CA1080241A - Photoinitiators - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Halogen onium salts, and onium salts of Group VA and V1A elements having an MF6-anion, where M is selected from P, As and Sb, have been found to exhibit unusual activity under ultraviolet light. These onium salts can be employed as cationic photoinitiators when used with a variety of organic resins and cyclic organic compounds.

Description

10~0;~41 The present invention relates to onium salt photoini-tiators of halogen, Group Va and Vla elernents having an MF6 anion, where M is an element selected from P, As and Sb.
The photoinitiator compositions of the present invention can be used in combination with various organic resins, such as epoxy resins, to produce UW curable com-positions. The photoinitiators of the present invention are included by the formula (1) Y (MF6) where M is an element selected from P, As and Sb, and Y
is a cation selected from ~(R)a(R )b Q ~ :

~R)C(R )d(R3)e X~
' ~ .
~R) f(R ) (R )h Z ~ ' 10 ~ RD-7735 where R is a monovalent aromatic organic radical, R is a divalent aromatic organic radical, R2 is a monovalent organic aliphatic radical selected from alkyl, cycloalkyl and substituted alkyl, R3 is a polyvalent organic radical forming a heterocyclic or fused ring structure selected from aliphatic radicals and aromatic radicals, R4 is a monovalent organic aliphatic radical selected from alkyl, alkoxy, cycloalkyl and substituted derivatives thereof, R5 is a pol~valent organic radical forming an aromatic heterocyclic or fused ring structure with Z, Q is a halogen radical such as I, Br, Cl, etc., X
is a Group Vla element selected from sulfur, selenium and tellurium, Z is a Group Va element selected from ~, P, As, Sb and Bi, a is a whole number equal to 0 or 2, b is a whole number equal to 0 or 1, the sum of a + b is equal to 2 or the valence of Q, c is a whole number equal to 0 or 3, d is a whole number equal to 0 to 2 inclusive, e is a whole number equal to 0 or 1, where the sum of c + d + e times the valency of the radical associated therewith is equal to 3;
f is a whole number equal to 0 to 4 inclusive, g is a whole number equal to 0 to 2 inclusive, and h is a whole number equal to 0 to 2 inclusive, and the sum of f + g + h times the valency of the radical associated therewith is equal to 4.
Radicals included by R can be the same or different, aromatic carbocyclic or heterocyclic radical having from 6 ~O~Z41 to 20 carbon atoms, which can be substituted with from 1 to 4 monovalent radicals selected from C(l 8) alkoxy, C(l 8) alkyl, nitro, chloro, etc., R is more particularly phenyl, chlorophenyl, nitrophenyl, methoxyphenyl, pyridyl, etc.
Radicals included by Rl are divalent radicals such as ~CH ) R2 radicals include C(l 8) alkyl such as methyl, ethyl, etc., substituted alkyl such as -C2H4OCH3, -CH2COOC2H5, -CH2COCH3, etc. R radicals include such structures as:

J ' ~OJ

~, 13 . ~ . etc /
., ' ~
R4 radicals include C(1_8) alkyl, C(3_8) y stituted alkyl such as haloalkyl, for example, chloroethyl;
alkoxy such as OCH2C6H5 and OCH3; alkoxyalkyl such as -C2H4OCH3 , etc Radicals included by R5 are, for example, ~ 3 lV8l)241 RD- 773s X~ ' ~ <Q~' R'-N
N ~ N

R' where Q' is selected from 0, CH2, N, R and S; Z is selected : R' from -O-, -S- and -N-, and R' is a monovalent radical selected from hydrogen and hydrocarbon.
Halonium salts included by Formula 1 are, for example, CH3 C H5 ~) _ [~ _ : I PF6 ~ AsF6 ~' ~

' 1080Z4~

PF6 ~ SbF6

2 ~) AsF6 Group VIa onium salts included by Formula 1 are, for example, : ~H3 H3 HO ~ ~ AsF6

3 H3 ~ C-CH2- ~ PF6-02N{~C-CH2-~ A9F6-Br ~ C-CH2- ~ 6 <~ S bF6 , ~

~ -C-CH2S O PF ~

~ ~ A9F6 SbF6 , etc.

