Thiophenium Salts as New Oxidant for Redox Polymerization under Mild- and Low-Toxicity Conditions

In mild conditions (under air, room temperature, no monomer purification and without any energy activation), redox free radical polymerization (RFRP) is considered as one of the most effective methods to polymerize (meth)acrylate monomers. In the past several years, there has been a growing interest in research on the development of new redox initiating systems (RISs), thanks mainly to the evolution of toxicity labeling and the stability issue of the current RIS based on peroxide and aromatic amine. In this study, a new, low-toxicity RIS based on thiophenium salt as the oxidant species is presented with various reductive species. The reactivity and the stability of the proposed RISs are investigated and the synthesis of new thiophenium salts reported.


Introduction
Since the discovery of redox initiating systems (RISs) by German researchers trying to improve thermal initiating systems using peroxide as a thermal accelerator [1], studies have been carried out to optimize this new method of polymer synthesis.
In this context, new RISs were discovered. Most of these new systems rely on lowenergy bond dissociation, such as the O-O bond (peroxide) or the S-S bond (disulfide) with various reducing agents (metallic ions, thiols, carboxylic acids, amines, etc.), as the oxidizing agent [2]. Among these possible oxidizing agents, dibenzoyl peroxide (BPO) has been extensively studied for its ability to work with a wide panel of reducing agents in both aqueous and nonaqueous media [2].
Redox free radical polymerization (RFRP) using an RIS is one of the mildest methods to reduce energy consumption in polymer synthesis [3,4]. Indeed, no additional stimuli are needed to trigger the initiation step of the polymerization. In addition, RFRP can be performed in mild conditions [5][6][7][8] (under air, no further purification of the monomers and carried out at room temperature). The general mechanism of RFRP is illustrated in Scheme 1.
The polymerization of monomers by using the RFRP method is based on the mixing of an oxidizing agent and a reducing agent, which are able to generate in situ radical species through a redox mechanism (Scheme 1, K r , generation of radical species).
The radicals generated from the redox reaction (R • ) may react with a monomer (M), leading to a propagating polymeric radical (Scheme 1, K i , initiation step). This small polymeric radical is then able to react with other monomer units to yield a longer polymeric radical (Scheme 1, K p , propagation step). Finally, the polymerization stops when a polymeric radical reacts with another macroradical in combination or through disproportionation (Scheme 1, K t , termination step). Even though the system is very effective, it has some drawbacks, such as the low stability of peroxide in monomers [10] and the relative toxicity of the aromatic amine class [11]. The toxicity issue of the aromatic amine class may be overcome by playing on the chemical structure of the amine. Indeed, 2-(N-methyl-p-toluidino)ethanol, for example, is nontoxic and yields similar results compared with 4-N,N-TMA. However, the instability of BPO in monomers is a dead end. Industrial formulations use BPO in an inert plasticizer to avoid side reactions leading to instability. Given the major disadvantage of inert plasticizers in formulations (unreacted material in the final polymer altering the mechanical Scheme 1. General mechanism of RFRP. The most common RIS used at the academic and industrial scales is based on dibenzoyl peroxide and an amine compound. Dibenzoyl peroxide is used as the oxidizing agent for the reasons mentioned above, and the amine compound is widely used as a reductive reagent thanks mostly to its (i) low cost, (ii) high reactivity and (iii) possibility to polymerize in solvent-free conditions (bulk), which is more challenging for other reductive reagents, such as metallic complexes (solubility issue) [9]. Studies conducted on the amine structure revealed the criteria needed to optimize the efficiency of the RIS [9]: (i) the primary and secondary amines classes are not suitable for RFRP, because proton transfer is more favorable, resulting in low yields in initiating radicals; (ii) among the tertiary amines class, aromatic amines with the minimal steric hindrance have the best efficiency, but they also have higher toxicity.
Thus, the most widely used RIS at the industrial scale is based on a mixture of dibenzoyl peroxide (BPO) and a tertiary aromatic amine such as 4-N,N-Trimethylaniline (4-N,N-TMA) (Figure 1), which fulfills all the criteria mentioned above. Scheme 1. General mechanism of RFRP.
The polymerization of monomers by using the RFRP method is based on the mixing of an oxidizing agent and a reducing agent, which are able to generate in situ radical species through a redox mechanism (Scheme 1, Kr, generation of radical species).
The radicals generated from the redox reaction (R • ) may react with a monomer (M), leading to a propagating polymeric radical (Scheme 1, Ki, initiation step). This small polymeric radical is then able to react with other monomer units to yield a longer polymeric radical (Scheme 1, Kp, propagation step). Finally, the polymerization stops when a polymeric radical reacts with another macroradical in combination or through disproportionation (Scheme 1, Kt, termination step).
