Upcycling polyethylene into closed-loop recyclable polymers through titanosilicate catalyzed C-H oxidation and in-chain heteroatom insertion | Nature Communications

Oct 25, 2024

Nature Communications volume 15, Article number: 9188 (2024) Cite this article

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Polyolefins are the most widely produced type of plastics owing to their low production cost and favorable properties. Their polymer backbone consists solely of inert C-C bonds, making them resistant and durable materials. Although this is an extremely useful attribute during their use phase, it complicates chemical recycling. In this work, different types of polyethylenes (PEs) are converted into ketone-functionalized PEs with up to 3.4% functionalized carbon atoms, in mild conditions (≤100 °C), using a titanosilicate catalyst and tert-butyl hydroperoxide as the oxidant. Subsequently, the introduced ketones are exploited as sites for heteroatom insertion. Through Baeyer-Villiger oxidation, in-chain esters are produced with yields up to 73%. Alternatively, the ketones can be converted into the corresponding oxime, which can undergo a Beckmann rearrangement to obtain in-chain amides, with yields up to 75%. These transformations allow access to polymers that are amenable to solvolysis, thereby enhancing their potential for chemical recycling.

The production and use of plastics continue to grow at high pace since the start of their large-scale implementation around the 1950s1. Annual plastic production has now exceeded 400 million tons, with polyolefins constituting the largest share (>180 million tons). Since polyolefins are mainly applied as single-use packaging materials, this implies that millions of tons of polyolefin waste are generated annually1,2,3,4. Consequently, efficient collection and recycling of polyolefins are crucial5. Whereas heteroatom containing plastics such as polyesters and polyamides can easily be depolymerized to their monomers and repolymerized to materials of equal quality (closed-loop recycling)6,7,8, the polyolefin backbone consists exclusively of inert C-C bonds, rendering chemical recycling through solvolysis impossible9,10. Alternatives like pyrolysis, gasification or hydrocracking are highly energy-intensive and do not selectively yield the monomers (open-loop recycling)3,11,12,13. Consequently, large-scale recycling of polyolefins is predominantly performed mechanically, with an unavoidable decrease of product quality4,9,13,14,15,16. Given the rate of polyolefin waste generation, this results in a rapid devaluation of the material stream. Clearly, redesigning the polyolefin backbone for chemical recycling is crucial for sustainable production5,17,18,19,20.

Polyethylene-like materials containing small concentrations of in-chain functionalities have been extensively researched by the group of Mecking. In 2021, they accomplished the synthesis of an aliphatic PE-18,18 polyester from plant oil based monomers6. Despite the presence of ester groups, the polymer showed highly similar properties compared to virgin HDPE. In addition, the polymer could easily be chemically recycled to a material of identical quality through methanolysis and repolymerization. In a subsequent publication, a polyketone, obtained through co-polymerization of CO and ethylene, was converted via Baeyer-Villiger oxidation into a chemically recyclable ester-containing polymer17. In addition to esters, polyethylene-like materials with small amounts of in-chain amides were obtained through ADMET copolymerization of long α,ω-diene monomers21. However, instead of producing such materials de novo, from building blocks like CO and specific olefins, it would be beneficial to directly convert existing PE (waste) into similar heteroatom containing polymers.

The efficient introduction of ketones (and alcohols) onto a PE chain was reported earlier by Hartwig et al.22. The increased polarity enhanced properties such as paintability and adhesion without compromising the mechanical strength of PE. However, a perfluorinated Ru-porphyrin complex was used as the catalyst, along with 2,6-dichloropyridine N-oxide as an atom-inefficient oxidant and a chlorinated solvent (1,2-dichloroethane).

In this work, ketone-functionalized PEs (kf-PEs) were produced directly from virgin PEs in a sustainable way, employing an easily recyclable heterogeneous titanosilicate catalyst in a non-chlorinated solvent. TiITQ-6, a delaminated titanosilicate with FER topology, showed an exceptional catalytic activity in the C-H bond oxidation of both long alkanes, such as dodecane, and of PEs using tert-butyl hydroperoxide (TBHP) as the oxidant. Next, the kf-PEs were exploited as platform polymer for heteroatom insertion (Fig. 1). First, ester-functionalized PEs (ef-PEs), which were shown to be completely solvolyzable, were obtained through Baeyer-Villiger oxidation (BVO)17,23,24 of the ketones. Additionally, we demonstrate the transformation of the ketones into in-chain amides, through reaction with hydroxylamine25 and subsequent liquid-phase Beckmann rearrangement26 to obtain oxime-functionalized PEs (of-PEs) and amide-functionalized PEs (af-PEs) respectively. The molecular weight of the starting material was completely retained throughout all modifications, which is crucial to maintain the beneficial physical properties of the starting material. Additionally, the thermal properties were shown to remain appropriate for common PE applications after functionalization. Other beneficial properties of the virgin PE such as mechanical strength can also be maintained well despite the introduction of a small concentration of in-chain functional groups21,27,28.

