Mechanochemical upcycling of poly (vinyl chloride) for alcohol halogenation

Domain: chemistry
Published: 2026-01-02
Editorial outcome: major_revision

Abstract
Polyvinyl chloride (PVC) has raised significant environment concerns due to its chlorine-rich composition. Mechanochemical dechlorination of PVC produces waste chlorides that remain largely unutilized. Herein, we demonstrate that contact-electro-catalysis-mediated dechlorination, in tandem with alcohol chlorination can enable the upcycling of PVC to produce value-added organic chlorides and enhance atomic efficiency. Anatase TiO2, as contact-electro-catalyst, mediates the single electron transfer process that promotes the dechlorination of PVC under ball milling, thereby enabling the cascade alcohol chlorination through in-situ generated HCl. This strategy is applicable to a variety of benzylic, alicyclic, aliphatic and heterocyclic alcohols, yielding the corresponding organic chlorides with yields up to 95% in 4 hours. This mechanochemical approach can be scaled up to a 40 g scale of PVC and applied to real PVC samples. Structural analysis confirms the effective degradation of PVC accompanied by dechlorination with molecular weight decreasing from 65.0 kDa to 4.6 kDa after five cycles. This mechanochemical approach opens avenues for the application of PVC as a convenient and safe chlorine source in chemical transformations.

Introduction
Polyvinyl chloride (PVC) is one of the top five commodity polymers, accounting for 9.6% of global plastic production1–3. Despite their enormous utility, PVC waste management has emerged as a global challenge, since common industrial approaches, such as direct landfill, incineration, and pyrolysis lead to the release of hazardous by-products in ecosystem. In particular, conventional thermal treatment is inappropriate for chlorine-rich PVC due to production of corrosive HCl, which not only cause damage to equipment but also poses new environmental risks. Catalytic hydrocracking has been demonstrated as an effective approach for various commodity plastics recycling, but its application to PVC remains limited due to catalyst deactivation caused by chlorine species4,5. Recent advancements in the chemical recycling of PVC have explored its conversion into polyethylene-polystyrene-like copolymers and polyethylene-like polymers or its use as a chlorine source to facilitate various chemical reactions via electrocatalysis, thermal, or photothermal catalysis6–13. Despite these advancements, incompleted dechlorination as well as the use of costly metal catalysts and organics solvents which may pose challenges for large scale and cost-effective practical applications.

Mechanical recycling, one of the dominant industrial methods for waste plastic management, commonly involves the repurposing of plastics by sorting, grinding, and reprocessing. During grinding process, degradation occurs via force induced polymer chain scission14. The mechanochemical degradation of polymers via force induced chain scission has been documented since the late 20th century15,16. Recent studies on mechanochemical degradation of polyesters and polyolefins have demonstrated significant advantages, including operation at room temperature, solvent-free condition, and scalability17–22. Mechanochemical dechlorination of PVC was also accomplished by ball milling treatment23–26. However, in most cases, HCl is produced and typically neutralized with excessive amounts of base, raising significant environmental concerns (Fig. 1a). Using ball milling and piezoelectric nanoparticles provides a way of conducting mechanochemical reactions. This piezoelectric-mediated mechanoredox reactions have realized the synthesis of small organic molecules and polymers27–33. However, the requirement for an asymmetric central structure in piezoelectric catalysts greatly constrains the choice of available materials. By contrast, contact-electro-catalysis (CEC) utilizes the ubiquitous phenomenon of contact-electrification at dielectric interfaces to stimulate redox reactions, offering a broad range of material choices (e.g., polymers, metal oxides, non-metallic oxides)34,35. Owing to this unique catalytic activity, various cheap and readily available triboelectric materials have recently been employed in the degradation of organic pollutants36–38. In addition, the acid stability of triboelectric materials allows the subsequent chlorination reaction.

Herein, we demonstrate a tandem mechanochemical reaction, employing commercial anatase TiO2 (TiO2-A) as contact-electro-catalyst to promote dechlorination of PVC while activating the alcohol to facilitate cascade chlorination reactions. In comparison to base-mediated PVC dechlorination approach, our strategy valorizes chlorine by synthesizing value-added organochlorides. Notably, conventional solution methods typically require corrosive and expensive chlorinating reagents (Fig. 1b)39,40, our tandem mechanochemical reaction, which uses PVC as a Cl source, can be performed under solvent-free and mild conditions (Fig. 1c). This strategy was applicable to a variety of benzylic, alicyclic, aliphatic and heterocyclic alcohols, yielding the corresponding value-added organic chlorides with yields up to 95% in 4 h. Structural analysis of degraded product (d-PVC) revealed almost complete dechlorination, resulting in a Cl-free polymer with molecular weight decreased from 65.0 kDa to 4.6 kDa after five cycles. A high yield of benzyl chloride (99%) could also be achieved using 400 mg PVC label as Cl source. The resulting Cl-free degradation products can be further used as a potential photothermal material. Finally, we elucidated the mechanism of the reactions and clarified the role of TiO2-A through DFT calculations and experimental validation. This tandem catalytic approach utilizes chlorine from waste PVC to transform a range of alcohols into valuable Cl-containing chemicals, with the potential for application to other chlorination reactions.

Results
Optimization of the reaction conditions
In this study, we used a commercial PVC (Mn = 65.0 kDa) as chlorine source and benzyl alcohol as model substrate for the chlorination of alcohol. Initially, our experiments were conducted in a 10 mL ZrO2 milling jar with two 10 mm ZrO2 balls, operating at 30 Hz (Supplementary Fig. 1). The yield of benzyl chloride was determined by gas chromatography mass spectrometry (GC-MS). After 3 h of ball milling with a 10 wt% TiO2-A loading, a 13% yield of benzyl chloride was observed. Higher TiO2-A loadings resulted in improved yields (Supplementary Table 1, Entries 1–4), with a maximum yield of 83% at 50 wt% TiO2-A (Supplementary Table 1, Entry 5). A control experiment without TiO2-A confirmed only a 10% yield of benzyl chloride (Supplementary Table 1, Entry 6). We believe that this phenomenon may results from contact-electrification effect of ZrO2 balls and jars. Therefore, we conducted control experiments using a stainless-steel jar, which lack the potential to act as electro-catalysts (Supplementary Table 2). The results revealed that no product was observed in the absence of catalyst. The use of ZrO2 resulted in only a trace amount of benzyl chloride, whereas the application of TiO2 as an abrasive agent increased the yield to 16%. The gel permeation chromatography (GPC) was used to characterize the change of number-average molecular weight (Mn) of PVC during ball milling. We observed a gradual decrease in Mn with the increase of TiO2-A content (Supplementary Fig. 2).

Subsequently, the reaction parameters were further explored by varying the milling frequency and the size of milling balls. It was observed that reducing either the milling frequency or the size of the milling balls resulted in low yields and low degradation rates of PVC (Supplementary Table 1, Entries 7–11 and Supplementary Figs. 3 and 4). However, as the 10 Hz reaction progressed over time, significant increases in PVC degradation and benzyl chloride yield were observed between 3 and 4 h of milling, after which the degradation rate decreased with continued milling (Supplementary Figs. 5 and 6). These findings underscore the crucial role of mechanical force and TiO2-A in facilitating the halogenation reaction. Therefore, our subsequent investigation was centered using TiO2-A (50 wt%) as contact-electro-catalyst with two ZrO2 balls (10 mm) at 30 Hz. As shown in Supplementary Figs. 2–4, the higher molecular weight peak suggested the inter-chain coupling occurring during the ball milling process, particularly under conditions with a high yield of benzyl chloride. We conjecture that a greater extent of dehydrochlorination leads to a more unsaturated polymer structure for inter-chain coupling.

