The chemically inert nature of fully saturated hydrocarbon backbones endows vinyl polymers with desirable durability, but it also leads to their significant environmental persistence. Enhancing the sustainability of these materials requires a pivotal yet challenging shift: transforming the inert backbone into one that is degradable. Here, we present a versatile platform for mechanochemically editing the fully saturated backbone of polyacrylates towards a degradable polymer chain by integrating cyclobutene-fused succinimide (CBS) units along the polymer backbone through photo-iniferter reversible addition-fragmentation chain-transfer (RAFT) copolymerization. Significantly, the evenly insertion of CBS units does not compromise thermal or chemical stability but rather offers a means to adjust the properties of polymethylacrylate (PMA). Meanwhile, reactive acyclic imide units can be selectively introduced to the backbone through mechanochemical activation (ultrasonication or ball-milling grinding) when required. Subsequent hydrolysis of the acyclic imide groups enables efficient degradation, yielding telechelic oligomers. This approach holds promise for inspiring the design and modification of more environmentally friendly vinyl polymers through backbone editing.
Article
Mechanochemical Backbone Editing for Controlled Degradation of Vinyl Polymers
https://doi.org/10.21203/rs.3.rs-4310783/v1
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Vinyl polymers, especially polyacrylates, are extensively utilized in coatings, adhesives, and biomedical materials owing to their exceptional adhesion, durability, and weather resistance, highlighting their crucial role and adaptability in modern society.1 However, the all-saturated hydrocarbon backbone of polyacrylates poses significant challenges for chemical recycling or upcycling, thus exacerbating their environmental persistence (Fig. 1a). Amidst pressing environmental and sustainability concerns, the demand for degradable polyacrylates has surged within academia and industry.2 Recently, significant efforts are being directed towards developing methodologies for chemical recycling polymethacrylates, either by reverting them to monomers or inducing in-chain breakpoints.3-10 However, in contrast, there has been notably less progress in the deconstruction and chemical recycling of polyacrylates due to their high ceiling temperature.11
One strategy for addressing the persistent nature of polyacrylates and enhancing their degradability and sustainability is to incorporate cleavable functional groups along the polymer backbone. The most widely used approach to produce chemically degradable polyacrylates is the free-radical copolymerization of acrylates with a comonomer (Fig. 1b). Various monomers including cyclic ketene acetals,12,13 macrocyclic thionolactone,14,15 macrocyclic allylic sulfones/sulfides,16,17 isocyanides18 and α-lipoic acid19 have sequentially been utilized as comonomers to incorporate degradable units such as ester, thioester, carbonyl and disulfide groups into the polyacrylate backbone. While cleavable groups facilitate desirable degradability in response to mild triggers (e.g. base, light), the inherent instability resulting from unintentional degradation raises concerns about potentially compromising polymer properties during material application. On-demand tuning of polymer microstructures presents a potential solution to reconcile conflicting demands: maintaining high resistance to unintentional degradation while achieving desirable degradability.
Skeletal editing of polymer backbone has been considered as an attractive way for tailoring the structure and properties of polymer materials.20-22 This approach offers new opportunities to improve polymer sustainability by modulating their backbone degradability in response to specific stimuli. In this regard, chemical, thermal and force-induced technologies have been employed to promote de-cyclization editing of polymer backbone, leading to the incorporation of degradable functionalities into the main polymer chain. Notably, force-induced cycloreversion has been elegantly utilized to incorporate degradable functional groups such as acetal and ester into the backbone of unsaturated linear polymers obtained through ring-opening metathesis polymerization (ROMP).23,24 Despite the de-cyclization editing of unsaturated linear polymers, it is also possible to alter the degradability of fully saturated hydrocarbon backbones.25 Methods such as alkyl lithium promoted Skattebøl rearrangements of in-chain gem-dihalocyclopropane units26,27 and thermally induced retro-Diels–Alder reactions of in-chain norbornadiene-derived units28,29 have subsequently been employed to introduce unsaturated carbon-carbon bonds into fully saturated hydrocarbon chains. Unfortunately, backbone editing of commodity vinyl polymers, which can insert degradable groups into inert hydrocarbon chain, has not yet been successfully demonstrated.
Photo-controlled reversible addition-fragmentation chain-transfer (RAFT) polymerization has emerged as a powerful synthetic strategy for the preparation of vinyl polymers.30-32 To maintain the stability of the inert hydrocarbon backbone of vinyl polymers, a critical challenge lies in evenly inserting cyclic units that conceals the breakpoints with C-C bonds. Additionally, the modification of the all-saturated hydrocarbon backbone must be carefully targeted, particularly under specific stimuli, to reveal pre-installed cleavable functionalities. Herein, we present a versatile platform for mechanochemically editing the all-saturated hydrocarbon backbone to facilitate the degradation of vinyl polymers, where a cyclobutene-fused succinimide (CBS) comonomer is evenly incorporated into the backbone of polyacrylates through photo-iniferter RAFT polymerization (Fig. 1c). The key aspect is that the cyclobutane units preserve the integrity of the polymer backbone during material application, while the cleavable imide groups can be selectively exposed through backbone editing upon mechanochemical activation.
