Novel Peroxide-Free redox initiator based on
Saccharin/Electron Donor/Copper salt for radical
polymerization

Issei Takahashi, Xavier Allonas

► To cite this version:

Issei Takahashi, Xavier Allonas. Novel Peroxide-Free redox initiator based on Saccharin/Electron Donor/Copper salt for radical polymerization. European Polymer Journal, 2023, 188, pp.111900. 10.1016/j.eurpolymj.2023.111900. hal-05073941

HAL Id: hal-05073941

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Submitted on 9 Jul 2025

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Distributed under a Creative Commons Attribution 4.0 International License
# Novel Peroxide-Free Redox Initiator Based on Saccharin/Electron
Donor/Copper Salt for Radical Polymerization

Issei Takahashi,<sup>a, b</sup> Xavier Allonas<sup>b*</sup>

a) Mitsui Chemicals, Inc., 580-32, Nagaura, Sodegaura-City, Chiba 299-0265, Japan

b) Laboratoire de Photochimie et d'Ingénierie Macromoléculaires, Institut Jean Baptiste Donnet,
3b rue Alfred Werner, 68093 Mulhouse Cedex, France

**ABSTRACT:** In this work, a peroxide-free redox initiating system for radical polymerization of acrylate is suggested, based on the combination of saccharin, aromatic amine and copper salt. Free radical redox polymerization was investigated through differential scanning calorimetry (DSC), highlighting the efficiency of the redox initiating system. The role of the different components was studied mainly through liquid chromatography-coupled mass spectrometry (LC-MS), evidencing the involvement of aminoalkyl radical as initiating species. When p-toluenesulfonate is added as accelerator, the corresponding radical, as well as the oxygenated derived one, were found to act also as initiating radical. The oxidation state of the copper catalyst during the reaction is studied by UV-visible spectroscopy experiments, using bathophenanthroline as a probe for the involvement of Cu(I). From these experiments, a mechanism is proposed to account for the initiation reaction.

**Keywords:** redox initiating system ; radical polymerisation ; hydroperoxide ; copper salt ; dental material

## 1. Introduction
Radical polymerization is widely used in many industries such as color printing, 3D printing, coating, cosmetic and dental field. There are various initiating systems such as photoinitiators (PIs), thermal initiators (TIs), and redox initiators (RIs) for radical polymerization. RIs can initiate the radical polymerization at room or moderate temperature without external thermal or light source. With the aim to decrease the temperature for initiating the polymerization process, there has been a considerable interest in redox initiators these last years.¹ Indeed, RIs doesn't require any thermal or light activation devices, which doesn't influence the penetration of external energies to contribute the desired polymer. These redox initiators are strongly used in the dental field where no thermal polymerization could be used. Generally, RIs are based on thermally unstable peroxide, which is no more desirable today because of potential hazards. One of the well-known RIs is based on the combination of dibenzoyl peroxide (BPO) and tertiary aromatic amine.² This BPO / amine system is widely used in many applications such as dental care. However, BPO is not thermally stable,³ which is required to be stored at relatively low temperature, a fact which impacts product stability and risk of storage costs. Nevertheless, peroxides were found quite useful in many developments. For example, polymerization has been found to be quite effective when using a combination of barbiturate, peroxide, copper and chloride.⁴ Some alternatives are based on the decomposition of hydroperoxides (ROOH) by a transition metal salt. For example, thiourea has been used in combination with an hydroperoxide-polyvinylpyrrolidone complex.⁵ The presence of vanadium as catalyst with thiourea and hydroperoxide in the presence of acidic monomer has been found quite effective.⁶ Following this approach, ascorbic acid, known as natural reductant, has been used instead of thiourea with hydroperoxide-polyvinylpyrrolidone complex in the presence of acidic monomer.⁷

A typical example is the system ROOH/copper salt (Cu(II)). In this system, hydroperoxide reduces Cu(II) to Cu(I),⁸ generating ROO⁻ radical which is known to have a poor initiating ability.⁹⁻¹¹ Then, Cu(I) is oxidized by hydroperoxide, generating RO⁻ radical⁴ which is able to initiate the polymerization reaction.¹¹ In this system, both ROOH and copper salt act as both oxidant and reductant in a Haber-
Weiss cycle, as shown in Scheme 1a. Although more stable than the peroxide/amine systems, the ROOH/Cu(II) system is still suffering from short shelf-life.

