Thioxanthone based one-component polymerizable visible light photoinitiator for free radical polymerization

Abstract

An acrylate functionalized thioxanthone based one-component polymerizable visible light photoinitiator, 4-((methyl(9-oxo-9H-thioxanthen-2-yl)amino)methyl)phenyl acrylate (TX-PA) has been designed and synthesized and its structure has been confirmed by ¹H NMR, ¹³C NMR and MS. TX-PA could successfully initiate photopolymerization of 1,6-hexanedioldiacrylate (HDDA), trihydroxymethylpropyl triacrylate (TMPTA) and pentaerythritol triacrylate (PETA) under xenon light exposure (λ > 400 nm) both in the presence and absence of N-methyldiethanolamine (MDEA). The synergistic effect of the acryloxy group and tertiary amine in the thioxanthone molecule helps TX-PA exhibit a higher photopolymerization activity and better migration stability than that of 2-(benzyl(methyl)amino)-9H-thioxanthen-9-one (TX-B). The results demonstrate that TX-PA is an effective one-component polymerizable visible light photoinitiator for free radical polymerization with excellent migration stability, which has great potential to be widely used in the food packing or biomedical fields.

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Introduction

Visible light photopolymerization has attracted much attention not only due to the cheap, safe and high penetration ability of the visible light source, but also due to the targeted applications, such as printing plates,¹ integrated circuits,² photoresists,³ dental filling materials,⁴ laser-induced 3D curing,⁵ holographic recordings,⁵ and nanoscale micromechanics.⁶ As the most important part in the photopolymerization system, visible light photoinitiators absorb visible light and then generate active species to initiate polymerization, which could be classified as radical or cationic photoinitiators based on the type of active species. Although there is an increasing interest in the study of cationic photoinitiators, radical photoinitiators have more potential applications not only due to their versatile structures based on acrylates, styrene, and unsaturated polyesters etc., but also due to their potential ability to be easily transferred to cationic initiators through an oxidization procedure in the presence of oxidants, such as diphenyliodonium hexafluorophosphate (Ph₂I⁺·PF₆⁻).⁷ In comparison to UV radical photoinitiators, visible light radical photoinitiators are generally type II since the visible light is not strong enough to carry out the α-cleavage in type I photoinitiator. Typical type II photoinitiators could not produce radical alone, but could take effect in the presence of hydrogen donors, such as amines, alcohols, thiols and ethers.

In order to make the photopolymerization to proceed as completely as possible, the amount of photoinitiator used is usually more than it actually needs. After the photopolymerization procedure, thus, only a small amount of photoinitiators have been actually consumed and a large quantity of unused photoinitiators has been left. The left unused photoinitiators, hydrogen donors, and some species transferring from the consumed photoinitiators still exist in systems which may bring about unfavorable properties, such as odor, toxicity, yellowing and migration problems, and the decreased hardness of the post-cured films and even could be harmful in some particular application field, such as dental filling materials.

Macromolecular photoinitiators in which both the chromophoric and hydrogen-donating groups are incorporated into the polymer chains could efficiently solve all these problems.⁸ However, problems such as low reactivity and poor compatibility with many resin systems arise. Many researches have been concentrated on developing one-component visible light photoinitiators to overcome these problems. One-component visible light photoinitiators are composed of visible light chromophores and hydrogen donors in the same molecule or even some new developed chromophores which could produce photo-induced radicals only in the presence of oxygen. This method could efficiently eliminate odor, toxicity and migration problems, but still could not get rid of unused photoinitiators and the species transferring from them. Polymerizable photoinitiators⁹ with active polymerizable groups in the molecules could not only play the role of photoinitiators but also take part in the polymerization procedure along with the photocurable monomers/oligomers, which would definitely decrease the content of free photoinitiators and their residues. To our best knowledge, this method has been used in the design of thioxanthone based UV photoinitiator systems,^10–17 but only one literature¹⁸ involved in the visible light system.

