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# New Generation of Thermoplastic Elastomers from Narrowly Dispersed All-(Meth)acrylic ABA Triblock Copolymers Made by RAFT

Maksym Odnoroh, Oleksandr Ivanchenko, Guillaume Fleury, Marc Guerre, Mathias Destarac

► To cite this version:

Maksym Odnoroh, Oleksandr Ivanchenko, Guillaume Fleury, Marc Guerre, Mathias Destarac. New Generation of Thermoplastic Elastomers from Narrowly Dispersed All-(Meth)acrylic ABA Triblock Copolymers Made by RAFT. Macromolecules, 2024, 57 (20), pp.9565-9575. 10.1021/acs.macromol.4c01233 . hal-04829948

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Submitted on 29 Oct 2025

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# New generation of thermoplastic elastomers from narrowly dispersed all-(meth)acrylic ABA triblock copolymers made by RAFT

Maksym Odnoroh,[a] Oleksandr Ivanchenko,[a] Guillaume Fleury,[b] Marc Guerre,[a]* Mathias Destarac[a]*

[a] Laboratoire SOFTMAT, CNRS UMR 5623, Université of Toulouse, Université Toulouse III-Paul Sabatier, 118 route de Narbonne, F-31062 Toulouse, France

[b] Université de Bordeaux, CNRS, Bordeaux INP, LCPO, UMR 5629, F-33600, Pessac, France

E-mail: marc.guerre@cnrs.fr; mathias.destarac@univ-tlse3.fr

**Keywords:** RAFT, trithiocarbonate, thermoplastic elastomer, triblock copolymer, acrylic

**For table of contents use only**

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<figure><img src="image_3.png" /></figure>

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MMA

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /></figure>

All-meth(acrylic) triblock copolymer

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

iBoMA

<figure><img src="image_9.png" /></figure>

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

nBA

✓ $M_n > 100$ kg mol⁻¹, $D < 1.10$

✓ Tunable Tgs

✓ Stable after 5 cycles at 180 °C for 30 min

✓ TPE properties

## Abstract

Symmetrical trithiocarbonate bis(2-cyanopropan-2-yl)trithiocarbonate (TTC-bCP) was investigated in reversible addition-fragmentation chain transfer (RAFT) polymerization for the synthesis of a library of well-defined all-meth(acrylic) ABA triblock copolymers with thermoplastic elastomeric (TPE) properties. In addition to standard monomers such as methyl methacrylate (MMA) and *n*-butyl acrylate (*n*BA), bio-based monomers such as isobornyl methacrylate (*i*BoMA) and tetrahydrofurfuryl acrylate (THFA) were used. High molar masses (>$ 100$ kg mol⁻¹) with low dispersities (< 1.10) were achieved and variation of the hard block composition between MMA and *i*BoMA allowed to tune the service temperature between 120 and 180 °C. The obtained materials have been shown to exhibit invariant storage modulus (G') in the MPa range after 5 cycles of recycling at 180 °C for 30 min.

## Introduction
Thermoplastic elastomers (TPEs) share the advantages of both rubbery and thermoplastic materials. Although discovered in the 1950s,¹,² they recently gained increasing attention as they exhibit very competitive properties while still being recyclable.³⁻⁵ TPEs are widely used in various sectors of modern production as adhesives, automotive components, coatings, fibers, electronics, etc.⁶,⁷ Almost all TPEs contain two or more distinct polymeric phases, encompassing incompatible soft (glass transition temperature ($T_g$) < RT) and hard ($T_g$ or melting temperature ($T_m$) > RT) phases that form nanodomains with controlled sizes and geometries.⁸⁻¹⁰ Hard polymer domains function as thermally stable anchors. The other phase is constituted by soft amorphous segments, which contributes to the rubbery properties. From this segregated state, a homogeneous material can be reformed provided that the polymer is heated above its characteristic order-disorder transition temperature ($T_{ODT}$). The synthesis of these elastomers, either by chain-growth or step-growth polymerization, results in a wide spectrum of materials. They usually consist of copolymers with various macromolecular structures, e.g. random,¹¹ graft,¹² star¹³ and ABA (hard-soft-hard) triblock copolymers.¹⁴ Varying the ratio of hard and soft segments allows the design of materials with tailor-made properties.

Styrenic block copolymers (SBCs) are a well-known family of TPEs that are industrially available under the Kraton® trademark.¹⁵ SBCs are synthesized via sequential anionic copolymerization of styrene and dienes and have found interest in applications owing to their unique properties and low cost. However, they suffer from several drawbacks such as sensitivity to UV and oxidation due to the presence of unsaturations in the rubbery phase.⁴ In addition, this class of TPEs has a limited service temperature range due to the relatively low $T_g$ of the styrenic phase (≈ 100 °C). Some efforts have been undertaken to increase the upper service temperature of SBCs by introducing adamantyl¹⁶⁻¹⁸ or 1,1-diphenylethylene¹⁹,²⁰ groups, which changes the polymerization conditions and can have a significant influence on the interaction parameter, $\chi$, with respect to polydienes. In this regard, poly(meth)acrylate-based TPEs have the potential to replace traditional SBCs for some applications, where optical properties and durability are important, as well as the need to increase the upper service temperature without impairing the elastomeric properties. High $T_g$ materials can be more easily
obtained by statistical copolymerization of common MMA monomer ($T_g$ (PMMA) ≈ 100 °C)²¹ with commercially available methacrylates of higher $T_g$ such as isobornyl methacrylate (iBoMA, $T_g$ (iBoMA) ≈ 190-200 °C).²² Regarding the soft block, poly(n-butyl acrylate) (PnBA, $T_g$ (PnBA) ≈ -54 °C)²³ is widely used in the context of all-(meth)acrylic TPEs.²⁴

These materials were recently developed on the industrial scale by Kuraray using living anionic polymerization (LAP).²⁵ There were several reports for the synthesis of such TPEs by reversible-deactivation radical polymerization (RDRP) methods, such as nitroxide-mediated polymerization (NMP) by Arkema,²⁶ atom transfer radical polymerization (ATRP),²⁷ and reversible addition-fragmentation chain transfer (RAFT)²⁸,²⁹ polymerization. However, RDRP methods were unable to compete with the LAP process, which showed superior control over the macromolecular architecture and resulted in polymers with lower dispersities.²⁷,²⁸

Until recently, all the RAFT approaches considered to produce (meth)acrylic hard-soft-hard ABA triblocks suffered from strong limitations due to the low reactivity of available symmetrical RAFT agents towards methacrylates. Indeed, trithiocarbonates (TTCs) with tertiary leaving groups such as carboxy or ester functions resulted into relatively ill-defined triblock copolymers with dispersity $D > 1.5$.²⁹⁻³² Recently, our group filled this gap by synthesizing a new symmetrical trithiocarbonate (TTC-bCP) comprising 2-cyanopropan-2-yl groups known to provide an optimal fragmentation rate for methacrylate monomers.³³ For the first time, a low dispersity ($D < 1.1$) PMMA-b-PnBA-b-PMMA triblock copolymer with a relatively high number-average molar mass ($M_n$) value was synthesized by a convergent RAFT process in two steps. The copolymer exhibited a dispersity comparable to those of counterparts derived from living anionic polymerization.