The most common RIS used at the academic and industrial scales is based on dibenzoyl peroxide and an amine compound. Dibenzoyl peroxide is used as the oxidizing agent for the reasons mentioned above, and the amine compound is widely used as a reductive reagent thanks mostly to its (i) low cost, (ii) high reactivity and (iii) possibility to polymerize in solvent-free conditions (bulk), which is more challenging for other reductive reagents, such as metallic complexes (solubility issue) [9]. Studies conducted on the amine structure revealed the criteria needed to optimize the efficiency of the RIS [9]: (i) the primary and secondary amines classes are not suitable for RFRP, because proton transfer is more favorable, resulting in low yields in initiating radicals; (ii) among the tertiary amines class, aromatic amines with the minimal steric hindrance have the best efficiency, but they also have higher toxicity.
Thus, the most widely used RIS at the industrial scale is based on a mixture of dibenzoyl peroxide (BPO) and a tertiary aromatic amine such as 4-N,N-Trimethylaniline (4-N,N-TMA) (Figure 1), which fulfills all the criteria mentioned above. Even though the system is very effective, it has some drawbacks, such as the low stability of peroxide in monomers [10] and the relative toxicity of the aromatic amine class [11]. The toxicity issue of the aromatic amine class may be overcome by playing on the chemical structure of the amine. Indeed, 2-(N-methyl-p-toluidino)ethanol, for example, is nontoxic and yields similar results compared with 4-N,N-TMA. However, the instability of BPO in monomers is a dead end. Industrial formulations use BPO in an inert plasticizer to avoid side reactions leading to instability. Given the major disadvantage of inert plasticizers in formulations (unreacted material in the final polymer altering the mechanical Even though the system is very effective, it has some drawbacks, such as the low stability of peroxide in monomers [10] and the relative toxicity of the aromatic amine class [11]. The toxicity issue of the aromatic amine class may be overcome by playing on the chemical structure of the amine. Indeed, 2-(N-methyl-p-toluidino)ethanol, for example, is nontoxic and yields similar results compared with 4-N,N-TMA. However, the instability of BPO in monomers is a dead end. Industrial formulations use BPO in an inert plasticizer to avoid side reactions leading to instability. Given the major disadvantage of inert plasticizers in formulations (unreacted material in the final polymer altering the mechanical properties), new studies must be carried out to propose alternative redox agents that are stable in monomers and nontoxic.
To replace the oxidizing agent (e.g., BPO), this work will focus on thiophenium salts as new oxidant species. These salts have been used in organic chemistry but only for trifluoromethylation reactions [12]. Our interest in these salts emerged from the structural similarity with sulfonium salts such as bis [4-(diphenylsulfonio)phenyl]sulfide and bis(hexafluoroantimonate). Onium salts such as sulfonium and iodonium salts are well known in the literature to generate radical species upon irradiation or heat [2,13]. Some RISs were actually designed using iodonium salt as an oxidizing agent. [14,15]. In order to fully characterize the reactivity of thiophenium salts, this study was conducted using (i) different (meth)acrylate monomers (Figure 2), (ii) different reducing agents ( Figure 3) and (iii) different thiophenium salts ( Figure 4). The development of new RIS is a huge challenge [16,17] that can also require the synthesis of new compounds [18].
as new oxidant species. These salts have been used in organic chemistry but only for trifluoromethylation reactions [12]. Our interest in these salts emerged from the structural similarity with sulfonium salts such as bis [4-(diphenylsulfonio)phenyl]sulfide and bis(hexafluoroantimonate). Onium salts such as sulfonium and iodonium salts are well known in the literature to generate radical species upon irradiation or heat [2,13]. Some RISs were actually designed using iodonium salt as an oxidizing agent. [14,15]. In order to fully characterize the reactivity of thiophenium salts, this study was conducted using (i) different (meth)acrylate monomers (Figure 2), (ii) different reducing agents ( Figure 3) and (iii) different thiophenium salts ( Figure 4). The development of new RIS is a huge challenge [16,17] that can also require the synthesis of new compounds [18].   as new oxidant species. These salts have been used in organic chemistry but only for trifluoromethylation reactions [12]. Our interest in these salts emerged from the structural similarity with sulfonium salts such as bis [4-(diphenylsulfonio)phenyl]sulfide and bis(hexafluoroantimonate). Onium salts such as sulfonium and iodonium salts are well known in the literature to generate radical species upon irradiation or heat [2,13]. Some RISs were actually designed using iodonium salt as an oxidizing agent. [14,15]. In order to fully characterize the reactivity of thiophenium salts, this study was conducted using (i) different (meth)acrylate monomers (Figure 2), (ii) different reducing agents ( Figure 3) and (iii) different thiophenium salts ( Figure 4). The development of new RIS is a huge challenge [16,17] that can also require the synthesis of new compounds [18].      In a 25 mL two-neck round bottom flask, 0.5 g of sodium triflinate (1 eq., 3 mmol) are added under anhydrous and argon atmosphere conditions, followed by 10 mL of anhydrous nitromethane as solvent. After total dissolution, 1 mL of triflic anhydride (2 eq., 6 mmol) is added. After stirring for 10 min, the 4,4 -di-tert-butylbiphenyl (1 eq., 3 mmol, 0.799 g) is added as a solution in 5 mL of nitromethane. The reaction is monitored by TLC using cyclohexane as eluent.

