Some studies on the photo-initiated cationic polymerisation of epoxides

Eur. Polym. J. Vol. 18, pp. 589 to 595, 1982 Printed in Great Britain.All rights reserved

0014-3057/82/070589-07503.00/0 Copyright © 1982Pergamon Press ktd

SOME STUDIES ON THE P H O T O - I N I T I A T E D CATIONIC P O L Y M E R I S A T I O N OF EPOXIDES R. S. DAVIDSONand J. W. GOODIN Department of Chemistry, The City University, Northampton Square, London EC1V 0HB (Received 10 November 1981) Abstract -Several triarylsulphonium and diaryliodonium compounds have been synthesised. Alkylarylsulphonium compounds were obtained by alkylating diaryl sulphides with trialkyloxonium salts and by reaction of diaryl sulphides with alkyl halides in the presence of silver tetrafluoroborate. Photolysis of the sulphonium salts in methanol gave diaryl sulphides and, in the case of triarylsulphonium compounds, the appropriate aromatic hydrocarbon and its methyl ether. Diaryliodonium tetrafluoroborates and pentafluorophosphates gave aryl fluorides, biaryls and aromatic hydrocarbons. The decomposition of the salts is suggested as occurring via both free radical and ionic pathways. The ability of the compounds to sensitise the polymerisation of epoxy resins was found to be dependent upon the counterion (hexafluorophosphates being more efficient than tetrafluoroborates) and upon the structure of the cation. Sensitised cationic initiated polymerisation was also investigated and it was found that both excited singlet state and triplet state sensitisers were effective.

INTRODUCTION

with triethylorthoformate and trimethylorthoformate respectively. Triethyloxonium hexafluorophosphate was found to react with diphenylsulphide to give a

It is well known that epoxides are polymerised on reaction with a Lewis acid. v ~ /

O ~

O

v

BF 3 C--C--O

e,g.

/ I X OBF 3

Considerable effort has been applied to devising photochemical systems which will produce Lewis acids suitable for initiating this type of polymerisation [1]. Systems which have been found suitable are aryl~ diazonium salts [2], diaryliodonium salts [3] and triarylsulphonium compounds [4]. Some of the advantages of photo-initiated cationic polymerisations over other u.v. curing processes are (i) the high reaction rates, which allow low viscosity monomers to be polymerised to high viscosity materials, and (ii) since cations do not react with 02, the polymerisations are not retarded by 02. Furthermore, since the polymerisation involves living polymers, polymerisation continues after the light source has been removed. Thus "post-curing" is often observed with these systems. This paper describes some approaches to the synthesis of diaryl iodonium compounds and triphenylsulphonium compounds, an investigation of the photochemistry of these compounds and an assessment of their value as photo-initiators in the u.v. curing of epoxides. (i) Synthetic aspects Trialkyloxonium salts are known to be powerful alkylating agents [-5] and recently they have been used to S-alkylate penam sulphides. Triethyl and trimethyloxonium hexafluorophosphate can be prepared in high yield by reaction of phosphorus pentafluoride

I

+ BF~

I

low yield diphenylethylsulphonium hexafluorophosphate. Thiophenol was successfully akylated with the reagent to give diethylphenylsulphonium hexafluorophosphate. Thioxanthene, thioxanthen-9-ol and thioxanthone were also alkylated with the oxonium salts. The use of the described alkyloxonium salts only allowed the preparation of S-methyl and S-ethyl compounds. Other methods of S-alkylating were therefore explored. Thiophene has been successfully methylated by a mixture of iodomethane and silver perchlorate [7]. As a result of the explosive nature of perchlorates and silver perchlorate, the reaction was re-examined using silver tetrafluoroborate [8] (Note: this material is not without risk) [9]. In principle this method can be utilised for the preparation of a wide variety of compounds. RzS + R1Hal + AgBF4---* Rz~R1BF4 + AgHal Successful alkylations of thioxanthene and thioxanthone were carried out using this method. Usually only moderate yields of the desired products were obtained and in some cases the products were found to be unstable under laboratory conditions. (ii) Photo-induced decomposition of sulphonium and iodonium salts Triarylsulphonium iodides are known to decompose on irradiation in chloroform [10]. The course of 589