In this work, the TTC-bCP RAFT agent was used to synthesize all-(meth)acrylic triblock copolymers (Figure 1) with tunable hard and soft phases with enhanced service temperature and bio-based content. To do so, iBoMA was used either as single hard monomer or as comonomer with MMA for the polymerization of the hard block. In addition, the bio-based tetrahydrofurfuryl acrylate (THFA) was
evaluated as soft monomer. Narrowly dispersed methacrylate-acrylate-methacrylate triblock copolymers of controlled high molar masses were synthesized, and resulted into materials with modulated T<sub>g</sub> values and stable TPE properties over an extended range of temperature inherent to the formation of segregated microdomains.

<figure><img src="image_11.png" /><figcaption>**Figure 1.** General scheme describing the synthesis of ABA triblock copolymers using **TTC-bCP** RAFT agent.</figcaption></figure>

# Experimental section

## Materials.

Methyl methacrylate (MMA, Aldrich, 99 %), *n*-butyl acrylate (*nBA*, Aldrich, 99 %) and isobornyl methacrylate (*iBoMA*, Aldrich, technical grade) were purified by passing through a basic alumina column. Tetrahydrofurfuryl acrylate (THFA, Aldrich, 99 %) was purified by vacuum distillation. 2,2'-Azobis(2-methylpropionitrile) (AIBN, Aldrich, 99 %) was purified by recrystallization in methanol. 2,2'-Azobis(4-methoxy-2,4-dimethylvaleronitrile) (V-70, 95%), tetrahydrofuran (THF, Aldrich, HPLC grade), toluene (PhMe, Aldrich, 99 %), acetonitrile (MeCN, Fisher, HPLC grade), acetone (Me<sub>2</sub>CO, Aldrich, HPLC grade), petroleum ether (Aldrich, ACS reagent), methanol (MeOH, Aldrich, HPLC grade), 2-aminopropane (*iPr*-NH<sub>2</sub>, Aldrich, 99 %) and 1,3,5-trioxane (Triox, Aldrich, 99 %) were used as received.

## Characterization.

*Nuclear Magnetic Resonance (NMR)*. ¹H and ¹³C NMR spectra were recorded on a Bruker AV 300 MHz spectrometer. Deuterated chloroform was used as solvent and reference in all samples (CDCl<sub>3</sub>, δ = 7.26 ppm). Chemical shifts are given in parts per million (ppm).
**Size Exclusion Chromatography (SEC).** Number- ($M_n$) and weight-average ($M_w$) molar masses and corresponding dispersities ($D$) were determined by size-exclusion chromatography (SEC) on a system composed of a Waters 515 HPLC pump, an Agilent 1260 autosampler, a Varian ProStar 500 column valve module, a set of three Waters columns (Styragel Guard Column, 20 µm, 4.6 mm × 30 mm, Styragel HR3, 5 µm, 7.8 mm × 300 mm and Styragel HR4E, 5 µm, 7.8 mm × 300 mm), a Varian ProStar 325 UV-Vis detector set at 290 nm, Wyatt Optilab rEX differential refractive index detector and a Wyatt MiniDawn TREOS multiangle light scattering detector. Tetrahydrofuran (THF) was used as eluent for all samples at a flow rate of 1.0 mL/min (35 °C). Samples were diluted to a concentration about 5 mg/mL and filtered through 0.45 µm Nylon syringe filters before injection. The column system was calibrated with PMMA standards (ranging from 0.96 to 265.3 kg mol⁻¹) for the determination of the average molar masses of one single low molar mass PMMA sample (Table 1, entry 1). For all the other polymers, average molar masses were determined using RI-MALS detection. The following refractive index increment (dn/dc) values were used: (dn/dc)<sub>PMMA</sub> = 0.085,<sup>34</sup> (dn/dc)<sub>PnBA</sub> = 0.067,<sup>35</sup> (dn/dc)<sub>p/bMA</sub> = 0.108,<sup>36</sup> (dn/dc)<sub>pTHFA</sub> = 0.085.<sup>37</sup> Average molar masses were determined using the Astra software.

**Differential Scanning Calorimetry (DSC).** The glass transition temperature ($T_g$) of the polymers was measured by DSC using Mettler Toledo STARe DSC in nitrogen atmosphere using aluminum crucibles of 40 µL at heating rates of 10 °C/min. Samples in 5-10 mg portions were used for all analyses. All $T_g$ values were obtained from the second or third ramp to remove the thermal history of the samples.

**Thermogravimetric Analysis (TGA).** TGA analyses were conducted on a TGA/DSC 3+ Mettler with a temperature ramp of 10 °C/min under N₂ (50 mL/min).

**Dynamical mechanical analysis.** Isochronal temperature ramp tests were performed on an Anton Paar MCR 302. The experiments were performed in parallel plate geometry using 8 mm sample disks. A frequency of 1 rad s⁻¹, and a strain of 1 % were used to record the evolution of the dynamic moduli as function of the temperature with a heating rate of 2.5 °C/min.
*Small Angle X-ray Scattering (SAXS)*. SAXS experiments were performed after thermal annealing under vacuum at 180 °C for 8 hours. SAXS data were acquired in transmission mode on a high-resolution X-ray spectrometer Xeuss 2.0 (Xenocs) operating with a radiation wavelength of λ = 1.54 Å at 150 °C. The resulting 2D images were found to be isotropic, and the data were azimuthally averaged to yield curves of the scattering intensity, *I(q)*, as function of the scattering vector, *q*. The beam center position and the angular range were calibrated using silver behenate as standard.

**Synthesis.**

RAFT agent synthesis (TTC-bCP).

Bis-(2-methylpropanenitrile)-trithiocarbonate (TTC-bCP) was prepared following an adaptation of our previously published procedure.³³ A solution of 2-mercapto-2-methylpropanenitrile (200 mg, 1.98 mmol) in 5 ml of dry MeCN was purged with argon and added slowly (dropwise) during 2 h to a solution of 1,1'-thiocarbonyldiimidazole (171 mg, 0.96 mmol) in 5 ml of dry MeCN purged with argon. The reaction mixture was allowed to stir for an additional hour. A change of color of the reaction solution was observed from yellow to bright orange. Upon completion the reaction mixture was transferred on the silica column under argon. TTC-bCP was recovered from column chromatography under argon atmosphere (50 % ethyl acetate / 50 % cyclohexane R<sub>f</sub> = 0.9). After column purification the purity was about 90 % (10 % of disulfide impurity), and the yield was 68 %. To obtain a 99.9 % pure compound, a recrystallization from CHCl<sub>3</sub>/n-pentane was necessary.

¹H NMR (300 MHz, CDCl₃) δ (ppm): 1.91 (s, 6H, S-C(CH₃)₂-CN). ¹³C NMR (75 MHz, CDCl₃) δ (ppm): 211.01 (S-(_C=S)-S); 119.70 ( _CN ); 42.93 (S- _C(CH₃)₂-CN); 26.95 (S-C(_CH₃)₂-CN).

