Photobases

Abstract

The present invention relates to the use of a triphenylphosphonium carboxylate salt of formula (I) below as a photobase generator in a photopolymerisable composition comprising at least one type of monomer in order to prepare photopolymers by curing the composition by irradiation with light of a suitable wavelength: (I) in which R1 and R2 are each independently selected from -H and -OCH3, wherein at least one of R1 and R2 on each of the three phenyl groups is -OCH3, and in which R3 is selected from -H and -CH3, and X is either absent or is selected from a chemical bond, -CH2-, -O-CH2-, -CH2-O-, -C(=O)-, -O- and -S-; and corresponding new triphenylphosphonium carboxylate salts of formula (I).

Description

[0001] PHOTOBASES

[0002] The present invention relates to the use of novel carboxylate salts as photobase generators in photopolymerizable compositions.

[0003] STATE OF THE ART

[0004] The concept of organic photobase generators (PBGs) was first introduced in 1990 by Cameron and Frechet, who utilized a photolabile carbamate group to generate basic amines (J.F. Cameron, J.M.J. Frechet, J. Org. Chem. 55, 5919-5922 (1990)). Later reports described the light-induced release of primary, secondary, or tertiary amines, which are rather weakly basic and therefore inefficient in activating anionic polymerization. The use of salts for base generation first appeared in a 1998 report in the form of quaternary ammonium salts (Sarker et al., J. Phys. Chem. A 102, 5375-5382 (1998)), which significantly increased the storage stability of the formulations. In recent years, numerous carboxylates have been described as counterions of the bases in the salts used as PBGs.For example, one article discloses salts of 2-(3-benzoylphenyl)-, 2-(xanthon-2-yl)-, and 2-(thioxanthon-2-yl)propionic acid or -acetic acid as carboxylate-functional chromophores for PBG. These are characterized by short-wavelength absorption maxima below 400 nm and are therefore suitable for curing the formulations using UV irradiation. Cations mentioned for the salts include those of peralkylated guanidines and phosphazenes (Zivic et al., Angew. Chem. Int. Ed. 58(31), 10410-10422 (2019)).

[0005] Recently, the suitability of triarylphosphines as nucleophilic catalysts for oxa-Michael additions was also revealed. Specifically, triphenylphosphine, mono-, and trimethoxylated triphenylphosphine were investigated for their catalytic activity on 16 different combinations of Michael donors and acceptors. The use of tris(4-methoxyphenyl)phosphine tended to yield the best conversions, but in almost half of the experiments, hardly any differences were observed between the three catalysts (Fischer et al., Beilstein J. Org. Chem. 17, 1689-1697 (2021)). Against this background, the aim of the invention was to prepare new carboxylate salts and their use as photobase generators in photopolymerizable compositions.

[0006] DISCLOSURE OF THE INVENTION

[0007] The present invention achieves this object in a first aspect by providing a novel photopolymerization process, namely one using a triphenylphosphonium carboxylate salt of the following formula (I) as a photobase generator in a photopolymerizable composition comprising at least one type of monomer for producing photopolymers by curing the composition by irradiation with light of a suitable wavelength: in which the R 1 and R 2 are each independently selected from -H and -OCH3, wherein at least one of R 1 and R 2 represents -OCH3, and wherein R 3 is selected from -H and -CH3 and X is either absent or selected from a chemical bond, -CH2-, -O-CH2-, -CH2-O-, -C(=O)-, -O- and -S-.

[0008] The inventors discovered that salts consisting of such a triphenylphosphonium cation and the anion of a phenylacetic acid or phenylpropionic acid derivative defined above, all of which were prepared for the first time by the inventors, are sometimes excellently suited as photobase generators, as the subsequent examples clearly demonstrate. It was, however, highly surprising that, in contrast, salts consisting of the same triphenylphosphonium cations but with different anions, or of the same carbonate anions but with different cations, sometimes proved completely unsuitable under identical photopolymerization reaction conditions and failed to produce any conversion of the reactants.

[0009] According to the invention, conversions have already been achieved with simply methoxylated phenyl residues, with the methoxy group in the ortho position, i.e. with an -OCH3 as R 1gave slightly better results than in para-position, ie with -OCH3 as R 2 However, the triphenylphosphonium cations with multiply methoxylated phenyl residues were significantly superior. In preferred embodiments, therefore, at least one R is present on each of the three phenyl residues. 1 for -OCH3, even more preferably both R 1 for -OCH3, and in particular both R 1 and R 2 for -OCH3. That is, the phosphonium cation of the salt is preferably the tris(2-methoxyphenyl)phosphonium, more preferably the tris(2,6-dimethoxyphenyl)phosphonium, and especially the tris(2,4,6-trimethoxyphenyl)phosphonium cation.

[0010] Regarding the carboxylate anion, in preferred embodiments, X is absent or is selected from a chemical bond, -O-, and -S-. More preferably, it is the anion of one of the following carboxylic acids: 2-(3-benzoylphenyl)propionic acid (ketoprofen) or -acetic acid, 2-(xanthon-2-yl)propionic acid or -acetic acid, or 2-(thioxanthon-2-yl)propionic acid or -acetic acid. And in particular, the photobase generator is selected from the following triphenylphosphonium carboxylate salts: tris(2,6-dimethoxyphenyl)phosphonium 2-(3-benzoylphenyl)propionate, tris(2,4,6-trimethoxyphenyl)phosphonium 2-(3-benzoylphenyl)propionate, tris(2,6-dimethoxyphenyl)phosphonium 2-(xanthon-2-yl)acetate, tris(2,4,6-trimethoxyphenyl)phosphonium 2-(xanthon-2-yl)acetate, tris(2,6-dimethoxyphenyl)phosphonium 2-(thioxanthon-2-yl)acetate or tris(2,4,6-trimethoxyphenyl)phosphonium 2-(thioxanthon-2-yl)acetate, with which excellent results have been obtained.