590

R. S. DAVIDSONand J. W. GOODIN Table 1. Photo-induced decomposition of sulphonium salts ( ~ 2 x 10-2M) degassed methanol usinR 254 nm radiation

in

Photolysis time (hr)

Unchanged starting material (~o)

61

23.8

61

22.0

BF 4

61

15.5

Thloxanthene (83), Benzene (41) Fluorobenzene (25)

1BF4--

61

16.9

Thioxanthene (81)

PF6""

61

17.5

Thioxanthene (82)

Bg'4

61

8.2

Thioxanthene (88)

B'F4

61

23.1

Thioxanthone (75)

61

14.0

Thioxanthone (84)

Salt

Identified products and yields

(Vo)

t -

Ph3SBF4 + -

PhaSPF6

I

Benzene (12), Anisole (8), Biphenyl (1), Diphenyl sulphide (60.5), Fluorobenzene (30) Benzene (15), Anisole (8), Biphenyl (0.8), Diphenylsulphide (65.0) Fluorobenzene (35)

Ph

I

Me

I

CH2CH 3

I

CH2CH2CH 3 O

I

Me O

I

PF 6

I

CH2CH 3

the reaction appears to be determined by charge transfer from the halide ion to the sulphonium group. This pathway is not likely to be available for sulphonium compounds having non-nucleophilic counter ions such as tetrafluoroborate and hexafluorophosphate ions. The oxidation potential of these

ions is probably so high as to preclude them from participating in electron transfer reactions. The photo-induced decompositions of triarylsulphonium and alkyldiarylsulphonium salts were investigated and the results are shown in Table I. In all cases it was found that, on termination of irradiation,

591

Cationic polymerisation of epoxides Table 2. Photo-induced decomposition of diaryliodonium salts (~ 1.4 × 10-2 M) in degassed methanol using 254 n.m. radiation

Salt

Photolysis time (hr)

Products (~o)

+

Ph21 BF4

61

Benzene (68) Fluorobenzene (17) Biphenyl (10) Benzene (70) Fluorobenzene (16) Biphenyl (12)

+

Ph21 PF6

61

the inside of the reaction vessel was coated with an uncharacterised insoluble yellow film. Furthermore, the photolysis solutions were found to be acidic. Presumably, the lack of mass balance is due to formation of the yellow film. Sulphonium salts are thought to decompose by the following routes [11]. +

Ar3SX-

hv )

+

(AraSX-)*

+

(Ar3SX-)*

, Ar2S + Ar" + X-

Ar2S* + Y - - H - - ~ A r 2 ~ - - H + Y' Ar2S÷H + X- --~Ar2S + HX Ar" + Y--H--~ ArH + Y" Y--H -- solvent

In the case of methanol,

Ar2S* + CHaOH ~ Ar2~H + CH2OH

photolysis of vinyl halides [15]. The extent to which the photo-induced solvolysis occurs in relation to the mechanism involving radical cations may be subtly dependent upon solvent polarity. Undoubtedly radicals are involved, even in such polar solvents as methanol. The decomposition of the alkyl diarylsulphonium compounds via the ionic process should be more favourable than for the triarylsulphonium compounds. Indeed, the instability of S-benzylthioxanthene tetrafluoroborate and 4-benzoylbenzyl diphenylsulphonium tetrafluoroborate may well be due to the ease with which these compounds fragment to give reasonance stabilised carbocations [16]. A brief study was made of the photo-induced decomposition of diaryliodonium compounds in methanol and the results are summarised in Table 2. The photolysis led to the formation of an unidentified acid. Mechanisms put forward to explain the observed products include the following [11].

This scheme does not, however, account for all the products e.g. the formation of fluorobenzene, anisole and biphenyl. The formation of anisole suggests that the decomposition of the excited salt may give rise to aryl cations. (AraS÷X)*--~Ar2S + Ar ÷ + XAr ÷ + CHaOH--~ArOCH3 + H ÷

Ar2][X- -~ (Ar2tX)* (Ar2tX-)*--~ArI* + Ar" + XArI t + S--H--~AR][H + S" Ar]~H + X---~ ArI + HX This mechanism does not however explain the formation of fluorobenzene and biphenyl. The fluorobenzene may well arise via a photo-induced ionic process.

