Photoinitiator and uses thereof

Abstract

The present invention relates to a compound having the general formula (I) wherein R1 is a linear or branched C1 to C4 alkyl group, (C=O) -CH3, -CH2-CF3, or -CH2-C6H5, and R2 is -H or -OR3, wherein R3 is C1 to C4 alkyl group, - (C=O) -CH3, -CH2-CF3, or -CH2-C6H5.

Description

[0001] PHOTOINITIATOR AND USES THEREOF

[0002] TECHNICAL FIELD

[0003] The present invention relates to the field of photoinitiators .

[0004] BACKGROUND ART

[0005] The choice of photoinitiator is crucial in the process of curing photoreactive resins . When exposed to UV or visible light , the photoinitiator absorbs the light and forms the species that initiate the polymeri zation . For radical photopolymeri zation, these species are free radicals .

[0006] There are two classes of photoinitiators based on the chemical mechanism of radical formation . Norrish Type I photoinitiators form free radicals by breaking a chemical bond when irradiated . Norrish Type I I photoinitiators , on the other hand, undergo a reaction with a second molecule ( known as a coinitiator ) in the excited state , resulting in the formation of polymeri zation-initiating radicals through electron and proton trans fer . Type I and Type I I photoinitiators are used for UV light curing, with Type I I being the more commonly used in the visible range , except for bisacyl dialkyl germanium compounds .

[0007] UV curing is most ef fective for transparent coatings with thin layers due to the short wavelength of UV light . However, it is not suitable for strongly colored, pigmented or composite materials , as well as thicker layers , as these conditions prevent complete curing with UV light . In such cases , visible light is typically utili zed for irradiation, especially for applications like light-curing dental filling composites , where deeper curing is required .

[0008] Ivocerin (bis ( 4-methoxybenzoyl ) diethylgermane ) is a commonly used photoinitiator for various polymeri zation processes , particularly in dental materials such as dental composites , adhesives , and sealants . Ivocerin is usually activated at a wavelength of 390-445 nm . WO 2023 / 186782 discloses tetrakis ( 2 , 6-dimethoxybenzoyl ) germane as photoinitiator which can be activated up to a wavelength of 480 nm.

[0009] US 2019 / 0194362 Al discloses polymerizable compositions which contain an acyltin compound as a photoinitiator.

[0010] US 2021 / 0261578 Al discloses aromatic acylgermanium and acyltin compounds suitable as photoinitiators for radical polymerization and especially for the production of dental materials .

[0011] US 2023 / 0364832 Al discloses the use of an acyltin photoinitiator, especially tetrakis (2, 4, 6-trimethylbenzoyl) stannane, involved in the production of UV-absorbing silicone hydrogel contact lenses.

[0012] Radebner, J et al. Chemistry 2018, 24, 8281-8285, disclose tetraacylstannanes as photoinitiators exhibiting absorbance maxima at around 400 nm and low cytotoxicity.

[0013] Mitterbauer, M et al. Macromol Mat Eng 2017, 302, 1600536, disclose tetrakis ( 2 , 4 , 6-trimethylbenzoyl ) stannane as a photoinitiator which can be activated up to a wavelength of 550 nm. However, it turned out that tetrakis ( 2 , 4 , 6-trimethylben- zoyl ) stannane shows a rather low stability.

[0014] It is an object of the present invention to provide stable compounds which overcome the drawbacks of photoinitiators known in the art and which show efficient absorbance at long wavelengths .

[0015] SUMMARY OF THE INVENTION

[0016] The present invention relates to a compound having the general formula (I) wherein Ri is a linear or branched Ci to C4 alkyl group, - ( C=O) -CH3, -CH2-CF3, or -CH2-C6H5, and R2is -H or -0R3, wherein R3is Ci to C4alkyl group, - ( C=0) -CH3, -CH2-CF3, or -CH2-C6H5.

[0017] It turned surprisingly out that compounds having general formula ( I ) have an absorption range for visible light which is clearly shifted towards larger wavelengths compared with known storage stable photoinitiators . They thus allow the polymeri zation to be initiated with light of higher wavelengths and make larger depths of cure possible . The compound of the present invention can be cleaved using light with up to 560 nm . This is unexpected since germanium-based photoinitiators having a comparable structure can only be cleaved at a wavelength up to 480 nm ( see WO 2023 / 186782 ) . On the other side , structurally similar stannane-based photoinitiators show a signi ficantly reduced stability compared to the compounds of the present invention ( see example 5 below) . Increased stability of the compounds of the present invention is not only advantageous but in fact essential for their use as photoinitiators in most polymeri zation processes .

[0018] Furthermore , the compounds of the present invention are characteri zed by very high extinction coef ficients of the absorption in the visible range . In low concentration they are therefore already ef fective as photoinitiators for photopolymeri zation initiated by visible light .

[0019] Another aspect of the present invention relates to a composition comprising at least one compound according to the present invention as photoinitiator and at least one radically polymeri zable monomer .

[0020] The compounds of the present invention produce radicals when exposed to radiation . Hence , said compounds can be used as photoinitiators for initiating radical polymeri zation processes . Together with one or more radically polymeri zable monomers in a composition they may be used for producing polymers .

[0021] A further aspect of the present invention relates to a kit comprising at least one container comprising at least one com- pound of the present invention and at least one further container comprising at least one radically polymerizable monomer of the present invention.

[0022] Hence, another aspect of the present invention relates to a method for the preparation of a polymer comprising the step of exposing a composition according to the present invention to electromagnetic radiation at a wavelength up to 560 nm.

[0023] A further aspect of the present invention relates to the use of a compound or a composition according to present invention for preparing or restoring dental restorations, prosthe- ses, dentures, inlays, onlays, crowns or bridges; as a dental material, preferably as a dental cement, a filling composite material or a facing material; as photocurable composition for 3D printing; highly filled or pigmented formulations, or the curing of very thick layers.