[0011] The method of curing the photopolymerizable composition is not specifically limited. However, in preferred embodiments, the photopolymerizable composition is either applied to a substrate and cured by irradiation to form a coating, or cured in a generative manufacturing process by layer-by-layer irradiation to form a three-dimensional object. In both cases, curing can be carried out with heating, and the resulting photopolymer can be subjected to a thermal post-treatment to optimize its mechanical properties.

[0012] As a generative manufacturing process, hot lithography is preferably carried out by heating to a temperature of at least 50 °C or at least 70 °C or at least 80 °C, in which high conversions can be achieved in a particularly short time.

[0013] The use according to the invention is also not limited to polymerizations based on Michael additions, although in preferred embodiments, at least two types of monomers polymerizable by Michael addition reactions are used in the photopolymerizable composition, which are even more preferably polymerizable by oxa-ene additions, oxa-yne additions, or CC addition reactions of CH-active compounds, in particular by oxa-ene additions. Particularly preferred examples of the monomers are combinations of (meth)acrylates or (meth)acrylamides and alcohols, which, especially with regard to additive manufacturing processes, are preferably reacted in bulk without solvent in the presence of the photobase generator.

[0014] In addition, the photopolymerizable compositions may further comprise at least one photosensitizer, preferably 9,10-dibutoxyanthracene, and / or at least one radical scavenger, preferably 2,6-di-tert-butyl-p-cresol (butylhydroxytoluene, BHT), in order to improve the reaction rate and conversion and to increase the storage stability of the composition.

[0015] In a second aspect, the present invention also provides a photopolymer obtainable by curing a photopolymerizable composition as described above. In a third aspect, the present invention naturally also relates to the novel salts prepared for the first time by the inventors and usable as photobase generators in the process according to the invention, ie a triphenylphosphonium carboxylate salt of the following formula (I) in which the R 1 and R 2 are each independently selected from -H and -OCH3 and wherein R3 is selected from -H and -CH3 and X is either absent or selected from -O- and -S-, wherein the salt is specifically selected from the group consisting of the following:

[0016] Tris(4-methoxyphenyl)phosphonium 2-(3-benzoylphenyl)propionate (1): Tris(2-methoxyphenyl)phosphonium 2-(3-benzoylphenyl)propionate (2):

[0017] Tris(2,6-dimethoxyphenyl)phosphonium 2-(3-benzoylphenyl)propionate (3): Tris(2,4,6-trimethoxyphenyl)phosphonium 2-(3-benzoylphenyl)propionate (4): Tris(2,4,6-trimethoxyphenyl)phosphonium 2-(xanthon-2-yl)acetate (5):

[0018] Tris(2,4,6-trimethoxyphenyl)phosphonium 2-(thioxanthon-2-yl)acetate (6):

[0019] Finally, the invention also relates to the use of one of these novel triphenylphosphonium carboxylate salts as a photobase generator in photopolymerizable compositions for the preparation of photopolymers.

[0020] BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The present invention will now be described in more detail with reference to specific examples and the accompanying drawings, in which: Fig. 1 is a graphical representation of the reaction conversions achieved by varying the reaction temperature in Example 14.

[0022] Fig. 2 is a photo-DSC diagram of the photopolymerization carried out in Example 16.

[0023] And Figs. 3A-C, 4A-B, 5A-B and 6A-B are photographs of the four three-dimensional bodies produced in Example 22 by hot lithography.

[0024] EXAMPLES

[0025] Synthesis examples - Examples 1 to 6, Comparison examples 1 to 5

[0026] To prepare the novel phosphonium carboxylate salts, commercially available reagents were largely purchased, i.e., the respective triphenylphosphine (or, in Comparative Example 2, BINAP, i.e., 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl) and the corresponding carboxylic acid (or, in Comparative Example 3, triethylammonium tetraphenylborate), which were dried under high vacuum and used for salt formation without further purification. For Comparative Example 3, Example 5, and Example 6, the acid yielding the respective anion (i.e., triethylammonium tetraphenylborate, 9-oxo-9 / 7-xanthene-2-acetic acid, or 9-oxo-9 / 7-thioxanthene-2-acetic acid, respectively) was prepared, purified, and dried according to the literature.

[0027] Salt formation was carried out by initially preparing a solution or suspension of the acid in absolute THF, adding an equimolar amount of the base (or, for Comparative Example 2, half the molar amount of the bisphosphine BINAP), stirring for one hour, removing the solvent under high vacuum, washing the residue with an absolute solvent, and drying the salt under high vacuum. Quantitative conversions (> 99% of theory, "quant.") were achieved for the majority of the inventive examples. Characterization was carried out in each case by NMR spectroscopy. Comparative Example 1 (C1)

[0028] Unsubstituted triphenylphosphine and 2-(3-benzoylphenyl)propionic acid gave

[0029] Triphenylphosphonium 2-(3-benzoylphenyl)propionate (V1):

[0030] (V1 )

[0031] 2-(3-Benzoylphenyl)propionic acid (1 eq., 2 mmol, 0.509 g) and triphenylphosphine (1 eq., 2 mmol, 0.525 g) in abs. THF (6 ml) gave, after removal of the solvent, a sticky solid residue, which was washed with petroleum ether and dried to give a white solid (yield: 0.896 g, 87% of dry matter).