Through a titanosilicate catalyzed C-H bond oxidation of PE, a ketone-functionalized PE is obtained, which can be further exploited for heteroatom insertion. The inset shows a SEM micrograph of TiITQ-6.

Although titanosilicates are renowned for their application in the epoxidation of olefins29,30, their ability to selectively oxidize alkanes to secondary alcohols and ketones using hydrogen peroxide and tert-butyl hydroperoxide (TBHP) has only sporadically been explored31,32,33,34,35. The hypothesized oxygen rebound mechanism involves alkyl radicals, but these stay within the Ti coordination sphere. This way, undesired side reactions such as chain fragmentation or cross-linking can be avoided35. While significant efforts have been devoted to increasing the external surface area of titanosilicates for their application in epoxidation of bulky olefins29,30,36,37,38, these materials were not investigated in C-H bond oxidation of alkanes.

We initially hypothesized that TiITQ-6 could be a promising catalyst for the C-H bond oxidation of polyethylene. As confirmed through SEM measurements (Fig. 1 and Supplementary Fig. 6), the material consists of thin zeolite layers obtained through exfoliation of a TiPREFER precursor38,39. Consequently, it exhibits a large external surface area (Supplementary Fig. 5 and Supplementary Table 3), thereby facilitating the access of the polymer to the Ti-sites at its outer surface.

The catalytic performance of TiITQ-6 was first investigated in the C-H bond oxidation of different alkanes and compared with that of a commercial microporous TS-1, with pore diameters ≤ 0.55 nm (Fig. 2a). Tert-butylbenzene (TBB) was chosen as the solvent, since it is non-chlorinated and highly stable towards oxidation itself. It can dissolve PE from 80 °C onwards, while autoxidation of linear alkanes only becomes important at temperatures above 130 °C40,41. TBHP is the preferred oxidant because of its superior solubility in organic solvents compared to H2O2, and was employed in an equimolar amount with respect to the alkane. In control reactions without a catalyst, only minimal quantities of oxidation products were detected after 24 h. With TS-1 as the catalyst, increasing chain lengths cause the yield to decrease, which is attributed to the zeolite’s microporous structure. While hexane can readily enter the pores, longer alkanes encounter a progressively larger diffusional limitation, resulting in decreased yields. In contrast, TiITQ-6 shows a different trend, as the increase of chain length results in an increased yield. In this case, the substrates do not experience strong diffusional limitations since the active sites are predominantly present on the zeolite’s outer surface. The increased yield and peroxide efficiency observed for longer alkanes may be ascribed to an increased adsorption affinity of the longer alkanes for the catalyst surface42. Importantly, chain-splitting side-products (acids) which would be expected in free radical oxidation were not observed at all.

a Comparison of the catalytic performance of TiITQ-6 with that of a commercial TS−1 in the C-H bond oxidation of alkanes with increasing chain length. b Recyclability of TiITQ-6 (10 mg) in the C-H bond oxidation of dodecane. After the 4th reaction cycle, the titanosilicate was recalcined for 7 h at 580 °C. Reaction conditions: 1 mmol of TBHP (351 µL of 29.2 wt% solution in TBB), 1 mmol of alkane, TBB as solvent to obtain a total volume of 600 µL, 0 to 50 mg of titanosilicate catalyst, 24 h, 80 °C, 500 rpm. Reactions were conducted in triplicate and 95% confidence intervals are indicated for each result.

A further distinction between both zeolites is found in the product distribution. In reactions with TiITQ-6, a high selectivity for ketones is observed, indicating a rapid conversion of the formed alcohols to the respective ketones. TS-1 on the other hand, produces a considerable amount of alcohols. Furthermore, TS-1 shows strong selectivity for oxidation at the 2-position of the alkane, especially concerning ketone formation. This can again be explained by the catalyst’s shape selectivity, directing oxidation to the least sterically hindered position. Conversely, TiITQ-6 exhibited no clear preference for the position of oxidation.

