# Calcium/lithium and calcium/magnesium systems for the anionic copolymerization of butadiene and styrene

Thomas La Porta, Renaud Perrin, Mélanie Ray, Simon Harrisson, Benoit Gadenne, Charlotte Dire, Stéphane Carlotti

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Thomas La Porta, Renaud Perrin, Mélanie Ray, Simon Harrisson, Benoit Gadenne, et al., Calcium/lithium and calcium/magnesium systems for the anionic copolymerization of butadiene and styrene. Polymer, 2025, 332, pp.128560. 10.1016/j.polymer.2025.128560. hal-05177010

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# Calcium/lithium and calcium/magnesium systems for the anionic copolymerization of butadiene and styrene

Thomas La Porta<sup>a,b</sup>, Renaud Perrin<sup>b</sup>, Mélanie Ray<sup>b</sup>, Simon Harrison<sup>a,b</sup>, Benoit Gadenne<sup>b</sup>, Charlotte Dire<sup>b</sup>, Stéphane Carlotti<sup>a,*</sup>

## ABSTRACT

The synthesis of styrene-butadiene copolymers is widely used in rubber materials. The microstructure of polybutadiene segments and the amount of styrene incorporated into it are key parameters to tune the rubber properties. Copolymerization using an anionic initiator such as an alkyllithium tends to produce block copolymers in an apolar medium. Polar additives can be used to increase the amount of styrene into the copolymer but with an increase amount of 1,2 units of the polybutadiene part. Herein, we report a controlled anionic copolymerization of butadiene and styrene using a bimetallic [(THF)₂Ca{N(SiMe₃)₂}₂]:[n-BuLi] initiator in cyclohexane. The insertion of styrene units into polybutadiene chains is increased due to a decrease of the butadiene reactivity ratio by the presence of the calcium complex at the chain ends. The polybutadiene part is mainly composed of 1,4-*trans* units and keeps low vinyl content. A less efficient lithium-free approach using [(THF)₂Ca{N(SiMe₃)₂}₂]:[(n,s)-Bu₂Mg] initiator for the butadiene and styrene copolymerization is also discussed.

## 1. Introduction

Butadiene monomers are widely used to prepare rubber materials and the copolymerization methodology is known as a powerful route to modulate and control the properties of the materials [1–4]. The use of anionic initiators, such as alkyllithium, for the copolymerization of butadiene with other monomers like styrene tend to prepare block copolymers in apolar medium, the first block being mainly composed of butadiene units [5,6]. Thermomechanical properties of the copolymers are mainly dependent on styrene contents and the distribution of styrene units into the copolymers (block, tapered, statistical) [7–10]. Controlling those parameters are widely studied for decades for specific purposes, in particular rubber materials [1,5,11,12]. The addition of polar additives and Lewis bases enables to increase the styrene content resulting in more statistical copolymers. As an example, the addition of diglyme or TMEDA increased the styrene content in the polybutadiene first block [13]. It was reported that for an initial butadiene/styrene composition of 65/35, the copolymer composition at 40 % of conversion was 5–6 % of styrene units at 32 °C and 66 °C. The addition of diglyme, with [diglyme]/[BuLi] = 0.1/1, increased the styrene content at 12 % at 32 °C and 15 % at 63 °C at the same conversion. For [diglyme]/[BuLi] = 1, the styrene content was 23 % and 29 % at 32 °C and 66 °C respectively. Similar values were found with the use of TMEDA. The microstructure of polybutadiene units was also modified by the amount of