_5_ ~08(~Z~l Group Va onium salts included by Formula 1 are, for example, L ~ 2 C `\ 1 SbF6 ~ ~ N-CH2C ~ I PF-N -CH2C ~ AsF6-H ~ ~ As ~ ; ~ ~ 3 r ~ ¦ 6 L CH -C ~
+

P-CH--C-(~ I AsF ~

~(( ~ CH 2--c o o c 2 H~3 PF 6 As shown in my U.S. Patent 3,981,897, dated September ~
21, 1976, and assigned to the same assignee as the present inven-tion, where Y in Formula 1 contains a Q radical, the photoinitia~r can be made by effecting contact under aqueous conditions between an arylhaloniumbisulfate and the corresponding hexafluoro acid -~ -- 6 --~08~4~

or salt, s~ch as yl M F6, where yl can be hydrogen, an alk~li metal ion, alkaline earth metal ion or transition metal ion.
In additlon to the aboYe-described metathesis for making the corresponding halonium salts, the halonium salts of the present invention, also can be prepared by using silver compounds, such as silver oxide, or silver tetra-fluoroborate, which were reacted with the appropriate diarylhalonium salt, as shown by M. C Ca~erio et al., J.
Am Chem. Soc 81, 336 (19S9) or M C. Beringer et al., J Am Chem. Soc 81, 342 (1959). Methods for making Group VIa compounds, such as sulfonium, selenium and tellurium compounds, where Y of Formula 1 contains an X radical can be made by procedures shown in J W. Knapczyk and W E
McEwen, J Am Chem. Soc., 91 145, (1969); A L Maycock and G A Berchtold, J Org Chem., 35 No 8,2532 (1970);
H. M Pitt, U S Patent 2,807,648, E Goethals and P.
De Radæetzky, Bul. Soc Chim Belg , 73 546 ~1964); H. M.
Leichester and F. W. Bergstrom, J Am Chem. Soc , 51 3587 (1929), etc.
Among the procedures which can be used to make Group Va onium salts, arsonium, antlmonium and bimuthonium salt, where Y in Formula 1 is a Z radical can be found in Goerdeler, Methoden der Organishen Chimie 1l/2 , 591-640 (1958) and K Sasse, idid, 12/1 79-112 (1963).

108~)2~

The fol~owing examples are given by way of illu9-tration and not by way of limitation. All parts are by weight.
Example 1 There was added a cooled solution of about 100 ml of acetic ac i d and 70 ml of concentrated sulfuric acid to a suspension of 100 g of potassium iodate in 100 ml of acetic anhydride and 90 ml of benzene During the addition, the mixture was stirred and maintained below 5C. When the addition was complete, the reaction mixture was allowed to warm to room temperature and stirred for 48 hours There was then added 400 ml of distilled water.
The aqueous portion of the reaction mixture was extracted three times with diethyl ether and petroleum ether to re-move unreacted organic materials A pale yellow crystalline product formed upon addition of ammonium chloride to the aqueous reaction mixture There was obtained a 48% yield of diphenyliodonium chloride having a m.p. of 180-185C.
After recrystallization, the pure salt had a m.p. of 228-- A photoinitiator was prepared within the scope of Formula 1, having a Q containing Y cation as follows:
A mixture of 20 g of moist, freshly prepared Ag20, 10 ml of water and 31.7 g of diphenyliodonium chloride was ground together in a slurry. The wet mixture was filtered and washed with water to produce 360 ml of filtrflte. The filtrate was cooled until a substantial amount of the solu-tion had frozen. 'rhere was slowly added 25 ml 45-50% HBF4 cooled to -15C, The cold solution was stirred and allowed to warm to room temperature, A white crystalline solid separated and was collected by filtration, There was obtained a 60% yield of diphenyliodonium fluoroborate, m,p, 136C when the solid was dried overnight in vacuo at 60C, Replacement in the above procedure of HBF4 with HPF6 and HSbF6, yielded the corresponding diphenyliodonium hexafluorophosphate mp, 138-141C, and diphenyliodonium hexafluoroantimonate mp 58-57C, The salts were dissolved in acetonitrile (3 g salt to 10 ml acetonitrile) and then added to 4-vinyl-cyclohexene dioxide such that there was present 3% by weight of the salt, The solutions were coated as 3 mil films onto glass slides and exposed to W irradiation from a GE H3T7 lamp at a distance of 6 inches, The minimum time required to produce a tack-free film was recorded as the cure time, ~ 20 By tack free is meant that the film was no longer tacky - and an imprint was not left when a thumb was impressed on its surface, Under these conditions, the following results were recorded:

Salt Cure Time (sec.) Clearly, those salts po~sessing the MF6 anion were much faster than the salts having the BF4 anion.
Example 2 A solution of about 200 parts of sulfuric acid in about 300 parts of acetic acid was added at a temperature between 0 to 3C to a mixture,while it was being agitated, of 200 parts of potassium iodate, about 300 parts of toluene, about 900 parts of acetic acid, and about 400 parts of acetic anhydride. The mixture was then stirred for 11 hours after all of the sulfuric acid and the acetic acid had been added.
The resulting inorganic salts were removed by filtration and then washed with a small amount of cold glacial acetic acid. A pale yellow solution was obtained which was diluted to twice its volume with water and extracted three times with ether. A small amount (0~3 part) of sodium sulfide was added as a reducing agent.
Based on method of preparation, there was obtained a quantitative yield of 4,4'-dimethyldiphenyliodonium bisulfate. A slightly warm solution of substantially equal RD- 773s molar amounts of 4,4'-dimethyldiphenyliodonium bisulfate and potassium hexafluoroarsenate was allowed to cool.
There was obtained a white crystalline deposit. The pro-duct was filtered and washed with distilled wa~er. A second crop of crystals was obtained on further standing. After the crystals were dried overnight, there was obtained 27 parts of a product having a melting point of 148-152C.
Recrystalli~ation of the product from a water ethanol mix-ture resulted in a product having a melting point of 163-166C Based on method of preparation and NMR spectra and elemental analysis for C14H14IAsF6 calculated: percent C, 33 74; percent H, 2 81; percent As, 15.06, found: percent C, 33 70; percent H, 2.92; percent As, 15.28, the product was 4,4'-dimethyldiphenyliodonium hexofluoroarsenate By a similar procedure, treatment of 4,4'-dimethyldiphenyliodonium bisulfate with KSbF6 and KPF6 resulted in 4,4'-dimethyldiphenyliodonium hexafluoro-antimonate m.p. 75-80C and 4,4'-dimethyldiphenyliodonium hexafluorophosphate mp 169-172C.

4,4-dimethyldiphenyliodonium tetrafluoroborate was prepared by treatment of the 4,4'-dimethyldiphenylio-donium bisulfate with ammonium chloride in water and isolat-ing the resulting 4,4'-dimethyldiphenyliodonium chloride.
The dry chloride salt (34.5 g) was slurried together with 20 g freshly prepared silver oxide and 10 ml water. The mixture was then filtered and washed to produce 360 ml filtrate The filtrate was cooled until nearly the entire solution had been frozen, and then a cold solution of 25 ml 45-50% HBF4 was added. The mixture was stirred snd 810wly allowed to warm to room temperature. The white crystalline product which separated was 4,4'-dimethyldiphenyliodonium fluoroborate mp 95-100C
A comparative study of the cure ratio was performed on 3% of each of the salts dissolved directly in 4-vinyl-cyclohexene dioxide, as described in Example 1. The results obtained were a~ follows:

Salt Cure Time (~ec.
H3 ~ 1 BF4 60 " AsF6 5 : :
" SbF6 3 The above results chow that the MF6 salts provided unexpected results over the prior art BF4 salt.

Example 3 Triphenylselenonlum chloride was prepared accord-ing to the procedure of H. M Leicester and F. W Bergstrom, J Am. Chem Soc., 51 3587 (1929) starting with diphenyl selenide. The corresponding fluoroborate, hexafluoroarsenate -1~-and hexafluoroantimonate salts were prepared by adding sodium hexafluoroarsenate, sodium tetrafluoroborate or potassium hexafluoroantimonate to an aqueous solution of triphenylselenonium chloride. The products which contained Y radicals as shown in Formula l,where Y was an X contain-ing cation, were white crystalline solids which were dried in vacuo.
Three percent solutions of the above salts in 4-vinylcyclohexene dioxide were cured as 2 mil films at a distance of six inches from a GE H3T7 lamp. The following cure times were observed:
Salt Cure Time ( 6 5)3 4 10 sec.

( 6 5)3 s 6 5 sec (C6H5)3Se sbF63 sec.