[0032] 1 H-NMR (600 MHz, C6D6) ö: 7.85 (t, 1 H), 7.70 (dd, 2H), 7.54 (dt, 1 H), 7.47-7.33 (m, 6H), 7.23 (dt, 1 H), 7.14-7.09 (m, 1 H), 7.08-7.00 (m, 11H), 6.98 (t, 1H), 3.40 (q, 1H), 1.22 (d, 3H).

[0033] 13 C-NMR (151 MHz, C6D6) ö: 195.31, 180.22, 140.21, 138.26, 137.73, 137.63, 133.91, 133.78, 131.96, 131.07, 129.99, 129.42, 128.98, 128.54, 128.51, 128.46, 128.42, 128.08, 45.04, 17.74.

[0034] 31 P-NMR (243 MHz, C6D6) δ: -5.41. Example 1

[0035] Tris(4-methoxyphenyl)phosphine and 2-(3-benzoylphenyl)propionic acid gave tris-(4-methoxyphenyl)phosphonium 2-(3-benzoylphenyl)propionate (1 ):

[0036] 2-(3-Benzoylphenyl)propionic acid (1 eq., 1.8 mmol, 0.458 g) and tris(4-methoxyphenyl)phosphine (1 eq., 1.8 mmol, 0.634 g) in abs. THF (7 ml) gave, after removal of the solvent, a white solid residue, which was washed with THF and dried to give a white solid (yield: 1.092 g, quant.).

[0037] 1 H-NMR (600 MHz, C6D6) δ: 7.85 (s, 1H), 7.70 (dd, 2H), 7.53 (dt, 1H), 7.41 (dd, 5H), 7.23 (dt, 1H), 7.13-7.07 (m, 1H), 7.05-7.00 (m, 4H), 6.96 (t, 1H), 6.76-6.71 (m, 5H), 3.38 (q, 1H), 3.23 (s, 9H), 1 .21 (d, 3H).

[0038] 13C-NMR (151 MHz, C6D6) ö: 194.42, 178.94, 159.48, 139.46, 137.39, 136.89, 134.40, 134.26, 131.08, 130.21, 129.11, 128.75, 128.58, 128.09, 127.55, 127.19, 1 13.43, 1 13.38, 53.49, 44.17, 16.93.

[0039] 31 P-NMR (243 MHz, C6D6) δ: -10.32. Example 2

[0040] Tris(2-methoxyphenyl)phosphine and 2-(3-benzoylphenyl)propionic acid gave tris-(2-methoxyphenyl)phosphonium 2-(3-benzoylphenyl)propionate (2):

[0041] 2-(3-Benzoylphenyl)propionic acid (1 eq., 1.8 mmol, 0.458 g) and tris(2-methoxyphenyl)phosphine (1 eq., 1.8 mmol, 0.634 g) in abs. THF (8 ml) gave, after removal of the solvent, a sticky solid residue, which was washed with diethyl ether and dried to give a white solid (yield: 0.900 g, 82% of dry matter).

[0042] 1H-NMR (600 MHz, C6D6) ö: 7,85 (s, 1 H), 7,72-7,67 (m, 2H), 7,53 (dt, 2H), 7,23 (dt, 1 H), 7,12-7,07 (m, 6H), 7,06-7,00 (m, 2H), 6,97 (t, 1 H), 6,76 (td, 3H), 6,51 (m, 3H), 3,39 (q, 1 H), 3,17 (s, 9H), 1 ,21 (d, 3H).

[0043] 13 C-NMR (151 MHz, C6D6) ö: 195,29, 179,91 , 161 ,85, 161 ,73, 140,28, 138,26, 137,74, 134,05, 131 ,96, 131 ,08, 129,95, 129,77, 128,96, 128,42, 128,08, 125,54, 125,43, 120,94, 1 10,06, 54,76, 45,03, 17,77.

[0044] 31 P-NMR (243 MHz, C6D6) ö: -37,31 . Beispiel 3

[0045] Tris(2,6-dimethoxyphenyl)phosphin und 2-(3-Benzoylphenyl)propionsäure ergaben

[0046] Tris(2,6-dimethoxyphenyl)phosphonium-2-(3-benzoylphenyl)propionat (3):

[0047] 2-(3-Benzoylphenyl)propionic acid (1 eq., 1.5 mmol, 0.381 g) and tris(2,6-dimethoxyphenyl)phosphine (1 eq., 1.5 mmol, 0.664 g) in abs. THF (7 ml) gave, after removal of the solvent, a white solid residue, which was washed with THF and dried to give a white solid (yield: 1.045 g, quant.).

[0048] 1 H-NMR (600 MHz, C6D6) δ: 7.92 (d, 1H), 7.74-7.70 (m, 2H), 7.55 (dt, 1H), 7.38 (dt, 1H), 7.15-7.10 (tt, 3H), 7.03 (m, 3H), 7.10-7.02 (t, 1H), 6.31 (dd, 6H), 3.58 (q, 1H), 3.22 (s, 18H), 1 .32 (d, 3H).

[0049] 13 C-NMR (151 MHz, C6D6) δ: 195.44, 178.09, 162.73, 162.67, 141.32, 138.10, 137.84, 131.89, 131.41, 129.98, 129.49, 128.64, 128.59, 128.33, 128.03, 127.97, 104.35, 55.42, 45.40, 18.25.

[0050] 31 P-NMR (243 MHz, C6D6) δ: -37.31 . Example 4

[0051] Tris(2,4,6-trimethoxyphenyl)phosphine and 2-(3-benzoylphenyl)propionic acid gave

[0052] Tris(2,4,6-trimethoxyphenyl)phosphonium 2-(3-benzoylphenyl)propionate (4):

[0053] 2-(3-Benzoylphenyl)propionic acid (1 eq., 2.38 mmol, 0.605 g) and tris(2,4,6-trimethoxyphenyl)phosphine (1 eq., 2.38 mmol, 1.266 g) in abs. THF (9 ml) gave, after removal of the solvent, a beige, viscous solid residue, which was washed with diethyl ether and dried to give a light beige solid (yield: 1.252 g, 67% of the dry matter).

