Photoinitiators for light-curable compositions

Abstract

Compounds of formula (I) are photoinitiators or photosensitizers in a photopolymerizable composition: ##STR00001##
R.sub.1 represents a monovalent, linear, branched or cyclic, aliphatic hydrocarbon group having 1 to 20 carbon atoms, optionally substituted with substituent(s) selected from —Cl, —Br, —OH, ═O, —NH—CO—OR.sub.2, —NH—CO—R.sub.2 or free-radically or ionically polymerizable groups. Each R.sub.2 is independently —H or C.sub.1-6 alkyl; n is ≥1. If n=1, Z and Y are absent and X represents —OR.sub.3; if n is >1, Z represents —OR.sub.4—, Y represents —OR.sub.5— and X represents —H or —OH. R.sub.3 represents —H or R.sub.1; and R.sub.4 and R.sub.5 each independently represent a bivalent hydrocarbon group. The polymerizable moieties as optional substituents of R.sub.1 are polymerizable double or triple bonds, lactam, lactone and epoxide moieties, which are subjectable to ring-opening polymerization; and two of R.sub.1 to R.sub.5 may be linked to one another to form a ring or a dimer.

Claims

1. A photoinitiator or photosensitizer of the following formula (I) for use in a photopolymerizable composition: ##STR00073## wherein R.sub.1 represents a monovalent, linear, branched or cyclic, aliphatic hydrocarbon group having 1 to 20 carbon atoms, which is optionally substituted with one or more substituents selected from —Cl, —Br, —OH, ═O, —NH—CO—OR.sub.2, —NH—CO—R.sub.2 or free-radically or ionically polymerizable groups, wherein each R.sub.2 radical is independently selected from —H or C.sub.1-6 alkyl; wherein n is >1; Z represents —OR.sub.4—; Y represents —OR.sub.5—; X represents —H or —OH; and R.sub.4 and R.sub.5 each independently represent a bivalent hydrocarbon group for which otherwise the same options as mentioned for R.sub.1 apply; wherein the polymerizable moieties as optional substituents of R.sub.1 are selected from polymerizable double or triple bonds as well as lactam, lactone and epoxide moieties, which are subjectable to ring-opening polymerization; and wherein any two of R.sub.1 to R.sub.5 may be optionally linked to one another to form a ring or a dimer.

2. The photoinitiator or photosensitizer according to claim 1, wherein, in formula (I): a) R.sub.1 represents a linear, branched or cyclic aliphatic hydrocarbon group having 1 to 20 carbon atoms, in which one or more carbon atoms may have been replaced by oxygen atoms and which is optionally substituted with one or more substituents selected from —Cl, —Br, —OH and —SH; and/or n is >1 and is selected from the range from 2 to 100 or from 2 to 50 or from 2 to 20, Z represents —OR.sub.4—, Y represents —OR.sub.5—, and X represents —OH.

3. The use according to claim 1, wherein the compound of formula (I) is used as a type I or type II photoinitiator.

4. The photoinitiator or photosensitizer according to claim 1, wherein the compound of formula (I) is used in photopolymerizable compositions as a photoinitiator in combination with one or more co-initiators.

5. The photoinitiator or photosensitizer according to claim 4, wherein one or more compounds is/are used as co-initiator/s selected from mono- or polyhydric alcohols (—OH), thiols (—SH), amines (—NR—), silanes (═SiH—), germanes (═GeH—), phosphines (—PRR′R″—), ethers (>CH—O—CH<), iodonium (—I.sup.+—) and sulfonium (═S.sup.+—) salts and compounds based on derivatives thereof.

6. The photoinitiator or photosensitizer according to claim 5, wherein one or more mono- or polyhydric alcohols selected from sugars, glycerol, thiols, polyethylene glycol and polypropylene glycol are used as co-initiators.

7. The photoinitiator or photosensitizer according to claim 1, wherein the compound of formula (I) is used in an amount of 0.1 to 10 parts by weight per 100 parts by weight of polymerizable monomers.

8. The photoinitiator or photosensitizer according to claim 1, wherein the compound of formula (I) is used for curing acrylate or methacrylate monomers.

9. The photoinitiator or photosensitizer according to claim 1, wherein the compound of formula (I) is used as a photoinitiator or photosensitizer in compositions to be cured inside the human body or in curable compositions from the food sector.

