A polylactic acid chemical recovery method with high monomer recovery rate

Abstract

The invention discloses a polylactic acid chemical recovery method with a high monomer recovery rate, including adding alcohol compounds and 2,4-pentanedione iron in polylactic acid plastic under normal pressure, inert gas protection, under high temperature conditions, under the catalysis of a catalyst, to obtain lactate; 2,4-pentanedione iron is continued to be added to the reaction system, reacted under high temperature and high pressure conditions, and an intermediate is obtained. The present invention is based on 2,4-pentanedione iron catalyzing polylactic acid depolymerization and repolymerization, 2,4-pentanedione iron has good thermal stability, and is nontoxic to human body, and the lactide prepared by methyl lactate or ethyl lactate has stable performance, high selectivity, and high yield, solves the problem of difficult purification of lactide synthesized from lactic acid in industry, and then by using CHO as a co-initiator, Zn as a catalyst, it is well suitable for catalyzing lactide ring-opening polymerization, meets the needs of staff, and the present invention is conducive to promotion and use.

Description

Technical Field

[0001] The invention relates to the technical field of polylactic acid chemical recovery, and in particular to a polylactic acid chemical recovery method with high monomer recovery rate. Background Art

[0002] Polylactic acid (PLA) has good biocompatibility and biodegradability, can replace petroleum-derived polymers for industrial production, and is one of the most promising green biopolymers. PLA is widely used because of its good biocompatibility, biodegradability, transparency, mechanical properties and good plasticity. It is often used as medical dressings (such as hydrogels), surgical sutures, connectors, agricultural products (such as mulch), packaging plastic films, textile materials, automotive parts, etc. The annual output of PLA is about 200,000 tons, which can be used in food packaging and pharmaceuticals. It is worth noting that the degradation cycle of PLA is very long in the natural environment. The annual utilization rate of waste PLA products is only 2%. The stacking or landfill of waste PLA takes up space, which affects the environment and wastes resources. Therefore, the recycling and reuse of PLA has become a research hotspot.

[0003] Polylactic acid (PLA), a substitute for petrochemical products, is a biodegradable plastic that can be recycled by physical methods, biodegradation or chemical methods. However, the recycling efficiency of the first two methods is low, and the harsh conditions can only generate water and carbon dioxide, which still causes waste of resources. Chemical recycling (such as alcoholysis, pyrolysis, and hydrolysis) can not only quickly and effectively degrade the discarded PLA, but also reuse the products obtained by degradation. Therefore, the chemical recycling of PLA, especially alcoholysis and aminolysis, has become a hot spot in the research of PLA degradation. However, the existing chemical recycling methods still have great limitations, such as long reaction time, large amount of nucleophilic reagent added (usually more than 350wt.% of PLA), not gentle enough reaction temperature (temperature above 100°C), single structure of the generated product (mainly methyl lactate), etc. Summary of the invention

[0004] In order to solve the above technical problems, a polylactic acid chemical recovery method with high monomer recovery rate is provided. This technical solution solves the problems that the existing chemical recovery method proposed in the above background technology still has great limitations, such as long reaction time and large amount of nucleophilic reagent added.

[0005] In order to achieve the above purpose, the technical solution adopted by the present invention is:

[0006] A polylactic acid chemical recovery method with high monomer recovery rate, comprising:

[0007] S1. Under normal pressure, inert gas protection, high temperature and catalyst, alcohol compounds and 2,4-pentanedione iron are added to polylactic acid plastic to obtain lactic acid ester;

[0008] S2, continue to add 2,4-pentanedione iron to the reaction system, react under high temperature and high pressure conditions to obtain an intermediate;

[0009] S3, reacting the intermediate under high temperature and high pressure conditions to obtain a crude lactide product, and then purifying the crude lactide product to obtain a lactide monomer;

[0010] S4, adding lactide monomer and 2,4-pentanedione iron into the reaction system, and preparing polylactic acid by bulk polymerization.

[0011] Preferably, the alcohol compound is one or more of methanol, ethanol, isopropanol, ethylene glycol, butanediol, propylene glycol, phenol, phenyl-1,2-ethanediol, 3,3-dimethyl-1,2-butanediol, and cis-1,2-cyclopentanediol mixed in any proportion.

[0012] Preferably, the high temperature condition in S1 is to react at 150-200° C. for 30-60 min.

