Monomer for polymerization of photodegradable polymer, photodegradable polymer, production method and decomposition method thereof, and method for recovering lactone

Abstract

To provide a monomer for the polymerization of a photodegradable polymer having only one radical-decomposable site which is easily decomposed by light, a photodegradable polymer containing the monomer in the main chain of the polymer, a method for producing the polymer and a method for decomposing the polymer, and a method for recovering an organic compound (lactone) by the decomposition of the polymer.SOLUTION: The monomer for polymerization of a photodegradable polymer has only one radical-decomposable site and is represented by formula (1). Wherein R1 and R2 each independently represent an alkyl group or an aryl group. ) SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a monomer for polymerization of a photodegradable polymer having a radically decomposable site, a photodegradable polymer obtained by polymerizing a monomer containing such a monomer, a method for producing the photodegradable polymer, a method for decomposing the photodegradable polymer by irradiating it with light, and a method for decomposing the photodegradable polymer by irradiating it with light and recovering the corresponding lactone. [Background technology]

[0002] The global plastics (polymer) market is expected to reach US$756.9 billion by the end of 2030, up from US$599.4 billion in 2022. Synthetic polymers have traditionally been designed with an emphasis on performance and durability, without sufficient consideration of the degradability and recyclability of the polymers after use. Currently, post-consumer polymer waste is disposed of by landfilling, incineration, or mechanical recycling. Landfilling, however, poses the risk of serious environmental pollution due to the persistence of most polymer waste, resulting in environmental accumulation and degradation. While thermal recycling is used for incineration, it is difficult to call it recycling in its truest sense, due to the insufficient energy recovery efficiency for heat utilization, including power generation, and the resulting carbon dioxide emissions. On the other hand, mechanical recycling, for example, in the case of PET bottles, involves sorting, crushing, and cleaning, followed by high-temperature diffusion of contaminants from the resin, decontamination, and drying to produce flakes. This is the most efficient method. However, there are problems in that the polymers to which mechanical recycling can be applied are limited, and processing polymer waste such as composite materials, laminated materials, coating materials, and contaminated materials is difficult and expensive.

[0003] Meanwhile, from the perspectives of environmental conservation, including the microplastics issue, and carbon circulation, the market for biodegradable polymers, or "green polymers" (GPs), is expected to reach US$5.8 billion in 2020 and US$16.8 billion in 2030. However, the rate of GP biodegradation is significantly affected by environmental conditions (e.g., soil, water, air), and not all GPs are always biodegradable under favorable environmental conditions. Furthermore, GPs derived from nonedible biomass such as bagasse and cellulose remain expensive, making their practical application slow. Therefore, most biodegradable polymers are derived from food crops such as corn and sugarcane. Mass production of biodegradable polymers is feared to cause serious food shortages. Therefore, relying on plant-based raw materials for biodegradable polymer production could lead to a trade-off between solving environmental conservation issues and worsening food shortages.

[0004] To address these issues, we must rethink our current approach to recycling and bring about a revolution in our approach to polymer materials, particularly in terms of the monomer design that makes up the polymer. In other words, a logical and ideal solution to minimize environmental impact and address the easy chemical recycling of synthetic polymers is to replace polymers that can be produced from readily available raw materials with "degradable polymers" by incorporating new molecular structures that facilitate recycling. This would enable a circular economy approach to sustainable polymers that can be easily chemically recycled. The required properties of the designed degradable moieties include simple structure, proper decomposition, and the incorporation of the degradable moieties without destabilizing the polymer. Designing degradable plastics that meet these requirements is necessary. By adding degradable functionality to inexpensive petroleum-based plastics, we can address the recycling and microplastic issues.