additive. In the presence of one equivalent of diglyme or TMEDA, 1,2 units content reached 50 %. Less 1,2 units were also reported using THF. Indeed, styrene-butadiene copolymers with 34 % of styrene content and 15 % of 1,2 units were prepared in heptane with [THF]/[Li] = 0.42. Increasing the [THF]/[Li] ratios until 2 gave the same styrene incorporation (around 30 %) with an increased 1,2 content (29 %). As also well known, polybutadiene microstructure has a strong influence on thermomechanical properties [12,14], the control of 1,4-*trans*, 1,4-*cis* and 1,2 units being a major issue in the styrene-butadiene copolymerization [1,5,15]. Styrene-diene copolymers with high 1,4-*cis* [16–18] or 1,4-*trans* [19–22] content was reported in the literature using a coordinative mechanism with relatively broad chains distributions. Anionic mechanism is known to produce such types of copolymers with narrow dispersities and controlled architectures. However, producing high 1, 4-*trans* styrene-butadiene copolymers is quite limited, especially due to a lack of control and livingness of the polymerization. We recently published the efficient use of calcium-based initiating systems to prepare high 1,4-*trans* polybutadiene [23,24]. Herein, we report the styrene-butadiene anionic copolymerization using those systems. The styrene content, reactivity ratios, control and microstructure are presented and discussed.

<footnote><sup>a</sup> Univ. Bordeaux, CNRS, Bordeaux INP, LCPO, UMR 5629, Pessac, F-33600, France</footnote>

<footnote><sup>b</sup> Manufacture Française des Pneumatiques Michelin Centre de Technologie / Ladoux, 23 Place des Carmes Déchaux, cedex 9, 63040, Clermont-Ferrand, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: carlotti@ensmac.fr (S. Carlotti).</footnote>
## 2. Experimental section

### 2.1. Materials

Butadiene (99.5 % from Shell Chemicals), distilled under vacuum and stored under vacuum at room temperature in graduated glass tubes until use. Styrene (99 % from Sigma Aldrich) was first dried over freshly crushed CaH₂, stored over (n,s)-dibutylmagnesium, and distilled before use. Hexamethyldisilazane (HMDS, 99 %), calcium granular (99 %) and (n,s)-dibutylmagnesium ((n,s)-Bu₂Mg) solution in hexane (0.7 mol L⁻¹) were purchased from Sigma Aldrich and used without further purification. Cyclohexane (99 %, J.T. Baker) was purified by polystyryllium seeds, distilled under vacuum, and then stored in graduated glass tubes under vacuum until use. THF (Sigma Aldrich, 99.9 %) was dried over sodium/benzophenone, distilled under vacuum and stored under vacuum at room temperature in graduated glass tubes until use. Calcium bis [bis(trimethylsilyl)amide] [(THF)₂Ca{N(SiMe₃)₂}] was synthesized according to literature [23].

[(THF)₂Ca{N(SiMe₃)₂}] : ¹H NMR (400 MHz, C₆D₆, 25 °C): δ 3.54 ppm (t, 8H, THF), δ 1.21 ppm (t, 8H, THF), δ 0.37 ppm (s, 36H, N (SiMe₃)₂). ¹³C NMR (400 MHz, C₆D₆, 25 °C): δ 69.7, 24.9 ppm (THF), δ 5.87 ppm (N(SiMe₃)₂).

### 2.2. Procedures

All copolymerizations were performed at 40 °C or 60 °C under argon in a high-pressure glass reactor equipped with a magnetic stirrer and fitted with PTFE stopcocks previously flamed under vacuum. The calcium complex [(THF)₂Ca{N(SiMe₃)₂}] was weighed in a glovebox, then added in a schlenk previously flamed under vacuum to make a solution in cyclohexane. In another schlenk previously flamed under vacuum, styrene, butadiene and cyclohexane were introduced under vacuum through connected glass tubes to obtain a solution at the desired concentration. A desired amount of calcium was injected in this schlenk followed by n-BuLi in hexane via a syringe under argon flow (or (n,s) Bu₂Mg in hexane for Ca:Mg systems). After a desired reaction time, degassed methanol was added to stop the reaction, and the possible remaining monomers and cyclohexane were stripped off under vacuum. Conversions were determined using ¹H NMR (integration of butadiene vs. polybutadiene and styrene vs. polystyrene) with an aliquot taken during the run of polymerization (Fig. S1 for details).

