (disproportion)
– Head-to-head (coupling)
Chain Transfer Reaction (CTR)
• Transfer of reactivity from growing polymer
chain to another species.
– One chain is terminated but at the same time
generate new radical
• The results is low Mw products and broad distribution
• Typical for all free radical polymerization
• Chain end radical abstract hydrogen from a
chain leading to a reactive site for branching
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Chain Transfer Reaction (CTR)
• The hydrogen abstracting may also occur in
the intramolecular level, causes ‘backbiting’
– Leads to small chain branching in polyethylene
Chain Transfer Reaction (CTR)
• CTR can occur with monomer or initiator
• PS CTR with solvent (carbon tetrachloride)
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Chain Transfer Reaction (CTR)
Chain Transfer Reaction (CTR)
• CTR in monomer contain allylic hydrogen (PP)
is very important, since the formation of a
resonance stable allylic radical is favorable
– Difficult to achieve high molecular weight PP with
free radical technique
– Not a problem in PMMA since addition is more
favorable
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Chain Transfer Reaction (CTR)
• CTR is used to prevent polymerization (for
example in styrene monomers)
– Inhibitors, like alkylated phenols, transfer
hydrogen to form a new radical that undergo
coupling reaction.
– Before polymerization the inhibitors need to be
distilled or consume by adding excess of initiator
• The time till reaction start called induction period
R
R
R
OH
Free Radical Kinetics
• Assumptions:
– Rates of the different stages are different.
– Each propagation stage is head-to-tail and occur at
the same rate (true when chain is longer then n=4)
– Rate of termination is independent of chain length
– Rate of decomposition is very slow compare to
addition
• Initiation:Initiator
KdR.
R.Ki
+ M M1.
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Free Radical Kinetics
• Taking into account that two radical are
formed:[ ]
[ ]Ifkdt
MdR di 2==
•
f : Initiator efficiency.(fraction of initiator that initiate polymerization)
f usually in the range of 0.3-0.8
kd : Decomposition rate constant.
[I] : molar concentration of initiator.
[M ·] : molar concentration of radical.
f = radicals formed from initiator
radicals that initiate a polymer chain
Free Radical Kinetics
• Termination rateRt= dt
-d[M·]= 2kt[M·]
2
kt = ktc+ ktd
Rp = dt
-d[M]= kp[M][M·]
Rp = dt
-d[M]= kp[M]
t
d
k
Ifk ][[M·]=t
d
k
Ifk ][
Ri=Rt
][2][2 ⋅= MkIfk td2
Propagation rate (Rp) = Polymerization rate
Early on in the process, the rates of initiation and termination become equal.(from empirical observations)
We reach a Steady state concentration of free radicals.(The steady state assumption)
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Free Radical Kinetics
• Polymerization rate is therefore the
propagation rate:
Rp is proportional to the square root of [I].
Rp is proportional to [M].
Rp = dt
-d[M]= kp[M]
t
d
k
Ifk ][
Free Radical Kinetics
• Average kinetic chain length (ν) is used to
described the average number of monomer
units polymerized per chain initiated = rate of
polymerization over rate of initiation
ν = Ri
Rp=
Rt
Rp
ν =2kt[M·]
2
kp[M][M·]=
2kt[M·]
kp[M]=
kp[M]
][2 Ikfk dt
Steady state: Ri = Rt
initiation of rate
tionpolymeriza of rateν =
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Free Radical Kinetics
• So, in the absence of side reaction:
• But we already seen that CTR change the
chain growth kinetics
Disproportionation :
Combination :
DP = ν
DP = 2ν
Free Radical Kinetics
• In case CTR occur we need to combined rates
of termination and CTR
trt
ptr
RR
Rv
++++==== ]][[ TMkR trtr ⋅⋅⋅⋅====
Transfer agent
eventually, plugging (CT = chain transfer constant) we get:p
trT
K
KC ====
][
][11
M
TC
vv
T
tr
Σ++++====
As [T] increase or the rate of transfer increase the Mw is decrease
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Free Radical Kinetics
• There are cases where no termination or CTR
occur. This known as living (free radical)
polymers
– Adding more monomer, after monomer is fully
consumed, will increase Mw
– Adding different monomer will create block
copolymer
• Example for this kind of reaction is the atom
transfer radical polymerization (ATRP)
Free Radical Kinetics
• In the case of living (free radical) polymers
the degree of polymerization is simply:
– Since all chain are initiated at the same time we get
a low polydispersity (as low as 1.05)
0
0
][
][
I
MDP ====
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Free Radical Kinetics
• The kinetic of emulsion polymerization is
different then bulk or solution polymerization
• Increasing rate of polymerization increases
Mw
– Due to the fact that, not like in bulk, the initiation
is not competing with termination, but rather with
diffusion rate of terminator radicals.
– Polymerization depends on rate and number of
particles (N)
Free Radical Kinetics
• Emulsion kinetics
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Free Radical Kinetics
• Auto acceleration
– Also known as “gel effect”
• The increase viscosity in bulk polymerization
(or precipitating in solution) make it hard to
macro radicals to act as terminators
– Therefore polymerization rate is increase and high
Mw obtained
– High heat is generated
– May cause even cross linking
Free Radical Kinetics - Summary
Average chain lengthRate of PolymerizationMechanism
Simple
polymerization. No
CTR.
CTR polymerization
Living free radical
polymerization
(ATRP)
Emulsion
polymerization
Rp = dt
-d[M]= kp[M]
t
d
k
Ifk ][ ν =kp[M]
][2 Ikfk dt
DP = ν DP = 2ν
For disproportion For combination
][
][11
M
TC
vv
T
tr
Σ++++====
0
0
][
][
I
MDP ====
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Stereochemistry
• Free radical mechanism lack significant steric
and electro repulsive forces to generate a
regular stereochemistry.
• Most free radical polymers are atactic
• There are some special cases of syndiotactic
structure
• This is based on balance between propogation
energy and free rotation energy
• Regularity decreases as temperature
increases
Stereochemistry of Dienes
• Cyclopolymerization
• 1,3-butadiene goes 2 kinds of additions:
– 1,2
– 1,4
CH2 CH CH CH2RCH2 CH CH CH2
R .
CH2CH
CH
CH2
CH2
C C
CH2
H H
CH2
C C
H
H CH2
1,4-Addition cis 1,4-Addition trans
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Stereochemistry of Dienes