Next, the recyclability of TiITQ-6 was evaluated in the C-H bond oxidation of dodecane (Fig. 2b). After each reaction, the zeolite was isolated through centrifugation and dried overnight at 100 °C. The obtained yield remains within 85% of the initial value throughout at least four reaction cycles, highlighting the catalyst’s stability. A possible explanation for the slightly decreased yield is the presence of residual reaction products adsorbed on the outer surface42. This hypothesis is confirmed by the result obtained after re-calcination of the recycled zeolite (run 5), in which the yield again increases up to 93% of the initial value. Remarkably, the peroxide efficiency increases throughout the consecutive reaction cycles (42.5% peroxide efficiency in run 1 to 50.5% in run 4) and is maintained in run 5.

Based on the results for C-H bond oxidation of alkanes, TiITQ-6 could be a suitable catalyst for the oxyfunctionalization of PE. Table 1 shows that this catalyst not only consumes the peroxide more efficiently than TS-1; also the dispersion of functional groups throughout the carbon chain is more homogeneous, as reflected in a lower selectivity for formation of acetyl termini. This is crucial to obtain a polymer carrying functional groups over the full length of its backbone. Additionally, the formation of chain-splitting side-products is not observed, which allows to selectively functionalize PE without compromising the molecular weight.

First, a low molecular weight PE (LMWPE), with Mw = 4000 g/mol and Mn = 1900 g/mol, was tested as a substrate. As evidenced by the 1H-NMR spectrum of the obtained ketone-functionalized PE (kf-PE), both internal ketones and methyl ketones are introduced (Fig. 3ai). Of course, internal ketones are most desired to attain sufficient polymer degradability after the subsequent step(s). By varying the peroxide concentration and the amount of TiITQ-6 (kf-PE 1a to 1e), the degree of functionalization (FD) could be controlled from 3.44% (about 1 in 30 C-atoms is functionalized) to 1.08% (1 in 93 C-atoms is functionalized). The incorporated functional groups are almost exclusively ketones; alcohols were only retrieved in negligible amounts. Conversely, C-H bond oxidation with TS-1 (kf-PE 1 f) resulted in a much lower FD of 0.63%, and mainly yielded methyl ketones.

a 1H-NMR spectra of functionalized LMWPE samples (600 MHz, CDCl3, 50 °C): (i) kf-PE 1b, (ii) ef-PE 2a, (iii) solid residue after methanolysis of ef-PE 2a, (iv) of-PE 3a, (v) af-PE 4a. b 1H−13C HSQC NMR spectrum of ef-PE 2a (600 MHz, CDCl3, 50 °C). c GPC results of functionalized LMWPE samples compared to the non-functionalized LMWPE. The molecular weight distributions are offset for clarity. d DSC heating curves of functionalized HDPE samples compared to the non-functionalized HDPE. e FTIR-ATR spectra of functionalized LMWPE samples, compared to the non-functionalized LMWPE. The spectra are offset for clarity.

Similar FDs were obtained for both low-density PE (LDPE) and high-density PE (HDPE), indicating that the increasing molecular weight and polymer density do not compromise the catalytic activity of TiITQ-6, nor its interaction with the polymer. However, the peroxide efficiency is significantly lower in reactions with HDPE. This may be due to the decreased polyethylene concentration and the increased temperature required to completely dissolve HDPE, as the latter accelerates unproductive peroxide decomposition. It is noteworthy that significantly higher FDs could be achieved with TiITQ-6 as the catalyst compared to the work by Hartwig and co-workers, where a maximal functionalization of 4% per ethylene unit was obtained (1:1 ratio alcohols and ketones), corresponding to about 1 functional group per 50 C-atoms22.

To verify the applicability of the established method for the C-H bond oxidation of PE waste instead of pristine PE, the C-H bond oxidation was also applied on a post-consumer LDPE bag (pcLDPE; Supplementary Fig. 2). Although the PE concentration needed to be slightly decreased for solubility reasons, the obtained degree of functionalization in the resulting kf-PE 1o aligns very well with the results obtained with pristine LDPE.

