Light cured dental composite resins: History
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Subjects: Materials Science, Biomaterials
Contributor:
Monika Topa
,
Joanna Ortyl

The photoinduced polymerization of monomers is currently an essential tool in various industries. The photopolymerization process plays an increasingly important role in biomedical applications. It is especially used in the production of dental composites. It also exhibits unique properties, such as a short time of polymerization of composites (up to a few seconds), low e tes with their limitations and disadvantages. nergy consumption, and spatial resolution (polymerization only in irradiated areas). This entry describes a short overview of the history and classification of different typical monomers and photoinitiating systems such as bimolecular photoinitiator system containing camphorquinone and aromatic amine, 1-phenyl-1,2-propanedione, phosphine derivatives, germanium derivatives, hexaarylbiimidazole derivatives, silane-based derivatives and thioxanthone derivatives used in the production of dental composites with their limitations and disadvantages.

  • photopolymerization
  • light-cured composites
  • cationic photoinitiator
  • free radical photoinitiator
  • iodonium salt
  • polymerization shrinkage

1. Introduction

Nowadays, the most modern technologies for the production of polymeric materials are based on photochemically initiated processes. The synthesis of polymeric materials carried out by photopolymerization is one of the most efficient methods, thanks to which it is currently a very widespread and dynamically developing technique[1][2][3][4][5]. Compared to other methods, photopolymerization is considered environmentally friendly due to its low energy consumption, no use of solvents, and high speed at ambient temperature. In industrial practice, two types of photochemically initiated polymerization are most commonly used, namely radical and cationic polymerization[6][7][8][9]. Due to the presence of oxygen inhibition in the case of free radical photopolymerization, much attention is currently paid to the cationic, thiol-ene, and hybrid photopolymerization processes[10][11].

Polymerization using light, mainly ultraviolet (UV) light, was initially used in the coating industry, especially in varnishing for solvent-free paints and varnishes for the furniture and automotive industries[11]. Achieving high polymerization rates in fractions of seconds, resulting from the rapid formation of radicals or initiating ions, allows for high throughput of the production line[12]. Besides, the possibility of conducting photopolymerization processes at ambient temperature prepares polymeric materials carried out by photoinduced polymerization process one of the most efficient photochemical technologies. Currently, this type of polymerization is also used in many other industries, namely in photolithography for the production of printed circuits, in micro-replication for the production of spherical lenses, for photocuring polymeric adhesives, and in microelectronics for encapsulating integrated circuits[13]. The dynamically developing printing industry is a different direction of the application of photopolymerization, which enables printing on plastic or metal materials. Moreover, in recent years, a particular emphasis has been put on the use of photopolymerization processes for 3D-printing technology[14][15][16][17][18][19][20][21][22][23], including stereolithography in the design and formation of three-dimensional models[24].

All this means that not only has recently been an astonishingly rapid growth in the applications of technologies based on photopolymerization processes, but also the development of new materials determining the pace of this development[25]. Dynamic progress in the field of chemistry and technology of photoinitiated processes leads to the emergence of more and more sophisticated solutions in this field, an example of which may be successively developed new generation monomers [26][27], new, more effective photoinitiation systems[28][29][30] or new light sources[31] and methods of monitoring the online polymerization processes[32][33][34][35].

Photopolymerization processes play an increasingly important role in biomedical applications, for instance, in obtaining hydrogel polymer materials[36][37][38][39] or in vivo photocurable dental composites[40][41][42]. Applying photochemically initiated polymerization for obtaining dental polymer composites enables the use of unique and innovative features. The most important are:

  • Short time of monomer/filler compositions curing (up to a few seconds);
  • Conducting the reaction at room temperature;
  • Low energy consumption;
  • Spatial resolution (polymerization only in irradiated areas).

Nevertheless, obtaining polymer composites of the demanded properties, that is, above all, of favourable mechanical properties and reduction polymerization shrinkage, is still a significant challenge for the researchers. This is because many different factors, such as the selection of appropriate monomers, initiators, inorganic fillers, photopolymerization process time range, or the source and power of a light source, influence the quality of the composite obtained (Figure 1)[43].

Recently, iodonium salts have become of particular interest and are used as a component of initiating systems for the preparation of dental composites. This is directly due to the relatively good solubility of these salts in non-polar monomers. In addition, this group of compounds photodissociate with high initiation efficiency are thermally stable and show long-term stability under storage conditions[44][45].