Styrene-butadiene copolymer: ¹H NMR (400 MHz, CDCl₃, 25 °C): δ 6.6–7.2 ppm (m, 5H, aromatic protons from polystyrene), δ 5.57 ppm (m, 1H, vinyl proton from 1,2 unit), δ 5.43 ppm (s, 2H, vinyl protons from 1,4-trans unit), δ 5.39 ppm (s, 2H, vinyl protons from 1,4-cis unit), δ 4.96 ppm (m, 2H, vinyl protons from 1,2 unit), δ 2.6, 2.3, 2.05–2.08, 1.9 ppm (5s, 8H, protons from 1,4 unit).

As an example, for [Ca]/[Li] = 0.5 (molar ratio), and styrene/butadiene = 50/50, 32 mg of [(THF)₂Ca{N(SiMe₃)₂}] (63 µmol) was introduced into the glass reactor followed by 9 mL of cyclohexane. 1.3 mL of styrene (11.4 mmol) was introduced into the glass reactor followed by 1 mL of liquid butadiene (11.4 mmol). 50 µL of nBuLi (125 µmol) solution in hexane (2.5 M) was then added via a syringe under argon flow to run the polymerization, then heated at 40 °C.

### 2.3. Analysis

Polymer molar masses were determined by Size Exclusion Chromatography (SEC) using tetrahydrofuran (THF) as the eluent. Measurements in THF were performed on an Ultimate 3000 system equipped with differential refractive index detector dRI from Wyatt Technology. Polymers were separated on three G2000, G3000 and G4000 TOSOH HXL gel columns (300 × 7.8 mm) (exclusion limits from 1000 Da to 400 000 Da) at a flowrate of 1 mL/min. Columns temperature was held at 40 °C. Polybutadiene molar masses were determined using polystyrene standards.

Liquid-state ¹H NMR (400.2 MHz) and ¹³C NMR (Inversed gated decoupling, 100.6 MHz, D1 = 10 s, 4096 scans) spectra were recorded at 298 K on a Bruker Avance 400 spectrometer in CDCl₃ or in C₆D₆. A detailed description of the method used to determine polybutadiene microstructure is presented in ESI Fig. S2.

## 3. Results and discussions

[(THF)₂Ca{N(SiMe₃)₂}] complex was first synthesized according to our previous study [23] based on the work of Krieck et al. [25]. Styrene-butadiene copolymerizations were carried out in the presence of n-BuLi and the calcium complex at different [Ca]/[Li] molar ratios (0, 0.25 and 0.5) and with different proportions of styrene. The results obtained in the presence of calcium are compared with n-BuLi initiator alone. The evolutions of conversion as a function of polymerization time for an equimolar styrene-butadiene composition at 40 °C and 60 °C were plotted on Fig. 1a (and Fig. S3 for 10/90 and 25/75 compositions).

Styrene-butadiene copolymerization proceeds to full conversion of both monomers, regardless of [Ca]/[Li] ratios and the initial proportion of styrene in the mixture (Fig. 1a and S3). The hierarchy of polymerization times as a function of [Ca]/[Li] molar ratios is maintained in copolymerization as observed with the homopolymerization of butadiene [23]. The two graphs show a break in the slope at around 50 % conversion in the case of n-BuLi-initiated copolymerization. This is explained by the reaction of butadiene in the first part, followed by the styrene polymerization with a slight induction time. The resulting copolymer is a block-like structure as known with an anionic mechanism [5]. The slope break is less visible in the case of calcium initiating systems. A change in the butadiene/styrene reactivity ratios can therefore be envisaged. As shown in Fig. 1b, there is more polymerized styrene at equivalent butadiene conversion using Ca:Li systems.