[0054] 1 H-NMR (600 MHz, C6D6) δ: 8.05 (t, 1 H), 7.79-7.74 (m, 2H), 7.64 (dt, 1 H), 7.57 (dt, 1 H), 7.16-7.10 (m, 3H), 7.10-7.03 (m, 3H), 6.1 1 (d, 6H), 3.88 (q, 1H), 3.42 (s, 9H), 3.28 (s, 18H), 1 .48 (d, 3H).

[0055] 13 C-NMR (151 MHz, C6D6) δ: 195.82, 177.51, 163.72, 163.67, 143.51, 138.07, 137.69, 132.09, 131.73, 130.07, 129.71, 129.20, 128.10, 127.94, 91.49, 55.60, 55.48, 55.23, 54.62, 46.70, 19.12.

[0056] 31 P-NMR (243 MHz, C6D6) δ: -64.88. Comparison Example 2

[0057] 2,2'-Bis(diphenylphosphino)-1 , 1 '-binaphthyl and 2-(3-benzoylphenyl)propionic acid gave 2,2'-bis(diphenylphosphonium)-1 ,1 '-binaphthyl-bis[2-(3-benzoylphenyl)propionate] (V2):

[0058] 2-(3-Benzoylphenyl)propionic acid (2 eq., 1.6 mmol, 0.4068 g) and 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl) (1 eq., 0.8 mmol, 0.498 g) in abs. THF (5 ml) gave, after removal of the solvent, a white solid residue, which was washed with THF and dried to give a white solid (yield: 0.690 g, 76% of dry matter).

[0059] 1 H-NMR (400 MHz, C6D6) O: 7.95 (t, 2H), 7.89-7.70 (m, 7H), 7.70-7.43 (m, 1 1 H), 7.41 -6.93 (m, 25H), 6.78 (m, 2H), 3.48 (q, 2H), 1 .31 (d, 6H).

[0060] 13C-NMR (101 MHz, C6D6) Ö: 195.24, 180.07, 140.20, 138.29, 137.75, 134.56, 134.34, 133.08, 132.98, 132.88, 131.92, 131.03, 129.94, 129.39, 128.96, 128.39, 128.06, 126.35, 125.72, 45.01, 17.18.

[0061] 31 P-NMR (162 MHz, C6D6) Ö: -15.04. Comparison Example 3

[0062] Tris(2,4,6-trimethoxyphenyl)phosphine and triethylammonium tetraphenylborate gave tris(2,4,6-trimethoxyphenyl)phosphonium tetraphenylborate (V3):

[0063] Triethylammonium tetraphenylborate (1 eq., 1.0 mmol, 0.421 g) prepared according to the literature (Faulkner et al., J. Am. Chem. Soc. 137(22), 7224-7230 (2015)) and tris-(2,4,6-trimethoxyphenyl)phosphine (1 eq., 1.0 mmol, 0.535 g) in abs. THF (7 ml) gave, after removal of the solvent, a white solid residue, which was washed with hexane and dried to give a white solid (yield: 0.464 g, 54% of the dry matter).

[0064] 1H-NMR (600 MHz, acetone-de) δ: 7.35 (m, 8H), 6.94 (t, 8H), 6.79 (tt, 4H), 6.38 (d, 6H), 3.91 (s, 9H), 3.77 (s, 18H).

[0065] 13 C-NMR (151 MHz, C6D6) δ: 205.32, 164.26, 136.16, 125.09, 121 .36, 91 .45, 91 .40, 56.1 1 , 55.40.

[0066] 31 P-NMR (243 MHz, acetone-de) δ: -52.03. Comparison Example 4

[0067] Tris(2,4,6-trimethoxyphenyl)phosphine and phenylglyoxylic acid yielded tris(2,4,6-trimethoxyphenyl)phosphonium phenylglyoxylate (V4):

[0068] Phenylglyoxalic acid (1 eq., 1.0 mmol, 0.150 g) and tris(2,4,6-trimethoxyphenyl)phosphine (1 eq., 1.0 mmol, 0.533 g) in abs. THF (3 ml) gave, after removal of the solvent, a beige, sticky solid residue, which was washed with THF and dried to give a slightly beige, sticky solid (yield: 0.683 g, quant.).

[0069] 1H-NMR (400 MHz, acetone-de) δ: 7.90-7.83 (m, 2H), 7.46-7.38 (m, 1H), 7.32 (tt, 2H), 6.15 (d, 2H), 3.71 (d, 9H), 3.53 (s, 18H).

[0070] 13 C-NMR (101 MHz, acetone-de) ö: 170.08, 163.19, 132.52, 129.39, 128.19, 91.35, 91.30, 55.79, 55.41, 55.08.

[0071] Comparison example 5

[0072] Tris(2,4,6-trimethoxyphenyl)phosphine and phthalimidoacetic acid gave tris(2,4,6-trimethoxyphenyl)phosphonium phthalimidoacetate (V5):

[0073] N-Phthaloglycine (1 eq., 1.0 mmol, 0.205 g) and tris(2,4,6-trimethoxyphenyl)phosphine (1 eq., 1.0 mmol, 0.533 g) in abs. THF (3 ml) gave, after removal of the solvent, a beige solid residue, which was washed with THF and dried to give a light beige solid (yield: (0.738 g, quant.).