When X- = BF£ Ar + + BF£ --~ ArF + BF3

+_

This photo-induced ionic dissociation will be particularly favoured in a polar solvent such as methanol. Such reactions have been encountered in the photolysis of benzylammonium halides [12], benzylhalides [13] and tertiary halides [14]. Perhaps even more surprising is that vinyl cations can be generated by

(Ar2IX)*--*Arl + Ar + + X When X- = BF~ Ar + + BF2 --~ArF + BF3 The formation of biphenyl cannot have arisen via dimerisation of phenyl radicals since this, if it ever occurs, will only take place in totally inert solvent. A possible mechanism is

Ar

l~Ar

Ar + Ar 2 1+

H

H H*

Ar

1 A r - - A r + ( A r l ~°

Ar

H~

H

I+Ar Ar--Ar+ArI+H

E.P.J. 18/'7 -¢"

÷

592

R.S. DAVIDSONand J. W. GOODIN Table 3. Efficiency of iodonium and sulphonium salts to sensitise the curing of a white ink containing an epoxy resin Photoinitiator Photoinitiator Cation Diphenyliodonium Diphenyliodonium Triphenylsulphonium S-methylthioxanthonium S-methylthioxanthylium S-ethylthioxanthylium S-n-hexylthioxanthylium S-ethylthioxanthylium S-ethyl-9-ethoxythioxanthylium

The fact that iodobenzene was not found in the reaction mixture is probably due to its ready photoinduced decomposition which would account for some of the benzene produced in the reaction. ArI---, Ar" + I' Ar" + CHaOH ~ ArH + CH2OH It appears that the photodecompositions of sulphonium and iodonium tetrafluoroborates and pentaluorophosphates lead to the formation of hydrogen fluoride and Lewis acids (boron trifluoride and phosphorus pentafluoride respectively).

(iii) The use of sulphonium and iodonium tetrafluorobor-

ates and pentafluorophosphates as photo-initiators The efficiency of the salts as photo-initiators was tested in a commercial white ink containing a mixture of high and low viscosity epoxy resins (45~o), titanium dioxide (45~o) and cycloaliphatic diepoxide diluent to make the mixture up to 100~o. A study was undertaken using diphenyliodonium tetrafluoroborate, to discover the optimum photo-initiator content for the ink. This was found to be 3~o. Using this loading of initiator, a series of sulphonium and iodonium salts were tested in a white ink containing an epoxy resin (see Table 3). The results in Table 3 substantiate the view that the rate of cationic photo-initiation of epoxide polymerisation is more dependent on the nature of the anion than upon the type of cation. That hexafluorophosphates initiate curing more readily than tetrafluoro-

Anion

Lamp passes Lamp passes to to tack free through cure

BF,~ PF6 PF 6 BF~, BF2 BF~BF2 PF6 PF6

4 1-2 2 12 16 10 13 2 4

9 5 15 20 15 15 6 7

borates is in accord with previous findings 1-17,18]. Table 3 shows that there is some dependence upon the structure of the cation e.g. the thioxanthonium salt is more efficient than the thioxanthylium salt. Furthermore, the efficiency is affected by the nature of the alkyl groups for alkylthioxanthylium compounds, e.g. the S-ethyl compound is markedly more efficient than the S-methyl compound. In all cases it was found that the inks underwent considerable postcure. The latter is favoured by an increase in temperature. (iv) Photosensitised decomposition of cationic photo-

initiators Diaryliodonium and sulphonium salts absorb little radiation beyond 275 nm. Since in most commercial applications the light source is a medium pressure Hg lamp or a tungsten halogen doped lamp, it would be advantageous if the initiators could be sensitised by compounds with absorption spectra matching the output of the lamps. Crivello and Schroeter [19] have found that a number of dyes e.g. acridine orange, sensitise the decomposition of iodonium salts. Sensitisation by compounds absorbing between 300-400 nm was investigated by examining how these compounds affected the cure of a white ink. The results are shown in Tables 4, 5 and 6. In all cases the level of the sensitiser was 1~ w/w. In order to discover the optimum amount of sensitiser for effecting the most rapid speed of cure, a study was made of the curing of an ink initiated by diphenyliodonium tetrafluoroborate and sensitised by 2-ethyl-9,10-dimethoxyanthracene. It was found that a 3~o w/w loading was