[0005] Photolysis technology as a polymer decomposition technique is relatively old, having been known since the 1960s, and some photolysis systems for polystyrene and ketone polymers have been put to practical use (Non-Patent Document 1). Furthermore, since the 1970s, photolysis of polymers using enzymes and photocatalysts such as titanium oxide has also been reported (Non-Patent Document 2). Recently, a technology for decomposing polymers with special cyclic structures by cleaving carbon-oxygen bonds using an organic photocatalyst system has been reported (Non-Patent Document 3), but the target is limited to polymers with specific structures. It has also been reported that introducing a small amount of vinyl ether into a (meth)acrylic polymer decomposes the polymer through a hydrogen atom transfer reaction upon irradiation with light (Non-Patent Document 4), but the vinyl ether is present in the side chain, not in the main chain of the polymer. Furthermore, Patent Document 1 describes a photodegradable polymer compound that uses a group containing a nitro-substituted benzyl as the photodegradable moiety, but because the polymer chain is linked via the photodegradable moiety and the photodegradable moiety is not used as a monomer, the degree of freedom in designing the photodegradable polymer compound is limited.

[0006] The present inventors have been engaged in research into alkyl radicals for many years, and in the course of their research, have discovered that alkyl radicals can be easily produced by cleaving the carbon-oxygen bond of an ester through photocatalytic action. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-233137 [Non-patent literature]

[0008] [Non-Patent Document 1] N. Grassie, et al., Journal of Applied Polymer Science, The Photooxidation of Polymers II. Photolysis of Polystyrene,Vol.9, pp.975-986 (1965) [Non-patent document 2] Masao Kato, Polymers, Easily Degradable Polymers, Vol.5, No.7·8, pp.409-411(1972) [Non-patent document 3] Adam M. Freiberg et al.,Triggered Transience of Plastic Materials by a Single Electron Transfer Mechanism, ACS Cent Sci. 2020, 6. 266-273 [Non-patent document 4] Taichi Kimura, Makoto Ouchi, Angewandte Chemie International Edition, Photocatalyzed HAT-Degradation of Vinyl Polymers: Cleavage of CC Bond in Backbone Triggered by Radical Activation of CH Bond in Pendant, 10.1002 / anie.202305252(DOI), 26 June 2023 Summary of the Invention [Problem to be solved by the invention]

[0009] The present invention aims to provide a monomer for polymerization of a photodegradable polymer that has only one radically decomposable site that is easily decomposed by light, a photodegradable polymer that contains such a monomer in the main chain of the polymer, a method for producing the same, a method for decomposing the same, and a method for recovering an organic compound (lactone) by decomposing the polymer. [Means for solving the problem]

[0010] The present inventors identified a radically decomposable site that is easily decomposed by light, and designed and synthesized a polymer having a radically decomposable site based on a monomer containing the radically decomposable site. They confirmed that such a polymer decomposes upon irradiation with light under photocatalytic conditions. Based on this finding, they discovered that by incorporating a monomer having only one radically decomposable site into the polymer chain, it is possible to design a polymer that can be decomposed upon irradiation with light in the presence of a photocatalyst and an acid. They also discovered an efficient method for decomposing such photodecomposable polymers and a method for recovering the organic compound (lactone) obtained by decomposition, thereby completing the present invention.

[0011] That is, the present invention is specified by the following items. [1] A photodegradable polymer polymerization monomer having only one radically decomposable site, represented by the following formula (1): [ka] (In the formula, R1 and R2 independently represent an alkyl group or an aryl group.) [2] The photodegradable polymer polymerization monomer according to [1], wherein R1 and R2 are both methyl groups. [3] A photodegradable polymer obtained by polymerizing a monomer containing the monomer for photodegradable polymer polymerization according to [1] or [2]. [4] The photodegradable polymer according to [3], wherein the photodegradable polymer is a polyester-based polymer. [5] A method for producing a photodegradable polymer by polymerizing a monomer containing a photodegradable polymer polymerization monomer represented by the following formula (1), which has only one radically decomposable site: [ka] (In the formula, R1 and R2 independently represent an alkyl group or an aryl group.) [6] A method for decomposing a photodegradable polymer by irradiating a photodegradable polymer containing a monomer for photodegradable polymer polymerization represented by the following formula (1), which has only one radically decomposable site, with light in the presence of a photocatalyst and an acid. [ka] (In the formula, R1 and R2 independently represent an alkyl group or an aryl group.) [7] A method for recovering the corresponding lactone by decomposing a photodegradable polymer containing a monomer for photodegradable polymer polymerization represented by the following formula (1), which has only one radically decomposable site, by irradiating the polymer with light in the presence of a photocatalyst and an acid: [ka] (In the formula, R1 and R2 independently represent an alkyl group or an aryl group.) [Effects of the Invention]

