Single-Component Cationic Photoinitiators: History
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With the advantages offered by cationic photopolymerization (CP) such as broad wavelength activation, tolerance to oxygen, low shrinkage and the possibility of “dark cure”, it has attracted extensive attention in photoresist, deep curing and other fields in recent years. The applied photoinitiating systems (PIS) play a crucial role as they can affect the speed and type of the polymerization and properties of the materials formed. Much effort has been invested into developing cationic photoinitiating systems (CPISs) that can be activated at long wavelengths and overcome technical problems and challenges faced. 

  • photopolymerization
  • cationic photoinitiating systems
  • onium salt
  • UV/visible LED lights
  • long wavelength

1. Introduction

Single-component cationic photoinitiators (CPIs), also known as photoacid generators, absorb photons and in the excited state generate Bronsted acid directly. Single-component CPIs generally possess two components: cationic and anionic, which play different roles in CP. As a photosensitive group, cation part absorbs photons, then undergo energy level transition, and also determines the molar extinction coefficient, spectral absorption range, quantum yield and thermal stability of the PI [1][2]. The anionic part also affects the polymerization efficiency as termination occurs with highly nucleophilic counter anions. Therefore, the anion has to be sufficiently nonnucleophilic to prevent the termination of a growing chain–cation combination. In general, the common anion reactivity is increased in the order (C6H5)4B > SbF6 > AsF6 > PF6 > BF4 > CF3SO3 [3][4]. In the following section, single-component PIs are discussed according to their ionic or nonionic structures.

2. Ionic Photoinitiators

The ionic PIs have a hetero atom salt structure with cationic centers on the heteroatoms coupled with non-nucleophilic counter anions.

2.1. Aryl Diazonium Salts

As one of the earliest reported CPIs, aryl diazonium salts can release N2 and Lewis acids, and form Bronsted acids by reacting with proton donors (RH) under the irradiation of light source. As is well known, Lewis acids and Bronsted acids formed in the photolysis process can be used as active species to initiate CP [5] (Scheme 1). Despite their high initiation efficiency, aryl diazonium salts have some disadvantages, such as the release of nitrogen from the initiating system during photolysis, which can have a large impact on the properties of the final materials. Meanwhile, the poor thermal stability further limits their industrial application of aryl diazonium salts [6].
Scheme 1. Photolysis mechanism of aryl diazonium salts [5]. Red color represent the active species that can initiate polymerization.

2.2. Iodonium Salts

Diaryl iodonium salts as CPIs were first reported by Crivello et al. [2] in the 1970s. With the advantages of excellent photoinitiated activity, easy synthesis, no gas generation and excellent stability, diaryl iodonium salts have been well developed and widely used in CP. The photolysis mechanism of diaryl iodonium salt is depicted in Scheme 2.
Scheme 2. Photolysis mechanism of iodonium salt [2]. Red color represent the active species that can initiate polymerization.
However, the absorption wavelength of diaryl iodonium salts is usually below 300 nm, which greatly limits their applications. There are two common ways to extend their spectral sensitivity to longer wavelengths. The first approach concerns the extension of conjugation by introducing additional chromophoric groups to the structure. H. Schroder et al. [7] replaced the aryl group of diphenyl iodonium salt with 9-fluorenone, resulting in the extension of the conjugation. The new compound had two weak absorption bands at 380 nm and 394 nm with molar extinction coefficients of 430 and 400 L mol−1 cm−1, respectively.
Following this study, a series of iodonium salts with different chromophores have been synthesized and used as PIs. For example, J. Lalevée et al. [8] reported a novel iodonium salt containing naphthalimide (naphthalimide-Ph-I+-Ph) as a single-component PI, which causes the red-shift of the absorption wavelength of the initiator, allowing polymerization to occur at longer and safer wavelengths (i.e., violet-emitting diodes at 365, 385 and 395 nm). This PI can effectively initiate the polymerization of various formulations (methacrylates, epoxides, vinyl ethers). Due to the excellent photophysical and chemical properties of coumarin, it has also been used as a chromophore introduced into iodonium salts. Four new coumarin-based PIs are described by Ortyl et al. [9] as a single-component PI for the CP of epoxides, vinyl ethers, oxiranes and glycerol-based monomers, as well as hybrid formulations under visible light. The hybrid polymerizations (HP) applied were successfully used to construct interpenetrating polymer networks (IPN) polymer materials in the field of 3D printing. Optical microscopy experiments show that structures can be printed with good resolution in these hybrid resins by a 3D stereolithography process. In 2021, Orty et al. [10] introduced a series of novel one-component iodonium salt photoinitiators based on benzylidene scaffolds, which contain double bonds and dialkylamino groups, and were synthesized in one step through a classical aldol condensation reaction. Novel benzylidene iodonium salts can photoinitiate the polymerization of vinyl ether and epoxy monomers under LED@ 365 nm and LED@ 405 nm illumination. The investigated compounds can simultaneously initiate and monitor the polymerization process based on changes in fluorescence during photocuring. Formulations prior to photopolymerization show no fluorescence; during photopolymerization, the fluorescence is “turned on”. This phenomenon can be used to monitor photopolymerization in an “online” manner.
Beside the commonly used iodonium salts, dialyl chloronium salts and dialyl bromonium salts can also be used as CPIs [11]. These salts have a similar skeleton structure, UV absorption spectrum and photolysis mechanism to iodonium salts. It is worth noting that the change of the central halogen atom results in a lower difference, the synthesis yields of dialyl chloronium salts and dialyl bromonium salts are relatively low, and the materials obtained by using these PIs are dark colored, which limits their practical applications.

