Chemically recyclable polyesters from CO2, H2, and 1,3-butadiene

Chemically recyclable solid polymeric materials with commercializable properties only using CO2 and inexpensive bulk chemicals as chemical feedstock can open a brand-new avenue to economically viable, large-scale fixation of CO2 over a long period of time. Despite previous great advancements, development of such a kind of CO2-based polymers remains a long-term unsolved research challenge of great significance. Herein, we reported the first methodology to polymerize six-membered lactone with two substituents vicinal to the ester group (HL), a compound previously found to be non-polymerizable. The present methodology enables the first synthesis of chemically recyclable solid polyesters (polyHL) with a high CO2 content (28 wt %) and large molecular weights (Mn up to 613.8 kg mol−1). Transparent membranes with promising pressure-sensitive adhesive (PSA) properties comparable with their commercial counterparts can be conveniently fabricated from the polyesters. Mechanistic studies indicate that rigorous removal of water impurity is the key to the successful polymerization of the relatively inert disubstituted six-membered lactone. A complete monomer recovery from polyHL was also successfully achieved under mild catalytic conditions. The synthesis of polyHL only requires CO2 and two inexpensive bulk chemicals, H2 and 1,3-butadiene, as the starting materials, thus providing a new strategy for potential scalable chemical utilization of CO2 with desirable economic values and concomitant mitigation of CO2 emissions. This work should inspire future research to make useful new solid CO2-based polymers that can meaningfully increase the scale of chemical utilization of CO2 and promote the contribution of chemical utilization of CO2 to global mitigation of CO2 emissions.


INTRODUCTION
The scale of annual global production of synthetic polymers, mostly carbonaceous solid materials, is ca. 0.4 gigaton currently and is projected to reach ca. 1.2 gigaton in 2050. 1 Utilization of CO 2 as a main chemical feedstock to synthesize commercial solid polymers has great potentials in mitigation of CO 2 emissions. 2 To date, however, only ca. 0.001 gigaton of CO 2 has been used as a chemical feedstock in the production of commercial CO 2 -based polymers, primarily polycarbonates and polyols. 3,4 Thus, development of scalable new CO 2 -based polymers represents a significant challenge in the area of mitigating CO 2 emissions.
We reason that three requirements should be met to achieve large-scale production of new CO 2 -based polymers. Firstly, the co-feedstock to synthesize the polymers should be inexpensive bulk chemicals in order to meet both economic and scalable requirements. Secondly, the polymers can also be conveniently processed into desirable solid shapes with commercializable properties. Finally, to address the aggravating pollution issues from increasing polymer wastes, the polymers should be recyclable. Unfortunately, CO 2 -based polymers satisfying all the three requirements, which can potentially fix large-scale CO 2 in solid polymeric materials for a long period of time, are still unprecedented.
Polymers containing heteroatoms in their backbones, 5-7 especially polyesters, [8][9][10] featured with readily cleavable carboxylic ester backbone linkages, are excellent candidates for recycling. Therefore, making new polyesters from CO 2 and cheap bulk chemicals as co-feedstocks, especially large-volume olefins, such as ethylene and 1,3-butadiene, that can be derived from biomass, has long been pursued over the past several decades. [11][12][13][14][15][16] Although ester-containing solid polyolefins from CO 2 and 1,3-butadiene have been previously achieved in multiple-step fashions, the lack of processability, commercializable properties, and recyclability limits its potential large-scale utilization. 14,15,17,18 Herein, we report the first successful synthesis of polyesters using only CO 2 and cheap bulk chemicals as the co-feedstock (Figure 1). The synthe-sis was achieved via ring-opening polymerization (ROP) of an intermediate derived from CO 2 , H 2 , and 1,3-butadiene-a diethyl-substituted sixmembered lactones (HL) that was previously thought to be non-polymerizable. 16 Solid polyesters with high molecular weight (MW) (M n up to 613.8 kg mol À1 ) and high CO 2 content (28 wt %) were obtained. Remarkably, the polyesters can be conveniently processed into transparent and flexible membranes with pressure-sensitive adhesive (PSA) properties comparable with their commercial counterparts. A series of catalytic methods were also developed for clean chemical recycling of the polyesters back to their starting monomer HL.