Table 4. Comparison of the efficienciesof compounds to sensitise the curing of a white ink containing an epoxy resin and a diphenyliodonium pentafluorophosphate as initiator Sensitizer None Thioxanthone Anthracene 9,10-dimethylanthracene 2-ethyl-9,10-dimethoxyanthracene 2,6-dimethylnaphthalene 2,5-diphenyl-1-3-4-oxadiazole

Lamp passes Lamp passes to to tack free through cure 1-2 l 1 1 l 1 1

5 4~5 4 3 2 4-5 4

Cationic polymerisation of epoxides

593

Table 5. Comparison of the etiiciencies of compounds to sensitise the curing of a white ink containing an epoxy resin and diphenyliodonium tetrafluoroborate compound as initiator Sensitizer

Lamp passes to tack free

Lamp passes to through cure

4 4 4

9 7 5

None Anthracene 2-ethyl-9,10-dimethoxyanthracene

optimal. At higher concentrations of the sensitiser, the sensitising efficiency began to diminish. This type of effect has been previously noted [19]. Tables 4-6 show that many of the compounds were effective sensitisers for both the iodonium and sulphonium compounds (e.g. from a consideration of the number of lamp passes to effect a through cure). Most of the sensitisers exhibit fluorescence e.g. the anthracenes and in these cases the sensitisation may be occurring from the excited singlet state. In the case of the oxadiazole, which has a quantum yield of fluorescence of close to unity, the excited singlet state is the only one available for sensitisation. A brief study showed that sulphonium and iodonium compounds quench the fluorescence of 9,10-dimethylanthracene in dichloromethane at close to diffusion controlled rates (Table 7). For compounds such as 9-nitroanthracene and thioxanthone, the sensitisation possibly occurs via the triplet state since neither of the compounds fluoresce efficiently. The question arises as to whether the sensitisation involves energy or electron transfer.

For this question to be answered, it will be necessary to determine if there is any correlation between the redox properties of the sensitisers and the quantum yield for decomposition of the iodonium and sulphonium compounds.

EXPERIMENTAL

Melting points uncorrected were determined on either a Kofler Block or a Gallenkamp MFB-600. i.r. Spectra for samples as thin films or KBr discs were obtained with Perkin Elmer 237 and 457 grating spectrophotometers. 'H nmr were recorded for solutions in CDCI3 (with tetramethylsilane as a internal standard) on a Varian T60 and a Jeol PS100 spectrometer. Mass spectra were recorded using a Micromass MS 30/76 Kratos instrument fitted with a g.l.c./mass spectrometer interface, g.l.c. Was performed on a Pye-Unicam 104 and a Perkin Elmer Sigma 3 instruments. Elemental analyses were determined by using a Carlo-Erber Model 1106 C,H,N. analyser. Fluorescence spectra were recorded on a Perkin Elmer MPF-4 spectrofluorimeter.

Table 6. Comparison of the efficiencies of compounds to sensitise the curing of a white ink containing an epoxy resin and a triphenylsulphonium pentafluorophosphate compound as initiator Sensitizer None Xanthpinacol Perylene 2,6-dimethylnaphthalene 1,2-benzanthracene Anthracene 9-nitroanthracene 9,10-dimethylanthracene 2-ethyl-9,10-dimethoxyanthracene Thioxanthone 2,5-diphenyl- 1-3-4-oxadiazole

Lamp passes to tack free

Lamp passes to through cure

4 4 4 3 3 2 3 2 1 4 2

6 6 4 6 4 3 5 3 2 4 2

Table 7. Rate constants for quenching the fluorescence of 9,10-dimethyl anthracene by sulphonium and iononium salts Quencher Diphenylmethylsulphonium Triphenylsulphonium S-methylthioxanthylium Diphenyliodonium

BF2 PF6 BF~BF~-

Slope*

kq ( x 10 t°)

