Method for producing aliphatic polycarbonate having crosslinked structure

Abstract

To provide a method for producing an aliphatic polycarbonate with a crosslinked structure, the method enabling improvement of functions including heat resistance and being effective in reducing environmental impact by chemical recycling.SOLUTION: A cyclic alkylene carbonate monomer is subjected to ring-opening polymerization in the presence of a compound having at least two cyclic alkylene carbonate structures, thereby producing an aliphatic polycarbonate having a crosslinked structure.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing an aliphatic polycarbonate having a crosslinked structure, and a method for decomposing the resulting crosslinked aliphatic polycarbonate with ammonia. [Background technology]

[0002] Polymer materials (plastics) have become indispensable in everyday life, but most of them are discarded, with the recycling rate of materials remaining at less than 15%. While measures to address environmental issues are urgently needed in relation to the Sustainable Development Goals (SDGs), the demand for plastics remains high, and there is a need to develop a new recycling system that balances the use of plastics with the conservation of the global environment.

[0003] Plastic recycling processes are mainly divided into material recycling and chemical recycling, with the former accounting for the majority of modern recycling. Material recycling is a method of collecting, processing, and reusing used plastics, but the issue is that quality can deteriorate due to the introduction of foreign matter during the processing process and a decrease in molecular weight. In contrast, chemical recycling is a method of returning plastics to their original raw materials through depolymerization and reusing them, which has the advantage of avoiding quality deterioration.

[0004] Polycarbonate is one of the general-purpose polymer materials that has excellent physical properties such as heat resistance, mechanical properties, and melting characteristics. However, in the trend of responding to the environmental issues mentioned above, aliphatic polycarbonates, which have a low environmental impact, have been attracting attention (Patent Documents 1 to 5), and polycarbonates have also been synthesized by ring-opening polymerization of cyclic alkylene carbonates (Patent Documents 6 to 7). The present inventors have also proposed a recycling system in which polycarbonate synthesized from sugar-derived raw materials is decomposed with ammonia to convert it into fertilizer (Non-Patent Documents 1 and 2).

[0005] In recent years, 3D printer-related technologies have been attracting attention from many different angles. Stereolithography, which crosslinks areas irradiated with laser light, has become mainstream, while uncrosslinked areas are removed in a cleaning process. For this reason, there has been active development of photoresponsive polymer materials that allow for control of a wide range of physical properties. However, once photoresponsive polymers are photocrosslinked, an insoluble and infusible three-dimensional network structure is formed, making it difficult to recycle the crosslinked resin. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-146019 [Patent Document 2] Japanese Patent Application Laid-Open No. 2009-046519 [Patent Document 3] Patent No. 4351675 [Patent Document 4] Patent No. 5708087 [Patent Document 5] Japanese Patent Application Laid-Open No. 2003-292603 [Patent Document 6] Special Publication No. 2012-522098 [Patent Document 7] Japanese Patent Application Laid-Open No. 2012-232909 [Non-patent literature]

[0007] [Non-Patent Document 1] "Development of a recycling system that converts plastic into fertilizer", [online], published October 28, 2021, Tokyo Institute of Technology and Japan Science and Technology Agency, Internet<URL:https: / / www.jst.go.jp / pr / announce / 20211028 / pdf / 20211028.pdf> [Non-patent document 2] Aoki et al., Green Chem., 2021, 23, 9030-9037 Summary of the Invention [Problem to be solved by the invention]

[0008] Under these circumstances, the present inventors have focused in particular on aliphatic polycarbonates, among polycarbonates which are considered to be representative recyclable polymeric materials, and have conducted extensive research into the development of materials that can be recycled by decomposition even when crosslinked. As a result, they have found that aliphatic polycarbonates obtained by ring-opening polymerization of a cyclic alkylene carbonate monomer in the presence of a compound having at least two cyclic alkylene carbonate structures can be decomposed into precursors of the raw material compounds and urea when treated with ammonia, thereby arriving at the present invention. That is, the present invention provides a novel method for producing an aliphatic polycarbonate having a crosslinked structure, and a method for recovering a precursor of a cyclic alkylene carbonate monomer, a precursor of a compound having at least two cyclic alkylene carbonate structures, and urea by decomposing the aliphatic polycarbonate with ammonia. [Means for solving the problem]