[0024] Another aspect of the present invention relates to the use of a compound according to the present invention as a photoinitiator .

[0025] BRIEF DESCRIPTION OF THE FIGURES

[0026] Fig. 1 shows UV / Vis spectra of tetrakis ( 2 , 6-dimethoxyben- zoyl ) stannane (1) , tetrakis ( 2 , 6-dimethoxybenzoyl ) germane (VI) , tetrakis ( 2 , 4 , 6-trimethylbenzoyl ) stannane (V2) and ivocerin (V3) with a concentration of 1 -lCU3M in acetonitrile

[0027] Fig. 2 shows stability of 1 -lCU3M solutions of compounds 1, VI, V2 and V3 in acetonitrile with 200 ppm H2O, absorbance at the absorption maximum of each compound normed to 100 % for day 0 over time.

[0028] Fig. 3 shows steady state photolysis experiments of l -10~3M solutions of compounds 1, VI, V2 and V3 in chloroform, absorbance at the absorption maximum of each compound normed to 100 % at the beginning over time during irradiation with a 460 nm LED (130 mW / cm2) .

[0029] DESCRIPTION OF EMBODIMENTS

[0030] According to a preferred embodiment of the present invention the linear or branched Ci to C4 alkyl group is a methyl group or an ethyl group. According to another preferred embodiment of the present invention Ri and / or R3 is a methyl group.

[0031] According to a further preferred embodiment of the present invention Ri is a methyl group and R2 is -H or -OCH3.

[0032] It is particularly preferred that the compound of the present invention is tetrakis ( 2 , 6-dimethoxybenzoyl ) stannane having formula (II)

[0033] (ID

[0034] The compound of the present invention can be synthesized, for instance, by derivatizing a potassium intermediate obtainable by reacting tetrakis ( trimethylsilyl ) stannane with potassium tert-butoxide (KOtBu) with an acid fluoride according to formula (III)

[0035] (HI) according to Haslinger, C. et al. ChemPhotoChem 2022, 6, e202200108. Tetrakis ( trimethylsilyl ) stannane is synthesized as described in Buerger, H.; and Goetze, U. Angew. Chem. Int.

[0036] Ed. Engl. 1968, 7 (3) , 212-13 and the acid fluoride can be synthesized according to Kaduk, C. et al. Lett. Pept. Sci. 1996, 2 (5) , 285-288. As these reactions are moisture sensitive, it is preferred to utilize Schlenk techniques.

[0037] Another aspect of the present invention relates to a composition comprising at least one compound according to the present invention as photoinitiator and at least one radically polymerizable monomer.

[0038] "Radically polymerizable monomer", also known as a radical monomer or a free-radical monomer, is a chemical compound that is capable of undergoing a radical polymerization process, which involves the formation of covalent bonds between monomer units through the use of radicals, formed in particular by photoinitiators .

[0039] According to a preferred embodiment of the present invention the composition comprises 0.001 to 10 wt%, preferably

[0040] 0.01 to 8 wt%, more preferably 0.01 to 6 wt%, more preferably

[0041] 0.01 to 5 wt%, more preferably 0.01 to 4 wt%, more preferably

[0042] 0.01 to 3 wt%, more preferably 0.01 to 2 wt%, of the at least one compound according to the present invention. In a particular preferred embodiment of the present invention the composition comprises 0.01 to 2 wt% of the at least one compound according to the present invention.

[0043] According to another preferred embodiment of the present invention the composition comprises at least one further photoinitiator from the group of Type I initiators and / or Type II initiators. Type I initiators may be selected from the group consisting of, but not limited to, acylgermanium compounds, (bis ) acylphosphine oxides and alpha-hydroxy ketones. Type II initiators may be selected from the group consisting of, but not limited to, camphorquinone, benzophenone, thioxantone and similar compounds.

[0044] The compositions of the present invention comprise at least one radically polymerizable monomer. Said at least one monomer may comprise two or more, preferably two to three, radically polymerizable groups. Polyfunctional monomers have crosslinking properties.

[0045] Hydrolysis-stable monomers, such as hydrolysis-stable mono (meth) acrylates , e.g. mesityl methacrylate or 2- (alkoxyme- thyl) acrylic acids, e.g. 2- ( ethoxymethyl ) acrylic acid, 2- (hydroxymethyl ) acrylic acid, N-mono- or -disubstituted acrylamides, such as e.g. N-ethylacrylamide, N, N-dimethacrylamide, N- ( 2-hydroxye thyl ) acrylamide or N-methyl-N- ( 2-hydroxy- ethyl ) acrylamide, N-monosubstituted methacrylamides, such as e.g. N-ethylmethacrylamide or N- ( 2-hydroxyethyl ) methacrylamide as well as N- vinylpyrrolidone or allyl ether can advantageously also be used as radically polymerizable monomers.

[0046] Preferred examples of hydrolysis-stable crosslinking monomers are urethanes of 2- (hydroxymethyl ) acrylic acid and diisocyanates, such as 2 , 2 , 4-trimethylhexamethylene diisocyanate or isophorone diisocyanate, crosslinking pyrrolidones, such as e.g. 1 , 6-bis ( 3-vinyl-2-pyrrolidonyl ) -hexane, or commercially available bisacrylamides such as methylene or ethylene bisacrylamide, bis (meth) acrylamides , such as e.g. N,N'-di- ethyl-1, 3-bis (acrylamido) -propane, 1, 3-bis (methacrylamido) - propane, 1 , 4-bis ( acrylamido ) -butane or 1 , 4-bis ( acryloyl ) -piperazine, which can be synthesized by reaction of the corresponding diamines with (meth) acrylic acid chloride. Monomers that are liquid at room temperature, which can be used as diluting monomers, are preferred. Besides these hydrolysis stable monomers, all other monomers based on acrylate and methacrylates are suitable as well.

