The present invention relates to substituted derivatives of alpha-diketones which can be used as photoinitiators in LED photocuring and to a process for photocuring compositions comprising said alfa-diketones.

BACKGROUND OF THE ART

Photopotymerizable systems contain photoinitiators that possess in the molecule a functional group which, by exposure to fight radiation of appropriate wavelength, generate radicals able to initiate the polymerization.

Among the light radiation sources used in this field, light emitting diodes (LED), a semiconductor light source, have been the subject of significant development over the past few years because of the advantages of low temperature operation and extremely long life in comparison with conventional medium pressure mercury arc curing lamps. LED lamps are advantageous because of the inherently small size of LED units, their longer lifetime, their robustness and their ability to be easily engineered, for example into commercial printing systems.

When using LED lamps to photocure inks and coatings, it is necessary to use selected photoinitiator systems that are tuned to the wavelength of this light source. Whiie Mercury arc lamps typically have a polychromatic emission spectrum, emitting light in all regions of the UV-visible spectrum from 200 to 450 nm, LED lamps usually have only a single emission band in the range 365-420 nm. Photoinitiators, absorbing in the region between 365 nm and 420 nm, are thus required to make full use of the recent development of LEDs with increasing power. Thioxanthones, such as isopropyl thioxanthone (ITX) and its derivatives, and acyl phosphine oxides are photoinitiators absorbing in this spectral region and are commonly used in the field.

Unfortunately, the thioxanthone derivatives commonly used both as photoinitiators and sensitizers are prone to yellowing upon exposure, thereby forming degradation products with limited stability. As a result, the original yellowing can shift unpredictably upon storage. Especially in imaging, e.g. inkjet printing, this unstable yellowing behavior makes quite difficult the control of the image tone in the final image .

Acyl phosphine oxides initiators, on the other hand, result in medium volatile aldehyde type of degradation products, producing a background smell of the cured coatings or the printed image, which is no more acceptable. Moreover, the use of high amounts of acyl phosphine oxides initiators creates several health and safety problems.

Therefore, there is an increasing demand for the development of different photoinitiators, absorbing in the region between 365 nm and 420 nm, having a stable yellowing behavior, good photochemical reactivity, no odorous degradation products and no health or safety drawbacks.

Aipha-diketones, such as camphorquinone and its derivatives and 1- phenyl propanedione, are examples of these photoinitiators and they have been widely used in combination with LED light sources, particuiarly for dental applications. US 4,992,351 discloses a photopolymerizable composition containing at least a pofymerizabie compound having an unsaturated double bond (ethylenic unsaturation) and, as photopolymerization initiator, an alpha- diketone represented by the following formula (A) :

wherein Ar _! and Ar ₂ are each an aromatic ring or a heterocyclic ring, substituted or unsubstituted. These photopolymerization initiators are activated using a fluorescent light source emitting at wavelength below 360 nm (peak wavelength 335 nm), since, as stated in the patent, the photopolymerization initiators represented by the formula (A) have no sensitivity to the lamps which work at longer wavelength (above 360 nm). In this patent, different photoinitiators, such as thioxanthone and coumarine derivatives, are proposed for longer wavelengths (360-430 nm).

Surprisingly, we have found that derivatives of aromatic alpha-diketone substituted with a sulfur containing group show a high cure speed on exposure to LED light source with wavelength comprised between 365 and 420 nm compared with photoinitiators known to those skilled in the art, do not show yeiiowing behavior, and maintain at the same time superior compatibility with photopolymerizable systems.

Therefore the object of the present invention are specific aromatic derivatives of alpha-diketone containing sulfur and a process for photocuring compositions including these components as photoinitiators. Such photopolymerizable compositions have been found to be suitable for inclusion in ink or coating compositions which are curable on exposure to radiation from a LED light source.