[0009] The present invention has the following aspects. [1] A method for producing an aliphatic polycarbonate having a crosslinked structure by ring-opening polymerization of a cyclic alkylene carbonate monomer in the presence of a compound having at least two cyclic alkylene carbonate structures. [2] The method according to [1] above, wherein the cyclic alkylene carbonate monomer is trimethylene carbonate. [3] The method according to [2] above, wherein the compound having at least two cyclic alkylene carbonate structures is di(trimethylolpropane) carbonate. [4] The method according to any one of the above [1] to [3], wherein the ring-opening polymerization is carried out by a photocrosslinking reaction. [5] A method for recovering a precursor of the cyclic alkylene carbonate monomer, a precursor of the compound having at least two cyclic alkylene carbonate structures, and urea from the resulting decomposition products by treating an aliphatic polycarbonate having a crosslinked structure, which is produced by ring-opening polymerization of a cyclic alkylene carbonate monomer in the presence of a compound having at least two cyclic alkylene carbonate structures, with ammonia. [6] The method according to [5] above, wherein the cyclic alkylene carbonate monomer is trimethylene carbonate. [7] The method according to [6] above, wherein the compound having at least two cyclic alkylene carbonate structures is di(trimethylolpropane) carbonate. [8] The method according to any one of the above [5] to [7], wherein the ring-opening polymerization is carried out by a photocrosslinking reaction. [Effects of the Invention]

[0010] According to the present invention, by crosslinking an aliphatic polycarbonate, it is possible to tune the physical properties according to the application, and it is also easy to improve the functionality such as heat resistance. In addition, while it is generally difficult to decompose crosslinked polymers, the aliphatic polycarbonate of the present invention, which has a carbonate bond as its basic skeleton, can be decomposed with ammonia, and this can lead to the chemical recycling of raw material monomers and crosslinking agents and the recovery of urea, including those obtained as photoresponsive polymers by stereolithography using a 3D printer. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a diagram showing the 1H NMR spectrum of di(trimethylolpropane) carbonate obtained in Synthesis Example 1. [Figure 2] FIG. 1 shows the gelation state of P(TMC-co-DTMPC) having a crosslinked structure obtained in Example 1. [Figure 3] FIG. 1 shows the appearance of P(TMC-co-DTMPC) having a crosslinked structure obtained in Example 2. [Figure 4]FIG. 1 is a diagram showing a 1H NMR spectrum of a product obtained by decomposing Sample 1 of Example 1 with ammonia. [Figure 5] FIG. 1 is a diagram showing a 1H NMR spectrum of a product obtained by decomposing Sample 2 of Example 1 with ammonia. [Figure 6] FIG. 1 is a diagram showing a 1H NMR spectrum of a product obtained by decomposing Sample 3 of Example 1 with ammonia. [Figure 7] FIG. 1 is a diagram showing a 1H NMR spectrum of a product obtained by decomposing Sample 4 of Example 1 with ammonia. [Figure 8] FIG. 1 shows a 1H NMR spectrum of the product obtained by decomposing Sample 5 of Example 1 with ammonia. [Figure 9] FIG. 1 is a diagram showing the 1H NMR spectrum of the product obtained by decomposing the sample of Photo-1 in Example 2 with ammonia. [Figure 10] FIG. 1 is a diagram showing the 1H NMR spectrum of the product obtained by decomposing the sample of Photo-2 in Example 2 with ammonia. [Figure 11] FIG. 1 is a diagram showing the 1H NMR spectrum of the product obtained by decomposing the sample of Photo-3 in Example 2 with ammonia. DETAILED DESCRIPTION OF THE INVENTION

[0012] [Method for producing aliphatic polycarbonate having a crosslinked structure] The method for producing an aliphatic polycarbonate having a crosslinked structure of the present invention involves ring-opening polymerization of a cyclic alkylene carbonate monomer in the presence of a compound having at least two cyclic alkylene carbonate structures.