Results and Discussion
After 24 h, the nitromethane is distilled off under reduced pressure and washed several times with toluene (4 × 3 mL) to remove as much nitromethane as possible.
The crude is then diluted in 3.5 mL of distilled water, followed by 3.5 mL of diethyl ether, causing the precipitation of the product after stirring overnight.
After filtration and several washings with diethyl ether (4 × 3 mL), the product is dried under reduced pressure at 40 • C, yielding a white solid (0.167 g, 21% yield).
After evaporation of acetonitrile under reduced pressure, the crude is dissolved in 10 mL of dichloromethane and washed with a saturated solution of NaHCO 3 (3 × 5 mL). The dichloromethane is distilled off; 5 mL of acetonitrile is added; and a large excess of diethyl ether is added until precipitation of the product occurred (approx. 20 mL). The product is obtained as a white powder (0.413 g, 83%).

Effect of the Reducing Agent
Thiophenium salts and reducing agents alone are stable in (meth)acrylic monomers. Polymerization occurs only by mixing the two formulations (cartridges), which clearly highlights a radical generation through the reaction between thiophenium salts and reducing agents.
Finally, compared with BPO/4-N,N-TMA, all Thiophenium (III) systems with different reducing agents yield better results, namely a faster gel time, a higher exothermic peak ( Figure 5: curve 5 compared with curves 1, 2, 3 and 4) and better polymer appearance (gel-like polymer for reference BPO/4-N,N-TMA with a very poor surface curing).
Finally, compared with BPO/4-N,N-TMA, all Thiophenium (III) systems with different reducing agents yield better results, namely a faster gel time, a higher exothermic peak ( Figure 5: curve 5 compared with curves 1, 2, 3 and 4) and better polymer appearance (gellike polymer for reference BPO/4-N,N-TMA with a very poor surface curing). This first study on the nature of the reducing agent shows excellent results for reducing agents selected from aromatic compounds (i.e., electron-rich compounds). For the remaining work, Na-Tol-sulfinate was used instead of DHPP, for availability reasons. This first study on the nature of the reducing agent shows excellent results for reducing agents selected from aromatic compounds (i.e., electron-rich compounds). For the remaining work, Na-Tol-sulfinate was used instead of DHPP, for availability reasons.