[0074] 1H-NMR (400 MHz, acetone-de) δ: 7.89-7.81 (m, 4H), 6.25-6.14 (m, 6H), 4.34 (s, 2H), 3.83 (s, 9H), 3.59 (s, 18H).

[0075] 13 C-NMR (101 MHz, acetone-de) δ: 168.74, 167.56, 163.54, 163.47, 133.99, 122.83, 91,18, 91,16, 55.46, 54.73, 40.96.

[0076] Example 5

[0077] Tris(2,4,6-trimethoxyphenyl)phosphine and 2-(xanthon-2-yl)acetic acid gave tris-

[0078] (2,4,6-trimethoxyphenyl)phosphonium 2-(xanthon-2-yl)acetate (5):

[0079] 9-Oxo-9H-xanthene-2-acetic acid (1 eq., 0.2 mmol, 51 mg) and tris(2,4,6-trimethoxyphenyl)phosphine (1 eq., 0.2 mmol, 107 mg) in abs. THF (0.6 ml), prepared according to the literature (Blake et al., Org Lett. 8(6), 1057-1060 (2006); with the exception that K2CO3 was used instead of CS2CO3), and tris(2,4,6-trimethoxyphenyl)phosphine (1 eq., 0.2 mmol, 107 mg) in abs. THF (0.6 ml) gave, after removal of the solvent, a beige solid residue, which was washed with THF and dried to give a light beige solid (yield: 158 mg, quant.).

[0080] 1 H-NMR (400 MHz, acetone-de) ö: 8.13 (dd, 1 H), 8.06 (d, 1 H), 7.77-7.66 (m, 2H), 7.51 -7.40 (m, 2H), 7.38-7.29 (m, 1 H), 6.03 (s, 6H), 3.66 (s, 9H), 3.61 (s, 2H), 3.39 (s, 18H).

[0081] 13 C-NMR (101 MHz, acetone-de) δ: 171.64, 163.39, 136.66, 135.07, 126.60, 126.21, 124.05, 118.10, 117.98, 91.11, 55.32, 54.59, 39.67. Example 6

[0082] Tris(2,4,6-trimethoxyphenyl)phosphine and 2-(thioxanthon-2-yl)acetic acid gave

[0083] Tris(2,4,6-trimethoxyphenyl)phosphonium 2-(thioxanthon-2-yl)acetate (6):

[0084] 9-Oxo-9H-thioxanthene-2-acetic acid (1 eq., 0.15 mmol, 41 mg) and tris(2,4,6-trimethoxyphenyl)phosphine (1 eq., 0.15 mmol, 80 mg) in abs. THF (0.5 ml), prepared according to the literature (Yilmaz et al., Macromol. Rapid Commun. 37(13), 1046-1051 (2016)), yielded a light green solid residue after removal of the solvent, which was washed with THF and dried to give a pale green solid (yield: 121 mg, quant.).

[0085] 1 H-NMR (400 MHz, acetone-de) δ: 8.42-8.29 (m, 2H), 7.63-7.54 (m, 4H), 7.49-7.35 (m, 1H), 5.97 (d, 6H), 3.84 (s, 0.3H), 3.69 (s, 1.7H), 3.64 (s, 9H), 3.37 (s, 18H).

[0086] 13 C-NMR (101 MHz, acetone-de) ö: 163.25, 134.28, 132.58, 129.95, 129.35, 126.43, 126.31, 126.19, 91.06, 55.22, 54.48, 40.28.

[0087] Examples 7 to 12, Comparison Examples 6 to 10 - Photocatalysis "Oxa-Ene"

[0088] The novel compounds (1) to (6) according to the invention prepared as described above and those of Comparative Examples (V1) to (V5) were first investigated for their suitability as photobase generators in a photoinitiated model reaction. The course of the reaction was monitored using photo-DSC (model: DSC 204 F1 Phoenix from Netzsch), and the conversions achieved were subsequently determined using 1 H-NMR spectra of the reaction mixtures dissolved in CDCh were determined. An oxa-Michael addition between a

[0089] alcohol and an acrylate, specifically between benzyl alcohol as Michael-

[0090] Donor and benzyl acrylate as Michael acceptor according to the following reaction:

[0091] To determine the conversion, the chemical shift of the two hydrogen atoms (highlighted in the scheme above) of the methylene group of the benzyl alcohol was used. 1 H-NMR spectrum shows a singlet, the peak of which is at 4.63 ppm for the benzyl alcohol contained as starting material in the reaction mixture, but after successful addition to the acrylate double bond, it appears to be shifted downfield to 4.43 ppm (while that for the methylene group of the benzyl alcohol bound as ester is at over 5 ppm).

[0092] The percentage turnover was therefore calculated according to the following formula:

[0093] U (%) = IP / (IP + l E ) x 100 where lp is the integral of the singlet peak in the product (ie at 4.43 ppm) and IE is that of the peak in the reactant (at 4.63 ppm).

[0094] The reaction mixtures for the model reactions were (expressed as molar amounts) 1 eq. benzyl alcohol, 1 eq. benzyl acrylate, 2 mol% of the potential photobase generator, 0.02 mol% 9,10-dibutoxyanthracene (BAnt) as a photosensitizer, and 2 mol% 2,6-di-tert-butyl-p-cresol (butylhydroxytoluene, BHT) as a radical scavenger. 10 to 15 mg of each of the formulations, mixed at room temperature, were placed in an aluminum DSC crucible and covered with a quartz glass plate. The reaction mixtures were subsequently heated to 80 °C, then irradiated at this temperature for 50 s using the medium-pressure mercury lamp of the DSC instrument with light having a wavelength between 320 nm and 500 nm (at 133 mW / cm 2), after which the temperature was held for a further 850 s, and then the irradiation for 50 s and the holding for a further 850 s were repeated once each (ie the reaction time was 2 x 15 min). The reaction mixtures were then briefly cooled and dissolved in CDCh, after which their 1 H-NMR spectra were recorded.