Inspired by another interesting system based on saccharin, hydroperoxide, tertiary aromatic amine with transition metal used in anaerobic adhesion industry.¹²⁻¹⁴ we decided to set up a new system as free-peroxide redox initiator. In this novel system, the hydroperoxide in the ROOH/CU(II) system was replaced by two molecules: the first one being a reductant, such as an aromatic amine, and the second one being the oxidant, such as saccharin (see Scheme 1b). We found that this system is prone to initiate the radical polymerization of (meth)acrylates. Therefore, the effect of the oxidant was tested by employing several saccharin derivatives. Some change in the copper salt were also tested as well as some variations of the amine. This allows to assess the reactivity and to relate the latter to the redox potentials of the components. In addition, LC-MS and UV-visible spectroscopy was used to confirm the mechanism of initiation. Moreover, it is shown that the addition of sodium p-toluene sulfinate significantly enhanced the radical polymerization, providing the desirable polymerization time.

<figure><img src="image_2.png" /><figcaption>**Scheme 1.** a) Mechanism for the initiation reaction with hydroperoxide/copper salt system and b) expected mechanism of saccharin/copper salt/aromatic amine system.</figcaption></figure>
## 2. Experimental

### 2.1. Materials

All chemicals were purchased with high purity and used as received. Saccharin and the analogs studied in this work are shown in Scheme 2.

<figure><img src="image_3.png" /><figcaption>**Scheme 2.** Structures of the analogs of saccharin and the amines used in this study.</figcaption></figure>

Saccharin (SAC1, CAS# 81-07-2), di-toluene-4-sulfonamide (SAC2, CAS# 3695-00-9), 3H-1,2-benzodithiol-3-one-1,1-dioxide (SAC3, CAS# 66304-01-6), 2-sulfobenzoic anhydride (SAC4, CAS# 81-08-3), dimethyl-p-toluidine (DMPT, CAS# 99-97-8), sodium p-toluenesulfinate, (pTSS, CAS# 824-79-3), methyl methacrylate (MMA, CAS# 80-62-6), chloroform-d (CAS# 865-49-6), acetonitrile (CAS# 75-05-8), acetonitrile-d (CAS# 2206-26-0), trimethylolpropane triacrylate (TMPTA, CAS# 15625-89-5),
sodium saccharinate (CAS# 128-44-9) and bathophenanthroline (BTP, CAS# 1662-01-7) were purchased from Tokyo Chemical Industry (Tokyo, Japan).

Copper(II) acetylacetonate (Cu(acac)₂, CAS# 13395-16-9), copper (II) acetate monohydrate (Cu(OAc)₂, H₂O, CAS# 6046-93-1), copper(II) 2-ethylhexanoate (Cu(EH)₂, CAS# 149-11-1), copper(II) bromide (CuBr₂, CAS# 7789-45-9), and copper(II) chloride dihydrate (CuCl₂, 2H₂O, CAS# 10125-13-0) were purchased from Fujifilm Wako Pure Chemical Corporation (Osaka, Japan).

1-(4-Methoxy-phenyl)-pyrrolidine (MPP, CAS# 54660-04-7), tetramethylguanidine (TMG, CAS# 80-70-6), 2,2' -(4-methylphenyl-imino)-diethanol) (DEPT, CAS# 3077-12-1) were purchased from Sigma-Aldrich (Missouri, St. Louis, USA).

Copper saccharinate complex was synthesized using a procedure described in the literature.¹⁵

## 2.2. Methods

In order to avoid premature gelation of the polymerizable resin, 2K-system were prepared, one included copper and saccharin (or its analogs), and the other included amine and possible additives. These two formulations were mixed just before analysis with a ratio of 1:1.