Effect of the Oxidizing Agent
The reactivity of several thiophenium salts is investigated in the monomer EDMA using Na-Tol-sulfinate as the most effective reducing agent identified in Section 2.2.1 ( Figure 6). The reactivity of several thiophenium salts is investigated in the monomer EDMA using Na-Tol-sulfinate as the most effective reducing agent identified in Section 2.2.1 (Figure 6).
Thiophenium (III)/Na-Tol-sulfinate compared with Thiophenium (II)/Na-Tol-sulfinate (Figure 7: curves 3 and 2, respectively) shows higher reactivity (31 s vs. 52 s for gel time and 104 °C vs. 72 °C for maximum temperature reached). This may be due to the mesomeric effect of fluorine [19], which leads to the electroenrichment of the sulfur and thus to a less-potent oxidizer agent.
Thiophenium (II)/Na-Tol-sulfinate compared with Thiophenium (IV)/Na-Tol-sulfinate ( Figure 6: curves 2 and 4, respectively) shows lower reactivity (52 s vs. 21 s for gel time and 72 °C vs. 98 °C for maximum temperature). This is due to the effect of the counter ion, as described by the hard and soft acids and bases (HSAB) theory [20]. Indeed, the PF6 − counter ion is considered as soft, whereas CF3SO3 − is listed as hard, resulting in a higher ion-dissociation rate for the PF6 − counter ion and thus a higher reactivity. The study of the chemical structure of the thiophenium itself and its associated counter ion revealed (i) better reactivity with soft counter ions (according to the HSAB theory), (ii) better solubility of the thiophenium salts in a monomer with lipophilic substituents (tert-butyl groups) and (iii) lower reactivity with the electroenrichment of the sulfur. (1) Thiophenium (I) 1% wt // Na-Tol-sulfinate 1% wt , (2) Thiophenium (II) 1% wt /Na-Tol-sulfinate 1% wt , (3) Thiophenium (III) 1% wt /Na-Tol-sulfinate 1% wt , (4) Thiophenium (IV) 1% wt /Na-Tol-sulfinate 1% wt . Thiophenium (III)/Na-Tol-sulfinate compared with Thiophenium (II)/Na-Tol-sulfinate (Figure 7: curves 3 and 2, respectively) shows higher reactivity (31 s vs. 52 s for gel time and 104 • C vs. 72 • C for maximum temperature reached). This may be due to the mesomeric effect of fluorine [19], which leads to the electroenrichment of the sulfur and thus to a less-potent oxidizer agent. the optimum usage, the effect of the thiophenium salt concentration was studied, using Na-Tol-sulfinate as the reducing agent with EDMA as a monomer (Figure 7). The RIS at 1%wt (29 s gel time, 146 °C maximum temperature reached) compared with the RIS at 0.25%wt (58 s gel time, 122 °C maximum temperature reached) showed that the concentration can be significantly decreased (Figure 7: curves 1 and 2, respectively).
However, the RIS at 0.1%wt (159 s gel time, 72 °C maximum temperature reached) and the RIS at 0.01%wt (no polymerization occurs) showed that the concentration of the initiating system cannot be lowered too much (Figure 7: curves 3 and 5, respectively).

Effect of the Monomer
The respective reactivities of the various monomers were compared ( Figure 2) to determine the best RIS (e.g., Thiophenium (III)/Na-Tol-sulfinate) (Figure 8).
The RIS in EDMA (29 s gel time, 146 °C maximum temperature) compared with the RIS in GFMA (176 s gel time, 88 °C maximum temperature) (Figure 8: curves 1 and 2, respectively) showed a large difference in reactivity between the acrylate and methacrylate monomers [21].