10. A photopolymerizable composition comprising at least one compound of formula (I) as a photoinitiator or photosensitizer, at least one photopolymerizable monomer and optionally at least one co-initiator, fillers, solvents, further initiators or sensitizers, plasticizers and/or flow enhancers: ##STR00074## wherein: R.sub.1 represents a monovalent, linear, branched or cyclic, aliphatic hydrocarbon group having 1 to 20 carbon atoms, which is optionally substituted with one or more substituents selected from —Cl, —Br, —OH, ═O, —NH—CO—OR.sub.2, —NH—CO—R.sub.2 or free-radically or ionically polymerizable groups, wherein each R.sub.2 radical is independently selected from —H or C.sub.1-6 alkyl; wherein n is >1; Z represents —OR.sub.4—; Y represents —OR.sub.5—; X represents —H or —OH; and R.sub.4 and R.sub.5 each independently represent a bivalent hydrocarbon group for which otherwise the same options as mentioned for R.sub.1 apply; wherein the polymerizable moieties as optional substituents of R.sub.1 are selected from polymerizable double or triple bonds as well as lactam, lactone and epoxide moieties, which are subjectable to ring-opening polymerization; and wherein any two of R.sub.1 to R.sub.5 may be optionally linked to one another to form a ring or a dimer.

11. The photoinitiator or photosensitizer according to claim 1, wherein n is selected from the range from 2 to 100.

12. The photoinitiator or photosensitizer according to claim 11, wherein n is selected from the range from 2 to 50.

13. The photoinitiator or photosensitizer according to claim 12, wherein n is selected from the range from 2 to 20.

Description

EXAMPLES

(1) The present invention will now be described in more detail with reference to represen-tative examples that only serve the purpose of illustrating the invention without limiting its scope.

(2) Materials and Procedures

(3) If not stated otherwise below, all reagents and photoinitiators were obtained from commercial sources and used without further purification. .sup.1H NMR and .sup.13C NMR spectra were mostly recorded using a Bruker DPX-200 Fourier Transform spectrometer at 200 MHz or 50 MHz, respectively, and some measurements were carried out using a BrukerAvance at 400 MHz (.sup.1H) and 100 MHz (.sup.13C). Mass spectra were recorded using a Thermo Fisher Scientific ITQ 1100 and a silica capillary column (30 m×0.25 mm).

(4) The following photoinitiators of formula (I) were tested in the Examples of the present invention:

(5) 2-Oxopropanoic acid ethyl ester (pyruvic acid ethyl ester, ethyl pyruvate) (1):

(6) ##STR00010##

(7) 2-Oxobutanoic acid methyl ester (2-oxobutyric acid methyl ester) (2):

(8) ##STR00011##

(9) 2-Oxo-3-methylbutanoic acid ethyl ester (2-oxoisovaleric acid ethyl ester) (3):

(10) ##STR00012##

(11) 2-Oxo-3,3-dimethylbutanoic acid methyl ester (4):

(12) ##STR00013##

(13) 3-Bromo-3-methyl-2-oxobutanoic acid ethyl ester (5):

(14) ##STR00014##

(15) 3-Hydroxy-3-methyl-2-oxobutanoic acid ethyl ester (6):

(16) ##STR00015##

(17) N,N′-Dimethylaminopyruvate (7)

(18) ##STR00016##

(19) 4,4-Dimethyldihydrofuran-2,3-dione (8):

(20) ##STR00017##

(21) α-Ketoglutaric acid diethyl ester (9):

(22) ##STR00018##

(23) N,N′-Dimethylaminopyruvate (10)

(24) ##STR00019##

(25) α-Ketoglutaric acid di(hydroxyethylmethacrylate) ester (11):

(26) ##STR00020##

(27) α-Ketoglutaric acid-hexanediol polyester (Mn ˜10000 Da) (12)

(28) ##STR00021##

(29) Urethane methacrylate-terminated α-ketoglutaric acid-hexanediol polyester (13)

(30) ##STR00022##

(31) The initiators (1) to (4) were obtained from commercial sources, while the novel initiators (5) to (13) were synthesized by the inventors, as described in more detail in the Synthesis Examples later on.

(32) In the Comparative Examples, the following four known, commercially available initiators were tested:

(33) Benzophenone (BP):

(34) ##STR00023##

(35) Ethyl phenylglyoxylate (PGO)

(36) ##STR00024##

(37) 2-Bromo-2-methyl-1-phenylpropan-1-one (2-bromoisobutyrophenone) (“bromo darocure”, BD):

(38) ##STR00025##

(39) 3-(4-Benzoylphenoxy)-2-hydroxy-N,N-dimethylpropane-1-amine hydrochloride (BPQ)

(40) ##STR00026##

Synthesis Example 1

Synthesis of 3-bromo-3-methyl-2-oxobutanoic acid ethyl ester (5)

(41) ##STR00027##

(42) Into a 50 ml three-necked flask, 15 ml of CCl.sub.4 were placed. Thereto, 1 equivalent (5.62 g, 39 mmol) of 3-methyl-2-oxobutanoic acid ethyl ester was added, followed by 1 eq. (6.24 g, 39 mmol) of bromine. Then, 1.5 ml of acetic acid was used for acidifica-tion. After decoloring the solution, the reaction was quenched using approx. 30 ml of sat. NaHCO.sub.3 solution, and the mixture was transferred into a separating funnel. There, 100 ml diethyl ether and another 20 ml of NaHCO.sub.3 were added. The aqueous layer was discarded, and the organic layer was re-extracted with 50 ml of NaHCO.sub.3. The ether layer was washed with sat. NaCl solution and dried over sodium sulfate. The solvent was then removed on a rotary evaporator.