Subsequently, we conjectured that increasing the basicity of the organocatalyst might promote the polymerization reactivity. Thus, three common phosphazene bases (PBs) in association with BnOH were investigated. For a 50/1 ratio of [HL]/[BnOH] with 1 mol % of t Bu-P 1 (tert-butyliminotris(dimethylamino)phosphorane) or t Bu-P 2 (1-tert-butyl-2,2,4,4,4-pentakis-(dimethylamino)-2l 5 ,4l 5 -catenadi (phosphazene)) at À25 C in THF ([HL] 0 = 5.3 M), no polymer was obtained for t Bu-P 1 after 72 h ( Table 1, run 1), whereas a 41% conversion was observed for t Bu-P 2 after 120 h ( Table 1, run 2; M n = 5,304 g mol À1 , Ð = 1.07). Encouragingly, the ROP process was dramatically improved when t Bu-P 4 (1-tert-butyl-4,4,4-tris(dimethylamino)-2,2-bis[tris(dimethylamino)phosphoranylide-namino]-2l 5 ,4l 5 -catenadi(phosphazene)) was employed, leading to 87% conversion after 12 h (Table 1, run 3). Corresponding polyHL with M n = 19,880 g mol À1 and moderate Ð = 1.90 was obtained. The difference in catalytic reactivities might be a consequence of the large basic differences among the three PBs 21 (pK a = 26.9, 33.5, and 42.7 in acetonitrile 22 for t Bu-P 1 , t Bu-P 2 , and t Bu-P 4 , respectively). Next, altering the t Bu-P 4 loading from 2 mol % to 0.2 mol % led to a much more controlled polymerization (Table 1, runs 4-7). Particularly, polyHL with M n = 9,154 g mol À1 and a much narrower Ð = 1.09 was obtained when the t Bu-P 4 loading was reduced to 0.2 mol % ( Table 1, run 7), suggesting a living polymerization behavior. However, as the system was gradually diluted ([HL] 0 = 2.0, 1.6 and 1.3 M in THF), the ROP of HL became less controlled: the conversion and M n were significantly decreased, along with a broader Ð ( Table 1, runs 8-10). Besides, elevating the reaction temperature from À25 C to 41 C also led to a less controlled polymerization (  (Table  S2). The polymerization kinetics data strongly support that the polymerization is living: the monomer conversion grows linearly with the reaction time ( Figure 2A Figure 2B); a linear correlation of the M n of polyHL with the monomer conversion was observed, and the dispersity Ð of the obtained polyHL remained low (Ð $ 1.1) during the chain propagation stage ( Figure 2C). In addition, The GPC curves for polyHL obtained at varied times were all confirmed to be unimodal ( Figure 2D). In addition, The M n of the resultant polymers increased linearly with increased [HL]/[BnOH] ratio, and the Ð of the polymers remained low, which further confirmed the living polymerization behavior ( Table 1, (Table 1, run 14) showed only one group of molecular ion peaks with the same spacing of 156.1 g mol À1 (M n = n•156.1 + 131.5), which could be assigned to the linear-polyHL initiated by BnOH ( Figure 3A). The NMR spectroscopies showed that both the methylene signal of BnO À (d 5.12 ppm) and methine signal (d 3.49 ppm) next to the hydroxyl end group were observed in the 1 H NMR spectrum, and corresponding 13 C NMR signals were also detected ( Figures 3B and S5).