Differential scanning calorimetry (DSC 3500 Sirius, Netzsch, Bayern, Germany) was used to characterize polymerization reactions. The samples were kept at 37°C during the measurement to mimic the behavior of the system used in the dental field. From these experiments, the peak time $t_{peak}$ necessary to start the polymerization was obtained, as well as the value of the exotherm. The latter was used to calculate the double bond conversion (DoC) based on the theoretical enthalpy value for a complete conversion of the acrylate double bonds ($ΔH^{theor}=78$ kJ/mol for ethyl acrylate).¹⁶

For Fourier transform infrared spectroscopy, the formulations were assembled at room temperature (20-25°C) and filled into a 2 mm thick silicon mold, and the degree of double bond conversion (DoC)
was measured using a FT-IR Spectrum Two (PerkinElmer, Waltham, MA, USA) in ATR mode. DoC was calculated from the decrease of the height in the C=C absorption peak observed at 1638 cm⁻¹.

$$ \text{DoC (\%)} = \left( 1 - \frac{\text{Height}(t)_{1638}}{\text{Height}(0)_{1638}} \right) \times 100\% $$

where t indicates the measurement time, and 0 stands for t=0 min after mixing the formulations. Height(t)₁₆₃₈ is the peak height at 1638 cm⁻¹ at time t.

For liquid chromatography mass spectrometry (LC-MS), the polymerizations were performed in acetonitrile, using 1wt% of MMA dissolved in MeOH with 10 mol% of Cu(acac)₂, and 1 eq of SAC1, DMPT and pTSS. The analysis was performed on a Waters SQ Detector 2 (Waters, Milford, MA, USA).

¹H-NMR spectra was recorded on a JASCO 280 MHz spectrometer (JASCO, Hachioji, Japan). Materials were dissolved in the solvent chosen from acetonitrile-d or chloroform-d, appropriately.

For UV-visible spectroscopy, copper salts were dissolved in MeOH at a concentration of 5 mmol/L and the other compounds at a concentration of 10 mmol/L and measured by Evolution 201 (Thermofisher, Waltham, MA USA).

# 3. Results and Discussion

## 3.1 Investigation of new peroxide-free redox initiators

The investigated peroxide-free were based on copper salt, tert-aromatic amine, and analogs of saccharin, namely saccharin itself, di-toluene sulfonamide, 3H-1,2-benzodithiol-3-one 1,1-dioxide and 2-sulfobenzoic anhydride (Table 1 and Figure 1).

<caption>**Table 1.** Formulations with copper salt, tert-aromatic amine, saccharin and its analogs (unit: mol).</caption>
<table><thead><tr><th></th><th>E<sub>red</sub> (V/SCE)</th><th>Entry 1</th><th>Entry 2</th><th>Entry 3</th><th>Entry 4</th></tr></thead><tbody><tr><td>TMPTA</td><td></td><td>2</td><td>2</td><td>2</td><td>2</td></tr><tr><td>Cu(AcAc)<sub>2</sub></td><td></td><td>10<sup>-4</sup></td><td>10<sup>-4</sup></td><td>10<sup>-4</sup></td><td>10<sup>-4</sup></td></tr><tr><td>DMPT</td><td></td><td>0.01</td><td>0.01</td><td>0.01</td><td>0.01</td></tr><tr><td>SAC1</td><td>-0.47</td><td>0.01</td><td>-</td><td>-</td><td>-</td></tr><tr><td>SAC2</td><td>-0.34</td><td>-</td><td>0.01</td><td>-</td><td>-</td></tr><tr><td>SAC3</td><td>-0.74</td><td>-</td><td>-</td><td>0.01</td><td>-</td></tr><tr><td>SAC4</td><td>-1.21</td><td>-</td><td>-</td><td>-</td><td>0.01</td></tr><tr><td>DoC (%)</td><td></td><td>61.5</td><td>69.3</td><td>-</td><td>-</td></tr><tr><td>t<sub>peak</sub> (min)</td><td></td><td>2.0</td><td>1.1</td><td>-</td><td>-</td></tr></tbody></table>