DESCRIPTION OF THE INVENTION

It is an object of the present invention a process for photocuring compositions, which process comprises :

I) preparing a photopolymerizable composition comprising :

a) from 50 to 99.9 % by weight, preferably from 70 to 98.9% by weight, of at least one ethylenically unsaturated compound;

b) from 0.1 to 35% by weight, preferably from 0.1 to 20% by weight, and more preferably from 0,2 to 15% by weight of at least one aipha-diketone of formula I : in which :

R _x is Ar or is selected among substituted or unsubstituted alkyi or C ₄ -C ₁₂ cycloalkyi groups, substituted or unsubstituted alkenyl groups and substituted or unsubstituted phenyl, aryl or heteroaryl groups;

Ar is an aromatic group, selected among one of the following formulas :

(a) (b)

(c) (d)

(e) (f)

wherein

m is 2 or 3;

n is 1, 2 or 3;

Z is a direct bond, -CH ₂ -, -CH ₂ CH ₂ -, -0-, -S-, N-alkyl or -(CO) ;

J is N-alkyl, -N-, -0-, -CH ₂ - or -CH-;

K is -CH - or -CH ₂ - ;

R ₂ , R ₃ , R ₄ , R ₅ and R ₆ are independently of one another hydrogen, haiogen, Ci-Ci ₂ alkyl, C ₃ -Ci ₂ alkenyl, C ₅ -C ₆ cycloalkyl, substituted or unsubstituted aryl or heteroaryl, Ci-C ₄ aikoxy, Ci-C ₄ hydroxyalkoxy, phenoxy, -COOH, -COO(Ci-C ₄ alkyl), alkyiamino, dialkylamino, -S-R ₇ , -SO-R ₇ or -SO2R7, where R ₇ is selected among : hydrogen, Ci-C ₁₂ alkyl, C ₃ -Ci ₂ alkenyl; substituted or unsubstituted phenyl, aryl or heteroaryl; C ₅ -C ₆ cycloalkyl, C1-C12 alkyl which is substituted by SH, ~N(Ci-C ₆ alkyl) ₂ , piperidino, morpholino, piperazino, -OH, -0(Ci-Ci ₂ alkyl), -COOH ; or R ₇ is a group of formula : in which R ₈ is a substituted or unsubstituted alkyl or cycloalkyi group, substituted or unsubstituted alkenyl group and substituted or unsubstituted phenyl, aryl or heteroary! group; p is from 0 to 4 and Y is a direct bond, -O- or -S-; or R ₇ is a group of formula :

wherein R ₈ has the same meaning as described above;

with the proviso that at least one of the substituents from R ₂ to R ₅ is a group -S-R ₇ or -SO-R ₇ or -S0 ₂ R ₇ ;

) photopoiimerizing the photopolymerizab!e composition so obtained with a LED light source emitting at wavelengths comprised between 365 nm and 420 nm.

It is a further object of the present invention alpha-diketones of formula I wherein R _x is Ar and each Ar is independently an aromatic group of formula (a), in which at least one of the substituents from R ₂ to R _s is a group -S-R _? and the other are hydrogen and R ₇ is n-propyl, a linear or branched C ₄ -Ci ₂ a!kyl or a C ₄ -Ci ₂ cycloalkyl group.

Another object of the present invention is a LED photopolymerizable ink comprising : a) from 50 to 98 % by weight of at least one ethylenically unsaturated compound; b) from 0.1 to 35% by weight of at least one alpha-diketone of formula I:

I

in which ;

Ri is Ar or is selected among substituted or unsubstituted C ₁ -C ₁₂ alkyl or C ₄ -Ci ₂ cycloalkyl groups, substituted or unsubstituted alkenyl groups and substituted or unsubstituted phenyl, aryl or heteroaryl groups;

Ar is a aromatic group, selected among one of the following formulas:

(e) (f)

wherein

m is 2 or 3;

n is 1, 2 or 3;

Z is a direct bond, -CH ₂ -, -CH ₂ CH ₂ -, -0-, -S-, N-a!kyl or -(CO)- ;

J is N-a!kyl, -N-, -0-, -CH ₂ - or -CH-;

K is -CH- or -CH ₂ -;

