Applications: Free Radical Initiators
Thermal Initiators: Decomposition Rate and Half-Life
Ideally, a thermal free radical initiator should be relatively
stable at room temperature but should decompose rapidly
enough at the polymer-processing temperature to ensure a
practical reaction rate. In addition to temperature, the
decomposition rate (kd) of the initiator will depend on the
solvent/monomer system used. The confining effect of
solvent molecules (the cage effect) causes secondary
“wastage” reactions including recombination of radicals to
regenerate the initiator. The cage effect becomes more
significant as viscosity increases.
Assuming first order decomposition kinetics, which is true
for most free radical organic initiators, the half-life (t1/2) is
related to the initiator decomposition rate (kd) as follows:
t1/2 = ln2/kd
Table I lists the decomposition rate (kd) data1 for several
commonly used free radical initiators, at specific temperatures and solvents. To further assist you in your selection of
a thermal initiator, the temperature corresponding to a 10
hour half-life in a specific solvent is also shown. Fig. 1
relates the initiator decomposition rate (kd) in s-1 to the
half-life for a broad range of kd values, including those found
in Table I.
The most important indicator of activity of an initiator is its
half-life (t1/2). It is the time required to reduce the original
initator content of a solution by 50%, at a given
temperature.
Fig 1: Relationship of Half Life (t F ) to Decomposition Rate (k d).
10 = k
70s
20h
t1/2=85d
120m
7s
0.07s
12m
8.5d
0.75s
9
80s
20d
10d
24h
8
7
1s
10s
6
200m
20m
120s
0.12s
5
40h
200d
20d
4
30m
5h
200s
60h
20s
3
40m
1y
0.25s
30s
40d
2
10h
k x 108
80m
60d
x 107
x 106
0.35s
40s
120h
2y
3s
x 105
x 104
5s
10m
0.5s
1
x 103
x 102
x 101
x 100
This chart is reprinted with permission from Wiley Interscience.
How to read the chart:
See circled example: Half lives are to
the left of each vertical line.
A half life of 200s has a rate constant
of 3.4 x 10-3
a
Key:
y =
d =
h =
m =
s =
years
days
hours
minutes
seconds
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Applications: Free Radical Initiators
Thermal Initiators: Decomposition Rate and Half-Life (continued)
Table I: Decompositon Rates and 10-Hour Half-lif e Temperatures of Common Thermal Initiators
(arranged alphabetically by initiator)
Aldrich
Cat. No.
Initiator
Solvent
T(°C)
kd(s-1)
70
69
80
80
95
102
50
70
100
60
78
100
4.6 x 10-5
1.9 x 10-5
9.0 x 10-5
6.5 x 10-6
5.4 x 10-5
1.3 x 10-4
2.2 x 10-6
3.2 x 10-5
1.5 X 10-3
2.0 x 10-6
2.3 x 10-5
5.0 x 10-4
93
115
145
1.9 x 10-5
1.1 x 10-5
4.7 x 10-4
441465
118168
tert-Amyl peroxybenzoate
4,4-Azobis(4-cyanovaleric acid)
380210
1,1'-Azobis(cyclohexanecarbonitrile)
Acetone
Water
Water
Toluene
441090
2,2'-Azobisisobutyronitrile (AIBN)
Benzene
179981
Benzoyl peroxide2
Benzene
441694
388149
388092
416665
2,2-Bis(tert-butylperoxy)butane
1,1-Bis(tert-butylperoxy)cyclohexane
2,5-Bis(tert-butylperoxy)-2,5dimethylhexane
2,5-Bis(tert-Butylperoxy)2,5-dimethyl-3-hexyne
Bis(1-(tert-butylperoxy)-1methylethyl)benzene
1,1-Bis(tert-butylperoxy)-3,3,5trimethylcyclohexane
tert-Butyl hydroperoxide
Benzene
388076
tert-Butyl peracetate
Benzene
168521
tert-Butyl peroxide
159042
tert-Butyl peroxybenzoate
Benzene
Benzene
Benzene
Benzene
441473
247502
tert-Butylperoxy isopropyl carbonate
Cumene hydroperoxide
Benzene
289086
329541
290785
Cyclohexanone peroxide
Dicumyl peroxide
Lauroyl peroxide
Benzene
Benzene
Benzene
441821
269336
216224
2,4- Pentanedione peroxide
Peracetic acid
Potassium persulfate3
329533
441716
388084
10h Half-life °C (Solvent)
99 (benzene)
69 (water)
88 (toluene)
65 (toluene)
70 (benzene)
100 (benzene)
Benzene
Benzene
120 (benzene)
125 (benzene)
115 (benzene)
85 (dibutyl phthalate)
Water
0.1M NaOH
130
160
170
183
85
100
130
80
100
130
100
130
3 x 10-7
6.6 x 10-6
2.0 x 10-5
3.1 x 10-5
1.2 x 10-6
1.5 x 10-5
5.7 x 10-4
7.8 x 10-8
8.8 x 10-7
3.0 x 10-5
1.1 x 10-5
3.5 x 10-4
115
145
4.0 x 10-7
6.6 x 10-6
40
60
85
80
50
60
80
90
4.9 x 10-7
9.2 x 10-6
3.8 x 10-4
6.9 x 10-5
9.5 x 10-7
3.2 x 10-6
9.2 x 10-5
3.5 x 10-4
170 (benzene)
100 (benzene)
125 (benzene)
103 (benzene)
98 (aliphatic hydrocarbons)
135 (toluene)
90 (benzene)
115 (benzene)
65 (benzene)
125 (triethyl phosphate)
135 (toluene)
60 (H2O)
70 (0.1M NaOH)
”Polymer Handbook”, Eds. Brandrup, J; Immergut, E.H.; Grulke, E.A., 4th Edition, John Wiley, New York, 1999, II/2-69; Aldrich Catalog No. Z412473.
1
2
Amines can significantly increase the decomposition rates of peroxides, e.g., addition of N,N-dimethyl aniline to benzoyl peroxide causes the latter to
decompose rapidly at room temperature.
3
Persulfate decomposition is pH dependent.
4
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a