Acid-assisted depolymerization of RAFT-derived PMMA

Abstract

Chemical recycling of vinyl polymers to their constituent monomers offers a route to closed-loop reuse, but depolymerization of polymethacrylates typically requires very high temperatures or catalysts. Here, we show that simple Brønsted acids accelerate the depolymerization of RAFT-derived poly(methyl methacrylate). Addition of p-toluenesulfonic acid increased depolymerization by more than nine-fold within the first hour and by nearly an order of magnitude at longer reaction times relative to thermal depolymerization alone, and this acceleration compounded additively with green-light irradiation. The magnitude and duration of acceleration depended on the identity of the Brønsted acid, suggesting a role for the conjugate base beyond simple proton donation. These results suggest acid addition as a strategy for accelerating depolymerization of RAFT-derived polymers that is compatible with existing thermal and photochemical methods.

Simple Brønsted acids accelerate depolymerization of RAFT-derived PMMA, and the effect compounds additively with green-light irradiation.

Introduction

Effective recycling strategies are essential for reducing polymer waste while conserving valuable resources.^1–3 Mechanical recycling remains the most widely implemented recycling strategy; however, repeated processing often reduces molecular weight and degrades material properties, limiting the performance and value of recycled plastics.^4–6 Consequently, considerable attention has shifted toward chemical recycling, which seeks to recover valuable chemical feedstocks from polymers rather than simply reprocessing the existing material.^7–9 Depolymerization has emerged as a particularly attractive chemical recycling strategy because it regenerates constituent monomers that can subsequently be repolymerized into new materials, providing a pathway toward a circular polymer economy.^7,9

Despite the promise of chemical recycling, efficient depolymerization of vinyl polymers remains a challenge due to the exceptional stability of their carbon–carbon backbones.^10,11 Conventional depolymerization of these materials typically relies on high-temperature pyrolysis to generate the reactive species necessary to initiate chain depolymerization via unzipping.^12,13 Although effective, these harsh conditions are energy-intensive and frequently promote competing degradation pathways that reduce monomer selectivity and generate undesirable byproducts.^10–12,14 Consequently, effort has been devoted to developing depolymerization strategies that operate under milder and more controlled conditions.

Poly(methyl methacrylate) (PMMA) has become one of the most extensively studied model systems for developing depolymerization methodologies because of both its commercial importance and the potential to selectively regenerate methyl methacrylate (MMA).^15,16 Owing to its excellent optical transparency, weather resistance, dimensional stability, mechanical performance, and biocompatibility, PMMA is widely used in architectural glazing, automotive components, optical devices, medical applications, and dentistry.^17,18 Thus, the development of efficient methodologies to regenerate MMA under milder reaction conditions while maintaining high monomer recovery remains an important objective in polymer recycling.

A major advance in polymer depolymerization came with the recognition that polymers prepared by reversible-deactivation radical polymerization (RDRP) retain dormant chain-end functionalities,^19,20 which can be selectively reactivated to initiate depolymerization.^21,22 Unlike polymers prepared by conventional radical polymerization, RDRP-derived polymers retain reactive chain ends after polymerization, enabling radical generation at the polymer terminus under appropriate activation conditions.²¹ These chain-end radicals initiate sequential depropagation, allowing monomer regeneration to proceed from the polymer chain end rather than through random backbone cleavage.^22,23 Among the various RDRP techniques, reversible addition–fragmentation chain-transfer (RAFT) polymerization has proven particularly well suited for depolymerization because the preserved thiocarbonylthio chain end provides a convenient handle for radical generation.^21,24–26 Initial studies by Gramlich established that preserved chain-end functionality could indeed be exploited to regenerate monomers under relatively mild conditions, thereby laying the foundation for modern RAFT depolymerization.²⁷ Parallel advances in atom transfer radical polymerization (ATRP) have similarly enabled monomer regeneration from polymethacrylates, extending depolymerization strategies beyond the RAFT platform.^23,28–33