R ₂ , R ₃ , R ₄ , R ₅ and R ₆ are independently of one another hydrogen, haiogen, C _! -C ₁₂ alkyl, C ₃ -Ci ₂ alkenyl, C ₅ -C ₆ cycSoalkyl, substituted or unsubstituted aryl or heteroaryf, Ci-C ₄ alkoxy, Ci-C ₄ hydroxyalkoxy, phenoxy, -COOH, -COO(C _! -C ₄ alkyl), alkylamino, dialkylamino, -S-R ₇ , ~SO-R ₇ or -S0 ₂ R ₇ , where R ₇ is selected among : hydrogen, C ₁ -C ₁₂ alkyl, C ₃ -Ci ₂ alkenyl; substituted or unsubstituted phenyl, aryl or heteroaryl; C ₅ -C ₆ cycloa!kyl, C ₁ -C ₁₂ alkyl which is substituted by -SH, -N(Ci-C ₆ alkyl) ₂ , piperidino, morpholino, piperazino, -OH, -0(Ci-Ci ₂ alkyl), -COOH; or R ₇ is a group of formula :

in which R ₈ is a substituted or unsubstituted alkyl or cycloa!kyl group, substituted or unsubstituted alkenyl groups and a substituted or unsubstituted phenyl, aryl or heteroaryl group; p is from 0 to 4 and Y is a direct bond, -O- or -S- ; or R ₇ is a group of formula : wherein R ₈ has the same meaning as described above;

with the proviso that at least one of the substituents from R ₂ to R ₆ is a group -S-R ₇ or -SO-R ₇ or -S0 ₂ R ₇ ;

c) from 0.01 to 30 % by weight of colorants.

DETAILED DESCRIPTION OF THE INVENTION

In the present text the expressions "alkyl" or "a!kyl group" mean, where not differently indicated, a linear or branched chain containing from 1 to 12 carbon atoms and includes all possible variants for each number of carbon atoms in the alkyl group i.e. for three carbon atoms: n-propyl and isopropyl; for four carbon atoms : n-butyl, isobutyl and tertiary-butyl ; for five carbon atoms: n-pentyl, 1, 1 -dimethyl-propyl, 2,2-dimethylpropyl and 2-methyi-butyl etc.

The expressions "cycloalkyl" or "cycloalkyl group" mean, where not differently indicated, a aliphatic ring containing from 4 to 12 carbon atoms which can be, for example, cyclopenty!, cyclohexyl, cyclooctyl, cyclododecyl and the like.

The expressions "aryl" or "aryl group" mean for example substituted or unsubstituted phenyl group, substituted or unsubstituted naphthyl group, anthracenyl group, indenyi group, fluorenyl group and others.

The expressions "heteroaryl" or "heteroaryl group" mean for example furan, thiophene, pyrrole, oxazole, isooxazole, thiazole, isothiazole, imidazole, pyrazole, pyrane, pyridine, pyrrolidine, piperidine, indole, quinoline, isoquinoline, xanthene, carbazole, acridine, indeline, julolidine and others.

"Alkenyl" or "alkenyl group" mean an unsaturated group containing from 3 to 12 carbon atom which can be, for example, ally!, methallyl or undecenyl.

The term substituted means that a group bears a substituent that can be hydrogen atom, halogen atom, an alkyl, cycloalkyl, alkoxy, a!kylamino, dialkylamino, alkyithio or aryithio group, heterocyclic groups, more specifically, hydrogen atom, methyl, ethyl, isopropyi, tert-butyl, phenyl, trifluoromethyl, cyano, acetyl, ethoxycarbonyj, carboxyl, carboxylate, amino, methylamino, dimethylamino, ethylamino, diethylamino, isopropylamino, diisopropylamino, cyclohexylamino, dicyclohexylamino, acetylamino, piperidino, pyrrolidyl, methoxy, ethoxy, propoxy, isopropoxy, butoxy, pentyloxy, phenoxy, hydroxyl, acetoxy, -P0 ₃ H, methylthio, ethylthio, i-propylthio, n-propylthio, phenyitio, mercapto, acetylthio, thiocyano, methyisu lfinyl, methylsulfonyi, dimethylsu lfony!, sulfonate groups, fluorine atom, ch lorine atom, bromine atom, iodine atom, tri methylsilyl, triethyisilyl, trimethylstannyi, furyl, thienyl , pyridyl, piperidino, morpholino, pyrrolidyl groups and so on .

Among these substituents, electron donating groups such as al koxy groups, for exa mple methoxy, ethoxy, isopropoxy, tert-butoxy or phenoxy groups; methyi, ethyl, isopropyl, hyd roxyl, acetoxy, benzoyloxy groups, etc, or a thioa lkyl group, such as methylthio, ethylthio, n-propylthio, i- propylthio, butylthio, pentylthio, or a a ryithio g roup, such as phenylth io, are prefera bly contained .