[0013] The term "cyclic alkylene carbonate" refers to a compound having a structure in which chain alkylene groups containing 2 to 20 carbon atoms are linked in a ring shape via a carbonate bond (-OC(=O)-O-). Such a cyclic alkylene carbonate can be prepared by a method such as reacting a chain aliphatic hydrocarbon having hydroxyl groups at both ends with phosgene. The alkylene portion of the cyclic alkylene carbonate is preferably a straight chain, and the number of carbon atoms therein is preferably 2 to 4. Trimethylene carbonate is particularly preferred.

[0014] The term "compound having at least two cyclic alkylene carbonate structures" refers to a compound having at least two structures in which alkylene group portions of two cyclic alkylene carbonate monomers are bonded to each other via a chain hydrocarbon group that may contain a heteroatom in the chain. An example of such a compound is di(trimethylolpropane) carbonate when trimethylene carbonate is used as the "cyclic alkylene carbonate" monomer. Di(trimethylolpropane) carbonate can be obtained by the method of Endo et al. (H. Matsukizono and T. Endo et al., Journal of Applied Polymer Science., 2015, 132, 41956), which uses di(trimethylolpropane) and a large excess of diphenyl carbonate through an intramolecular cyclization reaction accompanied by the formation of a carbonate bond.

[0015] The aliphatic polycarbonate having a crosslinked structure of the present invention is produced by polymerizing a cyclic alkylene carbonate monomer and a compound having at least two cyclic alkylene carbonate structures using a reaction initiator and an ester exchange reaction catalyst, or by using a photoanion generator. The polymerizable compound can be produced by polymerizing the polymerizable compound by a photocrosslinking reaction. The reaction molar ratio of the cyclic alkylene carbonate monomer to the compound having at least two cyclic alkylene carbonate structures, which acts as a crosslinking agent, can be in the range of 1:1 to 10:1, and preferably in the range of 4:1 to 10:1.

[0016] An example of the reaction initiator is 3-phenyl-1-propanol, but the reaction initiator is not limited to this. Examples of the transesterification catalyst include, but are not limited to, diphenyl phosphate and 1,8-diazabicyclo[5.4.0]-7-undecane. An example of the photoanion generator is 2-(9-oxoxanthen-2-yl)propionic acid 1,8-diazabicyclo[5.4.0]undec-7-ene salt, but is not limited thereto.

[0017] In the case of transesterification, examples of the reaction solvent include ethyl acetate, chloroform, dichloromethane, and toluene, with toluene being preferred. The aliphatic polycarbonate having a crosslinked structure of the present invention can be obtained by carrying out the reaction at a temperature of 0 to 60°C, preferably 30 to 60°C, for 1 to 24 hours, preferably 12 to 24 hours.

[0018] In the case of the photocrosslinking reaction, the crosslinking is promoted by irradiation with light using a UV irradiator. The general conditions are the same as those in the case of the above transesterification reaction, but in the case of irradiation with 365 nm light, the aliphatic polycarbonate having a crosslinked structure of the present invention can be obtained by irradiating for 1 minute or more.

[0019] [Method for recovering a precursor of a cyclic alkylene carbonate monomer, a precursor of a compound having at least two cyclic alkylene carbonate structures, and urea] The "cyclic alkylene polycarbonate monomer" and the "compound having at least two cyclic alkylene carbonate structures" used in the production of the aliphatic polycarbonate having a crosslinked structure of the present invention can be recovered as their precursors together with urea by treating the aliphatic polycarbonate having a crosslinked structure with ammonia.

[0020] The method for treating the aliphatic polycarbonate having a crosslinked structure with ammonia may be the same as a known method, and for example, a method in which the aliphatic polycarbonate is brought into contact with an aqueous ammonia solution can be mentioned. There are no particular restrictions on the concentration of ammonia, but it is preferably 5 to 15%, and more preferably 10 to 15%. The temperature when treating the aliphatic polycarbonate with ammonia can be, for example, 10 to 100°C, preferably 30 to 100°C, and particularly preferably 80 to 100°C. The time for treating the aliphatic polycarbonate with ammonia varies depending on the treatment temperature, but may be, for example, 1 to 72 hours, and is preferably 24 to 72 hours. In this way, a reaction product containing a decomposition product of the aliphatic polycarbonate with ammonia can be obtained. After the reaction, the reaction product may be dried, if necessary, to remove ammonia, water, etc. remaining in the reaction product.

