The chemistry of photoinitiators — some recent developments

J. Phofochem.

Photobid.

A: Chum,

81

73 (1993) 81-96

Review

The chemistry

of photoinitiators

-

some recent developments

R. S. Davidson Universiy of Kent at Canterbury Dept. ofChemistry, Canterbury, Kent CT2 7NH (UK) (Received

December

21, 1992; accepted

February

4, 1993)

Abstract Developments since 1987, of photoinitiators for curing radiation (e.g. from CW lasers), water compatible and of these reactions are discussed. Some new types of photoinitiators, bifunctional photoinitiators and those

1. Introduction This review is intended to highlight the developments and trends in the chemistry of photoinitiators for the period 1987-1992. A few new commercial photoinitiators have been launched during this period, but it is clear that the high cost of registering new materials is deterring the few European and US manufacturers of photoinitiators from bringing new products to the market place. An area which is still of current interest, is the development of photoinitiators for use in pigmented coatings. Such initiators need to show some absorption at greater than 350 nm when used at reasonable concentrations. Two type I photoinitiators which fall into this class are 1 [l] and 2 [2]: c”,

CH,S

;-L-N I CH,

0 (I

I

0

u

c”, (2)

Acylphosphine oxides [3] which are also type initiators are gaining an increasing share of the market despite their high cost. An advantage of the acylphosphine oxides is that they are photobleached and hence can be used to cure thick

lOlO-6030/93/$6.00

pigmented films, photoinitiators which respond to visible cationic photoinitiators are reviewed. Mechanistic aspects photoinitiators which have been introduced e.g. anionic containing peresters are also described.

(50-100 pm) coatings. Acylphosphonates [4] which, apart from a few exceptions, mainly initiate cure via a type II process are, because of their absorption characteristics, becoming more popular as initiators. A very strong stimulus for developing initiators for use with visible radiation (and particularly that produced by continuous wave lasers) has been the drive to produce direct write lithoplates. In such a system, a computer holding the text is used to direct a laser beam so that the latter writes the text on the plate, thereby removing the necessity to produce a negative. Needless to say, the ability of dyes to act as sensitizers (usually in combination with a synergist) has been reinvestigated [5]. A new type of initiator, the titanocenes [6], has emerged and, like the dye-amine systems, is used in conjunction with a formulation based on acrylates. Two other aspects of free-radical polymerization processes that have attracted interest are the watercompatible systems [7] and the polymerizable [8] and polymeric photoinitiators [9]. Interest in watercompatible systems arises because many curing systems have some water in their formulations, e.g. screen inks. In other cases, curing in aqueous solution, e.g. grafting acrylics onto natural and synthetic textiles, emulsion polymerization and coating flatstock, is being considered. Not so long ago, a radiation-curable formulation that could

0 1993 - Elsevier Sequoia. All rights reserved

R. S. Davidson I Chemisby

82

not be considered as 100% solids was regarded as an anathema. The growth of interest in polymerizable and polymeric photoinitiators stems from impending legislation in Europe and USA which will require that food-packaging materials shall not contain any m&ratable species. This change in the regulations poses a real challenge to the surface coatings industry. The curing of materials via ionic processes is still an area of active research and new concepts are being advanced. For the bulk user, the iron arene complexes, e.g. 3 [lo], offer an alternative to the more classical onium salt chemistry:

(3)

The chemistry of 3 and of the onium salts is still far from being fully understood. This lack of understanding has not, however, stood in the way of developing some most innovative applications, particularly in the area of electronics. It has been shown that irradiation of hexafluorophosphate salts of onium compounds leads to the generation of hydrogen fluoride [ll] which is volatile. Thus irradiation of the salts in close proximity to a suitable polymerizable compound leads to surface coatings which do not contain any initiator residues or unused initiator. A number of latent acid (e.g. sulphonic [12], silicic [13] and boric acids [14]) generators have been described. Some ingenious new systems (in many cases based on peptide chemistry) have been described for the photogeneration of bases [15] and some indications as to how such systems may be used have been given. Outside the above more general areas, specific work has been carried out on assessing the utility of initiators containing perester groups [Ml, the use of iodonium salts [17] and ferrocene derivatives [18] in free-radical reactions and the application of dithiocarbamates [19, 201 to the production of telechelic methacxylates.

2. Photoinitiator

systems

2.1. Systems developed having absorption at greater than 350 nm for free-radical polymerization The emergence of new Type I systems having better absorption characteristics than the well-

of photoinitiators

Absorbance Benzil

I

Dimethyl

I

Ketal

(0.001% in MeOH) Fig. 1. Absorption

:

(nm)

spectra of 1 and 2 and benzil dimethyl ketal.