[0047] Low-shrinkage radically ring-opening polymerizable monomers such as e.g. mono- or multifunctional (i.e. polyfunctional) vinylcyclopropanes or bicyclic cyclopropane derivatives, preferably those described in DE 196 16 183 C2 or EP 1 413 569 Al, or cyclic allyl sulfides, preferably those described in US 6,043,361 and US 6,344,556, can furthermore also be used as radically polymerizable binders. These can advantageously also be used in combination with the di (meth) acrylate crosslinkers listed above. Preferred ring-opening polymerizable monomers are vinylcyclopropanes, such as 1 , 1-di ( ethoxycarbonyl ) - or 1 , 1-di (methoxycarbonyl ) -2-vinylcyclopropane or the esters of 1-ethoxycarbonyl- or l-methoxycarbonyl-2-vinylcyclopropanecar- boxylic acid with ethylene glycol, 1 , 1 , 1-trimethylolpropane, 1 , 4-cyclohexanediol or resorcinol. Preferred bicyclic cyclopropane derivatives are 2- (bicyclo [ 3.1.0 ] hex-l-yl ) acrylic acid methyl or ethyl esters and their disubstitution products in the 3 position, such as ( 3 , 3-bis ( ethoxycarbonyl ) bicyclo [ 3 . 1 . 0 ] hex-l-yl ) acrylic acid methyl or ethyl esters . Preferred cyclic allyl sul fides are the addition products of 2- (hydroxymethyl ) - 6-methylene- l , 4-dithiepane or 7-hydroxy-3- methylene- 1 , 5-dithiacyclooctane with 2 , 2 , 4-trimethylhexameth- ylene- 1 , 6-diisocyanate or the asymmetrical hexamethylene diisocyanate trimer ( Desmodur® VP LS 2294 from Bayer AG) .

[0048] Further preferred radically polymeri zable monomers are vinyl esters , vinyl carbonates and vinyl carbamates . Moreover, styrene , styrene derivatives , divinylbenzene , unsaturated polyester resins as well as allyl compounds or radically polymeri zable polysiloxanes , which can be produced from suitable methacrylsilanes , such as e . g . 3- (methacryloyloxy ) propyltrimethoxysilane , and are described e . g . in DE 199 03 177 C2 , can also be used as radically polymeri zable monomers . By styrene derivatives is meant compounds in which the phenyl group of the styrene , but not the vinyl group, is mono- or polysubstituted by simple groups , such as Ci to Cio alkyl , Cl , Br, OH, CH3O, CHO, C2H5O, COOH or carboxylic acid ester groups .

[0049] Moreover, mixtures of the above-named monomers with radically polymerizable , acid-group-containing monomers , which are also called adhesive monomers , can also be used as radically polymeri zable binders . Preferred acid-group-containing monomers are polymeri zable carboxylic acids , such as maleic acid, acrylic acid, methacrylic acid, 2- (hydroxymethyl ) acrylic acid, 4- (meth) acryloyloxyethyltrimellitic acid anhydride , 10-methac- ryloyloxydecylmalonic acid, N- ( 2-hydroxy-3-methacryloyloxypro- pyl ) -N-phenylglycine or 4-vinylbenzoic acid .

[0050] Radically polymeri zable phosphonic acid monomers , in particular vinylphosphonic acid, 4-vinylphenylphosphonic acid, 4- vinylbenzylphosphonic acid, 2-methacryloyloxyethylphosphonic acid, 2-methacrylamidoethylphosphonic acid, 4-methacrylamido- 4-methyl-pentyl-phosphonic acid, 2- [ 4- ( dihydroxyphosphoryl ) -2- oxa-butyl ] -acrylic acid or 2- [ 2-dihydroxyphosphoryl ) -ethoxymethyl ] -acrylic acid ethyl or 2 , 4 , 6-trimethylphenyl ester are particularly suitable as adhesive monomers . In addition, acidic polymeri zable phosphoric acid esters , in particular 2-methacryloyloxypropyl mono- or dihydrogen phosphate , 2-methacryloyloxyethyl mono- or dihydrogen phosphate , 2-methacryloyloxyethylphenyl hydrogen phosphate , dipen- taerythritol-pentamethacryloyloxy phosphate , 1 O-methacrylo- yloxydecyl dihydrogen phosphate , dipentaerythritol-pen- tamethacryloyloxy phosphate , phosphoric acid mono- ( 1-acryloyl- piperidin-4-yl ) ester, 6- (methacrylamido ) hexyl dihydrogen phosphate and 1 , 3-bis- (N-acryloyl-N-propyl-amino ) -propan-2-yl dihydrogen phosphate , are suitable as adhesive monomers .

[0051] Furthermore , polymeri zable sul fonic acids are also suitable as adhesive monomers , in particular vinylsul fonic acid, 4-vi- nylphenylsul fonic acid or 3- (methacrylamido ) propylsul fonic acid .

[0052] Thiol-ene resins which contain mixtures of mono- or multifunctional mercapto compounds and di- or multi functional unsaturated monomers , above all allyl or norbornene compounds , are particularly suitable as binders curable by polyaddition .

[0053] Examples of mono- or multi functional mercapto compounds are o- , m- or p-dimercaptobenzene and esters of thioglycolic or of 3-mercaptopropionic acid of ethylene , propylene or butylene glycol , hexanediol , glycerol , trimethylolpropane or pentaerythritol . Preferred multi functional mercapto compounds to produce thiol-ene resins include derivatives of thiocyanuric acid .