[0012] Since the monomer for photodegradable polymer polymerization of the present invention has only one radically decomposable site, synthesis by incorporating the radically decomposable site is easy. Furthermore, the photodegradable polymer of the present invention obtained by polymerizing a monomer containing the monomer for photodegradable polymer polymerization of the present invention is a polymer having a radically decomposable site incorporated into the main chain of the polymer, and is decomposed by cleavage of the radically decomposable site upon irradiation with light, etc. Furthermore, the polymer of the present invention is photodegradable, and since the monomer for photodegradable polymer polymerization of the present invention has only one radically decomposable site, it is possible to reduce the effect of the radically decomposable site on the polymer properties. Furthermore, the photodegradable polymer of the present invention further allows the substance obtained upon decomposition to be recovered as a lactone. DETAILED DESCRIPTION OF THE INVENTION

[0013] The present invention relates to a monomer for polymerization of a photodegradable polymer, which has only one radically decomposable site and is represented by the following formula (1): a photodegradable polymer obtained by polymerizing a monomer containing such a monomer; a method for producing and decomposing the photodegradable polymer; and a method for recovering an organic compound (lactone) obtained by decomposition. [ka] (In the formula, R1 and R2 independently represent an alkyl group or an aryl group.)

[0014] The photodegradable polymer polymerization monomer of the present invention is a monomer represented by the following formula (1) which has only one photocleavable group, ie, one radically decomposable site. [ka] (In the formula, R1 and R2 independently represent an alkyl group or an aryl group.) In the monomer represented by formula (1), R1 and R2 are alkyl groups such as linear alkyl groups like methyl, ethyl, n-propyl, and n-butyl, branched alkyl groups like isopropyl and isobutyl, and cycloalkyl groups like cyclopentyl and cyclohexyl. Aryl groups include phenyl. R1 and R2 may be the same or different.

[0015] The method for synthesizing the monomer for polymerization of the photodegradable polymer of the present invention is not particularly limited. As will be described in detail in the Examples, the monomer can be obtained by reacting 4-vinylbenzoic acid with 2-hydroxyisobutyric acid to produce an intermediate, and then reacting the intermediate with 4-vinylbenzyl chloride.

[0016] The photodegradable polymer of the present invention can be produced by polymerizing monomers including the monomer for polymerization of the photodegradable polymer of the present invention. The photodegradable polymer of the present invention can be produced as a homopolymer by polymerizing only the photodegradable polymer polymerization monomer represented by formula (1), and can also be produced as a copolymer in which the photodegradable polymer polymerization monomer of the present invention is arranged in any ratio by polymerizing the photodegradable polymer polymerization monomer represented by formula (1) with other monomers in any ratio.

[0017] The copolymer may be a polymer in which a monomer other than the monomer having a radically decomposable moiety serves as a base monomer to form the polymer main chain, and a monomer represented by formula (1) serves as a radically decomposable moiety and is incorporated into the main chain formed by the base monomer. The main chain of the polymer formed by the base monomer is not particularly limited, and examples thereof include polycondensation / polyaddition polymers such as polyester polymers, polyamide polymers, polyetheramide polymers, polyether polymers, and polyurethane polymers, addition polymerization polymers such as polyethylene polymers, polypropylene polymers, and polyvinyl chloride polymers, phenolic resins, and epoxy resins. However, polyester polymers are preferred, and polyester polymers whose base monomer is a monomer represented by formula (1) from which the radically decomposable moiety has been removed are particularly preferred from the viewpoint of polymerization.