The reactivity ratios of styrene-butadiene copolymerizations in cyclohexane as a function of the amount of calcium were then determined using the Meyer-Lowry equation, which is an integrated form of the Mayo-Lewis equation (see ESI Fig. S4, Equations S4 and S5). Several styrene-butadiene copolymerizations were carried out, with and without calcium, and with different initial monomer proportions, i.e. 10 %, 25 % and 50 % of styrene content. Reaction kinetics were monitored by proton NMR analysis to determine the conversion of butadiene and styrene, the proportion of each monomer and the amount of styrene incorporated into the copolymer. With these data, it was possible to determine the reactivity ratios for each system using a mathematical solver [26,27] (Table 1 and Fig. 2, S5, S6 and S7).

These reactivity ratios indicate that both styrenyl and butadienyl chain ends have a strong preference to react with butadiene. As a result, the copolymer formed in the initial stages of the reaction consists mainly of butadiene units, with a low level of styrene incorporation. When the butadiene has been consumed, the remaining styrene polymerizes, resulting in a pronounced gradient or even block-like copolymer. Increasing the Ca:Li ratio results in an increased trend for butadienyl chain ends to react with styrene (decrease in rB with a value around 7 instead of 26), and a corresponding increase in the styrene content of the copolymer formed in the initial part of the reaction. However, beyond the model used, experimental errors may occur but appear identical for all systems, permitting comparison and discussion, especially as the trend is clear and marked (Fig. 2 and S5). Based on our previous work using Ca:Li systems [23], two active bonds coexist within the bimetallic complex i.e. C–Ca bond and C–Li. The rapid equilibrium between these species allows the formation of homopolybutadiene with low dispersities. The structure of the complex and the difference of reactivities between C–Li and C–Ca bonds account for variations in the reactivity ratios in the copolymerization with styrene, in particular with high [Ca]/[Li] ratios where C–Ca bonds are predominant.

The microstructure of the polybutadiene within the styrene-butadiene copolymers was determined by quantitative carbon NMR. The results are reported in Table 2.
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** a) Evolution of conversion as a function of polymerization time - b) Evolution of styrene conversion (between 0 and 40 %) with butadiene conversion. Styrene-butadiene (50/50) copolymerization initiated by [(THF)Ca{N(SiMe₃)₂}₂]:n-BuLi. ([Ca]/[Li] = 0, 0.25, 0.5; 40 °C and 60 °C; cyclohexane; [Styr]₀ = [But]₀ = 1.25 mol.L⁻¹; [n-BuLi]₀ = 13.8 mmol.L⁻¹)].</figcaption></figure>

<caption>**Table 1**</caption>

Point estimates for reactivity ratios of styrene-butadiene copolymerization initiated by n-BuLi in presence of increasing amounts of [(THF)₂Ca{(N(SiMe₃)₂)}₂] in cyclohexane at 40 °C.

<table><thead><tr><th>[Ca]/[Li]</th><th>r<sub>S</sub></th><th>r<sub>B</sub></th></tr></thead><tbody><tr><td>0<sup>a</sup></td><td>≈0</td><td>26</td></tr><tr><td>0.25</td><td>≈0</td><td>7.4</td></tr><tr><td>0.5</td><td>≈0</td><td>7.5</td></tr><tr><td>0.7</td><td>≈0</td><td>5.4</td></tr></tbody></table>

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** 95 % joint confidence interval (in red) for reactivity ratios of butadiene and styrene initiated by a) n-BuLi and b) [(THF)Ca{N(SiMe₃)₂}₂]:n-BuLi ([Ca]/[Li] = 0.5). Internal contours indicate 50 % (green), 70 % (blue), and 90 % (orange) joint confidence intervals. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>

<caption>**Table 2**</caption>

Microstructures of styrene-butadiene copolymers obtained by n-BuLi in the presence of increasing amount of [(THF)₂Ca{(N(SiMe₃)₂)}₂] in cyclohexane at 40 °C.