[0013] Preferably, the high temperature and high pressure conditions in S2 are 130-180° C. and 75-95 MPa for 6-12 hours.

[0014] Preferably, the high temperature and high pressure conditions in S3 are 150-200° C. and 10-15 MPa for 2-5 h.

[0015] Preferably, the purification treatment in S3 is to recrystallize the crude lactide using ethyl acetate, acetone and petroleum ether.

[0016] Preferably, the lactate in S1 is methyl lactate or ethyl lactate.

[0017] Preferably, the molar ratio of the alcohol compound to 2,4-pentanedione iron in S1 is 5:13.

[0018] Preferably, the bulk polymerization method in S4 uses CHO as a co-initiator and Zn as a catalyst.

[0019] Preferably, during the bulk polymerization in S4, the ratio of lactide monomer to Zn catalyst is 5000:1.

[0020] Furthermore, bulk polymerization is the ring-opening polymerization of lactide, and the reaction is carried out at 150°C, with a monomer-to-catalyst ratio of 5000:1. However, the reaction rate is slow, with only 14% of the monomer conversion after one hour of reaction. CHO as a co-initiator can significantly increase the catalytic activity of the metal catalyst, and it can withstand high temperatures of 150°C, making it suitable for bulk polymerization of lactide. Therefore, CHO was first selected as a co-initiator and Zn as a catalyst to catalyze the ring-opening polymerization of lactide.

[0021]

[0022]

[0023] Conv. b Depend on 1 Calculated by HNMR; TOF c = the number of lactide monomers converted per unit time by a single metal active center; d was determined by GPC, the mobile phase was tetrahydrofuran, the standard sample was polystyrene, and the correction factor was 0.58; P m e Determined by the methyl region of the homonuclear decoupling H NMR spectrum: [mmm] = Pm2 + (1-Pm)Pm / 2, [mmr] = [rmm] = (1-Pm)Pm / 2, [rmr] = (1-Pm)2 / 2, and [mrm] = [(1-Pm)2 + Pm(1-Pm)] / 2.

[0024] Under the same reaction conditions, when 300 equivalents of CHO were added, the conversion rate of lactide monomer increased to 35%. In addition, increasing CHO to 2000 equivalents increased the monomer conversion rate to 83%, which is about 2.4 times that of 300 equivalents. This result reveals the effect of epoxide on the rate of lactide ring-opening polymerization. In addition, according to 1HNMR, CHO does not open the ring to form polyether under all reaction conditions.

[0025] It is speculated that during the reaction, the low CHO concentration will limit its interaction with Zn, resulting in a lower monomer conversion rate. CHO not only plays a key role in the initiation stage, but also inhibits the chain transfer reaction by coordinating with the metal, thereby promoting chain growth. The monomer conversion rate and the molecular weight of the obtained polylactic acid increase with the increase of the CHO ratio. This experimental result confirms our speculation. Figure 2 Further research found that the monomer conversion rate was significantly improved by ring-opening polymerization of the monomer under the same reaction conditions.

[0026] In order to verify whether other epoxides can also be used in this catalytic system to catalyze the ring-opening polymerization of lactide, several common epoxides, namely BO, PO, ECH and GMA, were selected and tested instead of CHO under the optimal reaction conditions. The results showed that compared with CHO, PO and BO had lower initiation activity, but the molecular weight of the resulting polylactic acid was high, which may be related to the steric hindrance effect of epoxides. ECH and GMA with polar groups showed higher initiation activity, but the molecular weight of polylactic acid was smaller. Therefore, this catalytic system can prepare polylactic acid of various molecular weights by selecting suitable epoxides.

[0027] In addition, benzyl alcohol, a traditional initiator for the ring-opening polymerization of lactide, was also tested. No polymer was detected under the same reaction conditions. This may be due to the different reaction initiation mechanisms of the two co-initiators. Benzyl alcohol follows the monomer activation mechanism, while CHO interacts with metal complexes to generate metal alkoxide active species to initiate the ring-opening polymerization of lactide. It is worth noting that under the same conditions, the catalytic system using CHO as a co-initiator exhibits higher polymerization activity compared to the industrial catalyst stannous octoate.

