Atom transfer radical depolymerization of polyolefins

Abstract

The present invention generally relates to a reverse atom transfer radical depolymerization method to depolymerize polyolefins into its corresponding alkyl monomers and short-chain alkyl halides with moderate depolymerization yield. Also, the present invention relates to repolymerizing of the depolymerized monomers to polyolefins. The depolymerization process only requires 1-2 steps and works under mild conditions. Moreover, the monomer, produced does not require any additional separation.

Description

[0001] ATOM TRANSFER RADICAL DEPOLYMERIZATION OF POLYOLEFINS

[0002] FILED OF THE INVENTION

[0003] The present invention generally relates to a reverse atom transfer radical depolymerization method to depolymerize polyolefin into corresponding alkyl monomers and short-chain alkyl halides with moderate depolymerization yield. More particularly, the present invention relates to repolymerizing of the depolymerized monomers to polyolefin.

[0004] BACKGROUND OF THE INVENTION

[0005] Plastic has gradually become an indispensable part of our day to day lives. The growth of plastics production in the past 70 years has left behind most of the manufactured materials to a great extent. The annual production of plastics worldwide has already surpassed 380 million metric tons which is expected to exceed 500 million metric tons by 2050. As of 2017, more than 9 billion tons of plastic have been produced since 1950s, approximately 10% of which had been recycled, 14% were incinerated, while the remaining 76% were simply thrown into the trash. If the current scale of plastics production and scenario of waste management do not change, approximately 12 billion tons of plastic waste will be accumulated in landfills or in the natural environment by 2050. Polyolefins are accounted for more than 50% of the global synthetic polymers produced, where polyethylene (PE), the most consumed single-use plastic in human society, itself comprises -34% of the total plastics market.

[0006] Improving the sustainability of polymers is a potential solution to the emerging environmental concerns associated with the end-of-life plastics and it encircles a wide range of approaches including development of biodegradable polymers, degradation, chemical recycling and upcycling. Depolymerization is the process of converting polymers back to constituting monomers, and can be considered as the most complete and ideal form of recycling. Unlike cleavable polymers such as polyesters, polyurethanes, polycarbonates etc. with cleavable bonds, polyolefins with inert and saturated C-C backbones cannot be easily broken down by chemical or enzymatic means and the release of polyolefin waste into nature or their incineration has been emerged as the most serious threat to environment. Over the past decade, a continuous effort has been observed for the development of innovative approaches to transform raw polyolefin wastes into short-medium carbon range chemicals for their use as either energy carriers or as circular feedstock to produce new chemicals and plastics. Different methods to depolymerize PE to gaseous and liquid hydrocarbons have been reported including pyrolysis (Ragaert, K. et al., Waste Manage. 2017, 69, 24-58), catalytic cracking (Munir, D. et al., Mater. Renew. Sustain. Energy. 2020, 9, 1-13), hydrogenolysis (Sanchez-Rivera, K. L. et al., ACS Cent. Sci. 2021, 7 (1), 17-19), tandem hydrogenolysis- / aromatization and alkane metathesis (Jia, X. et al., Sci. Adv. 2016, 2 (6), el501591). A low energy and cost-effective approach to depolymerize polyolefins, especially PE, would be a very challenging and valuable development from an environmental perspective.

[0007] The field of reversed controlled polymerization (RCP) is growing and evolving at an unprecedented rate. Polymers with chain-end functionalities (CEFs) synthesized via reversible deactivation radical polymerization (RDRP) techniques have been reported to undergo depolymerization reactions via unzipping mechanism. Halogen end capped methacrylate-based polymers have been already reported to initiate depolymerization using an atom transfer radical polymerization (ATRP) catalyst at elevated temperature (Jones, G. R. et al., J. Am. Chem. Soc. 2023, 145 (18), 9898-9915). Recently Matyjaszewski and co-workers (ACS Macro Lett. 2023, 12 (8), 1173-1178) reported the depolymerization of Cl-terminated poly methacrylates mediated by copper (II) chloride / tris(2-pyridyl methylamine (CuCh / TPMA) catalyst at 180- 230 °C. However, an atom transfer radical depolymerization (ATRdP) of (waste) polyolefins is nowhere disclosed in the literature till date.

[0008] There remains an unmet need of the efficient and cost-effective method for depolymerization of polyolefins specifically polyethylene and polypropylene.

[0009] OBJECTIVES OF THE INVENTION

[0010] In an object of the present invention is to provide a process of depolymerization of (waste) polymer into depolymerized monomer(s) and short-chain alkyl halide(s), and a subsequent repolymerization of the depolymerized monomer(s) into fresh and new polymer.

[0011] In another object of the present invention is to provide a process of depolymerization of (waste) C2-C4 polyolefin into depolymerized monomer(s) and short-chain alkyl halide(s), and a subsequent repolymerization of the depolymerized monomer(s) into fresh and new C2-C4 polyolefin.

[0012] In yet another object of the present invention is to provide a process of depolymerization of (waste) polyethylene or polypropylene into depolymerized monomer(s) ethylene or propylene and short-chain alkyl halide(s), and a subsequent repolymerization of the depolymerized monomer(s) into fresh and new polyethylene or polypropylene. Still another object of the present invention to provide an atom transfer radical depolymerization (ATRdP) strategy to depolymerize polyolefins to its corresponding monomers with effective depolymerization yield.