(43) Yield: 6.94 g (79% of theory)

(44) .sup.1H NMR (400 MHz, C.sub.6D.sub.6) δ ppm: 4.31 (q, 2H, J=7.02 Hz); 1.95 (s, 6H); 1.32 (t, 3H, J=7.2 Hz).

(45) GC-MS: 224.99 [M+H].sup.+, 143.07 [M−Br].sup.+, 122.93 [M−COCOOEt].sup.+, 70.20 [M−Br-COOEt].sup.+.

Synthesis Example 2

Synthesis of 3-hydroxy-3-methyl-2-oxobutanoic acid ethyl ester (6)

(46) ##STR00028##

(47) 1 equivalent (4.46 g, 20 mmol) of 3-bromo-3-methyl-2-oxobutanoic acid ethyl ester was added to 0.5 eq. silver(I) oxide in 40 ml moist acetonitrile (ACN). The mixture was stirred for 12 hrs, and the light-grey precipitate was removed by filtration. The solution was then diluted using 100 ml of water and extracted with 200 ml of ether, and the organic layer was dried over sodium sulfate. After removing the solvent, the crude product was subjected to fractional distillation (120° C., 25 mbar).

(48) Yield: 1.50 g (93% of theory)

(49) .sup.1H NMR (400 MHz, C.sub.6D.sub.6) δ ppm: 3.85 (q, 2H, J=7.31 Hz); 2.94 (bs, 1H); 1.30 (s, 6H); 0.83 (t, 3H, J=7.09 Hz).

(50) GC-MS: 161.16 [M+H].sup.+, 143.20 [M−OH].sup.+, 115.16 [M−OEt].sup.+, 87.09 [M−COOEt].sup.+, 59.06 [(CH.sub.3).sub.2OH].sup.+.

Synthesis Example 3

Synthesis of N,N′-dimethylamino pyruvate (10)

(51) ##STR00029##

(52) In a 100 ml three-necked round-bottom flask, 2,5 eq. N,N-dimethyl ethanolamine were added dropwise from a septum to a mixture of 1 eq. 3,3-dimethyl-2-oxobutanoyl chloride in 10 ml dichloromethane while cooling using an ice bath. After the addition had been completed, stirring was continued for 48 hrs at room temperature. After the reaction had been completed, the solvent was removed using a rotary evaporator. The product was obtained as a clear oil by Kugelrohr distillation at 1.2 mbar and 94° C.

(53) Yield: 177 mg (17% of theory)

(54) .sup.1H NMR (400 MHz, C.sub.6D.sub.6, ppm): 4.11 (t, 2H), 2.29 (t, 2H), 2.02 (s, 6H), 1.16 (s, 9H).

(55) 2.5 eq. N,N-dimethylethanolamine were added dropwise from a septum to a mixture of 1 eq. 3,3-dimethyl-2-oxobutanoyl chloride in 10 ml dichloromethane. After the addition had been completed, stirring was continued for 48 h at room temperature. After the reaction had been completed, the solvent was removed using a rotary evaporator. The product was obtained as a clear oil by Kugelrohr distillation at 1.2 mbar and 94° C.

(56) Yield 177 mg (17% of theory)

(57) .sup.1H NMR (400 MHz, C.sub.6D.sub.6, ppm): 4.11 (t, 2H), 2.29 (t, 2H), 2.02 (s, 6H), 1.16 (s, 9H).

Synthesis Example 4

Synthesis of α-ketoglutaric acid diethyl ester (9)

(58) ##STR00030##

(59) At first, α-ketoglutaric acid was re-crystallized from acetone. Subsequently, 1 eq. (3.662 g, 25 mmol) of α-ketoglutaric acid were weighed into a 250 ml three-necked round-bottom flask equipped with a condenser and a septum, followed by the addition of 100 ml of ethanol and 0.3 eq. (0.4 ml, 7.5 mmol) of sulfuric acid. The reaction mixture was magnetically stirred and refluxed. The oil bath was set to 100° C., and the reaction was left to proceed for 24 h. Reaction progress was monitors by thin-layer chromato-graphy (PE:EE, 5:1; Rf 0.36). The solvent was evaporated using a rotary evaporator, followed by the addition of 50 ml of de-ionized water and neutralization of the solution using 1N KOH solution. The aqueous layer was extracted three times with 100 ml of ehtylacetate, and the combined organic layers were dried over Na.sub.2SO.sub.4. The solvent was then evaporated using a rotary evaporator. 4.193 g (83% of theory) crude yield were obtained. The crude product was chromatographically separated by means of MPLC using a 214 g silica column (PE:EE, 9:1). The product was detected using a UV detector (254 nm) and identified by DC (PE:EE, 5:1; Rf 0.36). The fractions containing the product were combined, and the solvent was evaporated using a rotary evaporator. In total, 1.77 g (35% of theory) of α-ketoglutaric acid diethyl esther (9) were obtained as a colorless oil.