*Polymerization procedure.*

The initiator, RAFT (or polymethacrylate macro-RAFT) agent, solvent (if needed) and monomer were
mixed and the obtained solution was divided in ampoules (or Schlenk flasks), which were sealed after
degassing by three freeze-pump-thaw cycles and immersed in an oil bath at different temperatures for
the desired time. The polymerization was stopped by rapid cooling and the solution was immediately transferred to NMR tubes for conversion analysis. The remaining solution was dried under vacuum and used as such for the preparation of SEC samples. Monomer conversions were calculated by the integrations of vinyl protons compared either to polymer or to 1,3,5-trioxane standard (≈ 5.2 ppm). The polymers were purified by precipitation. The crude polymer was dissolved in the minimum amount of good solvent (Me₂CO for PMMA, MePh for others) and the resulting viscous oil was slowly added dropwise into a non-solvent in excess (petroleum ether for PMMA, MeOH for others). The obtained solid polymer was then filtered, washed and dried under vacuum. For some samples, the procedure was repeated several times to remove traces of residual monomer.

Using the above-described procedure, a typical polymerization was conducted as follows (Table 3, entry 1). AIBN (0.001 mmol, 0.17 mg), PMMA macro-RAFT agent (Table 1, entry 7) (0.01 mmol, 0.4 g) and nBA (8.64 mmol, 1.1 g) were mixed. The solution was transferred to a glass ampoule, which was flame sealed after degassing by three freeze-pump-thaw cycles and immersed in an oil bath at 60 °C for 20 h. The polymerization was stopped by rapid cooling. After opening, the solution was immediately transferred to an NMR tube for conversion analysis (86.6 %). The volatiles were removed by evaporation. The crude product was dissolved in PhMe and precipitated in MeOH.

¹H NMR (300 MHz, CDCl₃) δ (ppm): 1. PMMA part: 3.60 (s, 3H, O-CH₃), 1.82-2.02 (m, 2H, CH₂), 0.85-1.25 (s, 3H, CH₃); 2. PnBA part: 4.03 (s, 2H, O-CH₂), 1.90-2.28 (s, H, CH₂-CH-COOCH₂-), 1.55-1.62 (m, 2H, O-CH₂-CH₂-CH₂-CH₃), 1.49 (m, 2H, CH₂-CH-COOCH₂-), 1.31-1.43 (m, 2H, O-CH₂-CH₂-CH₂-CH₃), 0.91-0.96 (m, 3H, O-CH₂-CH₂-CH₂-CH₃). M_n = 174 kg mol⁻¹, D = 1.05.

NMR of polymers are presented in the ESI (Figure S3-S7).

## Results and Discussion

### Hard block synthesis
In a first communication,³³ the newly proposed symmetrical RAFT agent **TTC-bCP** (Figure 1) showed an excellent efficiency for the preparation of one single PMMA-*b*-PnBA-*b*-PMMA triblock copolymer with a very good control over molar masses and remarkably low dispersities (D = 1.04). The control and polymer definition offered by this RAFT agent was unprecedented compared to classically used symmetrical RAFT agents such as S,S'-bis(α,α'-dimethylacetic acid)trithiocarbonate.³¹ To better evaluate the potential of this RAFT agent for the design of TPE materials of higher and tunable service temperature, iBoMA was considered as high T<sub>g</sub> monomer (T<sub>g</sub> (iBoMA) = 190-200 °C), either alone or as a MMA/iBoMA 50/50 (mol.%) feed composition for a targeted M<sub>n</sub> of 30 kg mol<sup>-1</sup>. Results were compared to PMMA synthesis taken as reference. Polymerizations were followed by ¹H NMR to determine methacrylate conversion and SEC to follow the evolution of average molar masses (Table 1).

<table><caption>**Table 1.** Experimental conditions and results of TTC-bCP-mediated RAFT polymerization of methacrylates.</caption><thead><tr><th>Entry</th><th>[M]/[CTA]/[I]</th><th>Monomers</th><th>Time, h</th><th>Conversion, % [a]</th><th>M<sub>n</sub> (theor) kg mol<sup>-1</sup> [b]</th><th>M<sub>n</sub> (SEC), kg mol<sup>-1</sup> [c]</th><th>D (M<sub>w</sub>/M<sub>n</sub>) [c]</th></tr></thead><tbody><tr><td>1</td><td>330/1/0.3</td><td>MMA</td><td>1.5</td><td>8.5</td><td>3.0</td><td>4.5</td><td>1.26</td></tr><tr><td>2</td><td>330/1/0.3</td><td>MMA</td><td>4.5</td><td>23.7</td><td>8.0</td><td>8.6</td><td>1.24</td></tr><tr><td>3</td><td>330/1/0.3</td><td>MMA</td><td>8</td><td>40.7</td><td>13.5</td><td>13.3</td><td>1.21</td></tr><tr><td>4</td><td>330/1/0.3</td><td>MMA</td><td>18</td><td>72.4</td><td>23.8</td><td>23.2</td><td>1.15</td></tr><tr><td>5</td><td>330/1/0.3</td><td>MMA</td><td>25</td><td>94.3</td><td>31.0</td><td>29.8</td><td>1.12</td></tr><tr><td>6</td><td>330/1/0.3</td><td>MMA</td><td>40</td><td>99.9</td><td>32.8</td><td>31.2</td><td>1.12</td></tr><tr><td>7</td><td>450/1/0.3</td><td>MMA</td><td>28</td><td>92.6</td><td>42.3</td><td>39.0</td><td>1.10</td></tr><tr><td>8</td><td>1100/1/0.3</td><td>MMA</td><td>70</td><td>99.0</td><td>112</td><td>112</td><td>1.11</td></tr><tr><td>9</td><td>330/1/0.3</td><td>iBoMA</td><td>2</td><td>11.5</td><td>8.3</td><td>18.7</td><td>1.25</td></tr><tr><td>10</td><td>330/1/0.3</td><td>iBoMA</td><td>5</td><td>41.9</td><td>29.7</td><td>40.1</td><td>1.19</td></tr><tr><td>11</td><td>330/1/0.3</td><td>iBoMA</td><td>9</td><td>66.6</td><td>47.0</td><td>59.8</td><td>1.29</td></tr><tr><td>12</td><td>330/1/0.3</td><td>iBoMA</td><td>24</td><td>97.7</td><td>68.8</td><td>75.9</td><td>1.24</td></tr><tr><td>13<sup>[d]</sup></td><td>330/1/0.3</td><td>iBoMA</td><td>17</td><td>46.2</td><td>33.7</td><td>45.5</td><td>1.28</td></tr><tr><td>14</td><td>(125/125)/1/0.3</td><td>MMA/iBoMA</td><td>2</td><td>13.8</td><td>6.0</td><td>11.1</td><td>1.32</td></tr><tr><td>15</td><td>(125/125)/1/0.3</td><td>MMA/iBoMA</td><td>5</td><td>37.5</td><td>15.8</td><td>21.3</td><td>1.12</td></tr><tr><td>16</td><td>(125/125)/1/0.3</td><td>MMA/iBoMA</td><td>8</td><td>56.9</td><td>23.9</td><td>29.4</td><td>1.12</td></tr><tr><td>17</td><td>(125/125)/1/0.3</td><td>MMA/iBoMA</td><td>15</td><td>72.2</td><td>30.3</td><td>34.2</td><td>1.13</td></tr><tr><td>18</td><td>(125/125)/1/0.3</td><td>MMA/iBoMA</td><td>22.5</td><td>97.6</td><td>40.8</td><td>47.8</td><td>1.09</td></tr><tr><td>19</td><td>(46/138)/1/0.3</td><td>MMA/iBoMA</td><td>28</td><td>100</td><td>44.6</td><td>41.7</td><td>1.13</td></tr></tbody></table>