78.02 296.94 63.08 164.66

4.82 18.32 3.89 10.16

* Derived from a Stern-Volmer plot of Io/I against concentration of salt. Fluorescence lifetime of 9,10-dimethylanthracene taken as 16.2 n.sec. 2°

594

R.S. DAVIDSON and J. W. GOODIN

Synthesis Ethyl diphenylsulphonium hexafluorophosphate [21]. Diphenyl sulphide (1.5 g) was added to triethyloxonium hexafluorophosphate (1.65 g). The reaction vessel containing the mixture was evacuated to remove the diethylether as it was formed. After 0.5 hr, petroleum ether was added to the reaction mixture and the product filtered off. Recrystallisation from n-butanol gave colour crystals of the hexafluorophosphate (0.15 g) m.p. 216-217 3 (CDC13) 7.65 (10 Hm), 4.1 (2 Hq), 1.45 (3 Ht). Dieth ylphen ylsulphonium hexafluorophosphate [21]. Thiophenol (0.8 g) in acetonitrile (10 ml) was reacted with triethyloxonium hexaftuorophosphate (3.9g) under an atmosphere of dry N2. After 1 hr the solvent was removed under vacuum to give a red oil which was triturated with diethylether. The yellow-brown solid thus obtained was recrystallised from ethanol to give the product (0.59 g) m.p. 89-90 °, t~(CD2CI2) 7.95 (5H, broads), 3.6 (4 Hq), 1.4 (6H, t), V~x 1410, 970, 830cm -1. S-Ethylthioxanthylium hexafluorophosphate. Thioxanthene (0.5 g) was dissolved in dichloromethane (15 ml) and triethyloxonium hexafluorophosphate (0.65 g) added. The solution was stirred for 12 hr under a blanket of dry N 2. The solvent was removed in vacuo to leave an orange oil which on trituration with ether gave a solid. Recrystallisation from ethanol gave the product as a white crystalline material m.p. 118-199 °, C, 47.94; H, 4.02 C15H15F6PS requires C, 48.39%, H, 4.06~o, ~((CD3)2CO ) 7.9 (8H, m) 4.7 (2H, s) 3.9 (2H, q), 1.4 (3H, t), vm~x1400, 970, 825, 750 cm- 1. S-Ethylthioxanthonium hexafluorophosphate [21]. Thioxanthone (0.2 g) was dissolved in a mixture of acetonitrile (20 ml) and chloroform (2 ml). Triethyloxonium hexafluorophosphate (0.6 g) was added and the mixture stirred under dry N 2 for 12 hr. During the stirring the product crystallised. The product was filtered off and recrystallised (acetonitrile-chloroform) m.p. 222-222.5 °. ~((CD3)2CO), 8.3 (8H, m) 3.8 (2H,q) 1.6 (3H,t), Vmax 1680, 1580, 1440, 1320, 1150, 1106, 815, 745cm -1.

S-Ethyl-9-ethoxythioxanthytium

hexafluorophosphate.

Thioxanthen-9-ol (0.Sg) was added to triethyloxonium hexafluorophosphate (1.5g) in dichloromethane (12ml). The solution was stirred under dry N 2 for 12 hr. The solvent was removed in vacuo and the remaining solid recrystallised from ethanol to give the product (0.47g) m.p. 237-239 ° 6(CD2C12) 8.8 (2H, m), 7.8 (4H, m), 7.2 (2H, m) 4.4 (4H, m), 1.4 (6H, m), v.... 1580, 1430, 1310, 1000, 725 cm- 1. S-Methylthioxanthanthylium tetrafluoroborate. Thioxanthene (0.5 g) and iodomethane (2.5 g) were dissolved in 1,2-dichloroethane (20 ml). A solution of silver tetrafluoroborate (0.5g) in 1,2-dichloroethane (15ml) was added dropwise with stirring under N 2 in the dark. After the addition the mixture was stirred for a further 12hr. The mixture was filtered through a Celite filter and the solvent removed on a rotary evaporator to leave a yellow solid. Recrystallisation from n-butanol gave the product (0.22 g) m.p. 184°, 55.95, H 4.29 C~4H13F4BS requires C 56.03, H 4.36%, 6((CD~)2CO ) 7.8 (8H, m), 4.7 (2H, s), 3.5 (3H, s), v~x 2930, 1410, 1280, 1040, 750cm -1. S-Methylthioxanthonium tetrafluoroborate [21-1. To thioxanthone (0.5g) in 1,2-dichloroethane (25ml) was added under N 2 a 1,2-dichloroethane (10ml) solution of iodomethane (3.1 g) and silver tetrafluoroborate (0.5 g). The mixture was stirred in the dark for 24 hr. The reaction mixture was filtered through celite filter aid and the solvent removed from the filtrate under vacuum. The solid so obtained was crystallised (1,2-dichlorethane/diethylether) to give the product as white needles (0.45 g) m.p. 219-220 °. 6(CFaCO2H ), 8.3 (8H, m), 3.8 (3H, s) vm,x(mull) 1680, 1580, 1440, 1325, 1310, 1150, 1100, 810, 750cm -1.