Although ketone-functionalized PEs are photodegradable under stimulated sunlight43,44, their solvolytic recyclability is not enhanced compared to that of the native PE. However, the ketone groups can be further modified to introduce heteroatoms inside the carbon chain, which does allow solvolysis. A first method is the BVO of the ketone groups with a peracid, yielding in-chain esters17. The reaction conditions for the BVO with meta-chloroperbenzoic acid (mCPBA, 70–75%) and peracetic acid (AcOOH, 35 wt% in dilute acetic acid) were first optimized using 4-heptanone as a model substrate (supplementary discussion, section 3). Next, the optimized conditions were applied on the kf-PE obtained through C-H bond oxidation of the different PE types with TiITQ-6 and TBHP (Table 2). An ef-LMWPE with ester yield of 73% could be obtained when 4 eq. of mCPBA are employed (ef-PE 2a). Both internal ketones and methyl ketones are equally well converted (Fig. 3aii). Note that the latter are exclusively converted to acetate esters. Methyl esters are not formed, due to the inferior migratory aptitude of the methyl group as compared to the alkyl chain23,24. The use of AcOOH on the other hand seems less efficient, with a maximal yield of 20%. In the BVO of kf-HDPE (ef-PE 2g), a similar ester yield of 73% could be obtained, though the temperature was increased to 90 °C to achieve complete solubility. For kf-LDPE, the obtained yield was lower (ef-PE 2f, 63%). This may be due to the higher degree of branching found in LDPE, sterically hindering the ketone functions.

The degradability of the ef-PE was evaluated by acidic methanolysis in toluene17, through which the in-chain esters are split into methyl esters and primary alcohols (Fig. 4). As indicated by the signal at 4.10 ppm in the 1H-NMR spectrum of the solid material obtained after methanolysis of ef-LMWPE 2a (Fig. 3aiii), the amount of residual internal esters after methanolysis is negligible, proving complete depolymerization of the polymer. Besides the expected degradation products, also methoxyethers are found, originating from the condensation of the primary alcohols with methanol. The obtained mixture can be repolymerized through transesterification, thereby enabling closed-loop recycling (supplementary discussion, section 4).

Through acidic methanolysis of the ef-PE, the incorporated ester groups are split into methyl esters and primary alcohols. pTSA para-toluenesulfonic acid, MeOH methanol.

In addition to oxygen, also nitrogen can successfully be introduced into the polymer chain. The oximation of ketone-functionalized PE has been reported earlier by Hartwig et al.25. Using 1.5 eq. of hydroxylamine (50 wt% in water) with respect to the amount of ketone groups, these are quantitatively converted into the corresponding oximes (Fig. 3aiv). Next, a Beckmann rearrangement in acidic conditions yields in-chain amides (Table 3). Some hydrolysis of the oximes to ketones is noticed, especially for terminal methyl oximes; however, internal oximes are less sensitive to this side reaction. Note that methyl oximes are exclusively converted into N-substituted acetamides (Fig. 3av). Based on previous work by our group it is envisioned that the amide-functionalized PE (af-PE) can be efficiently depolymerized, for instance through ammonolysis45.

As indicated throughout the results, 1H-NMR was used to identify the introduced functional groups (Fig. 3a) and quantify the degree of functionalization. Further confirmation was obtained through 1D 13C, 2D 1H-1H COSY and 2D 1H-13C HSQC NMR experiments (Fig. 3b and Supplementary Figs. 42–51), as these showed the expected signals and couplings for the proposed types of functionalization.

GPC analysis of functionalized LMWPE samples showed that the molecular weight of the polymer was well-retained throughout all modifications (Fig. 3c). The product obtained after methanolysis of ef-PE 2a shows a clear decrease in molecular weight, which again proves the successful depolymerization. The typical Gauss curve is preserved but is simply shifted to lower molecular weight values which suggests that functionalization occurs randomly throughout the full backbone. The Mn of 1.3 kDa corresponds to fragments of about 90 carbon atoms, which corresponds well with the expected value based on the percentage of internal esters (1.38%, or 1 internal ester per 72 carbon atoms).

DSC was employed to evaluate the thermal properties of the functionalized PEs (Fig. 3d and Supplementary Table 5), as these are crucial for the materials’ applicability and processability. As established in previous literature, the melting temperature slightly decreases with increasing amount of ketone groups inserted in the PE crystal structure, following the simplified Sanchez-Eby model (Supplementary Fig. 52)17,22,46. After conversion to either esters or oximes, a more pronounced decrease of the melting temperature is observed due to a stronger disruption of the crystal structure6,17,25. Although the melting point decreases upon functionalization, it remains appropriate (>110 °C for functionalized HDPE and >90 °C for functionalized LDPE) for common PE applications. After Beckmann rearrangement of the of-PE samples, the melting temperature again increases, which can at least partly be ascribed to the formation of amides, since these can form interchain hydrogen bonds21.