[0095] The sales calculated according to the above formula were as shown in Table 1 below.

[0096] Table 1

[0097] From this, it is clear that none of the comparison substances was able to generate conversions, i.e., to be cleaved upon irradiation and trigger the Michael addition reaction. This means that neither the unsubstituted triphenylphosphonium nor the BINAP cation in combination with anions according to the invention nor the tris(2,4,6-trimethoxyphenyl)phosphonium

[0098] Cation in combination with alternative anions are suitable as photobase generators. In the inventive combinations of methoxy-substituted triphenylphosphonium cations and anions of phenylacetic acid or phenylpropionic acid derivatives, it was shown that both the number of substituents and their position significantly influence the reactivity: The ortho position of the methoxy group on the phenyl residues of compound (2), ie as substituent R 1 , is opposite the para-position of compound (1 ) as substituent R 2 to be preferred. However, the conversions achieved from 10% and only 6%, respectively, can be multiplied by increasing the number of substituents to two and three, respectively: 58% conversion was obtained with the dimethoxy- and 69% with the trimethoxy-substituted compound (3) and (4), respectively.

[0099] On the other hand, it can also be seen that a ring closure between the two benzene rings in the aromatic anion further improves the suitability of the salt as a photobase generator: In combination with the tris(2,4,6-trimethoxyphenyl)phosphonium cation, 81% conversion was achieved with the 2-(xanthon-2-yl)acetate anion and 84% conversion with the 2-(thioxanthon-2-yl)acetate anion for compounds (5) and (6), respectively. Therefore, the inventors assume that similar results can be achieved with a chemical bond, -CH2-, -O-CH2-, -CH2-O-, or -C(=O)- as substituent X in formula (I) – as well as with a longer alkyl radical as substituent R. 3 (such as ethyl or propyl). However, increasing the molecular weight of the photobase generator is obviously not preferable unless significantly better results can be achieved.

[0100] Example 13, Comparison Example 11 - Photocatalysis "CC"

[0101] The new compound (4) according to the invention and a known catalyst for CC Michael addition, namely CGI 1193 (from BASF) of the following formula: were investigated in photo-DSC experiments analogous to those above for the achievable conversions in a model reaction with diethyl malonate as Michael donor and again benzyl acrylate as Michael acceptor according to the following reaction:

[0102] To determine the conversion, the chemical shift of the two hydrogen atoms (highlighted in the scheme above) of the benzylmethylene group of benzyl acrylate in the reactant and the product was used. The reaction mixtures in this case were 1 eq. diethyl malonate, 2 eq. benzyl acrylate, 2 mol% of compound (4) or CGI 1193, 0.02 mol% BAnt as a photosensitizer, and 2 mol% BHT as a radical scavenger. The reaction conditions for Example 13 were the same as before (80 °C, 2 x 50 s irradiation, each followed by 850 s holding). The 1 The conversions calculated from H-NMR spectra are given in Table 2 below.

[0103] Table 2 The new compound (4) according to the invention thus surpassed the known compound as a photobase generator in terms of reaction conversion by 75%, which proves its excellent suitability also as a catalyst for CC Michael additions.

[0104] Example 14 - Reaction temperature

[0105] Since it was assumed that the conversions could be further increased by increasing the concentration of the photobase generator, the sensitizer or the reaction temperature, the inventors carried out further photo-DSC experiments with compound (4), with which a conversion of 69% had previously been achieved in the oxa-ene experiments.

[0106] The concentration of the photobase generator was increased from 2 to 5 mol% and that of the sensitizer from 0.02 to 1 mol%, and the temperature was increased in each batch from room temperature (25 °C) in steps of 25 °C to 150 °C. The 1 Conversions calculated from H-NMR spectra as described above are given in Table 3 below and graphically shown in Fig. 1.

[0107] Table 3

[0108] From these results, it can be concluded, on the one hand, that the optimal reaction temperature for this reaction mixture is likely to be in a range between 80 °C and 90 °C. On the other hand, it can be seen that the reaction conversion could be significantly increased, namely by more than 25%, compared to the 69% previously achieved with compound (4) in Example 10, primarily due to the increase in the concentration of the photobase generator.

[0109] From this, it can also be deduced that the monomethoxy-substituted compounds (1) and (2), which had yielded comparatively poor results under the reaction conditions of Examples 7 and 8, can also be used industrially as photobase generators after optimization of the parameters. This naturally also applies to ortho / para-disubstituted variants, which have not yet been tested—especially in combination with a ring-closed anion of formula (I).

[0110] Example 15 - Bearing stability

[0111] To investigate the storage stability of the formulations, the model formulation previously investigated in Example 10 was used again. This formulation comprised 1 eq. of benzyl alcohol, 1 eq. of benzyl acrylate, 2 mol% of compound (4), 2 mol% of BHT as a radical scavenger, and BAnt as a photosensitizer. In this example, only the amount of sensitizer was increased from 0.02 mol% to 0.1 mol%. Several batches were stored for up to 14 days at room temperature (RT) or at 60 °C in the absence of light, after which their compositions were again analyzed by 1H NMR, and any conversions were calculated as previously. The results obtained are shown in Table 4 below.

[0112] Table 4 This clearly shows that no reaction occurred at room temperature for 14 days. Even at 60 °C, only slight conversions were observed after 4 days, and even after 14 days at this elevated temperature, only 7% of the reactants had reacted. Overall, this demonstrates a more than satisfactory storage stability of the formulations according to the present invention.