As a part of our continuing interest in the design and development of thioxanthone (TX) based visible light photoinitiating systems for radical photopolymerization, three one-component TX visible light photoinitiators (TX-A, TX-B and TX-C in Chart 1) have been explored and showed very good visible light photoinitiating properties in the absence of additional hydrogen donors.¹⁹ If unused TX photoinitiators and their residues could be diminished, this series of TX visible light photoinitiators would obtain more widely applications. Thus, polymerizable photoinitiator TX-PA (Chart 1) incorporated with a TX moiety, N-benzylmethylamine moiety and polymerizable acrylate double bond has been designed and prepared. Its photopolymerization behavior and migration in cured polymers after photopolymerization have been investigated.

Chart 1

Experimental

Light source

The light source was assembled from xenon lamp (laite optics, XD 300, cold light source) with a filter (λ > 400 nm). The light intensity was determined using a SRC-1000-TC-QZ-N reference monocrystalline silicon cell system (Oriel, USA), which was calibrated by National Renewable Energy Laboratory, A2LA accreditation certificate 2236.01.

Fluorescence experiments

The fluorescence properties of photoinitiators were determined in THF using a spectrometer (RF-5301PC) and the quenching constants were obtained from a Stern–Volmer treatment I₀/I = 1 + K_sv[Q] = 1 + k_qτ[Q], where I and I₀ stand for the fluorescence intensity of the photoinitiator in the presence and absence of the quencher Q, respectively. K_sv is the Stern–Volmer quenching constant in M⁻¹, k_q is quenching rate constant and τ is fluorescence lifetime.

Lifetime measurements

Lifetime measurements were carried out on an Edinburgh FLS920 which is equipped with a 450 W xenon lamp combined fluorescence lifetime and steady state spectrometer at room temperature. The chi square χ² values are 1–1.1. Lifetimes in the absence of quencher were measured.

Visible light photolysis experiments

The photodecomposition of photoinitiators was studied by the analysis of the changes in the absorption of the maximum wavelength in visible region. The absorption spectra of the photoinitiators in THF were measured with a UV-visible spectrometer (Agilent 8453) under xenon lam exposure (I = 57 mW cm⁻²) at room temperature.

Photopolymerization experiments

1,6-Hexanediol diacrylate (HDDA), trihydroxymethylpropyl triacrylate (TMPTA) and pentaerythritol triacrylate (PETA) were used as active monomers. N-Methyldiethanolamine (MDEA) was used as hydrogen donors if needed. The film polymerization experiments were carried out in laminated conditions. The photosensitive formulations were deposited on a KBr pellet in laminate for irradiation with xenon lamp (I = 28 mW cm⁻²). The evolution of the double-bond content was continuously monitored by real time FT-IR spectroscopy (Nicolet IS 10) at 1610–1650 cm⁻¹. The degree of conversion was calculated from the equation:
where A₀ and A_t represent the area of the IR absorption peak at 1610–1650 cm⁻¹ of the sample before and after exposure during time t. The rate–time curve was obtained by differentiating the conversion–time profile.

Migration study

HDDA polymer samples for migration study were prepared by a photopolymerization procedure under argon. The samples were made from HDDA by using the same amounts of photoinitiator (3 × 10⁻⁵ mol g⁻¹) and irradiating with visible light (I = 200 mW cm⁻²) for 20 min. After that, the samples were grounded into small particles then immersed in 10 mL of acetone for 2 days at room temperature. The solids were filtered off and the solution was taken to measure the UV-vis absorption of the leached photoinitiator. The molar concentration (C) and quality (m) of the extracted initiator were calculated based on equations:

where ε is molar absorption coefficient of initiator in acetone, b is optical path length, here is 1 cm, M is molecular weight of photoinitiator.