Inspired by the excellent chain-end fidelity of polyHL sample, telechelic polyHL catalyzed by t Bu-P 4

Mechanistic studies for the ROP of HL to polyHL
To investigate the possible mechanism in our system, ROPs at a feed ratio of [HL]/[BnOH] = 50/1 with varied loadings of t Bu-P 4 (2, 1, 0.5, 0.4, and 0.2 mol %; Table 1, runs 3-7) were performed. Notably, the M n of the polyHL produced increased significantly as the loading of t Bu-P 4 increased, and the trends of bimodal distribution of the GPC traces have become more apparent as the t Bu-P 4 loading increased, which might be due to the existence of other competitive initiation mechanisms ( Figure S7). MALDI-TOF spectrum of low MW analogs catalyzed by 1 mol % of t Bu-P 4 disclosed three sets of molecular ion peaks with the same MW spacing of 156 g mol À1 . The signals were assignable to BnOH end-capped, with no chain end, and water end-capped polyHL ( Figure S8), respectively. As the t Bu-P 4 loading increased to 2 mol % relative to HL, the major signal set changed from BnOH chain end to no chain end, and the observed MW increased significantly, and the signals of the oligomers initiated by residual water became negligible ( Figure S9). We next investigated the feasibility for PBs to directly catalyze the ROP of HL without the addition of any alcohol initiator. The ROP reactions were carried out at À25 C in THF with 2 mol % of t Bu-P 1 , t Bu-P 2 , and t Bu-P 4 (Table S3, run 1-3). After 12 h of reaction, no polymer was generated for t Bu-P 1 and t Bu-P 2 . To our surprise, maximum conversion of 88% was achieved for t Bu-P 4 after 12 h and a solid pol-yHL sample with unexpected ultrahigh MW of M n = 613.8 kg mol À1 and moderate Ð = 1.45 was obtained (Table S3, run 3). Adjusting the concentrations of phosphazene base only had small effects on M n and Ð of the polyHL samples (Table S3, runs 4-6).
To test the controllability of the ROP of HL catalyzed by t Bu-P 4 alone, kinetic experiments were performed at À25 C in THF ([HL]/[ t Bu-P 4 ] = 50/1, [M] 0 = 4.0 M) ( Figure S10; Table S4). The ln[M] 0 /[M] versus time plot revealed that the monomer conversion achieved up to 63% in 4 h, then the polymerization rate tended to slow down and the conversion reached 84% in the subsequent 6-8 h ( Figures  S10A and S10B). Notably, the M n of polyHL has a clear linear correlation with the monomer conversion throughout the polymerization process, but the dispersity Ð significantly broadened after half monomer conversion ( Figure S10C). The GPC curves also exhibited a gradually emerging bimodal distribution, presumably due to inevitable transesterification reactions at higher monomer conversion (Figure S10D). These data indicated that relatively controlled ROP manners could be realized at which the reactions were quenched within the first 4 h.
MALDI-TOF spectrum for polyHL generated by t Bu-P 4 alone only showed one set of mass peaks assignable to cyclic-polyHL mass peak ( Figure S11). No characteristic signals of chain end could be seen in the 1 H and 13 C NMR spectra either, further supporting the idea that the polymer obtained might have cyclic components (Figures S12 and S13). Notably, high abundance of water-initiated signals could only be detected by MALDI-TOF when the residual water was deliberately reserved in the ROP system ( Figure S14).
The topologies of putative cyclic-and linear-polyHLs were further studied by intrinsic viscosity measurements. According to the Mark-Houwink plot (Figure S15), the ratio of [h] cyclic /[h] linear was estimated to be 0.79, which is slightly higher than the theoretically predicted value. 10,23 This phenomenon might be attributed to the presence of minor linear-polyHL in the putative cyclic products. Besides, the Mark-Houwink exponent a values were 0.70 for cyclic-polyHL and 0.72 for linear-polyHL, suggesting that both polymers were random coils in THF solution.