Building upon these pioneering studies, considerable effort has been focused on developing increasingly effective strategies to accelerate RAFT depolymerization by improving chain-end activation (Scheme 1A and B). Thermal depolymerization (Scheme 1A) demonstrated that RAFT-derived PMMA could be efficiently depolymerized under catalyst-free conditions at temperatures substantially lower than those required for conventional pyrolysis, showing that thermal activation is a practical route toward monomer regeneration.^21,34 The scope of thermal depolymerization was further expanded by systematically investigating the effects of reaction conditions, solvent, polymer concentration, chain-end structure, oxygen tolerance, and depolymerization kinetics, thereby significantly improving both the mechanistic understanding and the practical implementation of these systems.^35,36 Light-driven approaches provided a means to more directly promote chain-end radical generation (Scheme 1B). Direct irradiation of the thiocarbonylthio end group in combination with heat substantially accelerated RAFT depolymerization relative to thermal activation alone,²⁶ while the incorporation of photocatalysts such as eosin Y or zinc tetraphenylporphyrin further demonstrated that light-mediated activation could be leveraged to facilitate thiocarbonylthio chain-end activation and monomer regeneration.^37,38 Subsequent studies have explored alternative energy inputs and activation modes to improve the efficiency, concentration, and control of depolymerization. Microwave-assisted depolymerization, for example, enabled rapid monomer recovery at substantially higher repeat-unit concentrations under catalyst-free conditions,³⁹ while electrochemical approaches provided complementary control over radical generation through activation of chain-end functionalities or electrochemically cleavable pendent groups.^40,41 These studies trace a progression from thermal chain-end activation toward increasingly diverse strategies for controlling radical generation and promoting vinyl polymer depolymerization. The utility of controlled depolymerization has also expanded beyond straightforward bulk monomer recovery, including selective sculpting of polymer molecular weight distributions, differentiation of copolymers and sequence-controlled multiblocks,⁴² simultaneous degradation–depolymerization of bioderived comb copolymers,⁴³ etching of microphase-separated materials,⁴⁴ and solution disassembly of polymer nanoparticles.⁴⁵

Scheme 1. Schematic illustration of previously reported (A) thermal and (B) photothermal RAFT depolymerization strategies and (C) the Brønsted acid-assisted depolymerization strategy reported in this work.

Beyond modifying the physical mode of activation, complementary strategies have introduced radical-generating triggers within the polymer structure or through post-polymerization modification.⁴⁶ Pendent-group triggers have enabled efficient bulk depolymerization of conventionally prepared polymethacrylates, while mechanochemical and chemical retrofitting strategies have extended this concept to pre-existing and post-consumer polymers.^47–50 Comonomer-based strategies have enabled efficient depolymerization across multiple vinyl polymer classes, including the incorporation of thermally labile radical triggers into polystyrene and α-methylstyrene doping of PMMA, with the latter subsequently extended toward the upcycling of recovered MMA into value-added chemical feedstocks.^51–53 More recently, macromolecular architecture has been shown to influence the efficiency of both chain-end- and pendent-triggered PMMA depolymerization.⁵⁴ However, relatively little attention has been devoted to manipulating the chemical reaction environment itself as a means of accelerating chain-end activation.

Recent studies by Anastasaki and co-workers demonstrated the ability of Brønsted acids to promote radical polymerization through several distinct mechanisms. Initial work demonstrated acid-triggered radical polymerization through protonation-mediated monomer autoinitiation,⁵⁵ while subsequent studies showed that acid addition could accelerate RAFT polymerization, facilitating the synthesis of well-defined multiblock copolymers with reduced initiator concentrations.⁵⁶ More recently, Brønsted acid addition dramatically enhanced visible-light photoiniferter polymerization of thiocarbonylthio RAFT agents by eliminating the induction period, increasing chain-transfer-agent consumption, and accelerating polymerization.⁵⁷ Acid addition enhanced photolysis of the thiocarbonylthio compound, demonstrating that simple proton donors can influence the photochemical reactivity of RAFT agents.⁵⁷ We therefore questioned whether Brønsted acids could similarly accelerate RAFT depolymerization (Scheme 1C).