The above results show that the hexafluoro salt is a superior photosensitizer with respect to cure time as compared to the tetrafluoroborate salt.

Example 4 Additional curable compositions were prepared using the epoxy resin mixture of Example 1 and a variety of phosphonium salt~ as shown as follows where cation is the organic portion corresponding to Y of Formula 1, where Y contains a Z radi~al, anion is the Lewis Acid por~ion, 1~8024~

m.p. is the melting point of the crystalline onium salt and "cure tLme" is as previously defined.

~ .

.. . . . .

.

.
- .

'' ' :' , .

`'` ~. .

108VZ~

RD- 773s U~
a cr~ O

~) ,~ r~ c~r~ ~ C`
c~ ~ ~a~ o o ~
O o ~ ~ .~ ~ o C~l U~ I I ~ I I ~ I
. ~ U~ O~ ~ ~ I`
o ~
a ol, , ' ~ ' ~D, C ~ ~ D a~
¢ ~ ' ¢ P~ ~ ~

'1 o I o ~ 0 ~ I
i~ ~
V ~, +p~ +~ +~, +~, +~ _ ~ ~ ~

1--1 H H ~ ~ H H ~
H H ~ 1 H
~>

108~Z4~

Example 5 In accordance with the procedure of Example 1, additional iodonium salts were prepared as sh~wn by the ~ I J) CH3 (~ / O
following table, where t is -C-CH3 and

5 3 i~ ~ H . .

... . .
..

!

o o. oo.~ ~
~ '`' . o ~
U~ ~ ~D
a~ o ~ _l ~ ~
a~ C a~ ~ o u, ~ ~ . ~; ~ ~ ~ . ~ .~ . . ~n . .
~ U~ U~ ~ _~ o o~ ~ ~ ~ e C~ C`~ C~l _ X ~ ~ ~ ~ ~ ~:
~-n _~ ~ ~ oo ~ ~ U~ ~ C~l ~ U~
'D ~ C~ ~ ~~ C~ I~ ~o ~ ~ C~ ~ ~ C~ ~ ~ ~ o E3 ~ `J `;t ~ ~ .. ~ ~ C`~
~ C U~ U ~ ~ ~
~ ~C ~ ~ ~ tO~

.` X O O C~ O O O
~ O I~ C~J O oo~ ~S) X O O C~l _l O~ ~I
~ CO~ 0 ~ t'O CO ~0 ct: oC') r` t` Ir~ ~ u ~ ~ l l l ll l H ~) ~O ~ ~D ~
O . -I _1 ~ ~ ~1 a ~ , o ~0 ~'0 1:~ ~`S) ~o t~
~C P~ ¢ ~ P~ ~1: ¢

,~ + I 1 + I + I ~ I + I
~ ~1 ~ ~ ~ ~

C.?

11~8(3'~1 ~o V~ ~00 C`J~ ¢
.,, ~ ~ ~ o~ U~
o o I~ ~ C~ ~ ~ ~ ~ u~ U~
~:: C`J ~ O r` ~ o I Z;0~ 0~ ~ ~ ~t O I~ U~ l oo O r~ oo l ~ O
E3 t) . . ~ ~) O O ~ u~ ~
.... .... .... ....
U ~ U
C ~ ~ ~ ~ ~ ~
o ~ o ~ o C~ o X ô ô ô ôô
o o o ~ o o~
_ ~ ô ~ , u~ r~
X _, _, ~_ ~ `_ _, ~ ~o Ul ~ U~

_ U~ o C~l ~ ~ ~o U~
- ~ ~ ~ ~
~D ~o ~: ~ ~ c`l ~

r~ ~ ~ ~ k~ ~
g ;~

10802~1 The above halonium salts were found to exhibit substantially the same utillty with respect to a faster rate of cure of 4-vinylcyclohexene dioxide a~ compared to prior art photolnitiators as shown for Example 1.

ExamPle 6 Several sulfonium hexafluoroarsonium salts,.
sulfonium hexafluoroantimonate salts and selenium hexafluoro-arsonium salts, where Y in Formula 1 is an X containing cation, were prepared by adding the corresponding anion in the form of the acid or salt, such as sodium hexafluoro-arsenate to the corresponding cation structure, such as an aqueous solution of tri(3,5-dimethyl-4-hydroxy)phenyl sulfonium chloride. The procedure described by H. M
Leicecter and F, W, Bergstrom, J, Am. Chem Soc 51 3587 (1929) was employed. The following table shows the results obtained . ~ , ~ , . _ _ _ : .