[0054] Examples of di- or multi functional allyl compounds are esters of allyl alcohol with di- or tricarboxylic acids , such as malonic, maleic, glutaric, succinic, adipic, sebacic, phthalic, terephthalic or gallic acid, as well as mono- or tri functional allyl ethers , such as e . g . diallyl ether, a, w- bis [ allyloxy ] alkanes , resorcinol or hydroquinone diallyl ether as well as pyrogallol triallyl ether, or other compounds such as e . g . 1 , 3 , 5-triallyl- l , 3 , 5-triazine-2 , 4 , 6- ( 1H, 3H, 5H) -trione , tetraallylsilane or tetraallyl orthosilicate .

[0055] Examples of di- or multi functional norbornene compounds are Diels-Alder addition products of cyclopentadiene or furan with di- or multi functional (meth) acrylates , as well as esters and urethanes of 5-norbornene-2-methanol or 5-norbornen-2-ol with di- or polycarboxylic acids , such as e . g . malonic, maleic, glutaric, succinic, adipic, sebacic, phthalic, terephthalic or gallic acid, with di- or polyisocyanates , such as hexamethylene diisocyanate or its cyclic trimer, 2 , 2 , 4-trimethylhexa- methylene diisocyanate , toluylene diisocyanate or isophorone diisocyanate .

[0056] According to a preferred embodiment of the present invention the at least one radically polymeri zable monomer is a monofunctional (meth) acrylate or a polyfunctional (meth) acrylate .

[0057] The polyfunctional (meth) acrylate is preferably a di (meth) acrylate , preferably selected from the group consisting of urethane di (meth) acrylates , alkyl di (meth) acrylate , polyether- and polyesterdi (meth) acrylates or epoxy (meth) acrylates .

[0058] Other preferred (meth) acrylates are selected from the group consisting of methyl (meth) acrylate , ethyl (meth) acrylate , n- propyl (meth) acrylate , isopropyl (meth) acrylate , n-butyl acrylate , isobutyl acrylate , tert-butyl (meth) acrylate , n- pentyl (meth) acrylate, iso-pentyl (meth) acrylate , n-hexyl (meth) acrylate , iso-hexyl (meth) acrylate , cyclohexyl (meth) acrylate , phenyl (meth) acrylate , octyl (meth) acrylate , iso-octyl (meth) acrylate, 2-octyl (meth) acrylate , 2-ethylhexyl (meth) acrylate, decyl (meth) acrylate , lauryl (meth) acrylate , 2- propylheptyl (meth) acrylate , stearyl (meth) acrylate , isobornyl acrylate , benzyl (meth) acrylate , octadecyl acrylate , nonyl acrylate, dodecyl acrylate , isophoryl (meth) acrylate , (meth) acrylate esters of 2-alkyl alkanols wherein the molar carbon number average of the 2-alkyl alkanols is 12 to 32 , and any combinations or mixtures thereof .

[0059] According to a particularly preferred embodiment of the present invention the composition comprises 1 to 99 wt% , preferably 5 to 95 wt% , more preferably 10 to 90 wt% , more preferably 15 to 85 wt% , of the at least one radically polymeri zable monomer .

[0060] According to another preferred embodiment of the present invention the composition of the present invention may further contain at least one organic or inorganic filler or fiber or additives, preferably pigments, dyes and / or stabilizers.

[0061] The composition according to the present invention can advantageously comprise one or more organic or preferably inorganic fillers. Fibrous and in particular particulate fillers are preferred.

[0062] Nanofibres, glass fibres, polyamide fibres and carbon fibres are preferred as fibrous fillers. By nanofibres is meant fibres with a length of less than 100 nm. Fibrous fillers are particularly suitable for the production of composite materials .

[0063] Preferred inorganic fillers are amorphous spherical nanoparticulate fillers based on oxides, such as pyrogenic silica or precipitated silica, ZrCh and TiCh or mixed oxides of SiO2, AI2O3, ZrO2 and / or TiCh, lithium disilicate, microfine fillers, such as quartz, glass ceramic or glass powder, and radiopaque fillers, such as ytterbium trifluoride, nanoparticulate tantalum (V) oxide or barium sulfate. Preferred inorganic fillers include also phosphates like hydroxyapatite and tricalciumphosphate, nitrides such as silicon nitrides, and metals like titanium, iron and copper. The ytterbium trifluoride preferably has a particle size of from 200 to 800 nm.

[0064] Particulate fillers preferably have a particle size of from 0.01 to 15 pm. Nanoparticulate fillers preferably have a particle size of from 10 to 100 nm and microfine fillers preferably have a particle size of from 0.2 to 5 pm. Radiopaque fillers, unless they are nanoparticulate fillers, preferably have a particle size of from 0.2 to 5 pm.

[0065] Unless otherwise indicated, all particle sizes are weightaverage particle sizes (D50 values) , wherein the particle size determination in the range of from 0.1 pm to 1000 pm is preferably effected by means of static light scattering, for example using an LA-960 Static Laser Scattering Particle Size Distribution Analyzer (Horiba, Japan) . Here, a laser diode with a wavelength of 655 nm and an LED with a wavelength of 405 nm are used as light sources. The use of two light sources with different wavelengths makes it possible to measure the entire particle size distribution of a specimen in only one measurement pass, wherein the measurement is carried out as a wet measurement. For this purpose, a 0.1 to 0.5% aqueous dispersion of the filler is produced and the scattered light thereof is measured in a flow cell. The scattered light analysis for calculating particle size and particle size distribution is effected in accordance with the Mie theory according to DIN / ISO 13320. The measurement of the particle size in the range of from 5 nm to 0.1 pm is preferably effected by dynamic light scattering (DLS) from aqueous particle dispersions, preferably using an He— Ne laser with a wavelength of 633 nm, at a scattering angle of 90°, and at 25° C., e.g. using a Malvern Zetasizer Nano ZS (Malvern Instruments, Malvern UK) .