Among those mentioned above, Ri of formula I is preferably Ar, a substituted or unsubstituted alky! group or a substituted or unsubstituted aryl group.

Preferred for the realization of the present invention are alpha-diketones of formula I wherein Ar is an aromatic group of formula (a) in which at least one of the substituents from R ₂ to R ₆ is a group -S-R ₇ and wherein Ri, R2, R3, R4, R5, R-6 and R ₇ have the sa me mea nings reported above. In a most preferred embodiment of the present invention Ar is a n aromatic group of formula (a ) in which at least one of R ₂ to R ₆ is -S-R ₇ and Ri is an unsubstituted C1-C12 a lkyl group or a phenyl group.

In another most preferred embodiment in the alpha-di ketones of formula I Ri is Ar and each Ar is independently an aromatic group of formula (a) in which at least one of the substituents from R ₂ to R ₆ is a group -S-R ₇ and R ₇ is a C1-C12 alkyl group or a phenyl group.

The compounds represented by formula I can be prepared according conventional methods known to the expert in the art. In particular, they can be synthesized, as schematized below, primarily by Friedel-Crafts acylation with the corresponding acyl chloride, nitrosation to give the 1,2- dione monooxime and subsequent hydrolysis.

An alternative preparation method is reported for example in Syn!ett, 13, 2315-2318, 2004.

The photopo!ymerizable compositions of the invention can also conveniently include a co-initiator, which is a molecule that acts as hydrogen donor that increases the polymerization rate. The co-initiators are known in the art and they are typically alcohols, thiols, amines or ethers that have an available hydrogen, bonded to a carbon adjacent to the heteroatom. Such co-initiators are generally present in an amount comprised between 0.2 and 15% by weight, preferably from 0.2 to 8% by weight, Suitable co-initiators include, but are not limited to, aliphatic, cycloaliphatic, aromatic, aryl-aiiphatic, heterocyclic, oligomeric or polymeric amines. They can be primary, secondary or tertiary amines, for example butyl amine, dibutyl amine, tributyl amine, cic!ohexyl amine, benzyldimethyl amine, di-cyclohexyl amine, N-phenyl glycine, triethyl amine, phenyi-diethanol amine, triethanolamine, piperidine, piperazine, morpholine, pyridine, quinoline, esters of dimethyiamino benzoic acid, Michler's ketone (4,4'-bis-dimethyl aminobenzophenone) and corresponding derivatives.

As the amine co-initiator, an amine-modified acrylate compound can be used, examples of such amine-modified acrylate include acrylates modified by reaction with a primary or secondary amine that are described in US 3,844,916, EP 280222, US 5,482,649 or US 5,734,002. Preferred co-initiators are Esacure A198 (bis-N,N-[4- dimethyiaminobenzoyl) oxyethylen-l-yl]-methylamine) and Esacure EDB (ethyl-4-dimethylamino benzoate) both from Lamberti S.p.A., IT and 2- ethylhexyl-4-dimethylaminobenzoate.

The photopolymerizable compositions of the invention can also conveniently include other photoinitiators commonly used in the field. Examples of photoinitiators which can be used in combination with the alpha-diketones of formula I include acylphosphine oxides, both monoacylphosphine oxides and bisacylphosphine oxides, coumarines or ketocoumarines, aromatic onium salt compounds, organic peroxides, thioxanthones, hexaaryl bisimidazoles, ketoxime esters, borate compounds, azinium compounds, metaliocene compounds, benzophenones, ketosulfones, a-aminoketones, benzoin and benzoin ethers, benzii ketals, a-hydroxyketones and mixture thereof.

Examples of thioxanthone derivatives are thioxanthone, diethylthioxanthone, 2-isopropylthioxanthone, 2-ch!orothioxanthone, 2,4- diethyithioxanthone or those described in the patent application PCT/EP2011/069514, such as n-dodecyl-7-methy!-thioxanthone-3- carboxylate and N,N-disobutyl-7-methyi-thioxanthone-3-carbamide.