~P~

' , . .

1~8l)241 U~
u~ u~ ~ ~ In .,, o~ ~ U~ U~
U~ o ~ o~
~:
o ~ a~
. ~ o~
. . U
al ~ ~ ~ a~
~1 .~................... ....
t. ~ ~ ~
~ g ~d ~
u ~o u o ô~
o o o o o o o o o o o ~ I~ U~
~ o ~ ~o ~ .
_ ~ ~ o ~ ~ ~ CS~ ~ C~l, X ~ , ~ o ~ a~ ~ ~ o ,~
X o oo 1~ u~ `D 00 ~: ~ ~ ~ C~ C~l C`J
E~
¢ C~ U~ U~
V~ o . l U~
:~ . U~
o ~:
tn o ~o ~r~
~¢ ¢ ¢

:~ ~ +U~ .

U V ~ W

1080Z4~

~ ~ ~ U~
~ ::C o a~
s ~) .... ....
t~
~aO ~

X o ~ U~ o ~ o o o o o o o~ o~ oo oo oo a~ oo ~ o~ I~ e~l r~
..
_ o~ o~C~l~ ~o~ ~o ~
X ~ ~ _,_, _,_, ~ ~, o~ o~ o oo o X o X
~: u~ u~ O ~ O ~ O
~ C~ ~ ~ C~I ~ ~ ~ ~
., ~ ~ ~ o ~ ~
~) 00 ~ ~ ~D

~ ~ o ~ ~ o :, . o~ ~ ~ ~ ~D

~ l O ~D
.~ ~ ~ ~D ~ ~

e ~

-21 _ 1080Z4~

. It was found that 4-vinylcyclohexene dioxide compositions containing the above onium salts exhibited a faster rate of cure, as compared to comparable prior art tetrafluoroborate anium salts.

Example 7 In addition to the aforementioned haloni~m salt - -and sulfonium ~alts falling within the scope of Formula 1, where Y is a Q containing and X containing cation, several phosphonium salt~ and ammonium salts were prepared by the procedure shown by J Goerdeler, Methoden der Organishen Chimie, ltl2 591-640 (1958) The latter onium salts, where Y in Formula 1 is a cation containing a Z radical are shown as follows:
.. . . .... .. . . . . _ , ', ',,.' ~ . ~ ' . ~ : .
; ~

~ .

108~)~4~ .

0~ ~ ~o (n P~ . ~ ~ P~ O
.~ ~ ~ U~ U~U~ U~
~ o o C~l ~1 ~O ~ `D 1~
~ X . . ~ ~ . .
¢ ~ ~ . .
~ o J~ ~ u~ I~ o~ co r~
~ C~ . . C~ . . C~ .
a~ a~ o~. ~ ~ o o U~ U~ U~ U~ ~ U~
.... .... ....
tJ ~ U ~ U
,-1 ~ ~ ~
o ~J o t~ o ~ 4~ ~U

,~
.' ~d O
. C~ . _ X

U~ _` ~D I~ ~
C~ o cr~

X ~ o P~
~ l ~
~1 ~ ~D ~
~C U~

~1 -'1 ~

1~)802 ~ ~ 0~l ~ Z ~1 ~ ~ ~r ~ ~ ~ ~
~ or~ oo ~o o ~1 ~ ~ ~ ~ ~ '.
$ Ln ~D ~ ~ a~
~ ~ ~ o~i ~ .... .... ....
~,~ V~ ~,~
~, o ~, o ~, o æ~ O

æ U~
~ ~9 ~ ~ .

: ol , I~D '~
~1 ~9 108~4~ RD-7735 In addition to using the photoinitiators of the present invention for curing epoxy resins, these onium salts of halogen, Group Va and Vla elements can be used to polymerize a varity of cyclic organic compounds and cyclic organo-silicon compounds as shown in Canadian Serial Numbers 226,108, 226,107, and 266,109 filed May 2, 1975 and assigned to the same assignee as the present invention.