established benzoin ether and benzil ketal derivatives is a most welcome development [l, 21. The absorption spectra of 1 and 2 and that of benzil dimethylketal are shown in Fig. 1. That 1 and 2 undergo a-cleavage has been unequivocally demonstrated using the technique of chemically induced dynamic nuclear polarization (CIDNP) and radical-trapping experiments [2,21]. A further useful aspect of these materials is that their initiation efficiency is increased by sensitization using thioxanthones as triplet sensitizers. Introduction of the electron-donating substituents methylthio and morpholino into the 4-positions of the benzoyl group in a benzoin lowers the triplet energy to about 60 kcal mol-l and the triplet state character has changed from n + r* to GTTT--, r*. Efficient Type I cleavage of benzoins is usually associated with the lowest excited triplet state having n+p* character. This raises the question as to why 1 and 2 undergo cycleavage. The cleavage process produces a-amino alkyl radicals which are likely to be more stable than cu-alkoxyalkyl radicals and consequently the bond which cleaves in 1 and 2 is weaker than that in a typical benzoin ether. If the C-C bond energy in 1 and 2 is lower than 60 kcal mol-‘, then at least one can understand why 1 and 2 undergo (Y cleavage but it is not so easy to see how the excited T--K* triplet state energizes this bond. The observation that LYcleavage of 1 and 2 is sensitized by thioxanthones raises the question as to whether the process is a triplet-triplet energy process or involves energy from the thioxanthone passing directly into the C-C which rapidly undergoes cleavage. The triplet energies (from phosphorescence measurements) of isopropylthioxanthone, 2-methyl-2-carbethoxythioxanthone and 1 are 61.4 kcal mol-‘, 58.4 kcal mol-’ and 61 kcal mol-’ respectively, which probably means that the energy transfer process is

R. S. Davidson I Chemishy of photoinitiators

isoenergetic or even slightly endothermic and therefore clearer evidence needs to be provided before one can accept that the sensitization is an energy transfer process. The interactions between thioxanthones and 1 have been the subject of laser flash photolysis studies [23, 241. In the early work it was claimed that in non-polar solvents the sensitization process is via energy transfer and in polar solvents via electron transfer with the morpholino group acting as an electron donor [23]. In recent related experiments [25] it has been shown that N-isopropylmorpholine acts as a physical quencher of many triplet states and is a most inefficient hydrogen donor, This would suggest that the electron transfer process makes little positive contribution to the efficiency of 1 when thioxanthones are used as sensitizers. Recent CIDNP experiments have shown unequivocally that thioxanthones do sensitize the LYcleavage reaction of 1:

These results were backed up by the finding that 1 quenches the triplet state of thioxanthones with rate constants of 2.4~ lo* M-’ SK’ and 6.3 x 108 M-l s-’ in toluene and methanol respectively. The CIDNP experiments also indicate that electron transfer between the morpholino group of 1 and triplet 2-carbomethoxy-thioxanthone occurs. It is not known to what extent this reaction contributes to the initiation process. Some other Type I sensitizers e.g. 4 carrying substituents at the 4-position of the benzoyl group have been investigated [26]:

83

for acleavage maywell be offset, when the materials are used in the curing of thin films, by the better absorption properties conferred upon the initiators by these groups. Acylphosphine oxides (5) [3] have been shown to undergo IYcleavage from their triplet states by the use of time-resolved chemically induced dynamic electron paramagnetic resonance spectroscopy [27, 281:

CH,

Radical trapping experiments also clearly identified the radicals produced by LY cleavage of these compounds [29]. Both the substituted benzoyl and the substituted phosphinoyl radicals were found to be reactive towards acrylates, styrenes and related compounds [30]. It has been estimated that the phosphinoyl radical is twice as effective as the 2,4,6_trimethylbenzoyl radical in initiating polymerization [31]. The initiating efficiency is dramatically reduced by oxygen [31] and this leads to a poor performance by 5 when used to cure thin films 1321. Since oxygen inhibition is not such a problem in thick films, compounds such as 5 can be used with benefit. Compound 5 decomposes to give products which absorb to the blue of 5 and thus the decomposition is attended by bleaching and this facilitates the through cure of thick films. An important finding for formulators is that compounds such as 5 undergo dark reactions with some amine synergists, thereby reducing the shelf life of formulations [33]. Compound 5 has found use for making radiation-curable adhesives [34] and 6 has been claimed as a useful alternative to 5 [35]: CH,O

CHs

hv

R, 0

CH.

R,

;

CHs

0

%

CH,O (6)

(41

The thiomethyl and dimethylamino groups increase the triplet lifetime and reduce the efficiency of the LYcleavage reaction. These effects are explainable since the substituents change the nature of the lowest triplet state from n+-r* to +rr*. The obvious deleterious effect that these electron donating substituents have on the quantum yield

Acylphosphonates have been shown, in the main to act as type II photoinitiators with tertiary amines acting as synergists [4,36]. Radical-trapping studies have shown that 2,4,6-trimethylbenzoyl diethylphosphonate will undergo efficient LYcleavage and can initiate polymerization in this way [37]. Flash photolysis studies have shown that some ketophosphonates undergo intramolecular hydrogen

R. S. Davidson / Chemistry of photoinitiators

84

TABLE 1. Quantum yield & of triplet formation and quantum yield &, for a cleavage for some acylphosphonates 1381 Acyl phosphonate