In this paper, we aim to present commonly used monomers and photoinitiating systems for the photocurable dental composites and indicated their main disadvantages. Recent developments and progress in the future of photocurable resins have also been shown. A particular emphasis was placed on novel photoinitiating systems containing iodonium salts applied in dental adhesive resin.

Figure 1. Illustrative diagram showing the influence of factors on the quality of obtained dental composites.

2. Monomers Used for the Production of Dental Composites

The mechanism of photopolymerization depends on using monomers. There are two main types of photopolymerization: radical polymerization and cationic polymerization (Figure 2). The type of organic matrix has a considerable impact on the properties of dental composites. It primarily affects mechanical strength, sorption, solubility, polymerization shrinkage, abrasion resistance, colour stability, and biocompatibility[43][46]. Generally, the organic components of a typical photocuring composition constitute about 10%–30% wt.[47] The remainder is inorganic filling in the form of microparticles (≥0.4 μm) or a mixture of micro and nanoparticles (50 nm > 400 nm)[47]. In addition to photoinitiators, adhesion promoters and possibly antibacterial compounds are also added to dental composites[48][49][50].

Figure 2. Mechanism of free radical polymerization and cationic ring-opening polymerization with their corresponding photoinitiation systems.

2.1. Monomers for Free Radical Photopolymerization Processes in Dental Adhesive Resin Application.

The most popular materials for obtaining dental composites through photopolymerization are (meth)acrylate monomers (RCB—resin-based composites) characterized by high reactivity, which form an organic matrix[51][52][53]. They guarantee obtaining networks with a high degree of crosslinking[53][54]. By free radical polymerization of the matrix monomers, a three-dimensional network is formed. Among the currently available dental composites, the most common are 2,2-bis[4-2-hydroxy-3-methacryloyloxypropyl)phenyl]propane (BisGMA) and triethylene glycol dimethacrylate (TEGDMA)[55].

The use of BisGMA in dental materials, due to the presence of the aromatic structure of Bisphenol A in the core of the molecule, ensures low volatility of the composition and high modulus of the light-cured composite[56][57][58]. In turn, the use of the low-viscosity TEGDMA monomer, which is the so-called active diluent, allows the introduction of an appropriate amount of inorganic filler[59]. The weight proportions of both monomers are usually 7/3 or 8/2, where BisGMA is the main component. Another commonly used acrylate monomer is 1,6-bis-[2-methacryloyloxyethoxycarbonylamino]-2,4,4-trimethylhexane (UDMA)[60]. The content of rigid urethane groups guarantees dental composites with favorable strength properties[61][62][63]. In addition to the aforementioned BisGMA, TEGDMA, and UDMA, other common dental monomers polymerized via the free radical process are also ethoxylated BisGMA (BisEMA). This monomer is used for reducing water absorption by the organic matrix. In addition, the lack of -OH also causes this monomer to be less viscous than BisGMA. An array of monomer structures for the base dimethacrylate materials, as well as new monomers, are given in Figure 3[64][65][66][67][68][69][70].

Figure 3. Examples of methacrylate monomers used in commercial and conventional dental composites based on free radical photopolymerization mechanism.

All dental composites based on crosslinking dimethacrylates exhibit an inherent problem of 2%–14% volumetric shrinkage during the photopolymerization process[71]. These stresses may produce defects in the composite–tooth bond, leading to bond failure, microleakage, postoperative sensitivity, and recurrent caries. Such shrinkage stresses could also cause deformation of the surrounding tooth structure when the composite–tooth bond is strong, predisposing the tooth to fracture[72]. The polymerization shrinkage of low molecular monomers is more pronounced when compared to that of high molecular monomers; however, high molecular monomers are very viscous (Table 1). For these reasons, polymerization shrinkage is dictated by a complex interplay among resin viscosity, polymerization rate, degree of conversion, and network structural evolution, where each of these properties cannot be individually manipulated and studied without having a significant impact on other properties.

Table 1. Properties of the popular free radical monomers to obtained dental composites[53].