Results and discussion

Synthesis and characterization

As the results we have reported that the TX-B showed the best photopolymerization efficiency, benzyl amine could accelerate the photoinitiating procedure.¹⁹ In order to achieve a polymerizable TX photoinitiator, the acryloxy was introduced into 2-[(4-hydroxybenzyl)(methyl)amino]-9H-thioxanthen-9-one (TX-HB) to come into being TX-PA (Scheme S1 in ESI†). In order to evaluate the impact of the introduction of acryloyl to the property of photoinitiator, TX-Ac (Chart 1) with acetate instead of acryloyl was synthesized for comparison. The structures of TX-PA and TX-Ac have been confirmed by ¹H NMR, ¹³C NMR and MS (ESI).

TX-PA shows good solubility in common organic solvents, such as ethyl acetate, dichloromethane, trichloromethane, ethanol, acetonitrile, acetone and THF, implying that it should have excellent compatibility with other components of the photopolymerization system.

Absorption and fluorescence

UV-vis absorption spectra of TX-PA and TX-B in THF have been shown in Fig. S8 (ESI†) and the absorption maximum (λ_max) and the molar extinction coefficients at λ_max are summarized in Table 1. From Fig. S8 (ESI†) and Table 1, TX-PA exhibits the similar UV-vis absorption property to TX-B, which shows that the introduction of acryloxy has no significant effect on the absorption of the thioxanthone moiety. TX-PA shows broad absorptions evidenced by the fact that its absorption edge reaches to 485 nm. Broad absorption and high molar extinction coefficients in the visible region are favorable to absorb visible light and produce active radicals to initiate radical polymerization as TX-B.

Table 1 Photophysical data of TX-B and TX-PA

Photoinitiators	λmax^a (nm)	ε^b (M^−1 cm^−1)	λex^c (nm)	λem^d (nm)
a λmax is the maximum absorption wavelength in visible region.b ε is the molar extinction coefficiency at λmax.c λex is the excitation wavelength.d λem is the emission wavelength.
TX-B	439	4848	448	512
TX-PA	438	4713	447	514

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Fluorescence spectra of TX-B and TX-PA were measured in THF (Fig. S9 in ESI†) and the excitation wavelength (λ_ex) and emission wavelength (λ_em) are summarized in Table 1. Luminescence analysis of TX-PA indicates weak fluorescence with a maximum around 514 nm, but the emission intensity of TX-PA is obviously weaker than TX-B. There may be two reasons for this: one might be the intramolecular quenching between TX moiety and amine group in TX-PA, which is more effective than that in TX-B; the other is that the acryloxy group may increase the rate of intersystem crossing to the triplet state and lead to the weaker fluorescence emission of TX-PA which will greatly affect their photoefficiency.

Fluorescence quenching

The fluorescence quenching study is helpful to understand the quenching process between the singlet excited state of the photoinitiator and the quencher. The quenching mechanism has been proposed as a photoinduced electron transfer by Tamaki.²⁰ The fluorescence emission spectra of TX-PA at different concentration of MDEA are shown in Fig. S10 (ESI†). However, the fluorescence quenching results do not match well with Stern–Volmer equation. A possible reason is that the polarity of solution increases with the addition of MDEA, a high polarity compound, which makes the maximum emission wavelength of TX-PA to be red-shifted. So N,N-dimethylaniline (DMA), a compound with a low polarity in comparison to MDEA, was used as the quencher in this system. Quenching results by DMA are presented in Fig. S11 (ESI†). The Stern–Volmer representation of this quenching process is shown in Fig. S12 (ESI†). Both their Stern–Volmer plots are linear over a broad range of amine concentrations (R² > 0.99). Stern–Volmer quenching coefficient, K_sv, obtained from the slope, is 3.42 M⁻¹ for TX-PA and 2.09 M⁻¹ for TX-B, respectively. The fluorescence quenching rate constant (k_q) allows a real evaluation of the interaction of the TX singlet with the amine in the polymerization medium and is given by the equation of k_q = K_sv/τ. Fluorescence lifetime (τ) values were measured to be 15.4 ns for TX-PA and 15.0 ns for TX-B in the absence of quencher. And the quenching rate constants (k_q) are calculated to be 2.22 × 10⁸ s⁻¹ M⁻¹ for TX-PA and 1.39 × 10⁸ s⁻¹ M⁻¹ for TX-B. The quenching rate constant k_q could relate to the energetics of electron transfer between the singlet excited state of photoinitiator and the quencher DMA which is usually regarded as the diffusion-controlled process.²¹ The k_q values are correlated with the rate constant of the diffusion-controlled reaction rate. So, the rate constants of electron transfer and proton transfer of TX-PA may be bigger than that of TX-B.

