Free Radical Initiators

Chemistry

Chemical Synthesis

Biocompatible/Biodegradable

Display and Optoelectronics

Energy Source Materials

Metals and Ceramic Science

Micro and Nanoelectronics

Free Radical Initiators

Stable Isotopes (ISOTEC)

FREE RADICAL INITIATORS
	Polymerization Tools

Photoinitiators

Thermal Initiators

Initiator/Stabilizer FAQs

Photoinitiators: Classification

A photoinitiator is a compound especially aded to a formulation to convert absorbed light energy, UV or visible light, into chemical energy in the form of initiating species, viz, free radicals or cations. Based on the mechanism by which intiating radicals are formed, photoinitiators are generally divided into two classes:

Type I photoinitiators undergo a unimolecular bond cleavage upon irradiation to yield free radicals.
Type II photoinitiators undergo a biomolecular reaction where the excited state of the photoinitiator interacts with a second molecule (a coinitiator) to generate free radicals.

UV photointiators of both Type I and Type II are available. However, visible light photoinitiators belong almost exclusively to the Type II class of photoinitiators. Table I summarizes the various classes of available Type I and Type II photoinitiators and their common applications.

Table I: Common Applications for Different Classes of Photoinitiators

	UV-Photoinitiators								Visible Photoinitiators
	Type I Photoinitiators						Type II		Visible Photoinitiators
Photoinitiator Class	Benzoin ethers	Benzil ketals	α-Dialkoxy-aceto-phenones	α-Hydroxy-alkyl-phenones	α-Amino- alkyl-phenones	Acyl-phosphine oxides	Benzo-phenones/ amines	Thio-xanthones/ amines	Titanocenes
Application	Benzoin ethers	Benzil ketals	α-Dialkoxy-aceto-phenones	α-Hydroxy-alkyl-phenones	α-Amino- alkyl-phenones	Acyl-phosphine oxides	Benzo-phenones/ amines	Thio-xanthones/ amines	Titanocenes
Wood coatings (polyester)	x	x	x			x
Paper coatings				x			x
Clear coatings (on metal, wood, plastic)		x	x	x	x²
Printing Plates		x			x	x			x
Offset inks				x¹	x		x	x
Screen inks					x³		x	x
Pigmented coatings					x			x
White lacquers					x⁴	x
Photo resists					x³			x	x
(1) In combination with benzophenone. (2) Only systems of high reactivty. (3) Partially in combination with thioxanthones. (4) Thin layers only. This chart is reprinted with permission from SITA Technology Ltd.

Photoinitiators

Product #

Product Name

Add to Cart

A10701

Acetophenone ReagentPlus, 99%

415952

Diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide 97%

A88409

4,4′-Dimethoxybenzoin 95%

A90004

Anthraquinone 97%

123242

Anthraquinone-2-sulfonic acid Sodium salt 97%

119318

Benzene-chromium(0) tricarbonyl 98%

B5151

4-(Boc-aminomethyl)phenyl isothiocyanate ~95%

B5151

Benzil 98%

399396

Benzoin purified by sublimation, ≥99.5%

172006

Benzoin ethyl ether 99%

195782

Benzoin isobutyl ether technical grade, 90%

B8703

Benzoin methyl ether 96%

B9300

Benzophenone ReagentPlus, 99%

B9300

Benzoic acid meets USP testing specifications

405620

Benzophenone/1-hydroxycyclohexyl phenyl ketone, 50/50 blend

262463

Benzophenone-3,3′,4,4′-tetracarboxylic dianhydride 98%, purified by sublimation

B12601

4-Benzoylbiphenyl 99%

405647

2-Benzyl-2-(dimethylamino)-4′-morpholinobutyrophenone 97%

160326

4,4′-Bis(diethylamino)benzophenone ≥99%

147834

Michler’s ketone 98%

124893

(±)-Camphorquinone 97%

C72404

2-Chlorothioxanthen-9-one 98%

D31737

5-Dibenzosuberenone 97%

227102

2,2-Diethoxyacetophenone >95%

D110507

4,4′-Dihydroxybenzophenone 99%

196118

2,2-Dimethoxy-2-phenylacetophenone 99%

149349

4-(Dimethylamino)benzophenone 98%

146706

4,4′-Dimethylbenzil 97%

D149675

3,4-Dimethylbenzophenone 99%

405663

Diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide/2-hydroxy-2-methylpropiophenone, blend

275719

4′-Ethoxyacetophenone 98%

E12206

2-Ethylanthraquinone ≥97%

F408

Ferrocene 98%

328103

3′-Hydroxyacetophenone ≥99%

278564

4′-Hydroxyacetophenone 99%

220434

3-Hydroxybenzophenone 99%

H20202

4-Hydroxybenzophenone 98%

405612

1-Hydroxycyclohexyl phenyl ketone 99%

405655

2-Hydroxy-2-methylpropiophenone 97%

157538

2-Methylbenzophenone 98%

198056

3-Methylbenzophenone 99%

M30507

Methyl benzoylformate 98%

405639

2-Methyl-4′-(methylthio)-2-morpholinopropiophenone 98%

156507

9,10-Phenanthrenequinone ≥99%

290742

4′-Phenoxyacetophenone 98%

T34002

Thioxanthen-9-one 97%

407216

Triarylsulfonium hexafluorophosphate salts, mixed 50% in propylene carbonate

405736

3-Mercapto-1-propanol 95%

447528

11-Mercapto-1-undecanol 97%

328375

1-Mercapto-2-propanol 95%

264792

3-Mercapto-2-butanol, mixture of isomers 97%

Thermal Initiators

Product #

Product Name

Solvent

T(°C)

k_d(s^-1)