Examples of a-hydroxyketones and -aminoketones are 1-hydroxy cyclohexylphenyl ketone, 2-hydroxy-2-methyl-l-phenyl-propane-l-one, l-[4-(2-hydroxyethoxy)pheny!]-2-hydroxy-2-methyl-l-propane-l -one, 2-hydroxy-l-{4-[4-(2-hydroxy-2-methyl-propionyl)-benzyl]phen yl}-2- methyl-propane-l-one), 2-methyl-l-(4-methylthiophenyl)-2- morphoiinopropane-l-one), 2-benzyi-2-dimethylamino-l-(4- morpholinophenyl)-butanone-l, and (2-(dimethylamino)-2-[(4- methylpheny I) methyl ]-l-[4-(4-morpholinyl)phenyl]- l-butanone).

Examples of oxime-based photoinitiators are l,2-octanedione,l-[4-

(phenylthio)phenyf]-,2-(0-benzoyloxime) and ethanone,l-[9-ethyl-6-(2- methylbenzoyl)-9H-carbazoSe-3-yl],l-(0-acetyioxime).

Examples of the acylphosphine-based photoinitiators inciude, but are not limited to, bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide, 2,4,6- trimethylbenzoyl-diphenyl phosphine oxide and bis(2,6- dimethoxybenzoyl)-2,4,4-trimethylpentyl phosphine oxide.

Examples of coumarine derivatives may include 4-methyl-7-hydroxy coumarine, 4-methyl-7-dimethylamino coumarine, 4-methyl-7-ethylamino coumarine, 4-methylpiperidino[3.2-g]coumarine, 4-methyl-7-cyclohexyl amino coumarine, 4-trifiuoromethyl-7-diethylamino coumarine, 3-phenyi- 4-methyl-7-diethylamino coumarine, 3-(2'-N-methyi benzimidazoyl)-7- diethylamino coumarine, 4-trifiuoromethyl-6-methyl-7-ethylamino coumarine and 3-pheny!-7-amino coumarine. Other specific examples of photoinitiators include acetophenone, acetophenone benzil ketal, 2,2-dimethoxy-2-phenylacetophenone, xanthone, fluorenone, anthraquinone, 3-methyiacetophenone, 4- chlorobenzophenone, 4,4'-dimethoxybenzophenone, 4,4'-diaminobenzo phenone, Michler's ketone, benzoin propyl ether, benzoin ethyl ether, benzil dimethyl ketal.

Preferred additional photoinitiators are acylphosphine oxides, a- hydroxyketones, -aminoketones, ketosulfones, ketocoumarines and bi- functional photoinitiators, for example Esacure 1001 and Esacure ONE (both commercialized by Lamberti S.p.A., IT).

The additional photoinitiators or mixture of different photoinitiators can be added to the photopolymerizabie compositions of the invention in an amount comprised between 0.5 and 15% by weight, preferably between 1 and 8% by weight.

By ethylenical!y unsaturated compound we mean a monomer, oligomer, prepolymer having at least one unsaturated double bond, or a mixture thereof, capable of undergoing radical polymerization. Also monomer combinations, oligomers and prepolymers with different degrees of unsaturation can be used .

The monomers suitable for the realization of the present invention can be chosen for example among vinyl ethers; N-vinyl pyrrolidone; N-vinyl caprolactam; mono- and poly-functional allyl ethers such as trimethylol propane dial!yl ether; styrenes and alpha-methyl styrenes; esters of (meth)acrylic acid with aliphatic alcohol, glycols, polyhydroxylated compounds such as pentaerythritoi or trimethylol propane; esters of vinyl aicoho! with acrylic or aliphatic acid;, derivatives of fumaric and maleic acids.

Suitable oligomers or prepolymers for the present invention comprise, for example, polyesters, polyacrylates, polyurethanes, epoxy resins, polyethers with acrylic, maleic or fumaric functionalities.

Monomers, oligomers and prepolymers, which are commonly used in photopolymerizable ink, in particular for ink-jet printing, are preferred. These compounds are well known to the expert in the art and are described for example in EP 1911814, US 2012/029108, US 2011/0074897 and WO 2006/102524. Specific examples thereof include monofunctional, difunctional and polyfunctional monomers such as the compounds represented by the formulas reported below:

Besides the above-mentioned compounds, other components normally used in the field and known to the experts in the art can be added to the photopo!ymerizable compositions of the invention. For example, thermal stabilizers, photo-oxidation stabilizers, anti-oxidants, fillers, dispersants, coloring and/or opacifying substances and other additives of general use. Others components of the photopolymerizabie compositions of the invention can be non-photopolymerizable polymers present as chemically inert substances, as an example nitrocellulose, polyacryiic esters, polyolefins etc.