[0028]

[0029] Industrial production of polylactic acid is usually carried out under more stringent conditions, such as polymerizing industrial-grade lactide monomers at a high temperature of 180°C and a low catalyst concentration, increasing the proportion of monomers at the optimal ratio of CHO, and studying how to maintain the high activity of the catalytic system at a low catalyst concentration.

[0030] The results show that when the monomer to catalyst ratio is 1000:1, the reaction time can still reach 68% conversion rate after 20 minutes, and obtain polylactic acid with a molecular weight of 50.1kg / mol. If the monomer to catalyst ratio is further increased to 5000:1, the reaction time can reach 59% conversion rate after 60 minutes, and high molecular weight polylactic acid can be obtained.

[0031] Given that the catalytic system exhibits extremely high activity at low catalyst concentrations, we raised the reaction temperature to 180°C, which is the industrial production condition, and the catalytic efficiency was still significantly improved. Specifically, compared with the reaction at 150°C, the catalytic activity at 180°C was significantly improved, with a monomer conversion rate of up to 93%. When the ratio of monomer to catalyst was 5000:1, the TOF value of 106500 h-1 was achieved when 10000 equivalents of CHO and 5000 equivalents of CHO were used as co-initiators for 20 minutes. -1 and 39000h -1, and in the case of transesterification, the molecular weight of the obtained product can still reach 32.2kg / mol and 18.7kg / mol. The above experimental results show that the catalytic system can replace stannous octoate and bring new options for the industrial production of environmentally friendly polylactic acid. However, the transesterification reaction will accelerate at high temperatures, and long-term reaction may cause partial degradation of polylactic acid, so that the molecular weight of the obtained polymer will decrease over time and the molecular weight distribution will also become wider.

[0032]

[0033]

[0034] The ring-opening polymerization of lactide was studied. The main chain of polylactic acid has five sequence links, including: mmm, mmm, rmm, mmr and rmr. Among them, m means that the configuration of two adjacent lactic acid units is the same, and r means that the configuration of two adjacent lactic acid units is different. The stereoregularity of polylactic acid is expressed as the probability of racemic or meso lactide ring opening to form syndiotactic or isotactic, that is, Pm or Pr. These values ​​can be calculated from the homonuclear decoupling NMR spectrum of polylactic acid. Pm represents the probability of polymerizing a lactide monomer of one configuration and then polymerizing another lactide monomer of the same configuration. Pm can be calculated by the following formula: [mmm] = Pm2 + (1-Pm)Pm / 2, [mmr] = [rmm] = (1-Pm)Pm / 2, [rmr] = (1-Pm)2 / 2, and [mrm] = [(1-Pm)2 + Pm(1-Pm)] / 2. The homonuclear decoupling NMR spectrum was measured to determine its stereoselectivity, such as Figure 3 As shown. The calculated Pm value of the polymer is 0.76, indicating that the catalytic system has a certain stereoselectivity in catalyzing the ring-opening polymerization of lactide. In addition, the homonuclear decoupling NMR spectra of other polymers obtained were also measured, and the results showed that the molecular chain of the obtained polylactic acid is mainly isotactic, which may be due to the chain end control mechanism of the catalytic system for the ring-opening polymerization of lactide.

[0035] The main physical properties of the synthesized polylactic acid (T g , T m ) were tested to study the effect of the isotacticity and molecular weight of the polymer on it. The results are shown in the following table. From the table below, it can be concluded that the transition temperature (T) of polylactic acid obtained at different ratios and temperatures g ) is between 45.8 and 60.7°C. m The increase of T value of polylactic acid g The T of polylactic acid increases with the increase of gIt depends on both the molecular weight of the polymer and the isotacticity of the polymer. In addition, the melting point (T m ) value can reach 176.8℃.

[0036]

[0037] Further research was conducted on the ring-opening polymerization of lactide to investigate the effects of monomer type and reaction temperature on the polymerization kinetics. Figure 3 shown. Figure 3 This indicates that the conversion rate of the monomer is linearly related to time, indicating that the reaction rate conforms to the first-order reaction kinetics and follows the coordination insertion mechanism. Figure 3 It can be seen that the reaction rate is slow in the early stage of the polymerization reaction, and it is difficult to detect the formation of polymers. This may be because it takes time for the monomer to melt, and it also takes time for the catalyst and CHO to interact to form active intermediates, resulting in a prolonged induction period. Increasing the temperature can accelerate the melting of the monomer and the interaction between the catalyst and CHO, shorten the induction period, and thus accelerate the reaction, so the reaction rate at 180°C is faster than that at 150°C.