CH,

br

&

0.6

0.3

0.6

0.3

0.9

0.03

0.5

0.2

%

Bu’C P[OEt), II il

00 PhC P(OE tbn II II

amines they form a highly efficient Type II photoinitiating system [43, 441. A distinct disadvantage of the ketocoumarins is their low solubility in most pre-polymer mixes. The absorption characteristics of the ketocoumarins allows some of them to be used in conjunction with visible-UV lasers, e.g. argon ion lasers (488 nm). Curing can be quite efficient, and efficiency is further increased by the addition of iodonium salts to the formulation [45, 461. The role of the iodonium salt in these systems is believed to be that of a scavenger of the ketyl radical (7) which has the possibility of acting as a chain terminator:

abstraction as well as (Ycleavage [38]. The efficiency of the a! cleavage is dependent upon the structure of the acyl fragment (Table 1). Benzoyl diarylphosphonates and related compounds react in a similar way to the benzoyl dialkylphosphonates [39]. A contribution to the absorption properties of the cu-ketophosphonates probably stems from lone pair-lone pair interactions as is the case with (Ydiketones: ;O

;O

&

41 II \ 0%

01

Camphorquinone, which is a yellow material and can be excited by radiation from black-light lamps can be used with an amine synergist (e.g. alkyl dimethylaminobenzoates) as a l$pe II initiator and has found application in dentistry [4O]. For this purpose, methacrylate-based pre-polymers are usually employed. It has also been claimed that silanes can be used as synergists [41]. Another well-known well-tried Type II system is that of thioxanthones plus an amine synergist [42]. It is difficult, however, to vary substituents in the thioxanthone nucleus so as to move the absorption to much beyond 400 nm. Ketocoumarins and bisketocoumarins are a class of compounds which were originally developed as sensitizers for [Z + 21 cycloaddition processes based on cinnamate esters. When used in conjunction with tertiary

on

0

00

Some advantage may also accrue from the fact that this process regenerates the ketocoumarin. Iodonium salts, as a class of onium salts, are usually considered in the context of UV curing as forming part of a cationic curing system. However, iodonium salts have low irreversible reduction potentials and are potent quenchers of excited states by an electron transfer process. Triplet ketones, e.g. benzophenone xanthones and thioxanthones are quenched by powerful electron acceptors such as methylviologen [47] (for an analogous reaction with cr-keto acids, see ref. 48) and iodonium salts [49, 501. The rate constants for quenching triplet ketocoumarins are high - often diffusion controlled (about lOlo M-l s-l). Thus, when a ketocoumarin is used in conjunction with equimolar amounts of a tertiary amine and an iodonium salt, reaction may occur via mechanisms other than that just outlined, as shown in Scheme 1: tKK*+

Ar,IX

-

t.

Kl$tA;tArI

+X

A,bKKtAm+’ A&KkH

Scheme

%

Am’+KK+Ar’+ArIX

1.

where KK* is the ketocoumarin excited, triplet state, KKH is the ketyl radical derived from ketocoumarin, Am is the tertiary amine, &-i is the initiating amino allcyl radical and Ar,IX is the iodonium salt.

R. S. Davidson I Chemistry

A similar problem obtains in the initiator system consisting of N-phenylglycine, thioxanthene dye and diphenyliodonium salt [51]. It is believed that the role of the iodonium salt is to oxidize the semireduced dye. Dyes which are obvious candidates for sensitizers which will operate in the visible region of the spectrum are also oxidized by iodonium salts and systems have been devised for curing both freeradical and cationic systems [52]. A wide variety of xanthene dyes when used in conjunction with tertiary amines act as initiators operating in the visible region [43, 45, 531. An interesting twist of this system has been described in which an ester of rose bengal 8 is used to generate radicals not only from the reducing agent but also by frag mentation of the sensitizer [54]: CI

CI

1

CH$O

+

0 I

I

This system, it is claimed, works efficiently with light of wavelengths 514 nm (argon ion laser), is useful for curing acrylates and consequently has application for producing three-dimensional objects via stereo lithography. Another reducing agent for xanthene dyes is the iodate ion [%I. Another important aspect of the use of dyes is that they are less efficient as initiators when used in pigmentary form 1561. The use of an alkyltriphenylborate anion as an electron donor which fragments to generate a radical has previously been alluded to [54]. However, it was the earlier elegant work by Schuster and coworkers [57, 581 that showed how this chemistry can be exploited. They prepared a series of cyanine borates whose absorption spectra covered much of the visible spectrum. The sensitizers were used to polymerize acrylates which were encapsulants for colour formers. Since the unpolymerized capsules are softer than the polymerized capsules, nipping the capsules causes the softer capsules to fracture, thereby allowing an

of photoinikztors

8.5

appropriate colour to be produced. This chemistry laid the foundations for a new colour printing process. Cyanine dyes, which are cationic are powerful electron acceptors and therefore efficiently oxidize the borate anion. For efficient electron transfer to occur, the salts need to exist as tight ion pairs and electron transfer occurs to the twisted first excited singlet state of the dye to give the cyanine radical. Since oxidation of the borate anion leads to fragmentation, the electron transfer is driven in one direction. The efficiency of polymerization is determined by the radical released from the borate anion. The fact that the cyanine radical is a powerful reducing agent has led to the development of another system in which the tetrabutyl perester of benzophenone - 3,3’,4,4’-tetracarboxylic acid (9) - is used in conjunction with a cyanine borate [59]. Presumably in this system the photogenerated cyanine radical reduces the perester, thereby inducing fragmentation of the latter. Efficient polymerization of methacrylates was observed and this makes this initiator cocktail of potential value in dental applications. Peresters homolyse upon excitation and photosensitizers which absorb in the visible have been used to bring about homolysis of such compounds by energy transfer although in some cases electron transfer may be occurring, In many cases the perester employed has been 9:

0 &

‘CH

‘N

CH, Ph,&

c

One

of fG.Jr pollible

PrOdUEts \

lnitlotlnp

rodlcol

‘Sensitizers’ include 10 [60], 11 [61], 12 [62], 7amino-4-trifluoromethylcoumarin [63] and other coumarins [64]:

R. S.

86

DavidsonI Chemistry of photoinitiators

R&x

tx)y R

[II)

(IO) X = O&NH

R = EtzN

-

X=CH=CH+OEt I?= CN RZ R’ = 3t2-

benzimidozolyl)

R2 = ethorycarbonyl

Dyes including cyanines 1651 in combination [66] and in the absence of amines [67] have been used to sensitize the homolysis of peroxides. The rich photoredox chemistry of ruthenium(H) complexes such as 11 suggests that they should perform as good visible-absorbing initiators when used in combination with a tertiary amine. The system does work [68] although the efficiency leaves something to be desired [43]. A new class of free-radical initiators which absorb in the visible range are the titanocenes [6, 69, 701. Despite extensive investigation by such techniques as flash photolysis, the mechanism whereby they induce polymerization is not very clear [71]. Laser flash photolysis showed that 13 underwent isomerization to give a blue species with a quantum yield of unity:

4 0

,C&

(131

- hv

II41

The identity of this species is unknown but a contender is 14; it has been suggested that the acrylates in common with other carbonyl compounds ligate the titanium atom to give a biradical which induces polymerization [71]. This suggestion would account for the fact that photodecomposition of 13 in the presence of radical traps did not lead to any trapped products. Needless to say, variants upon the earlier titanocenes have been described, all apparently having some superior quality. Iron arene complexes have been introduced as cationic photoinitiators which absorb visible radiation. If the counterion is an alkyhriphenylborate anion, then irradiation leads to reduction of the

iron arene, oxidation of the anion, followed by its fragmentation to release an alkyl radical [73]. Such systems are capable of initiating the free-radical polymerization of acrylates [74]. However, when irradiated in the presence of halogenated alkanes containing pyrrole, polypyrroles having a low conductivity are produced [75]. Doped polypyrroles have also been prepared by using an iron arene complex [74]. A claim which is somewhat surprising is that chalcones, which absorb in the visible, sensitize the decomposition of iron arene complexes generating radicals which initiate acrylate polymerization [76]. Usually these complexes, on direct irradiation or electron beam radiation [77], release acid, thereby inducing cationic polymerization. Iron complexes such as 15 undergo Fe-C bond homolysis [78]:

+

hv Fe co z

1.

F**

1

CHtPh

PhiH, PhCH,Cl+Ph

co co

(15)

I @@---CN

/

co

T PhCH&H~CHCN

t

Me :+ co

For the compounds to be efficient sensitizers, the P-carbon atom of the alkyl group must lack hydrogen atoms; alkene formation takes place. It would also appear that, even if this constraint is met, hydrogen transfer from the growing polymer chain will occur, leading to low molecular weight products. It may be more advantageous to use complexes such as 16:

+

PhiH,

(I61

2.2. Cationic curing system.9 Much of the earlier work on these systems has been recently reviewed [79]. Attention has recently focused on a new type of initiator, namely the iron arene complexes 3 [lo]. If the arene is varied, the long-wavelength absorption band can be moved about the visible range of the spectrum, e.g. chang-

R. S. Davidson / ChemiFny of photoinitiators

ing from cumene to naphthalene to anthracene causes a red shift [80]. The mechanism put forward to explain the ability of complexes such as 3 to induce the polymerization of epoxides is shown in Scheme 2:

Q+ Fe

Interest has been generated salts, e.g. 18 [86]:

0

PF6

87

in benzylsulphonium

R

IIS)

hV

It has been proposed that such a compound initiate polymerization via the benzyl carbocation. Other related compounds have been prepared which contain acid-cleavable groups, e.g. 19 and photocleavable groups, e.g. 20:

Ft

/I\ 000

-Ii t f? R

WF,),

(19)

PFt

R

(20)

Several benzylsulphonium salts have been shown to be good initiators, e.g. 21 [87]: Scheme 2.

It will be noted that in this mechanism the arene is replaced by three epoxide groups which leads to the cationic species which is responsible for polymerization. This aspect of the mechanism appears to be backed up by some recent flash photolysis results [Sl]. In the proposed mechanism there is apparently no role for the counterion which is usually the hexafluorophosphate anion. It had been shown that iron arene complexes initiate polymerization of epoxides under electron beam radiation and the hexafluorophosphate anion undergoes decomposition [82]. Perhaps even more perturbing is the finding that irradiation of hexafluorophosphate salts of iron arene complexes in the absence of epoxides leads to the generation of hydrogen fluoride [41]. Clearly further work needs to be done to unravel the mechanism of these useful compounds. Another interesting aspect of the iron arene complexes is that they will cure epoxides in the presence of aromatic amines having pK, 2-5 [83]. Recent patents would suggest that iron arene complexes can be used to polymerize a variety of epoxides [84]. Much attention has been paid to producing sulphonium salts with better absorption properties than the simple triphenylsulphonium salts, e.g. 17 [85]:

(21)