The process claimed in the present invention is useful in the coating of metallic, wood, paper and plastic surfaces. The alpha-diketones of formula I work both in transparent photopoiymerizable compositions and in non-transparent or colored compositions and, in particular, are useful for the preparation of inks photopoiymerizable with a LED light source. These photoinitiators are particularly suited for the preparation of photopoiymerizable inks for ink- jet printing .

Among the alpha-diketones of formula I, those in which R _t is Ar and each Ar is independentiy an aromatic group of formula (a) in which at least one of the substituents from R ₂ to R ₆ is -S-R ₇ and the other are hydrogen, and R ₇ is n-propyl or a linear or branched C ₄ -Ci ₂ alky! or C ₄ -C ₁₂ cycloa!kyl group are particularly suitable for the preparation of photopoiymerizable inks.

Colorants which can be used in the LED photopoiymerizable inks of the invention are dyes, pigments or a combination thereof. Organic and/or inorganic pigments may be used. The colorants are preferably pigments or polymeric dyes, most preferably pigments. The pigments may be black, white, cyan, magenta, yellow, red, orange, violet, blue, green, brown, mixtures thereof, and the like.

Exemplary organic pigments include insoluble azo pigments, condensed azo pigments, azo lake, and chelate azo pigments; polycyclic pigments, such as phthalocyanine pigments, pery!ene and perinone pigments, anthraquinone pigments, quinacridone pigments, dioxane pigments, thioindigo pigments, isoindolinone pigments, and quinophthaione pigments; dye chelates, such as basic dye chelates and acid dye chelates; dye lakes, such as basic dye lakes and acid dye lakes; and nitro pigments, nitroso pigments, aniline black, and fluorescent pigments.

For LED photopotymerizable white inks, the white colorants are preferably present in an amount of 3% to 30% by weight of the ink composition, and more preferably 5% to 25%. Usually the other colorants are present in the LED photopolymerizable inks of the invention in the range of 0.01 to 10 % by weight, preferably in the range of 0.1 to 5 % by weight.

In addition to the main components, the LED photopolymerizable inks can contain also other specific ingredients such as coinitiators and other photoinitiators, such as those described in the preceding paragraphs and in the same amount, dispersants, surfactants and other additives which are well known to the expert in the art. The choice of these components is not particularly limited.

The dispersants are added to the inks so as to enhance the dispersibility of the pigment. For the realization of the present invention, a dispersant that is generally used for preparing a pigment-dispersed liquid, such as a polymeric dispersant, can be used. Examples of such polymeric dispersant include polyoxyalkylene, polyalkyiene polyamines, vinyl polymers and copolymers, acrylic polymers and copolymers, polyesters, polyamides, polyimides, polyurethanes, amino polymers, silicon-containing polymers, sulfur-containing polymers, fluorine-containing polymers, and epoxy resins.

Examples of preparation of alpha-diketones of formula I and photopolymerizable compositions according to the invention, only for illustrative purpose and not limitative, are reported in the following paragraphs.

EXAMPLES

Example 1

Preparation of l,2-bis(4-(propylthio)phenyl)ethane-l,2-dione

1.3 g 1-propanethiol (17 mmol) were added to a solution of 0.95 g KOH (17 mmol) in 20 ml DMSO and the solution was stirred at room temperature for 30 minutes. 2 g of 4,4'-difluorobenzil (8.1 mmol) were added in small portion and after 4 h at room temperature under stirring, the mixture was poured in water. 20 ml of dich!oromethane were added, the organic layer was separated and washed with water. The organic fraction was isolated, dried with Na ₂ S0 ₄ and evaporated under reduced pressure. The product was purified by flash chromatography (Si0 ₂ - eluent: CH ₂ CI ₂ / ETP 6:4) giving 2.3 g (6.4 mmol) of a yellow oil which tends to crystallize.