[0038] The MALDI-TOF spectrum of polylactic acid was obtained under the condition of lactide:CHO:Zn=5000:1000:1. Figure 4 As shown in the figure, the interval of the corresponding peak of PLLA is 72.02uma, which corresponds to the molecular weight of half a lactide monomer, indicating that an ester exchange reaction occurred during the polymerization reaction. After calculation, it can be determined that there are two main peaks related to linear polymers in the spectrum, as well as a peak corresponding to cyclic products. These three obvious peaks can be attributed to hydrolysis byproducts or ester exchange products within and between polymer molecules. Another minor peak can be attributed to cyclic products without end groups, which is the result of a backbiting reaction when another CHO is inserted into the growing molecular chain. In addition, no interval of 98.15m / z of the CHO repeating unit was observed, which once again proves that CHO only works in the initiation step and will not open the ring to form polyethers by itself. Through the analysis of the characterization results, it is proved that the main polymerization product is linear polylactic acid.

[0039] Based on the MALDI-TOF results, a coordination insertion mechanism for the synthesis of polylactic acid was proposed, such as Figure 6As shown, the coordination bond between the N-donor atom of the ligand and Zn is broken, thereby the initiation step occurs. Subsequently, the lactide ring-opening generates a new active species, in which the alkoxy bond connected to the carbonyl group in the lactide monomer is broken to form an eight-membered heterocyclic species. Finally, the first insertion product containing a five-membered zinc lactate heterocycle is generated, and the ligand is attached to the end of the newly generated polymer chain. This hypothesized structure is consistent with the previous analysis, and the subsequent chain growth step is achieved by the continuous insertion of lactide monomers into the metal-oxygen bond.

[0040] Compared with the prior art, the present invention provides a polylactic acid chemical recovery method with a high monomer recovery rate, which has the following beneficial effects:

[0041] The invention is based on 2,4-pentanedione iron catalyzing the depolymerization and repolymerization of polylactic acid. The 2,4-pentanedione iron has good thermal stability and is non-toxic and harmless to the human body. The lactide prepared by methyl lactate or ethyl lactate has stable performance, high selectivity and high yield, and solves the problem of difficult purification of lactide synthesized from lactic acid in industry. CHO is used as a co-initiator and Zn is used as a catalyst, so the invention is well suitable for catalyzing the ring-opening polymerization of lactide, meets the needs of the staff, and is conducive to popularization and use. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 Schematic diagram of the polylactic acid chemical recovery method of the present invention;

[0043] Figure 2 is a graph showing the conversion rate of lactide monomer and the molecular weight of the obtained polylactic acid in the present invention;

[0044] Figure 3 is the homonuclear decoupling NMR spectrum of the polylactic acid in the present invention;

[0045] Figure 4 It is a kinetic curve diagram of the ring-opening polymerization of lactide at 150° C. and 180° C. in the present invention;

[0046] Figure 5 Schematic diagram of the MALDI-TOF spectrum of polylactic acid in the present invention;

[0047] Figure 6 Schematic diagram of the coordination insertion mechanism in the present invention. DETAILED DESCRIPTION

[0048] The following description is used to disclose the present invention so that those skilled in the art can implement the present invention. The preferred embodiments described below are only examples, and those skilled in the art may think of other obvious variations.

[0049] Example 1

[0050] Please refer to Figure 1As shown, a polylactic acid chemical recovery method with high monomer recovery rate comprises:

[0051] S1. Under normal pressure and inert gas protection, at 150-200° C., under the catalysis of a catalyst, add an alcohol compound and 2,4-pentanedione iron in a molar ratio of 5:13 to polylactic acid plastic, and react for 30-60 minutes to obtain lactic acid ester, wherein the lactic acid ester is methyl lactate or ethyl lactate, and the alcohol compound is one or more of methanol, ethanol, isopropanol, ethylene glycol, butanediol, propylene glycol, phenol, phenyl-1,2-ethylene glycol, 3,3-dimethyl-1,2-butanediol, and cis-1,2-cyclopentanediol, mixed in any proportion;