[0018] The photodegradable polymer of the present invention can be decomposed by light irradiation; specifically, it can be decomposed by irradiation with artificial light, which is light of a specific wavelength, and can also be decomposed by irradiation with sunlight. Furthermore, when exposed to sunlight, the photodegradable polymer of the present invention can be decomposed in a natural environment, such as in the presence of air. When artificial light is used, the decomposition rate of the photodegradable polymer is easier to control and the decomposition rate can be increased compared to when sunlight is used for decomposition. As the artificial light, which is light of a specific wavelength, light of 365 nm or 405 nm is used. That is, the photodegradable polymer of the present invention is photodecomposed by radical decomposition of the radically decomposable moiety of the monomer represented by formula (1), which cleaves the carbon-oxygen bond.

[0019] In addition, in the method for decomposing a photodegradable polymer of the present invention, a photocatalyst is used during light irradiation. The photocatalyst is a catalyst having a reduction potential (E 0 Red ) is sufficient as long as it has a reduction potential lower than -1.29 V. There are no particular limitations on the substance as long as it has such a reduction potential, but taking into consideration the interaction with the substrate, BDB (bisdiphenylaminobenzene), NPB, BNPB, HPTPN, etc. are suitable examples. The chemical structural formulas of BDB, NPB, BNPB, and HPTPN are shown below. [ka]

[0020] The photodegradable polymer of the present invention allows for time-dependent control of the radical decomposition of the polymer by selecting the content ratio of the photodegradable polymer polymerization monomer having only one radically decomposable moiety in the polymer and the decomposition conditions (e.g., irradiation with light of a specific wavelength or exposure to sunlight). The amount of photocatalyst used is preferably about 1 mol% relative to the photodegradable polymer if decomposition to a certain extent is desired within a few days under irradiation with light of a specific wavelength. The photocatalyst can be added to the photodegradable polymer during decomposition, or it can be kneaded into or coated on the polymer before the photodegradable polymer is used for a specific purpose. Additionally, an organic or inorganic third component may be added to the photodegradable polymer together with the photocatalyst to facilitate fixation of the photocatalyst to the polymer.

[0021] Furthermore, in the method for decomposing a photodegradable polymer of the present invention, a catalytic amount of an acid is used as a co-catalyst to promote decomposition. The acid is not particularly limited, but examples thereof include Brønsted acids, Lewis acids, sulfuric acid, hydrochloric acid, PTSA (p-toluenesulfonic acid) / HO, and CSA (10-camphorsulfonic acid).

[0022] The environment in which the method for decomposing a photodegradable polymer of the present invention is carried out is not particularly limited, but the method can be carried out in a solvent. The solvent for carrying out the method for decomposing a photodegradable polymer of the present invention is not particularly limited, but is preferably one organic solvent or a mixed organic solvent of two or more selected from N,N'-dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP), N,N-diacetylacetamide (DMA), dioxane, toluene, ether, acetone, THF, dichloromethane, etc., and particularly preferably a mixed solvent of THF (tetrahydrofuran) and water.

[0023] The photodegradable polymer of the present invention is decomposed by irradiation with artificial light or sunlight in the presence of a photocatalyst and an acid. The other decomposition conditions in the method for decomposing such a photodegradable polymer are not particularly limited, but it is preferable that the decomposition temperature is higher than room temperature.

[0024] The photodegradable polymer of the present invention contains a monomer for photodegradable polymer polymerization represented by formula (1) having a radically decomposable moiety, and therefore, when decomposed by light irradiation, a compound derived from -OC(R1)(R2)-C(O)- is generated. The product derived from such a structure can be recovered as a lactone by allowing diphenylethane (DPE) or the like to coexist during the decomposition of the photodegradable polymer. The method for recovering lactone of the present invention can be carried out simultaneously with the method for decomposing a photodegradable polymer of the present invention.