A wide variety of others have been shown to be useful for inducing the polymerization of vinyl ethers [88]. Other derivatives which have been shown to act as initiators include 4-hydroxyphenyl sulphonium salts [89], phenacyl triphenylphosphonium salts [90] and onium bicarbonates [91]. The chemistry of benzylsulphonium salts has been the subject of much mechanistic interest and has led to the unravelling of another mechanism whereby triarylsulphonium salts lead to the generation of acid. Saeva er al. [92] showed that irradiation of the salt 22 undergoes rearrangement with concomitant release of acid:

HF t

PFJ

-

HPFe

+ CH:

Irradiation of the benzylsulphonium salt 23 led not only to the rearrangement product 24 and

88

R. S. Davidson

also to a product 25 which show carbocations are generated [93]:

that

I Chemishy

benzyl

of photuinitiafm

For applications to UV curing, triarylsulphonium salts are still very popular. The established mechanism whereby these salts generate acid in solution has been probed further by a laser flash photolysis study [97] and evidence for the intermediacy of a diarylsulphonium radical cation presented: fAr3SX

Ar*$+RH-

SC.b

(7.3) +aYl +

000

t

$=0.34 CHzNHCCH,

O 13 CN

L

quantum yield of acid production (0.77) was The based on the assumption that it was equal to the sum of the quantum yields of the other products. Further work based on 26 [94] has shown that the rearrangement process occurs via an excited singlet state:

NC 0 -x2+

=NCe&,2 CH,NHCOCH,

Hz0

o

--%

CHpSPh

7i?

It was proposed that the benzyl cation is generated via oxidation of the benzyl radical via the anthracene cation. A detailed study has been made of the quantum yield for acid production from 9anthryl phenyl methylsulphoniurn salt as a function of wavelength [95]. It was found that the quantum yield when the anthracene group was excited in an upper excited state (‘B band) was 28 times greater than when the lowest excited singlet state (‘L, state) was populated. This difference was attributed to leakage of energy from the second excited singlet state into the u* orbital of the S-C bond. Since anthracene can sensitize decomposition of sulphonium salts, sulphonium compounds have been prepared which contain anthracene groups linked to the sulphur atom via an alkyl chain [96]; the photolysis of these compounds has also been investigated. In such compounds the anthracene helps to extend the wavelength response of the onium salt.

Ar,iH-

At-$+

+X-

+Ai

Ar$H+R Ar$+H+

where R-H is the solvent. The diarylsulphoniurn radical cation reacts with nucleophiles such as vinyl ethers and epoxides (bimolecular rate constants, about lo5 M-l s-l) which led to the suggestion that the radical cations may initiate polymerization [98]. Of particular importance is the finding that acid generation can take place without postulating any role for the solvent [99]. It is proposed that following excitation the salt undergoes heterolysis to give an aryl cation and that via an in-cage reaction this reacts with diary1 sulphide or alternatively it undergoes an incage electron transfer reaction to give the diarylsulphide radical cation and an aryl radical. Within the solvent cage it is proposed that the intramolecular rearrangement products arise via both the aryl cation and the aryl radical. Two points need to be specifically addressed; namely does excitation lead directly to heterolysis and is there any evidence for the formation of aryl cations? The latter is the easier of the two to answer since irradiation of triarylsulphonium salts in acetonitrile give acetanilide [99]. Earlier work concerning the nature of the products formed on the photolysis of sulphonium salts in polar solvents had also led to the conclusion that aryl cations are generated [49]. The question as to what occurs directly after irradiation is more difficult to rationalize. From the proposed mechanism it would seem that the in-cage diarylsulphide radical cation-phenyl radical pair should have a lower energy than the in-cage diarylsulphide phenyl cation pair. The formation of products from the aryl cation lends some credence to the idea that such species are generated upon excitation. It would seem that the energies of both types of in-cage species are very similar. If this is the case, could it not be the solvating power of the solvent which determines whether heterolysis or homolysis is the primary step? This problem is reminiscent of that encountered earlier when a study was made of the photolysis of benzylhalides [ 1001. Not surprisingly, triplet sensitized

R. S. Duvidson I Chemise

decomposition of the arylsulphonium salts leads to products via homolysis. If spin is to be conserved, a triplet radical pair should be produced. The relative yields of diarylsulphide to the intramolecular rearrangement products 27-29 is a good probe as to the extent to which in cage reactions occur: t-

PhgX

hv

S

s

+

(27)

pG--

(281

PhtS

+

Ph@h+,/X---W

f. PhpS

Phil

Ph$+‘Ph’//X--

127)

t (28)+

-

t

Ph++

S

;h

+

HX

(291

X-

X-

Ph,S+‘+ Ph’t Reoci in the usud

(291

t

HX

Xway

If the solvent viscosity is increased, formation of the in-cage products should be favoured and this indeed is the case [ 1011. The ultimate situation attends when the salts are irradiated in the solid state and there it is found that in cage reaction is strongly favoured and that the proportion of 27 is increased at the expense of 2% and 29 [102]. Results from recent work are in accord with this finding [ll] and the possibility that formation of 27 occurs via a 11, 31 sigmatropic shift suggested. A most important application of sulphonium salts is in the area of photoresists and therefore the mechanism of decomposition of these salts in polymer matrixes is of interest. It appears that the extent of in-cage reaction is less in polymer films than in the solid state and that an important factor is the molecular composition of the resist [103, 1041. In a polystyrene carrying a t-butoxycarbonyl group in the 4-position the yield of incage products is lower than expected and it was suggested [103] that the polymer sensitizes decomposition of the sulphonium salt by an electron transfer process, i.e. polymer,, + Ar,$% polymei