In addition to NMR, the incorporation of functional groups was also confirmed through FTIR-ATR (Fig. 3e and Supplementary Figs. 10–12). In the kf-PE, the presence of ketones is clearly evidenced by the ketone νC=O at 1715 cm−1 47. After BVO, a second νC=O at 1740 cm−1 appears, originating from the ester carbonyl groups. Additionally, vibrations at 1180 cm−1 and 1244 cm−1 emerge, which are assigned to the ν(C=O)-O of internal esters and acetate esters respectively48. After reacting the kf-PE with NH2OH, the ketone νC=O largely disappears, while the oxime νN-O becomes visible at 960 cm−1. Also, the oxime νO-H is observed as the broad signal at 3300 cm−1. Subsequent Beckmann rearrangement results in the disappearance of the oxime νN-O, while the amide νC=O and δN-H appear at 1650 and 1550 cm−1 respectively48,49. Since some oximes are hydrolyzed to ketones, also the ketone νC=O at 1715 cm−1 regains some intensity.

Through XRD analysis, it was confirmed that the kf- and ef-polymers show the same crystal structure (orthorhombic) and similar crystallinity compared to non-functionalized PE (Supplementary Fig. 13)27. Consequently, crystallization is still dominated by the Van der Waals interactions between long aliphatic hydrocarbon segments28. However, the diffraction peaks turn broader in the of- and af-PE samples meaning that the PE crystal structure is somewhat perturbed in these materials. This could be explained by the capability of oximes and amides to form hydrogen bonds between themselves or with ketones. Therefore, the crystallization process is not only driven by Van der Waals interactions in these polymers, but also by hydrogen bonding21,28.

In summary, different types of polyethylenes were converted into polyketones through a titanosilicate catalyzed C-H bond oxidation. TiITQ-6, a layered titanosilicate with large external surface area, was found to be exceptionally active in the oxidation of polyethylene with tert-butyl hydroperoxide in mild conditions. Furthermore, the reaction showed a high ketone selectivity. The incorporated ketone groups can be exploited to introduce heteroatoms in the polymer chain, without compromising the molecular weight. Oxygen was incorporated through a Baeyer-Villiger oxidation yielding in-chain esters, while nitrogen was incorporated through oximation and subsequent Beckmann rearrangement yielding in-chain amides. These polymers retain the beneficial properties of the virgin polyethylenes, which are combined with an increased potential for chemical recycling. The implementation of the reported modifications on waste polyethylene can serve as an important step towards a sustainable, circular polyethylene economy.

The lamellar TiPREFER precursor was prepared following the procedure by Corma et al.38 using fumed silica (Cab-O-Sil M5) as the Si-source, titanium ethoxide (TEOT) as the Ti-source and 4-amino-2,2,6,6-tetramethylpiperidine as the structure directing agent (SDA). The molar composition of the synthesis mixture was 1 SiO2 : 0.02 TEOT : 1 SDA : 10 H2O : 1.5 NH4F : 0.5 HF. This solution was crystallized for 10 days at 135 °C in a Teflon-lined Parr reactor equipped with a mechanical stirrer (100 rpm). Subsequently, the as-synthesized material was recovered by centrifugation, extensively washed with milliQ water and dried at 70 °C overnight.

The obtained TiPREFER precursor can either be directly calcined to obtain the respective TiFER zeolite, or can be delaminated to prepare TiITQ-6. The performed procedure was also based on the protocol reported by Corma et al.38. First, the precursor is swollen in an aqueous solution containing 16.67 wt% cetyltrimethylammonium bromide and 8 wt% TPAOH for 24 h at 90 °C while stirring. Next, delamination is performed by placing the mixture in an ultrasound bath for 1 h. Afterwards, the solid phase is recovered by centrifugation and extensively washed with milliQ water. After drying at 70 °C overnight, the material is calcined at 580 °C for 7 h, with a temperature ramp of 1 °C/min.

The desired amount of titanosilicate and 1 mmol of the alkane substrate (hexane, octane, decane or dodecane) were added into a glass 1.8 mL crimp cap vial. 1 mmol of tert-butyl hydroperoxide (TBHP) was introduced by adding 351 µL of a 29.2 wt% solution of TBHP in tert-butylbenzene (TBB). This solution was obtained through extraction of an aqueous 70 wt% TBHP solution. Next, the reaction mixture was diluted with additional TBB to obtain a total volume of 600 µL and a magnetic stirring rod was added. The reaction vials were then sealed and heated to 80 °C. After the required reaction time, the vials were cooled and the titanosilicate was centrifuged (5 min, 1318 x g). The reaction mixture was analyzed through GC-FID using dodecane or decane as an external standard (ES).