Monomer

Molecular Weight [g/mol]

ρmona [g/cm3]

ρpolb [g/cm3]

ΔVp [%]

Viscosity [mPa•s]

TEGDMA

286

1.072

1.250

−14.3

100

UDMA

470

1.110

1.190

−6.7

5000–10,000

Bis-GMA

512

1.151

1.226

−6.1

500,000–800,000

ρmona—density of monomer

ρpolb—density of polymer

Moreover, due to the inhomogeneous network architecture, which is obtained during a free radical photopolymerization process, the final materials tend to show a somewhat brittle behavior, and the occurring shrinkage stress could lead to delamination, deformation or mechanical failure of the final composites materials. The observed shrinkage stress evolves during polymerization reaction upon transitioning of the applied formulation from the liquid to solid-state (i.e., gel point) and is built up upon vitrification until the final conversion is reached. Before free radical photopolymerization, the monomers are situated at van der Waal’s distance towards each other (approximately 3.4 Å)[73]. The occurring shrinkage stress upon gelation is partially due to the formation of covalent bonds between the respective monomers, where the revealing distance is only 1.5 Å[74]. Incomplete free radical photopolymerization, volumetric shrinkage, and stress are some of the primary disadvantages of current methacrylates resin-based dental composites. Generally, attempts to increase the double-bond conversion and reduce polymerization shrinkage and stress have been conducted[74].

2.2. Monomers for Cationic Photopolymerization.

In recent years, the application of ring-opening cationic photopolymerizable epoxy–monomer-based compositions for dental fillings have found increasing attention in different articles and patent applications[53][75][76].

Thus, based on the cationic photopolymerization process, new-generation photocuring dental materials, including oxiranes[53], siloranes[77], oxetanes, and spiro-orthocarbonate [76], were developed (Figure 4). Dental materials based on these monomers have achieved clinical success because they have significantly reduced polymerization shrinkage to below 1% and minimized polymerization stress compared to traditional methacrylate materials[75]. The mechanism of compensation for systolic stress in this system was achieved by the phenomenon of opening the oxirane rings during the cationic photopolymerization process, which proceeds with a small change in the volume of the system[78].

Crosslinking cycloaliphatic epoxy compounds were particularly of interest because they demonstrate significantly lower shrinkage than dental methacrylate resins (e.g., cycloaliphatic epoxide 3,4-epoxycyclohexyl-methyl-3,4-epoxycyclohexane carboxylate and the diglycidyl ether of bisphenol A, which improved the mechanical properties of the cured composite). Moreover, these epoxy resins were reactive enough to be cured by cationic photopolymerization in an acceptable time frame and to an adequate depth using a dental Vis-LED light source. In addition to epoxy resins, oxetanes were evaluated for dental applications[79]. The reactivity of oxetanes is mainly controlled by the ring stress and the basicity of the ring oxygen. However, oxetanes demonstrate higher basicity. The ring-opening cationic photopolymerization of oxetanes was also characterized by a significantly lower shrinkage in comparison to methacrylates. From the investigated oxetanes, the hydroxy group containing monomer possessed the highest polymerization rate[80].

In turn, spiroorthoesters (SOEs) and spiroorthocarbonates (SOCs) (Figure 4) are other monomers that are polymerized via to the cationic mechanism and are increasingly used in dental applications. Spiroorthoesters (SOEs) and spiroorthocarbonates (SOCs) are the most widely studied expanding monomers. SOCs are double-cyclic acetals that polymerize under acidic catalysis but are stable under basic conditions. When these compounds polymerize by double ring-opening photopolymerization (ROP), poly(ether-carbonates) are produced. In general, bi-cyclic compounds cured by ROP shrink less as they harden because of an increase in the excluded free volume associated with the ring-opening process. Bailey[81] investigated bi-cyclic compounds, such as spiro-orthocarbonates (SOCs), that can be used as an expanding co-monomer in RBC formulations. Ring-opening reactions with SOCs produce expansion (3.5%), which could counteract normal shrinkage[82]. However, SOCs exhibit incomplete ring-opening, as well as limited solubility and minimal copolymerization in dimethacrylate resins, resulting in minimal shrinkage reduction.

Nevertheless, compared to traditional composite materials, spiroorthocarbonate-based composites show less polymerization shrinkage and twice as much adhesion to enamel[76]

However, the most recent modification on the polymer matrix is based on using ring-opening polymerization of the silorane molecules, instead of free radical polymerization of dimethacrylate monomers[83]. They are built of a siloxane backbone, which gives them hydrophobic and cycloaliphatic oxirane molecules responsible for low polymerization shrinkage. These monomers have provided particularly interesting and commercially viable results. Such monomers “open” their molecular structures with local volumetric expansion, and this may partly or totally compensate for volumetric shrinkage from C=C or similar polymerization[84][85]. Based on the literature reports, the use of siloranes has been shown to guarantee a reduction in the polymerization shrinkage to 0.94%[77].