Visible light photolysis

As demonstrated for previously reported highly conjugated thioxanthone derivatives,¹⁹ TX-PA is expected to undergo visible light photolysis (photobleaching) in the absence of hydrogen donors. Photobleaching experiments (Fig. 1) of different photoinitiators systems in THF were performed upon the irradiation of xenon lamp under air atmosphere. As seen from Fig. 1, there is little photolysis observed in TX-B and TX-Ac during the first 0.5 h and the photolysis becomes faster during the following 0.5 h. After that it is slower and slower. The possible procedure of the photolysis of these thioxanthone compounds is proposed in Scheme 1. Upon the exposure of visible light, the photoinitiator (PI) absorbs photon to produce an excited singlet state which could pass into an excited triplet state PI* through intersystem crossing. The excited triplet state molecule diffuses in close vicinity and works on the tertiary amine group of another molecule, which forms an encounter pair. Then all two following competitive processes correspond to the formation of the contact radical-ion pairs. One is the direct electron transfer process which resulted in the formation of a solvent-separated radical-ion pair. The other involves in the yield of a contact pair intermediate firstly and then the electron transfers to a contact radical-ion pair. Back electron transfer could turn radical-ion pair back to the ground state of photoinitiator. The proton transfer in contact radical-ion pair could produce the aminoalkyl radical and thioxanthyl ketyl radical. Then the ketyl radicals undergo the disproportionate or oxidation or coupling reacting with themselves or aminoalkyl radicals. Accordingly, the conjugated system is damaged and the photolysis happens. Since the back electron transfer process is thermodynamic favorable and is clearly faster than the proton transfer process especially in the presence of dissolved oxygen,²² only little aminoalkyl radical and thioxanthyl ketyl radical is produced. Therefore, the photolysis is little at the first 0.5 h. After the dissolved oxygen is gradually consumed, the proton transfer process becomes faster and faster. So Fig. 1b and c show a faster photobleaching period after 0.5 h. With the consume of photoinitiator, the photolysis is slower and slower. But the photolysis in TX-B is faster than that in TX-Ac in the whole photobleaching process. The possible reason is that the steric hindrance in TX-Ac makes the formation of solvent-separated radical-ion pair faster in TX-Ac system than that in TX-B system. Solvent-separated radical-ion pair could lead the photoinitiator to keep its structure so that the slower yield of radical in TX-Ac than that in TX-B system finally. So the photolysis is slower in TX-Ac in the whole system than that in TX-B system.

Fig. 1 The normalized UV-vis spectral changes during irradiation of (a) TX-PA; (b) TX-B; (c) TX-Ac and (d) TX-Ac/n-butylacrylate (molar ratio = 1 : 1) in THF [2 × 10⁻⁴ mol L⁻¹] under xenon lamp exposure (I = 57 mW cm⁻²).

Scheme 1 Proposed initiation mechanism for the photolysis of polymerizable photoinitiator in the absence of hydrogen donors.