4-Benzoylbenzyl

diphenylsulphonium

tetrafluoroborate

[21]. To 4-benzoylbenzyl diphenylsulphonium (0.2g) in 1,2-dichloroethane (10ml) was added under N2 a 1,2dichloroethane (10ml) solution of iodomethane (1.45g)

and silver tetrafluoroborate. The mixture was stirred under N 2 in the dark for 12 hr and then filtered through celite filter aid. The solvent was removed under vacuum to give a solid which was recrystallised (1,2-dichloroethane/diethylether) to give the product (0.08 g) 6(CDC13) 7.8 (14H, m), 3.8 (3H, s). The compound decomposes in less than 1 hr at room temperature. S-Benzylthioxanthonium tetrafluoroborate [21]. Thioxanthone (0.2g) was dissolved in dichloromethane (25ml). Benzyl bromide (1.71 g) and silver tetratluoroborate (0.22 g) dissolved in dichloromethane (1 aml) were added with stirring in the dark. Stirring was continued for a further 12 hr. After filtering through Celite filter aid, the solvent was removed under vacuum. The remaining solid was recrystallised (from dichloromethane/diethylether) to give the product (0.11 g) m.p. 215-216 °. 6(CDC13), 8.6(2H, m) 7.4 (13 H, m) V,~x 2800, 1630, 1585, 1430, 1320, 1160, 1100, 935, 735 cm- 1. The compound was found to decompose in less than 1 hr in the presence of light and air at room temperature. S-n-Butyl thioxanthylium tetrafluoroborate [21]. Thioxanthene (0.5 g) was dissolved in 1,2-dichloroethane (20 ml). A suspension of 1-bromobutane (3.43 g) and silver tetrafluoroborate (0.51g) in 1,2-dichloroethane (15ml) was added with stirring, under N 2 in the dark. The mixture was filtered through Celite filter aid and the solvent removed in oacuo to leave a red acid. Triturated with petroleum ether removed unreacted thioxanthene and the residue was recrystallised (from n-butanol) to give the product (0.05 g) m.p. 179-180 °, ~(CDCI3), 7.7 (8H, m), 4.5 (2H, s), 3.6 (2H, t), 1.4 (4H, m), 1.1 (3H, t) v,,ox 2970, 1640, 1585, 1430, 1315, 1170, 730 cm- i. S-n-Hexylthioxanthylium tetrafluoroborate [21]. This material was prepared in a similar way to the S-n-butyl compound from thioxanthene (0.5 g) 1-bromohexane (4.15g) and silver tetrafluoroborate (0.5g). The product (0.04g) was recrystallised from n-butanol m.p. 182-183 °. tS(CDCI3) 7.7 (8H, m), 4.5 (2H, s) 3.6 (2H, t) 1.6 (8H, m), 0.9 (3H, t).

Photolyses The sulphonium and iodonium compounds ( ~ 7 x 1 0 - S m o l ) i n methanol (3ml) in 12mm diameter quartz tubes were subjected to three freeze-pump-thaw cycles and then the tubes sealed off. Irradiations were carried out with an Applied Photophysics photochemical reactor fitted with germicidal fluorescent tubes. The solutions were irradiated for 61 hr. On removal from the reactor the tubes were opened; 1 ml was removed and added to diethyl ether (40ml) to precipitate unreacted starting material. After several hours the mixture was filtered through a preweighed sintered crucible. The crucibles were dried in vacuo and reweighed, to give the weight of unreacted starting material--g.l.c, analysis and g.l.c, mass spectrometry (10% SE 30 column) was used to analyse the reaction mixture and confirm the identity of the products. The yields of products were determined by the use of appropriate standard solutions.