[0066] Particle sizes smaller than 0.1 pm can also be determined by means of SEM or TEM micrographs. The transmission electron microscopy (TEM) is preferably carried out using a Philips CM30 TEM at an accelerating voltage of 300 kV. For the specimen preparation, drops of the particle dispersion are applied o to a 50 A thick copper grid (mesh width 300 mesh) , which is coated with carbon, and then the solvent is evaporated. The particles are counted and the arithmetic mean is calculated. To improve the bond between the filler particles and the crosslinked polymerization matrix, the fillers are preferably surface-modified. SiCh-based fillers are preferably surface- modified with methacrylate-functionalized silanes, particularly preferably with 3-methacryloyloxypropyltrimethoxysilane . For the surface modification of non-silicate fillers, e.g. of ZrO2 or TiO2, functionalized acidic phosphates, such as e.g. 10-methacryloyloxydecyl dihydrogen phosphate, can also be used .

[0067] Moreover, the composition according to the invention may contain further additives and solvents. The additives are preferably selected from stabilizers, chain transfer reagents, UV absorbers, dyes or pigments, rheological additives and lubricants. Preferred solvents are water, ethanol, acetone, ethyl acetate and mixtures thereof. A further aspect of the present invention relates to a kit comprising at least one container comprising at least one compound of the present invention and at least one further container comprising at least one radically polymeri zable monomer of the present invention .

[0068] The kit of the present invention comprises at least two containers , wherein one of said at least two containers contains at least one compound of the present invention and the other contains at least one radically polymeri zable monomer of the present invention . Furthermore , the kit may also comprise instructions on the use of the components for polymeri zation reactions .

[0069] Another aspect of the present invention relates to the use of a compound according to the present invention as a photoinitiator .

[0070] The compounds of the present invention and / or the compositions of the present invention can be used in various technical fields . They can be used in inks , coatings , and varnishes for curable printing processes , such as screen printing, flexography, and lithography; in adhesives and sealants to ensure fast curing, strong bonding, and improved performance ; in wood coatings for furniture , flooring, and wooden surfaces to achieve rapid curing and enhanced durability; in electronics and optics for manufacturing of electronic components , such as printed circuit boards ( PCBs ) , semiconductor devices , and optical films , to enable precise curing processes and reliable performance ; in the automotive industry in the production of automotive coatings , such as paints , clear coats , and topcoats , to provide quick curing, scratch resistance , and protection against UV radiation; in the packaging industry as or in inks and coatings for packaging materials like cartons , labels , films , and plastics , ensuring ef ficient and high-quality printing and surface protection; in medical and dental applications in dental composites , orthodontic adhesives , and other medical devices that require fast curing, biocompatibility, and high-performance properties ; in curable ink formulations for 3D printing technologies , enabling rapid layer-by-layer curing of resin-based materials ; in the textile industry in inks and coatings for textile printing, ensuring vibrant colors , fast curing, and excellent wash resistance ; and in cosmetics and personal care in formulations of curable nail gels and coatings for manicures and pedicures , enabling quick drying and long-lasting ef fects . It is particularly preferred to use the compounds and compositions of the present invention for dental applications and 3D printing .

[0071] Thus , a further aspect of the present invention relates to the use of a compound or a composition according to present invention in inks , in coatings , in varnishes , in curable printing processes , preferably in lithography, in adhesives , in sealants , in electronics , in optics , in the automotive industry, in the packaging industry, in medical applications , in dental applications ( e . g . for preparing or restoring dental restorations , prostheses , dentures , inlays , onlays , crowns or bridges , dental material , preferably as a dental cement , a filling composite material or a facing material ) , for preparing bone material , in 3D printing processes or in the textile industry .

[0072] A further aspect of the present invention relates to a method for the preparation of a polymer comprising the step of exposing a composition according to the present invention to electromagnetic radiation at a wavelength up to 560 nm

[0073] One of the maj or advantages of the compounds of the present invention is that they can be cleaved using light with up to 560 nm allowing increased curing depths . A further advantage is the stability of the compounds of the present invention in regard to their decomposition in the darkness .

[0074] According to another preferred embodiment of the present invention, the composition is exposed to electromagnetic radiation at a wavelength ranging from 320 to 560 nm, preferably from 430 to 560 nm, more preferably from 470 to 560 nm, more preferably from 480 to 560 nm, more preferably from 490 to 560 nm .

[0075] The present invention is further illustrated in the following examples , however, without being restricted thereto .

[0076] EXAMPLES Example 1

[0077] Synthesis of 2r6-dimethoxybenzoyl fluoride

[0078] Since this reaction involved moisture-sensitive compounds, Schlenk techniques were used to exclude even traces of oxygen and moisture during the reaction. The synthesis of 2, 6-dimethoxybenzoyl fluoride was carried out according to Kaduk, C. et al. Lett. Pept. Sei. 1996, 2 (5) , 285-288.

[0079] 2 , 6-Dimethoxybenzoic acid (1.154 g, 5.5 mmol, 1 eq.) was dried in HV (high vacuum) and 25 mL dry DCM (dichloromethane) were added. The suspension was cooled to 0 °C with an ice bath. Diethylaminosulfur trifluoride (DAST, 1.12 g, 5.8 mmol, 1.05 eq.) was added dropwise over 5 min, resulting in a clear solution. After stirring at 0 °C for 1 h, the solution was poured onto iced saturated NH4CI solution. The aqueous phase was extracted three times with each 25 mL DCM, the combined organic phases were dried over Na2SO4 and filtered. The solvent was removed in vacuo, and the beige solid was dried in HV. The product was characterized via NMR spectroscopy.