(60) Rf: 0.36 (PE:EE, 5:1)

(61) .sup.1H NMR (400 MHz, CDCl.sub.3, ppm): 4.32 (q, J=7.2 Hz, 2H), 4.13 (q, J=7.20 Hz, 2H), 3.14 (t, J=6.6 Hz, 2H), 2.65 (t, J=6.6 Hz, 2H), 1.36 (t, J=7 Hz, 3H), 1.24 (t, J=7 Hz, 3H).

(62) .sup.13C NMR (400 MHz, CDCl.sub.3, ppm): 192.8, 172.0, 160.6, 62.6, 60.9, 34.2, 27.8, 14.2, 14.0.

(63) GC-MS: 202.99 [M+H], 129.94 [M+H, —(CO—O—CH.sub.2—CH.sub.3)], 128.95 [M−(CO—O—CH.sub.2—CH.sub.3)], 101.99 [M+H, —(CO—CO—O—CH.sub.2—CH.sub.3)], 101.00 [M−CO—CO—O—CH.sub.2—CH.sub.3], 74.11 [M+H, —(CO—CH.sub.2—CH.sub.2—CO—O—CH.sub.2—CH.sub.3)], 73.14 [M−(CO—CH.sub.2—CH.sub.2—CO—O—CH.sub.2—CH.sub.3)], 55.13 [M+H, —(O—CH.sub.2—CH.sub.3), —(CH.sub.2—CH.sub.2—COO—CH.sub.2—CH.sub.3)].

Synthesis Example 5

Synthesis of α-ketoglutaric acid di-2-hydroxyethylmethacrylic acid ester (11)

(64) ##STR00031##

(65) At first, α-ketoglutaric acid was re-crystallized from acetone. Subsequently, 1 eq. (3.943 g, 27 mmol) of α-ketoglutaric acid, 2 eq. (7.290 g, 54 mmol) of 2-hydroxymethyl methacrylate, 0.7 wt % (78.5 mg) lipase (Candida antarctica) immobilized on acrylic resin (<5.000 U/g) and 565 ppm (10.1 mg) of 2,6-di-tert-butyl-4-methylphenol (BHT) were placed in a 50 ml single-necked round-bottom flask. The flask was equipped with a drying tube filled with calcium chloride. The mixture was magnetically stirred in an oil bath at 65° C. Reaction progress was monitored by NMR and TLC (PE:EE, 8:2; Rf 0.62), and after a reaction time of 140 hrs, the reaction was diluted with PE:EE (1:1) and chromatographically separated using a 90 g silica column by MPLC (PE:EE, 8:2). The combined product fractions were treated with BHT and evaporated using a rotary evaporator. 2.991 g (30% of theory) of the pure product were obtained as a colorless, transparent oil.

(66) Rf: 0.62 (PE:EE, 8:2)

(67) .sup.1H NMR (400 MHz, (acetone-d.sub.6, ppm): 6.05 (s, 2H, 2 CHH), 5.62-5.59 (m, 2H), 4.54-4.48 (m, 2H), 4.43-4.38 (m, 2H), 4.30 (s, 4H), 3.15 (t, J=6.6 Hz, 2H), 2.63 (t, J=6.6 Hz, 2H), 1.87 (t, J=1.2 Hz, 6H).

Synthesis Example 6

Synthesis of ketoglutaric acid-hexanediol polyester (12)

(68) ##STR00032##

(69) At first, α-ketoglutaric acid was re-crystallized from acetone and p-toluene sulfonic acid was re-crystallized from chloroform. Subsequently, 1 eq. (11.18 g, 80 mmol) of pure α-ketoglutaric acid, 1.01 eq. (9.54 g, 81 mmol) 1,6-hexanediol and 0.0025 eq. (30.2 mg, 0.2 mmol) of pure p-toluene sulfonic acid were weighed into a 100 ml three-necked flask equipped with a magnetic stirrer and a Dean Stark trap. 30 ml abs. toluene were added, and the oil bath was heated to 125° C. Reaction progress was monitored by NMR, and the reaction was stopped after 24 hrs. The dissolved polyester was diluted with 30 ml abs. toluene and precipitated in 800 ml cold diethyl ether. The result obtained was a slightly yellow polyester that was dried in a vacuum drying oven at 50° C., followed by dissolution in 60 ml THF, filtering the solution and re-precipitation in 800 ml cold diethyl ether. After drying under vacuum, 11.36 g (62%) of the polyester were obtained as a white polymer.

(70) Molecular weight, Mn (by GPC and NMR)≈10,000; n≈40

(71) .sup.1H NMR (400 MHz, CDCl.sub.3, ppm): 4.26 (t, J.sub.HH=6.6 Hz, 40H), 4.08 (t, J.sub.HH=6.6 Hz, 40H), 3.65 (t, J.sub.HH=6.6 Hz, 4H), 3.15 (t, J.sub.HH=6.6 Hz, 40H), 2.67 (t, J.sub.HH=6.6 Hz, 40H), 1.78-1.72 (m, 40H), 1.68-1.60 (m, 40H), 1.45-1.36 (m, 80H).