To assess the impact of particle size and morphology on the catalytic performance of TiO2-A, spherical TiO2-A nanoparticles (100 nm) and TiO2-A nanorods were selected as control catalysts. Their morphologies were characterized by transmission electron microscopy (TEM) (Supplementary Fig. 7). After 2 h of reaction, product yields of 18% and 42% were achieved using the 100 nm TiO2-A nanoparticles and TiO2-A nanorods, respectively (Supplementary Table 3). This significant difference underscore that the specific surface area plays a critical role in the catalytic activity of the system.

With the optimized conditions in hand, we set to examine the kinetics of PVC degradation and benzyl alcohol chlorination. We extended the reaction time and observed that the yield of benzyl chloride reached 95% after 4 h (Fig. 2a). In contrast, the yield was significantly lower in the absence of TiO2-A. In addition, proton nuclear magnetic resonance (1H NMR) spectrum also confirmed a substantial amount of benzyl alcohol remains unreacted in the absence of TiO2-A (Supplementary Fig. 8). The conversion of benzyl alcohol was determined to be 20.3% without the catalyst, as assessed by NMR integration, which is consistent with the yield determined by GC-MS analysis.

Next, we estimated the degree of dechlorination by 1H NMR (see the Supplementary information for more details). After 4 h of ball milling, a 35% dechlorination was achieved, while the absence of TiO2-A significantly inhibited dechlorination, resulting in only 2.8% after the same duration (Supplementary Fig. 9). We speculated that TiO2-A functions not only as a contact-electro-catalyst but also as a Lewis acid to activate C-Cl bond and promote the dechlorination of PVC. This mechanochemical activation is intrinsically linked to the degradation of PVC. We noted a continuous decreased in the Mn of d-PVC under different milling time. Specifically, in the presence of TiO2-A, Mn decreased from 65.0 kDa to 25.5 kDa, while the Mn only decreased to 33.9 kDa without TiO2-A (Fig. 2b, c). Further degradation of PVC will be discussed later in the paper.

To investigate the impact of ball milling on the CEC, we performed structural characterizations of TiO2-A before and after the reaction. Scanning electron microscopy revealed noticeable agglomeration of the TiO2-A particles after reaction (Supplementary Fig. 10). To further investigate the microstructural changes, TEM analysis was performed. The TEM images reveal that the TiO2-A particles display a slightly flattened morphology after reaction, indicating partial particle deformation (Supplementary Fig. 11). The observed interplanar spacing of 0.35 nm corresponds to the (101) plane of TiO2-A. This result aligns with the X-ray diffraction analysis shown in Supplementary Fig. 12 (PDF #211272), suggesting that the crystal structure did not undergo significant changes during the reaction. The residual percentages (% Residual) of TiO2-A were quantified by comparing the calculated and observed peak positions before and after the reaction (Supplementary Table 4). The lower %Residues indicate the crystal structure of the sample is in good agreement with the theoretical model. We also conducted Pseudo-Voigt function fitting on the diffraction peaks, and the results show that the diffraction peaks broaden after the reaction, which proves that mechanical force reduces the particle size of TiO2-A (Supplementary Table 5). The O1s spectra of the TiO2-A by X-ray photoelectron spectroscopy (XPS) analysis were listed in Supplementary Fig. 13. The oxygen species can be categorized into two components, lattice oxygen (530.0 eV) and oxygen vacancy components (531.7 eV). Following mechanochemical treatment, an increase in oxygen vacancy content was observed on the surface of the TiO2-A, which may facilitate the catalytic electron transfer process41.

Substrate scope
We explored the substrate scope by using different benzylic, alicyclic, aliphatic, and heterocyclic alcohols. As shown in Fig. 3, benzylic alcohols were efficiently converted into the desired products (2a and 2j) in 53–81% isolated yields. However, the low isolated yield of 2k can be ascribed to the electron-withdrawing group at the meta position that adversely affects intermediate stability. For compound 2l, the low yield likely derives from both the electron-withdrawing and steric hindrance effect of ortho-chloro substituent. 2-Naphthalenemethanol gave the corresponding product 2m in a moderate isolated yield of 68%. The tertiary alcohols provided the corresponding products in 67–93% yields (2n–2q) due to the strong stability of its carbocation (Supplementary Figs. 14–16). Alicyclic alcohols, including cyclopentanol, cyclohexanol, and cycloheptanol gave 47–68% GC yields (2r–2t) (Supplementary Figs. 17–19). We also explored the chlorination of diols and found 1,2-benzenedimethanol preferentially underwent disubstitution, yielding 2u in 58% isolated yield. We also examined the chlorination of heterocyclic alcohols, achieving isolated yields of 64% (2v) and 61% (2w), respectively. Product 2v serves as an intermediate in the synthesis of imidacloprid and acetamiprid, whereas 2w constitutes a pivotal building block for second-generation neonicotinoid insecticides.

Recycle experiments
We opted to recycle and reuse PVC multiple times to maximize the utilization of residual chlorine. After each reaction cycle, we washed and dried the solid mixture in a vacuum oven at 40 °C. Benzyl alcohol was then added for subsequent reaction under the same conditions. As shown in Fig. 4a, the yield of benzyl chloride remained above 90% after the second cycle but gradually decreased in subsequent cycles. This decline in yield also aligns with the gradual reduction in residual chlorine content of PVC observed during the recycle experiments. During the degradation process, a trace amount of toluene was detected (Supplementary Fig. 20). We conjectured that intramolecular cyclization or intermolecular cross-linking reaction occurs from the conjugated polyene chain to form aromatic substances42,43. In addition, we analyzed the low-molecular-weight byproducts after five cycles by HRMS, and no dioxin was detected (Supplementary Fig. 21). We also employed GPC to track the Mn change of d-PVC throughout the dehydrochlorination process. As the number of cycles increased, the Mn of d-PVC exhibited a continuous decrease, ultimately reaching a value of 4.6 kDa after five cycles (Fig. 4b).

Fourier transform infrared (FT-IR) curve and NMR spectrum further analyze the structure of d-PVC. As depicted in Fig. 4c, the IR signal of the C–Cl bond stretch at 610 cm−1 gradually decreased, while the signal of the C=C bond stretch at 1630 cm−1 and the C=O bond stretch at 1710 cm−1 emerged. The appearance of C=O bond indicates that the oxidation reaction occurred during the degradation of PVC. As shown in Fig. 4d, the 1H NMR signals corresponding to the -CHCl- of PVC (4.3–4.8 ppm) decreased with cycle experiments, while the characteristic peaks at 5.2–5.6 ppm and 7.0–7.9 ppm attributed to the protons of olefin group and aromatic group emerged. Besides the 1H NMR, 13C NMR analysis was performed to further identify the structure of d-PVC. In Fig. 4e, the characteristic peaks at 125–127 ppm and 137 ppm confirmed the formation of olefin and aromatic group, and the peak at 155 ppm was attributed to carbonyl group. Ultraviolet–visible diffuse reflectance spectroscopy was employed to examine light absorption ability of d-PVC (Supplementary Fig. 22). The broad peak at above 300 nm corresponds to the conjugated polyene structures44. In addition, the signal intensified with the increase of the ball milling cycles, implying its application as a potential photothermal material45.