Photoinitiation of polymerisation Curing was effected with an M.B. Laboratory Conveyor Drier fitted with a single Primarc medium pressure Hg lamp (200 W per inch). The conveyor speed for the polymerisations was 600 feet per min. Following each pass through the drier, the inks were tested for tack-free and through-cure (see previous paper). The consistency of the white ink is given in the text. The constituents were milled on a three roll mill to obtain an even dispersion of pigment and photoinitiator. The ink (0.5 ml) was printed onto flamed tin plate using a DuncanLynch proof print tester. Two coatings were applied, wet on wet.

Cationic polymerisation of epoxides For studying the effect of photosensitiser, the consistency of the ink was titanium dioxide (45~o) cycloaliphatic diepoxide diluent (6~o), photoinitiator (3~) and sensitiser (1~o). The inks were ground on a three roll mill to ensure an equal dispersion of photo-initiator and sensitiser.

REFERENCES

1. J. V. Crivello, U.V. Curing.. Science and Technology (Edited by S. P. Pappas). Technology Marketing Corporation, Norwalk (1980). 2. W. R. Watt, U.S. Pat. 3, 794, 576 (1974). J. H. Feinberg, U.S. Pat. 3, 817, 845 (1974). S. I. Schlessinger, Polym. Eng. and Sec. 14, 513 (1974). 3. J. V. Crivello, Macromolecules 10, 1307 (1977). G. H. Smith, Belg. Patent. 828, 841 (1975). I.C.I. Industries, Belg. Patent. 837, 782 (1976). 4. J. V. Crivello, U.S. Patent. 466, 374 (1974). 5. H. Meerwein, G. Hinz, P. Hofman, E. Kronig and E. Pfeil, d. Prakt. Chem. 147, 257 (1937). P. Dennerly and E. Thomas, Tetrahedron Lett. 71 (1977). 6. G. A. Olah, J. A. Olah and J. Svoboda, Synthesis 490 (1973). 7. G. C. Brumlick, A. I. Kosak and R. Pitcher, d. Am. chem. Soc. 86, 5630 (1964). 8. R. Acheson and D. Harrison, J. chem. Soc. (c) 1764 (1970).

595

9. D. M. Lemal and A. J. Fry, Tetrahedron Lett. 775 (1961). 10. S. L. Nickol and J. A. Kampmeier, J. Am. chem. Soc. 95 1908 (1973). 11. A. L. Maycock and G. A. Berchtold, J. org. Chem. 35, 2532 (1970). 12. D. C. Appleton, D. C. Bull, R. S. Givens, V. Lillis, J. McKenna, J. M. McKenna, S. Thackeray and A. R. Walley, J. chem. Soc. Perkin 11 77 (1980). 13. D. C. Appleton, B. Brocklehurst, J. McKenna, J. M. McKenna, S. Thackeray and A. R. Walley, J. chem. Soc. Perkin 11 87 (1980). 14. P. J. Kropp, G. S. Poindexter, N. J. Pienta and D. C. Hamilton, J. Am. chem. Soc. 98, 8135 (1976). 15. S. A. McNeely and P. J. Kropp, J. Am. chem. Soc. 98, 4319 (1976). 16. J. March, Advanced Organic Chemistry, Chap. 5. McGraw-Hill, New York (1977). 17. A. D. Ketley and J. H. Tsao, J. Rad. Curin(4 6, 22 (1979). 18. J. V. Crivello, J. H. W. Lam and C. N. Volante, J. Rad. Curing 4, 2 (1977). 19. J. V. Crivello and S. H. Schroeter, U.S. Patent 4,026,705 (1977). 20. J. B. Birks, Photophysics of Aromatic Molecules, p. 128. Wiley, London (1970). 21. It was found that the salts had a deleterious effect upon the column and its packing in the C, H and N analyser leading to large errors. Characterisation of these compounds is therefore based on n.m.r, data.