The structure of TX-PA is similar with TX-Ac, but the photolysis was found obviously more rapidly than TX-Ac. This change only results from the structural difference of double bonds in acryloxy. In order to evaluate the impact of acryloxy on the photolysis process in TX-PA system, the photolysis of TX-Ac/n-butylacrylate was performed for comparison. TX-PA and TX-Ac/n-butylacrylate show a similar photolysis procedure with some differences in comparison to TX-B and TX-Ac. Both of them exhibit evident photolysis during the first 0.5 h (Fig. 1a and d). Meanwhile, TX-PA degrades more rapid than TX-Ac/n-butylacrylate. As aforementioned in Scheme 1, the proton transfer competes with the back electron transfer in the presence of dissolved oxygen. Since the aminoalkyl radical could transfer the radical to acryloxy rapidly, this competitive balance between them is broken. Accordingly, the proton transfer changes to be faster and more thioxanthyl ketyl radicals would be yielded. Thus, more photolysis was observed in the first 0.5 h in Fig. 1a and d. The big surprise is an obvious shoulder peak around 384 nm observed in TX-PA system. It changes to be smaller in TX-Ac/n-butylacrylate system. This shoulder peak could be attributed to the yield of thioxanthyl ketyl radical. Amirzadeh²³ reported the absorption peak of the thioxanthyl ketyl radical around 330–370 nm. The thioxanthyl ketyl radicals are always not reactive towards vinyl monomers due to steric hindrance and delocalization of unpaired electrons. Aminoalkyl radicals could transfer radicals to acryloxy which accelerated consume of aminoalkyl. The consume of aminoalkyl radicals and ketyl radicals is out-off-balance. There is not enough time for ketyl radicals to be used up so that we observed the absorption peak around 384 nm. The red-shift in our systems may be attributable to the aminoalkyl which could act as electron donors. Comparison of the photolysis between TX-PA and TX-Ac/n-butylacrylate implies that the transfer is faster in TX-PA than that in TX-Ac/n-butylacrylate system.

As seen from Fig. 1, TX-PA shows highest photolysis level followed by TX-Ac and TX-Ac/n-butylacrylate. The acrylate group in molecule should be responsible to this big difference between TX-PA and TX-Ac/n-butylacrylate. When the photolysis level is about half, most of the remainder photoinitiator has been linked with acryloxy by electron transfer between aminoalkyl radical and acryloxy in the presence of acryloxy. Taking benzyl radicals as examples, Chart 2 presents the main existing species with TX moiety at the time of a half photolysis level. Since all of them contain TX moiety, they could act as the photoinitiator again. They could absorb visible light, form a new contact pair and then yield radicals by electron transfer and proton transfer. So, the photolysis goes on until complete. In TX-Ac solution, the produced benzyl radical A had almost the same structure as TX-Ac. It is understandable that it exhibits a gently drop in absorption with consume of TX moiety. In the TX-Ac/n-butylacrylate system, the benzyl radical performs the electron transfer to n-butylacrylate and yields radical B. A steric hindrance in B makes it form the new contact pair more difficult than TX-Ac itself. Therefore, the photolysis level after 3 h in TX-Ac/n-butylacrylate system is less. Bulker radical C is also yielded in TX-PA system. In comparison to B, C has additional tertiary amine in the molecule, which helps to form an intramolecular contact pair. This is independent from the diffusion process, and is as rapidly as possible. Thus, the photolysis in TX-PA is the most complete among them.

Chart 2 The main existing species of the remainder photoinitiator taking benzyl radicals as examples.