Synthesis Example 7

Synthesis of α-ketoglutaric acid-1,6-hexanediol polyester having 2-isocyanatoethyl-methacrylate-modified terminal groups (13)

(72) ##STR00033##

(73) In a first step, 1 eq. of the polyester (0.98 g, 0.2 mmol), 2 drops dibutyltin dilaurate as a catalyst and 20 ml of abs. toluene were charged into a 50 ml three-necked round-bottom flask. The flask was flushed with argon and equipped with a septum and an argon balloon. Subsequently, 2.05 eq. (0.6 ml, 0.4 mmol) of 2-isocyanatoethylmeth-acrylate were added dropwise via the septum. The mixture was stirred for 14 hrs at room temperature and then quenched using 5 ml of methanol. 20 ml of distilled acetone were added, and the polyester was precipitated in 300 ml of cold diethyl ether, yielding a white polymer (0.36 g, 34% of theory).

(74) Molecular weight, Mn (by GPC and NMR)≈10,000; n≈40

(75) .sup.1H NMR (400 MHz, CDCl.sub.3, ppm): 6.12 (s, 2H), 5.59 (s, 2H), 5.342 (s, 2H), 4.26 (t, J.sub.HH=6.6 Hz, 80H), 4.15 (t, 4H, J.sub.HH=6.4 Hz), 4.08 (t, J.sub.HH=6.6 Hz, 80H), 3.65 (t, J.sub.HH=6.4 Hz, 4H), 3.15 (t, J.sub.HH=6.6 Hz, 80H), 2.67 (t, J.sub.HH=6.6 Hz, 80H), 1.95 (s, 3H) 1.78-1.72 (m, 80H), 1.68-1.60 (m, 80H), 1.45-1.36 (m, 160H).

(76) In the following examples, of the inventive use of compounds of formula (I) as photoinitiators, reaction mixtures containing the respective photoinitiator, the specified liquid monomer and optionally a specified co-intiator were produced and cured for 10 min under a nitrogen atmosphere by exposure to an OmniCure® S2000 mercury lamp having a wave length filter of 320 to 500 nm and an UV light intensity of 1 W/cm.sup.2, while the progress of the reactions was monitored by photo-DSC type NETSCH DSC 204 F1 Phoenix.

(77) All measurements were carried out at least twice, the respective tables showing the average values for the respective initiators.

(78) In the tables, R.sub.P represents the polymerization rate and is thus an indicator of the reactivity of a system. A high value means that many monomer groups are reacted at the same time and that the curing process is generally shorter. t.sub.max is the time (in s) it takes until maximum heat development is reached and is thus an indicator of how long it takes to reach the gelling point and thus a certain initial solidity. Short times are thus more desirable. t.sub.95% is the time (in s) after which 95% of the entire reaction heat have been released and is thus an indicator of the rate at which a reaction occurs, lower values again being advantageous. DBC is the double bond conversion rate calculated based on the reaction heat released during polymerization (in J per g) of the respective formulation. For the conversion rate, it is desirable to achieve values that are as high as possible.

(79) In all experiments, acrylates and methacrylates that are typically used in the field of coating were used as monomers.

(80) In all formulations, the respective photoinitiator was used in an equimolar amount with respect to 1 wt % of ethyl pyruvate (1), which was the initiator having the lowest molecular weight, and weighed in accordingly, and co-initiators, if contained, were weighed in equimolar amounts with respect to the respective initiator. Subsequently, 12±0.5 mg of the reaction mixtures were weighed into DSC aluminum pans, and the pans were covered with cover glasses.

Examples 1 to 5, Comparative Example 1—Type II Initiators

(81) In this group of experiments, the hydrogen abstractors of the Examples 1 to 4 (E1 to E4) and one Comparative Example (C1) were tested in combination with a co-initiator serving as a hydrogen donor.

(82) Initiators:

(83) ##STR00034##

Comparative Example 1

(84) ##STR00035##
Monomer:

(85) ##STR00036##
Co-Initiator:

(86) 4-dimethylaminobenzoic acid ethyl ester (DMAB)

(87) ##STR00037##
Results:

(88) TABLE-US-00001 TABLE 1 R.sub.P t.sub.max t.sub.95% DBC [mmol .Math. l.sup.−1 .Math. s.sup.−1] [s] [s] [%] C1  96 10.1 85.5 68.9 E1 230  7.7 41.7 66.5 E2 204  8.5 40.5 60.9 E3 241  7.5 36.6 65.8 E4 170 10.6 42.5 59.1 E5 218  6.0 73.0 63.1

(89) Table 1 shows that the novel α-ketoesters according to the present invention achieve surprisingly high polymerization rates when compared to benzophenone (BP), the industrial reference. This is mainly shown by the fact that the time until 95% of the overall conversion is reached (t.sub.95%) is significantly shorter. The final conversion (DBC) remains at a level comparable to that using the industrial BP/DMAB system. In particular, the compounds of Examples 1 to 3 are distinguished by their high reactivity and a particularly high conversion rate, among which compounds, methylethyloxobutanoate (3) of Example 3 achieved outstanding values.