Reactions conditions: CTA = **TTC-bCP**, I = AIBN, Temperature = 60 °C, monomer concentration = 70 wt.% (in MeCN for MMA, and in PhMe for MMA/iBoMA and iBoMA). a Determined by ¹H NMR. b Determined by the following equation: M<sub>n</sub> (theor) = (m<sub>(mon.)</sub>×conv.<sub>(mon.)</sub>×M<sub>(RAFT agent)</sub>/m<sub>(RAFT agent)</sub>) + M<sub>(RAFT agent)</sub>. c Determined by SEC-RI-MALS in THF. d Polymerization
performed in a Schlenk flask on the 1.5 g scale for the synthesis of the triblock copolymer. Entry 1-6 were taken from ref 33

As we previously published,³³ MMA polymerization was well-controlled with a linear evolution of M~n~ with conversion and gradual decrease of dispersity values down to 1.10 during polymerization (Table 1, entry 1-7, Figure 2A). M~n~ values determined by SEC-RI-MALS analysis were in excellent agreement with those expected for a controlled process, suggesting negligible irreversible termination or transfer reactions. This observation was confirmed by monomodal chromatograms obtained by SEC over the whole range of conversion (Figure 3A). To evaluate the possibility of controlling RAFT polymerization of MMA for much higher M~n~, a PMMA of 110 kg mol⁻¹ was targeted (Table 1, entry 8). Satisfactorily, a comparable level of control was obtained with a M~n~ of 112 kg mol⁻¹ (M~n~ (theor) = 112 kg mol⁻¹) and a dispersity of 1.11.

<figure><img src="image_12.png" /><figcaption>**Figure 2.** M~n~ and D~r~ vs conversion during methacrylate RAFT polymerization with **TTC-bBP** at 60 °C. Values were determined by SEC-MALS in THF (except for the lowest conversion data (orange symbols) where PMMA calibration has been used). MMA (A), iBoMA (B) and MMA/iBoMA (50/50 mol.%) (C). Polymerization conditions are reported in **Table 1**. Data for A was taken from ref 33</figcaption></figure>

PiBoMA possesses interesting characteristics such as a much higher T~g~ than PMMA (190-200 °C vs 100 °C), and is partially bio-sourced as isoborneol is a terpene derivative. Surprisingly, studies reporting the RAFT homopolymerization of iBoMA are rare and showed only moderate control over polymerization.³⁸ Therefore, RAFT polymerization of iBoMA was conducted with RAFT agent **TTC-bCP** (Table 1, entry 9-13). Using our conditions with 70 wt.% monomer in PhMe at 60 °C with AIBN initiation, dispersities obtained for PiBoMA (D=1.19-1.29) are among the lowest reported for this monomer by RDRP³⁹ (Figure 2B). However, dispersities are higher than for PMMA (1.24 versus 1.10 at a conversion > 97 %). This increase of dispersity could be related to the relatively high steric hindrance
of the isobornyl group compared to methyl in MMA, which could decrease the transfer constant of PiBoMA propagating radicals to **TTC-bCP**. Nonetheless, monomodal chromatograms were obtained. A linear increase of M~n~ with conversion was observed, while M~n~ values were slightly greater than theoretical ones (**Figure 3B**). The overall higher reactivity of iBoMA compared to MMA can be mostly explained by the greater polymerizability of iBoMA. Indeed, it was reported that the propagation rate constant k~p~ of MMA in bulk at 60 °C equals 830 L mol⁻¹ s⁻¹,⁴⁰ while k~p~ = 1436 L mol⁻¹ s⁻¹ for iBoMA.⁴¹ In addition, it can be assumed that the termination rate constant k~t~ of iBoMA is lower than that of MMA due to steric considerations. Finally, the fact that the nature of the polymerization solvent (MeCN for MMA and PhMe for iBoMA) may have an effect on the rate of polymerization cannot be ruled out.

Controlling the T~g~ of the hard block is an important objective since a too high T~g~ such as the one of PiBoMA might be irrelevant for all-(meth)acrylic TPE materials as it would impose even higher processing temperatures which may be too close to the degradation temperature of polymethacrylates. Therefore, an improperly adjusted T~g~ of the outer hard blocks could potentially restrain the use of common processing techniques usually found in thermoplastics such as extrusion or injection molding. As a consequence, the RAFT polymerization of MMA/iBoMA mixtures was also evaluated. According to the Kwei equation,⁴² a T~g~ value in the range of 100 to 200 °C is expected for an iBoMA content between 0 % to 100 % of iBoMA.⁴³ A feed ratio of 50/50 (mol.%) MMA/iBoMA was first selected as it provides intermediate T~g~. By following monomers consumptions by ¹H NMR, it was found that MMA and iBoMA are isoreactive throughout the reaction (**Figure S8-S9**), thus forming a copolymer with statistical composition. Control of M~n~ and D~v~ values was in the same order that MMA polymerization, showing a strong beneficial effect of the MMA comonomer in iBoMA polymerization (**Figure 2C** and **3C**, and **Table 1**, entry 14-18). This implies that the propagating radicals preferably transfer to **TTC-bCP** through MMA⁴⁴ radicals resulting in an overall greater apparent transfer constant, resulting in a better polymerization control. The linear increase of M~n~ with conversion was observed, while M~n~ values were slightly greater than theoretical ones, but were intermediate between those obtained for pure PMMA and PiBoMA. Nonetheless, a T~g~ of 131 °C was obtained for 50/50
MMA/iBoMA polymerization, close to the $T_g$ of pure PMMA. Thus, a polymerization of 25/75
MMA/iBoMA was also conducted (Table 1, entry 19) and resulted in a $T_g$ of 155 °C (see the Thermal
properties section for detailed discussion). Thus, MMA/iBoMA comonomer mixtures were also
selected for the preparation of ABA triblock copolymers with various $T_g$ of the hard phase.

<figure><img src="image_13.png" /><figcaption>**Figure 3.** SEC-RI chromatograms of MMA (A), iBoMA (B) and iBoMA/MMA (50/50) (C) polymerization in the presence of TTC-bCP as RAFT agent. Polymerization conditions of Table 1. Data for A was taken from ref 33</figcaption></figure>

## Soft block (homopolymer) synthesis

RAFT homopolymerizations of nBA and THFA monomers were first studied with TTC-bCP in toluene to optimize the polymerization conditions for the synthesis of the soft mid-block. As we previously discussed,³³ polymerization of nBA was much faster than that of MMA under similar conditions (Table 2, entry 1-6) owing to the greater intrinsic reactivity of nBA, although an induction time of about 1 h is noteworthy (Table 2, entry 1-6). A linear evolution of M_n with conversion and excellent match of values with those estimated for a controlled RAFT process was observed. Dispersities are low with values down to 1.06 after 88 % conversion, and with a slight increase to 1.10 after 94 % conversion, suggesting a minor contribution of chain transfer to solvent as evidenced by the appearance of a shoulder in the lower molar mass area of the corresponding chromatograms (Figure S10, 9h).