[0013] SUMMARY OF THE INVENTION

[0014] The present invention generally relates to a reverse atom transfer radical depolymerization method to depolymerize polyolefin into corresponding alkyl monomers and short-chain alkyl halides with moderate depolymerization yield. Also, the present invention relates to repolymerizing of the depolymerized monomers to polyolefin.

[0015] In an aspect, the present invention relates to a process of depolymerization and repolymerization of a polymer, comprising the steps of: a) halogenating the polymer by reacting and treating a polymer solution containing said polymer with a halogenating agent, a catalyst, an aqueous acid solution and an oxygen flow under stirring and LED light irradiation at temperature in the range of 35-45 °C for time period in the range of 16-20 h to obtain the halogenated polymer; b) depolymerizing the halogenated polymer by reacting and treating a halogenated polymer solution containing said halogenated polymer of step a) with a catalyst solution mixture comprising a copper-based catalyst, a ligand and a solvent, under reflux at temperature in the range of 90-120 °C for time period in the range of 22-25 h to obtain crude mixture of monomer and / or short-chain alkyl halide(s); c) separating and purifying the crude mixture of step b) to obtain depolymerized monomer and short-chain alkyl halide(s) separately; and d) re-polymerizing the depolymerized monomer of step c) by treating and reacting with a polymerization catalyst solution at temperature in the range of 20-35 °C for time period in the range of 22-25 h to obtain the re-polymerized polymer; the polymer is selected from but not limited to C2-C4 polyolefin.

[0016] In an embodiment, the polymer is selected from but not limited to polyethylene, polypropylene, ethylene vinyl acetate (EVA), polyolefin elastomers (POEs), plastomers (POPs) and polybutylene.

[0017] In an embodiment, the polyethylene is selected from but not limited to low density polyethylene (LDPE), high density polyethylene (HDPE), linear LDPE (LLDPE), ultra-low density polyethylene (ULDPE), Metallocene Polyethylene (mPE), medium density polyethylene ( MDPE),

[0018] In an embodiment, the polypropylene is selected from but not limited to polypropylene homopolymer (PPH), polypropylene copolymer (PPC), and polypropylene random copolymer (PPC random).

[0019] In an embodiment, the polymer solution of step a) is prepared by mixing and dissolving polymer in o-xylene, p-xylene or toluene as solvent.

[0020] In an embodiment, the halogenating agent is selected from but not limited to potassium bromide, sodium bromide and so on.

[0021] In an embodiment, the catalyst is selected from but not limited to sodium nitrite, potassium nitrite and so on.

[0022] In an embodiment, the aqueous acid solution is prepared by mixing an acid in water to obtain at least 30% v / v concentration of aqueous acid solution.

[0023] In an embodiment, the acid is selected from but not limited to hydrochloric acid, sulfuric acid, nitric acid, and so on.

[0024] In specific embodiment, the halogenating of step a) is done by: i. providing a polymer solution in a beaker or container; ii. adding the halogenating agent, the catalyst, and the aqueous acid solution to said beaker / container under stirring in a sealed environment; iii. purging the reaction mixture of ii) with oxygen at temperature in the range of 25-35 °C for time period in the range of 10-20 minutes followed by irradiating the reaction mixture with the LED module light having wavelength in the range of 410-450 nm) under continuous stirring in presence of oxygen at temperature in the range of 35-45 °C for time period in the range of 16-20 h to obtain crude halogenated polymer; iv. the crude halogenated polymer is precipitated in excess C1-C4 alcohol to obtain precipitated halogenated polymer; v. centrifuging the precipitated halogenated polymer followed by decanting of solvent to obtain a residue; vi. washing the residue at least 3 times with C1-C4 alcohol followed by centrifugation, and drying under vacuum at temperature in the range of 70-90 °C to obtain the pure halogenated polymer. In an embodiment, the amount / concentration of the polymer is in the range of 30-40 mmol w.r.t monomer repeating unit.

[0025] For example: the concentration of polyethylene is given with respect to ethylene's repeating unit because as polyethylene is made by polymerizing ethylene monomers (C2H4), where the “30-40 mmol” range means that for every 30-40 millimoles of polymer concentration, it corresponds to that many ethylene monomers. The monomer here is ethylene with a molecular weight of 28 g / mol

[0026] In an embodiment, the amount / concentration of the polymer is at least 35.72 mmol w.r.t monomer repeating unit.

[0027] In an embodiment, the amount / concentration of the catalyst is in the range of 5-15 mmol w.r.t monomer repeating unit.

[0028] In an embodiment, the amount / concentration of the catalyst is at least 10 mmol w.r.t monomer repeating unit.

[0029] In an embodiment, the amount / concentration of the halogenating agent is in the range of 5-10 mmol.

[0030] In an embodiment, the amount / concentration of the halogenating agent is at least 7.14 mmol.

[0031] In an embodiment, the amount / concentration of the acid solution is in the range of 30-50 mmol.

[0032] In an embodiment, the amount / concentration of the acid solution is at least 42 mmol.

[0033] In an embodiment, the LED light is selected from but not limited to blue LED, green LED and red light.

[0034] In an embodiment, the copper based catalyst is selected from but not limited to Cu(I)bromide, Fe(II)bromide, Pd(II)bromide, Ni(II)bromide, Os(II)bromide, Ru(II)bromide and so on.