Examples 6 and 7, Comparative Example 2—Type II Initiators

(90) Two of the above experiments were repeated using the same two initiators and BP as a Comparative Example, using a non-aromatic co-initiator (MDEA) instead of the aromatic amine (DMAB).

(91) Initiators:

(92) ##STR00038##
Monomer:

(93) ##STR00039##
Co-Initiator:

(94) ##STR00040##
Results:

(95) TABLE-US-00002 TABLE 2 R.sub.P t.sub.max t.sub.95% DBC [mmol .Math. l.sup.−1 .Math. s.sup.−1] [s] [s] [%] C2 109 10.8 77.0 69.3 E6 228  6.7 56.5 65.2 E7 234  7.7 41.0 64.6

(96) It was also possible to achieve good results using the non-aromatic co-initiator with the compounds (3) and (4). The two ketoesters showed a significantly higher reactivity and lower t.sub.95% than the reference system.

Examples 8 and 9, Comparative Example 3—Type II Initiators

(97) The compounds (1) and (4) were compared to the commercial initiator ethylphenyl glyoxylate (PGO).

(98) Initiators:

(99) ##STR00041##
Monomer:

(100) ##STR00042##
Results:

(101) TABLE-US-00003 TABLE 3 R.sub.P t.sub.max t.sub.95% DBC [mmol .Math. l.sup.−1 .Math. s.sup.−1] [s] [s] [%] C3 204 14.1 63.0 75.1 E8 210  8.5 56.0 69.3 E9 214  8.3 53.5 64.6

(102) Surprisingly, the novel compounds (1) (Example 8) and (4) (Example 9) achieved significantly higher polymerization rates than the known phenyl glyoxylate initiator (PGO) (Comparative Example 3). The maximum polymerization rate was reached in almost half the time. The final conversion was also achieved up to 10 s earlier when using the novel α-ketoesters.

Example 10—Type II Initiator

(103) The compound that had achieved the best results in the experiments so far, compound (3), was tested using thiol as a co-initiator; the achieved results were compared to those of Example 3 where DMAB had been used as a co-initiator.

(104) Initiator:

(105) ##STR00043##
Monomer:

(106) ##STR00044##
Co-Initiator:

(107) ##STR00045##
Results:

(108) TABLE-US-00004 TABLE 4 R.sub.P t.sub.max t.sub.95% DBC [mmol .Math. l.sup.−1 .Math. s.sup.−1] [s] [s] [%] E3  241.0 7.5 36.6 65.8 E10 233.5 8.0 45.0 68.0

(109) It can be seen that also the thiol is very well suited for use as a co-initiator for type II α-ketoester initiators.

Example 11, Comparative Example 4—Type I Initiators

(110) In this experiment, the brominated ketoester (5) from Synthesis Example 1 was compared to a known initiator that also contained bromide, “bromo darocur” (BD). As these two initiators were type I initiators, polymerization was carried out without any co-initiators.

(111) Initiators:

(112) ##STR00046##
Monomer:

(113) ##STR00047##
Results:

(114) TABLE-US-00005 TABLE 5 R.sub.P t.sub.max t.sub.95% DBC [mmol .Math. l.sup.−1 .Math. s.sup.−1] [s] [s] [%] E11 239 5.9 40.0 62.1 C4  146 7.3 62.5 57.6

(115) The use of the novel α-ketoester (5) according to the invention provided clearly better results than the common bromide-containing type I initiator according to the state of the art, the novel compound (5) surprisingly even achieving significantly better reactivity results than the latter.

Examples 12 to 15, Comparative Example 5—Type II Initiators

(116) In this group of experiments, the experiments from Examples 1 to 4 and Comparative Example 1 were repeated, using a di-methacrylate mixture from the field of dental engineering as a monomer instead of the diacrylate that had been used above.

(117) Initiators:

(118) ##STR00048##
Monomers:

(119) ##STR00049##
Co-Initiator:

(120) ##STR00050##
Results:

(121) TABLE-US-00006 TABLE 7 R.sub.P t.sub.max t.sub.95% DBC [mmol .Math. l.sup.−1 .Math. s.sup.−1] [s] [s] [%] C5  21.4 18.5 146.5 50 E12 74.5 11.9  83.0 57 E13 71.2 12.2  85.5 57 E14 81.5 11.0  58.5 57 E15 55.9 14.9  69.5 51

(122) It becomes clear that the methacrylate monomer can also be cured more efficiently using α-ketoesters as initiators than when using benzophenone. The reactivity of the ketoesters (1) to (4) of the invention is surprisingly several times—up to 2.5 times—higher than that of benzophenone, and the time until the peak maximum is reached is much shorter than in the Comparative Example. In Examples 12 to 15 of the invention, the double bond conversion was much higher using the methacrylate monomer than when using the reference initiator, and maximum conversion was even reached 2 to 3 times faster, which further underlines that the present invention is superior to the state of the art.