<table><caption>**Table 2.** Experimental conditions and results of TTC-bCP-mediated RAFT polymerizations of acrylates.</caption><thead><tr><th>Entry</th><th>[M]/[CTA]/[I]</th><th>Monomers</th><th>Time, h</th><th>Conversion, % [a]</th><th>M<sub>n</sub> (theor), kg mol<sup>-1</sup> [b]</th><th>M<sub>n</sub> (SEC), kg mol<sup>-1</sup> [c]</th><th>Φ (M<sub>w</sub>/M<sub>n</sub>) [c]</th></tr></thead><tbody><tr><td>1</td><td>330/1/0.3</td><td>nBA</td><td>1</td><td>2.0</td><td>1.1</td><td>2.3</td><td>1.37</td></tr><tr><td>2</td><td>330/1/0.3</td><td>nBA</td><td>1.5</td><td>18.0</td><td>7.9</td><td>10.1</td><td>1.08</td></tr><tr><td>3</td><td>330/1/0.3</td><td>nBA</td><td>2</td><td>42.3</td><td>18.3</td><td>17.4</td><td>1.06</td></tr><tr><td>4</td><td>330/1/0.3</td><td>nBA</td><td>3</td><td>68.2</td><td>29.3</td><td>28.1</td><td>1.05</td></tr></tbody></table>
<table><tr><td>5</td><td>330/1/0.3</td><td>nBA</td><td>6</td><td>88.2</td><td>37.8</td><td>35.7</td><td>1.06</td></tr><tr><td>6</td><td>330/1/0.3</td><td>nBA</td><td>9</td><td>94.3</td><td>40.4</td><td>34.3</td><td>1.10</td></tr><tr><td>7</td><td>1500/1/0.1</td><td>nBA</td><td>1</td><td>1.0</td><td>2.1</td><td>4.7</td><td>1.40</td></tr><tr><td>8</td><td>1500/1/0.1</td><td>nBA</td><td>1.5</td><td>6.1</td><td>11.6</td><td>10.7</td><td>1.21</td></tr><tr><td>9</td><td>1500/1/0.1</td><td>nBA</td><td>8</td><td>55.7</td><td>104</td><td>95.7</td><td>1.03</td></tr><tr><td>10</td><td>1500/1/0.1</td><td>nBA</td><td>13</td><td>67.5</td><td>125</td><td>120</td><td>1.03</td></tr><tr><td>11</td><td>1500/1/0.1</td><td>nBA</td><td>24</td><td>83.5</td><td>155</td><td>141</td><td>1.02</td></tr><tr><td>12</td><td>1500/1/0.1</td><td>THFA</td><td>1</td><td>22</td><td>50.3</td><td>62.8</td><td>1.29</td></tr><tr><td>13</td><td>1500/1/0.1</td><td>THFA</td><td>1.5</td><td>40</td><td>91.2</td><td>97.0</td><td>1.40</td></tr><tr><td>14</td><td>1500/1/0.1</td><td>THFA</td><td>2</td><td>58</td><td>132</td><td>184</td><td>2.13</td></tr><tr><td>15</td><td>1500/1/0.1</td><td>THFA</td><td>4</td><td>90</td><td>205</td><td>247</td><td>26.34</td></tr></table>

Reactions conditions: CTA = **TTC-bCP**, I = AIBN (entry 1-11) or V-70 (entry 12-15), Temperature = 60 °C (entry 1-11), 30 °C (entry 12-15), monomer concentration = 70 wt.% in PhMe for entry 1-6 and bulk for entry 7-15. a Determined by ¹H NMR. b Determined by the following equation: $M_n(\text{theor})=(m_{\text{mon.}})\times\text{conv.}(\text{mon.})\times M_{\text{(RAFT agent)}}/m_{\text{(RAFT agent)}})+M_{\text{(RAFT agent)}}$. c Determined by SEC-RI-MALS in THF. Entry 1-6 were taken from ref 33

To evaluate the influence of solvent regarding the occurrence of side reactions, the polymerization of nBA was carried out in toluene at different dilutions. We applied relative nBA/TTC-bCP/AIBN concentration conditions of 800/1/0.1 as it is typical for the targeted length of the soft block during chain extension from polymethacrylate-TTC-bCP (see triblock copolymer synthesis section). nBA conversion after 7 h of reaction at 60 °C reaches 85.9, 72.2 and 67.1 % for 77 wt.% of nBA in toluene, 50 wt.% and bulk conditions, respectively (Table S1, entry 1-3). The use of a limited amount of solvent decreases the viscosity of the reaction medium and improves the reactants diffusion allowing access to higher conversion in comparison to bulk, but an increase of dilution slows down the polymerization kinetics as expected. Noteworthy, a marked bimodal molar mass distribution is obtained in solution polymerization in contrast to bulk polymerization (Figure S11). The peak mass $M_p$ of the population of lower mass is roughly half of that of the main population. These observations are characteristic of chain transfer to solvent occurring in a RAFT process using a symmetrical RAFT agent with a Z-group approach, for which deactivated polymer chains have roughly half the length of the main RAFT population. For this reason, the syntheses of triblock copolymers were all conducted in bulk.
<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /><figcaption>**Figure 4.** *M*<sub>n</sub> and *D* vs conversion (A) and corresponding SEC-RI chromatograms (B) of *n*BA polymerization with **TTC-bCP** as RAFT agent at 60 °C in bulk.</figcaption></figure>

On the basis of literature, a hard/soft molar ratio of 2:5 (1 hard-5 soft-1 hard)⁴⁵ with *M*<sub>n</sub> of PMMA segments higher than 10 kg mol<sup>-1</sup> is suited to give materials with good TPE properties. Due to the incomplete polymerization of *n*BA in bulk as a result of the high viscosity of the medium, the reaction was conducted with a *n*BA to RAFT agent ratio of 1500 (Table 2, entry 7-11) for the later targeted PMMA<sub>20K</sub>-*Pn*BA<sub>100K</sub>-PMMA<sub>20K</sub> structure. Relatively high molar masses were targeted in order to induce enough chain entanglement and therefore good mechanical properties.⁴⁶ An excellent control of the polymerization was obtained with control of *M*<sub>n</sub> with well-defined molar mass distributions and dispersities as low as 1.02. (Figure 4). The desired molar mass of about 125 kg mol<sup>-1</sup> was obtained after 13 h of polymerization.
In addition to nBA, tetrahydrofurfuryl acrylate (THFA), a bio-based monomer was also evaluated as constituting soft block ($T_g$ = -11 °C).³⁷ Standard conditions of 60 °C with AIBN provided poor RAFT control probably due to the presence of many acidic protons which are prone to transfer reactions inducing branching and eventually crosslinking reactions (Table S1, entry 4-8). Reasonably good $M_n$ control with low dispersities ($D = 1.25$) were solely obtained at conversions up to 30 %, making this monomer unlikely suitable for the preparation of TPE materials. V-70 initiation at 30 °C was considered with the idea of reducing the contribution of side reactions (Table 2, entry 12-15). Under these initiation conditions in bulk and for THFA conversions not greater than 40 %, the best control ever reported for this monomer using a RDRP approach was achieved. Indeed, monomodal SEC chromatograms were obtained (Figure S12, 1.5h) with $M_n$ of 97 kg mol⁻¹ and $D = 1.40$ at 40 % conversion. THFA has already been used to create poly(1-adamantyl acrylate)-b-poly(tetrahydrofurfuryl acrylate)-b-poly(1-adamantyl acrylate) triblock copolymer with TPE properties by RAFT polymerization.²⁸ However, the best dispersity achieved was 1.66 with a relatively short soft block of 23 kg mol⁻¹ and a bimodal peak observed in SEC. Another possibility to reduce dispersity may have been to copolymerize THFA statistically with nBA (Table S1, entry 9-13), but no significant improvement was noticed.

