Thermal Solution Depolymerization of RAFT Telechelic Polymers

Thermal solution depolymerization is a promising low-temperature chemical recycling strategy enabling high monomer recovery from polymers made by controlled radical polymerization. However, current methodologies predominantly focus on the depolymerization of monofunctional polymers, limiting the material scope and depolymerization pathways. Herein, we report the depolymerization of telechelic polymers synthesized by RAFT polymerization. Notably, we observed a significant decrease in the molecular weight (Mn) of the polymers during monomer recovery, which contrasts the minimal Mn shift observed during the depolymerization of monofunctional polymers. Introducing Z groups at the center or both ends of the polymer resulted in distinct kinetic profiles, indicating partial depolymerization of the bifunctional polymers, as supported by mathematical modeling. Remarkably, telechelic polymers featuring R-terminal groups showed up to 68% improvement in overall depolymerization conversion compared to their monofunctional analogues, highlighting the potential of these materials in chemical recycling and the circular economy.


Methods 1.Materials
All materials were purchased from either Sigma Aldrich or Fischer Scientific unless otherwise stated.

NMR Spectroscopy
1 H-NMR spectra were recorded on a Bruker Avance-300 spectrometer using acetone-d6, dimethyl sulfoxide-d6, or CDCl 3 as the NMR solvent.Chemical shifts are given in ppm downfield from tetramethylsilane and referenced to residual solvent proton signals.
The resulting solution was filtered, and the solvent partially evaporated to yield a dark-brown residue.This residue was treated with a large excess of dilute HCl to precipitate the 1,4-Benzenedicarbodithioic acid (8.5 g).
Next, a mixture of 1,4-Benzenedicarbodithioic acid (3.00 g, 0.0130 mol) and α-methylstyrene (3.87 g, 0.0326 mol) in chloroform (50 mL) was heated at 75 °C for 20 hours.The reaction mixture was then extracted with sodium hydroxide solution to remove unreacted 1,4-Benzenedicarbodithioic acid.The remaining organic layer was concentrated using a rotary evaporator to remove the solvent.The resulting solid was washed with hexane and dissolved in a 1:1 mixture of ethyl acetate and hexane.This solution was then passed through a basic alumina column, and the volatiles were removed under reduced pressure to obtain a purple solid of bis(2-phenylpropan-2-yl) benzene-1,4-bis(carbodithioate).The 1 H NMR spectrum (300 MHz, CDCl3) displayed peaks at δ (ppm): 2.02 (s, 12H), 7.34 (m, 4H), 7.49 (d, 6H), and 7.87 (m, 4H).In a 10 mL round bottom flask with a stirrer bar, 29.8 mg of 1,4-Bis(2-(thiobenzoylthio)prop-2yl)benzene (1 equivalent, 0.064 mmol) dissolved in 500 µL of benzene was combined with 2.1 mg of AIBN (0.2 equivalents, 0.013 mmol) , and 2.00 g of methyl methacrylate (312.5 equivalents, 0.020 mol).The flask was sealed with a septum, and nitrogen bubbling for 15 minutes followed by polymerization in an oil bath at 80 °C.Samples were periodically withdrawn under a nitrogen blanket for 1 H-NMR analysis and filtered through a syringe filter (0.45 μM PTFE membrane) before SEC analysis.Polymerization was halted at 40% conversion by removing the reaction from the oil bath and removing the septum.The quantities specified here are for the synthesis of DP125 polymer, adjusted according to the targeted DPs.In a 10 mL round bottom flask equipped with a stirrer bar, 29.8 mg bis(2-phenylpropan-2-yl) benzene-1,4-bis(carbodithioate) (1 equivalent, 0.064 mmol) dissolved in 500 µL of benzene was combined with 2.1 mg of AIBN (0.2 equivalents, 0.013 mmol) and 2.00 g of methyl methacrylate (312.5 equivalents, 0.020 mol) and 29.8 mg bis(2-phenylpropan-2-yl) benzene-1,4bis(carbodithioate) (1 equivalent, 0.064 mmol).The flask was sealed with a septum, and deoxygenation was carried out by bubbling nitrogen for 15 minutes before conducting the polymerization in an oil bath at 80 °C.Samples were periodically withdrawn under a nitrogen blanket for 1 H-NMR analysis and filtered through a syringe filter (0.45 μM PTFE membrane) before SEC analysis.Polymerization was halted at 40% conversion by removing the reaction from the oil bath and removing the septum.The quantities specified here are for the synthesis of DP125 polymer and were adjusted according to the targeted DPs.In a 10 mL round bottom flask equipped with a stirrer bar, 2.1 mg of AIBN (0.2 equivalents, 0.013 mmol) was combined with 2.00 g of methyl methacrylate (312.5 equivalents, 0.020 mol), 17.4 mg of 2-Phenylpropan-2-yl benzodithioate (1 equivalent, 0.064 mmol) and 1.00 mL of Acetonitrile.