2.3. Sulfonium Salts

Triarylsulfonium Salts

Following the first report on triarylsulfonium salts in 1970s, Crivello and his collaborators have made outstanding contributions to the development of highly efficient PIs [12][13][14][15]. Among the various onium salts, triarylsulfonium salts have excellent photosensitivity, photoacid quantum yields (between 0.6 and 0.9), and excellent thermal stability (the decomposition temperature exceeds 120 °C.) [13].
With the development of LED light technology and the improvement of environmental requirements, triarylsulfonium salts also face the challenges of short absorption wavelength and further improve the initiating efficiency. Accordingly, a large amount of work is devoted to extending the absorption wavelength. The main strategy was to increase the number of aromatic rings and introduce a chromophore on the benzene ring, which could increase the electron delocalization of the PI, resulting in a red-shift on the absorption wavelength of the PI. At the same time, it could also increase the molar extinction coefficient of the absorption of the PI [16]. For example, the introduction of a phenylthio group to a triarylsulfonium salt results in a red-shift of the maximum absorption wavelength from 227 nm to 313 nm [17].
Triarylsulfonium salts produce active species through two photolysis mechanisms: hetero- and homo-cleavage. Upon irradiation, the C-S bond of the salt is cleaved to generate a phenyl cation or a diarylsulfonium cation radical, which generates reactive protonic acid after the subsequent interaction with the proton donor. The overall mechanism is shown in Scheme 3 [13][18][19][20].
Scheme 3. Photolysis mechanism of triarylsulfonium salt [6]. Red color represent the active species that can initiate polymerization.
Another possible mechanism was found when the researchers analyzed the photolysis products. As photolysis products, phenyl cations and phenyl radicals can react with diphenyl sulfide to form three positionally substituted products [6] in the decreased order of ortho > meta > para positions (Scheme 4). In this process, the form of proton is still the core active species for initiating CP. These cationic photoinitiators suffer from poor solubility in epoxy resins and emit foul odors. These shortcomings greatly limit the application of photoinitiators. Based on this, Sun et al. [21] reported three new polysiloxane-modified 5-arylsulfonium salt cationic photoinitiators (1187-Si-A/B/C), whose polysiloxane-modified cations can be considered environmentally friendly. The photoinitiator was synthesized based on 9-(4-hydroxyethoxyphenyl) hexafluorophosphate (Esacure 1187) and polysiloxane. Cationic photoinitiators not only exhibit excellent solubility in epoxy resins, but also do not release odors and toxic by-products under UV irradiation.
Scheme 4. Photolysis products of triarylsulfonium salts [6]. Red color represent the active species that can initiate polymerization.

Aryl-Alkylsulfonium Salts

Trialkylsulfonium salts have poor thermal stability (thermal polymerization can occur at 50 °C) and could spontaneously initiate the polymerization of reactive monomers [6]. Fortunately, aryl-alkylsulfonium salts do not have the above problems. The photolysis mechanism of these compounds is relatively similar. Under irradiation, the sulfonium salt will undergo homo- or hetero-cleavage of the C-S bond to generate sulfur radical cations and carbon radicals. Sulfur radical cations can undergo substitution with carbon radicals or electron transfer with hydrogen donors, essentially producing Bronsted acid (Scheme 5). Jin et al., reported several strategies in this line [16][22][23][24][25][26][27]. The main strategy for designing such CPIs is to make the framework of PIs contain long conjugated structures such as D-π-A or D-π-A-D, which can not only cause a red-shift in the absorption, but also excellent acid production efficiency. In 2021, Li et al. [28] reported for the first time that a single component sulfonium salt photoinitiator combined with up-conversion nanoparticles successfully achieved near-infrared-induced cationic and free radical/cation hybrid photopolymerization. The resulting luminescent polymer material has a curing depth of more than 10 cm, good network uniformity and dual-wavelength responsiveness, which have great potential in sensing and anti-counterfeiting applications. 
Scheme 5. Proposed photolysis mechanism of aryl-alkylsulfonium salts [25]. Red color represent the active species that can initiate polymerization. * means that compound is in the excited state.