Effect of the Concentration
Thiophenium salts are not commercially available in large quantities. In order to find the optimum usage, the effect of the thiophenium salt concentration was studied, Molecules 2023, 28, 627 7 of 14 using Na-Tol-sulfinate as the reducing agent with EDMA as a monomer (Figure 7). The RIS at 1% wt (29 s gel time, 146 • C maximum temperature reached) compared with the RIS at 0.25% wt (58 s gel time, 122 • C maximum temperature reached) showed that the concentration can be significantly decreased (Figure 7: curves 1 and 2, respectively).
However, the RIS at 0.1% wt (159 s gel time, 72 • C maximum temperature reached) and the RIS at 0.01% wt (no polymerization occurs) showed that the concentration of the initiating system cannot be lowered too much (Figure 7: curves 3 and 5, respectively).

Effect of the Monomer
The respective reactivities of the various monomers were compared (Figure 2) to determine the best RIS (e.g., Thiophenium (III)/Na-Tol-sulfinate) ( Figure 8).
Molecules 2023, 28, x FOR PEER REVIEW 8 of 15 As mentioned in Section 2.2.2., the replacement of the CF3SO3 − counter ion by PF6drastically increased the reactivity of the system, as shown in Figure 9 (gel time of 84 s and maximum temperature reached of 75 °C for Thiophenium (IV) vs. no polymerization for Thiophenium (II)-curves 2 and 1, respectively).  The RIS in EDMA (29 s gel time, 146 • C maximum temperature) compared with the RIS in GFMA (176 s gel time, 88 • C maximum temperature) (Figure 8: curves 1 and 2, respectively) showed a large difference in reactivity between the acrylate and methacrylate monomers [21].
The RIS in isobornyl acrylate ( Figure 8: curve 3, no polymerization) highlighted the lack of solubility of thiophenium and sulfinate salts in a low-polar monomer. To improve this solubility, blends of EDMA and IBA were tested (Figure 8: curve 4, blend of 50% wt EDMA and 50% wt IBA, gel time of 131 s for 132 • C maximum temperature; Figure 8: curve 5, blend of 30% wt EDMA and 70% wt IBA, gel time of 312 s for 98 • C maximum temperature). However, the reactivity is lower thanks to the limited solubility of the RIS in these monomer blends.

Effect of Light Activation
For the different RISs based on Thiophenium (III)/Na-Tol-sulfinate (Figure 3), the conversion of (meth)acrylate functions were followed by RT-FTIR with and without light irradiation ( Figure 10). The conversion of (meth)acrylate functions is greatly improved by light excitation ( Figure 10B: curve 1 (0% of acrylate function conversion without light) vs. curve 2 (68% of acrylate function conversion with light excitation); Figure 10C: curve 1 (72% of methacrylate conversion without light) vs. curve 2 (84% of methacrylate conversion with light excitation)).
Also, the rate of conversion seems to be improved (i.e., the slope of the curves) ( Figure 10C: curve 1 (72% of methacrylate function conversion without light after 283 s) vs. curve 2 (80% of methacrylate function conversion with light excitation for the same time), as an example). Nonetheless, the impact of light irradiation remains unclear because the interaction of light in EDMA monomer results in a lower rate of acrylate function conversion ( Figure 10A: curve 1 (72% of acrylate function conversion without light after 62 s) vs. curve 2 (37% of acrylate function conversion with light excitation for the same amount of time)).
As mentioned above, the benefits of light irradiation on an RIS remains doubtful: in IBA, light irradiation drastically improved polymerization, whereas in EDMA, light irradiation slightly decreased the reactivity. An explanation may be the quantity of inhibitor present in the monomers (900 ppm of 4-methoxyphenol in EDMA, 250 ppm in IBA and 200 ppm in GFMA).
The light absorption properties of thiophenium salts were investigated (available in the Supplementary Materials (Part III)). Weak but significant absorption was observed between 380 and 400 nm, corresponding to one part of the emission spectrum of the LED used during RT-FTIR experiments, in good agreement with the ability of this system to perform photoactivation. Also, the rate of conversion seems to be improved (i.e., the slope of the curves) (Figure 10C: curve 1 (72% of methacrylate function conversion without light after 283 s) vs. curve 2 (80% of methacrylate function conversion with light excitation for the same time), as an example). Nonetheless, the impact of light irradiation remains unclear because the interaction of light in EDMA monomer results in a lower rate of acrylate function conversion ( Figure 10A: curve 1 (72% of acrylate function conversion without light after 62 s) vs. curve 2 (37% of acrylate function conversion with light excitation for the same amount of time)). As mentioned above, the benefits of light irradiation on an RIS remains doubtful: in IBA, light irradiation drastically improved polymerization, whereas in EDMA, light irradiation slightly decreased the reactivity. An explanation may be the quantity of inhibitor present in the monomers (900 ppm of 4-methoxyphenol in EDMA, 250 ppm in IBA and 200 ppm in GFMA).
The light absorption properties of thiophenium salts were investigated (available in the Supplementary Materials (Part III)). Weak but significant absorption was observed between 380 and 400 nm, corresponding to one part of the emission spectrum of the LED used during RT-FTIR experiments, in good agreement with the ability of this system to perform photoactivation.