10th Half-life °C(Solvent)

Add to Cart

118168

4,4′-Azobis(4-cyanovaleric acid) ≥75%

Acetone
Water
Water

70
69
80

4.6x10^-5
1.9x10^-5
9.0x10^-5

69 (water)

380210

1,1′-Azobis(cyclohexanecarbonitrile) 98%

Toluene

80
95
102

6.5x10^-5
5.4x10^-5
1.3x10^-4

88 (toluene)

441090

2,2′-Azobis(2-methylpropionitrile) 98%

Benzene

50
70
100

2.2x10^-6
3.2x10^-5
1.5x10^-3

65 (toluene)

179981

Benzoyl peroxide reagent grade, 97%

Benzene

60
78
100

2.0x10^-6
2.3x10^-5
5.0x10^-4

70 (benzene)

441694

2,2-Bis(tert-butylperoxy)butane Solution 50 wt. % in mineral oil

100 (benzene)

388092

2,5-Bis(tert-butylperoxy)-2,5-dimethylhexane technical grade, 90%

Benzene

1.9x10^-5

120 (benzene)

441716

Bis[1-(tert-butylperoxy)-1-methylethyl]benzene 96%

115
145

1.1x10^-5
4.7x10^-4

115 (benzene)

416665

tert-Butyl hydroperoxide Solution 5.0-6.0 M in decane

Benzene

130
160
170
183

3x10^-7
6.6x10^-6
2.0x10^-5
3.1x10^-5

170 (benzene)

388076

tert-Butyl peracetate Solution 50 wt. % in odorless mineral spirits

Benzene

85
100
130

1.2x10^-6
1.5x10^-5
5.7x10^-4

100 (benzene)

168521

tert-Butyl peroxide 98%

Benzene

80
100
130

7.8x10^-8
8.8x10^-7
3.0x10^-5

125 (benzene)

159042

tert-Butyl peroxybenzoate 98%

Benzene

100
130

1.1x10^-5
3.5x10^-4

103 (benzene)

247502

Cumene hydroperoxide technical grade, 80%

Benzene

115
145

4.0x10^-5
6.6x10^-4

135 (toluene)

329541

Dicumyl peroxide 98%

Benzene

115 (benzene)

290785

Lauroyl peroxide 97%

Benzene

40
60
85

4.9x10^-8
9.2x10^-7
3.8x10^-5

65 (benzene)

269336

Peracetic acid Solution 32 wt. % in dilute acetic acid

135 (toluene)

216224

Potassium persulfate ACS reagent, ≥99.0%

Water
0.1M NaOH

80
50
60
80
90

6.9x10^-8
9.5x10^-7
3.2x10^-6
9.2x10^-5
3.5x10^-4

60 (H₂O)
70 (0.1M NaOH)

Initiator/Stabilizer FAQ

Dr. S.S. Newaz

Polyorganix Inc.
Houston, TX

Q. How does one choose the appropriate initiator?

In a free radical addition polymerization, the choice of polymerization initiator depends mainly on two factors: a) its solubility and b) its decomposition temperature. If the polymerization is performed in an organic solvent, then the initiator should be soluble in that solvent and the decomposition temperature of the initiator must be at or below the boiling point of the solvent. Commonly, 2,2′-Azobis(2-methylpropionitrile) (441090) and 2-(4-Biphenyl)-5-phenyloxazole (216984) suit these requirements. If the desired polymerization occurs at or below 20 °C, then special, low temperature free radical initiators need to be used. Various AZO- type initiators can be chosen for range of decomposition temperature requirement.

For emulsion polymerization or polymerization in an aqueous system, a water soluble initiator like potassium persulfate (379824) or an organic, water-soluble initiator 4,4′-Azobis(4-cyanovaleric acid) (118168) would be suitable.

Q. How does one determine the reactivity of monomer?

Determination of monomer reactivity is not always obvious or straightforward. Researchers rely on their experience and published data on individual monomers. In general, extent of conjugation in the molecular structure can be viewed as indicative of its tendency to form the initial free radical required for propagating a free radical polymerization. Usually, a more conjugated system is more likely to undergo the free radical polymerization.

Q. When is it necessary to remove a stabilizer prior to polymerization and how does one do so?

Usually, it is not necessary to remove stabilizers. They are typically present in ppm level and the use of a free radical initiator at the polymerization temperature will overwhelm the effect of the stabilizer. In worst-case scenarios, one may need to add extra amounts of initiator to sustain an acceptable polymerization rate. In most cases, once a polymerization initiates, the rate can be sustained without much difficulty. If, however, it is absolutely necessary to remove the stabilizer, column chromatography is the preferred method.

Q. How does one remove residual initiator, stabilizer, and/or unreacted monomer after polymerization?

It is a common practice to dissolve the polymers in a solvent prior to end use, followed by precipitating the polymer using a co-solvent. Usually the residual initiators and stabilizers will remain in solution and the polymers will separate out as a solid (powder, gum or fibers). This process may be repeated until desirable polymer characteristics are obtained. This fractional precipitation is also effective in removing lower molecular weight polymers, resulting in narrower molecular weight distribution – of course accompanied with a loss of yield. Typical solvent/co-solvent pairs could be: Toluene/hexane, toluene/methanol, THF/water, etc.

FREE RADICAL INITIATORS

Photoinitiators: Classification

Dr. S.S. Newaz