To achieve performance and sustainability goals in material design, there are two scientific requirements: (1) the optimal ratio of cleavage group incorporation to maintain desired properties, and (2) ensuring homogeneous distribution of cleavage groups along the polymer backbone to prevent the formation of large microplastic fragments during degradation. Therefore, the reactivity of corresponding monomers in radical polymerization is crucial for controlling polymerization kinetics and tailoring molecular architecture. The photochemical [2+2] cycloaddition of succinimides with acetylene or 1,2-dichloroethylene has been reported to has been reported to efficiently produce cyclobutene-fused succinimides (Table 1).33,34 However, only activated cyclobutene-based monomers such as alkyl cyclobutenecarboxylates were reported to produce radical copolymers with n-butyl acrylate.35,36 Unfortunately, little was known about the reactivity of nonactivated cyclobutene-based monomers in radical polymerization reactions. Hence, we embarked on exploring the radical copolymerization of methyl acrylate (MA) with a cyclobutene-fused succinimide (CBS) comonomer. The CBS comonomer consists of a strained cyclobutene and a fused succinimide; the former may serve as the polymerization site moiety, while the latter acts as the preinstalled cleavable group. To establish a controllable polymerization system, the photo-iniferter RAFT polymerization of MA was initially conducted with a [MA]/[CTA] ratio of 200/1 in DMSO under blue-light irradiation. The kinetic plot exhibited a first-order relationship between ln([M]0/[M]t) over reaction time, strongly indicating of a constant radical concentration (Supplementary Fig. 6a). Despite variations in monomer conversions, the number average molecular weight (Mn,SEC) of the resulting polymethylacrylate (PMA) agreed well with theoretical values (Mn,NMR), yielding low dispersities (Ð = Mw/Mn < 1.08, Supplementary Figs. 6b and 6c).
Afterwards, photo-iniferter RAFT copolymerization of MA and CBS were conducted with different feed ratios (Table 1, entries 1-9). After precipitated from methanol, the composition of the purified product was determined by 1H NMR spectrum through comparison of the resonances of PMA segments and CBS units, respectively (Fig. 2a). To unambiguously verify the insertion of CBS, the diffusion-ordered spectroscopy (DOSY) NMR of the resultant products exhibited that all the characteristic signals shared single one diffusion coefficient with a sharp peak (Fig. 2b). Meanwhile, the size-exclusion chromatography (SEC) curve gave a monomodal distribution (Supplementary Figs. 8, 12, 14, 16, 21, and 24), indicating the formation of PMA bearing in-chain CBS units. With different comonomer feed ratio, a range of PMA copolymers incorporating CBS units (0.9-9.7 mol%) were synthesized to evaluate the impact of the comonomer content on the thermal properties. Differential scanning calorimetry (DSC) analysis of PMA exhibited a singular glass transition temperature (Tg) at 7.8 °C. The incorporation of CBS units augmented the thermal properties of PMA. With an increase of CBS incorporation ratio from 0.9 to 9.7 mol%, the glass transition temperatures rose from 19.5 to 38.8 °C (Fig. 2c).37 Thermogravimetric analysis (TGA) displayed a thermal decomposition profile closely mirroring that of commercial PMA materials, with a 5 wt% loss observed at 360-365 °C, comparable to their Td value of 355 °C (Fig. 2d). Consequently, the inclusion of CBS units does not compromise thermal stability but rather offers a means to modulate the thermomechanical properties of PMA. Furthermore, the synthesized copolymer in Table 1, entry 6 was subjected to treatment with trifluoroacetic acid water (TFA/H2O) at 40 oC for 24 hours. The SEC (Fig. 2e) and 1H NMR (Supplementary Fig. 44) analyses revealed no alternations, indicating that polyacrylates bearing in-chain CBS units displayed desired chemical stability with no unintended chain scission under acidic conditions, owing to the relative stability of cyclic imide.