[0021] The decomposition products include a precursor of a cyclic alkylene carbonate monomer, a precursor of a compound having at least two cyclic alkylene carbonate structures, and urea. The precursor of the cyclic alkylene carbonate monomer is a compound having a structure in which the ring of the cyclic alkylene carbonate monomer is opened, and is a compound having hydroxyl groups at both ends. The precursor of the compound having at least two cyclic alkylene carbonate structures is a compound having at least two cyclic alkylene carbonate structures. It is a compound having a structure in which a portion having the same structure as the alkylene carbonate monomer is ring-opened, and has hydroxyl groups at both ends of the ring-opened portion.

[0022] The urea obtained from the aliphatic polycarbonate having a crosslinked structure by the recovery method of the present invention can be reused as a fertilizer component. Furthermore, the precursor of the cyclic alkylene carbonate monomer obtained by this recovery method can be reused as a monomer for an aliphatic polycarbonate by reforming the cyclic structure, if necessary. Similarly, a precursor of a compound having at least two cyclic alkylene carbonate structures can be reused for crosslinking an aliphatic polycarbonate by reforming the cyclic structure as needed.

[0023] The present invention will be described in more detail below based on examples. However, the following examples are provided for illustrative purposes only and are not intended to limit the present invention. [Example]

[0024] Synthesis Example 1: Synthesis of crosslinkable compound Based on the report by Endo et al., whose flow is shown below, di(trimethylolpropane) carbonate (DTMPC) was synthesized by an intramolecular cyclization reaction involving the formation of a carbonate bond, using di(trimethylolpropane) as the starting material and a large amount of diphenyl carbonate as the carbonyl source. [ka]

[0025] The details of the synthesis method of di(trimethylolpropane) carbonate are shown below. 1.00 g (3.99 mmol) of di(trimethylolpropane) and 8.57 g (40.0 mmol) of diphenyl carbonate were added to a 200 mL recovery flask and stirred in a water bath at 140 °C for 48 hours. After the reaction was completed, a fraction with an Rf value of 0.28 was separated using silica gel column chromatography (developing solvent: n-hexane:ethyl acetate = 1:4), and the solvent was evaporated under reduced pressure and the product was dried in vacuo. The product was further recrystallized (mixed solvent: n-hexane:ethyl acetate = 1:3), and the solvent was evaporated under reduced pressure and dried in vacuo to obtain a white solid (1.28 mmol, 31.8%).

[0026] The obtained di(trimethylolpropane) carbonate 1 The results of 1 H NMR measurement are shown in Figure 1. A signal consistent with the target compound was confirmed, confirming the production of the target di(trimethylolpropane) carbonate (DTMPC).

[0027] Example 1: Synthesis of cross-linked P(TMC-co-DTMPC) Using the DTMPC obtained in the previous section, 3-phenyl-1-propanol (PPA) as an initiator, trimethylene carbonate (TMC) as a monomer, and diphenyl phosphate (DPP) or 1,8-diazabicyclo[5.4.0]-7-undecane (DBU) as a catalyst for the transesterification reaction, P(TMC-co-DTMPC) with a crosslinked structure was synthesized according to the flow chart shown below. [ka] [ka]

[0028] The details of the synthesis method of P(TMC-co-DTMPC) with a crosslinked structure by transesterification are shown below. TMC, PPA, DTMPC, DPP or DBU, and 2.0 ml of dehydrated dichloromethane (DCM) were added to a 10 ml recovery flask and stirred at room temperature. After gelation was confirmed, the crosslinked polymer P (TMC-co-DTMPC) was obtained by washing twice with methanol and then vacuum drying. The amounts of reagents used, reaction time, and yield are summarized in Table 1. Figure 2 shows the gelation of the reaction product. [Table 1]

[0029] Samples 1 to 3 shown in Table 1 were prepared using acid-catalyzed DPP, with a molar ratio of TMC to DTMPC of 30:7. Samples 4 and 5 were prepared using base-catalyzed DBU, with a molar ratio of TMC to DTMPC of 20:7 (4) and 30:5 (5). Table 1 and Figure 2 reveal that the gelation time is shorter when using base-catalyzed DBU than when using acid-catalyzed DPP. It also became clear that DTMPC acts as a crosslinking agent, producing a transparent gel.