<table><thead><tr><th>[Ca]/[Li]</th><th>1,2 (%)</th><th>1,4-trans (%)</th><th>1,4-cis (%)</th></tr></thead><tbody><tr><td>0</td><td>6</td><td>53</td><td>41</td></tr><tr><td>0.25</td><td>12</td><td>67</td><td>21</td></tr><tr><td>0.5</td><td>12</td><td>70</td><td>18</td></tr></tbody></table>

The ratio of 1,4-trans units increases with the molar ratio [Ca]/[Li] as described with butadiene homopolymerization initiated by these systems [23]. The 1,2-units content is slightly increased (12 %) and is attributed to styrene which probably interacts with the active complex by its aromatic ring with π interactions making complexed THF more labile, which slightly modifies the microstructure obtained.

The control of the styrene-butadiene copolymerization in cyclohexane initiated by [(THF)₂Ca{(N(SiMe₃)₂)}₂]:n-BuLi system was investigated and is presented **Table 3**.

All styrene-butadiene copolymerizations were carried out in a controlled way up to full conversion. The experimental molar masses obtained by SEC are given for information only, as they are based on a polystyrene calibration. Nevertheless, they are all in the expected range with low dispersities, close to 1, with a slight increase for the ratio [Ca]/[Li] = 0.5 and in the case of copolymers initiated by n-BuLi alone, which are relatively broad. Chromatograms obtained by SEC analysis for the latter show that two polymer populations coexist (Fig. 3a). This phenomenon can be explained by the large difference in reactivity ratios, as well as by the high speed of styrene homopolymerization. Indeed, once butadiene is almost entirely consumed, some styryllithium-terminated chains are expected to start the homopolymerization of styrene to form the second block, while butadienyllithium-terminated chains have not yet reacted with styrene units ($r_B = 26 \gg 1$). This hypothesis is

<footnote>a Initiated by n-BuLi alone.</footnote>
<table><caption>**Table 3**

Styrene-butadiene copolymerization initiated by Ca:Li systems with different monomers and [Ca]/[Li] ratios in cyclohexane.</caption><thead><tr><th>[Ca]<br>[Li]</th><th>[But]<br>[Styr]</th><th>T (°C)</th><th>Conv.<sup>a</sup> (%)</th><th>M&#773;<sub>n,th</sub><sup>b</sup><br>(styr)<br>(g.mol<sup>-1</sup>)</th><th>M&#773;<sub>n,th</sub><sup>b</sup><br>(but)<br>(g.mol<sup>-1</sup>)</th><th>M&#773;<sub>n,exp</sub><sup>c</sup><br>(g.mol<sup>-1</sup>)</th><th>D</th></tr></thead><tbody><tr><td>0</td><td rowspan="3">90/10</td><td>60</td><td>100</td><td>2000</td><td>10 000</td><td>15 300</td><td>1.84</td></tr><tr><td>0.2</td><td>40</td><td>100</td><td>2000</td><td>10 000</td><td>17 000</td><td>1.05</td></tr><tr><td>0.5</td><td>40</td><td>100</td><td>2000</td><td>10 000</td><td>13 900</td><td>1,16</td></tr><tr><td>0</td><td rowspan="6">75/25</td><td>40</td><td>100</td><td>5800</td><td>10 000</td><td>19 000</td><td>1.22</td></tr><tr><td>0</td><td>60</td><td>100</td><td>5800</td><td>10 000</td><td>17 500</td><td>1.17</td></tr><tr><td>0.2</td><td>40</td><td>100</td><td>5800</td><td>10 000</td><td>16 200</td><td>1.06</td></tr><tr><td>0.2</td><td>60</td><td>100</td><td>5800</td><td>10 000</td><td>18 000</td><td>1.06</td></tr><tr><td>0.5</td><td>40</td><td>100</td><td>5800</td><td>10 000</td><td>19 000</td><td>1.17</td></tr><tr><td>0.5</td><td>60</td><td>100</td><td>5800</td><td>10 000</td><td>18 600</td><td>1.30</td></tr><tr><td>0</td><td rowspan="6">50/50</td><td>40</td><td>100</td><td>10 200</td><td>5000</td><td>16 800</td><td>1.42</td></tr><tr><td>0</td><td>60</td><td>100</td><td>10 200</td><td>5000</td><td>18 600</td><td>1.18</td></tr><tr><td>0.2</td><td>40</td><td>100</td><td>10 200</td><td>5000</td><td>17 200</td><td>1.06</td></tr><tr><td>0.2</td><td>60</td><td>100</td><td>10 200</td><td>5000</td><td>16 900</td><td>1.05</td></tr><tr><td>0.5</td><td>40</td><td>100</td><td>10 200</td><td>5000</td><td>18 900</td><td>1.13</td></tr><tr><td>0.5</td><td>60</td><td>100</td><td>10 200</td><td>5000</td><td>17 900</td><td>1.34</td></tr></tbody></table>