To gain deeper insights into the impact of monomer composition on copolymerization kinetics and controllability, we closely monitored the evolution of monomer conversion and molecular weight over time for two distinct monomer feed ratios (MA : CBS = 9 : 1 and 7 : 3). When the feed ratio of CBS was higher, the conversions of MA and CBS both increased slower throughout the reaction (Fig. 3a, Supplementary Tables 2 and 3). As a typical controlled character of the copolymerization, a proximate first-order kinetic plots suggested constant radical concentrations during copolymerization, discernible from the slope (Fig. 3b, kobs, rate = kobs × [monomer], kobs= kp × [radical]). However, the polymerization rate increased with decreasing CBS fraction in the feed (kobs, rate = 0.61 and 0.33 h-1 for MA : CBS = 9 : 1 and 7 : 3, respectively). Furthermore, the linear relationships observed between the molecular weights and total monomer conversions of propagating polymers, along with monomodal SEC traces, indicated controlled polymerizations (Fig. 3c). Taking the advantage of photo-iniferter RAFT copolymerization, “ON/OFF” experiments were designed under a periodic process. At regular intervals, samples were withdrawn from the reaction mixture for 1H NMR and SEC analyses (Supplementary Figs. 29 and 30). Fig. 3d demonstrates that light exposure enables precise "ON/OFF" control over the conversions of MA with CBS simultaneously. Mn rises gradually with reaction time indicating the continuous chain propagation, which can be entirely terminated under dark conditions (Fig. 3e). The narrow dispersities of SEC traces demonstrated that our systems behaved perfect temporal control.
To resolve the monomer distribution and composition of the resultant copolymers, we determined the reactivity of MA and CBS by conducting a series of copolymerizations with varied monomer feed ratios (0.3 ≤ fMA ≤ 0.9) and low monomer conversion to prevent significant composition drift (<10%, Supplementary Table 5). After purification, the Mayo–Lewis plot was generated from the MA content in the copolymer (FMA) against the MA molar fraction in feed (fMA, Fig. 3f). It is evident that the MA content in copolymer surpasses the monomer feed ratio, indicating a preference for MA insertion over CBS. The value of rMA was calculated as 19.5, which compares with the value 18.1 and 18.8 Fineman-Ross (FR) and Kelen-Tüdös (KT) methods, respectively (Supplementary Figs. 32 and 33). Nevertheless, the value, rCBS = 0, suggests that the consecutive radical insertions of CBS are assumed to be impossible, which is consistent with the poor radical homopolymerization reactivity of CBS (Table 1, entry 9). Additionally, copolymerizations with high monomer conversions (feed ratio: MA : CBS = 7 : 3) demonstrated consistent incorporation ratios of CBS units (Table 1, entries 3-6), implying that the CBS units are evenly distributed along the PMA chain. This even distribution is crucial for maximizing the influence of cleavage groups from the degradable moieties.
Inspired by these results, we expanded the applicability of this methodology beyond MA by exploring copolymerization with other vinyl monomers. Acrylate derivatives such as n-butyl acrylate (nBA), benzyl acrylate (BnA), and N,N-dimethylacrylamide (DMA) were all able to copolymerize with CBS in high reactivity, resulting in the formation of corresponding polymers containing in-chain CBS units (Table 1, entries 10-12). Although similar CBS incorporation ratios were observed, the copolymerization with BnA gave the highest molecular weight (Table 1, entry 11, Mn = 103.5 kg/mol). Unfortunately, low polymerization reactivity and no CBS incorporation were observed in the case of styrene (Sty), possibly due to the radical chain termination after radical addition to a CBS monomer (Table 1, entry 13).
Cyclobutane-fused units on polymer backbone have the potential to undergo force-induced cycloreversion, leading to the formation of unsaturated linear polymers.38-42 Although the force-induced cycloreversion of CBS units have not been reported so far, efforts were taken to study the mechanochemical cycloreversion reactivity of the obtained polyacrylate copolymers. Ring-opening of cyclobutane were explored through pulsed ultrasonication of the THF solutions at 0 oC. Aliquots were removed at regular time intervals and analyzed by both 1H NMR and SEC spectra (Supplementary Figs. 46, 47, 49 and 50). As sonication time increased, 1H NMR analysis revealed a continuous cycloreversion of cyclobutane, yielding linear acyclic imide segments (Fig. 4a). The rate of ring-opening exhibited a dependency on molecular weight, which is contributed to the nature of mechanochemical activation.43 Meanwhile, decrease in molecular weight was observed due to elongational forces causing chain scission (Fig. 4b).