The desired amount of titanosilicate and PE (70 to 300 mg) were added into a glass 11 mL crimp cap vial. Next, the TBHP in TBB solution was added, and diluted with additional TBB to obtain the desired peroxide concentration. The reaction vials were sealed and heated to 90 °C or 100 °C. After the required reaction time, the vials were centrifuged (1 min, 586 x g) without cooling, to separate the titanosilicate from the polymer, as the latter remains dissolved. The supernatant was transferred to a crimp cap vial with ethanol (EtOH), which acts as the anti-solvent. Centrifugation (5 min, 1318 x g) makes the polymer precipitate, so that it can be separated from the remaining solution. The polymer was washed twice with EtOH to remove solvent and reaction products, and finally dried overnight in a vacuum oven.

The desired amount of peracid and kf-PE were added into a glass 11 mL crimp cap vial. 2 mL of tert-amyl methyl ether (TAME) was then added, after which the reaction vials were sealed and heated to the desired reaction temperature. After the required reaction time, the polymer is precipitated by addition of EtOH and separated through centrifugation. The obtained ef-PE is washed twice with EtOH to remove solvent and reaction products, and finally dried overnight in a vacuum oven.

The desired amount of kf-PE was added into a glass 11 mL crimp cap vial. Subsequently, 1.5 eq. of hydroxylamine (50 wt% in water) with respect to the amount of ketone groups was added, along with 2 mL of TBB or toluene as the solvent. The reaction vials were then sealed and heated to 80 °C. After the required reaction time, the polymer is precipitated by addition of EtOH and separated through centrifugation. The obtained of-PE is washed twice with EtOH to remove solvent and reaction products, and finally dried overnight in a vacuum oven.

The desired amount of of-PE was added into a glass 1.8 mL crimp cap vial. Subsequently, 600 µL of TBB was added, along 3 eq. of H2SO4 with respect to the amount of oxime groups. After sealing the reaction vials, these were heated to 100 °C. After the required reaction time, the polymer is precipitated through decantation of the reaction mixture in an 11 mL crimp cap vial containing EtOH, and separated through centrifugation. The obtained af-PE is washed twice with EtOH to remove solvent and reaction products, and finally dried overnight in a vacuum oven.

Methanolysis experiments of ef-PE were performed in small pressure reactors containing a glass liner. 50 mg of polymer, 5 mg of para-toluenesulfonic acid monohydrate (pTSA), 2 mL of toluene and 1 mL of methanol (MeOH) were added, after which the reactor was sealed and heated to the reaction temperature of 150 °C. After the required reaction time, the reactor was cooled, and the reaction mixture was transferred to an 11 mL crimp cap vial containing MeOH. After centrifugation and decantation of the supernatant, the solid residue was washed twice with MeOH and dried overnight in a vacuum oven.

The findings of this study are available within the article and its supplementary information. All data are available from the authors upon reasonable request.

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R.L. and J.V. thank the Fonds Wetenschappelijk Onderzoek (FWO) for funding (scholarship no. 1S85822N and 1263522N respectively). G. O’Rourke, M. Houbrechts and V. Lemmens are thanked for their help with DSC, GPC and nitrogen physisorption measurements respectively. C. Marquez and A. Lauwers are thanked for their assistance with the ICP measurement.

Centre for Membrane Separations, Adsorption, Catalysis and Spectroscopy (cMACS), KU Leuven, Celestijnenlaan 200F, post box 2454, 3001, Leuven, Belgium

Robin Lemmens, Jannick Vercammen, Lander Van Belleghem & Dirk De Vos

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Experiments were devised by R.L. and D.D.V. Titanosilicate synthesis was performed by R.L. and J.V. Optimization of the Baeyer-Villiger oxidation was performed by R.L. and L.V.B. Other experimental work was performed by R.L. The manuscript was written by R.L. and D.D.V.

Correspondence to Dirk De Vos.

The authors declare no competing interests.

Nature Communications thanks the anonymous reviewers for their contribution to the peer review of this work. A peer review file is available.

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Lemmens, R., Vercammen, J., Van Belleghem, L. et al. Upcycling polyethylene into closed-loop recyclable polymers through titanosilicate catalyzed C-H oxidation and in-chain heteroatom insertion. Nat Commun 15, 9188 (2024). https://doi.org/10.1038/s41467-024-53506-9

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Received: 12 July 2024

Accepted: 14 October 2024

Published: 24 October 2024

DOI: https://doi.org/10.1038/s41467-024-53506-9

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