Example 2

Preparation of l -(4~(phenylthio)phenyl)propan-l-one

30 g (0.161 mol) of diphenyl sulfide and 14.9 g (0.161 mol) of propionyl chloride were dissolved in 300 ml methylene chloride. The reaction mixture was cooled to 0 °C and 25.78 g (0.193 mol) of aluminum chloride were added portion wise, maintaining the temperature below 5°C. The reaction was allowed to continue for 3 h at room temperature. The reaction mass was then poured into 500 ml of ice. The organic fraction was isolated, dried with Na ₂ S0 ₄ and evaporated under reduced pressure. The intermediate obtained was used without any further purification in the subsequent step.

Preparation of l -(4-(phenylthio)phenyi)proparte-l,2-dione 25 ml of HCI were added dropwise at room temperature and under vigorously stirring to a solution of 15 g of l-(4-(phenyithio)phenyl) propan-l-one (61.8 mmol) and 6.4 g of NaN0 ₂ (92.8 mmol) in 150 ml of THF. After 4 h at room temperature the solvent was evaporated under vacuum and 50 mi of methylene chloride were added. The solution was washed with water, dried with Na ₂ S0 ₄ and evaporated under reduced pressure. 25 ml of HCI were added and the mixture was heated for 3 h refluxing. The organic phase was separated with 50 ml of methylene chloride, washed with water, dried and purified by flash chromatography (Si0 ₂ - eluenti Toluene) giving 11.1 g (43.2 mmol) of a yellow oil. Examples 3-4 The compounds of Examples 3-4 were prepared according to the procedure described in Example 2 from the corresponding ketones. The structure and ^-NMR-data of the compounds of Examples 1-4 are given in Table 1. Table 1.

Examples Structure ¹ H-NMR (CDCI ₃ )

1.05 (t, 6H), 1.72 (m,

Example 1 4H), 2.97 (t, 4H), 7.3

(d, 4H), 7.85 (d, 4H)

2.42 (s, 3H), 7.13 (d,

2H), 7.38 (m, 3H),

Example 2 a,xxV 7.47 (m, 2H), 7.81 (d,

2H). 7.19 (d, 2H), 7.48 (m,

Example 3 7H), 7.65 (t, 1H), 7.82

(d, 2H), 7.96 (d, 2H)

2.49 (s, 3H), 2,51 (s,

Example 4 3H), 7.27 (d, 2H), 7.92

(d, 2H)

Evaluation tests Photo DSC tests

The compositions for the DSC test were prepared dissolving the alpha- diketones photoinitiators described in the Examples 1-4 at a concentration of 0.1% w/w and EDB (ethyl 4-dimethyiamino benzoate) at a concentration of 0.1% w/w in tripropylene glycol diacrylate.

About 10.0 mg of composition, weighed with a precision of ± 0.3 mg, were transferred into a DSC aluminum panel and analyzed by a ettler DSC1 calorimeter equipped with a 400 nm LED with a power of 450 mW. The LED parameters were set in order to irradiate the composition with an intensity of 24.3 mW/cm ² .

During the exposition to LED light the heat developed from the polymerization of the compositions was recorded as peak height and peak area (ΔΗ). The peak height is proportional to the rate of polymerization : the higher the peak, the faster the polymerization. l-phenyl-l,2-propanedione and 4,4'-dimethoxybenzil were chosen as reference photoinitiators.

The results are reported in Table 2.

The compositions for the test were prepared dissolving the alpha- diketones photoinitiators described in Examples 1-4 at a concentration of 4% w/w and Esacure EDB (commercialized by Lamberti S.p.A) at a concentration of 4% w/w in a mixture 99.5 :0.5 w/w of Ebecryl 605 and Ebercyl 350 (Cytec Industries Inc.).

Table 2

* Comparative

Evaluation of alpha-diketones photoinitiators by FT-IR

The sample, placed in the sample lodgment of a FT-IR (FT-IR 430-Jasco), is exposed to a LED (400 nm) source set at a distance of 65 mm from the sample and at an angle of 30°.

IR spectra are acquired at constant time intervals during the photopolymerization and the reduction over the time of the peak area at 1408 cm ^"1 assigned to the acrylic double bond was determined using the IR software.

This allows to quantify the degree of polymerization and therefore the efficiency of the photoinitiator.

l-phenyl-l,2-propanedione and 4,4'-dimethoxybenzil were chosen as reference photoinitiators.

The results as % degree of polymerization over the time are reported in Table 3.

Tabie3

* Comparative