[0035] In an embodiment, the ligand is selected from but not limited to tris(2-pyridylmethyl)-amine (TPMA), _2,2'-Bipyridyl (Bpy); 1,1,4,7,10,10-Hexamethyltriethylenetetramine (HMTETA); N,N,N',N'-Pentamethyldiethylenetriamine (PMDETA); 1,4, 8,11 -Tetramethyl- 1,4, 8,11- tetraazacyclotetradecane (Me4Cyclam); Tris [2-(dimethylamino)ethyl] amine (MeeTREN), and so on.

[0036] In an embodiment, the solvent is selected from but not limited to toluene, anhydrous toluene, o-xylene, p-xylene, dodecane, and so on. In an embodiment, the halogenated polymer solution as solution A is prepared by mixing and dissolving the halogenated polymer in toluene solvent 5.2 under inert conditions followed by degassing the solution mixture by at least three freeze-pump-thaw cycles and heated at temperature in the range of 70-90 °C for time period in the range of 20-40 minutes to obtain the solution A of the halogenated polymer solution.

[0037] It is to be noted that oxygen must be removed from the ATRP reaction system because it inhibits radical polymerization, reacting with free radicals and causing termination or side reactions that disrupt controlled polymerization. The freeze-pump-thaw cycle is a technique used to remove oxygen from a reaction vessel. First, the vessel is cooled in a liquid nitrogen bath, freezing the contents. Next, a vacuum pump is applied to the vessel to remove trapped gases. After pumping, the vessel is allowed to warm up (thaw) to room temperature, causing any remaining volatile components to evaporate. This process is typically repeated several times to ensure thorough degassing.

[0038] In specific embodiment, the depolymerization of the halogenated polymer of step b) is done by: i. preparing the catalyst solution mixture by mixing copper-based catalyst, ligand and solvent, under stirring for 10-20 minutes till the solution forms a light orange colour due to complex formation, and then degassed by at least three times; ii. providing said solution, A containing halogenated polymer solution; iii. the solution A of ii) is transferred via a cannula into the container containing the catalyst solution mixture of i) to obtain a reaction mixture; and iv. refluxing the reaction mixture of iii) under an inert atmosphere at temperature in the range of 90-120 °C for time period in the range of 22-25 h to obtain crude depolymerized mixture of monomer and / or short-chain alkyl halide(s).

[0039] In specific embodiment, the separating and purifying the crude mixture of step c) is done by: i. centrifuging the crude depolymerized mixture followed by decanting of brownish clear solvent solution to obtain a residue; ii. washing the residue at least 3 times with C1-C4 alcohol followed by centrifugation, and drying under vacuum or reduced pressure at temperature in the range of 50-70 °C to obtain the pure depolymerized material i.e., monomer and / or short-chain alkyl halide(s). In an embodiment, the amount / concentration of the copper-based catalyst is in the range of 0.03-0.15 mmol.

[0040] In an embodiment, the amount / concentration of the copper-based catalyst is at least 0.05 mmol.

[0041] In an embodiment, the amount / concentration of the ligand is in the range of 0.01-0.10 mmol.

[0042] In an embodiment, the amount / concentration of the copper-based catalyst is at least 0.075 mmol.

[0043] In an embodiment, the amount / concentration of the halogenated polymer is in the range of 4- 15 mmol.

[0044] In an embodiment, the amount / concentration of the copper-based catalyst is at least 5.2 mmol.

[0045] In an embodiment, the polymerization catalyst solution is prepared by mixing a polymerization catalyst complex in excess dry toulene, dry hexane under stirring for at least 10 minutes till it forms a dissolved solution.

[0046] In an embodiment, the polymerization catalyst is selected from but not limited to palladium-phosphinesulfonate complex, Titanium-iminocarboxylate complex, Nickel- napthoxyimine complex, Palladium-imine sulfonate complex, phenoxy-imine Titanium complex, known as FI (Fujita Invent)-Ti catalyst, metallocene and post-metallocene complexes, and so on.

[0047] BRIEF DESCRIPTION OF THE DIAGRAMS

[0048] Figure 1: Shows the schematic representation of the atom transfer radical depolymerization and repolymerization pathway of polyethylene.

[0049] Figure 2: Represents 1H NMR spectra (toluene-d8, 400 MHz, 80 °C) of (a) polyethylene (PE) and (b) brominated polyethylene (BPE).

[0050] Figure 3: Represents ATR-FTIR spectrum of brominated polyethylene (BPE).

[0051] Figure 4: Represents plausible mechanism of the atom transfer radical depolymerization (ATRdP) of Br-end capped polyethylene (top), and the structure of ligand and Cu-complexes participated in the depolymerization of BPE in this present work.

[0052] Figure 5: Represents 1H NMR (CDCI3, 500 MHz, 25 °C) of bulk reaction mixture after 24 h of ATRdP (* solvent impurity).

[0053] Figure 6: Represents GC / MS identification of depolymerization products present in toluene solution.

[0054] Figure 7: Represents1H NMR (toluene-d8, 500 MHz, 80 °C) of solid polymer residue obtained after 24h of ATRdP (*Solvent peak).

[0055] Figure 8: Represents differential scanning calorimetric (DSC) analysis of PE, BPE and solid residue obtained after 24h ATRdP of BPE.

[0056] Figure 9: Represents GC chromatogram of ethylene standard (bottom) and ethylene generated after depolymerization of PE (top).

[0057] Figure 10: Represents ’ H NMR spectra (CDC13, 500 MHz, 25 °C) of standard ethylene gas (blue line) and gaseous reaction mixture after 72 hours of depolymerization (red line).