Photopolymerization

TX-PA was used as the photoinitiator for the polymerization of HDDA in laminate in the presence and absence of hydrogen donors. TX-B, TX-Ac and TX-PA/THF were used as photoinitiator systems for comparison. Fig. 2 presents the conversion–time and rate–time plots for the polymerization of HDDA photoinitiated by them under the same TX moiety molar concentration. Among them, TX-PA exhibits the highest photoinitiation activity evidenced by the decreased t_max (time to reach the maximum polymerization) and the improved R_max (maximum rate of polymerization) in comparison to those of TX-Ac and TX-B. The polymerization rates and final conversions photoinitiated by TX-PA and TX-Ac are higher than those of TX-B in the absence of hydrogen donors. One reason for this difference is the introduced ester-group in their molecular structures which acts as the electron donor and makes the tertiary amine to be as a stronger hydrogen donor. This could be further illustrated by the chemical shifts of carbon (–NCH₂) in ¹³C NMR. The other reason is their better solubility of TX-PA and TX-Ac in HDDA. It is so surprised that TX-PA exhibits a better activity with a large decrease in t_max value and a sharp increase in R_max value compared with that in TX-Ac. To our knowledge, this phenomenon is still not reported as a comparison between one-component polymerizable photoinitiator and its one-component photoinitiator counterpart. Nie reported one-component benzophenone-sesamol photoinitiaotor²⁴ and polymerizable one-component photoinitiator,²⁵ but it still had not a clear contrast. This phenomenon in our experiment could be caused by the synergistic effect between amino thioxanthone and acryloxy in TX-PA. Upon visible light exposure, TX-PA absorbs photons to yield aminoalkyl active free radicals (presumably as fast as in TX-Ac system). Then they diffuse in close vicinity of acryloxy of TX-PA or monomers to make the chain initiation to be possible. The diffusion of active free radicals plays an important role in this photopolymerization process. In the first stage of polymerization (low conversions), the conversion–time curve follows the “classical” free radical polymerization. After a certain point in the region of 20% to 30% conversion, an increase in the reaction rate takes place followed by an increase in the conversion values. This is the well-known auto-acceleration or gel effect²⁶ and is attributed to the effect of diffusion-controlled phenomena on the termination reaction. As the polymerization is in progress, the viscosity of photopolymerization system increases and causes a significant reduction in the mobility of large molecules, they could no longer diffuse into close proximity enough to react with other macroradicals and as a result the termination rate decreased.²⁷ This decrease in termination leads to a build-up in macroradical concentration, which subsequently causes a sudden increase in the rate of polymerization. Afterwards, the reaction rate falls significantly and the curvature of the conversion–time changes. At this stage, the conversion from 30% to 65%, the observed decrease in the termination reaction is not abrupt but only gradual. In TX-PA system, aminoalkyl active free radical is priority to transfer to acryloxy in TX-PA rather than monomers which is proved by the comparison between TX-PA and TX-Ac/n-butylacrylate systems in visible light photolysis experiments. So, the aminoalkyl active radical in TX-PA acts as one crossing point and acryloxy in TX-PA could act as another type crosslinking point, that is to say, the synergistic effect between the tertiary amine group as hydrogen donor and the acryloxy group as another type crosslinker of the photoinitiator molecule could yield more propagation macroradicals rapidly. Therefore, the auto-acceleration is more evident in TX-PA system with a decreased t_max value and an increased R_max value compared with that in TX-Ac system. Finally, at conversion about 65%, the reaction rate decreases rapidly. While the reaction rate tends asymptotically to zero, the polymerization is termination with a final conversion about 75%. The polymerization in TX-PA in the presence of solvent THF was also performed. The results indicate that the addition of THF increases mobility of the system with declined t_max by delaying the onset of polymerization but improved R_max by accumulation of reactive radicals with a final conversion about 78%. The photoinitiation activity in TX-PA/THF system is still faster than that in TX-Ac. The result is in accordance with that the acryloxy in TX-PA can make a contribution to the autoacceleration effect through a participation in polymerization. Thus, TX-PA is an effective one-component visible light photoinitiator. All these results imply that the synergistic effect of tertiary amine and acryloxy in the same molecule is responsible for the high polymerization ability. The more amino alkyl groups and acryloxy groups in a photoinitiator, the higher photoinitiation activity would be resulted in because of more crossing points.

Fig. 2 (a) Conversion–time and (b) rate–time plots for the photopolymerization of HDDA in laminate in the presence of TX-B, TX-PA, TX-Ac and TX-PA/THF (THF% = 25 wt%) ([PI] = 3 × 10⁻⁵ mol g⁻¹, I = 28 mW cm⁻²).