+ + Ar’+ Ar,S + 2

Such a process cannot occur in poly(methy1 methacrylate) and in this polymer the ratio of cage to

of photoinitiatom

89

escape products attains the expected value. When the resist was a poly(all@ methacrylate), the yield of acid was found to be dependent upon the alkyl group with the highest yield being obtained with the smallest alkyl group (methyl) used [104]. This difference was attributed to the varying ability of the polymers to accommodate the salts with the least polar leading to more extensive aggregation of the sulphonium salt. Another factor which influences the extent of in-cage and out-of-cage reaction is the nature of the anion and this shows up particularly well when one looks at the product distribution from irradiation of the salts in the solid state. The finding that sulphonium salts decompose in the solid state to give acid has led to the development of a process called remote cure. Sulphonium hexafluorophosphates as well as iodonium and iron arene hexafluorophosphates liberate hydrogen fluoride upon irradiation [ll] and are used in the process. The initiator is applied to an inert support and the polymerizable material held a short distance away from the initiator. Irradiation of the initiator liberates the acid which diffuses into the polymerizable layer, leading to cure. The efficiency of acid release is determined by, amongst other parameters, the surface density of the initiator. There is ari optimum density for attaining the highest efficiency and, if a greater density is used, the rate of evolution of acid is not increased. If the concentration of the initiator is dropped below the optimum value, the rate of acid liberation is decreased. The optimum concentration of initiator undoubtedly corresponds to the conditions where the initiator absorbs the light most effectively and, above this loading, inner filter effects to the upper layer confine reaction too close to the surface. One of the advantages of the remote cure process is that the film does not contain any photoinitiator residues apart from hydrogen fluoride and this can be readily removed by vacuum treatment. In the last few years a number of new types of sulphonium compound have been explored [85-89, 91, 10.51 and new substrates, e.g. E-caprolactone [l!X]. Clearly an area for further development is that of producing initiators which absorb at greater than 350 nm. The mechanism of decomposition of iodonium compounds has been explored and both the classical and the newer mechanism involving intramolecular rearrangement found to operate [99, 1071, i.e.

R. S. Davidson I Chembtry of photoinitiators

90

+

PhNHCOCHS

+

HX

Parallel work has been carried out on the photochemistry of diaryliodonium halides although these are not of course used as cationic initiators [108]. The finding that diacetoxyiodobenzene initiates the polymerization of a butyl vinyl ether has led to the suggestion {albeit experimentally unsubstantiated) that this compound acts as a cationic initiator [log]. Undoubtedly these iodonium compounds act as initiators for free-radical polymerization. One of the problems encountered with many cationic photoinitiators is their lack of solubility in pre-polymer mixes. In the case of iodonium compounds this problem has been addressed and some substituted compounds described which have better solubility than the parent compound [llO]. The preparation of a polymeric iodonium compound 30 has been described [ill] although it should be noted that irradiation of 30 will release low molecular weight species unless all the reaction occurs via the intramolecular process:

where TX is thioxanthone. It is this reaction which is proposed as being responsible for the ability of thioxanthanes to act as sensitizers for the decomposition of iodonium compounds. The thioxanthone radical cation may possibly react with the pre-polymer, e.g. an epoxide, and thereby initiate polymerization although it should be noted there is no firm evidence to support this proposal. Some very thorough quantitative work has been done on the polymerization of 2,3-epoxypropyl phenyl ether by diphenyliodonium hexafluorophosphate using a number of sensitizers [115]. The relative efficiency of initiators studied (when acetone was used as solvent) was found to be anthracene > benzoin isopropyl ether > phenanthra-9,10quinone > benzophenone. When methanol was used as solvent the order of reactivity changed to anthracene = benzophenone > benzoin isopropylether B phenanthraquinone. Clearly a number of factors control the benzoin efficiency of sensitization. In the case of isopropyl ether, sensitization follows the following route:

Ph C CHPh II I 0 OPi PhiHOP:

OCH,

Other interesting variations on the classical iodonium salt structure includes iodonium 9,10-d& methoxy-anthracene-2-sulphonates [112] and employment of a mixture of tetrabutylammonium hexafluorophosphate and 1-iodonaphthalene [113]. Flash photolysis studies have shown that irradiation of diphenyliodonium compounds produces the iodobenzene radical cation which reacts far more readily with nucleophiles (e.g. butyl vinyl ether and cyclohexene oxide) than the diphenylsulphide radical cation [97]. lodonium salts are powerful oxidizing agents and have been found to oxidize the triplet state of thioxanthone [114]:

TX,,+Ph,;PF,-

TX.++PhI+Ph-+PF,,

+

Ph&,

PheO

-.

t

PhtHOP:

Ph&iOPi

+

Pi

t

Yhl

t

%,

It has also been proposed that benzoyl radicals can be oxidized to benzoyl cations 1981. This idea appears to have been followed up since recently it has been claimed that irradiation of acylphosphine oxides in the presence of N-alkoxypyridinium hexafluoroarsenates includes the cationic polymerization of epoxides [116]. N-Alkoxypyridinium compounds have recently been shown to oxidize silyl radicals (generated from polysilanes) to give silyl cations which have been used to initiate cationic polymerization. In the systems considered so far, the acid to be released is hydrogen fluoride which is an excellent catalyst for polymerizing epoxides and vinyl ethers. To achieve rapid cure it is sometimes necessary to apply a thermal bump after the UV cure. Interest has also been shown in initiators which liberate weaker acids and therefore the curing process is much slower. This type of initiator is sometimes called a latent acid initiator. It seems strange that the onium and diazonium salts are not included in this classification. It has been shown that the benzoin ethers of the type 31 release a sulphonic acid upon irradiation [117]:

R. S. Davidwn 0

I Chemishy

hv

Ph-LLPh

FhtO

I ;-Ph

t

CH&lSOeR

CH,OSO,R

I

(31) PhCOiH,

:

RSO,H

Although these materials were designed to counteract oxygen inhibition which occurs in free-radical curing, their potential as “latent acid” generators was soon recognized [1X3]. A series of somewhat closely related compounds, e.g. 32, have also been described [12]: 0 O-H Ph !-A-

SO Ar L

2

Benzoin tosylates have been used as sulphonic acid generators which serve to catalyse melamine formation [119]. Although silicic acids are very weak acids, their effective acidity can be increased by interaction with aluminium acetoacetonate. Thus irradiation of 0nitrobenzyl triphenylsilylether (33) in the presence of the aluminium catalyst 34 leads to the polymerization of cyclohexene oxide [120]: NO CHO -----)

PhsSiOCH2

91

has undoubtedly been due to the lack of suitable initiators. The first type of initiator to be described were tertiary amine salts 36 of a-ketocarboxylic acids, which were used in formulations where hardening of a polyurethane were required [121]:

HO

OH

ofphotoinitiafon

Ph$iOH

t

NOL’

(361

Some newer systems, based on peptide chemistry, have been developed which, with further refinement, may well put anionic curing on a par with cationic curing. The carbamates 37 and 38 undergo fragmentation on irradiation, leading to the free base [15, 122, 1231:

(381

f

WCw%

The photolysis of 37 and 38 is believed to occur via a heterolytic pathway. In another system, the orthonitrobenzyl group, e.g. 39, has been used as the photoactive component [122-1241:

CHn o-c’ ‘A/ Ph3SioH

+ ’

--t \o

>l-OSiP~PhsioH

b

Pf%$, 8=0

O-H

-2 OWH,

\c /AI-

(341

62

1391 OSiPh,

Triarylsilylperoxides may also be used as photoinitiators in the presence of an aluminium catalyst [ 131. The chemistry of the U-nitrobenzyl silyl compounds has been extended to boron compounds 35 which in photodecomposition in the presence of zirconium acetylacetonate polymerize epoxides [144]:

A wide range of compounds of this type in which the number of nitro substituents and nitroaryl groups have been varied have been described. A report from another group of workers also claims that ortho nitrobenzylcarbamates can be used for photogeneration of bases and that ortho nitrobenzyl tosylates can be used for acid generation.

CHO

hv BPh ----w

2

0 (5

No

+

PhB(OH12

(35)

2.3. Anionic curing systems Hitherto anionic curing systems have been the poor relations to cationic curing systems and this

2.4. Water-compatible free-radical initiators Water-compatible free-radical initiators are of value in those systems where the presence of water is a necessity or during processing water may enter the formulation, e-g. in some printing processes. There is also interest in photografting synthetic polymers onto natural polymers such as cotton. Until recently there had been a lack of cleavable initiators in this field but recently Bunte salts (e.g.

R. S. Davidson / Chemtitry of photoinitinton

92

40) [126] and some phosphinates have been shown to be effective:

(e.g. 41) [127]

CH, (I.035 (411

In the latter case both the benzoyl- and the phosphorus-containing radical were found to be reactive towards the olefins. Water-soluble type I cleavable initiators based on some classical systems have been described (e.g. 42) [128]:

--a-

y3

R

\

C_C.OH

/I

0

-

I

R

+cYF

CH,

!

C-W

i

0

*

I 3

(421

The following were obtained: R R R R

= = = =

rate

OCH,CO,H 0CH,C02-Na+ OCHzCHzOH SCH,CH,OH

constants

k, for LXcleavage

k,= 1.8x 10’ s-r’k, = 0.8 x lo7 s-r k,=0.8x107 s-l k,=1x105 s-l

It was confirmed by laser flash photolysis studies that cleavage occurs from the first excited triplet state and the triplet states have sufficiently long lifetimes that they can be quenched by tertiary amines provided that the latter are used at a concentration in excess of lo-’ M. It is claimed that thioxanthones will sensitize cleavage of 42 via a triplet-triplet energy transfer process. For a number of years, much effort has been put into the synthesis of water-soluble Type II photoinitiators such as thioxanthones and benzophenones [7, 1291. It is usual to use a tertiary amine as the synergist and there is some evidence that thioxanthones can react with the amines via their first excited singlet as well as their triplet states [129, 1301. Photocalorimetry has been used to study the polymerization of a water-soluble acrylate initiated by a water-soluble benzophenone, and good correlation was found between these results and those obtained using a classical kinetic procedure [131]. A mechanistic investigation led to the view that the amines react with the benzophenone by an electron transfer process [132]. The fact that the radical anion of the benzophenone