We conducted the thermogravimetric analysis (TGA) to compare the thermal stability and decomposition behavior of d-PVC and PVC. Pristine PVC was found to be thermally stable up to 290 °C, after which it underwent dehydrochlorination in the temperature range of 290–350 °C and carbonization above 400 °C. In contrast, the TGA curves of d-PVC exhibited a continuous mass loss over the temperature range of 100–500 °C, suggesting that the thermal stability of d-PVC is compromised due to the introduction of carbonyl structure during ball milling. After complete pyrolysis, it was found that d-PVC had a carbon yield of 30–40 wt%. We also noticed that as the number of ball milling cycles increased, the amount of insoluble solid increased from 218.3 mg to 263.5 mg (containing 200.0 mg TiO2-A). We selected the insoluble solid from the last cycle to analyze its structure by solid-state cross-polarization magic-angle-spinning 13C-NMR spectroscopy. Two new peaks appeared in the region of 110–130 ppm and 130–150 ppm, which were assigned to the olefin and aromatic structures. Simultaneously, the peak at 55 ppm, corresponding to the C-Cl group, nearly disappears completely (Supplementary Fig. 23). XPS was performed to further validate this result (Supplementary Fig. 24). The signal corresponded to the oxidized products (carbonyl group) was absent in the insoluble solid.

This resulting Cl-free insoluble solid was an idea precursor for carbon/TiO2 composites. The following calcination was conducted at 700 °C for 3 h under Ar atmosphere. After naturally cooled to room temperature, the composites were obtained for further characterization. In the Raman spectroscopy analysis, the peak at 1580 cm−1 corresponds to the G-band, characteristic of graphitic carbon, while the D-band at 1350 cm−1 is associated with disordered carbon (Supplementary Fig. 25). The intensity ratio of D-band and G-band (ID/IG) was determined to be 0.9, indicating a moderate degree of graphitization. We also observed peaks in the range of 100–800 cm−1 that are associated with residual TiO2-A. The TEM images suggested the carbon nanosheets were successfully generated through the calcination process (Supplementary Fig. 26). Besides, high-angle annular dark field scanning transmission electron microscopy and corresponding energy-dispersive spectroscopy mappings showed the distribution of the elements of C, Ti, and O (Supplementary Fig. 26). Considering the unique photothermal properties of carbon and TiO2, we investigated the photothermal performance of the composite material by measuring the temperature change using a digital thermometer. The aqueous solution containing a 15 mg sample experienced a temperature increase of nearly 30 °C after 10 min irradiation, while the temperature change in deionized water was significantly lower. (Supplementary Fig. 27).

Synthetic applications
To evaluate the applicability of this approach using real plastics, we used PVC bag and label as chlorine source. These materials were subjected to initial size reduction via a pulverizer to enable efficient ball milling. Due to the presence of a large amount of plasticizer, we increased the amount of PVC to 400 mg to ensure sufficient chlorine content. During the ball milling process, the PVC pieces underwent direct dechlorination, resulting in the transformation into gray powder (Fig. 5a). The mechanochemical chlorination of benzyl alcohol could be carried out effectively, achieving a benzyl chloride yield of 74% and 99% when used PVC bag and PVC label, respectively. To demonstrate the practical application of this method, we conducted a scale-up experiment of chlorination of 2-naphthalenemethanol in four 250 mL ZrO2 ball-milling jars with planetary ball milling (550 rpm) (Fig. 5b and Supplementary Fig. 28). The combination of milling balls with varying sizes and quantities aims to enhance impact energy and ensure effective mixing of the reactants46. A mixture of balls with different size can randomize the motion of the milling media and thus prevent the abrasion of the jar47. The reaction time was extended to 24 h to accommodate the increased amounts of reactants. Upon completion of the reaction, an isolated yield of 68% was achieved. The high purity of the product was confirmed by 1H NMR analysis.

Mechanistic investigations
Previous work has demonstrated that strong bases facilitate the dechlorination of PVC via an E2 pathway25,26. In our system, we conjectured that the C-Cl bond was activated via the single electron transfer (SET) mechanism to form free radical intermediate. In situ formation of HCl was supported by the pH change after reaction (Supplementary Fig. 29). We employed 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO), a paramagnetic electron scavenger, to gain further insight into the mechanism of CEC via electron paramagnetic resonance (EPR) spectroscopy. TEMPO would be reduced to TEMPOH in aqueous solution under the action of TiO2-A, resulting in a reduced EPR signal (Supplementary Fig. 30). A control experiment without the catalyst showed no significant change in the EPR signal after ball milling. When TEMPO was introduced into the reaction system, the appearance of TEMPO-trapped products supported the formation of carbon radical intermediates. However, PVC can undergo force-induced bond homolysis to form carbon radicals during ball milling. This makes it challenging to distinguish whether TEMPO-captured species originate from SET-mediated bond cleavage or force-induced bond homolysis. Consequently, we used 1,3-dichlorobutane as a small-molecule analogue of PVC to verify the carbon radical intermediate. The TEMPO-trapped product was detected by GC-MS, indicating that TiO2-A plays a crucial role in the generation of free radical intermediates through a CEC mechanism (Supplementary Fig. 31). Next, we utilized 1,3-dichlorobutane as the Cl source together with TiO2-A for mechanochemical chlorination of benzyl alcohol. Following 4 h of ball milling reaction, benzyl chloride was obtained in a good yield of 83% (Supplementary Fig. 32).

The internal temperature of the jar during the reaction were determined to be 43.4–55.9 °C (Supplementary Fig. 33). To investigate the thermal contribution from ball milling, we also conducted the chlorination reaction in solution at 100 °C for comparison. As a result, we did not detect any products in the presence of PVC (Supplementary Table 6). In addition, we compared the solution-based approach using HCl-Et2O as the chlorine source with stirring at 35 °C (Supplementary Table 7). Notably, less reactive alcohols (e.g., n-heptanol and cyclohexanol) gave no product in the absence of TiO2-A, and only trace amount in its presence. Even for more reactive benzyl alcohol, the yield was only 3% in the presence of TiO2-A. These findings suggest that CEC approach exhibits better performance than conventional solution-based method.

The density functional theory (DFT) was employed to evaluate the dissociation energy of the C-Cl bond under CEC condition. Figure 6a show the tendency of the C-Cl bond breakage of PVC oligomer with 3 repeat units, serving as a model molecule for PVC. The bond length of the C-Cl bond increased from 1.827 Å to 4.188 Å, suggesting its tendency for the formation of alkyl radicals. Based on the aforementioned results, a plausible SET mechanism of CEC was proposed (Fig. 6b). First, PVC undergoes dechlorination via a TiO2 mediated SET pathway, resulting in the formation of carbon radical intermediate. Subsequently, the β-hydrogen atom from the carbon radical is eliminated, leading to the formation of C=C bond and HCl. Concurrently, alcohol reacts with HCl to yield chlorinated product.

As shown in Fig. 6b, we propose a mechanistic hypothesis for the reaction. TiO2 not only promotes the dechlorination step of PVC but also adsorbs hydroxyl groups and HCl at its titanium sites, thereby facilitating the subsequent chlorination reaction. Consequently, DFT calculations were carried out to help elucidate the underlying mechanisms using benzyl alcohol as the representative. The (101) crystal plane of TiO2-A exhibits a high surface energy, with a significant number of oxygen vacancies and unsaturated Ti atoms exposed on the surface48. This structure feature endows the surface with excellent adsorption capability for the hydroxyl group of alcohols. Calculated adsorption energies for benzyl alcohol on the TiO2-A (101) surface indicated a preferential adsorption in the horizontal orientation (Supplementary Fig. 34). As illustrated in Fig. 6c, the reaction pathway involves the adsorption of alcohol and HCl onto the titanium sites. The alcohol is protonated to form R-OH2+, then generating a carbocation intermediate. Cl- attacks the carbocation to produce benzyl chloride, which finally desorb from the surface. Notably, all reaction steps exhibit low energy barriers, highlighting the efficiency of this catalytic process. We also tested other Ti-based metal oxides for CEC, including BaTiO3 and rutile TiO2 (TiO2-R). As shown in Supplementary Fig. 35, the application of BaTiO3 led to a significant reduction in benzyl chloride yield from 95 to 23%. The degree of dechlorination using BaTiO3 was determined to be 12.1%, which is higher than that without catalyst but lower than that with TiO2-A (Supplementary Fig. 36). This confirms our conjecture that Lewis acid of TiO2-A can effectively activate C-Cl bonds. In contrast, replacing TiO2-A with TiO2-R did not result in a significant difference in the yield of benzyl chloride (85%). In view of the low yield of BaTiO3 as catalyst, we speculate that HCl cannot be adsorbed on titanium sites of BaTiO3 as evidenced by the DFT result. Consequently, the calculated energy barrier for generating carbocation and leaving group raises to 1.05 eV.