Example 16, Comparative Example 6—Type II Initiators

(123) In this Example, a ketoester, compound (1), was directly compared by means of photo-DSC measurements to benzophenone (BP) as a known type II initiator when it comes to curing a known difunctional vinyl ester, i.e. divinyl adipate (DVA), that was used as a monomer.

(124) Initiators:

(125) ##STR00051##
Monomer:

(126) ##STR00052##
Co-Initiator:

(127) ##STR00053##
Results:

(128) TABLE-US-00007 TABLE 8 DSC t.sub.max Area [mW .Math. mg.sup.−1] [s] [J.g.sup.−1] E16 1.7  194 466 C6  4.37  55 642

(129) It becomes evident that, when using the vinyl ester monomer DVA that is rather unreactive when compared to (meth)acrylates, the ketoester (1) according to the present invention achieves clearly worse results than benzophenone in the Comparative Example. This comparison, however, still shows that it is generally possible to cure biocompatible monomers such as DVA using food-safe α-ketoesters. As benzophenone and other commercially available initiators may be harmful to health, the advantage of their usability in this context will offset the lower reactivity of vinyl esters.

Examples 17 and 18, Comparative Example 7—Type II Initiators in Hydrogels

(130) Using the partially water-soluble ketoesters in a hydrogel formulation with PEG700DA (polyethylene glycol diacrylate with Mn 700) and 50 wt % water for polymerizing hydrogels worked surprisingly well. PBQ was used as a commercially available reference initiator, with MDEA as a co-initiator. The ketoesters ethyl pyruvate (1) and dimethyl-furandione (8) can be used as novel initiators without any co-initiator. Due to its long-wave absorption maximum (λ.sub.max=378 nm instead of 330 nm for compound (1)), compound (8) can also be used using visible light which is harmless for cells.

(131) Initiators:

(132) ##STR00054##
Monomer:

(133) ##STR00055##
Co-Initiator:

(134) ##STR00056##
Results:

(135) TABLE-US-00008 TABLE 9 R.sub.P t.sub.max t.sub.95% DBC [mmol .Math. l.sup.−1 .Math. s.sup.−1] [s] [s] [%] C7   30  8.8 87 50 E17* 33  8.5 83 48 E18* 26 10.3 78 56 *without co-initiator

(136) As becomes clear from Table 9, the biocompatible initiators of the invention proved surprisingly reactive and achieved results that were comparable to those achieved using the commercial initiator. In this connection, it has to be stated that in a study using the mouse fibroblast cell line L929 and the commercial compound BPQ, significant toxic effects were already observed at a concentration of 8 mmol/1, while the compounds (1) and (8) did not show any cytotoxicity even at concentrations that were twice as high. This may constitute an additional advantage for biological applications in which hydrogels are commonly used.

Examples 19 and 20, Comparative Examples 8 and 9—Type II Initiators

(137) Surprisingly, the biocompatible compound (8) from Example 18 was equally soluble in aqueous and non-aqueous monomer systems. For this reason, reactivity was tested in acrylates (Example 19) and methacrylates (Example 20); in the latter case, DMAB was additionally used as co-initiator because of the low reactivity of the methacrylates. The new system was compared with the industrial standard initiator benzophenone (BP) in acrylates (Comparative Example 8) and methacrylates (Comparative Example 9).

(138) Initiators:

(139) ##STR00057##
Monomers:

(140) ##STR00058##
Co-Initiator:

(141) ##STR00059##
Results:

(142) TABLE-US-00009 TABLE 10 R.sub.P t.sub.max t.sub.95% DBC [mmol .Math. l.sup.−1 .Math. s.sup.−1] [s] [s] [%] C8   93 11.1  69 69 C9   21 18.2 143 49 E19 189 11.6  58 66 E20  52 18.2  90 54

(143) As can be seen in table 10, the reactivity of compound (8) of the invention without any co-initiator in HDDA (Example 19) was surprisingly more than twice as high as that of the reference BP without co-initiator (Comparative Example 8). A comparable final conversion is also achieved more rapidly. In methacrylates, the use of DMAB as a co-initiator (Example 20) of compound (8) yields not only a polymerization rate that is more than twice as high, but, at the same time, also a significantly higher final conversion much more rapidly than in the reference example (Comparative Example 9).

Example 21 and 22, Comparative Example 10—Type II Initiators

(144) Ketoglutaric acid was esterified with simple alcohols (Example 21) and hydroxyethyl methacrylate (HEMA) Example 22) to produce polymerizable initiators, and their reactivity was tested in the di-acrylate HDDA. Reactivity was again compared to that of a benzophenone/amine system (Comparative Example 10).