[0080] Yield: 1.15 g (99 %) Characterization: beige solid

[0081] Melting point: 65.9 - 67.1 °C iH-NMR (400 MHz, CDCI3) : 5 7.41 (t, J = 8.5 Hz, 1H, Ar) , 6.59 (dd, J = 8.5, 1.0 Hz, 2H, Ar) , 3.87 (s, 6H, CH3) .

[0082] 13C NMR (101 MHz, CDCI3) : 5 159.28, 157.31, 153.83, 133.98, 104.03, 56.29.

[0083] 19F-NMR (376 MHz, CDCI3) : 5 53.64.

[0084] Synthesis of tetrakis (trimethylsilyl) stannane

[0085] As also this reaction involved moisture-sensitive compounds, Schlenk techniques were used to exclude even traces of oxygen and moisture during the reaction. The synthesis of tetrakis ( trimethylsilyl ) stannane was conducted according to Buerger, H.; and Goetze, U. Angew. Chem. Int. Ed. Engl. 1968, 7 (3) , 212-13.

[0086] Lithium foil (1.99 g, 288 mmol, 10 eq.) was cut into small pieces and suspended in 59 mL dry THF (tetrahydrofuran) in a Schlenk flask. TMSC1 (trimethylsilyl chloride; 22.5 mL, 180 mmol, 6.25 eq.) was mixed with SnC14 (3.4 mL, 28 mmol, 1 eq.) and filled into a dropping funnel equipped to the Schlenk flask. The flask was cooled to -78 °C before the TMSCl / SnC14 mixture was added dropwise over 50 min. The reaction was stirred overnight during reaching RT . The black suspension was heated up to reflux for 3 hrs and after cooling to RT filtered over Celite®, resulting in a clear solution. The liquid was quenched with 50 mL cooled 1 N H2SO4, the aqueous phase was washed three times with each 25 mL diethyl ether. The combined organic phases were washed three times with each 25 mL water and dried over Na2SO4. The solvent was evaporated in vacuo, resulting in a grey solid. It was dissolved in diethyl ether, filtered and the solvent was evaporated again in vacuo. The white solid was stored under argon at 4 °C.

[0087] Yield: 1.49 g (13 %) Characterization: white solid

[0088] Melting point: 181.3 - 199.1 °C 2H NMR (400 MHz, Benzene-ds, ppm) : 5 0.41 - 0.33 (s, 27H) . 119Sn NMR (149 MHz, Benzene-ds, ppm) : 5 -664.06. 29Si NMR (79 MHz, Benzene-ds, ppm) : 5 -9.69 (s, 4Si) . Synthesis of tetraktis (2 6-dimethylbenzoyl) stannane (1) The synthesis of tetrakis ( 2 , 6-dimethoxybenzoyl ) stannane was conducted in orange light under exclusion of wavelengths below 520 nm within the use of Schlenk techniques to exclude even traces of oxygen and moisture during the reaction. The reaction is based on Marschner, C.; Eur. J. Inorg. Chem. 1998, 221-226 and Haslinger, C. et al. Ch einPho toChem 2022, e202200108.

[0089] Tetrakis ( trimethylsilyl ) stannane (500.8 mg, 1.22 mmol, 1 eq.) and dry KOtBu (potassium tert-butoxide ; 150.5 mg, 1.34 mmol, 1.1 eq.) were weighted in the glovebox into a dry brown glass vial. 12.5 mL dry DME (dimethoxyethane) was added and it was stirred for 2 hrs. The potassium intermediate was confirmed via119Sn-NMR. In the meantime, 2 , 6-dimethoxybenzoyl fluoride (914.2 mg, 4.98 mmol, 4.1 eq.) was dissolved in 6.7 mL dry DME in another dry brown glass vial. Both solutions were cooled to 0 °C before the ( TMS ) 4Sn / KOtBu solution was added dropwise to the acid fluoride solution. The addition ended in a dark red suspension that was allowed to reach RT overnight. The following day, it appeared as orange suspension where the solvent was evaporated in vacuo. The orange residue was extracted with 18 mL DCM, centrifuged and the solution was reduced to 2 mL . Further, it was precipitated into 75 mL cold n-pentane, centrifuged and dried in HV. Further by-products were separated using flash chromatography, where the impurities were eluated first using PE : EE 1:1 and the pure product was eluated using pure EE. The final product was dried in HV.

[0090] Yield: 169.1 mg (9 %) Characterization: orange solid

[0091] Rf: 0.21 (pure EE)

[0092] 3H NMR (400 MHz, CDCls, ppm) : 5 7.10 (t, J = 8.4 Hz, 4H, Ar-H) , 6.39 - 6.30 (m, 8H, Ar-H) , 3.83 (s, 24H, - OCH3) .

[0093] 13C NMR (101 MHz, CDCls, ppm) : 5 236.73 (C=O) , 158.02, 133.14, 121.85, 103.76, 55.91 (-OCH3) .

[0094] 119Sn NMR (149 MHz, CDCls, ppm) : 5 -547.04.

[0095] Example 2

[0096] Synthesis of VI

[0097] Synthesis of tetrakis ( 2 , 6-dimethoxybenzoyl ) germane (VI) was carried out according to Haslinger, C. et al. ChemPhotoChem 2022, 6, e202200108.

[0098] Example 3

[0099] Synthesis of V2

[0100] Synthesis of tetrakis ( 2 , 4 , 6-trimethylbenzoyl ) stannane (V2) was carried out according to Mitterbauer, M. et al. Angew. Chem. Int. Ed. 2018, 57, 12146.

[0101] Compound V3 was kindly provided from Ivoclar AG.

[0102] Example 4

[0103] UV / Vis measurements

[0104] The absorbance of a photoinitiator defines the wavelengths, with which the photoinitiator can be cleaved for the initiation of a radical polymerization process. A longer wavelength can penetrate deeper into a formulation and therefore cure thicker layers.