To assess catalytic effect of different catalysts, control experiments were performed under ball milling using both TiO2-A and BaTiO3 catalysts with identical HCl loading (Supplementary Table 8). The yield achieved with the TiO2-A catalyst was higher than that obtained using BaTiO3 (<1%). This finding demonstrates the potential catalytic effect TiO2 for alcohol chlorination reactions under ball milling condition.

Methods
Dechlorination of PVC and chlorination of alcohol
The ball milling reaction was conducted in a 10 mL ZrO2 jar (with 2 × 10 mm ZrO2 ball) using a Retsch MM 400 miller at 30 Hz without inert gas protection. After the reaction, add 30 μL chlorobenzene as internal standard and 3 mL methanol, then milling for another 1 min. The supernatant could be obtained by centrifugation, which was measured by GC-MS. Next, the precipitate was washed with methanol for three times. Drying the precipitate in a vacuum drying oven at 40 °C, dissolving d-PVC with THF and then centrifugating. The d-PVC was obtained from supernatant by vacuum-rotary evaporation. This process is repeated three times until the supernatant is colorless and transparent.

Chlorination of alcohol using PVC real plastic as the chlorine source
The PVC real plastic (PVC bag or PVC label) was comminuted in a pulverizer for 3 min to achieve preliminary size reduction. Then, 400 mg PVC real plastic, 200 mg TiO2-A, and 31 μL benzyl alcohol were added to 10 mL ZrO2 jar (with 2 × 10 mm ZrO2 ball) and reacted at 30 Hz for 4 h. After the reaction, add 30 μL chlorobenzene as internal standard and 3 mL methanol, then milling for another 1 min. The supernatant could be obtained by centrifugation, which was measured by GC-MS.

Scale-up experiment
Scale-up experiment was performed using planetary ball milling (QM-3SP2) equipped with four 250 mL ZrO2 ball milling jars. 10 g PVC, 10 g TiO2-A, and 2.375 g of 2-naphthalenemethanol were added to each jar. Then 10 × 15 mm milling balls 20 × 12 mm milling balls and 50 × 10 mm milling balls were added to each jar. The mixtures were then milled at 550 rpm for 24 h. After the reaction, the resulting mixture was washed several times with anhydrous methanol, followed by centrifugation. The supernatant was subsequently subjected to rotary evaporation and recrystallization to yield a pure white product.

UV–Vis experiment
The PVC (or d-PVC) (15 mg) was dissolved in 1 mL THF, then diluted it to 150 μg/mL. The solution was placed in a quartz sample chamber for subsequent testing.

EPR experiment
The TEMPO solution with a concentration of 2 mmol/L was prepared by dissolving 6.24 mg TEMPO in 20 mL deionized water. 200 mg catalyst as well as 1 mL of the above solution were placed into a 10 mL ZrO2 milling jar and reacted at 30 Hz for 1 h. The solution was centrifuged for the removal of catalyst. Supernatant was collected with 0.5 mm capillary tube and analyzed using EPR. Among them, the control sample contained 2 mmol/L TEMPO solution. The “TiO2-A sample” refers to the supernatant obtained using TiO2-A as catalyst. The “no additive sample” refers to the supernatant obtained without any catalyst.

Radical trapping experiment
1,3-dichlorobutane (1.6 mmol, 188 μL), TiO2-A, and TEMPO (0.4 mmol, 50.4 mg) were added to a 10 mL ZrO2 jar and allowed to react for 4 h. After the reaction, the mixture was diluted with anhydrous acetonitrile, and the catalyst was removed by centrifugation. The supernatant was then filtered through a 0.45 μm PTFE filter and analyzed by GC-MS.

Photothermal performance of the recycled insoluble solid
15 mg solid was dispersed in 2 mL deionized water by ultrasound. The suspension was then transferred to a quartz sample chamber, where a thermocouple temperature probe (accuracy of 0.1 °C) was inserted. The suspension was then irradiated with an NIR laser (808 nm) at a power density of 2.0 w cm–2 for 600 s and then naturally cooled. The temperature was recorded every 30 s with a digital thermometer.