As sonication time increased, the activation of units and the occurrence of unintended chain scission both escalated. To assess the balance between mechanical activation of CBS and chain scission, we introduced a function of the scission cycle (SC, eq. 1).43-47
Where, Mn,0 is the initial molecular weight, Mn,t is the current molecular weight after specific sonication time, the slope of the obtained plot ϕi serves as a quantitative parameter of the competition between mechanophore activation and polymer chain scission. A higher slope (larger ϕi value) indicates a greater degree of mechanophore reactivity under sonication conditions, implying more ring-opening events per scission cycle. Notably, copolymer with two distinct molecular weights showed similar ϕi values (Fig. 4c, Mn = 47.4 kg/mol, ϕi = 0.29; Mn = 63.2 kg/mol, ϕi = 0.28), which is higher than the previous observations by Bowser et al. with copolymer bearing in-chain alkyl cyclobutanecarboxylate units (Mn = 114 kg/mol, ϕi = 0.24).36 This result indicates that cyclobutane-fused succinimide mechanophores are more responsive to the reported alkyl cyclobutanecarboxylate mechanophores, which is beneficial for efficient backbone editing via mechanochemical cycloreversion.
The activation of CBS units leads to the unveiled acyclic imide groups into the polymer backbone (Fig. 4d, red line). It is known that acyclic imides exhibit substantially greater activity in hydrolysis compared to cyclic imides.48 The sonication activated products were then subjected to a solution of TFA/H2O (10/1 v/v) for 24 h at 40 oC. The 1H NMR spectrum of crude product revealed the absence of acyclic imide groups, which indicates the successful hydrolysis of the polymer backbone. After precipitation, the 1H NMR of polymeric material only showed the unactivated CBS units, similarly with the initial polymers. The SEC trace consistently shifts to longer elution time and the relative intensity of oligomer fraction increased obviously (ca. 4.7 kg/mol, Fig. 4e). The oligomers were collected and analyzed by high-resolution electrospray ionization mass spectrometry (HR-ESI-MS), and three types of bifunctional molecules were detected: diacid, diamide, and w-amide-a-acid telechelic oligomers (Supplementary Tables 6 and 7), which constitute important intermediates in recycling materials, such as preparation of polyamides and polyesters.
Significantly, it is also possible to achieve bulk mechanochemical activation of the obtained polyacrylate copolymers using ball-milling grinding. After 1.0 h of ball-milling, 1H NMR analysis revealed a 13% cycloreversion of the CBS units for the copolymer in Table 1, entry 6 (Fig. 4d, orange line). Meanwhile, a greater reduction in molecular weight (Fig. 4f, from 63.2 kg/mol to 19.6 kg/mol after 1.0 h) due to mechanically induced chain scission was observed comparing to that using ultrasonication activation (Fig. 4b, from 63.2 kg/mol to 30.5 kg/mol after 20 h). Moreover, hydrolysis of the unveiled acyclic imide groups under acidic conditions (10/1 v/v TFA/H2O) or basic conditions (0.5 M NaOH aq.) both led to evident degradation to low molecular weight oligomers (Fig. 4f, green and purple lines). This degradation ability holds significant importance, particularly when these materials are released into seawater, where they undergo prolonged exposure to mechanical forces from tides and waves.49
In summary, we have presented an unprecedented mechanochemical backbone editing protocol for the controlled degradation of vinyl polymers. A series of new vinyl polymers bearing in-chain cyclobutane-fused succinimides have been synthesized via a photo-iniferter RAFT copolymerization pathway. Kinetic studies suggest that the copolymerization of CBS and MA proceeds in a well-controlled way and behaves perfect temporal control. Additionally, the consistent incorporation ratios throughout the reaction process reveals the homogeneous distribution of CBS units along the backbone. Importantly, this evenly incorporation of CBS units does not compromise the thermal and chemical stability of the polymer; instead, it has been shown to elevate the glass transition temperatures of PMA copolymers. Meanwhile, backbone editing via mechanochemical cycloreversion of CBS units introduces reactive acyclic imide units into the main polymer chain of polyacrylates. The followed hydrolysis can recycle the polymers into diacid, diamide, and ω-amide-α-acid telechelic oligomers. The stability of the all-saturated backbone and accessible degradability when subject to mechanical forces (such as ultrasonication or ball-milling grinding) are both important for the practical application of these polymers. They may serve as environmentally friendly alternatives to commodity vinyl polymers, owing to the presence of in-chain CBS units. This research holds the potential to inspire the design and modification of vinyl polymers through backbone editing.
Data availability
All the other data supporting the findings of this study are available within the article and its Supplementary Information.
Acknowledgements
This research was supported by the Fundamental Research Funds for the Central Universities (23X010301599) and Shanghai Jiao Tong University 2030 Initiative (WH510363002/002). We are grateful to Prof. Ye Sha and Dr. Junfeng Zhou for the helpful discussions on mechanical activation.
Author Contributions
S.T. and Y.Z. contributed to the conception and design of the experiments. Z.L. performed the polymer synthesis and characterization. Z.L. and X.Z. conducted the polymer mechanochemical activation and degradation. Y.Z. and S.T. cowrote the manuscript. All authors participated in discussion and S.T. directed the project.
Competing financial interests
The authors declare no competing financial interests.
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