Synthesis of PMMA polymers using cumyl dithiobenzoate
The flask was then sealed with a septum, and deoxygenation was carried out by bubbling nitrogen for 15 minutes before conducting the polymerization in an oil bath at 80 °C.Samples were periodically withdrawn under a nitrogen blanket for 1 H-NMR analysis and filtered through a syringe filter (0.45 μM PTFE membrane) prior to SEC analysis.Polymerization was stopped at 40% conversion by removing the reaction from the oil bath and removing the septum.The quantities specified here are for the synthesis of DP125 polymer and were adjusted according to the targeted DPs.

Typical Depolymerization Procedure
In a 250 ml schlenk flask, 20 mg of PMMA was dissolved in 40 ml 1,4-dioxane (5 mM repeat unit concentration).25µL of poly(ethylene glycol) monomethyl ether (M n =350 Da) was then added as an internal standard for 1 H NMR analysis.The schlenk tube was sealed with a rubber septum and deoxygenated by nitrogen bubbling for 20 minutes.The schlenk flask was then placed in an oil bath at 120 °C (submerged ~2 cm below the surface of the oil bath) to start the reaction.To take samples, the reaction was periodically removed from the oil bath and quickly added to a water bath until the solution cooled to room temperature.The solution was then sampled under a nitrogen blanket.For SEC samples, ~3 ml of the sample solution was blow-dried, dissolved in 1.5 ml DMAc, and passed through a syringe filter (0.45 μm PTFE membrane) prior to analysis.

Depolymerization with addition of CTA
In order to study the control of the depolymerization reactions, the DP 240 monofunctional polymer and Z-terminal polymer were mixed with 1 and 5 equivalents of chain transfer agent (cumyl dithiobenzoate) and carried out the depolymerization reactions using the same procedure stated in 2.6.

End group removal experiment
A 15 mL glass vial containing 50 mg of PMMA polymer was sealed with a rubber septum and deoxygenated for 15 minutes.The deoxygenated vial was then placed in an oil bath at 180°C and heated for 45 minutes.During this period, the pink-colored polymer turned into a yellow-colored solid, indicating the removal of the end group.The end group removal was subsequently characterized by SEC through the loss of the Z group UV signal.

Mathematical Modelling
Mathematical modeling was conducted utilizing data acquired through SEC analysis.

Kinetics:
- Solving the system of differential equations gives: Depolymerization conversion: %UV variation during the depolymerization

Species:
A - Solving the system of differential equations gives:

Average MW:
[B] + [C] : M n : Depolymerization conversion: %UV variation during the depolymerization:            UV signal before and after the end group removal.(Typically, during thermolysis, there should be no noticeable shift in the SEC traces before and after the process, as only the RAFT Z groups are removed.However, the significant shift observed in the SEC results can be attributed to a unique aspect of the R-terminal bifunctional polymers.In these polymers, the Z groups are situated in the middle of the chain.Upon the removal of these Z groups, the chain undergoes cleavage into two halves, each with half the original length.This cleavage results in a substantial alteration in the apparent molecular weight of the polymer, as detected by SEC analysis.)
Scheme S 5: Synthesis of monofunctional polymers 1. Depolymerization %=     0          0 2. UV loss %=     0          0In formulating the mathematical model, we assumed that the chains undergo either instantaneous termination following the activation of the Z group or depolymerization until completion or until they encounter an end.The rate coefficient for depolymerization was designated as k 2 , while the rate coefficient for termination was labeled as k 1 .Based on these assumptions, rate equations were formulated for each species to track their concentrations throughout the process.The values of k 1 and k 2 were then precisely estimated to optimize the model's alignment with experimental data, particularly %depolymerization and %UV loss, minimizing discrepancies.Once the model achieved a superior fit, the optimized k 1 and k 2 values were applied to accurately determine the concentrations of each species at time t.Subsequently, the M n was estimated utilizing the concentrations of each species.