Other methods are included in the Supplementary Information.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The manuscript by Wang and colleagues develops a mechanochemical route to upcycle waste PVCs, based on the emerging concept of contact electrocatalysis. The manuscript uses a variety of characterisations to study and characterise the reaction, complemented by DFT. Whilst I agree that the work is of potential importance in further developing mechanochemical recycling of polymers (though the novelty atop existing routes/literature needs to be much better rationalised), I do not think it is not yet sufficiently robust for publication in Nature Commun. I would ask the authors to please consider some potentially major aspects of the work and I look forward to seeing a revised version:\nLine 40 – PE-like materials not defined\nReferences for ‘contact electro catalysis’ should be expanded to include work on piezoelectric mechanocatalysis, e.g. by H Ito, who pioneered this direction of research\nSimilarly, this area of polymer recycling by mechanochemistry is extensive, and has been a field of active research since the late 20th century (e.g. back to Oprea). The reference list does not reflect this extensive history in any way. A quick google of 'PVC recycling mechanochemistry' reveals an extensive list of papers in the area, including those dealing with dechlorination (which appears to be the main selling point of the present manuscript. This does query the novelty of the present manuscript (note I just copy a small number of examples here: https://pubmed.ncbi.nlm.nih.gov/30616090/ ; https://www.jstage.jst.go.jp/article/kona/18/0/18_2000027/_article; https://advanced.onlinelibrary.wiley.com/doi/10.1002/adma.202304113\nIn Suppl. Table 1, the authors find Yields of 10% even without any TiO2 added (and figure 2a up to 20% after 4 hours). This suggests that the process is either not contact electrocatalytic, or that the authors may not have correctly cleaned their set-up between runs (which questions their overall results). Clarification of the origin of this 10% transformation in the absence of TiO2 is essential. I note that ZrO2 is also piezoelectric (balls and jar), which throws into question what the electrocatalyst actually is in this system. The authors should perform further control experiments with non-piezoelectric jars/balls, adding ZrO2 powder as a catalyst, and then another with non-piezoelectric set-up and with hard inorganic additive that is not piezoelectric.\nThe supplementary figures that discuss reacted/unreacted reagent (e.g. Suppl. Figure 5) are not particularly helpful as no integration is reported. Integrations for all of these NMRs should be given to help support discussion in the main text\nThroughout the manuscript there are a lot of physical data, including phase quantification, and structural quantifications (e.g. XPS). However, none of these data have errors associated with them, making it difficult to assess the validity of and meaning of the numbers. Errors need to be provided (and explained) for all physical data that are presented. This includes on all graphs where individual data points are plotted.\nThe authors have performed analysis of the TiO2 particles after ball milling and find no change in the particle structure. This is a bit concerning, and suggests that the ball milling is not powerful enough to generate any distortion of the TiO2 particles, which begs the question how there is enough energy to initiate electrocatalytic behaviour in these particles (Which requires mechanical deformation). This microscopy finding does not seem consistent with the spectroscopic evidence of vacancy formation, which would require mechanical deformation of the particles. How representative are the samples used / analysed in each experiment of the bulk? The single SEM images in the ESI (Fig 7-8) do not suggest it is particularly representative of the whole bulk\nThe SI does not provide sufficient information on computational aspects to assess the validity of the results. There is no description for how any of the calculations were performed: e.g. how did the authors simulate ‘contact-eletrocatalysis conditions?). How did they observe tendency for C-Cl bond breakage and a ca 2.5 A extension of the C-Cl bond? Specifically, there is no discussion for the set-up of the slab models used to study the surface chemistry of the reaction. From the figures presented in ESI Fig 46, the vacuum layer appears to be far too small to avoid interaction between periodic images. If this is indeed the case, then the calculations must be redone after full convergence of the vacuum layer.", "reviewer_2": "The authors have developed a mechanochemical approach for the halogenation of alcohols, utilizing waste PVC in conjunction with contact-electro-catalysis. The dechlorination of PVC has been comprehensively validated through a series of spectral analyses following the recycling and reuse of PVC. Importantly, the resulting d-PVC shows promise as a photothermal material. Both experimental findings and DFT calculations confirm that the dechlorination of PVC proceeds via a TiO₂-mediated SET pathway. Furthermore, the subsequent chlorination reaction is facilitated by the activation of alcohol by TiO₂-A and enhanced by mechanical force. This work not only advances our understanding of sustainable chemistry but also highlights the innovative use of waste materials and novel catalytic mechanisms. However, several issues need to be addressed before it can be published.\n-In the introduction, the sentence \"Structural analysis confirmed the effective degradation of PVC accompanied by dechlorination, with the molecular weight decreasing from 65.0 kDa to 4.6 kDa,\" lacks information regarding the reaction cycle.\n-The catalytic efficiency requires enhancement. To address this, it is necessary to supplement the comparative activity analysis of TiO₂ with different particle sizes/morphologies, or explore composite catalysts (such as TiO₂/carbon materials) to reduce the required dosage.\n-The details of the GPC experiments, including the PDI value, should be provided. This information is pertinent to both Figure 2 and Supplementary Figures 2-4.\n-According to the results presented in Supplementary Table 1, the yield of benzyl chloride was less than 1%. It would be helpful to know the degradation rate of PVC at such a low milling frequency of 10 Hz.\n-The chlorination products are confirmed solely by the GC-MS method, without cross-validation of their structures through NMR and HRMS/elemental analysis. The isolation of the target products is also crucial to demonstrate the practicality of this work. Additionally, the current approach may underestimate impurities such as ethers or olefins, which could interfere with the results.\n-The work only mentioned the production of trace amounts of toluene, without systematically analyzing the potential generation of toxic by-products such as dioxins during the dechlorination process.\n-Add detailed mechanochemical parameters to the conditions section of Figure 3. Also, include the method for calculating the product yield.\n- In the scale-up experiment, the authors should provide an explanation for their use of milling balls of varying sizes. Additionally, they should support their rationale by including relevant citations.\n-For Supplementary Figures 11 to 31, please provide complete GC-MS analysis information, including retention times, peak areas, concentrations, and the MS data.\n- \"In Supplementary Figure 42, the detailed conditions for each experiment should be clearly noted. Additionally, detailed method for the radical trapping experiments should be included in ESI.\n-In page 2, line 55, “allow” should be “allows”.\n-All Compounds number should better be bolded.\n-Fig. 6 “c” should be bolded.", "reviewer_3": "The manuscript describes implementation of mechanochemical ball milling in the upcycling of PVC to alkyl chlorides where the process is envisaged as a tandem contact-electro-catalysis - alcohol halogenation sequence performed in a one-pot fashion. Reaction optimization revealed TiO2-anatase as a suitable piezoelectric material to facilitate PVC dechlorination in rather high 50 wt% loading. The products were characterized in detail using appropriate analytical techniques, and the authors also present mechanistic considerations based on experiments and DFT calculations. In terms of methodology, there are several inconsistencies and additional experiments should be carried out to support the conclusions. In the Supp. Info., some aspects need more detail. In general, the topic is relevant in the field of mechanochemistry but the potential application in broader sense, even on the lab scale, is limited. Thus I find this manuscript more suitable for publication in a more specialized journal dedicated to mechanochemistry, sustainability, polymer recycling etc.\nWhile the presented work is interesting in combining effects of mechanoredox polymer degradation with subsequent chlorination of alkyl alcohols using in situ generated HCl, its originality and novelty required by the high standards of Nature Communications is compromised by a number of papers on the topic of piezoelectric-mediated mechanoredox reactions, not included in the references list (e.g. Science 366, 2019, 1500-1504; Chem.─Eur. J. 27, 2021, 9721– 9726, Curr. Org. Chem. 28, 2024, 905-913; Faraday Discuss., 2023,241, 79-90 to name only a few). As pointed out in the introduction, the application of ball milling in recycling of different polymers is extensively studied, but the citation on mechanoredox reactions is poor and should be brought to reader's attention in the revised version. I also wonder about the general utility of the presented approach, with the substrate scope being rather limited and yields good to moderate (≥90% in just 3 cases). Also, it is not clear if the reported yields are isolated or calculated from GC chromatograms (the isolation procedure - rotavap/recrystallization is described only for the scale up experiment 2g).