Transcript RadicalChemistry - LSU Macromolecular Studies Group

Types of Vinyl Polymerization
Method
Advantages
Disadvantages
Bulk (Neat)
Simple equipment
Rapid reaction
Pure polymer isolated
Heat buildup
Gel effect
Branched or crosslinked product
Solution
Good mixing
Ready for application
Lower mol. Wt.
Low Rpoly
Solvent Recovery
Suspension
(Pearl)
Low viscosity
Direct bead formation
Removal of additives
Emulsion
High Rpoly
Low Temperatures
High Mol. Wt.
High surface area latex
Removal of additives
Coagulation needed
Latex stability
Inverse Emulsion Water in oil latex
formed
Inversion promotes
dissolution in water
Fate of Initiator Radicals
• Radical reactions
Recombination in solvent cage
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
•Chain initiation, Ri = 2 f kd [I]
R
R
+
X
ki
X
•Efficiency factor, f = 0.1 - 0.9
Radical Initiators
• Azo Initiators
H3C
CH3
CN
H3C
N
CH3
N
N
CN
CN
CN
1,1'-azobis(1-cyclohexanenitrile)
azobisisobutyronitrile, AIBN
HOOCCH2CH2-
N
H3C
H3C
N
CN
N
CH2CH2COOH
CN
4,4'-azobis(4-cyanovaleric acid)
Decomposition of Azo Initiators
• 2- bond cleavage to liberate nitrogen
H3C
CH3
H3C
N
CN
heat
CH3
N
2
or light
CH3
CH3
C
+
N2
CN
CN
Cage Recombination ---Side reaction- irreversible coupling of
succinonitrile radicals, efficiency decreases at high conversion
N
H3C
H3C C
CH3
CH3
C CH3
N
CH3
C
C
N
H3C
CH3
C
C
N
CH3
CH3
N
C
N
CH3
C
CH3
Peroxy Initiators
• High temperature initiators
O
C O
O
C O
H
O
C
R
Hydroperoxides Dialkyl Peroxides
Td = 155-175 C
100-135 C
O
C
O
O
O
O
Diacyl Peroxides
Td = 35-80 C
R
R
Peresters
110-130 C
Moderate temperature initiators
O
O
O
O S O O S O
O
O
Persulfates
50-90 C
Peroxy Initiators
• Low temperature initiators, 35-60 C
O
O
C
O
2
O
O
O
C
O
O
R
Peroxycarbonates
O
3-bond cleavage process?
O
O
O
O
O
2
O
+ 2 CO2
O
Di-t-butyl peroxylate, DBPOX
O + CH3
b-cleavage to carbon centered radical
Redox Initiation
0-5 C in water
O
O
O
O S O O S O
+ Fe++
O
O
O
O S O +
O +
Fe+++
O
S O
O
0-5 C in organic/aqueous phase
OOH
+ Fe++
cumyl hydroperoxide
O
+
OH + Fe+++
Decomposition of Peroxy Initiators
• 1-bond cleavage process
O
R
O
O
heat
O
R
2
R
O
O
- CO2
O
R
O
R
R
If R = aryl, acyl radical initiates
= alkyl, CO2 lost before initiation
occurs
Reaction of benzoyloxy radicals with styrene
O
Ph
O
heat
O
2 PhCO2
Ph
O
PhCO2-CH2 CH
80%
Sty
PhCO2
Sty
PhCO2
-CO2
H
C CH2
Sty
H2C CH
Ph
1%
14%
PhCO2
6%
Chain Transfer
• Hydrogen transfer to growing polymer chain
H
H
R
P C
H
X
X
+
R S
H
ktr
H
R
P C
H
X
R
+
X
S
•Reinitiation of growing chain using transferred radical
R
S
+
X
ka
R
S
X
kp
Effect of Chain Transfer on Rp and DP
Relative
rate
constants
kp.>> ktr
ka ~ kp
Type of effect
Effect on
Rp
Effect on DP
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
Control by Chain Transfer
• Chain transfer depends upon nature and
concentration of chain transfer agent.
1
=
DP
1
[SH]
Ctr
+
[M]
DPo
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
Common Chain Transfer Agents
Transfer agent
Toluene
Di-nbutyldisulfide
Carbon
tetrabromide
n-butyl
mercaptan
Styrene,
Ctr x 104
Vinyl Acetate
Ctr x 104
0.125
21.6
24
10,000
22,000
390,000
210,000
480,000
Additional Chain Transfer Processes
• Chain transfer to monomer, Ctr x 104
– Ethylene, 0.4- 4.0; Styrene, 0.3-0.6
Vinyl acetate, 1.75-2.8
Vinyl chloride, 10.8-16
Allyl systems, 50-100
Chain transfer to polymer--branching
Polyethylene
Vinyl acetate
Vinyl chloride
Transfer to Polymer
• Polyethylene branching
Long branches
H
H
ktr
ka
M
M
Short branches
H H
H
M
H
H
M
Inhibition of Radical Polymerization
• Must stop oxygen- and carbon centered
radicals
Radicals generated by auto oxidation
RH + O2
RH + HOO
R + O2
ROO + RH
ROOH
R
+ HOO
R
+
HOOH
ROO
ROOH + R
RO +
OH
May be metal catalyzed
•Oxygen centered radicals stopped by hydrogen transfer
Carbon centered radicals stopped by addition
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
Trapping Oxygen Centered Radicals
O
OH
RO
+ ROH
BHT
O
O
RO
OR
Trapping carbon centered radicals
• Carbon centered radicals stopped by addition
to oxygen or carbon
O
O
R.
R
O
O
Benzoquinone
O
O
R
O
R
Tautomerize
HO
R
H
R
O
R H
+
O
O
O
R
Typical Inhibitors
OH
OH
OH
OH
OCH3
Monomethylhydroquinone, MEHQ
3,5-ditert-butyl catechol
Cl
Cl
O
O
O
Benzoquinone
O2
O
Cl
FeCl3
OH
HQ
Cl
CuCl2
Chloranil
S
Stable Radical Inhibitors
O2N
N N
NO2
N
O
O2N
TEMPO
Diphenylpicrylhydrazyl, DPPH
N N
O
N
O
Galvanoxyl
N
Triphenylverdazyl