[0030] Example 2: Synthesis of cross-linked P(TMC-co-DTMPC) P(TMC-co-DTMPC) was synthesized by photocrosslinking using DTMPC as a crosslinker, 2-(9-oxoxanthen-2-yl)propionic acid 1,8-diazabicyclo[5.4.0]undec-7-ene salt as a photoanion generator, and TMC as a monomer. [ka] [ka]

[0031] The details of the synthesis method of P(TMC-co-DTMPC) having a crosslinked structure by photocrosslinking reaction are shown below. TMC, DTMPC, 2-(9-oxoxanthen-2-yl)propionic acid 1,8-diazabicyclo[5.4.0]undec-7-ene salt, and 1.5 ml of dehydrated DCM were added to a vial. After confirming complete dissolution, the vial was irradiated with UV light for 10 minutes and allowed to stand, yielding the crosslinked polymer P(TMC-co-DTMPC). The amounts of reagents used, reaction time, and yield are shown in Table 2. The appearance of the resulting crosslinked polymer is shown in Figure 3. [Table 2]

[0032] Photo-1 shown in Table 2 was prepared without adding 1,3-propanediol, which is a polymerization initiator, so that the ratio of [TMC]:[DTMPC] was 50:5. Photos 2 to 4 were prepared by adding 1,3-propanediol to give a [TMC]:[DTMPC] ratio of 50:5 (Photo 2) / 50:6 (Photo 4) / 50:7 (Photo 3). For Photos 5 and 6, 1,3-propanediol was diluted to 1% by volume with dehydrated DCM, and 0.2 ml was added to the vial to prepare a [TMC]:[DTMPC] ratio of 50:5 (Photo 5) / 50:6 (Photo 6). For Photos 7 and 8, 1,3-propanediol was diluted to 1% by volume with dehydrated DCM, and 0.1 ml was added to a vial to prepare a [TMC]:[DTMPC]=50:5 mixture. For Photos 9 to 11, 1,3-propanediol was diluted to 1% by volume with dehydrated DCM, and 50 μl was added to the vial to prepare a [TMC]:[DTMPC] ratio of 50:6 (Photo 9) / 50:5 (Photo 10) / 50:7 (Photo 11).

[0033] Example 3: Decomposition of cross-linked P(TMC-co-DTMPC) The decomposition reaction of P(TMC-co-DTMPC) with ammonia was carried out for Samples 1 to 5 in Table 1, which were synthesized in Example 1. As the initial condition for the decomposition reaction, 30 equivalents of ammonia were added relative to the carbonyl groups of P(TMC-co-DTMPC), and the reaction was carried out in an aqueous ammonia solution at 90°C. [ka]

[0034] When observing the reaction solution of Sample 1, P(TMC-co-DTMPC) had accumulated at the bottom at the start of the reaction, but after 24 hours, nothing was visible at the bottom and oil droplets and solids were visible on the surface. After 48 hours, the solids had disappeared, but oil droplets were visible on the surface. After 112 hours, the oil droplets had also disappeared and the solution had turned into a completely homogeneous solution. These findings suggest that decomposition by ammonia was progressing.

[0035] When observing the reaction solution of Sample 2, P(TMC-co-DTMPC) settled at the bottom at the start of the reaction, but after 24, 48, and 72 hours, oil droplets were observed there. However, after 114 hours, the oil droplets were no longer visible and the solution had turned into a completely homogeneous solution. These findings suggest that decomposition by ammonia is progressing.

[0036] Observation of the reaction solution for samples 3 to 5 showed that P(TMC-co-DTMPC) had settled at the bottom at the start of the reaction, but after 96 hours it had turned into a completely homogeneous solution. This suggests that decomposition by ammonia was progressing.

[0037] Similarly, the samples synthesized in Example 2, Photo-1 to Photo-3 in Table 2, were subjected to a decomposition reaction of P(TMC-co-DTMPC) with ammonia.