\nNext, the authors report that all experiments were carried out with pristine PVC while they also demonstrate in one example that commercial PVC storage bag can be used as well. In this case, the bag was pre-treated with ethanol to remove the plasticizer (phthalate as seen in the Supp. Info.) to yield benzyl chloride in 89%. Why was it necessary to wash the PVC bag with ethanol and remove the phthalate plasticizer, does it have any effect of the reaction performance? Have the authors tested other waste PVC material like PVC lables from waste bottles or PVC pipes used in construction as the major sources of waste PVC? In this respect, I find the title of the manuscript misleading since all of the experiments with different alcohols were actually done on pristine PVC and not on waste PVC.\nThe mechanochemical reactions were carried out in ZrO2 media which is rather expensive - what is the effect of other milling materials, specifically stainless steel as the most frequently used?\nMechanistically, the proposed explanation of PVC dechlorination is sound and supported by TEMPO trapping of the radical species formed upon milling TiO2-A with 1,3-dichlorobutane analogue of PVC. Have the authors tested the performance of TiO2-A for PVC dechlorination in the absence of an alcohol substrate? An experiment with PVC/TiO2-A/TEMPO might provide a more direct evidence for SET reaction and mechanoredox effects of TiO2-A.\nParallel test reactions in solution with PVC as Cl source also showed that the temeprature alone is not sufficient to promote the dechlorination reaction and that ball milling is required. However, it is not clear how the authors carried out 2M HCl-Et2O (b.p. 35 oC!) reactions at 100 oC described in the Supp. Info, Table 3. Under pressure, in ampoule? There is inconsistency in the manuscript, lines 226-229 with the data in Table S3, entry 6 - the reported yield is 95%, but the authors state \"... only a negligible amount of product was observed for the alcohol, even with the presence TiO2-A.\"\nThe data in Table S4 in fact suggest that the chlorination of alcohols proceeds efficiently even in the absence of TiO2-A (88%), what makes the first step - the dechlorination - the key one. However, the experiment was performed with 1.5 mL of Et2O solution, and this can hardly be termed as a mechanochemical reaction. How relevant is the proposed catalytic effect of TiO2-A vs. BaTiO3 (adsorption of alcohol and HCl) in this respect? Is the lower benzyl chloride yield with BaTiO3 after 4 h of milling just the result of lower amount of in situ generated HCl, rather than poor adsorption of HCl onto BaTiO3 and its potential catalytic effect (lines 257-266)? The authors should try to resolve experimentally the difference in TiO2-A vs. BaTiO3 \"catalyzed\" alcohol chlorination reactions under ball milling conditions and design a control experiment to further support and strenghten their mechanistic proposal."}, "author_response": "Response: We thank the reviewer for evaluation and valuable suggestions. We have carefully addressed his/her concerns as below.\nComment 1: Line 40 – PE-like materials not defined\nResponse: Thanks a lot for this valuable comment. We have revised PE-like materials to polyethylene-polystyrene-like copolymers and polyethylene-like polymers. This clarification has been revised in the main text, lines 40-41.\nComment 2: References for ‘contact electro catalysis’ should be expanded to include work on piezoelectric mechanocatalysis, e.g. by H Ito, who pioneered this direction of research. Similarly, this area of polymer recycling by mechanochemistry is extensive, and has been a field of active research since the late 20th century (e.g. back to Oprea). The reference list does not reflect this extensive history in any way. A quick google of 'PVC recycling mechanochemistry' reveals an extensive list of papers in the area, including those dealing with dechlorination (which appears to be the main selling point of the present manuscript. This does query the novelty of the present manuscript (note I just copy a small number of examples here: https://pubmed.ncbi.nlm.nih.gov/30616090/; https://www.jstage.jst.go.jp/article/kona/18/0/18_2000027/_article; https://advanced.onlinelibrary.wiley.com/doi/10.1002/adma.202304113\nResponse: Thanks a lot for this helpful suggestion. Contact-electro-catalysis operates via a distinct mechanism that does not rely on the polarization of piezoelectric materials. However, recognizing the significance of piezoelectric mechanocatalysis, we have added those references ([Science, 2019, 366, 1500-1504], [Angew. Chem. Int. Ed., 2020, 59, 22570–22576], [Angew. Chem. Int. Ed., 2023, 62, e202311531], [Chem. Eur. J., 2021, 27, 9721–9726], [Curr. Org. Chem., 2024, 28, 905–913], [Faraday Discuss., 2023, 241, 79–90], [Chem. Sci., 2023, 13, 4131–4138]) on page 2, lines 53-56 as following:\n“Using ball milling and piezoelectric nanoparticles provides a new way of conducting mechanochemical reactions. This piezoelectric-mediated mechanoredox reactions have realized the synthesis of small organic molecules and polymers27–33.”\nIn the context of mechanochemical recovery of polymers, we have referenced recent studies on the degradation of polyester and polyolefin, but inadvertently overlooked earlier research in this area. We have added those references ([Polym. Mech., 1979, 14, 783–792] and [J. Appl. Polymer Sci., 1986, 32, 5557]) on page 2, lines 47-49 as following:\n“The mechanochemical degradation of polymers via force induced chain scission has been documented since the late 20th century15,16.”\nWhile the mechanochemical treatment of PVC using ball milling and base can lead to the declorination of PVC, the utilization of chlorine atoms in PVC for organic transformations remains a challenge. In our approach, TiO acts as a contact-electrocatalyst, allowing PVC to be employed as a non-corrosive reagent for alcohol halogenation. Therefore, the primary focus of this manuscript is not on the declorination of PVC, but rather on its upcycling for value-added chemicals. To clarify this point, we have added related discussion on page 3, lines 63-67 as following:\n“In comparison to base-mediated PVC dechlorination approach, our strategy valorizes chlorine by synthesizing value-added organochlorides. Notably, conventional solution methods typically require corrosive and expensive chlorinating reagents (Fig. 1b)38,39, our tandem mechanochemical reaction, which uses PVC as a Cl source, can be performed under solvent-free and mild conditions (Fig. 1c).”\nComment 3: In Suppl. Table 1, the authors find yields of 10% even without any TiO2 added (and figure 2a up to 20% after 4 hours). This suggests that the process is either not contact electrocatalytic, or that the authors may not have correctly cleaned their set-up between runs (which questions their overall results). Clarification of the origin of this 10% transformation in the absence of TiO2 is essential. I note that ZrO2 is also piezoelectric (balls and jar), which throws into question what the electrocatalyst actually is in this system. The authors should perform further control experiments with non-piezoelectric jars/balls, adding ZrO2 powder as a catalyst, and then another with non-piezoelectric set-up and with hard inorganic additive that is not piezoelectric.\nResponse: Thank you for this helpful comment. We follow reviewer’s suggestion to conduct further control experiments with stainless steel jars/balls (non-piezoelectric), using different catalysts (TiO2, ZrO2) (Table R1). We found that the yield of benzyl chloride was negligible in the presence of ZrO2 powder, and 16% when using TiO2-A as the catalyst. This result suggests that the ZrO2 is unlikely to be the catalyst for our reaction. In fact, both the monoclinic and tetragonal phases ZrO2 only exhibit weak piezoelectricity, and the cubic phase ZrO2 is not piezoelectric [Appl. Mater. Today, 2023, 30, 101708]. Therefore, ZrO2 is not an effective piezoelectric material for piezo-catalysis. In addition, the classic piezoelectric catalysts BaTiO3 also demonstrated lower catalytic efficiency compared with TiO2 (Table R1).\nContact-electro-catalysis harnesses electron transfer occurring at contact-electrification effect to drive redox reactions [Chem. Soc. Rev., 2024, 53, 4349–4373]. In contrast to piezoelectric catalysis that impose specific structure requirements to enable polarization of material, contact-electro-catalysis allows most organic and inorganic material to function as a catalyst as long as it demonstrates contact-electrification capabilities. Therefore, we believe that the background activity may results from contact-electrification effect of ZrO2 balls and jar. We believe these results supported our assumption that the mechanism is based on contact-electrification effect rather than the piezoelectric effect. By contrast, the conductive characteristics of stainless-steel material are not conducive to charge accumulation in contact-electro-catalysis. It is also important to note that the in-situ generated HCl can corrode the stainless-steel jar (see the response to Reviewer 3 Comment 3), which is why the yield of the reaction using stainless steel jar was lower than that using ZrO2 jar.\nTable R1. Control experiments with stainless steel jars/balls. [a]\nEntry Catalyst Yield (%) [b]\n[a] Reaction conditions: PVC (200 mg, 3.2 mmol), substrate (0.3 mmol), catalyst (200 mg), 3 hours, 30 Hz, 10 mL stainless steel jar with two 10 mm stainless steel balls.\nWe have added relevant information on page 3, lines 86-87:\n“We believe that this background activity may results from contact-electrification effect of ZrO2 balls and jars.”