(145) Initiators:

(146) ##STR00060##
Monomer:

(147) ##STR00061##
Co-Initiator:

(148) ##STR00062##
Results:

(149) TABLE-US-00010 TABLE 11 R.sub.P t.sub.max t.sub.95% DBC [mmol .Math. l.sup.−1 .Math. s.sup.−1] [s] [s] [%] C10  93 11.1 69 69 E21 201  9.2 58 67 E22 203 11.6 53 64

(150) Surprisingly, the polymerization rate of these α-ketoesters was also at least twice as high as that of the BP/amine reference system in Comparative Example 10. A high final conversion is also achieved more rapidly. Additionally, the polymerizable groups of compound (11) make sure that any residual initiator will not be able to diffuse from the polymer after polymerization, which is particularly important for applications in the fields of medicine and food.

Examples 23 and 24 and Comparative Example 11—Type II Initiators

(151) α-Ketoglutaric acid esters can also be used for methacrylates. For this reason, substances (9) and (11) were tested in comparison with benzophenone (BP) in the dental formulation DDM, using DMAB as a co-initiator.

(152) Initiators:

(153) ##STR00063##
Monomers:

(154) ##STR00064##
Co-Initiator:

(155) ##STR00065##
Results:

(156) TABLE-US-00011 TABLE 12 R.sub.P t.sub.max t.sub.95% DBC [mmol * l.sup.−1 * s.sup.−1] [s] [s] [%] C11 21 18.2 143 49 E23 66 14.3  85 53 E24 57 14.5  93 52

(157) Surprisingly, the polymerization rate achieved in unreactive methacrylates using the novel α-ketoglutaric acid esters and DMAB as a co-initiator was 3 times as high as that of the reference system, as can be seen from table 12. Polymerization proceeds much more rapidly than in case of the reference substance BP and results more rapidly in higher final conversion rates.

Example 25, Comparative Example 12—Polymeric Type II Initiators

(158) As α-ketoglutaric acid is a dicarboxylic acid, it was also possible to produce polyesters from hexanediol and α-ketoglutaric acid. Polymeric initiators have the advantage that they do not migrate from the resulting polymer, which is why they are used in the fields of food and medicine. Additionally, polyesters can be modified in various ways, so that their solubility, functionality, and degradability can be adapted to specific applications.

(159) Initiators:

(160) ##STR00066##
Monomer:

(161) ##STR00067##
Co-Initiator:

(162) ##STR00068##
Results:

(163) TABLE-US-00012 TABLE 13 R.sub.P t.sub.max t.sub.95% DBC [mmol .Math. l.sup.−1 .Math. s.sup.−1] [s] [s] [%] C12  93 11.1 69 69 E25 144  9.2 71 64

(164) As illustrated in table 13, the polyester (12) of Example 25 shows an unexpectedly high reactivity when compared to benzophenone (BP) in Comparative Example 12. In spite of its high molecular weight and the associated slow diffusion, the polymerization rate is more than one third higher. α-Ketoglutaric acid polyesters are known for their bio-compatibility and their biodegradability. The use of this class of substances as photoinitiators is, however, completely new, and the high polymerization rates were very surprising.

Example 26—Type II Initiator, Curing Test

(165) Amines are used as co-initiators to increase the reactivity of type II initiators. It is also possible to bind the co-initiator covalently to the initiator in order to avoid two-compo-nent systems. Compound (10) is such an initiator that was tested using hexanediol diacrylate (HDDA) as a monomer in the present example.

(166) Initiator:

(167) ##STR00069##
Monomer:

(168) ##STR00070##

(169) 1 g of a mixture of HDDA and 1 wt % of compound (10) were irradiated in a silicone mold placed in an Intelli-Ray 400 W UV oven at 100% power. After a few seconds, a transparent, hard sample rod was obtained, proving the suitability of the present invention for synthesizing molded articles.

Example 27—Polymeric Type II Initiator, Curing Test

(170) Undesired migration of unreacted initiator molecules or their degradation products during irradiation constitutes a major problem, in particular in the fields of food and medicine. This is why macromolecular initiators and, in particular, macromolecular initiators having polymerizable terminal groups are of great interest. Its high molecular weight and reactive terminal groups prevent the initiator from diffusing out of the resulting polymer. In this connection, compound (13) was examined using hexanediol diacrylate (HDDA) as a monomer.

(171) Initiator:

(172) ##STR00071##
Monomer:

(173) ##STR00072##

(174) 1 g of a mixture of HDDA and 1 wt % of compound (13) was exposed in a silicone mold in an Intelli-Ray 400 W UV oven at 100% power. After a few seconds, a clear, hard sample rod was obtained, which proved that the polymeric initiator according to the present invention was suitable for synthesizing molded articles.

(175) The above examples clearly show that, depending on the specific reaction conditions, the use of α-ketoesters provides a number of advantages compared to known initiators, so that the present invention constitutes a valuable addition to the state of the art.