was seen by laser flash photolysis should not, however, be taken as evidence that the reaction occurs via electron transfer since in the presence of aliphatic amines the benzophenone ketyl radical ionizes to give the anion radical [133]. Sulphonated benzophenone, thioxanthone, acridone and methylacridone induce photopolymerization of acrylamide [134]. The triplet states of these compounds and in some cases the singlet state induce decomposition of iodonium salts via the electron transfer process outlined earlier [134, 1351. Interestingly it was also proposed that the initiator may add to the acrylamide, giving a 1,4biradical which undergoes oxidation by the iodonium compound to give a distal radical cation. Another application of water-soluble Type II initiators has been to the photopolymerization of emulsions of acrylates in water generated by means of ultrasound [136]. Latexes were obtained in which the particle size could be varied from 0.5 to 10 pm by changing the ultrasonic treatment. The polymers show a high polydispersivity and their properties are strongly affected by the type of amine synergist which is employed. Those having the greater water solubility lead to the higher molecular weight products because emulsion polymerization predominates over suspension polymerization. 2.5. Decomposition of peresters Since the early work was done on photosensitized decomposition of peresters [16], further work has been carried out on the mechanistic aspects of these reactions. A popular material to use is tetra t-butyl benzophenone-3,3’,-4,4’-percarboxylate (9). This material undergoes decomposition on direct irradiation and presumably this occurs via energy transfer from the benzophenone triplet into the perester bond. A comparison of the reactivity of this compound with t-butyl benzophenone-4-percarboxylate showed the latter to be more efficient based on the tests involving film formation and from real-time infrared studies [137]. Reference has already been made to the ability of sensitizers absorbing in the visible region, to trigger off the homolytic decomposition of the tetra-perester [60, 611. Peresters of fluorenone carboxylic acids have also been prepared [138] and their reactivities compared with those of their benzophenone counterparts. The benzophenone derivatives gave, when methylmethacrylate was used as the monomer, a higher molecular weight polymer than did the fluorenone compounds. Apparently the fluorenone derivatives exhibit a high degree of self-termination. It has been proposed that perester de-

R. S. Davidson / Chemistry of phoioinitiaiors

composition is aided by addition synergist 4-dimethylaminobenzaldehyde

of the amine [139].

2.6. Bifinctional photoinitiators In a sense the peresters described in Section 2.5 are bifunctional initiators since they contain a sensitizer and a group which will homolyse. Some very interesting and potentially useful compounds have been made which contain a thermally labile azo group and a benzoin group, e.g. 43 [140]: 0 K II PhC-;(CH,),OCCH,Cbi,~-N Ph

CH3 I

CH,

CH,

0

R

0

II

1

II

=N-;-CH,CH,CCKH&-7 -CPh C%

Ph

(43)

Thermal decomposition of this material in styrene gives a polymeric styrene capped with benzoin groups. Irradiation of such a material in a reactive monomer leads to a block copolymer [141]. An interesting variation of 43 is 44 [142]: Ph W-C-OH

i

I

0

(441

93

istry has been previously reviewed [9]. These initiators are classed as iniferters since the growing radical chain attacks a thiocarbamate group, thereby becoming terminated and at the same time releasing another initiating radical. In this way, telechelic photoinitiators have been prepared [144], including materials based on siloxanes [145]; dithiocarbamates have also found use in the production of radiation-curable adhesives. 2.8. The hexaarylbisimidazole system This is quite a remarkable system. The hexaarylbisimidazoles (HABIs) undergo homolysis on irradiation with W light and the radicals so generated are quite powerful hydrogen and electron acceptors. A careful laser flash photolysis study [147] has shown that, at 10 K, irradiation leads to population of the triplet HABI. At high temperatures the tist excited singlet state of the HABIs undergoes rapid homolysis. Homolysis can also be sensitized using dyes and it has been proposed that these reactions involve electron transfer [148]. When thioxanthones are used as sensitizers, it seems that homolysis is induced by triplet energy transfer from the thioxanthone to the HABI [149].

b CH =

CHZ

This material can be thermally polymerized in the presence of a variety of reactive monomers to give benzoin end-capped polymers which are compatible with either hydrophilic or hydrophobic resin systems. A different type of bifunctional initiator is one in which an aromatic ketone is covalently linked to an amine synergist, thus forming an intramolecular Type II system. A large number of alkylamino-substituted benzophenones have been prepared in which the length of the spacer arm (which separates the benzophenone from the amine) has been varied [143]. The amine groups perturb the triplet lifetime (usually increasing it!) and their effectiveness is dependent upon structure, with it increasing in the order alicyclic < aliphatic < aromatic. When low concentrations of initiator are used, the propoxy spacer arm, which will favour intramolecular reaction, appears to give the most efficient initiator. At higher initiator concentrations it is not possible to distinguish between intermolecular and intramolecular processes. 2.7. Use of dithiocarbamates A number of telechelic polymers have been made using these materials and some of the chem-

3. Conclusion In this and the companion review [9] the development of some new initiators has been described and some of the current mechanistic theories as to how photoinitiators react. Trends in photoinitiator design have been examined and avenues where further fruitful work may be carried out explored.

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52

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