\nComment 4: The supplementary figures that discuss reacted/unreacted reagent (e.g. Suppl. Figure 5) are not particularly helpful as no integration is reported. Integrations for all of these NMRs should be given to help support discussion in the main text.\nResponse: Thank you for this helpful comment. As suggested, we have integrated the peaks in all relevant NMR spectra (Suppl. Figure 8, Suppl. Figure 15 and the 1H NMR spectra in Suppl. Figure 37-82). Taking Suppl. Figure 8 as an example, the revised figure is shown below, and the corresponding discussion has been incorporated into the main text (page 4 lines 113-115) to strengthen our conclusions:\n“The conversion of benzyl alcohol was determined to be 20.3% without the catalyst, as assessed by NMR integration, which is consistent with the yield determined by GC-MS analysis.”\nComment 5: Throughout the manuscript there are a lot of physical data, including phase quantification, and structural quantifications (e.g. XPS). However, none of these data have errors associated with them, making it difficult to assess the validity of and meaning of the numbers. Errors need to be provided (and explained) for all physical data that are presented. This includes on all graphs where individual data points are plotted.\nResponse: Thanks a lot for this valuable comment. In term of the phase quantification, we have compared our experimentally observed peak positions of anatase TiO2 before and after the reaction with the standard XRD peaks (calculated using I4₁/amd space group symmetry). The %Residuals were calculated accordingly, and the results are summarized in Revised Suppl. Table 3. The %Residuals suggest that the difference between the observed peak position and the calculated peak positions are minimal, indicating that the crystal structure of the sample is in good agreement with the theoretical model. To clarify this point, we have added relevant discussion on page 5, lines 133-136 as following:\n“The residual percentages (%Residual) of TiO2-A were quantified by comparing the calculated and observed peak positions before and after the reaction (Supplementary Table 3). The lower %Residual indicate the crystal structure of the sample is in good agreement with the theoretical model.”\nFurthermore, we have added the error bars to all graphs containing individual data points to ensure clarity and transparency in the revised manuscript and supporting information.\nComment 6: The authors have performed analysis of the TiO2 particles after ball milling and find no change in the particle structure. This is a bit concerning, and suggests that the ball milling is not powerful enough to generate any distortion of the TiO2 particles, which begs the question how there is enough energy to initiate electrocatalytic behaviour in these particles (Which requires mechanical deformation). This microscopy finding does not seem consistent with the spectroscopic evidence of vacancy formation, which would require mechanical deformation of the particles. How representative are the samples used / analysed in each experiment of the bulk? The single SEM images in the ESI (Fig 7-8) do not suggest it is particularly representative of the whole bulk\nResponse: Thank you for your insightful comments and constructive feedback on our manuscript. Regarding the concern that “the single SEM images do not suggest it is particularly representative of the whole bulk.”, we performed additional SEM analysis, which revealed noticeable agglomeration of the TiO2-A particles after the reaction. To further explore the changes of microstructure, we extended our analysis with TEM imaging. The TEM results indicate that the TiO2-A particles exhibit a slightly flattened morphology after reaction, suggesting a certain degree of particle deformation. The observed interplanar spacing about 0.35 nm corresponds to the (101) plane of TiO2-A, suggesting that the crystal structure did not undergo significant changes during the reaction.\nIn addition to the electron microscopy images, subtle changes are also evident in the XRD patterns (Supplementary Fig. 12). We performed Gaussian fitting on the XRD signals of each observed crystal plane to determine the full width at half maximum (FWHM), with the results summarized in Revised Suppl. Table 4. The FWHM values slightly increased after the reaction. According to the Scherrer equation (D = Kλ / βcosθ, D is the grain size, λ=0.154 nm, K is the Scherrer constant, β is the FWHM and θ is the Bragg diffraction angle), the grain size of crystalline materials is inversely proportional to the peak width. Therefore, the observed peak broadening indicates a reduction in grain size, suggesting that ball milling leads to deformation of the TiO2-A crystallites.\nWe have revised the manuscript in term of the mechanical deformation of the TiO2 particles on page 5, lines 126-133 and 136-138 as follow:\n“Scanning electron microscope (SEM) revealed noticeable agglomeration of the TiO2-A particles after reaction (Supplementary Fig. 10). To further investigate the microstructural changes, TEM analysis was performed. The TEM images reveal that the TiO2-A particles display a slightly flattened morphology after reaction, indicating partial particle deformation (Supplementary Fig. 11). The observed interplanar spacing of 0.35 nm corresponds to the (101) plane of TiO2-A. This result aligns with the X-ray diffraction (XRD) analysis shown in Supplementary Fig. 12 (PDF #211272), suggesting that the crystal structure did not undergo significant changes during the reaction (Supplementary Table 3).”\n“We also conducted Gaussian fitting on the diffraction peaks, and the results show that the diffraction peaks broaden after the reaction, which proves that mechanical force reduces the grain size of TiO2-A (Supplementary Table 4).”\nComment 7: The SI does not provide sufficient information on computational aspects to assess the validity of the results. There is no description for how any of the calculations were performed: e.g. how did the authors simulate ‘contact-eletrocatalysis conditions?). How did they observe tendency for C-Cl bond breakage and a ca 2.5 A extension of the C-Cl bond? Specifically, there is no discussion for the set-up of the slab models used to study the surface chemistry of the reaction. From the figures presented in ESI Fig 46, the vacuum layer appears to be far too small to avoid interaction between periodic images. If this is indeed the case, then the calculations must be redone after full convergence of the vacuum layer.\nResponse: Thanks a lot for this valuable comment. In this study, all the molecular models were constructed using the Material Studio simulation package. In contact-electro-catalysis, electrons play a crucial role. Electrons accumulated at TiO2 surface are captured by PVC, thus activating the C-Cl bond. To simulate ‘contact-electro-catalysis’ conditions, we calculated the change in the C-Cl bond length before and after introducing an electron into a PVC oligomer. To facilitate the calculation, we set the initial structure of PVC oligomer with 3 repeat units and obtain the bond length of the C-Cl bond. The convergence thresholds for electronic energy, maximum atomic displacement, and maximum force per atom were set at 10-5 Hartree, 0.005 Å and 0.002 Hartree Å-1, respectively. After introducing an electron, we obtain a new structure with the bond length of C-Cl bond increased from 1.827 Å to 4.188 Å. This indicates that the C-Cl bond undergoes facilitated dissociation under contact-electro-catalysis conditions.\nOur surface model was cleaved from a six-layer TiO2-A slab to expose the (101) surface. Periodic slab images were isolated using 15 Å vacuum gaps, following the method reported in literatures [Nat. Commun., 2023, 14, 4479].\nWe have revised the simulation details in the SI as follow:\n“3.1 Polarization of C-Cl bond\nIn this study, all the molecular models were constructed using the Material Studio simulation package. The geometrical structures and electronic properties were investigated using the Dmol3 code based on the density functional theory (DFT) employing the generalized-gradient-approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional. The double numerical plus polarization (DNP) basis set and the DFT-D approach for van der Waals dispersion corrections, utilizing the TS scheme, were implemented to accurately describe the expansion of occupied molecular orbitals and to precisely quantify electrostatic interactions. The convergence thresholds for electronic energy, maximum atomic displacement, and maximum force per atom were set at 10-5 Hartree, 0.005 Å and 0.002 Hartree Å-1, respectively.\nTo facilitate the calculation, we set the initial structure of PVC oligomer (repeat unit of 3) and obtain the bond length of the C-Cl bond. After introducing an electron, we obtain a new structure of PVC oligomer and a revised C-Cl bond length, and the bond length of C-Cl bond increased from 1.827 Å to 4.188 Å.”\n“3.2 Chlorination reaction of alcohol on catalyst surface\nDFT calculations for this segment were performed in the Vienna ab initio simulation package (VASP) based on the plane-wave basis sets with the projector augmented-wave method. The exchange-correlation potentials were processed using the GGA and the PBE parameter. The energy cut-off of 450 eV was applied. The Brillouin zone integrals were sampled with a 2 × 2 × 1 Γ-centered Monkhorst-Pack mesh. Structural optimizations were carried out until the maximum residual force on any atom was below 0.02 eV/Å, with an energy convergence threshold set at 10-5 eV. The adsorption energy can be defined as:\nE = E_total - (E_mol + E_slab)\nwhere E stands for the total energy, E_mol is the energy of a molecule, E_slab is the energy of a slab.