Claims (25)

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. Photoinitiators of the formula, Y+ (MF6)-where M is an element selected from the class consisting of P, As and Sb, and Y is a cation selected from the class consisting of , where R is a monovalent aromatic organic radical, R2 is a monovalent organic aliphatic radical selected from alkyl, cycloalkyl and substituted alkyl, R3 is a polyvalent organic radical forming a heterocyclic or fused ring structure selected from aliphatic radicals and aromatic radicals, R4 is a monovalent organic aliphatic radical selected from alkyl, alkoxy, cycloalkyl and substituted derivatives thereof, R5 is a polyvalent organic radical forming an aromatic heterocyclic or fused ring structure with Z, X is a Group V1a element selected from sulfur, selenium and tellurium, Z is a Group Va element selected from N, P, As, Sb and Bi, c is a whole number equal to 0 or 3, d is a whole number equal to 0 to 2 inclusive, e is a whole number equal to 0 or 1, where the sum c + d + e times the valence of the radical associated there-with is equal to 3; f is a whole number equal to 0 to 4 inclusive, g is a whole number equal to 0 to 2 inclusive, and h is a whole number equal to 0 to 2 inclusive, and the sum of f + g + h times the valence of the radical associated therewith is equal to 4.

2. Group VA salts of the formula, where M is an element selected from the class consisting of P, As and Sb, R is a monovalent aromatic organic radical, R4 is a monovalent organic aliphatic radical selected from alkyl, alkoxy, cycloalkyl and substituted derivatives thereof, R5 is a polyvalent organic radical forming an aromatic heterocyclic or fused ring structure with Z, Z is a Group VA element selected from N, P, As, Sb and Bi, f is a whole number equal to 0 to 4 inclusive, g is a whole number equal to 0 to 2 inclusive, and h is a whole number equal to 0 to 2 inclusive, and the sum of f + g + h times the valence of the radical associated therewith is equal to 4.

3. An ammonium salt in accordance with Claim 2, where M is phosphorus.

4. An ammonium salt in accordance with Claim 2, where M is arsenic.

5. An ammonium salt in accordance with Claim 2, where M is antimony.

6. An ammonium salt in accordance with Claim 2, selected from the formulae , , .

7. An ammonium salt in accordance with Claim 2 wherein the cationic portion thereof has the formula ,

8. A phosphonium salt in accordance with Claim 2, where M is phosphorus.

9. A phosphonium salt in accordance with Claim 2, where M is arsenic.

10. A phosphonium salt in accordance with Claim 2, where M is antimony.

11. A phosphonium salt in accordance with Claim 2, wherein the cationic portion has the formula .

12. A phosphonium salt in accordance with Claim 2, wherein the cationic portion thereof has the formula .

13. Group V1A salts of the formula, where M is an element selected from the class consisting of P, As and Sb, R is a monovalent aromatic organic radical, R2 is a monovalent organic aliphatic radical selected from alkyl, cycloalkyl and substituted alkyl, R3 is a polyvalent organic radical forming a heterocyclic or fused ring structure selected from aliphatic radicals and aromatic radicals, X is a Group V1A element selected from sulfur, selenium and tellurium, c is a whole number equal to 0 or 3, d is a whole number equal to 0 to 2 inclusive and 3 is a whole number equal to 0 to 1, where the sum of c + d + 3 times the valence of the radical associated therewith is equal to 3.

14. A sulfonium salt in accordance with Claim 13, where M is phosphorus.

15. A sulfonium salt in accordance with Claim 13, where M is arsenic.

16. A sulfonium salt in accordance with Claim 13, where M is antimony.

17. A sulfonium salt in accordance with Claim 13 having the formula .

18. A sulfonium salt in accordance with Claim 13 having the formula .

19. A sulfonium salt in accordance with Claim 13 having the formula .

20. A sulfonium salt in accordance with Claim 13 wherein the cationic portion has the formula .

21. A sulfonium salt in accordance with Claim 13 wherein the cationic portion has the formula .

22. A sulfonium salt in accordance with Claim 13 wherein the cationic portion is selection from the cations of the formulae , and .

23. A sulfonium salt in accordance with Claim 13 wherein the cationic portion has the formula .

24. A sulfonium salt in accordance with Claim 13 wherein the cation portion has the formula .

25. A sulfonium salt in accordance with Claim 20, 21 or 22 wherein the anionic portion is AsF6-.