Study of the System Stability
The stability of the system was investigated by aging the two formulations (for both Thiophenium (III) and Na-Tol-sulfinate cartridges) in an oven at 40 • C. The degradation of the system reactivity was followed by optical pyrometry every week during the aging period ( Figure 11).
Molecules 2023, 28, x FOR PEER REVIEW 10 of 15 The stability of the system was investigated by aging the two formulations (for both Thiophenium (III) and Na-Tol-sulfinate cartridges) in an oven at 40 °C . The degradation of the system reactivity was followed by optical pyrometry every week during the aging period ( Figure 11).
The Thiophenium (III)/Na-Tol-sulfinate system presents excellent stability with almost the same amount of gel time (around 28 s) and the same maximum temperature reached (around 141 °C ) before and after aging, with no visible change in the viscosity of the samples.

Mechanistic Consideration
Thiophenium salts are well studied in the field of organic chemistry for their effectiveness in trifluoromethylation reactions [22]. In particular, they are used together with sodium benzenesulfinate to produce phenyltrifluoromethylsulfone with good yields [23]. Although the thiophenium/sodium benzenesulfinate couple has never been used as an RIS, its mechanistic behavior is well known and takes place in three steps [23]: (1) mixing Na-Tol-sulfinate with thiophenium salts results in a counter ion exchange; (2) a single electron transfer occurs, leading to the formation of •CF3 and Tol-SO2• radicals, and at this stage, •CF3 is able to initiate the polymerization steps; whereas (3) Tol-SO2• evolves The Thiophenium (III)/Na-Tol-sulfinate system presents excellent stability with almost the same amount of gel time (around 28 s) and the same maximum temperature reached (around 141 • C) before and after aging, with no visible change in the viscosity of the samples.

Mechanistic Consideration
Thiophenium salts are well studied in the field of organic chemistry for their effectiveness in trifluoromethylation reactions [22]. In particular, they are used together with sodium benzenesulfinate to produce phenyltrifluoromethylsulfone with good yields [23]. Although the thiophenium/sodium benzenesulfinate couple has never been used as an RIS, its mechanistic behavior is well known and takes place in three steps [23]: (1) mixing Na-Tol-sulfinate with thiophenium salts results in a counter ion exchange; (2) a single electron transfer occurs, leading to the formation of •CF 3 and Tol-SO 2 • radicals, and at this stage, •CF 3 is able to initiate the polymerization steps; whereas (3) Tol-SO 2 • evolves toward the formation of Tol• (Scheme 2).
Molecules 2023, 28, x FOR PEER REVIEW 11 of 15 Scheme 2. Proposed mechanism of the RIS according to Zhou et al. [23].
ESR spin trapping experiments were conducted to confirm the proposed mechanism. Simulated spectra revealed the generation of • CF3 radical, whereas Tol-SO2• or Tol• were not detected. Indeed, the hyperfine coupling constants for the PBN-CF3 spin adduct (Figure 12: an = 14.0 ± 0.1 G; aH = 1.2 ± 0.1 G and aF = 1.7 ± 0.1 G) are in good agreement with the data in the literature [24].