[0025] The present invention will be specifically described below with reference to examples of the present invention, but the technical scope of the present invention is not limited to these examples. [Example]

[0026] [Synthesis of Monomers for Photodegradable Polymerization] (1a) Photodegradable Polymer An intermediate (1a) in the synthesis reaction of a monomer for polymerization was synthesized according to the following chemical reaction formula. [ka] A mixture of 4-vinylbenzoic acid (20 mmol, 1.0 equiv., 2963.2 mg) and dimethylformamide (DMF (0.5 mL)) in dichloromethane (20 mL, 1.0 M) was added with thionyl chloride (40 mmol, 2.0 equiv., 2.9 mL) and stirred overnight at 70 °C under reflux. After the reaction, the mixture was concentrated using a rotary evaporator, and acetonitrile (MeCN (20 mL)) was added and stirred at 0 °C. A mixture of 2-hydroxyisobutyric acid (24 mmol, 1.2 equiv., 2498.4 mg)) and pyridine (40 mmol, 2.0 equiv., 3.2 mL) in acetonitrile (MeCN (20 mL)) was added dropwise and stirred overnight. After the reaction, the mixture was separated into 1M HCl aqueous solution and a 4:1 volumetric solution of ethyl acetate and acetonitrile (MeCN) and then exposed to magnesium sulfate. After concentrating using a rotary evaporator, the mixture was transferred to a storage container and dried under reduced pressure to obtain the product (referred to as 1a in the above chemical reaction formula). The 1H NMR spectrum of product (1a) is shown below. 1HNMR (CDCl3); 7.99 (d, J = 8.4 Hz, 2H), 7.46 (d, J = 8.4 Hz, 2H), 6.77-6.72 (m, 1H), 5.87 (d, J = 17.6 Hz, 1H), 5.39 (d, J = 11.0 Hz, 1H), 1.73 (s, 6H).

[0027] (2a) Photodegradable Polymer Monomer (2a) for polymerization was synthesized according to the following chemical reaction formula. [ka] A mixture of 1a (10 mmol, 1.0 equiv., 2342.5 mg), K2CO3 (20 mmol, 2.0 equiv., 2764.2 mg), and KI (30 mmol, 3.0 equiv., 4980.0 mg) in dimethylformamide (DMF, 20 mL, 0.5 M) was added with 4-vinylbenzyl chloride (20 mmol, 2.0 equiv., 2.8 mL) and stirred overnight at room temperature. After the reaction, the mixture was partitioned between HO and a 5:1 ethyl acetate:hexane solution and then exposed to magnesium sulfate. After concentration on a rotary evaporator, the mixture was purified by flash column chromatography (hexane:ethyl acetate (Hex:AcOEt) = 20:1) to give product 2a in 39% yield. The 1H NMR spectrum of the product (2a) is shown below. 1HNMR (CDCl3); 7.98 (d, J = 8.3 Hz, 2H), 7.46 (d, J = 8.4 Hz, 2H), 7.35 (d, J = 8.2 Hz, 2H), 7.27 (d, J = 8.2 Hz, 2H), 6.79-6.66 (m, 2H), 5.87 (d, J = 17.6 Hz, 1H), 5.73 (d, J = 17.7 Hz, 1H), 5.39 (d, J = 10.7 Hz, 1H), 5.25 (d, J = 10.9 Hz, 1H), 5.17 (s, 2H), 1.71 (s, 6H). [Example]

