Glyoxylates and Related Structures as Photoinitiators of Polymerization

Glyoxylates and Related Structures as Photoinitiators of Polymerization: Comparison

Please note this is a comparison between Version 1 by Fréderic Dumur and Version 2 by Sirius Huang.

The design of photoinitiators activable under low-light intensity is an active research field, supported by the energetic sobriety plans imposed by numerous countries in Europe. With an aim to simplify the composition of the photocurable resins, Type I photoinitiators are actively researched as these structures can act as monocomponent systems. In this field, a family of structures has been under-investigated at present, namely, glyoxylates.

glyoxylate
methyl benzoyl formate
water-soluble
Type I photoinitiators
low-light intensity
LEDs

1. Glyoxylate Derivatives

In 2021, a series of glyoxylate derivatives have been proposed by Sun and coworkers, bearing electron-donating or electron-accepting groups ^[1][229]. By means of this specific substitution, photopolymerization experiments could be carried out at 405 nm. In this series of dyes, dimethyl 1,4-dibenzoylformate (DM-BD-F) proved to be the most efficient photoinitiator during the free radical polymerization (FRP) of acrylates (tri (propylene glycol)diacrylate (TPGDA) or trimethylolpropane triacrylate (TMPTA)), resulting from its unique ability to produce twice more radicals than the nine other structures. To determine the real performance of the different glyoxylate derivatives, phenylbis (2,4,6-trimethylbenzoyl)phosphine oxide (BAPO), and dibenzoyl (DB) were used as reference photoinitiators.

Due to the weak absorption of the different dyes at 405 nm, deep layer photocuring could also be obtained and a polymer thickness of 6.5 cm could be polymerized within 30 s. Parallel to this, due to the weak absorption of glyoxylate derivatives at 385, 395, and 405 nm, almost colorless coatings could be produced. From the absorption viewpoint, major differences could be found between the different dyes in acetonitrile (See Table 1).

Table 1. Molar extinction coefficients (M⁻¹·cm⁻¹) of the different glyoxylate derivatives in acetonitrile, at the maximum absorption and different wavelengths used for photopolymerization.

Photoinitiator	λ	_max	(nm)	εmax	ε385	ε395
BAPO	-	-	1100	1020	770	0
MBF	255	17,430	30	10	10	0
F-MBF	257	16,760	30	20	10	0
S-MBF	326

Interestingly, compared to the parent methyl benzoyl formate (MBF), all derivatives exhibited a redshifted absorption, except for TF-MBF exhibiting the strong electron-withdrawing group. Logically, the most redshifted absorptions were found for all dyes comprising an electron-donating group inducing an efficient intramolecular charge transfer (ICT) through a push-pull effect. Thus, N-MBF and S-MBF both exhibited the most redshifted absorptions located at 356 and 326 nm respectively, together with the highest molar extinction coefficients (43,800 M⁻¹·cm⁻¹ and 29,660 M⁻¹·cm⁻¹ respectively). Compared to BAPO, N-MBF exhibited higher molar extinction coefficients at all wavelengths later used for photopolymerization. Photolysis experiments revealed the occurrence of a decarboxylation reaction using bromocresol green as the pH indicator. Consistent with the mechanism established in the literature, a decarboxylation reaction occurring subsequent to the photocleavage was proposed. By theoretical calculations, the bond dissociation energy (BDE) of the different derivatives could be determined, and values ranging between 108.40 kJ/mol for TF-MBF and 150.94 kJ/mol for N-MBF were calculated (See Table 2). Parallel to this, the ΔH of all MBFs was determined as being negative, meaning that the cleavage reaction was energetically favorable ^[2][230].

Table 2.

Bond dissociation energies (kJ/mol) were determined for different glyoxylates.

Photoinitiator	BDE
MBF	138.98
F-MBF	137.92
S-MBF	148.85
29,660	590
C-MBF	138.30	260	110	0
C-MBF	266	19,630
O-MBF	145.18	60	40	30	10
O-MBF	290	19,830	120	90	70	40
N-MBF	150.94	N-MBF	356	Cl-MBF43,800	12,180	6660	3600	140.5580
Cl-MBF	265	21,420	60	40	30
TF-MBF	0
108.40	TF-MBF	246	32,800	110	80	50	10
DM-BD-F	118.83	DM-BD-F	273	18,440	280	220	160	50
DF-MBF	134.76	DF-MBF	256	25,350	110
DC-MBF	140.28	90	70	20
DC-MBF	269	20,690	100	40	30	10