As can be seen in Figure 1, radical polymerization exotherms were observed by DSC for only two formulations comprising SAC1 (Entry 1) or SAC2 (Entry 2). It is worth noting that both compounds contain a sulfonamide group and exhibit the highest reduction potentials (-0.47 and -0.34 V/SCE for SAC1 and SAC2, respectively). This is consistent with a described three-component initiating system based on copper, hydroperoxide and sulfonamide.¹⁷ The fastest polymerization was observed with SAC2, with a t<sub>peak</sub> value of 1.1 min and DoC value of 69.3 % (Table 1). The saccharin based system exhibits a slightly lower reactivity, the t<sub>peak</sub> being 2.2 min and the DoC value 61.5 %. In the following, all experiments will be performed with SAC1 because of the too fast reactivity of SAC2.
<figure><img src="image_4.png" /><figcaption>**Fig. 1** Polymerization experiments monitored by DSC in isothermal conditions at 37°C for Entries 1 to 4.</figcaption></figure>

In order to clarify the role of each initiator components, different combinations were studied (Table 2 and Figure SI1). Interestingly, suppressing either the copper salt (Entry 5), saccharin (Entry 6) or the aromatic amine (Entry 7) does not lead any polymerization. This confirms the importance of each compound used in the RI.

<table><caption>**Table 2.** Minimum components for redox radical polymerization.</caption><thead><tr><th></th><th>Entry 1</th><th>Entry 5</th><th>Entry 6</th><th>Entry 7</th></tr></thead><tbody><tr><td>TMPTA</td><td>2</td><td>2</td><td>2</td><td>2</td></tr><tr><td>Cu(acac)<sub>2</sub></td><td>10<sup>-4</sup></td><td>-</td><td>10<sup>-4</sup></td><td>10<sup>-4</sup></td></tr><tr><td>DMPT</td><td>0.01</td><td>0.01</td><td>0.01</td><td>-</td></tr><tr><td>SAC1</td><td>0.01</td><td>0.01</td><td>-</td><td>0.01</td></tr><tr><td>DoC (%)</td><td>61.5</td><td>-</td><td>-</td><td>-</td></tr><tr><td>t<sub>peak</sub> (min)</td><td>2.0</td><td>-</td><td>-</td><td>-</td></tr></tbody></table>
For further investigation, various Cu complexes such as Cu(OAc)₂ monohydrate, CuEH₂, CuSO₄ pentahydrate, CuBr₂, and CuCl₂ dihydrate were investigated to clarify the influence of their ligand to the reactivity (Table 3 and Figure SI2).

<table><caption>**Table 3.** Formulations with various copper salts, amine, and saccharin.</caption><thead><tr><th></th><th>E<sub>red</sub> (V/)</th><th>Entry 1</th><th>Entry 8</th><th>Entry 9</th><th>Entry 10</th><th>Entry 11</th></tr></thead><tbody><tr><td>TMPTA</td><td></td><td>2</td><td>2</td><td>2</td><td>2</td><td>2</td></tr><tr><td>Cu(AcAc)<sub>2</sub></td><td>-1.01</td><td>10<sup>-4</sup></td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Cu(OAc)<sub>2</sub></td><td>-0.5</td><td>-</td><td>10<sup>-4</sup></td><td>-</td><td>-</td><td>-</td></tr><tr><td>Cu(EH)<sub>2</sub></td><td>-0.67</td><td>-</td><td>-</td><td>10<sup>-4</sup></td><td>-</td><td>-</td></tr><tr><td>CuBr<sub>2</sub></td><td>-0.57</td><td>-</td><td>-</td><td>-</td><td>10<sup>-4</sup></td><td>-</td></tr><tr><td>CuCl<sub>2</sub></td><td>-0.47</td><td>-</td><td>-</td><td>-</td><td>-</td><td>10<sup>-4</sup></td></tr><tr><td>DMPT</td><td></td><td>0.01</td><td>0.01</td><td>0.01</td><td>0.01</td><td>0.01</td></tr><tr><td>SAC1</td><td></td><td>0.01</td><td>0.01</td><td>0.01</td><td>0.01</td><td>0.01</td></tr><tr><td>DoC (%)</td><td></td><td>61.5</td><td>23.8</td><td>65.5</td><td>56.5</td><td>26.3</td></tr><tr><td>t<sub>peak</sub> (min)</td><td></td><td>2.0</td><td>6.4</td><td>1.5</td><td>2.3</td><td>5.2</td></tr></tbody></table>

a) not soluble in the electrochemical medium.

