types of vinyl polymerization methodadvantagesdisadvantages bulk (neat)simple equipment rapid...
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Types of Vinyl Polymerization
Method Advantages Disadvantages
Bulk (Neat) Simple equipmentRapid reactionPure polymer isolated
Heat buildupGel effectBranched or crosslinked product
Solution Good mixingReady for application
Lower mol. Wt.Low Rpoly
Solvent Recovery
Suspension(Pearl)
Low viscosityDirect bead formation
Removal of additives
Emulsion High Rpoly
Low TemperaturesHigh Mol. Wt.High surface area latex
Removal of additivesCoagulation neededLatex stability
Inverse Emulsion Water in oil latex formedInversion promotes dissolution in water
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Fate of Initiator Radicals
• Radical reactionsRecombination in solvent cage
R +
X
R
Xki
•Chain initiation, Ri = 2 f kd [I]
•Efficiency factor, f = 0.1 - 0.9
Reaction with polymer radicals (kt)--primary termination
Reaction with initiator (MIH)
Hydrogen abstraction from polymer chains (chain transfer to polymer)
Reaction with solvent or inhibitor
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Radical Initiators
• Azo Initiators
CH3
H3C
CN
NN
CH3
CN
H3C
azobisisobutyronitrile, AIBN
CN
NN
CN1,1'-azobis(1-cyclohexanenitrile)
H3C
CN
NN
CH2CH2COOH
CN
H3CHOOCCH2CH2-
4,4'-azobis(4-cyanovaleric acid)
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Decomposition of Azo Initiators
• 2- bond cleavage to liberate nitrogen
CH3
H3C
CN
NN
CH3
CN
H3C heat
or light
CH3 CCH3
CN
2 + N2
Cage Recombination ---Side reaction- irreversible coupling of succinonitrile radicals, efficiency decreases at high conversion
CH3
H3C
C
N
CH3
C
CH3
NC
CH3
C
N
H3CC
CH3
C
N
H3C CH3
CH3
C
N
N
C
CH3CH3
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Peroxy Initiators
• High temperature initiators
C OO
HC O
OR
C OO
R
O
Hydroperoxides Dialkyl Peroxides Peresters
Td = 155-175 C 100-135 C 110-130 C
C OO
O
R
ODiacyl Peroxides
SO
OSO O
O
OO
O
Persulfates
Td = 35-80 C 50-90 C
Moderate temperature initiators
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Peroxy Initiators
• Low temperature initiators, 35-60 C
CO
O
O
RO
O O
Peroxycarbonates
2 CO
O
O
OO
OO
O
O
Di-t-butyl peroxylate, DBPOX
O2 + 2 CO2
3-bond cleavage process?
-cleavage to carbon centered radical
O + CH3
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Redox Initiation
0-5 C in water
SO
OSO O
O
OO
O
+ Fe++ SOO
O
O
OS O
O
O
+Fe+++
+
0-5 C in organic/aqueous phase
OOH
cumyl hydroperoxide
Fe+++O
+ OH +Fe+++
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Decomposition of Peroxy Initiators
• 1-bond cleavage process
If R = aryl, acyl radical initiates = alkyl, CO2 lost before initiation occurs
R OO
O
R
O
heatR O
O
2
- CO2
RR O
O
R
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Reaction of benzoyloxy radicals with styrene
PhCO2-CH2 CH
80%
Sty
Ph OO
O
Ph
O
heat2 PhCO2
PhCO2
Ph
-CO2
1%
HC CH2PhCO2
6%Sty
CH
PhCO2
14%
Sty
H2C
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Chain Transfer
• Hydrogen transfer to growing polymer chain
R
X
P CH X
H+ R S
H R
X
P CH X
HH
+ R S
ktr
R S +
XR S
Xka
kp
•Reinitiation of growing chain using transferred radical
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Effect of Chain Transfer on Rp and DP
Relative rate constants
Type of effect Effect on Rp
Effect on DP
kp.>> ktr
ka ~ kp
Normal None Decrease
kp<< ktr
ka ~ kp
Telomerization None Large decrease
kp>> ktr
ka < kp
Retardation Decrease Decrease
kp<< ktr
ka << kp
Inhibition Large decrease
Large decrease
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Control by Chain Transfer• Chain transfer depends upon nature and
concentration of chain transfer agent.
1DP
=1
DPo+
[SH][M]Ctr
Where Ctr is the chain transfer constant that includes the rate constants for hydrogen abstraction and re-initiation of a new chain
Ctr is specific for a given monomer at a given temperature
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Common Chain Transfer Agents
Transfer agent Styrene,
Ctr x 104
Vinyl Acetate
Ctr x 104
Toluene 0.125 21.6
Di-n-butyldisulfide
24 10,000
Carbon tetrabromide
22,000 390,000
n-butyl mercaptan
210,000 480,000
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Additional Chain Transfer Processes
• Chain transfer to monomer, Ctr x 104
– Ethylene, 0.4- 4.0; Styrene, 0.3-0.6
Chain transfer to polymer--branchingPolyethyleneVinyl acetateVinyl chloride
Vinyl acetate, 1.75-2.8Vinyl chloride, 10.8-16Allyl systems, 50-100
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Transfer to Polymer
• Polyethylene branching
HH
M
M
ktr
ka
HH H
MH
H
M
Long branches
Short branches
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Inhibition of Radical Polymerization• Must stop oxygen- and carbon centered radicals
Carbon centered radicals stopped by addition
•Oxygen centered radicals stopped by hydrogen transfer
RH + O2 R +
HOORH + R
HOO
HOOH+
Radicals generated by auto oxidation
R + O2 ROO
ROO + RH ROOH + R
ROOH + OHRO May be metal catalyzed
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Critical Inhibitor Properties
An inhibitor should not add to, abstract from or otherwise reach with monomer or solvent
Inhibitors should not undergo self reaction or unimolecular decomposition
Inhibitors must react rapidly with the propagating and/or initiator derived radicals to terminate polymer chains
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Trapping Oxygen Centered Radicals
OH
BHT
O
+ ROHRO
OO
OR
RO
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Trapping carbon centered radicals
• Carbon centered radicals stopped by addition to oxygen or carbon
O O
Benzoquinone
OR
OR.
O OR
O O
R HR O O
R
R H+
HO O
R
Tautomerize
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Typical InhibitorsOH OH
OCH3
Monomethylhydroquinone, MEHQ
OH
3,5-ditert-butyl catechol
OH
OHHQ
O O
Benzoquinone
O O
Cl Cl
ClClChloranil
O2 FeCl3 CuCl2 S
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Stable Radical Inhibitors
N N
O2N
NO2
O2N
Diphenylpicrylhydrazyl, DPPH
N
O
TEMPO
O
O
Galvanoxyl
NN
NN
Triphenylverdazyl