## Triblock copolymer synthesis

PMMA-b-PnBA-b-PMMA triblock copolymers were synthesized according to the optimized conditions reported above for both monomers. In our previous work,³³ the chain extension from a PMMA macro-CTA (Figure 5A) with $M_n$ of 39 kg mol⁻¹ showed a clear shift of the chromatogram towards a higher molar mass of 174 kg mol⁻¹, proving the successful chain extension. Dispersity values decreased down to 1.05 over time, reflecting the excellent control of the polymerization (Table 3, entry 1). In order to confirm the formation of a symmetrical triblock structure, the copolymer was reacted with isopropylamine. This reaction induces the cleavage of C-S bonds resulting in the formation of the corresponding PMMA-b-PnBA diblock copolymer. This is clearly seen by SEC analysis where the molar
mass of the triblock copolymer is divided by two after aminolysis (see chromatograms and conditions in Figure S13).

<figure><img src="image_16.png" /><figcaption>**Figure 5.** SEC-RI chromatograms of triblock copolymers and corresponding methacrylate macro-CTAs. (A) PMMA-*b*-PnBA-*b*-PMMA (*Table 3*, entry 1), (B) PiBoMA-*b*-PnBA-*b*-P/iBoMA (*Table 3*, entry 3 and 4), (C) P(MMA-*co*-iBoMA)-*b*-PnBA-*b*- P(MMA-co-iBoMA) (*Table 3*, entry 5). Data for A was taken from ref 33</figcaption></figure>

The efficiency of chain extension was also verified for higher molar masses. To do so, a reaction with
[nBA]/[PMMA-TTC-bCP] = 15 000 was carried out, which corresponds to a targeted Mn of about
2 000 kg mol⁻¹ for the triblock copolymer, starting from a PMMA macro-CTA of 112 kg mol⁻¹ (Table 3,
entry 2). The chain extension was successful, leaving no residual PMMA precursor. The shape of the
peak remained monomodal but slightly broadened as a result of a long polymerization time (30 h) and
possible side reactions after 63.9 % conversion (Figure S14). Nevertheless, a PMMA-b-PnBA-n-PMMA
triblock copolymer with an ultra-high molar mass of about 2 000 kg mol⁻¹ and relatively low dispersity
(Ø = 1.43) was prepared. Overall, access to very high controlled Mn and still relatively low dispersity is
afforded owing to both extremely good control of the polymerization offered by TTC-bCP and
optimized polymerization conditions responsible for minimized side reactions. Such level of high molar
mass control was only reported in the case of photo-mediated RAFT polymerization,⁴⁷ emulsion⁴⁸,⁴⁹ or
gel polymerization,⁵⁰ or by RAFT polymerization at very high pressures.⁵¹

Higher T<sub>g</sub> PiBoMA-*b*-PnBA-*b*-PiBoMA triblock copolymers were also prepared (Table 3, entry 3-4).
Starting from a PiBoMA-TTC-bCP macro-CTAs, shifts in chromatograms are clearly visible for two
different molar masses of the soft mid-block, with M<sub>n</sub> in agreement with expected values and
significant decrease in dispersity down to 1.02 (Figure 5B). In the same way, a well-defined P(iBoMA-
*co*-MMA)-*b*-P*n*BA-*b*-P(*i*BoMA-*co*-MMA) with a controlled *M*<sub>n</sub> of about 140 kg mol<sup>-1</sup> and a low *D* of 1.05 was synthesized (**Table 3**, entry 5 and **Figure 5C**).
<table><caption>**Table 3.** Experimental conditions and results of the triblock copolymerizations</caption><thead><tr><th>Entry</th><th>Macro-CTA</th><th>[M]/[CTA]/[I]</th><th>Triblock copolymers</th><th>Time, h</th><th>Conversion, % <sup>[a]</sup></th><th>M<sub>n</sub> (theor), kg mol<sup>-1</sup> <sup>[b]</sup></th><th>M<sub>n</sub> (SEC), kg mol<sup>-1</sup> <sup>[c]</sup></th><th>Đ (M<sub>w</sub>/M<sub>n</sub>) <sup>[c]</sup></th></tr></thead><tbody><tr><td>1</td><td>PMMA<sub>40k</sub><br>(Entry 7, T1)</td><td>900/1/0.1</td><td>PMMA<sub>20k</sub>-b-PnBA<sub>135k</sub>-b-PMMA<sub>20k</sub></td><td>20</td><td>86.6</td><td>139</td><td>174</td><td>1.05</td></tr><tr><td>2</td><td>PMMA<sub>112k</sub><br>(Entry 8, T1)</td><td>15000/1/0.3</td><td>PMMA<sub>56k</sub>-b-PnBA<sub>1.9M</sub>-b-PMMA<sub>56k</sub></td><td>30</td><td>63.9</td><td>1291</td><td>1980</td><td>1.43</td></tr><tr><td>3</td><td>PiBoMA<sub>46k</sub><br>(Entry 13, T1)</td><td>1000/1/0.1</td><td>PiBoMA<sub>23k</sub>-b-PnBA<sub>117k</sub>-b-PiBoMA<sub>23k</sub></td><td>17</td><td>87.0</td><td>149</td><td>163</td><td>1.02</td></tr><tr><td>4</td><td>PiBoMA<sub>46k</sub><br>(Entry 13, T1)</td><td>2500/1/0.1</td><td>PiBoMA<sub>23k</sub>-b-PnBA<sub>285k</sub>-b-PiBoMA<sub>23k</sub></td><td>42</td><td>68.4</td><td>269</td><td>331</td><td>1.09</td></tr><tr><td>5</td><td>P(iBoMA-co-MMA)<sub>42k</sub><br>(Entry 19, T1)</td><td>900/1/0.1</td><td>P(iBoMA-co-MMA)<sub>21k</sub>-b-PnBA<sub>99k</sub>-b-P(iBoMA-co-MMA)<sub>21k</sub></td><td>40</td><td>75.6</td><td>144</td><td>141</td><td>1.05</td></tr><tr><td>6</td><td>PiBoMA<sub>46k</sub><br>(Entry 13, T1)</td><td>1600/1/0.1</td><td>PiBoMA<sub>23k</sub>-b-PTHFA<sub>34k</sub>-b-PiBoMA<sub>23k</sub></td><td>2</td><td>14.7</td><td>79.8</td><td>66.1</td><td>1.31</td></tr></tbody></table>