[0038] Observation of the reaction solution in Photos 1 to 3 showed that P(TMC-co-DTMPC) had settled at the bottom at the start of the reaction, but after 96 hours it had turned into a completely homogeneous solution, suggesting that decomposition by ammonia was progressing.

[0039] In order to confirm the decomposition products of Samples 1 to 5 / Photo-1 to 3, the solvent was distilled off from the reaction solution after decomposition under reduced pressure, and the solution was dried in vacuum. 1 1 H NMR was used to analyze the results. 1 The H NMR spectrum is shown. 1In the H NMR spectrum analysis, signals attributed to urea, 1,3-propanediol, and the DTMP and 3-phenyl-1-propanol used to synthesize P(TMC-co-DTMPC) were observed for Samples 1 to 5. This indicates that P(TMC-co-DTMPC) was decomposed by ammonia, and that the decomposition products were urea, 1,3-propanediol, DTMP, and 3-phenyl-1-propanol. The small signal from 3-phenyl-1-propanol after decomposition is due to the fact that only a small amount was added during the synthesis of P(TMC-co-DTMPC).

[0040] Also, samples 1 to 5 1 Table 3 shows the urea yield, the molar ratio of decomposition products, and the charging ratio of raw materials calculated from the H NMR spectrum. [Table 3]

[0041] From the results in Table 3, it can be seen that although the urea yield was below 20% for all samples 1 to 5, the molar ratio of the monomer precursors produced, 1,3-propanediol (derived from TMC) and DTMP (derived from DTMPC), was almost the same as the feed ratio, and therefore it was found that the monomer precursors could be obtained in relatively high yields by decomposition with ammonia.

[0042] Figures 9 to 11 show the results of Photos 1 to 3. 1 The H NMR spectrum is shown. 1 In the H NMR spectrum analysis, signals attributed to urea were observed for Photo-1 to Photo-3, while signals attributed to 1,3-propanediol and DTMP were partially observed. This indicates that photocrosslinked P(TMC-co-DTMPC) is decomposed by ammonia, and that urea, 1,3-propanediol, and DTMP are present as decomposition products. [ka]

[0043] Photo-1~3 1 Table 4 shows the urea yield, the molar ratio of decomposition products, and the ratio of raw materials used, all calculated from the H NMR spectrum. [Table 4]

[0044] From the results in Table 4, it can be seen that, like Samples 1 to 5, the yield of urea is low in Photos 1 to 3, but the monomer precursors produced are 1,3-propanediol (derived from TMC) and D Since the molar ratio of TMP (derived from DTMPC) produced was almost the same as the feed ratio, it was found that the monomer precursor could be obtained in relatively high yield by decomposition with ammonia.

Claims

1. A method for producing an aliphatic polycarbonate having a crosslinked structure by ring-opening polymerization of a cyclic alkylene carbonate monomer in the presence of a compound having at least two cyclic alkylene carbonate structures.

2. 10. The method of claim 1, wherein the cyclic alkylene carbonate monomer is trimethylene carbonate.

3. The method according to claim 2, wherein the compound having at least two cyclic alkylene carbonate structures is di(trimethylolpropane) carbonate.

4. The method according to any one of claims 1 to 3, wherein the ring-opening polymerization is carried out by a photocrosslinking reaction.

5. The method comprises treating an aliphatic polycarbonate having a crosslinked structure, which is produced by ring-opening polymerization of a cyclic alkylene carbonate monomer in the presence of a compound having at least two cyclic alkylene carbonate structures, with ammonia, and recovering a precursor of the cyclic alkylene carbonate monomer, a precursor of the compound having at least two cyclic alkylene carbonate structures, and urea from the resulting decomposition product.

6. 6. The method of claim 5, wherein the cyclic alkylene carbonate monomer is trimethylene carbonate.

7. The method according to claim 6, wherein the compound having at least two cyclic alkylene carbonate structures is di(trimethylolpropane) carbonate.

8. The method according to any one of claims 5 to 7, wherein the ring-opening polymerization is carried out by a photocrosslinking reaction.

Images