[0058] Figure 11: Represents ’ H NMR (toluene-d8, 500 MHz, 25 °C) of solid polymer residue obtained after 24h of polymerization.

[0059] DETAILED DESCRIPTION OF THE INVENTION

[0060] In one embodiment, the present invention relates to an atom transfer radical depolymerization of polyolefins to its corresponding monomers; the process comprising steps of: a) bromination of polyolefin; and b) depolymerizing the brominated polyolefin to its corresponding monomers.

[0061] In another embodiment, the present invention relates to a repolymerizing of the depolymerized monomer to polyolefin.

[0062] In one embodiment of the present invention, brominated polyolefin can be potentially depolymerized using a suitable ATRP catalyst yielding monomer units and / or oligomers.

[0063] In another embodiment, the present invention relates to a catalytic activation of polyolefin to add necessary functionality for catalytic interaction.

[0064] Yet in another embodiment of the present invention, polyolefin is activated through the catalytic bromination to add necessary functionality for catalytic interaction.

[0065] In one embodiment of the present invention, catalytic halogenation of the polyolefin is done under light using acidic PE solution in solvent for 16h-18h at 30 °C-40 °C in presence of appropriate catalyst and reagent.

[0066] In one embodiment of the present invention, polyolefin is selected from C2-C4 polyolefins. In another embodiment of the present invention, the polyolefin is selected from polyethylene, polypropylene, polybutylene and, etc.

[0067] In most preferred embodiment of the present invention, the polyolefin is polyethylene.

[0068] In an embodiment, the bromination is done to selectively functionalize polyethylene chains. Instead of conventional visible light, blue LED irradiation (400-450 nm) is employed for bromination to enhance the reaction efficiency. Additionally, the reaction temperature is kept in between 40-60°C and systematically covered solvent selected from toluene, o-xylene, and p-xylene, to optimize the process. Notably, this reaction achieves brominated C2-C4 polyolefin or specifically polyethylene with exceptionally low bromine content — an essential factor for enabling controlled depolymerization.

[0069] In another embodiment, the present invention relates to a repolymerizing of the depolymerized monomer to polyethylene.

[0070] In one embodiment of the present invention, brominated polyolefin can be potentially depolymerized using a suitable ATRP catalyst yielding monomer units and / or oligomers.

[0071] In another embodiment, the present invention relates to a catalytic activation of polyethylene to add necessary functionality for catalytic interaction.

[0072] Yet in another embodiment of the present invention, polyolefin is activated by introducing functionalities such as halides, -NO2, -NH2, -S2, etc.

[0073] Yet in another embodiment of the present invention, polyethylene is activated through the catalytic halogenation to add necessary functionality for catalytic interaction.

[0074] Yet in another embodiment of the present invention, polyethylene is activated through the catalytic bromination to add necessary functionality for catalytic interaction.

[0075] In one embodiment of the present invention, catalytic halogenation of the polyethylene is done under light using acidic PE solution in solvent for 16h-18h at 30 °C-40 °C in presence of appropriate catalyst and reagent.

[0076] Yet in another embodiment of the present invention, catalytic bromination of the polyethylene is achieved by applying blue LED irradiation on 12.5 wt.% acidic PE solution in o-xylene for 18h at 40 °C in presence of NaNCh (cat.), KBr and O2. In one embodiment, the present invention relates to catalytic bromination of polyethylene followed by the depolymerization of PE into corresponding monomers. Depolymerization followed by the repolymerization of monomers into PE.

[0077] In one embodiment of the present invention, depolymerization of PE is done using metal catalyst and appropriate solvent and reagents at 100 °C - 130 °C for 22 hr-26 hr.

[0078] In one embodiment of the present invention, depolymerization of PE is done using CuBr / TPMA catalyst, toluene as a solvent at 110 °C for 24hr.

[0079] Depolymerizing polyolefins (PE) into their monomers is inherently energy-intensive due to the exceptional thermal and chemical stability of their backbones. Thermal depolymerization typically requires temperatures exceeding 200°C, as the process begins with chain scission, generating a radical that triggers unzipping to yield monomers. This process is generally performed above the polymer’s ceiling temperature (Tc), a critical threshold indicating its tendency to revert to its monomeric form. e.g. in present case, polyethylene has an exceptionally high Tc exceeding 600°C. This high Tc is because of polyethylene consisting solely of carbon and hydrogen, forming a highly robust, unfunctionalized backbone that resists thermal degradation. Consequently, polyethylene presents a far greater challenge, necessitating extreme temperatures and harsher conditions to achieve depolymerization.

[0080] In present depolymerization step, there is no need for an activator regeneration step which is required conventionally, and in present case, the depolymerization is done by employing Cu(I) complex [CuCl / TPMA] for halogen abstraction from polyethylene chain ends. This fundamental distinction streamlines the process and eliminates reliance on an external electron donor for catalyst activation.

[0081] In one embodiment of the present invention, repolymerization of PE is done using metal catalyst and appropriate solvent and reagents at 20 °C - 30 °C for 22hr-26 hr.

[0082] Yet in another embodiment of the present invention, repolymerization of PE is done using Pd in acetonitrile as catalyst, toluene and DCM at 25 °C for 24 hr.

[0083] In one embodiment of the present invention, catalytic bromination of PE is done using NaNCh catalyst.