Regulatory Labeling of the New Redox Agents
According to the REACh European regulation, Thiophenium (III) and Na-Tol-sulfinate are both reported as skin and eye irritants (globally harmonized system (GHS) 07). Compared with BPO (GHS 01, 02 and 07) and 4-N,N-TMA (GHS 06 and 08), the new RIS, Scheme 2. Proposed mechanism of the RIS according to Zhou et al. [23].
ESR spin trapping experiments were conducted to confirm the proposed mechanism. Simulated spectra revealed the generation of • CF 3 radical, whereas Tol-SO 2 • or Tol• were not detected. Indeed, the hyperfine coupling constants for the PBN-CF 3 spin adduct ( Figure 12: a n = 14.0 ± 0.1 G; a H = 1.2 ± 0.1 G and a F = 1.7 ± 0.1 G) are in good agreement with the data in the literature [24].
Molecules 2023, 28, x FOR PEER REVIEW 11 of 15 Scheme 2. Proposed mechanism of the RIS according to Zhou et al. [23].
ESR spin trapping experiments were conducted to confirm the proposed mechanism. Simulated spectra revealed the generation of • CF3 radical, whereas Tol-SO2• or Tol• were not detected. Indeed, the hyperfine coupling constants for the PBN-CF3 spin adduct (Figure 12: an = 14.0 ± 0.1 G; aH = 1.2 ± 0.1 G and aF = 1.7 ± 0.1 G) are in good agreement with the data in the literature [24].

Regulatory Labeling of the New Redox Agents
According to the REACh European regulation, Thiophenium (III) and Na-Tol-sulfinate are both reported as skin and eye irritants (globally harmonized system (GHS) 07).

Regulatory Labeling of the New Redox Agents
According to the REACh European regulation, Thiophenium (III) and Na-Tol-sulfinate are both reported as skin and eye irritants (globally harmonized system (GHS) 07). Compared with BPO (GHS 01, 02 and 07) and 4-N,N-TMA (GHS 06 and 08), the new RIS, composed of Thiophenium (III)/Na-Tol-sulfinate, can be safer to use, leading to a huge advantage for practical applications.

Two-Cartridge System Configuration for RFRP
Redox formulations were prepared as follows: in a first glass vial, 0.03 g (1% wt based on total monomer mass after mixing) of an oxidizing agent is added to 1.47 g of a (meth)acrylate monomer. In a second glass vial, 0.03 g (1% wt based on total monomer mass after mixing) of a reducing agent is added to 1.47 g of a (meth)acrylate monomer. The two vials are agitated for 2 h at ambient temperature with a magnetic stirrer. Then, the two formulations are placed in a 1:1 Medmix mixpac mixing syringe dispenser ( Figure 13).

Two-Cartridge System Configuration for RFRP
Redox formulations were prepared as follows: in a first glass vial, 0.03 g (1%wt based on total monomer mass after mixing) of an oxidizing agent is added to 1.47 g of a (meth)acrylate monomer. In a second glass vial, 0.03 g (1%wt based on total monomer mass after mixing) of a reducing agent is added to 1.47 g of a (meth)acrylate monomer. The two vials are agitated for 2 h at ambient temperature with a magnetic stirrer. Then, the two formulations are placed in a 1:1 Medmix mixpac mixing syringe dispenser ( Figure 13). Polymerization experiments are carried out by mixing the two cartridges at ambient temperature (±25 °C) and dispensing the mixture under air (an oxygen inhibition is then expected, particularly at the surface of the polymer [16]).

Efficiency of the RIS Followed by Optical Pyrometry
Redox polymerization is strongly exothermic, and optical pyrometry is a well-established technique to follow redox polymerization (see [2,[13][14][15][16][17]). Therefore, the efficiency of an RIS can be easily followed qualitatively by optical pyrometry. An Omega OS552-V1-6 infrared thermometer (Omega Engineering, Inc., Stamford, CT, USA) with ±1 °C sensitivity was used to determine the temperature vs. time profiles, thus allowing for measur- Polymerization experiments are carried out by mixing the two cartridges at ambient temperature (±25 • C) and dispensing the mixture under air (an oxygen inhibition is then expected, particularly at the surface of the polymer [16]).