Phenacylsulfonium Salts

Crivello et al. [29] proposed a facile method for the synthesis of dialkylphenacylsulfonium salt CPIs. Dialkylphenacylsulfonium salts can be obtained by mixing 2-bromoacetophenone and sulfide to produce bromine salt followed by anion exchange reaction (Scheme 6) [30]. In this work, it was shown that the compatibility of dialkylphenacylsulfonium salts with non-polar monomers and oligomers can be improved by changing the length of the alkyl chain. Yagci and Crivello et al. designed and synthetized a series of phenacylsulfonium salts [25][26][27][28][29]. There are two cleavage pathways of phenacylsulfonium salts under light irradiation: (i) homolysis to produce phenylacetyl radicals and sulfur radical cations; (ii) heterolysis to produce phenylacetyl cationic species. Phenylacetyl radicals are capable of initiating FRP. These free radicals can also undergo dimerization and hydrogen abstraction reactions. Sulfur radical cations can further react with hydrogen donors to generate Bronsted acids, which subsequently initiate CP [30] (Scheme 7). Liu et al. [31] reported a novel broad-wavelength absorbing phenacylsulfonium salt possessing phenacylphenothiazine chromophore that can effectively initiate CP and FRP under UV, visible and NIR irradiation. Phenacylsulfonium salts have attracted much attention in the field of photopolymerization due to their simple synthesis and adjustable properties. In 2023, based on the guidance of theoretical calculations, Liu et al. [32] designed and synthesized sulfonium salt photoinitiators based on different alkyl chains of coumarin skeleton. The resulting coumarin sulfonium salt (CSS) was used as a single-component photoinitiator for cationic photopolymerization, free radical photopolymerization and hybrid polymerization. This work clarified the effect of aliphatic chain length on photoinitiation activity. The research results provide some guidance for the design of new efficient sulfonium salt photoinitiators with controllable activity and solubility. 
Scheme 6. General synthetic method of phenacylsulfonium salt [30].
Scheme 7. Proposed photolysis mechanism of phenacylsulfonium salts [30]. Red color represent the active species that can initiate polymerization. * means that compound is in the excited state.

2.4. Phosphonium Salts

Phosphonium salts can be obtained by chloro- or bromo-methylating reactions of the related aryl compounds with the corresponding phosphine-containing compounds [33][34][35]. For phosphonium salts, the active species that initiate CP are generally considered to be carbocations formed by the photochemical cleavage of the P-C bond (Scheme 8).
Scheme 8. Proposed photolysis mechanism of benzoylsulfonium salts [36]. Red color represent the active species that can initiate polymerization.
The phosphonium salts of pyrene methyl have good initiation ability for epoxy and vinyl monomers with almost quantitative conversion [36]. The UV-Vis and 1H-NMR spectral analysis of the obtained polymers provided clear evidence for the presence of aromatic end groups, confirming the involvement of pyrene methyl carbocations in the initiation process. 

2.5. Ammonium Salts

N-Alkoxypyridinium Salts

N-Alkoxypyridinium salts can be prepared from the corresponding pyridyl oxynitrides and can be obtained with a high yield. This type of onium salt usually produces alkaline by-products such as pyridine and isoquinoline, which can additionally consume protonic acid. The polymerization reaction may be terminated when the concentration of released base is above a certain level [37].
The UV absorption wavelength of most N-Alkoxypyridinium salts is below 300 nm, which does not match with the emission from green LED light source. The usual strategy to shift the absorption wavelength is to incorporate additional chromophores into pyridine ring. 
Upon irradiation, these salts undergo hemolytic cleavage to generate pyridinium cation radicals. These radical cations abstract hydrogen and then form protonic acids to initiate a CP reaction (Scheme 9) [37][38][39][40][41].
Scheme 9. Proposed photolysis mechanism of N-Alkylpyridinium salts [36]. Red color represent the active species that can initiate polymerization.