[0084] In another embodiment of the present invention, polyethylene depolymerized into its corresponding monomers selected from ethylene, short chain alkyl and alkyl halide. In some embodiment of the present invention, the short chain alkyl is ecosene, octadecene, hexadecane, tetradecene, dodecene, decene, octene, hexane or butene.

[0085] In some embodiment of the present invention, the short chain alkyl halide is ethyl bromide, propyl bromide, butyl bromide, pentyl bromide, hexyl bromide, heptyl bromide, octyl bromide, nonyl bromide, decyl bromide, dodecyl bromide, octadecyl bromide, tetradecyl bromide or hexadecyl bromide.

[0086] In one embodiment of the present invention, atom transfer radical depolymerization (ATRdP) of brominated polyethylene was carried out using a Cu-based ATRP catalyst: Cu(I)bromide / tris(2-pyridylmethyl)-amine (CuBr / TPMA) at 110 °C for 24 hours in toluene using standard Schlenk techniques.

[0087] In another embodiment of the present invention, the abstraction of bromine atom from the BPE chains via the reversible oxidation of the [CuzBr / TPMA] complex to [CunBr2 / TPMA] complex resulted in generation of radical at polymer backbone which ultimately led to unzipping of the whole polymer chains.

[0088] In one embodiment of the present invention, the depolymerization products are ethylene gas which generally escape the reaction vessel with argon during the course of depolymerization, short chain alkyl halide which goes into toluene solution and simple alkane which remains as solid residue.

[0089] In another embodiment of the present invention, the outlet of the depolymerization reactor is directly connected with a schlenk flask already charged with a polymerization catalyst: palladium-phosphinesulfonate complex [{PAO}PdMe(L)] (PAO=K2-P,O-Ar2PC6H4SO2O with Ar = 2-MeOC6H4;L=CH3CN) in toluene at room temperature. Initially, a 100 mL schlenk flask was charged with 5 mg of the above mentioned polymerization catalyst and dissolved in 1.5 mL dry DCM followed by addition of 10 mL dry toluene. The reaction mixture was stirred for 10 minutes and connected to the outlet of the depolymerization reactor via a cannula and kept at 25° C for 24 hours with stirring. Excess of ethylene and argon gas coming from the depolymerization reactor was properly vented throughout the polymerization.

[0090] In one embodiment of the present invention, the method of depolymerization of present invention achieved ~49 % depolymerization of brominated polyethylene to short chain alkane / alkyl halide. In a nutshell, the present invention provides a method to brominate polyethylene chains using blue LED under mild condition followed by the depolymerization of the brominated polyethylene using an ATRP catalyst at 110 °C in toluene. By this method, the inventors were able to achieve at least 49 % depolymerization of brominated polyethylene to short chain alkane / alkyl halide.

[0091] EXAMPLES:

[0092] Example 1: Depolymerization of polyolefin i.e., polyethylene:

[0093] Step A: Bromination of polyethylene:

[0094] Polyethylene (1g, 35.72 mmol w.r.t ethylene repeating unit, 5 equiv.) was taken in an oven- dried conical flask and dissolved in 8 ml of o-xylene. KBr (850 mg, 7.14 mmol, 1 equiv.), NaNO2 (246.44 mg, 10 mol % w.r.t polyethylene) and 35% HC1 (aq) (3.72 mL, 42.86, 6 equiv.) were added to it. The conical flask was equipped with a magnetic stir bar and sealed with a silicone rubber septum. The reaction mixture was purged with oxygen for 15 minutes and irradiated with 18W compact blue LED module light (410-450 nm) under continuous magnetic stirring in presence of oxygen for 18 h at 40 °C. The reaction mixture initially turned brownish orange and finally a dark brown coloured solution was phase- separated after completion of the reaction. Note that, to avoid the escape of HCL gas, HC1 was the last reagent to be added, followed by quick sealing of the conical flask. After completion of the reaction, the reaction mixture was cooled to room temperature and precipitated in excess MeOH (50 mL). The resulting suspension was centrifuged, the solvent was decanted and the resulting solid was washed several times with methanol followed by centrifugation. Finally, the resulting solid was dried under vacuum at 80 °C. Yield (%) = 52.

[0095] It is to be noted that the bromination reactions were performed inside a fume hood with adequate safety precautions as the reactions involved the production of bromine gas in situ.

[0096] Characterization Data:

[0097] NMR analysis of pristine polyethylene (in Figure 2a) which mainly showed characteristic signals of polyethylene backbone at 1.08-1.67 ppm. On the other hand, the ’ HNMR spectrum of the brominated polyethylene (in Figure 2b) showed new signals at around 3.88-4.25 and 1.95 ppm which were considered to correspond to -CH-Br protons and CfL-CHBr protons respectively, thus confirming the successful bromination of polyethylene. Additionally, multiple signals were observed in the region of 2.17-3.74 ppm originating from CH2 - protons in dissimilar environments due to other neighbouring electronegative bromine atoms. The percentage substitution of the CH protons by Br atoms on the polyethylene chain (% Br) were calculated from1H NMR spectra following a reported protocol: Assume that the integral for CHBr = x (based on the integral for the region between 3.81 and 5.42 ppm, Figure 1), for CH2 = y (based on the integral for the region between 1.07 and 1.88 ppm, 1.89 and 2.01ppm and 2.16 and 3.74 ppm, in Figure lb), and for CH3 = z (based on the integral for the region between 0.71 and 1.03 ppm in Figure lb).