Reactions conditions: macro-CTA, I = AIBN (entry 1-5), V-70 (entry 6), Temperature = 60 °C (entry 1-5), 30 °C (entry 6), bulk conditions. <sup>a</sup> Determined by <sup>1</sup>H NMR. <sup>b</sup> Determined by the following equation: $M_n(\text{theor})=(m_{\text{mon.}})\times\text{conv.}_{\text{(mon.)}}\times M_{\text{(RAFT agent)}}/m_{\text{(RAFT agent)}})+M_{\text{(RAFT agent)}}$. <sup>c</sup> Determined by SEC-RI-MALS in THF. Entry 1 was taken from ref 33
We evaluated the possibility to synthesize a fully bio-based PiBoMA-*b*-PTHFA-*b*-PiBoMA copolymer.

The synthesis of triblock copolymers from PiBoMA-*TTC-bCP* with THFA was difficult since it was
necessary to stop the reaction before the generation of polymer branching. Still, monomodal peaks
were observed only at around 15 % conversion, for the synthesis of PiBoMA<sub>23k</sub>-*b*-PTHFA<sub>34k</sub>-*b*-PiBoMA<sub>23k</sub>
*(Table 3, entry 6)* with a monomodal SEC chromatogram (Figure S15) and an acceptable *D* of 1.31.

However, it was decided not to study the TPE properties of this class of copolymer due to insufficient
length of the PTHFA soft block.

**Thermal properties.**

The T<sub>g</sub> values were measured by differential scanning calorimetry (DSC) (Table 4 and Figure S17A). In our previous study,<sup>33</sup> starting from polymethacrylates-*TTC-bCP*, a T<sub>g</sub> of 121 °C (instead of the expected 100 °C) was observed for PMMA. A similar value has already been reported for some PMMA-based acrylic block copolymers.<sup>52</sup> On the contrary, for PiBoMA, the T<sub>g</sub> value of 181 °C was lower than anticipated (190-200 °C according to literature).<sup>22</sup> For P(iBoMA-co-MMA), the dependence of T<sub>g</sub> values on the copolymer composition is known to be not linear.<sup>43</sup> That is why, for a iBoMA/MMA composition of 50/50 mol.%, a T<sub>g</sub> value of 131 °C was obtained, while for iBoMA/MMA = 75/25, a higher T<sub>g</sub> of 155 °C was achieved, which is in close agreement with literature.<sup>43</sup>

Due to relative low weight fraction of the hard block, the observation of the T<sub>g</sub> of the hard block in
triblock copolymers was not always possible (Figure S17B). This is especially true for PiBoMA which
contains much less monomer units in comparison to a PMMA of similar molar mass, due to the
molecular weight of iBoMA monomer being more than twice as high as that of MMA. The fact that T<sub>g</sub>
values of the hard block of PMMA-*b*-PnBA-*b*-PMMA and P(MMA-*co*-iBoMA)-*b*-PnBA-*b*-P(MMA-*co-*
iBoMA) hardly differed from those of polymethacrylate-*TTC-bCP* precursors, with T<sub>g</sub> of the soft block
being very close to that of nBA homopolymer hinted a proper microphase separation between the
blocks. (Table 4, entry 5-9).

**Table 4.** Thermal properties of the hard blocks and triblock copolymers
<table><thead><tr><th>Entry</th><th>Polymer</th><th>T<sub>g</sub> (hard block), °C</th><th>T<sub>g</sub> (soft block), °C</th></tr></thead><tbody><tr><td>1</td><td>PMMA</td><td>121</td><td>-</td></tr><tr><td>2</td><td>P(iBoMA-co-MMA) (50/50)</td><td>131</td><td>-</td></tr><tr><td>3</td><td>P(iBoMA-co-MMA) (75/25)</td><td>155</td><td>-</td></tr><tr><td>4</td><td>PiBoMA</td><td>181</td><td>-</td></tr><tr><td>5</td><td>PMMA-b-PnBA-b-PMMA</td><td>124</td><td>-57</td></tr><tr><td>6</td><td>PiBoMA-b-PnBA-b-PiBoMA</td><td>n.o.</td><td>-52</td></tr><tr><td>7</td><td>P(iBoMA-co-MMA)-b-PnBA-b-P(iBoMA-co-MMA)<br>(iBoMA/MMA = 50/50)</td><td>n.o.</td><td>-60</td></tr><tr><td>8</td><td>P(iBoMA-co-MMA)-b-PnBA-b-P(iBoMA-co-MMA)<br>(iBoMA/MMA = 75/25)</td><td>157</td><td>-48</td></tr><tr><td>9</td><td>PiBoMA-b-PTHFA-b-PiBoMA</td><td>172</td><td>-9</td></tr></tbody></table>

n.o.: not observable. Entry 1 and 5 were taken from ref 33

The thermal stability of the polymers was tested using thermogravimetric analysis (TGA) under air and inert atmosphere (Figure S16). PMMA-TTC-bCP showed the lowest stability with a 5 % weight loss at 175 °C. Similar results were previously obtained for PMMA with TTC end-groups.⁵³ In contrast, PnBA-TTC-bCP was much more thermally stable with a 5 % weight loss at 305 °C. The PMMA-b-PnBA-b-PMMA triblock copolymer, with mid-chain TTC groups surrounded by a PnBA environment, exhibit a stability similar to PnBA with a 5 % weight loss reached at 257 and 295 °C in air and in an inert atmosphere, respectively, which shows good potential for applications as TPE materials. The TGA data obtained are in good agreement with the results of a similar triblock copolymer synthesized by living anionic polymerization by Kuraray⁵⁴ (5 % loss at 276 °C and 316 °C in air and in an inert atmosphere, respectively), which shows the competitiveness of the RAFT process in relation to more stringent anionic polymerization. It is important to note that quantitative assessment of TTC stability in such triblock copolymers is not possible by TGA due to its low weight fraction (< 0.1 %).

RAFT thiocarbonylthio groups are known to exhibit limited thermal stability at high temperature. This stability is strongly dependent on the chemical nature of the Z-stabilizing group and neighboring groups. It was reported that trithiocarbonate elimination can be achieved at temperatures above 200 °C for PnBA-TTC.⁵⁵ To evaluate the possible negative impact of the TTC group on TPE properties, the PMMA<sub>20k</sub>-b-PnBA<sub>135k</sub>-b-PMMA<sub>20k</sub> triblock copolymer was heated five times at 180 °C for 30 min in air atmosphere. The 180°C value was selected arbitrarily because TTC degradation is likely to be
triggered around this temperature for polyacrylates.⁵⁵ A slight broadening of the molar mass distribution (from $D = 1.05$ to $1.07$) without a significant change of $M_n$ of the heat-treated sample was observed (Figure 6), attesting the good stability of the TTC link under these conditions.