Phenacyl Ammonium Salts

A series of phenacyl ammonium salts [42][43][44][45] as efficient PIs for FRP and CP was reported by Yagci, Phenacyl ammonium salts can be obtained by SN1 reaction of 2-bromoacetophenone with nitrogen compounds. In 2014, Yagci et al. [38] synthesized polystyrene-b-poly-2-vinylphenacylpyridine hexafluorophosphate (PS-b-PVPP) by phenacylation followed by anion exchange reactions of polystyrene-b-poly-2-vinylpyridine prepared by live anion polymerization. As can be seen from Scheme 10, PS-b-PVPP undergoes photolysis in two ways after absorption of photons. On one hand, benzoyl methyl radicals are produced by homolysis, which can initiate the FRP of methyl methacrylate (MMA). On the other hand, phenacylium cations formed by heterolysis initiate the CP of monomers such as epoxy cyclohexane (CHO), n-butyl vinyl ether (BVE) and N-vinyl carbazole (NVC) indicating excellent capability for HP and forming IPN. Their photochemical transition from cation to neutral state induces molecular association, leading to changes in the surface morphology of solid films and the formation of aggregates in solution. This photochemical behavior may provide new avenues for a variety of biological applications and new ways to control the surface properties and thickness of polymer films and multilayers (Figure 1). In 2023, Li et al. [46] prepared a coumarin acylaniline (CAA) onium salt by one-pot reaction. Compared with commercial iodonium salts, CAA salts exhibit excellent photoinitiation performance in cationic polymerization. Under visible light irradiation, the onium salt undergoes homolysis and cracking, followed by electron transfer and hydrogen extraction reactions to form reactive species that can initiate cationic polymerization of epoxides and vinyl monomers. After a short irradiation period, due to the non-nucleophilic nature of the counterion, polymerization also takes place in the dark. Near-infrared induced polymerization was successfully carried out on the basis of up-conversion photochemistry. CAA salt can also initiate the stepwise growth polymerization of N-ethylcarbazole (NEC) by photochemically formed aniline radical cationic oxidation monomer.
Scheme 10. Proposed photolysis mechanism of PS-b-PVPP [38]. Red color represent the active species that can initiate polymerization.
Figure 1. A schematic diagram of the interaction between PS-b-PVPP chains before and after 365 nm irradiation [38].

2.6. Ferrocenium Salts

Ferroceniums salts are attractive PIs for CP [36][47][48]. These compounds can be obtained from inexpensive raw materials by simple synthetic methods. Cyclopentadiene-iron-arene salt was prepared by the ligand exchange reaction of cycloferrocene with arenes according to the method described by Nesmeyanov et al. [49]. Ferrocenium salts, such as (η6-arene) (η5-cyclopentadiene) ferric hexafluorophosphate ArFe(+)Cp, have the advantage of having larger absorption peaks in the near-UV and UV-visible light spectrum region that can be facilitated, and by changing the structure of the ligands, their absorption can be changed between long-wavelength UV and visible wavelengths.
Ferrocene salts undergo photolysis to form iron-based Lewis acids with the loss of aromatic ligands. The latter species can coordinate with epoxy monomers, followed by ring-opening polymerization  [50][51][52][53][54]. The photoinitiated mechanism is shown in Scheme 11.
Scheme 11. Proposed photolysis mechanism of ferrocenium salts [49]. * means that compound is in the excited state.
These compounds with different substituents are single-component PIs (or key components of multi-component PIS) for the cationic ring-opening polymerization of epoxy compounds, showing activity under near-UV, blue or even green LEDs. Reviewing the recent developments in the field of CPIs, application opportunities for CP and HP based on LED irradiators may open new pathways and ferrocene salts hold great promise in this direction.

3. Nonionic Photoinitiators for Cationic Polymerization

Over the past few decades, various nonionic CPIs have been developed for the initiation and surface polymerization due to their better solubility in conventional solvents and resins [55][56][57][58]. Such compounds mainly include arene sulfonates, sulfamic acid esters and iminosulfonic acid esters. Under light exposure, they form structurally stable free radicals through the cleavage of C-O, S-O and N-O bonds, which can usually abstract hydrogen from the solvent, which releases acidic compounds, which are considered to be active species for initiating cationic polymerization (Scheme 12) [59]. So far, only aryl tosylates have been used in photopolymerization, capable of promoting the CP of epoxy groups in hybrid sol-gel photoresists [60].
Scheme 12. Proposed photolysis mechanism of nonionic CPs [59]. Red color represent the active species that can initiate polymerization.
Jin et al. [61] reported three novel thiophene oxime compounds containing sulfonic acid groups as non-ionic photoacid generators with large conjugated systems. The irradiation of near-UV/visible LED (365–475 nm) leads to the cleavage of two weak N-O bonds (Scheme 13), resulting in different sulfonic acids with good quantum yields and chemical yields. These PIs are highly active and do not require any additives. They were used for CP of epoxides and vinyl ethers under low concentrations of near-UV and visible LEDs. The conversion of cyclohexene oxide can reach 99 % in the light range of 365–475 nm.
Scheme 13. Proposed photolysis mechanism of thiophenoxime containing sulfonic acid compounds [61]. Red color represent the active species that can initiate polymerization.

This entry is adapted from the peer-reviewed paper 10.3390/polym15112524


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