All the copper salt tested exhibit some ability to polymerize the medium. Based on this result, the tolerance of this novel redox initiator is wide, and the polymerization time and the intensity are able to be controlled by the choice of copper complex. Interestingly, the reactivity depends on the reduction potential of the copper complex: short t<sub>peak</sub> values are found for copper complexes exhibiting values lower than -0.6 V/SCE (i.e. Cu(EH)₂, Cu(AcAc)₂, CuBr₂).

To expand this redox technology, two kinds of amines, tert-aromatic amine such as DEPT and MPP, and non-nucleophilic base such as TMG were investigated. Formulations including DMPT, DEPT or MPP give rise to polymerization reaction. The lack of reactivity of TMG could be attributed to its poor electron donor and hydrogen donor abilities. The lowest values of t<sub>peak</sub> were obtained for compounds
exhibiting the lowest oxidation potentials. This would indicate that the best candidates are tertiary aromatic amines exhibiting low oxidation potential. However, it should be noted that MPP, although characterized by the fastest polymerization, exhibits a low value of DoC. This could be attributed to a low ability to release a hydrogen after electron transfer due to sterically hindered two-orbital-three-electron stabilization.¹⁸

<table><caption>**Table 4.** Effect of the amine on the polymerization ability.</caption><thead><tr><th></th><th>Eox (V/SCE)</th><th>Entry 1</th><th>Entry 12</th><th>Entry 13</th><th>Entry 14</th></tr></thead><tbody><tr><td>TMPTA</td><td></td><td>2</td><td>2</td><td>2</td><td>2</td></tr><tr><td>Cu(AcAc)<sub>2</sub></td><td></td><td>10<sup>-4</sup></td><td>10<sup>-4</sup></td><td>10<sup>-4</sup></td><td>10<sup>-4</sup></td></tr><tr><td>SAC1</td><td></td><td>0.01</td><td>0.01</td><td>0.01</td><td>0.01</td></tr><tr><td>DMPT</td><td>0.64</td><td>0.01</td><td></td><td></td><td></td></tr><tr><td>DEPT</td><td>0.66</td><td></td><td>0.01</td><td></td><td></td></tr><tr><td>MPP</td><td>0.43</td><td></td><td></td><td>0.01</td><td></td></tr><tr><td>TMG</td><td>1.31</td><td></td><td></td><td></td><td>0.01</td></tr><tr><td>DoC (%)</td><td></td><td>61.5</td><td>48.3</td><td>6.1</td><td>-</td></tr><tr><td>t<sub>peak</sub> (min)</td><td></td><td>2.0</td><td>7.8</td><td>1.7</td><td>-</td></tr></tbody></table>

In order to confirm the fact that the mechanism is based on a redox reaction rather than involving electrophilic/nucleophilic reaction, p-toluenesulfinate (pTSS) was used instead of amine. Sulfinate has been known as a radical polymerization accelerator for many years. For example, it has been found that sulfinate accelerated the conventional radical initiator, BPO-amine in the presence of acidic monomer.¹⁹ pTSS was indeed known a good reductant ($E_{ox}$ = 0.43 V/SCE) and was used in many different initiating systems for free radical reactions.²⁰ Table 5 shows some combinations tested with pTSS as electron donor. As can be seen, pTSS worked well instead of DMPT. On the other hand, in the absence of SAC1 or copper (II), the formulation didn't show any polymerization on DSC (Entries 17 and 16, respectively), similarly to the system based on DMPT (Figure SI5).
<table><caption>**Table 5.** Effect of pTSS on the polymerization ability.</caption><thead><tr><th></th><th>Entry 15</th><th>Entry 16</th><th>Entry 17</th><th>Entry 18</th><th>Entry 19</th><th>Entry 20</th></tr></thead><tbody><tr><td>TMPTA</td><td>2</td><td>2</td><td>2</td><td>2</td><td>2</td><td>2</td></tr><tr><td>Cu(acac)<sub>2</sub></td><td>10<sup>-4</sup></td><td>-</td><td>10<sup>-4</sup></td><td>10<sup>-4</sup></td><td>10<sup>-4</sup></td><td>10<sup>-4</sup></td></tr><tr><td>Saccharin</td><td>0.01</td><td>0.01</td><td>-</td><td>0.01</td><td>0.01</td><td>0.01</td></tr><tr><td>DMPT</td><td>-</td><td>-</td><td>-</td><td>0.01</td><td>0.02</td><td>-</td></tr><tr><td>pTSS</td><td>0.01</td><td>0.01</td><td>0.01</td><td>0.01</td><td>-</td><td>0.02</td></tr><tr><td>DoC (%)</td><td>25.0</td><td></td><td></td><td>70.1</td><td>73.3</td><td>45.7</td></tr><tr><td>t<sub>peak</sub> (min)</td><td>2.2</td><td>-</td><td>-</td><td>0.7</td><td>1.4</td><td>0.7</td></tr></tbody></table>