<figure><img src="image_17.png" /><figcaption>**Figure 6.** SEC-RI chromatograms of PMMA<sub>20k</sub>-b-PnBA<sub>135k</sub>-b-PMMA<sub>20k</sub> before and after exposure of 180 °C for 30 min (5 times).</figcaption></figure>

The thermal stability of PMMA<sub>20k</sub>-b-PnBA<sub>135k</sub>-b-PMMA<sub>20k</sub> was also investigated through an isothermal treatment at 180 °C for 30 min, followed by measurements of the storage modulus between 20 °C and 100 °C. This cycle was repeated five times. Indeed, the cleavage of the mid-chain TTC group would generate a fraction of diblock copolymer with dangling PnBA blocks in the triblock copolymer, strongly deterring the envisioned TPE properties. **Figure S18** shows that such thermal treatment has no influence on the viscoelastic properties of PMMA<sub>20k</sub>-b-PnBA<sub>135k</sub>-b-PMMA<sub>20k</sub>, confirming the stability of the triblock copolymer architecture under these conditions.

The influence of the hard block composition over viscoelastic properties was further investigated through rheological measurements. **Figure 7A** displays the temperature dependence of $G'$ for representative triblock copolymers having similar hard/soft block ratio.

As previously reported,³³ the PMMA-based triblock copolymer (PMMA<sub>20k</sub>-b-PnBA<sub>135k</sub>-b-PMMA<sub>20k</sub>) behaves as a typical TPE material with storage modulus values in the order of $10^{5-6}$ Pa below the glass transition of the PMMA hard block (Figure 7A, black curve). Yet, this decrease is progressive and
suggests a well-defined phase separation of hard and soft domains above the glass transition of PMMA.

No order-disorder transition inherent to the mixing of the two phases is observed over the range of temperature investigated. This characteristic imparts excellent mechanical integrity to this particular triblock copolymer over a broad temperature range. Nevertheless, this feature could also limit the further development of this material, especially concerning its implementation in industrial processing techniques such as injection molding or extrusion, where low viscosity at high temperatures is a prerequisite. One solution to overcome this limitation would consist in decreasing the molar mass of the triblock copolymers, as T<sub>ODT</sub> decreases with polymer chain length for constant block chemistry and given composition.<sup>56,57</sup>

The statistical introduction of bulky groups such as isobornyl in the hard blocks ($P(iBoMA-co-MMA)_{21k}-b-PnBA_{99k}-b-P(iBoMA-co-MMA)_{21k}$) only slightly modifies the viscoelastic behavior of the triblock copolymers, with storage modulus values of the same order as PMMA$_{20k}-b-PnBA_{135k}-b-PMMA_{20k}$ and no order-disorder transition (**Figure 7A**, red curve). In contrast, when iBoMA is used as unique constituent of the hard phase ($PiBoMA_{23k}-b-PnBA_{117k}-b-PiBoMA_{23k}$), the decrease of G' with increasing temperature is steeper (**Figure 7A**, blue curve), which hints a lower segregation strength of the system under study. Indeed, a sudden drop of G' is observed after 200 °C, suggesting a probable order-disorder transition of the segregated structure.

Further investigation on the segregation behavior of these triblock copolymers was pursued by small
angle X-ray scattering (SAXS). **Figure 7B** shows the SAXS profiles obtained at 150 °C for a series of TPE
materials with similar composition and different hard blocks. As attested by the SAXS profiles, all
samples are segregated at 150 °C, confirming the phase separation of the systems in both hard and
soft domains (similar results **(Figure S19)**) were obtained at 25 °C, with no hints of order-order
transitions over this temperature span). A sequence of peaks at scattering wavevectors, indexed as
$q/q^* = 1, \sqrt{3}, \sqrt{4}, \sqrt{7}$, is clearly visible for PMMA$_{20k}$-b-PnBA$_{135k}$-b-PMMA$_{20k}$ which is consistent with a
cylindrical mesostructure (lattice parameter, *p* = 37.9 nm). The SAXS profiles of $P(iBoMA-co-MMA)_{21k}$-
$b$-PnBA<sub>99k</sub>-b-P(iBoMA-co-MMA)<sub>21k</sub> (*p* = 30.1 nm) and PiBoMA<sub>23k</sub>-b-PnBA<sub>117k</sub>-b-PiBoMA<sub>23k</sub> (*p* = 40 nm) samples only display a first order peak and, thus, it is difficult to assign a morphology without ambiguity.

<figure><img src="image_18.png" /><figcaption>**Figure 7.** (A) Isochronal temperature ramp tests upon heating at 2.5 °C/min for PMMA<sub>20k</sub>-b-PnBA<sub>135k</sub>-b-PMMA<sub>20k</sub>, PiBoMA<sub>21k</sub>-b-PnBA<sub>117k</sub>-b-PiBoMA<sub>23k</sub> and P(iBoMA-co-MMA)<sub>21k</sub>-b-PnBA<sub>99k</sub>-b-P(iBoMA-co-MMA)<sub>21k</sub> triblock copolymers. (B) SAXS spectra of PMMA<sub>20k</sub>-b-PnBA<sub>135k</sub>-b-PMMA<sub>20k</sub>, PiBoMA<sub>23k</sub>-b-PnBA<sub>117k</sub>-b-PiBoMA<sub>23k</sub> and P(iBoMA-co-MMA)<sub>21k</sub>-b-PnBA<sub>99k</sub>-b-P(iBoMA-co-MMA)<sub>21k</sub> triblock copolymers acquired at 150 °C. Data have been shifted vertically for clarity. **Data for PMMA<sub>20k</sub>-b-PnBA<sub>135k</sub>-b-PMMA<sub>20k</sub> was taken from ref 33.**</figcaption></figure>

## Conclusion

The symmetrical trithiocarbonate **TTC-bCP** was used to synthesize different high molar mass ($M_n$ > 150 kg mol<sup>-1</sup>) all-(meth)acrylic triblock copolymers exhibiting remarkably low dispersities ($D < 1.10$) by RAFT polymerization. Through a convergent two-step process and using MMA and iBoMA monomers either alone or in mixtures of various proportions, it was possible to tune the glass transition temperature of the hard domains between 120 and 180 °C. Then, the successful synthesis of the soft acrylate mid-block allowed the synthesis of hard-soft-hard triblock copolymers of controlled molar mass and composition. The thermal stability of the TTC mid-chain group in copolymers was verified by SEC analyses and after material exposure to 180 °C (5 cycles) without loss of TPE properties, demonstrating the integrity of the macromolecular triblock architecture. Both viscoelastic and scattering data concluded on the phase separation into hard and soft domains of these systems, with in particular accessible order-disorder transition for the triblock system based on pure PiBoMA hard
blocks. This clearly demonstrates the interest of fine structural design of the hard blocks in order to
tune the final viscoelastic and mechanical properties of this new class of TPEs.

## Acknowledgements

The authors thank Pascale Laborie, Institut de Chimie de Toulouse ICT-UAR 599, and Lucie Perquis, Assistante Ingénieur CNRS, for their valuable respective help with SEC and DSC analyses.

## Supporting Information

NMR spectra of studied compounds; additional SEC chromatograms, thermal analysis of the polymers.

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