[0028] [Production of photodegradable polyester polymer] The photodegradable polyester polymer (3a) of the present invention was produced according to the following chemical reaction formula. [ka] In a glove box, a flask was charged with 2a (2.0 mmol, 1.0 equiv., 700.8 mg) and the second-generation Grubbs catalyst (2 ndHoveyda-Grabbs catalyst (0.02 mmol, 1 mol%, 12.5 mg) and 1,2-dichlorobenzene (2 M, 1 mL) were added, a septum was placed, and the mixture was allowed to stand. The mixture was stirred under reduced pressure at 80 °C for 1 hour. After the reaction, the mixture was cooled to room temperature, dissolved in a small amount of tetrahydrofuran (THF), and then added dropwise to a beaker containing 200 mL of methanol, yielding a precipitate (represented as 3a in the above chemical reaction equation). The decomposition temperature of the resulting polymer was 325 °C, with a number-average molecular weight (Mn) of 21,629, a weight-average molecular weight (Mw) of 35,977, and a Z-average molecular weight (Mz) of 53,416. [Example]

[0029] [Decomposition of photodegradable polyester polymers] The photodegradable polyester polymer of the present invention (represented as 1 in the following chemical reaction formula) was decomposed according to the following chemical reaction formula. [ka] The photodegradable polyester polymer was dissolved in a tetrahydrofuran / water mixture (95:5, 0.25M). The photocatalysts used were BNPB (catalyst 1) and NPB (catalyst 2) at 5.0 mol%, and p-toluenesulfonic acid (PTSA)·HO (catalyst 10 mol%). The polymer was decomposed by irradiating the solution with 365 nm light at room temperature for 24 hours. The product of the polymer decomposition was recovered as lactone (represented as 2 in the above chemical reaction formula and table) by adding diphenylethane (DPE, 2 equivalents) to the decomposition reaction. The results are shown in Table 1. [Table 1] From the above results, it was confirmed that the molecular weight of the photodegradable polyester polymer of the present invention was reduced by light irradiation in the presence of a photocatalyst and an acid, and therefore decomposition of the polymer occurred. In addition, the presence of diphenylethane (DPE) in the decomposition reaction system confirmed the recovery of lactone as a product. [Industrial Applicability]

[0030] Polymers obtained by incorporating the photodegradable polymer polymerization monomer of the present invention, which has only one radically decomposable site, into the polymer chain were successfully decomposed by light irradiation in the presence of a photocatalyst and an acid, demonstrating their chemical recyclability. Because this monomer with only one radically decomposable site can be incorporated into condensation polymers, its range of applications is broad. Furthermore, lactones can be recovered as a result of decomposition, enabling the recovered lactones to be widely recycled.

Claims

1. A photodegradable polymer polymerization monomer having only one radically decomposable site and represented by the following formula (1): 【Chemistry 1】 (In the formula, R １ and R ２ are independently an alkyl group or an aryl group.

2. R １ and R ２ and are both methyl groups.

3. A photodegradable polymer obtained by polymerizing a monomer containing the monomer for polymerization of a photodegradable polymer according to claim 1 or 2.

4. The photodegradable polymer according to claim 3 , wherein the photodegradable polymer is a polyester-based polymer.

5. A method for producing a photodegradable polymer by polymerizing a monomer containing a monomer for photodegradable polymer polymerization represented by the following formula (1), which has only one radically decomposable site: 【Chemistry 2】 (In the formula, R １ and R ２ are independently an alkyl group or an aryl group.

6. A method for decomposing a photodegradable polymer, comprising irradiating a photodegradable polymer containing a monomer for photodegradable polymer polymerization represented by the following formula (1), which has only one radically decomposable site, with light in the presence of a photocatalyst and an acid: 【Transformation 3】 (In the formula, R １ and R ２ are independently an alkyl group or an aryl group.

7. A method for recovering the corresponding lactone by decomposing a photodegradable polymer containing a monomer for photodegradable polymer polymerization represented by the following formula (1), which has only one radically decomposable site, by irradiating the polymer with light in the presence of a photocatalyst and an acid: 【Chemistry 4】 (In the formula, R １ and R ２ are independently an alkyl group or an aryl group.

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