[0098] Assuming that there is no formation of Br-C-Br bonds, and then it is apparent that -CHBr signals resulted from the -CH2 or -CH3 moiety through substitution of one hydrogen atom by one bromine atom. The equation: % Br = [x / (2x+y+z)]xl00 was used to calculate the percentage substitution of CH by CBr. The %Br for the BPE was calculated to be 0.21. It is to be noted that, to perform ATRdP on BPE samples, only a single C-Br bond in the polymer chain is sufficient to initiate the depolymerization process. Hence, the depolymerization reaction was performed on BPE sample with very low bromine content to have a better control on the overall process.

[0099] FTIR spectrum (in Figure 3) of the BPE which showed characteristic absorption bands that can be assigned to C-Br stretching at 668 and 596 cm-1, along with the C-H asymmetric stretching at 2915 cm-1, C-H symmetric stretching at 2847 cm-1, CH2 -scissoring at 1458 cm-1, and CH2 rocking at 718 cm-1.

[0100] Step B: Depolymerization of brominated polyethylene (BPE):

[0101] First, CuBr (7.5 mg, 0.05 mmol), TPMA (23 mg, 0.075 mmol) and anhydrous toluene (4 mL) were charged in a 100 mL round-bottom Schlenk flask. This mixture was stirred for 15 minutes until it imparted a light orange colour due to complex formation and degassed by 3 freeze- pump-thaw cycles. In a second Schlenk flask, 145.3 mg BPE (5.2 mmol w.r.t ethylene repeating unit) was taken under argon followed by introduction of 4 mL anhydrous toluene into the vessel. This mixture was degassed by 3 freeze-pump-thaw cycles and heated to 80 °C for 30 min. After complete solubilization, the resulting mixture was then transferred via a cannula into the first Schlenk flask. The resulting mixture was then placed in an oil bath with a reflux condenser. The depolymerization took place under an argon atmosphere at 110 °C for 24 h. After 24h, the solution was quenched by cooling down the reaction medium to ambient temperature and exposition of the depolymerization medium to air. The bulk reaction mixture was then centrifuged and the brownish clear toluene solution was decanted. The solid residue recovered by centrifugation was washed several times with excess methanol and dried at 60 °C under reduced pressure. This solid residue was weighted to be 43 mg. The bulk reaction mixture was concentrated in vacuo yielding a brown solid (weighted to be 30.3 mg) which was soluble in chloroform at room temperature.

[0102] Characterization Data:

[0103] 1) NMR Analysis of the residue / reaction mixture obtained:

[0104] Due to the presence of very reactive bromine atoms, brominated polyethylene can be potentially depolymerized using a suitable ATRP catalyst yielding monomer units and / or oligomers. The atom transfer radical depolymerization (ATRdP) of BPE was carried out using a Cu-based ATRP catalyst: Cu(I)bromide / tris(2-pyridylmethyl)-amine (CuBr / TPMA) at 110 °C for 24 hours in toluene using standard schlenk techniques that are typically used in the ATRP reactions. The abstraction of bromine atom from the BPE chains via the reversible oxidation of the [Cu(I)Br / TPMA] complex to [Cu(II)Br2 / TPMA] complex resulted in generation of radical at polymer backbone which ultimately led to unzipping of the whole polymer chains (refer, Figure 4).

[0105] The1H NMR spectrum of the bulk reaction mixture (Figure 5) revealed signal, at 4.23-5.47 ppm originating from CHBr protons and another signal, at 1.93 ppm originating from the neighbouring CH2 protons along with characteristic signal at 0.98-1.64 ppm and2.32-3.76 ppm for backbone -CH2 protons with different environment. Using the same protocol as mentioned earlier for BPE, the bromine content (%Br) of the product present in the bulk reaction mixture was evaluated from the integration ratios between the signals of -CHBr and the -CH2 backbone and was calculated to be 0.89. There is a large decrease of integral value of -CH2 protons for the bromoalkane obtained after depolymerization of BPE while considering the integral of - CHBr protons to be unity. This clearly suggested the formation of short-chain bromoalkane as one of the depolymerization products.

[0106] Inventors have also tried to identify the depolymerization products present in the toluene medium through GC-MS analysis. The reaction mixture was passed through a silica column prior to GC-MS analysis. The GC-MS (in Figure 6) chromatogram revealed a molecular ion peak at m / z = 467.3 (corresponding to the large peak at retention time 12.8 min) which is consistent with a dibromoalkane structure with molecular formulae 22H44Br2. On the other hand, the solid residue recovered after depolymerization was subjected to high temperature1H NMR analysis which showed only characteristic peaks of backbone -CH2 protons of long chain alkanes (refer, Figure 7).

[0107] The DSC curves (in Figure 8) revealed that the solid residue obtained after depolymerization of BPE showed a little higher melting point temperature compared to both the pristine polyethylene and brominated polyethylene which somewhat indicated the formation of longer chain alkanes. This may be attributed to the fact of random termination of polymer chain radicals during the course of depolymerization due to increased rate of radical formation in this solvent thereby causing radical-radical coupling reaction.

[0108] Inventors have calculated the % depolymerization (% depolymerization) of BPE after 24h of ATRdP by using the equation: % depolymerization = [weight of BPE converted into monomers / initial weight of BPE] x 100, where the weight of BPE converted into monomers = [initial weight of BPE - (weight of solid residue + weight of polymer materials in toluene)]. The % depolymerization of BPE after 24h of ATRdP was calculated to be 48.9 %.