Herein, we demonstrate that the addition of simple Brønsted acids significantly accelerates the depolymerization of RAFT-derived PMMA under both thermal- and light-assisted conditions. Through systematic investigation of acid identity, acid loading, irradiation wavelength, and depolymerization kinetics, we identify conditions that significantly increase depolymerization rates and establish Brønsted acid additives as a simple strategy for accelerating RAFT depolymerization. We note that a related study examining acid-accelerated depolymerization of RAFT-derived polymethacrylates has been concurrently reported by Anastasaki and co-workers.⁵⁸

Results and discussion

PMMA bearing a trithiocarbonate chain end (PMMA-TTC) was synthesized by RAFT polymerization (Fig. S1) and subsequently subjected to thermal depolymerization in 1,4-dioxane at 100 °C at a repeat-unit concentration of 5 mM. Thermal depolymerization experiments were first performed to determine whether Brønsted acids could accelerate RAFT depolymerization. Reactions were conducted in the presence of 100 equiv. of p-toluenesulfonic acid (p-TsOH) relative to the trithiocarbonate chain-end alongside an acid-free control under otherwise identical conditions. The addition of p-TsOH resulted in markedly faster and more extensive depolymerization compared to the acid-free control (Fig. 1A). Whereas the control experiment exhibited only limited depolymerization, reaching approximately 7% conversion after 4 h, reactions containing p-TsOH afforded 38% depolymerization within the first hour and reached 65% conversion after 4 h. Thus, the addition of a simple Brønsted acid increased depolymerization by more than nine-fold within the first hour while increasing the final depolymerization conversion by nearly an order of magnitude at longer reaction times. ¹H NMR spectroscopy confirmed the regeneration of MMA, with the characteristic vinyl resonances increasing in intensity as the reaction progressed (Fig. 1B), confirming that the observed depolymerization results in recovery of the constituent monomer rather than nonspecific polymer degradation. Representative size exclusion chromatography (SEC) chromatograms further illustrated accelerated depolymerization in the presence of acid (Fig. 1C and D). Whereas only minor changes in the PMMA chromatographic traces were observed throughout the reaction in the absence of acid (Fig. 1C), reactions containing p-TsOH exhibited a progressive decrease in PMMA peak intensity with reaction time (Fig. 1D), consistent with the enhanced depolymerization observed in Fig. 1A. Notably, the residual PMMA peak retained a similar elution time and peak shape throughout depolymerization, and no new lower-molecular-weight polymer population was observed. These results confirmed that the presence of a Brønsted acid both accelerates the rate and enhances the extent of depolymerization relative to acid-free conditions.

Fig. 1. Brønsted acids accelerated the thermal depolymerization of RAFT-derived PMMA at 100 °C. (A) Depolymerization conversion versus time under thermal conditions with and without 100 equiv. of p-toluenesulfonic acid (p-TsOH). (B) Time-resolved ¹H NMR spectra showing methyl methacrylate regeneration in the presence of p-TsOH. (C and D) Representative SEC chromatograms showing a progressive decrease in the PMMA peak during depolymerization in the absence and presence of p-TsOH, with polystyrene used as an internal standard for determination of depolymerization conversion.

We hypothesized that the observed rate enhancement arises from Brønsted acid-assisted activation of the preserved RAFT chain end. One plausible explanation is that Brønsted acids facilitate homolytic cleavage of the chain-end C–S bond, thereby increasing the rate of radical generation required for depolymerization. Such activation may arise from transient protonation or hydrogen-bonding interactions involving the preserved RAFT chain end, thereby lowering the energetic barrier for C–S bond homolysis. Potential sites for these interactions include the thiocarbonylthio moiety itself, as well as ester carbonyl groups within the methacrylate polymer. Consistent with this general hypothesis, recent computational studies of acid-enhanced photoiniferter polymerization demonstrated that protonation of a trithiocarbonate-containing photoiniferter altered its excited-state energetics and lowered the calculated C–S bond dissociation enthalpy, thereby facilitating homolytic C–S bond cleavage.⁵⁷ Although those calculations were performed for an excited-state small-molecule photoiniferter under aqueous conditions, they nevertheless provide precedent that Brønsted acid-mediated protonation can lower the energetic barrier for C–S bond homolysis. We therefore propose that a related effect may facilitate activation of the preserved RAFT chain end in the present system, although the precise site of protonation or hydrogen bonding and the molecular origin of this activation remain unresolved.