[0105] UV / Vis measurements were performed using 1 -lCU3M solutions of the Pls (photoinitiators) 1 and V1-V3 in acetonitrile and a quartz cuvette (10 mm length) .

[0106] Table 1. Absorption maxima in acetonitrile and their corresponding extinction coefficients.

[0107] According to Table 1, 1 shows its absorption maximum at the highest wavelength compared to V1-V3 as well as the highest extinction coefficient at its maximum. Additionally, 1 can be cleaved using light with up to 550 nm, as it has the absorption with longest tail-out towards longer wavelengths (see Fig. 1) .

[0108] Example 5

[0109] Photo-DSC experiments

[0110] For the determination of the initiation kinetics and the further completeness of the photopolymerization process, photo-DSC experiments were executed by using 0.1 moll PI in hexanediol diacrylate (HDDA) with 500 ppm 2 , 6-di- tert-butyl-4- methylphenol (BHT) as stabilizer. With these experiments, parameters like double bond conversion (DBG) , rate of polymerization (Rp) , time to reach 95 1 heat flow ( tgs) and the time until heat flow maximum is reached (tmax) . The curing was carried out using a 460 nm LED with 10.0 mW / cm2and a 400 nm LED with 10.3 mW / cm2. The samples were irradiated for 300 s at 25 °C in triplicates.

[0111] To compare Pls with four chromophores (1, VI, V2) with Pls with only two chromophores (V3) , an additional formulation was investigated using double the molar amount of V3 (0.2 moll) .

[0112] Table 2. Photo-DSC results using 0.1 moll PI and 500 ppm BHT in HDDA, irradiated with 400 nm at 10.3 mW / cm2for 300 s at 25 °C.

[0113] DBC (%) Rp (mol / L S) tmax (s) t95 (s)

[0114] 1 74.2 ± 1.2 397 ± 15 3.2 ± 0.0 23.1 ± 0.8

[0115] VI 76.8 ± 0.4 404 ± 12 3.0 ± 0.2 24.3 ± 0.6

[0116] V2 74.0 ± 0.5 372 ± 10 3.4 ± 0.1 24.8 ± 0.4

[0117] V3 75.7 ± 1.1 393 ± 7 3.1 ± 0.1 24.2 ± 1.0

[0118] 2x 77.9 ± 0.2 403 ± 15 2.9 ± 0.1 24.3 ± 0.7

[0119] Table 3. Photo-DSC results using 0.1 mo 11 PI and 500 ppm BHT in HDDA, irradiated with 460 nm at 10.0 mW / cm2for 300 s at 25 °C. DBC (%) Rp (mol / L S) tmax (s) t95 (s)

[0120] 75.2 ± 0.3 426 ± 19 2.9 ± 0.0 22.3 ± 0.8

[0121] 68.5 ± 0.3 401 ± 1 3.8 ± 0.1 22.5 ± 0.4

[0122] 68.0 ± 0.2 415 ± 6 3.9 ± 0.1 20.8 ± 0.4

[0123] 63.0 ± 0.7 258 ± 14 5.7 ± 0.1 27.9 ± 1.8 69.0 ± 2.2 395 ± 6 4.3 ± 0.1 22.6 ± 0.1

[0124] 1 yields the highest DBG for the 460 nm experiments (Table 3) , exceeding even 2x V3, which has been the best DBG for the 400 nm experiments (Table 2) . Comparing Rp, for most of the Pls the Rpis increasing (1, V2) or staying similar (VI) for 460 nm compared to 400 nm, except for V3, where the Rpis lower for the longer wavelength. The best results regarding Rpat 460 nm are achieved also by 1, directly followed by V2.

[0125] Similar to DBC and Rp, also the values for tmax at 400 nm are very similar for all Pls. Rather unexpected are the results for tgs at 400 nm, as 1 needs the shortest amount of time; although all Pls achieved results in a similar range. The tmax values from the experiments at 460 nm are also here more differing, showing again 1 with the best results by far (2.9 s) , as it is nearly one second faster than VI (3.8 s) and V2 (3.9 s) .

[0126] Example 6

[0127] Stability study in solution

[0128] The stability of Pls in solutions or in formulations is important for further applications, as it is in most of the cases not possible to store it at inert and water-free conditions below RT . Solutions of 1 and the reference Pls VI, V2 and V3 with the concentration of l -10~3M using acetonitrile with a water content of 200.3 ppm as solvent. According to these concentrations, the molar ratio of water to PI was calculated to be 8.7:1. The solutions were filled into quartz glass cuvettes, flushed with argon and closed with the Teflon lid and parafilm. Besides the UV / Vis measurements after 0, 1, 2, 3, 4, 7, 9 and 14 days, the cuvettes were stored under light exclusion at RT . The resulting absorbance maxima of each compound were plotted over the investigated time period, with the absorbance of the first measurement on day 0 being standardized to 100 %.

[0129] Table 4. Change of the absorbance maximum of the Pls in solution (1 -10-3M in acetonitrile with 200.3 ppm H2O) over 14 days, normed to 100 % for day 0.

[0130] Table 5. Rates of decomposition during storage with light exclusion in % of absorbance per day.

[0131] The absorbance from V2 with the lowest stability decreased in one day by more than 80 % (Table 4, Fig. 2) . The other Pls VI, V3 and 1 show good stability for two weeks and most likely even longer. The absorbance of 1 is percentual remaining as high as the absorbance of V3, what makes it the first Sn-based PI that is as stable as commercially available photoinitiators. Looking at Table 6, the rates of decomposition can be compared even better, with the lowest rate for VI followed by 1 and V3 with similar rates.