\nIn all calculations, we allowed at least the top two atomic layer and the adsorbed species to fully relax, while fix the other layers to create a bulk environment to the surface region. A (3 × 3) supercell was built by cleaving the bulk in the (101) direction. The vacuum thicknesses were set to 15 Å to avoid the interactions between periodically repeated slabs in the vertical direction1. A Monkhorst-Pack k-point sampling of 2 × 2 × 1 was selected. Spin-polarized calculation was employed to describe the magnetic properties. DFT-D3 method was used to compute the surface energy.”"}, {"round": 2, "reviewer_comments": {"reviewer_1": "Overall the authors have made extensive effort to address the reviewer comments, which has significantly improved the manuscript. The manuscript is a nice piece of work demonstrating the use of ball milling to up-cycle polymer waste and I’m sure will inspire further work in this area.\nMy main concern with the manuscript is that the crux of the novelty is based on the ‘contact electro catalysis’, which in my view the authors have not rigorously demonstrated. Whilst this is a nice theory, the residual reactivity in the absence of the catalyst (though the authors suggest it might be due to some electrocatalytic behaviour or ZrO2) really throws this into question. The effect of particle shape/size could also be accounted for through discussion of local forces where in the TiO2 is used as an abrasive agent (which would also suggest why spherical particles are less effective). I would really like to see some control experiments done in jars that have no potential to act as electro-catalysts to really solidify the rigor of this theory. This said, the rest of the paper is in my opinion well documented and will make a nice contribution to the community.\nJust a few small things:\nThe meaning of this phrase is unclear, given an oxygen vacancy cannot exhibit an Oxygen 1s emission (there is no oxygen).\n‘’The oxygen species can be categorized into two components, lattice oxygen (530.0 eV) and oxygen vacancies (531.7 eV).’’\nUsing a Gaussian fit to Diffraction profiles to extract line widths is not particularly rigorous (usually a Pseudo-Voigt function is more rigorous)", "reviewer_2": "The authors have addressed the points raised in the initial review. The revised manuscript, accompanied by a point-by-point response, demonstrates efforts to improve the clarity and scientific rigor of the work. The presentation, analysis, and discussion of the data have been further refined. Overall, the authors have responded to most of the reviewers' comments. However, several minor issues (listed below) require clarification or correction prior to final acceptance. In principle, upon satisfactory resolution of these points, the manuscript would meet the publication criteria for Nature Communications.\n-In page 4, line 95，the statement, \"However, as the 10 Hz reaction progressed over extended time, the accelerated PVC degradation also led to a significant increase in benzyl chloride yield\" is inaccurate. The data indicate that PVC degradation was accelerated between 3 and 4 hours of milling; however, further prolonging the milling time resulted in a deceleration of the degradation process.\n-In the substrate scope study, the authors stated, \"However, the isolated yields of 2k and 2l were lower due to the strong electron-withdrawing effect of the trifluoromethyl and chlorine substituents.\" However, this explanation for the observed reaction outcomes is inadequate. The electron-withdrawing effect of the nitro group in 2f is significantly stronger than that of either the trifluoromethyl or chlorine substituents, yet 2f may exhibit a different yield trend. Therefore, the influence of substituent position, potentially affecting steric accessibility or intermediate stability, must be considered to rationalize the observed yield variations.\n-“isolate yield” should better be “ isolated yield”.\n-In the revised Suppl. Table 6, using \"n.d.\" (not detected) is more appropriate than “0” to indicate the absence of detectable product. Additionally, in the caption of Figure 3, the compound numbers should be formatted in bold typeface for consistency.", "reviewer_3": "After carefully reviewing the revised version of the manuscript by Chen and Wang, I am happy to see that the authors have addressed all points raised by the reviewers. Considerable changes to all sections have been implemented, most importantly to the experimental part and supplementary data, including verification of proposed mechanistic aspects of the studied reactions.\nAs a final note, and particularly since other reviewers also brought this up, I would suggest that before publication a few sentences in the Introduction section, 2nd paragraph, lines 56-57 are added, describing the contact-electro-catalysis in more detail to make the transition from piezoelectric-mediated mechanoredox reactions more clear for readers. Other than that, the manuscript is now of sufficient quality for publication in Nature Communications. I congratulate the authors for their excellent work!"}, "author_response": "Response: Thank you for the positive feedback. We sincerely thank the reviewer #1 again for your thoughtful and detailed suggestions previously provided.\nComment 1: My main concern with the manuscript is that the crux of the novelty is based on the ‘contact electro catalysis’, which in my view the authors have not rigorously demonstrated. Whilst this is a nice theory, the residual reactivity in the absence of the catalyst (though the authors suggest it might be due to some electrocatalytic behavior or ZrO2) really throws this into question. The effect of particle shape/size could also be accounted for through discussion of local forces where in the TiO2 is used as an abrasive agent (which would also suggest why spherical particles are less effective). I would really like to see some control experiments done in jars that have no potential to act as electro-catalysts to really solidify the rigor of this theory. This said, the rest of the paper is in my opinion well documented and will make a nice contribution to the community.\nResponse: Thank you very much for your valuable comment. In response to your suggestion regarding control experiments conducted in jars with no potential to act as contact-electro-catalysis, we performed additional experiments using a stainless-steel jar. Due to its high electrical conductivity, stainless-steel inhibits charge accumulation and therefore cannot function as electro-catalysts. The results of these control experiments using the stainless-steel jar have been incorporated into the revised manuscript (page 4, lines 89-93) and SI.\nSupplementary Table 2. Control experiments with stainless-steel jars/balls. [a]\nEntry Catalyst Yield (%) [b]\n1 / n.d.\n[a] Reaction conditions: PVC (200 mg, 3.2 mmol), substrate (0.3 mmol), catalyst (200 mg), 3 hours, 30 Hz, 10 mL stainless-steel jar with two 10 mm stainless-steel balls.\nHere, n.d. stands for not detected.\n“Therefore, we conducted control experiments using a stainless-steel jar, which lacks the potential to act as electro-catalysts (Supplementary Table 2). The results revealed that no product was observed in the absence of catalyst. The use of ZrO2 resulted in only a trace amount of benzyl chloride, whereas the application of TiO2 as an abrasive agent increased the yield to 16 %.”\nComment 2: The meaning of this phrase is unclear, given an oxygen vacancy cannot exhibit an Oxygen 1s emission (there is no oxygen). “The oxygen species can be categorized into two components, lattice oxygen (530.0 eV) and oxygen vacancies (531.7 eV).’’\nResponse: Thanks a lot for this valuable and insightful comment. We agree that oxygen vacancies is an inaccurate label for an XPS O 1s peak, as a vacancy contains no oxygen to generate the signal. We have carefully revised the relevant sentence in the manuscript on pages 5 and 6, lines 146-148.\n“The oxygen species can be categorized into two components, lattice oxygen (530.0 eV) and oxygen vacancy components (531.7 eV).’’\nComment 3: Using a Gaussian fit to Diffraction profiles to extract line widths is not particularly rigorous (usually a Pseudo-Voigt function is more rigorous)\nResponse: Thanks a lot for this valuable comment. In the revised manuscript, the diffraction peak has been refitted using a Pseudo-Voigt function, with the refined results presented in Supplementary Table 5."}, {"round": 3, "reviewer_comments": {"reviewer_1": "The authors have done, in my opinion, a good job at addressing the queries posed by both reviewers, and have provided adequate changes to their manuscript. Whilst I am still not fully convinced by the authors' interpretations of the phenomenon based on contact electrification, the control studies do provide good evidence that the effect only occurs in the presence of large quantities of the ZrO2 species (ball/jars). There could be much more rigorous work done around identifying the catalyst loading of this material needed to achieve the same effect, for example, and further investigations into the role of TiO2, and the reason wy ZrO2 balls/jars are needed as opposed to loose catalyst. That said, this mechanistic study is well suited as a dedicated follow up investigation.\nAside from this, the manuscript is now a solid piece of work that provides an interesting study into the decomposition of environmentally harmful polymer materials using mechanochemical techniques, and I think warrants publication.", "reviewer_2": "The revised manuscript by Chen and Wang has carefully and adequately addressed all reviewer comments. This well-executed study demonstrates the effective application of mechanochemical CEC technology for polymer waste upcycling.\nThe work is innovative, impactful, and recommended for publication."}, "author_response": ""}]

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