These initial results led us to investigate whether the acid-mediated enhancement could be further amplified by combining Brønsted acid addition with light-assisted depolymerization. We considered whether combining two activation strategies could provide complementary enhancements in depolymerization efficiency (Fig. 2A). At a monomer unit concentration of 5 mM in 1,4-dioxane at 100 °C, green-light irradiation (λ = 525 nm) significantly accelerated depolymerization compared with thermal conditions alone. More importantly, simultaneous application of green-light irradiation and Brønsted acids afforded the highest depolymerization conversions under the conditions investigated, outperforming either activation strategy alone.

Fig. 2. Brønsted acids and green light irradiation provide additive enhancements to RAFT depolymerization at 100 °C. (A) Depolymerization profiles of PMMA-TTC under thermal, thermal + p-TsOH, thermal + green light, and thermal + green light + p-TsOH conditions. (B) Pseudo-first-order kinetic plots of the corresponding depolymerization reactions. (C) Apparent depolymerization rate constants (k_dp,app) determined from the slopes of the linear fits shown in (B).

Four conditions were compared: thermal only, thermal with p-TsOH, thermal with green-light irradiation, and thermal with both green-light irradiation and p-TsOH. To quantitatively assess their relative rates, the data were analyzed assuming pseudo-first-order behavior by plotting −ln(1 − p) as a function of time, where p is the depolymerization conversion (Fig. 2B). Good linear correlations were obtained under all reaction conditions, allowing apparent depolymerization rate constants (k_dp,app) to be determined from the slopes of the corresponding linear fits. As summarized in Fig. 2C, green-light irradiation under heating increased the apparent depolymerization rate by approximately 8-fold relative to thermal depolymerization alone, while Brønsted acids with heat produced an even greater enhancement of approximately 17-fold. Combining acid exposure with green light and heat resulted in the highest apparent depolymerization rate constant, corresponding to more than a 20-fold enhancement over thermal control. Acid addition and light irradiation thus appeared to independently accelerate RAFT depolymerization, and their combination afforded the fastest rates and highest conversions of the four conditions examined.

To further probe whether the observed enhancement depends on activation of the RAFT chain end, conventionally prepared PMMA lacking a thiocarbonylthio chain end was subjected to the combined thermal, green-light, and p-TsOH conditions. No measurable depolymerization was observed over the course of the reaction (Fig. S8), whereas RAFT-derived PMMA readily underwent depolymerization under otherwise identical conditions. This control supports the involvement of the preserved RAFT chain terminus in the observed depolymerization and is consistent with acid-assisted chain-end activation.

Because the combination of acid and green light proved most effective, we next examined the influence of acid loading. Depolymerization reactions were conducted using 25, 50, and 100 equiv. of p-TsOH while maintaining all other reaction conditions constant, including thermal activation and green light irradiation (Fig. 3A). Increasing the p-TsOH loading progressively increased the depolymerization conversion, reaching approximately 53%, 70%, and 73% after 4 h with 25, 50, and 100 equiv., respectively. To determine whether these higher conversions originated from faster depolymerization kinetics, the experimental data were analyzed assuming pseudo-first-order depropagation behavior (Fig. 3B). Good linear correlations were obtained for all three acid loadings, allowing k_dp,app to be determined from the slopes of the corresponding linear fits. As summarized in Fig. 3C, the apparent depolymerization rate constant increased progressively from 0.131 h⁻¹ in the absence of acid to 0.200, 0.300, and 0.344 h⁻¹ for 25, 50, and 100 equiv. of p-TsOH, respectively. Higher acid loadings, therefore, systematically increase both the rate and final conversion under the optimized light-assisted conditions. Given that 100 equiv. of p-TsOH afforded both the highest depolymerization conversion and the largest apparent depolymerization rate constant, this loading was selected for all subsequent studies unless otherwise noted.