a Determined by ¹H NMR: butadiene vs polybutadiene and styrene vs. polystyrene.

b Calculated with one polymer chain per n-BuLi.

c Number-average molar mass estimated by SEC on the basis of polystyrene standards.

supported by the fact that the higher the initial molar styrene amount is, the more pronounced is the phenomenon (Fig. 3a).

The use of the calcium:lithium bimetallic systems for the styrene-butadiene copolymerization avoids this phenomenon by facilitating the butadiene-styrene cross-over reaction, even for an initial styrene composition of 50 % as shown in chromatograms obtained by SEC analysis of copolymers initiated by the [(THF)₂Ca{(N(SiMe₃)₂)₂]:n-BuLi in cyclohexane (Fig. 3b). A comparison with the addition of polar

additives can be made as such molecules modifies reactivity ratios. The Ca:Li complex is also shown to slightly increase the vinyl content.

A lithium-free system using a calcium-magnesium derivative was also shown to homopolymerize butadiene [24]. The copolymerization of styrene-butadiene using this initiator was also investigated using [(THF)₂Ca{N(SiMe₃)₂}₂]:n(s)-Bu₂Mg ([Ca]/[Mg] = 0,75) system in cyclohexane at 60 °C. This system did not permit styrene-butadiene copolymerization up to full conversion. Despite a reaction time of

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** a) SEC chromatograms of styrene-butadiene copolymers initiated by n-BuLi in cyclohexane at different initial compositions of monomers. b) SEC chromatograms of styrene-butadiene copolymers initiated by [(THF)₂Ca{(N(SiMe₃)₂)₂]:n-BuLi systems in cyclohexane at different [Ca]/[Li] ratios for equimolar ratio of monomers.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Evolution of conversion as a function of polymerization time for styrene-butadiene (50/50) copolymerization initiated by [(THF)Ca{N(SiMe<sub>3</sub>)<sub>2</sub>}<sub>2</sub>]:(n,s)-Bu<sub>2</sub>Mg ([Ca]/[Mg] = 0.75) and by n-BuLi alone. (Cyclohexane, [Styr]<sub>0</sub> = 0.3 mol.L<sup>-1</sup>, [But]<sub>0</sub> = 2.5 mol.L<sup>-1</sup>, [n-BuLi]<sub>0</sub> = 13.8 mmol.L<sup>-1</sup> = [(n,s)-Bu<sub>2</sub>Mg]<sub>0</sub>.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Evolution of styrene conversion between 0 and 40 % with butadiene conversion for styrene-butadiene (10/90) copolymerization initiated by [(THF)Ca{N(SiMe<sub>3</sub>)<sub>2</sub>}<sub>2</sub>]:n-BuLi ([Ca]/[Li] = 0 et 0.5) and [(THF)Ca{N(SiMe<sub>3</sub>)<sub>2</sub>}<sub>2</sub>]:(n, s)-Bu<sub>2</sub>Mg ([Ca]/[Mg] = 0.75). (Cyclohexane, [Styr]<sub>0</sub> = 0.3 mol.L<sup>-1</sup>, [But]<sub>0</sub> = 2.5 mol.L<sup>-1</sup>, [n-BuLi]<sub>0</sub> = [(n,s)-Bu<sub>2</sub>Mg]<sub>0</sub> = 13.8 mmol.L<sup>-1</sup>).</figcaption></figure>

several days, styrene polymerization does not exceed 60 % (Fig. 4) but the observed red color of the reaction medium suggests that the active species are still present.