[0109] 2) GC analysis of ethylene obtained as depolymerized product:

[0110] A conical flask, oven-dried and equipped with a stirring bar and glass stopper, was transferred into a glove-box. The flask was loaded with CuBr (15 mg, 0.1 mmol), TPMA (46 mg, 0.15 mmol), and 290.6 mg BPE (10.4 mmol with respect to the ethylene repeating unit). The apparatus was then taken out of the glove-box. Employing the Schlenk technique, 10 ml of anhydrous toluene was introduced into the flask. Subsequently, the flask was placed inside a 450 mL autoclave reactor, which had been pre-purged with argon gas. The autoclave reactor was tightly sealed and positioned in a pre-heated oil bath at 120 °C. The depolymerization reaction proceeded for 72 hours, generating a substantial amount of ethylene.

[0111] After 72 hours, the ethylene gas produced was carefully transferred from the autoclave reactor to a Tedlar gas sampling bag. This bag was then connected to a GC instrument for gas analysis. The GC analysis (in Figure 9) confirmed a signal at a retention time of 2.02 min. To verify whether this peak corresponded to ethylene gas, another Tedlar bag filled with standard ethylene gas (diluted with nitrogen gas) was injected into the GC instrument. The signal for the standard ethylene gas was observed at a retention time of 2.00 min (Figure 9), confirming that the observed signal in the reaction mixture indeed corresponded to ethylene gas.

[0112] 3) NMR analysis of ethylene obtained as depolymerized product: Following a methodology similar to the one previously outlined for GC analysis, the detection of ethylene gas produced during ATRdP of PE was carried out through1H NMR analysis. An oven-dried conical flask, equipped with a stirring bar and glass stopper, was meticulously transferred into a glove-box to maintain a controlled environment. Within this setting, the flask was loaded with CuBr (15 mg, 0.1 mmol), TPMA (46 mg, 0.15 mmol), and BPE (290.6 mg, 10.4 mmol concerning the ethylene repeating unit). Subsequently, the apparatus was removed from the glove-box. Utilizing the Schlenk technique, 10 ml of anhydrous toluene was introduced into the flask. The loaded flask was then carefully placed inside a 450 mL autoclave reactor, which had undergone purging with argon gas. The autoclave reactor was securely sealed and immersed in a pre-heated oil bath set at 120 °C, allowing the depolymerization reaction to progress for 72 hours and generating a substantial amount of ethylene.

[0113] After 72 hours, a high-pressure NMR tube charged with CDCI3 was connected to the autoclave reactor. The NMR tube was cooled to -78 °C. Subsequently, the outlet of the autoclave reactor was opened, and the produced ethylene was methodically transferred to the NMR tube. Following this, the NMR tube was sealed and disconnected from the reactor. To ensure proper dissolution of ethylene into CDCI3, the NMR tube was shaken at intervals of 15 minutes over 2-3 hours. Following extended shaking, theXH NMR spectrum was recorded, revealing a peak at 5.33 ppm (Top side graph of Figure 10). To validate this observation, theXH NMR of standard ethylene gas in CDCI3 was recorded, displaying a signal at 5.41 ppm (Bottom side graph of Figure 10). This comparison unequivocally confirms the presence of ethylene gas in the depolymerization reaction mixture.

[0114] Example 2: Re-polymerization of depolymerized polyethylene i.e., ethylene monomers:

[0115] The depolymerization products are ethylene gas which generally escape the reaction vessel with argon during the course of depolymerization, short chain alkyl halide which goes into toluene solution, and a simple alkane which remains as solid residue. Now, inventors tried to investigate the scope of further repolymerization of ethylene monomers generated during the depolymerization reaction (of example 1 provided above) by directly connecting the outlet of the depolymerization reactor with a schlenk flask already charged with a polymerization catalyst palladium-phosphinesulfonate complex [{PAO}PdMe(E)] (PAO=K2- P,O-Ar2PC6H4SO2O with Ar = 2-MeOCeH4; and E = CH3CN) in toluene at room temperature. Initially, a 100 mL schlenk flask was charged with 5 mg of the above mentioned polymerization catalyst and dissolved in 1.5 mL dry DCM followed by addition of 10 mL dry toluene. The reaction mixture was stirred for 10 minutes and connected to the outlet of the depolymerization reactor via a cannula and kept at 25° C for 24 hours with stirring. Excess of ethylene and argon gas coming from the depolymerization reactor was properly vented throughout the polymerization. After the completion of polymerization, the solvent was evaporated under high vacuum yielding 11 mg of solid residue. The solid residue was subjected to ’ H NMR analysis (Figure 11) which showed signals for -CH2 protons at around 1.34ppm which further confirmed the formation of very low molecular weight hydrocarbons.

[0116] ADVANTAGES OF THE PRESENT INVENTION

[0117] 1) Successfully developed a protocol to brominate polyolefin chains using blue LED under mild condition followed by the depolymerization of the brominated polyolefin. 2) The depolymerization process only requires 1-2 steps and works under mild conditions. Moreover, the monomer, produced does not require any additional separation.