Fig. 3. Effect of Brønsted acid loading on the depolymerization of RAFT-derived PMMA at 100 °C. (A) Depolymerization profiles obtained using 25, 50, and 100 equiv. of p-toluenesulfonic acid (p-TsOH) under optimized thermal/green light conditions. (B) Pseudo-first-order kinetic analysis of the corresponding depolymerization reactions. (C) Apparent depolymerization rate constants (k_dp,app) determined from the slopes of the linear fits shown in (B).

To determine whether other Brønsted acids could similarly promote depolymerization, a series of structurally distinct acids were evaluated under the optimized depolymerization conditions established above using 100 equiv. of acidic functionalities relative to the RAFT chain end. The acids examined included methanesulfonic acid (MSA), acetic acid, camphorsulfonic acid (CSA), citric acid, and p-TsOH (Fig. 4A). As shown in Fig. 4B, all acids substantially enhanced depolymerization relative to the acid-free control, although minor differences in the depolymerization profiles were observed. Among the acids investigated, p-TsOH proved to be the most effective. The final depolymerization conversions obtained after 4 h are summarized in Fig. 4C, demonstrating that all acids significantly outperformed the acid-free control, with p-TsOH affording the highest depolymerization conversion under the conditions investigated.

Fig. 4. Scope of Brønsted acid-assisted depolymerization of RAFT-derived PMMA at 100 °C. (A) Brønsted acids evaluated in this study, including acetic acid (AcOH), citric acid, camphorsulfonic acid (CSA), methanesulfonic acid (MSA), and p-toluenesulfonic acid (p-TsOH). (B) Depolymerization profiles obtained using the indicated Brønsted acids under the optimized thermal and green light irradiation conditions. (C) Comparison of the final depolymerization conversions after 4 h.

The differences in depolymerization profiles suggest that acid strength alone does not fully account for the observed behavior and that additional factors, such as acid structure, solubility, and interactions with the polymer or the RAFT chain end, may contribute to overall depolymerization performance. Because pK_a values are strongly solvent-dependent, literature pK_a values determined in other solvents provide only a qualitative comparison with the present reaction conditions in 1,4-dioxane. In particular, MSA afforded not only the fastest initial depolymerization but also the earliest reaction plateau, and increasing the MSA loading accelerated the initial depolymerization while shifting the onset of the plateau to progressively shorter reaction times (Fig. S13). These observations raise the possibility that, in addition to proton-assisted chain-end activation, the corresponding conjugate base influences the fate of the activated RAFT chain end. For example, the conjugate base may interact with the activated chain end through ion pairing, reversible association, or competing pathways that promote radical recombination, irreversible chain-end modification, or other deactivation processes. Differences in the conjugate-base structure, nucleophilicity, ion pairing, and solvation may therefore contribute to the distinct behaviors observed for the acids examined. The present data do not distinguish among these possibilities, and additional mechanistic and computational studies will be required to establish the respective roles of protonation, hydrogen bonding, and conjugate-base effects in Brønsted acid-mediated RAFT depolymerization.

We next asked whether the acid-mediated rate enhancement persisted across different irradiation wavelengths and sought to clarify the role of light in the depolymerization process. Thermal depolymerization reactions were conducted under green (λ = 525 nm), blue (λ = 440 nm), and UV irradiation (λ = 365 nm) using the conditions established above (100 equiv. of p-TsOH), and the resulting kinetics were compared with those under the corresponding acid-free reactions (Fig. 5A). Pseudo-first-order analysis revealed that acid addition increased the apparent depolymerization rate under all irradiation conditions investigated. Although the apparent depolymerization rates varied slightly with the irradiation wavelength, the highest k_dp,app values were consistently observed when acid and light irradiation were combined, demonstrating that light provides an additional kinetic enhancement beyond acid acceleration alone.