Examples of monomers that polymerize thorough to the cationic mechanism and have reduced polymerization shrinkage are shown in Figure 4.

Figure 4. Examples of monomers used in cationic photopolymerization (a) oxirane, (b) spiro-orthocarbonate, (c) silorane: a merger of siloxanes and oxiranes.

The development of new monomers polymerizing via the cationic mechanism contributed to a significant reduction in the polymerization shrinkage of dental composites and obtaining dental composites with better mechanical properties. Moreover, acrylate monomers, which often cause severe allergies, have been eliminated (Figure 5).

Figure 5. Comparison of the properties of monomers polymerizable via free radical mechanism with monomers polymerizable via cationic mechanism.

3. Commonly Used Photoinitiating Systems for Dental Application.

Photoinitiating systems for obtaining dental composites are particularly important. They affect such parameters as the efficiency of the photopolymerization process and the choice of a light source (Figure 6).

Figure 6. The range of emission spectra of UV and visible light-curing units and the range of the absorption of standard co-initiators camphorquinone used in the dental application (UV—ultraviolet, LCU—light-curing units, LED—light-emitting diode, PAC—plasma arc).

To date, several initiation systems for radical photopolymerization processes have been developed. In Figure 7, the absorption spectra of standard initiators in comparison with the emission characteristics of the commonly used light-curing units are presented. While table 2 shows the photoinitiators used in a dental application, their basic properties, and photoinduced cleavage of photoinitiators.

Figure 7. Comparison of the normalized molar extinction coefficient of standard type I initiators (top) and type II initiators together with amines (bottom) used in dental applications with the emission characteristics of standard light-curing units.

Table 2. Summary of the photoinitiators used in dental application, their basic properties, and photoinduced cleavage of photoinitiators.

Acronym of Photoinitiator

Structure, Together with a Scheme of Photoinduced Cleavage of Photoinitiator

Characteristic of Absorbance

Advantages

Disadvantages

Ref.

CQ

λmax = 468nm

wide absorption range based on the visible range

molar extinction coefficient in the range of 400–500 nm is only 40 [dm3 · mol–1 · cm–1], strongly yellow color

 [86][87]

PPD

λmax = 400nm

improve the color stability

necessary to use LED light sources with two violet emission bands (380–420 nm and blue 420–520 nm), dual peak LEDs

[88][89][90]

TPO

λmax = 382nm

high efficiency of generating radicals,

improve the color stability

low initiation efficiency, the need for UV light sources

[91][92][93]

IVO

λmax = 445nm

no cytotoxicity,

high initiation rate and excellent bleaching

initiators only for free radical polymerization

[94][95][96]

HABI

display extended absorption tails well into the visible spectral region

effective in initiating thiol-ene photopolymerization

poor absorption in the visible spectrum, sometimes requiring a photosensitizer, low solubility in standard resins used in a dental application and low solubility in organic solvents

[97][98][99]

Silane

derivatives

ultraviolent strong absorption in the 300–350 nm region

very effective in free radical photopolymerization, witch iodonium salts or pyridinium can be used for cationic photopolymerization

need for UV light sources

[100][101][102]

λmax = 425 nm

 

excellent bleaching properties,

in combination with an iodonium salt, can be useful for initiating cationic photopolymerization

low value of molar extinction

coefficient ε: 120 dm3 · mol–1 · cm–1 in toluene and 100 dm3 · mol–1 · cm–1 in acetonitrile

 [103][104]

λmax = 486nm for SED1

λmax = 468 nm for SED2

in toluene

suitable for free radical photopolymerization exposure to blue (@ 455 nm) and even green (@ 520 nm) LED

not suitable for methacrylate photopolymerization

 [105]

λmax = 442 nm in acetonitirle

excellent bleaching properties, a high water solubility, and a very good stability in acidic

not suitable for methacrylate photopolymerization

[106]

TX

λmax = ∼378 nm

water-soluble co-initiators,

instant bond strength to dentin

minimize the effects of concentration stress and phase separation in the aquatic environment

generally, less reactive than CQ/amine system

[96][107][108]

3. Onium Salts as an Innovative Component of Photoinitiating Systems for Photopolymerization Processes in Dental Applications.