Claims

We Claim:

1. A process of depolymerization and repolymerization of a polymer, comprising the steps of: a) halogenating the polymer by reacting and treating a polymer solution containing said polymer with a halogenating agent, a catalyst, an aqueous acid solution and an oxygen flow under stirring and LED light irradiation at temperature in the range of 35-45 °C for time period in the range of 16-20 h to obtain halogenated polymer; b) depolymerizing the halogenated polymer by reacting and treating a halogenated polymer solution containing said halogenated polymer of step a) with a catalyst solution mixture comprising a copper based catalyst, a ligand and a solvent, under reflux at temperature in the range of 90-120 °C for time period in the range of 22- 25 h to obtain crude mixture of monomer and / or short-chain alkyl halide(s); c) separating and purifying the crude mixture of step b) to obtain a depolymerized monomer and a short-chain alkyl halide(s) separately; and d) re-polymerizing the depolymerized monomer of step c) by treating and reacting with a polymerization catalyst solution at temperature in the range of 20-35 °C for time period in the range of 22-25 h to obtain re-polymerized polymer; wherein the polymer is selected from C2-C4 polyolefin.

2. The process as claimed in claim 1, wherein the polymer solution of step a) is prepared by mixing and dissolving polymer in a solvent selected from o-xylene, p-xylene, and toluene.

3. The process as claimed in claim 1, wherein the halogenating agent is selected from potassium bromide and sodium bromide; the catalyst is selected from sodium nitrite and potassium nitrite; and the aqueous acid solution is prepared by mixing an acid in water to obtain at least 30% v / v concentration of aqueous acid solution, wherein the acid is selected from but not limited to hydrochloric acid, sulfuric acid, nitric acid, and so on.

4. The process as claimed in claim 1, wherein the halogenation of polymer of step a) comprising the steps of: i. providing a polymer solution in a beaker or container; ii. adding the halogenating agent, the catalyst, and the aqueous acid solution to said beaker / container under stirring in a sealed environment;iii. purging the reaction mixture of ii) with oxygen at temperature in the range of 25-35 °C for time period in the range of 10-20 minutes followed by irradiating the reaction mixture with the LED module light having wavelength in the range of 410-450 nm) under continuous stirring in presence of oxygen at temperature in the range of 35-45 °C for time period in the range of 16-20 h to obtain crude halogenated polymer; iv. the crude halogenated polymer of step iii) is precipitated in excess C1-C4 alcohol to obtain precipitated halogenated polymer; v. centrifuging the precipitated halogenated polymer obtained in step iv) followed by decanting of solvent to obtain a residue; vi. washing the residue obtained in step v) at least 3 times with C1-C4 alcohol followed by centrifugation, and drying under vacuum at temperature in the range of 70-90 °C to obtain the pure halogenated polymer5. The process as claimed in claim 1, wherein the LED light is selected from blue LED, green LED and red LED.

6. The process as claimed in claim 1, wherein the copper based catalyst is selected fromCu(I)bromide, Fe(II)bromide, Pd(II)bromide, Ni(II) bromide, Os(II) bromide, and Ru(II) bromide; the ligand is selected from tris(2-pyridylmethyl)-amine (TPMA), 2,2'-Bipyridyl (Bpy); 1,1,4,7,10,10-Hexamethyltriethylenetetramine (HMTETA); N,N,N',N'- Pentamethyldiethylenetriamine (PMDETA); 1,4,8, 11 -Tetramethyl- 1, 4,8,11- tetraazacyclotetradecane (Me4Cyclam); and Tris [2-(dimethylamino)ethyl] amine (MeeTREN); the solvent is selected from toluene, anhydrous toluene, p-xylene, and dodecane.

7. The process as claimed in claim 1, wherein the halogenated polymer solution as solution A is prepared by mixing and dissolving the halogenated polymer in toluene solvent 5.2 under inert conditions followed by degassing the solution mixture by at least three freeze-pump- thaw cycles and heated at temperature in the range of 70-90 °C for time period in the range of 20-40 minutes to obtain the solution A of the halogenated polymer solution.

8. The process as claimed in claim 1, wherein the depolymerization of the halogenated polymer comprising the steps of: i. preparing the catalyst solution mixture by mixing copper based catalyst, ligand and solvent, under stirring for 10-20 minutes till the solution forms a light orange colour due to complex formation, and then degassed by at least three times;ii. providing said solution A containing halogenated polymer solution; iii. the solution A of step ii) is transferred via a cannula into the container containing the catalyst solution mixture of step i) to obtain a reaction mixture; and iv. refluxing the reaction mixture of step iii) under an inert atmosphere at temperature in the range of 90-120 °C for time period in the range of 22-25 h to obtain crude depolymerized mixture of monomer and / or short-chain alkyl halide(s).

9. The process as claimed in claim 1, wherein the separating and purifying the crude mixture of step c) comprising the steps of: i. centrifuging the crude depolymerized mixture followed by decanting of brownish clear solvent solution to obtain a residue; and ii. washing the residue at least 3 times with C1-C4 alcohol followed by centrifugation, and drying under vacuum or reduced pressure at temperature in the range of 50-70 °C to obtain the pure depolymerized material i.e., monomer and / or short-chain alkyl halide(s).

10. The process as claimed in claim 1, wherein the polymerization catalyst solution is prepared by mixing a polymerization catalyst complex in excess dry toluene or dry hexane under stirring for at least 10 minutes till it forms a dissolved solution, wherein the polymerization catalyst is selected from palladium-phosphinesulfonate complex, titaniumiminocarboxylate complex, nickel-napthoxyimine complex, palladium-imine sulfonate complex, phenoxy-imine titanium complex as FI (Fujita Invent)-Ti catalyst, and metallocene and post-metallocene complexes.

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