Plausible chain initiation mechanisms for ROP of HL catalyzed by t Bu-P 4 /BnOH and t Bu-P 4 alone were studied through NMR spectroscopy. Monitoring the stoichiometric reaction between t Bu-P 4 and BnOH at RT in 1 H NMR exhibited the formation of complex [ t Bu-P 4 H + $$$OBn]. The disappearance of the hydroxyl proton signal at d 0.92 ppm and chemical shift changes of t Bu-P 4 and BnOH were also observed ( Figure S16). In addition, in situ NMR tests were performed to gain further mechanistic evidence. By lowering the ratio of HL/BnOH to 1/1, the dominant product observed is consistent with the ring-opened intermediate generated in the initiation step ( Figure 5, path 2, and Figure S17). In contrast, only a trace amount of BnO À was observed ( Figure S17). These results suggest that chain propagation is slower than the initiation step, thus leading to the accumulation of the initial intermediate with only one repeat unit. The disfavored ring-opening steps might be attributable to the increased steric hindrance surrounding the proposed propagating secondary alkoxides relative to the initiating primary BnO À .
Subsequently, the ability of t Bu-P 4 to extract an acidic H from HL was also verified. HL and t Bu-P 4 were mixed at ratios of 1/1, 2/1, 4/1, and 8/1, respectively, in a J.-Young tube with added toluene-d 8 at RT (no polymerization occurred under these conditions), and corresponding 1 H and 31 P NMR spectra were taken after sufficient oscillations (Figures S18-S20). Characteristic signals of [ t Bu-P 4 H] + (d 7.92-8.07 ppm 1 H NMR; d 12.48 ppm and À23.64 ppm in 31 P NMR) were clearly observed. However, as the HL/ t Bu-P 4 feed ratio increased, the intensity of the [ t Bu-P 4 H] + signals increased slightly. Meanwhile, the characteristic signals of t Bu-P 4 (d 1.72, 2.70, and 2.72 ppm in 1 H NMR; d 4.96 and À25.31 ppm in 31 P NMR) remained unchanged; that is, t Bu-P 4 could not be fully consumed even in the presence of a great excess of HL. These data clearly indicated that t Bu-P 4 was only able to deprotonate a tiny portion of HL, which might account for the lack of direct correlation between t Bu-P 4 concentration and the M n of cyclic-polyHL produced in this system (vide supra).
Density  www.cell.com/the-innovation Figure S21). The free energy for the formation of a-H abstracted acylated anion is 26.5 kcal mol À1 less than that of the formation of d-H abstracted alkyl anion in THF. We also performed quantum-mechanical calculations to evaluate the Gibbs free energy for proton abstraction from BnOH, HL, and H 2 O by t Bu-P 4 (see supplemental information and Figure S22). The Gibbs free energy for the deprotonations follows the order of BnOH (3.9 kcal mol À1 ) < HL (5.7 kcal mol À1 ) < H 2 O (9.3 kcal mol À1 ). Such order suggests that BnO À is the most readily generated active initiating species, while deprotonation of HL is slightly disfavored. The feasibility of BnO À being the active initiating species was further supported by the observation that alkali metal alkoxides, such as KOMe, NaOMe, KOEt, NaOEt, KO t Bu, and NaO t Bu, could also initiate the ROP of HL at À25 C, giving solid polyHL with a MW of up to 475.7 kg mol À1 and narrow dispersity Ð (see in Table  S5). It is also worth mentioning that the ROPs initiated by alkali metal alkoxides were less rapid compared with t Bu-P 4 catalyst, which might be attributed to the observed poor solubility of the alkali metal alkoxides in the reactant mixture.
The presence of water impurity can presumably interfere with the abovementioned process. HL with approximately 100 ppm water was prepared for the ROP with t Bu-P 4 /BnOH at a ratio of [HL]/[ t Bu-P 4 ]/[BnOH] = 100/1/1 at À25 C for 12 h, [HL] 0 = 5.3 M in THF (Table S12, run 2). Intriguingly, the HL conversion dramatically dropped to 21%, and the polyHL produced had a lower MW of M n = 17,938 g mol À1 with a greatly broadening dispersity of Ð = 2.258.