[0052] S2, continue to add 2,4-pentanedione iron to the reaction system, react at 130-180° C., 75-95 MPa for 6-12 hours to obtain an intermediate;

[0053] S3, reacting the intermediate at 150-200° C. and 10-15 MPa for 2-5 hours to obtain a crude lactide product, and then purifying the crude lactide product to obtain a lactide monomer, and the purification process uses ethyl acetate, acetone, and petroleum ether to recrystallize the crude lactide product;

[0054] S4. Add lactide monomer and 2,4-pentanedione iron to the reaction system, and the ratio of lactide monomer to Zn catalyst is 5000:1. Use CHO as a co-initiator and Zn as a catalyst to carry out bulk polymerization to prepare polylactic acid.

[0055] Example 2

[0056] Bulk polymerization is the ring-opening polymerization of lactide. The reaction is carried out at 150°C, and the ratio of monomer to catalyst is 5000:1. However, the reaction speed is slow, with only 14% of the monomer conversion in one hour. CHO as a co-initiator can significantly increase the catalytic activity of the metal catalyst, and it can withstand high temperatures of 150°C, which is suitable for bulk polymerization of lactide. Therefore, CHO is first selected as a co-initiator and Zn as a catalyst to catalyze the ring-opening polymerization of lactide.

[0057]

[0058]

[0059] Conv. b Depend on 1 Calculated by HNMR; TOF c = the number of lactide monomers converted per unit time by a single metal active center; d was determined by GPC, the mobile phase was tetrahydrofuran, the standard sample was polystyrene, and the correction factor was 0.58; P m eDetermined by the methyl region of the homonuclear decoupling H NMR spectrum: [mmm] = Pm2 + (1-Pm)Pm / 2, [mmr] = [rmm] = (1-Pm)Pm / 2, [rmr] = (1-Pm)2 / 2, and [mrm] = [(1-Pm)2 + Pm(1-Pm)] / 2.

[0060] Under the same reaction conditions, when 300 equivalents of CHO were added, the conversion rate of lactide monomer increased to 35%. In addition, increasing CHO to 2000 equivalents increased the monomer conversion rate to 83%, which is about 2.4 times that of 300 equivalents. This result reveals the effect of epoxide on the rate of lactide ring-opening polymerization. In addition, according to 1HNMR, CHO does not open the ring to form polyether under all reaction conditions.

[0061] It is speculated that during the reaction, the low CHO concentration will limit its interaction with Zn, resulting in a lower monomer conversion rate. CHO not only plays a key role in the initiation stage, but also inhibits the chain transfer reaction by coordinating with the metal, thereby promoting chain growth. The monomer conversion rate and the molecular weight of the obtained polylactic acid increase with the increase of the CHO ratio. This experimental result confirms our speculation. Figure 2 Further research found that the monomer conversion rate was significantly improved by ring-opening polymerization of the monomer under the same reaction conditions.

[0062] In order to verify whether other epoxides can also be used in this catalytic system to catalyze the ring-opening polymerization of lactide, several common epoxides, namely BO, PO, ECH and GMA, were selected and tested instead of CHO under the optimal reaction conditions. The results showed that compared with CHO, PO and BO had lower initiation activity, but the molecular weight of the resulting polylactic acid was high, which may be related to the steric hindrance effect of epoxides. ECH and GMA with polar groups showed higher initiation activity, but the molecular weight of polylactic acid was smaller. Therefore, this catalytic system can prepare polylactic acid of various molecular weights by selecting suitable epoxides.

[0063] In addition, benzyl alcohol, a traditional initiator for the ring-opening polymerization of lactide, was also tested. No polymer was detected under the same reaction conditions. This may be due to the different reaction initiation mechanisms of the two co-initiators. Benzyl alcohol follows the monomer activation mechanism, while CHO interacts with metal complexes to generate metal alkoxide active species to initiate the ring-opening polymerization of lactide. It is worth noting that under the same conditions, the catalytic system using CHO as a co-initiator exhibits higher polymerization activity compared to the industrial catalyst stannous octoate.

[0064]

[0065] Industrial production of polylactic acid is usually carried out under more stringent conditions, such as polymerizing industrial-grade lactide monomers at a high temperature of 180°C and a low catalyst concentration, increasing the proportion of monomers at the optimal ratio of CHO, and studying how to maintain the high activity of the catalytic system at a low catalyst concentration.