In recent times, onium salts, i.e., sulfonium and iodonium salts, particularly in the form of diaryliodonium salts, have been playing an increasingly important role in initiating photopolymerization processes[109][110][111][112][113][114][115][116].

All the properties of ionic compounds supporting their commercial use as photoinitiators depend only on their structure. It has been shown that the cation of iodonium salt, absorbing electromagnetic radiation, is responsible for the photochemical properties of these compounds as photoinitiators. Thus, the structure of the cation determines the initiator’s properties, such as the location of the maximum absorption (λmax), molar absorption coefficient (ε), the quantum efficiency of the initiator, and even thermal stability. On the other hand, the nature of anion has a decisive influence on the suitability of the initiating system as a photoinitiator. The type of anion determines the power of protic acid generated during photolysis, directly affecting the efficiency of initiation and the kinetics of the polymerization process. However, the essential properties of iodonium salts, from their applications in cationic polymerization processes (in addition to solubility in monomers) are their optical properties, i.e., the location of the maximum absorption (λmax) and molar extinction coefficient (ε)[44][117]

3.1 Two- or Three-component Photoinitiating Systems Containing Iodonium Salt for Initiating Free radical Photopolymerization Processes for an Obtained Dental Composites.

The hydrophobicity of commonly used photoinitiating systems based on camphorquinone (CQ) and ethyl 4-(dimethylamino)benzoate (EDMAB) has limited their performance in the wet, oral environment. Therefore, to eliminate this limitation, a water-soluble iodonium salt is added mainly diphenyliodonium hexafluorophosphate (DPIHP)[118]. Iodonium salt as an accelerator in dental applications is usually found in a ternary initiating system containing CQ and a tertiary aromatic amine. However, it is also possible to use a two-component initiating system based on CQ and an iodonium salt (without a tertiary aromatic amine), except that, compared to the three-component system, slightly lower conversion rates are usually obtained.

In a two-component initiating system based on CQ/onium salt, after being irradiated with blue light, the exciplex state is formed; next, the onium salt is reduced by electron transfer. The resulting diphenyliodine free radical is unstable and quickly degrades to phenyliodine and phenyl free radical, which causes the reaction to be irreversible. These reactive phenyl forms are useful in initiating the photopolymerization. Radicals generated during polymerization propagation effectively cleave the C–I bond, releasing another radical and allowing the photopolymerization[119].

The three-component initiating system is usually based on CQ/aromatic amine/iodonium salt, and this system is characterized that the additional amine radicals are produced. In addition, CQ is regenerated through substitution of inactive and also termination radicals to active radicals in the form of phenyl radicals and the generation of positive active phenyl radicals[115].

This makes the photopolymerization process initiated by the ternary initiation system extremely efficient and fast. A similar degree of conversion and rate of polymerization compared to acylphosphine oxide (MAPO) or bis-acylphosphine oxide (BAPO) photoinitiators is even obtained[120][121][122].

The photoinitiating system based on CQ/iodonium salt or CQ/iodonium salt/amine have mainly found application in initiating traditional methacrylate monomers used in the production of dental composites[120][121][123][124][125][126]. The addition of iodonium salt to the photoinitiating systems used to prepare dental composites brings many benefits. The most important are the

  • Increase conversion in short photo-activation time;
  • Reduced inhibitory polymerization effect from an organic solvent;
  • Improved dentin bonding performance;
  • Improved reactivity and mechanical properties;
  • Decreased sorption and water solubility;
  • Reduced initial color and improved color stability.

3.2. Iodonium Salts as Photoinitiators for Cationic and IPN Photopolymerization to Obtain a Dental Composites.

In recent years, researchers from around the world have designed new initiating systems based on onium salts for cationic and thiol-ene photopolymerization processes for obtained dental composites.

In 2014, it was first described using composites based on cationic systems for dental applications[75]. It has been shown that the two-component system of the CQ/[4(1-methylethyl)phenyl][4-methylphenyl] iodonium tetrakis (pentafluorophenyl)borate, Rhodorsil 2074 is useful for initiating the cationic photopolymerization process of bis[2-(3,4-epoxycyclohexyl)ethyl]tetramethyldisiloxane, UV30. CQ promotes the photopolymerization process even in the absence of amines as a hydrogen donor. It can separate labile hydrogen from an epoxy monomer; carbon-concentrated radicals are formed, which are oxidized by onium salt. The complete conversion of the epoxy group was achieved after 50 s with blue irradiation.