Physical properties of polyHL
We next investigated the physical properties of polyHL. The thermostability of polyHL produced by t Bu-P 4 and t Bu-P 4 /BnOH systems were also analyzed through thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC). Both the linear-and cyclic-polyHL exhibited high thermal stability (T d,5% > 325 C). The TGA and derivative thermogravimetry (DTG) curves of cyclic-product showed T d,5% = 332.3 C and T max = 367.3 C ( Figure S23), which were 6 C and 12 C higher than those of the linear polymer with a similar MW ( Figure S24). These results are consistent with conclusions from another study that the thermal stability of cyclic polymers are generally higher than that of their linear analogs. 9,10,24,25 The DSC curves for the second heating scan curves (5 C min À1 ) of two polyHL specimens displayed two similar glass-transition temperatures (T g ) of À29.7 C and À30.6 C for cyclic-and linear-polyHL, respectively, and no crystalline peaks were observed (Figures S25 and S26). All the data confirmed that both the cyclic-and linear-polyHL are amorphous polymeric materials with excellent thermal stability.
Broadly speaking, polyHL is a new type of polyhydroxyalkanoate. Previous polyhydroxyalkanoates, such as poly(g-butyrolactone), poly(d-valerolactone), poly(e-valerolactone), and poly(lactic acid), have attracted widespread attention due to their degradability. However, their drawbacks, such as relatively high brittleness and low impact strength 9,26,27 limited their potential applications. In contrast, polyHL has relatively high flexibility, which provides new opportunities in applications, such as PSAs and polyester polyols used to produce polyurethanes.
The ability to obtain high MW solid polymers via the ROP of HL by t Bu-P 4 alone provides a promising avenue to obtain PSAs with potential utilities. A simple 180 peel test was carried out to measure the peel strength of polyHL. Cyclic samples were employed here because our current synthetic methodology can give much higher MW for cyclic-polyHL than a linear one. Glass slides were utilized as the rigid substrate, and a sheet of A4 paper (15 3 2.6 cm) was used as the face substrate ( Figure 4A). The polyHL samples were evenly coated on the glass slides by using a coating blade. Cross-sectional scanning electron microscope (SEM) showed that the thin film had a fair uniform thickness of 36.9 ± 1.2 mm ( Figure 4B). The test was performed at 25 C on an Instron 5966 universal testing instrument at 180 peel angle at a rate of 10 mm min À1 . To our delight, pol-yHL_319, polyHL_562, and polyHL_160 (the numbers refer to the sample with an M n of 319, 562, and 160 kg mol À1 , respectively) exhibited adhesion with peel strength of 3.8 ± 0.12, 3.5 ± 0.20, and 1.5 ± 0.65 N cm À1 , respectively ( Figure 4C; Table S6). The peel strength of polyHL_319 and pol-yHL_562 were relatively higher than those of commercialized 3M Scotch tapes tested under the same test conditions (3M 665, 2.4 ± 0.40 N cm À1 ; 3M 810, 1.9 ± 0.31 N cm À1 ) and vinyl electrical tape (3M 1,600, 0.8 ± 0.17 N cm À1 ). PolyHL_160 exhibited a peel strength of 1.5 ± 0.65 N cm À1 , comparable with that of the 3M 810 Scotch tape. In addition, a high MW polymer sample (M n = 613.8 kg mol À1 , Ð = 1.45; Table S3, run 3) was solvent-cast into a PTFE mold to form a transparent and colorless polymer film with good flexibility and viscoelasticity ( Figure 4D).

Development of methodology for chemical recycling of polyHL to HL
Van 't Hoff analysis was performed to calculate the thermodynamic parameters of the polymerization (Figures S27 and S28; Tables S7-S9). According to Dainton's equation, 28 the change in enthalpy ðDH p Þ and entropy ðDS p Þ were calculated to be À13.12 kJ mol À1 and À49.09 J mol À1 K À1 , respectively, which further gave a T c of À6 C in THF at [HL] 0 = 1.0 mol L À1 .