[0132] Example 7

[0133] Steady State Photolysis To investigate the photobleaching behaviour, steady state photolysis experiments were carried out . With these experiments , it can be shown how fast a compound is degrading during irradiation and therefore how fast the radicals are generated .

[0134] The samples were dissolved in chloroform to receive a l - 10~3M solution . The exact amount of 2 ml was pipetted into the cuvette and it was very important to stir the solution during the whole experiment with a small magnetic bar that would fit into the cuvette and a magnetic stirrer under the cuvette holder . As background, a spectrum of pure chloroform was recorded . Each sample was irradiated using a 460 nm LED with 130 mW / cm2for 900 s , recording a UV / Vis spectra every 5 s .

[0135] Table 6 . Rates of decomposition during irradiation with 460 nm

[0136] LED in % of absorbance per second .

[0137] The rates of decomposition during the irradiation with the 460 nm LED (Rd 460nm) in Table 6 show the highest value for 1 , directly followed by V2 , both Sn-based photoinitiators ( also shown in Fig . 3 ) . That means , that these compounds can initiate the curing of a formulation the fastest . Additionally, both compounds show good photobleaching behaviour, what means that after irradiation, no signi ficant absorbance is remaining and can yield therefore colourless polymers .

[0138] Example 8

[0139] Curing depth

[0140] The curing depth of a formulation is an important parameter, as it depends on the wavelength of the used light as well as the photoinitiator and the amount of the used filler . Therefore , two similar formulations only with di f ferent compounds as photoinitiator were compared in these experiments . The formulations used for all the following experiments consisted of 0.1 moll PI (1 or V2) , 500 ppm BHT as stabilizer, UDMA (urethane dimethacrylate) and D3MA (decanediol dimethacrylate) as monomer system (molar ratio 1:1) and 10 wt% dental glass as filler. For determination of the curing depth, a Teflon-mold was used covered with a glass plate at the bottom in combination with the green laser pointer (532 nm, 80 mW / cm2) . The Teflon-mold was completely filled with the formulation, the laser pointer was put under the glass plate to irradiate the formulation directly for a certain time. After irradiation, the liquid formulation was removed and the resulting polymer was cleaned with acetone and dried. The final polymers were characterized regarding their height and their mass using a fine scale and a thrust gauge.

[0141] Table 7. Results of the curing depth experiments of V2 and 1 by height and mass for different irradiation times using a 532 nm laser pointer with 80 mW / cm2.

[0142] For every single point of Table 7, both for height and mass, the formulation with 1 achieved more polymerization than the formulation with V2. Therefore, it can be stated that 1 is more reactive at 532 nm than V2. The highest difference can be seen in the mass for short irradiation times, especially from 30 to 90 s. For longer irradiation times, starting from 120 s, the parameters in height and mass assimilate for both initiators, but with 1 still exceeding.

Claims

CLAIMS :

1. A compound having the general formula (I)wherein Ri is a linear or branched Ci to C4 alkyl group, - (C=O)-CH3, -CH2-CF3, or -CH2-C6H5, and R2is -H or -0R3, wherein R3is Ci to C4alkyl group, - (C=0) -CH3, -CH2-CF3, or -CH2-C6H5.

2. The compound according to claim 1, wherein the linear or branched Ci to C4alkyl group is a methyl group or an ethyl group .

3. The compound according to claim 1 or 2, wherein Ri and / or R3 is a methyl group.

4. The compound according to any one of claims 1 to 3, wherein Ri is a methyl group and R2 is -H or -OCH3.

5. A composition comprising at least one compound according to any one of claims 1 to 4 and at least one radically polymerizable monomer.

6. The composition according to claim 5, wherein the composition comprises 0.001 to 10 wt%, preferably 0.01 to 8 wt%, more preferably 0.01 to 6 wt%, more preferably 0.01 to 5 wt%, more preferably 0.01 to 4 wt%, more preferably 0.01 to 3 wt%, more preferably 0.01 to 2 wt%, of the at least one compound according to one of claims 1 to 4.. The composition according to claim 5 or 6, wherein the composition comprises at least one further photoinitiator from the group of Type I initiators and / or Type IT initiators.

8. The composition according to any one of claims 5 to 7, wherein the at least one radically polymerizable monomer is a monofunctional (meth) acrylate and / or a polyfunctional(meth) acrylate.

9. The composition according to claim 8, wherein the polyfunctional (meth) acrylate is a di (meth) acrylate, preferably selected from the group consisting of urethane di (meth) acrylates, alkyl di (meth) acrylate, epoxy-, polyether or polyesterdi (methacrylates)10. The composition according to any one of claims 5 to 9, wherein the composition comprises 1 to 99 wt%, preferably 5 to 95 wt%, more preferably 10 to 90 wt%, more preferably 15 to 85 wt%, of the at least one radically polymerizable monomer.

11. The composition according to any one of claims 5 to 10, wherein the composition further comprises at least one organic or inorganic filler or fiber or additives, preferably pigments, dyes and / or stabilizers.

12. Use of a compound according to any one of claims 1 to 4 or a composition according to any one of claims 5 to 11 in inks, in coatings, in varnishes, in curable printing processes, preferably in lithography, in adhesives, in sealants, in electronics, in optics, in the automotive industry, in the packaging industry, in medical applications, in dental applications, for preparing bone material, in 3D printing processes or in the textile industry.

13. Use of a compound according to one of claims 1 to 4 as a photoinitiator .14 . A method for the preparation of a polymer comprising the step of exposing a composition according to any one of claims 5 to 11 to electromagnetic radiation at a wavelength up to 560 nm .15 . The method according to claim 14 , wherein the composition is exposed to electromagnetic radiation at a wavelength ranging from 320 to 560 nm, preferably from 430 to 560 nm, more preferably from 470 to 560 nm, more preferably from 480 to 560 nm, more preferably from 490 to 560 nm .

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