[0113] To confirm the suitability of the photobase generators according to the invention for the production of photopolymers, another photo-DSC experiment was first conducted using multifunctional monomers. Specifically, a reaction mixture was prepared from 1 eq. trimethylolpropane (TMP), 1 eq. trimethylolpropane triacrylate (TMPTA), 5 mol% of compound (4), 2 mol% BHT as a radical scavenger, and BAnt as a photosensitizer, the amount of which was then reduced to 0.02 mol%. The reaction temperature, however, was increased to 100 °C.

[0114] As can be seen in Fig. 2, a strong exotherm was observed just a few seconds after the start of exposure, which completely subsided after just over a minute, indicating complete conversion of the reactants. Thus, the corresponding photopolymer could be produced in an extremely short time using the photobase generator (4) according to the invention. Example 17 - Photopolymerization for the Production of a Coating

[0115] A reaction mixture of 1 eq. TMP, 1 eq. TMPTA, 2 mol% of compound (4), 2 mol% BHT as radical scavenger and 0.02 mol% BAnt as photosensitizer was applied to a glass plate with a doctor blade in a layer thickness of 100 pm.

[0116] This was heated to 80 °C in an INTELLI-RAY 600 UV oven from Uvitron International and irradiated with UV light at 320-500 nm using the oven's broadband mercury UV lamp. After an exposure time of just 1 minute, a hard, transparent coating was obtained on the glass plate.

[0117] Examples 18 to 21, Comparative Examples 12 and 13

[0118] To further determine the reaction parameters of the above photopolymerization reaction, further photo-DSC experiments were carried out using reaction mixtures analogous to Example 17, consisting of 1 eq. TMP, 1 eq. TMPTA, 2 mol% of the respective photobase generator, 2 mol% BHT as radical scavenger and 0.02 mol% BAnt as photosensitizer.

[0119] The performance of the compounds (1) to (4) according to the invention was investigated, ie the tris(methoxyphenyl)phosphonium 2-(3-benzoylphenyl)propionates substituted at different positions or with a different number of methoxy groups, as well as those from Comparative Examples 3 and 4 with different anions, ie tris(2,4,6-trimethoxyphenyl)phosphonium tetraphenylborate (V3) and tris(2,4,6-trimethoxyphenyl)phosphonium phenylglyoxylate (V4).

[0120] From the corresponding photo-DSC diagrams, the peak area (J / g) was determined as a measure of the achieved conversion, the peak height (mW / mg) as a measure of the polymerization rate, and the time to reach 95% conversion, tes (s), as a measure of the rate at which the overall reaction proceeds. The results are shown in Table 5 overleaf. Table 5

[0121] At first glance, it is evident that the two comparison substances, which in Comparative Examples 8 and 9 above failed to generate any conversion as catalysts for the oxa-ene Michael addition between benzyl alcohol and benzyl acrylate, also showed activity as photobase generators. However, they were clearly outperformed in all respects by the four compounds according to the invention. The latter produced peak areas approximately 1.5 and 2 times larger, respectively, peak heights approximately 2 and 4 times larger, respectively, and tes times approximately 20% shorter, demonstrating that they were capable of generating significantly higher conversions in a shorter period of time.

[0122] In comparison to the above results of the photo-DSC experiments for the reaction between benzyl alcohol and benzyl acrylate, it was not only surprising that (V3) and (V4) now also acted as photobase generators, but also that all four compounds according to the invention in the present Examples 18 to 21 gave comparable results: They did not differ by more than 10% in any of the three parameters, although the conversions achieved in the earlier Examples 7 to 10 had varied enormously (6%, 10%, 58% and 69% conversion, respectively).

[0123] Without wishing to commit to a theory, the inventors assume that both are due to the Trommsdorff effect or gel effect, which leads to an increase in the reaction rate with increasing conversion during polymerization. As the viscosity of the reaction mixture increases, the heat of reaction becomes increasingly difficult to dissipate, causing the exothermic polymerization reaction to cause a rise in temperature. This effect is, of course, particularly noticeable in bulk polymerizations, i.e., without the diluting effect of a solvent.

[0124] In any case, these results also demonstrate the superiority of the inventive combinations of formula (I) of methoxy-substituted triphenylphosphonium cations and the anions of defined phenylacetic acid or phenylpropionic acid derivatives over similar other salts whose cation and / or anion does not correspond to the definition according to the present invention.

[0125] Example 22 - Hot Lithography

[0126] Due to the fact that relatively high reaction temperatures were required or preferred for the tested reaction mixtures, they are extremely well suited for 3D printing using hot lithography, i.e. layer-by-layer exposure of the liquid mixtures with a laser while heating to elevated temperatures to produce three-dimensional photopolymers with predefined shape and structure.

[0127] For this purpose, four experiments were conducted, each using the same reaction mixture as in Example 16, i.e., 1 eq. TMP, 1 eq. TMPTA, 2 mol% of compound (4), 2 mol% BHT, and 0.02 mol% BAnt. The first three experiments were heated to 80 °C in a Cubicure Caligma 200® Hot Lithography System and cured layer by layer with a layer thickness of 50 pm using a 375 nm laser (60 mW) at a scan rate of 200 mm / s to form three-dimensional bodies. Photos of the resulting molded bodies are shown in Figs. 3 and 4.