Fig. 5. (A) Pseudo-first-order kinetic analysis of depolymerization under thermal and visible-light irradiation conditions with and without p-TsOH at 100 °C. (B) Comparison of the final depolymerization conversions obtained after 4 h. (C) Representative SEC chromatograms demonstrating depolymerization under green, blue, and UV irradiation in the presence of p-TsOH, with polystyrene used as an internal standard for determination of depolymerization conversion.

The final depolymerization conversions obtained after 4 h are summarized in Fig. 5B. In the absence of acid, thermal depolymerization afforded only 7% depolymerization, while green, blue, and UV irradiation increased the conversion to 38%, 27%, and 42%, respectively. Upon addition of p-TsOH, the depolymerization conversion increased substantially under every reaction condition examined, reaching 65%, 73%, 69%, and 73% for heat treatment and green, blue, and UV irradiation, respectively. Under all irradiation conditions, acid addition enhanced both the rate and extent of depolymerization, while light further increased the extent of depolymerization across the wavelengths investigated. Representative SEC chromatograms obtained under green, blue, and UV irradiation in the presence of p-TsOH are shown in Fig. 5C. In each case, a progressive decrease in the PMMA SEC signal was observed with reaction time, consistent with increased depolymerization conversions.

The magnitude of the light-induced enhancement increased as acid loading decreased. While green light increased the final depolymerization conversion from 65% to 73% at 100 equiv. of p-TsOH, the improvement increased from 53% to 70% at 50 equiv. of p-TsOH and from 34% to 53% at 25 equiv. of p-TsOH under otherwise identical conditions (Fig. S18 and S19, respectively). A similar trend was observed for CSA, where green light irradiation increased the final depolymerization conversion from 50% to 68% (Fig. S20). We propose this trend as a natural consequence of additive, independent rate contributions under pseudo-first-order kinetics, with a constant photochemical rate increment producing a larger conversion gain when the baseline (acid-only) conversion is lower. The two complementary activation modes lead to a contribution of light that becomes increasingly pronounced under conditions where acid acceleration alone is less effective. Brønsted acid addition is therefore broadly compatible with light-assisted depolymerization across multiple irradiation wavelengths, and combining the two activation modes offers a versatile route to faster RAFT depolymerization.

Conclusions

The experimental observations are consistent with the working hypothesis proposed above. The absence of measurable depolymerization for conventionally prepared PMMA under the combined acid and green-light conditions supports the importance of the preserved RAFT chain end. Moreover, Brønsted acid addition and light irradiation increased the apparent depolymerization rate, while their combination afforded the highest rate, suggesting that the two activation modes provide complementary contributions to chain-end radical generation. The enhancement observed across a range of structurally distinct Brønsted acids further demonstrates that this effect is not unique to p-TsOH. The persistence of acid-mediated acceleration across multiple irradiation wavelengths indicates that the acid effect is not restricted to a particular photochemical activation pathway and may instead operate alongside light-mediated chain-end activation. Although the precise molecular origin of the acid-mediated enhancement remains to be fully elucidated, these results support Brønsted acid-assisted activation of the preserved RAFT chain end as a plausible origin of the accelerated depolymerization.

Author contributions

B. K. S. and R. W. H. conceived the idea. B. K. S., S. B., U. A. N. and L. E. D. carried out the experiments. B. K. S. prepared the first draft of the manuscript. B. K. S., L. E. D., R. W. H. and B. S. S. edited and revised the draft. All authors aided with data curation and interpretation. B. S. S. supervised the project and acquired funding.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting the findings of this study are available in the supplementary information (SI). Additional data are available from the corresponding author upon reasonable request. Supplementary information is available. See DOI: https://doi.org/10.1039/d6sc06540f.