3. 1 .
Scheme S6: Schematic diagram describing the events during the depolymerization of the Z-bifunctional polymers: AA : Polymer with 2 living chain ends (M n =M) AC : Polymer with one living and one dead chain end, (M n =M), CC : Polymer with 2 dead chain ends, (M n =M) , A : Polymer with one living chain end (after depolymerization of one Scheme S7: Schematic diagram describing the events during the depolymerization of the R-bifunctional polymers: A: Initial polymer (M n = M), B : living polymer after depolymerization of one arm, (M n = M/2), C : dead polymer after depolymerization of one arm, (M n = M/2) , D : Depolymerization of both branches M n ~ 0, k 1 =rate of termination, k 2 =rate of depolymerization Figure S1: 1 H-NMR spectra of Z-terminal bifunctional CTA

Figure S4 :
Figure S4: SEC traces of polymers synthesized with Z-terminal bifunctional CTA

Figure S7 :
Figure S7: SEC traces obtained during the depolymerization of Z-terminal bifunctional polymers: a).DP 75, b).DP 125, c).DP 350 and d).DP 550.Table S 2: Depolymerization conversions, M n values (experimental) and M n shifts for the depolymerization of the DP 240 Z-terminal bifunctional polymer and monofunctional polymer.

Figure S8 :
Figure S8: SEC traces demonstrating the M n Shift which occurred during the depolymerization of DP 240 monofunctional (yellow) and Z-terminal bifunctional (purple) polymers.

Figure S10 :
Figure S10: a).Depolymerization kinetics of the Z-terminal bifunctional polymer (reaction conditions: 5 mM repeating unit concentration of polymer in dioxane at 120 °C) with addition of 0, 1 and 5 eq. of CTA, b).M n shifts comparison of the depolymerization reactions with different amount of CTA addition.

Figure S11 :
Figure S11: Comparison between the modeled (half-filled diamonds) and experimentally obtained (filled diamonds) M n values during the depolymerization of poly(benzyl methacrylate, poly(butyl methacrylate) and poly(methyl methacrylate) Z-terminal bifunctional polymer.(solid lines represent the variation of M n(exp) , where the dashed lined represent the variation of M n(model) )

Figure S13 :
Figure S13 : Comparison between the modeled and experimentally obtained M n values during the depolymerization of DP 75 Z-terminal bifunctional polymer.

Figure S14 :
Figure S14: End group removal of DP 550 R-terminal bifunctional polymer (conditions: the solid polymer heated at 180 °C for 45 min) a).Normalized RI signal before and after end group removal, b).UV signal before and after the end group removal.(Typically, during thermolysis, there should be no noticeable shift in the SEC traces before and after the process, as only the RAFT Z groups are removed.However, the significant shift observed in the SEC results can be attributed to a unique aspect of the R-terminal bifunctional polymers.In these polymers, the Z groups are situated in the middle of the chain.Upon the removal of these Z groups, the chain undergoes cleavage into two halves, each with half the original length.This cleavage results in a substantial alteration in the apparent molecular weight of the polymer, as detected by SEC analysis.)

Table S 1
: M n and Đ values of monofunctional, Z-terminal bifunctional and R-terminal bifunctional polymers

Table S 2
: Depolymerization conversions, M n values (experimental) and M n shifts for the depolymerization of the DP 240 Z-terminal bifunctional polymer and monofunctional polymer.

Table S3 :
Comparison between the depolymerization conversions, M n values (experimental) and M n shifts of DP 240 R-terminal bifunctional polymer and DP 240 Z-terminal bifunctional polymers.
* Depo% = (Area of RI signal at t 0 -Area of RI signal at t)/ Area of RI signal at t 0