[0128] First, a solid square pyramid (Fig. 3A) was produced, followed by a hollow cube (Fig. 3B). A hollow pyramid was then produced, as shown in Fig. 3C and Fig. 4. Fig. 4A highlights the area of ​​one of the four struts, of which Fig. 4B shows an enlarged view, demonstrating that the individual layers cured one after the other during 3D printing are visible even to the naked eye. Finally, a more complex cube shape, shown in Fig. 5A and in Fig. 5B (slightly larger), was 3D printed from the same reaction mixture as above using a system similar to the Cubicure Caligma 200®, namely the Blue Printer 10. This system incorporates an Ikarus II light engine from IN-VISION with a wavelength of 385 nm and a pixel pitch of 50 pm, and is thus capable of DLP hot lithography.

[0129] The printing was again carried out at 80 °C with an exposure time of 18 s and an intensity of 75 mW / cm2 In this case, however, Digital Light Processing, or DLP for short, was used, in which the light is directed onto the reaction mixture to be cured using reflective micromirrors instead of a laser with a projector or beamer.

[0130] Figures 6A and 6B show light microscopy images of this complex hollow cube at different magnifications, demonstrating the high resolution of this 3D printing process.

[0131] All four experiments, as well as the other examples, clearly demonstrate the usability of the triphenylphosphonium carboxylate salts according to formula (I) according to the invention as photobase generators.

Claims

PATENT CLAIMS 1. Use of a triphenylphosphonium carboxylate salt of the following formula (I) as a photobase generator in a photopolymerizable composition comprising at least one type of monomer for producing photopolymers by curing the composition by irradiation with light of a suitable wavelength: in which the R 1 and R 2 are each independently selected from -H and -OCH3, wherein at least one of R 1 and R 2 represents -OCH3, and wherein R 3 is selected from -H and -CH3 and X is either absent or selected from a chemical bond, -CH2-, -O-CH2-, -CH2-O-, -C(=O)-, -O- and -S-.

2. Use according to claim 1, characterized in that at each of the three phenyl radicals at least one R 1 stands for -OCH3.

3. Use according to claim 2, characterized in that the phosphonium cation of the salt is the cation of one of the following triphenylphosphines: tris-(2-methoxyphenyl)phosphine, tris(2,6-dimethoxyphenyl)phosphine or tris(2,4,6-trimethoxyphenyl)phosphine.

4. Use according to any one of claims 1 to 3, characterized in that X is absent or is selected from a chemical bond, -O- and -S-.

5. Use according to claim 4, characterized in that the carboxylate anion of the salt is the anion of one of the following carboxylic acids: 2-(3-benzoylphenyl)propionic acid or -acetic acid, 2-(xanthon-2-yl)propionic acid or -acetic acid or 2-(thioxanthon-2-yl)propionic acid or -acetic acid.

6. Use according to claim 5, characterized in that the photobase generator is selected from the following triphenylphosphonium carboxylate salts: tris(2,6-dimethoxyphenyl)phosphonium 2-(3-benzoylphenyl)propionate, tris(2,4,6-trimethoxyphenyl)phosphonium 2-(3-benzoylphenyl)propionate, tris(2,6-dimethoxyphenyl)phosphonium 2-(xanthon-2-yl)acetate, tris(2,4,6-trimethoxyphenyl)phosphonium 2-(xanthon-2-yl)acetate, tris(2,6-dimethoxyphenyl)phosphonium 2-(thioxanthon-2-yl)acetate and tris(2,4,6-trimethoxyphenyl)phosphonium 2-(thioxanthon-2-yl)acetate.

7. Use according to one of claims 1 to 6, characterized in that the photopolymerizable composition is applied to a substrate and cured by irradiation to form a coating or is cured in a generative manufacturing process by means of layer-by-layer irradiation to form a three-dimensional article, wherein the curing is optionally carried out with heating and wherein the photopolymer obtained is optionally subjected to a thermal aftertreatment.

8. Use according to claim 7, characterized in that hot lithography is carried out as the generative manufacturing process by heating to a temperature of at least 50 °C or at least 70 °C or at least 80 °C.

9. Use according to one of claims 1 to 8, characterized in that at least two types of monomers polymerizable by Michael addition reactions are used in the photopolymerizable composition.

10. Use according to claim 9, characterized in that the monomers are polymerizable by oxa-ene additions, oxa-yne additions or CC addition reactions of CH-active compounds, preferably by oxa-ene additions.

11. Use according to claim 10, characterized in that a combination of (meth)acrylates or (meth)acrylamides and alcohols is used as monomers, preferably without solvent in bulk.

12. Use according to one of claims 1 to 11, characterized in that the photopolymerizable composition further comprises at least one photosensitizer, preferably 9,10-dibutoxyanthracene, and / or at least one radical scavenger, preferably 2,6-di-tert-butyl-p-cresol (butylhydroxytoluene, BHT).

13. A photopolymer obtainable by curing a photopolymerizable composition as described in any one of claims 1 to 12.

14. Triphenylphosphonium carboxylate salt of the following formula (I) in which the R 1 and R 2 are each independently selected from -H and -OCH3 and wherein R 3 is selected from -H and -CH3 and X is either absent or selected from -O- and -S-, wherein the salt is selected from the group consisting of the following: Tris(4-methoxyphenyl)phosphonium 2-(3-benzoylphenyl)propionate (1): Tris(2-methoxyphenyl)phosphonium 2-(3-benzoylphenyl)propionate (2): Tris(2,6-dimethoxyphenyl)phosphonium 2-(3-benzoylphenyl)propionate (3): Tris(2,4,6-trimethoxyphenyl)phosphonium 2-(3-benzoylphenyl)propionate (4): 5 Tris(2,4,6-trimethoxyphenyl)phosphonium 2-(thioxanthon-2-yl)acetate (6):

15. Use of a triphenylphosphonium carboxylate salt according to claim 14 as a photobase generator in photopolymerizable compositions for the preparation of photopolymers.

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