In comparison to DMPT, the polymerization with pTSS (Entry 20) was accelerated, although that the intensity of the polymerization was decreased based on the height of each peak. Therefore, it could be interesting to study the combination of DMPT and pTSS. Entries 18, 19 and 20 show the results obtained for a combination of both DMPT and pTSS with comparison of each electron donor at the same weight ratio. As can be seen, the combination of 1 wt% of both compounds reacts as well as a formulation containing 2 wt% of DMPT. In comparison to formulations with DMPT or pTSS, respectively, the formulation containing both electron donors showed the highest peak intensity on DSC. The short t<sub>peak</sub> value for pTSS is indicating a fast polymerization. In the case of DMPT, the polymerization proceeds more slowly, possibly due to the inhibiting effect of oxygen. However, in this case, the DoC is higher than for pTSS, a fact that may be attributed to higher yield of initiating radicals. Overall, the used of both coinitiators is quite beneficial as short t<sub>peak</sub> and high DoC were observed.

In order to evaluate more deeply these novel RIs, FT-IR was performed at room temperature. Figure 2 shows that the formulation comprising pTSS started the radical polymerization within 60 seconds, a result close to that obtained by DSC at 37 °C (Entry 18 and 20). On the other hand, the formulation without pTSS (Entry 19) exhibited a slower polymerization, although the final conversion was found to be similar as for Entry 18.
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Double bond conversion profile with time for Entries 18 to 20 recorded at room temperature.</figcaption></figure>

It should be noted that the experimental conditions for DSC and FTIR experiments strongly differ. Indeed, in the case of FTIR, an important oxygen inhibition is possible via the large surface area at contact with air. As a consequence, one can see on Figure 2 that oxygen inhibition slows down the polymerization process when using the amine and limits the final conversion, by contrast to the polymerization operated in the presence of pTSS. This would suggest that pTSS is able to overcome oxygen inhibition.

**3.1 Identification of the initiating radical species**

In order to clarify the structure of the radical species which initiate the polymerization, experiments with low concentration of MMA in organic solvent was carried out. All of components were dissolved completely, the appearance was totally transparent at the beginning of the experiment. Two hours
later, the solution became slightly cloudy, showing that insoluble polymer was formed in methanol.

LC-MS analysis was conducted on the polymer after filtration and further dissolution in acetonitrile.

Complex LC-MS graphs were obtained, from which the identification was focused on SAC1, DMPT, pTSS derived radicals linked to MMA molecules (see SI). From this analysis, three different initiating radicals were detected deriving from: i) *p*-toluene sulfinate, ii) *p*-toluene sulfonate, and iii) dimethyl-*p*-toluidine (Scheme 3). In detail, we have found six kinds of peaks; poly-methyl methacrylate ((P)MMA) with a *p*-toluene sulfinate, (P)MMA with two *p*-toluene sulfinates, (P)MMA with a *p*-toluene sulfonate, (P)MMA with two *p*-toluene sulfonates, (P)MMA with a dimethyl-*p*-toluidine, and (P)MMA with a dimethyl-*p*-toluidine and *p*-toluene sulfinate. It was not possible to find any evidence of MS peaks corresponding to SAC1, suggesting that SAC1 does not participate to the initiation mechanism. As it has been seen that SAC1 is required to run the polymerization, it can be concluded that SAC1 participate to the mechanism for the formation of initiating radicals without leading itself to initiating radicals.