Although the copolymerization process does not allow full polymerization of styrene, this system also appears to modulate and reduce the reactivity ratio of butadiene. In other words, it enables styrene to be better incorporated into the copolymer as shown in Fig. 5, similarly to the Ca:Li system previously presented.

To better understand why full conversion of styrene monomer is not reached, styrene homopolymerization was studied using the calcium: magnesium system. Styrene polymerization was first studied in apolar solution, i.e. in cyclohexane or toluene. At 25 °C, polymerization did not take place (or was very slow) and required to be heated. Various styrene polymerizations were therefore carried out at 80 °C (Table S1). Full conversion was not achieved after 5 days and broad dispersities were obtained despite a red color of the medium, characteristic of residual active species. We previously shown that Ca:Li systems are more reactive than Ca:Mg systems [24].

To rationalize the low reactivity observed, the hypothesis of a carbanion stabilized by the aromatic ring (by delocalization of the anionic charge to the *ortho* and *para* positions) and the calcium complex (by the stabilizing orbital interactions of calcium and σ* Si–Me bonds) is proposed in Fig. 6. To support this hypothesis, it was shown that less conversion was reached when the [Ca]/[Mg] ratios were increased (Table S2).

Decreasing the amount of calcium as compared to magnesium was

<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Stabilization of the anionic charge by calcium complex within σ* interaction of Si–Me bonds.</figcaption></figure>

shown to increase the conversion of styrene (e.g. 83 % at [Ca]/[Mg] = 0.4, Table S2), thus supporting the hypothesis of the stability provided by the calcium complex of the anionic charge delocalized into the aromatic ring at the end of the active chains. Nevertheless, full conversion was still not reached and broad distributions were observed.

## 4. Conclusion

The copolymerization of styrene and butadiene can be achieved in a controlled way up to full conversion using [(THF)<sub>2</sub>Ca{N(SiMe<sub>3</sub>)<sub>2</sub>}<sub>2</sub>:n-BuLi systems. A blocky structure is still observed, but as main differences with copolymers prepared from n-BuLi alone, narrow chains distributions and increased contents of styrene into polybutadiene blocks can be obtained. Indeed, the alkaline earth-based complex decreases the butadiene reactivity ratio. Moreover, the microstructure of polybutadiene is mainly composed of 1,4-trans units (65–70 %) with about 12 % of 1,2 units which is unusual for an anionic system. A lithium-free system based on a Ca:Mg complex is shown to copolymerize butadiene and styrene with limited control. Full conversion of styrene monomers is not reached and high dispersities are obtained. The stabilization of active species by the calcium complex via the stabilizing orbital interactions of calcium and σ* Si–Me bonds is suggested.

### CRediT authorship contribution statement

**Thomas La Porta:** Writing – original draft, Visualization, Validation, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Renaud Perrin:** Visualization, Supervision, Methodology, Formal analysis, Conceptualization. **Mélanie Ray:** Visualization, Supervision, Methodology, Formal analysis, Conceptualization. **Simon Harrisson:** Visualization, Validation, Investigation, Formal analysis, Data curation. **Benoît Gadenne:** Validation, Supervision, Resources, Project administration, Methodology, Funding acquisition, Conceptualization. **Charlotte Dire:** Writing – original draft, Validation, Supervision, Resources, Project administration, Methodology, Conceptualization. **Stéphane Carlotti:** Writing – original draft, Visualization, Validation, Supervision, Resources, Project administration, Methodology, Funding acquisition, Conceptualization.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.polymer.2025.128560.
# Data availability

Data will be made available on request.

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