To further cut down the energy input during the chemical recycling process, La (La[N(SiMe 3 ) 2 ] 3 ) were also tested. At 50 C with [HL] 0 = 0.5 M in toluene, excellent HL recovery was detected: 47% in 3 h, 81% in 12 h, and 88% in 24 h (Table S11, runs 1-3), and no significant improvement of recovery rate was observed even in a system diluted to 0.1 M (Table S11, runs 4 and 5). To achieve a complete recovery of HL monomer, the temperature was elevated to 80 C for the reaction with [HL] 0 = 0.5 M in toluene, the recovery of HL monomer reached 85% within 3 h and    (Table S11, runs 6 and 7), indicating a much more rapid depolymerization process. When the system was diluted to 0.1 M at 80 C, 93% recovery of HL monomer was achieved in 3 h (Table S11, run 8), and 100% recovery was achieved in 12 h ( Figure S29; Table S11, run 9). Finally, the validity of our catalytic method to chemically recycle linear-polyHL was confirmed with ZnCl 2 , giving 100% recovery of HL in o-DCB at 160 C in 12 h ( Figure S30).

DISCUSSION
Our mechanistic studies may rationalize why HL was observed to be non-polymerizable previously. 16 Firstly, for the initiation step, our data support a multiple competitive initiating mechanism ( Figure 5), including (path 1) direct abstraction of the proton from the C-H bond vicinal to the carbonyl group of HL (the most acidic proton in the lactone, Figure S21) to generate highly reactive species, (path 2) hydroxyl deprotonation of alcohols to form alkoxides, and (path 3) deprotonation of residual water to form hydroxide. Compared with lactones without a substituent at the carbon vicinal to the carbonyl group, the ethyl substituent at the same carbon lowers the acidity of the C-H bond and also sterically hinders the approach of bulky bases, especially t Bu-P 4 , thus disfavoring the polymerization process through path 1. Secondly, for the subsequent nucleophilic ring-opening step, both ethyl substituents are vicinal to the ester group, which should sterically hinder the approach to the ester by any nucleophiles, thus increasing corresponding activation barriers no matter which initiating mechanism leads to the ring-opening nucleophile. The experimentally measured thermodynamic data for ROP of HL also clearly indicated that the two ethyl substituents of HL greatly increase the entropic penalty compared with non-/mono-substituted d-lactones, 28,29 thus resulting in significantly less thermodynamically favorable polymerization than the less-substituted counterparts (Table S9). Therefore, the ROPs of HL are both thermodynamically and kinetically challenging, requiring judiciously selected reaction conditions to allow for polymerization, including sub-zero temperatures, prolonged reaction times, and highly concentrated conditions (vide supra).
Our work subsequently showed that the polymerization of HL is highly sensitive to water impurity in the reaction system. In essence, the H 2 O-initiated oligomers contain a carboxylic acid end group, which should have a strong tendency to quench the active alkoxide species for the chain propagation process and form dormant species featured by a terminal carboxylate anion ( Figure 5, path 3). Potassium benzoate and cesium formate were tested as potential ROP initiators under our optimized conditions (Table S12, (Table S12, runs 5 and 6). However, no HL conversion was detected for all three species. These observations confirm that carboxylate is a dormant species.
As such, rigorous drying is the key to the present success of ROP of HL, or the formation of carboxylate would inhibit the reaction.

CONCLUSION
In conclusion, we have developed the first methodology to polymerize sixmembered lactone with two substituents vicinal to the ester group, which enables the first synthesis of chemically recyclable solid polyesters with a high CO 2 content, high MW, and promising adhesive properties comparable with commercial counterparts by only using CO 2 and bulk chemicals as the starting materials. This work provides a brand-new avenue to potential large-scale utilization of CO 2 as a main chemical feedstock, while the chemical recyclability may allow for fixation of CO 2 in solid polymeric materials over a long period of time.