<figure><img src="image_6.png" /><figcaption>**Scheme 3.** Initiator analysis on the polymerization with (P)MMA and peroxide-free redox initiators (n=2 or 3; * standing for H, pTSS derived radicals).</figcaption></figure>

From these experiments, it is not possible to decided which initiating radical is the most favourable for the initiation reaction. These results can also explain the beneficial effect of pTSS in the presence of oxygen (Fig. 2). It is known that oxygen inhibits radical polymerization due to formation of oxy radical which has no ability to initiate the polymerization.²¹ Therefore, subsequent polymerization is stopped. The detection of the initiating species (ii), which includes an oxygen atom, would suggest that sodium *p*-toluene sulfinate radical reacts with oxygen, leading to an efficient initiating radical.
### 3.3 Absorption spectra of copper complexes in the presence of additives

During the course of our experiments, an interesting change in the color of some formulations was observed, which may indicate some change in the copper ligand or some reduction process taking place. We further studied this effect by using bathophenanthroline (BTP) which is known as a ligand for Cu(I), the color of the resulting complex being characteristically brown with a maximum of absorption around 460-470 nm.²² Figure 3a shows the corresponding peak at 470 nm formed from the reduction of Cu(acac)₂ by hydroxy ammonium chloride (HAC), which is known to reduce Cu(II) to Cu(I).

Therefore, BTP was added to a slightly blue colored acetonitrile solution of Cu(acac)₂ with no change in the absorption spectrum. Figure 3a shows that when adding DMPT or pTSS, no change in color was noticed and no peak was detected around 470 nm. These experiments suggest that no reduction of Cu(acac)₂ takes place in the presence of DMPT or pTSS. Interestingly, the color of the solution was turned to emerald green by adding SAC1. However, this last solution didn't show any peak at 470 nm. We interpreted this color change as the formation of Cu(SAC1)₂ from Cu(acac)₂.

Therefore, the synthesized complex Cu(SAC1)₂ was dissolved in acetonitrile in the presence of BPT. Figure 3b shows that in that case, no peak was observed around 470 nm, and a peak was confirmed when HAC is used. Both DMPT or pTSS were found to reduce Cu(SAC1)₂ and reacted with BTP to yield a peak at around 470 nm. Interestingly, when SAC1 was subsequently added to a solution of Cu(II) once reduced, the solution was decolorized and the peak around 460 nm almost disappeared.
<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /><figcaption>**Fig. 3.** UV-visible spectra of a) Cu(acac)₂ and b) Cu(SAC1)₂ in the presence of different additives.</figcaption></figure>

From all the results obtained above, novel peroxide-free RIs based on saccharin, dimethyl-*p*-toluidine, and transition metal showed excellent redox polymerization at room temperature. The formulation comprises copper(II) and saccharin which may forms a copper/saccharin complex. Single electron transfer involving Cu/saccharin complex and DMPT or pTSS as reductant generates radical species detected by LC-MS analysis. The reduced Cu(I) is oxidized by saccharin to form Cu(II) again (Scheme 4).
<figure><img src="image_9.png" /><figcaption>**Scheme 4.** Suggested reaction mechanism for radical species.</figcaption></figure>

**4. Conclusion**

In this work, peroxide-free radical initiators based on saccharin, aromatic amine and transition metal is established. The system was found to perform quite well with double bond conversion higher than 60%. Remarkably, the use of an additive such as sodium *p*-toluene sulfinate accelerates the redox radical polymerization at room temperature. Identification of the role of the different species was made by use of LC-MS spectroscopy, although the role of the copper salt was assessed by UV-visible absorption spectroscopy in the presence of bathophenanthroline. The proposed reaction mechanism involves an oxidation/reduction cycle of copper salt using either saccharine and electron acceptor and aromatic amine as electron donor. This new redox initiating system would strongly contribute to the development of organic peroxide-free RIs in the future.

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Fast polymerization

<figure><img src="image_10.png" /></figure>

Human body temperature

New peroxide-free redox initiator for radical polymerization