The presence of an amine (MDEA) is important for effective photoreduction and photopolymerization. MDEA always plays dual roles in the polymerization. Besides acting as the hydrogen donation through the electron and proton transfer, MDEA can also react with oxygen thereby reducing the retarding effect of oxygen on the polymerization.^1,28 The conversion–time and rate–time plots for photopolymerization of HDDA initiated by TX-B, TX-PA, TX-Ac and TX-PA/THF in the presence of MDEA are shown in Fig. 3. It is seen that the photoinitiation activity is in the order of TX-PA, TX-B and TX-Ac in the presence of MDEA with an almost same final conversion about 73%. Meanwhile, the difference in the photoinitiation activity among them is obviously smaller than that of MDEA without. In these systems, both MDEA and the tertiary amine group of photoinitiator molecules could work on the excited state photoinitiators. The hindrance effect makes TX-Ac as a slower photoinitiation activity than TX-B. In TX-PA system, the hindrance effect results in a slower photoinitiation rate, but the synergistic effect between amino thioxanthone and acryloxy which improves the autoacceleration makes a faster photoinitiation rate. The synergistic effect is so stronger that the photoinitiation activity is still the fastest among the three photoinitiators. The photopolymerization study of TX-PA/MDEA/THF/HDDA system was also performed. The addition of THF increased mobility of the system with a dramatically increased t_max and decreased R_max. The final conversion is as high as 92% in TX-PA/MDEA/THF/HDDA system. This increase in the conversion is mainly due to the increased mobility of the reactive species. In comparison Fig. 3 to Fig. 2, it could be seen that polymerization takes place more rapidly in the presence of MDEA at the beginning, but the final conversion of the three in the absence of the solvent is almost the same as the polymerization of TX-PA in the absence of MDEA and solvent. It means that if the rate of polymerization is not the main concern, coinitiator is not necessary, because commonly commercial used coinitiators are always amine; the toxicity of the amines is really a big issue for the health problems. So, TX-PA could serve as an effective one-component visible light photoinitiator, which could avoid the use of large amounts of amines in the system and overcome some shortcomings involved.

Fig. 3 (a) Conversion–time and (b) rate–time plots for the photopolymerization of HDDA in laminate initiated by TX-B, TX-PA, TX-Ac and TX-PA/THF (THF = 25 wt%) in the presence of MDEA ([PI] = 3 × 10⁻⁵ mol g⁻¹, [MDEA] = 3 × 10⁻⁴ mol g⁻¹, I = 28 mW cm⁻²).

In order to further reveal the effect of monomer viscosity on photopolymerization, the higher viscosity monomers, such as the trifunctional acrylate compounds (TMPTA and PETA) were used as the comparable monomers to study the photopolymerization behaviors initiated by TX-PA and TX-PA/MDEA, respectively. Fig. 4 shows the conversion–time and rate–time profiles of HDDA, TMPTA and PETA initiated by TX-PA. It is clear that the degree of functionality of monomers has an influence on both the polymerization rate and final conversion of acrylate groups. With the increase of acrylate functionality, the content of residual unsaturations rises. In comparison to triacrylate TMPTA, diacrylate HDDA has lower double bond content and viscosity. The lower initial concentration of acrylate groups leads to declined t_max and the lower viscosity which results in the higher final conversion. As the increase of the functionality of acrylate groups, the viscosity of the resin increases and gel-effect and the higher cross-linking density are produced, which set a limit to the extent conversion.²⁹ The increased crosslinking level would eventually have limited the mobility of active species, and then the propagation reaction might become diffusion-controlled along with radical termination as the reaction continued. So the TMPTA has shorter induction time and lower double bond conversion than HDDA. As to PETA, which was also triacrylate, the lower polymerization rate and the final conversion are due to the intermolecular hydrogen bonding, which could lead to the increase of its viscosity thus set a limit to the mobility and the extent of conversion compared with that of TMPTA. Moreover, the double-bond conversion of the same monomer initiated by TX-PA in the presence and absence of MDEA is almost the same as shown in Fig. 4, that is to say, it is not necessary to need the additional hydrogen donor in these systems.