Examination of their photoinitiating abilities during the FRP of TPGDA revealed F-MBF to furnish a higher monomer conversion than BAPO. Excellent monomer conversions could also be obtained with the other MBFs, except S-MBF and O-MBF for which conversions lower than 40% could be determined. The low reactivity of these derivatives was confirmed during the FRP of TMPTA. However, contrary to what was observed in TPGDA, none of the MBFs could outperform BAPO. Thus, if a TMPTA conversion of 59.4% could be obtained with BAPO, the best conversion with MBFs was obtained with O-MBF, peaking at 49.6%. The lower monomer conversion obtained with TMPTA compared to TPGDA was assigned to the higher viscosity of TMPTA and its trifunctionality.

2. Cinnamoyl Formate Derivatives

In 2022, the same group examined a new family of dyes derived from methyl benzoylformate (MBF), namely ethyl cinnamoyl formates (ECFs) ^[3][231]. Four structures were investigated, two of them bearing an electron-donating group (S-ECF and O-ECF) and one structure with an electron-accepting group (F-ECF). The different dyes could be prepared by a two-step synthesis consisting first of a Claisen Schmidt condensation followed in the second step by an esterification reaction. F-ECF, S-ECF, and O-ECF could be prepared with reaction yields of 60, 55, and 59% for the two steps respectively. Examination of their absorption properties in acetonitrile revealed the shift of the absorptions to be comparable to that observed for the previous MBFs. Thus, the introduction of electron-donating groups redshifted the absorption (S-ECF and O-ECF) whereas the opposite effect was found in the presence of electron-accepting groups (F-ECF) (See Table 34). The most redshifted absorption was found for S-ECF, peaking at 362 nm. Irrespective of the substitution pattern, almost similar molar extinction coefficients could be found for the different dyes.

Table 34. Molar extinction coefficients of ECFs in acetonitrile at the maximum absorption, 405 nm, and 455 nm.

Photoinitiator	λ	_max	(nm)	ε
ECF	309	21,550	130	0
F-ECF	309	18,890	130	0
O-ECF	342	22,000	1730	10
S-ECF	362	22,930	7060	80
ITX	256	41,050	610	0

Photolysis experiments carried out in acetonitrile revealed the different ECFs to be unable to generate radicals alone. Upon addition of ethyl dimethylaminobenzoate (EDB), a fast photolysis process could be evidenced and the formation of α-aminoalkyl radicals was confirmed by electron spin resonance (EPR) experiments. The initiation mechanism is that of a type II photoinitiator. Thus, upon photoexcitation, a photoinduced electron transfer between EDB and ECFs can occur, generating EDB radical cations and ECF radical anions. In the second step, a hydrogen abstraction reaction can occur, generating α-aminoalkyl radicals on EDB and constituting the initiating species. It has to be noticed that the different radicals formed during photolysis have been identified by electron spin resonance (ESR) experiments.

3. Silyl Glyoxylates

In 2017, Lalevée and coworkers proposed a new family of glyoxylate, namely silyl glyoxylates ^[4][5][232,233]. Tert-butyl (tert-butyldimethylsilyl)glyoxylate (DKSi), ethyl(tert-butyldimethyl)silyl glyoxylate (Et-DKSi), and benzyl (tert-butyldimethyl)silyl glyoxylate (Bn-DKSi) were examined as monocomponent photoinitiating systems or in combination with additives for the FRP of a dental resin, namely a BisGMA/TEGDMA (70/30 w/w) blend (where BisGMA and TEGDMA stand for bisphenol A-glycidyl methacrylate and triethylene glycol dimethacrylate respectively) or urethane dimethacrylate (UDMA).

Examination of the absorption properties of DKSi in toluene revealed the absorption maximum to be located at 425 nm, therefore blueshifted compared to that of camphorquinone (CQ) (465 nm). Besides, compared to the previous MBF, a significant enhancement of the molar extinction coefficient could be evidenced at 405 nm. By theoretical calculations, the redshift of the absorption maximum was determined as originating from a strong participation of the d orbital of the Si atom to the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), decreasing the HOMO-LUMO gap compared to that of the previous MBF (4.19 eV for DKSi vs 4.71 eV for MBF). A good overlap between the emission of the LED emitting at 477 nm and DKSi and camphorquinone was thus found.