Fu et al. proposed the use a three-component photoinitiation system comprising 1 wt.% CQ (camphorquinone), 2 wt.% DMAEMA (2-(dimethylamino) ethyl meth acrylate) and 2 wt.% diphenyliodonium hexafluorophosphate to initiate the copolymerization of the matrix resins which combine bisphenol-S-bis(3-methacrylate-2-hydroxy propyl) ether (BisS-GMA) with the expanding monomer unsaturated spiro orthoesters 2-methylene-1,4,6-trispiro[4,4] nonane (MTOSN), for minimizing the volumetric shrinkage that generally occurs during polymerization. The results supported that the dental composites based on the expanding monomer and three-component photoinitiator system engendered a more significant decrease of volumetric shrinkage and better mechanical properties[127].

Danso, R. et al. proposed new resins (Oxirane-Acrylate IPN System—OASys) based on p-Cycloaliphatic diepoxide EPALLOY 5000TM (EP5000) and dipenta erythritol hexaacrylate (DPHA). A three-component initiating system in the form of (4-octyloxyphenyl)phenyliodonium hexafluoroantimonate (OPPI), CQ, and a co-reactant oligomeric diol 250 Mn poly(tetrahydrofuran) was used. These results demonstrate that OASys resins cure well, are more hydrophobic, and have lower shrinkage stress than BisGMA-based resins. However, they are mechanically weaker[128].

This new class of photoinitiators based on silyl glyoxylates to initiate cationic polymerization combined with an iodonium salt was presented in an article by Kirschner[129]. This system can be used to initiate free radical/cationic hybrid polymerization and for the synthesis of interpenetrating polymer networks. The system silyl glyoxylate/iodonium exhibits excellent polymerization performances and exceptional bleaching properties compared to other well-established photoinitiators (e.g., camphorquinone)[129]. This system is also suitable for initiating a hybrid monomer (2-vinyloxyethoxyethyl methacrylate [VEEM]). This monomer leads to a considerable improvement of the mechanical properties of the final polymer through hybrid polymerization[129].

Zang et al. proposed the use 1,2-diketone/iodonium salt (and optional NVK) systems to initiate cationic photopolymerization of epoxides or free radical photopolymerization of methacrylates. Most of the photoinitiating systems have exhibited higher initiation ability than the well-known CQ-based systems. Nevertheless, the study of the biocompatibility indicates that these materials exhibit cytotoxicity[130].

4. Challenges of Photoinitiator Systems for Dental Applications, Future Trends and Practical Aspects

In recent years, several new initiating systems for dental composites have been developed. However, these are mainly photoinitiating systems used to obtain dental composites by radical photopolymerization[131]. Most of them have several of the significant disadvantages mentioned earlier in this article. Despite such significant progress, new initiating systems with improved properties are still being sought, mainly to produce dental composites obtained by cationic photopolymerization, which

  • Are entirely safe for humans, eliminating the cytotoxic amines and acrylate monomers that often cause severe allergies;
  • Do not generate yellow color-eliminating camphorquinone, greater aesthetics, and quality of the final product;
  • Have better and/or comparable mechanical properties and, due to the use of polymerizable monomers via to the cationic mechanism, have reduced polymerization shrinkage;
  • Is possible to be used with dental lamps emitting radiation in the visible light range for the curing process, eliminating harmful UV radiation.

5. Conclusion

In conclusion, it can be stated that in the scope of initiating systems for photocuring dental composites according to the radical mechanism[131], a significant milestone towards solutions guaranteeing the active initiation of this type of process has now been realized. In addition, in recent years, new initiating systems containing iodonium salt to initiate cationic and/or IPN photopolymerization processes have been developed. In this way, dental composites with better mechanical properties and reduced polymerization shrinkage were obtained. Nevertheless, in most cases, these are camphorquinone-containing systems that generate yellow color or toxic co-initiators. In addition, the complete elimination of acrylate monomers that often cause severe allergies is still a significant challenge for researchers.

The literature review has presented previous achievements in the field of radical photoinitiators dedicated to the preparation of dental composites; their advantages and disadvantages are discussed. The advantages of iodonium salts and their potential to initiate cationic photopolymerization processes of silorane monomers to obtain new-generation dental composites were also indicated.

This entry is adapted from the peer-reviewed paper 10.3390/ma13184093

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