Fig. 4 (a) Conversion–time and (b) rate–time of different monomers such as HDDA, TMPTA and PETA initiated by TX-PA in the presence and absence of MDEA ([TX-PA] = 3 × 10⁻⁵ mol g⁻¹, [MDEA] = 3 × 10⁻⁴ mol g⁻¹, I = 28 mW cm⁻²).

Avci¹⁸ has reported five thioxanthone-functionalized methacrylates as copolymerizable photoinitiators and the best double conversion of TMPTA was 64% in the presence of MDEA after 90 s irradiation (I = 110 mW cm⁻²). Lalevée⁹ has reported naphthalimide based methacrylated photoinitiators, the best conversion of TMPTA was 48% in the presence of MDEA and phenacyl bromide at 800 s (I = 110 mW cm⁻²). In our case, the double bond conversion of TMPTA is 51% initiated by TX-PA in the absence of MDEA after 15 min irradiation (I = 28 mW cm⁻²). Thus, the photopolymerization properties of TX-PA are comparable to the reported polymerizable visible light photoinitiators.

Migration/extractability study

In order to investigate the migration stability of TX-PA and TX-B after the photopolymerization, HDDA polymers were prepared by a photopolymerization procedure and then extracted by acetone.^9,25 UV-visible spectrometer was used to determine the content of photoinitiators in the acetone solution. The UV-visible absorption of TX-PA and TX-B in acetone was shown in Fig. S13 (ESI†). As shown in Fig. 5, the apparent lower absorption at 440 nm for the extraction from TX-PA initiated HDDA polymer demonstrates that the residual free TX-PA is definitely lower than that of TX-B system. In comparison to the initial photoinitiator quality, the mass fraction of the extracted photoinitiator is 6.9% for TX-PA and 51.0% for TX-B. It may be because that the radical polymerization of HDDA predominantly occurred and allowed the chemical incorporation of TX-PA through a copolymerization of the acrylate function. The acrylate group in TX-PA could efficiently participate in the photopolymerization procedure in contrast with TX-B. Therefore, the introduction of acrylate groups in the TX structure reduced the possibility of their residual molecules diffusion in the solidified resin. The more amino alkyl group and acryloxy group could be responsible to the excellent migration stability. All these demonstrate that TX-PA could serve as an efficient one-component polymerizable visible light photoinitiator featured with the reduced toxicity of photoinitiator in the cured materials. It infers that TX-PA should be a potential visible light photoinitiator used in food packing or biomedical fields.³⁰

Fig. 5 (1) TX-B and (2) TX-PA extracted with acetone from the polymer prepared by photopolymerization of HDDA in the presence of PIs (3 × 10⁻⁵ mol g⁻¹) under Ar.

Conclusions

In summary, an acrylate functionalized thioxanthone based visible light photoinitiator (TX-PA) was designed and synthesized. The prepared TX-PA had better compatibility with acrylate resin than common TX photoinitiator. TX-PA had been proved as an efficient free radical photoinitiator for the photopolymerization of HDDA, TMPTA and PETA in the presence and absence of a hydrogen donor. Polymerization results indicate that TX-PA has higher photoinitiation activity and excellent migration stability. It could be contributable to the more possible crossing points resulting from the more amino alkyl groups and acryloxy groups in photoinitiator. These properties suggested that one-component polymerizable visible light photoinitiator TX-PA could find applications in a variety of practical visible light curing applications, especially has great potential use in food packing or biomedical fields.

Acknowledgements

The financial support from the National Science Foundation of China (21574101) is greatly appreciated.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Scheme S1; Experimental section: the synthesis and characterization of 4-(chloromethyl)phenyl benzoate, 4-((methyl(9-oxo-9H-thioxanthen-2-yl)amino)methyl)phenyl benzoate, TX-HB, TX-PA and TX-Ac; Fig. S1–S13. See DOI: 10.1039/c6ra15349f

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