During the FRP of the BisGMA/TEGDMA blend, the ability of DKSi to act as a monocomponent system was demonstrated in laminate. After 80 s of irradiation, a conversion of 40% could be determined. Upon the addition of EDB, the conversion drastically increased and a conversion of 68% was obtained. Under air and due to strong oxygen inhibition in thin films, DKSi alone was almost unable to initiate the FRP of the resin. Conversely, the three-component DKSi/EDB/DPI (2/1.4/1.6% w/w/w) system could furnish a monomer conversion of 33%, which could be improved by using the four-component DKSi/EDB/DPI/CQ (2/1.4/1.6/1% w/w/w/w) system (38%). Improvement of the monomer conversion obtained with the four-component system compared to the three-component system can be assigned to improved light absorption properties due to the concomitant presence of DKSi and CQ, both contributing to light absorption.

4. Water-Soluble Benzoylformic Acid Derivatives

The water solubility of photoinitiators is a property that is actively researched with the aim of developing greener polymerization processes ^{[6][7][8][9][10][11][12][13]}[57,96,225,235,236,237,238,239]. Indeed, polymerization in water becomes possible. This point was examined with a series of benzoylformic acid derivatives by the group of Sun and coworkers ^[14][240]. From the synthetic viewpoint, TF-BFA and CC-TFA could be prepared in one step, by oxidation of the acetyl groups with selenium oxide, and obtained reaction yields of 80 and 62%, respectively. As observed for MBFs and ECFs, the presence of the electron-accepting CF₃ group blueshifted the absorption compared with the parent structure BFA (244 nm for TF-CFA vs. 253 nm for BFA). Conversely, a redshift of the absorption was found for CC-BFA at 262 nm. Interestingly, a significant increase of the molar extinction coefficient was found, peaking at 18,480 M⁻¹·cm⁻¹ contrarily to 8640 M⁻¹·cm⁻¹ for BFA and TF-BFA. By theoretical calculations, the BDE of the different dyes could be determined and values of 154.8, 158.6, and 151.9 kJ/mol could be determined for BFA, TF-BFA, and CC-BFA, evidencing that the BDE was only slightly modified by the substitution pattern of benzoylformic acids. Polymerization experiments done at 405 nm for TPGDA and TMPTA revealed CC-TFA to outperform BFA and TF-BFA during the FRP of TPGDA. A conversion of 83.4% could be obtained after 120 s contrarily to 64.6 and 66.6% for BFA and TF-BFA. This is directly related to the ability of CC-TFA to produce twice more radicals. Noticeably, during the FRP of TMPTA, similar conversions could be obtained with the three derivatives (around 53%) and this result was assigned to the higher viscosity of TMPTA and the trifunctional character of the monomer speeding up the gelation process and adversely the double bond conversion. However, these monomer conversions remain lower than those previously obtained with DM-BD-F, with conversions of 79.1 and 46.8 being respectively obtained during the FRP of TPGDA and TMPTA. A similar trend was determined during the FRP of a water-soluble monomer, namely PEG diacrylate (PEGDA). Upon irradiation at 405 nm and by performing the polymerization experiments in water, a conversion of ca. 80% could be obtained within 180 s. Besides, a slower polymerization rate could be evidenced for CC-BFA, resulting from its poor water solubility.

5. Cytotoxicity of Glyoxylates

If polymerization efficiency is an important parameter governing the choice of photoinitiators, their toxicity is another major issue as it drastically impacts the scope of applications of polymers. Indeed, for biomedical applications or food packaging, the use of photoinitiators exhibiting low toxicity is required. This point was examined with a series of seven benchmark photoinitiators including methyl benzoylformate (MBF) ^[15][241]. Cytotoxicity tests carried out on four different tissue types of cells at concentrations ranging between 1 and 50 μM revealed phenylbis(acyl)phosphine oxide (BAPO), 2-benzyl-2-(dimethylamino)-4′-morpholinobutyrophenone (369), 4,4′-bis(diethylamino)benzophenone (EMK), diphenyl (2,4,6-trimethylbenzoyl)phosphine oxide (TPO), and 2-isopropylthioxanthone (ITX) to be more toxic than ethyl (2,4,6-trimethylbenzoyl)phenylphosphinate (TPOL) and methyl benzoylformate (MBF). In this series of photoinitiators, the most toxic structure was identified as BAPO, which is extensively used in industry. In the case of TPOL and MBF, the less toxic structure was identified as being TPOL. These different results can help for future developments of new photoinitiators in light of the low cytotoxicity of MBF.