[0066] The results show that when the monomer to catalyst ratio is 1000:1, the reaction time can still reach 68% conversion rate after 20 minutes, and obtain polylactic acid with a molecular weight of 50.1kg / mol. If the monomer to catalyst ratio is further increased to 5000:1, the reaction time can reach 59% conversion rate after 60 minutes, and high molecular weight polylactic acid can be obtained.

[0067] Given that the catalytic system exhibits extremely high activity at low catalyst concentrations, we raised the reaction temperature to 180°C, which is the industrial production condition, and the catalytic efficiency was still significantly improved. Specifically, compared with the reaction at 150°C, the catalytic activity at 180°C was significantly improved, with a monomer conversion rate of up to 93%. When the ratio of monomer to catalyst was 5000:1, the TOF value of 106500 h-1 was achieved when 10000 equivalents of CHO and 5000 equivalents of CHO were used as co-initiators for 20 minutes. -1 and 39000h -1 , and in the case of transesterification, the molecular weight of the obtained product can still reach 32.2kg / mol and 18.7kg / mol. The above experimental results show that the catalytic system can replace stannous octoate and bring new options for the industrial production of environmentally friendly polylactic acid. However, the transesterification reaction will accelerate at high temperatures, and long-term reaction may cause partial degradation of polylactic acid, so that the molecular weight of the obtained polymer will decrease over time and the molecular weight distribution will also become wider.

[0068]

[0069]

[0070] The ring-opening polymerization of lactide was studied. The main chain of polylactic acid has five sequence links, including: mmm, mmm, rmm, mmr and rmr. Among them, m means that the configuration of two adjacent lactic acid units is the same, and r means that the configuration of two adjacent lactic acid units is different. The stereoregularity of polylactic acid is expressed as the probability of racemic or meso lactide ring opening to form syndiotactic or isotactic, that is, Pm or Pr. These values ​​can be calculated from the homonuclear decoupling NMR spectrum of polylactic acid. Pm represents the probability of polymerizing a lactide monomer of one configuration and then polymerizing another lactide monomer of the same configuration. Pm can be calculated by the following formula: [mmm] = Pm2 + (1-Pm)Pm / 2, [mmr] = [rmm] = (1-Pm)Pm / 2, [rmr] = (1-Pm)2 / 2, and [mrm] = [(1-Pm)2 + Pm(1-Pm)] / 2. The homonuclear decoupling NMR spectrum was measured to determine its stereoselectivity, such as Figure 3 As shown. The calculated Pm value of the polymer is 0.76, indicating that the catalytic system has a certain stereoselectivity in catalyzing the ring-opening polymerization of lactide. In addition, the homonuclear decoupling NMR spectra of other polymers obtained were also measured, and the results showed that the molecular chain of the obtained polylactic acid is mainly isotactic, which may be due to the chain end control mechanism of the catalytic system for the ring-opening polymerization of lactide.

[0071] Example 3

[0072] The main physical properties of the synthesized polylactic acid (T g , T m ) were tested to study the effect of the isotacticity and molecular weight of the polymer on it. The results are shown in the following table. From the table below, it can be concluded that the transition temperature (T) of polylactic acid obtained at different ratios and temperatures g ) is between 45.8 and 60.7°C. m The increase of T value of polylactic acid g The T of polylactic acid increases with the increase of g It depends on both the molecular weight of the polymer and the isotacticity of the polymer. In addition, the melting point (T m ) value can reach 176.8℃.

[0073]

[0074] Example 4

[0075] Further research was conducted on the ring-opening polymerization of lactide to investigate the effects of monomer type and reaction temperature on the polymerization kinetics. Figure 3 shown. Figure 3This indicates that the conversion rate of the monomer is linearly related to time, indicating that the reaction rate conforms to the first-order reaction kinetics and follows the coordination insertion mechanism. Figure 3 It can be seen that the reaction rate is slow in the early stage of the polymerization reaction, and it is difficult to detect the formation of polymers. This may be because it takes time for the monomer to melt, and it also takes time for the catalyst and CHO to interact to form active intermediates, resulting in a prolonged induction period. Increasing the temperature can accelerate the melting of the monomer and the interaction between the catalyst and CHO, shorten the induction period, and thus accelerate the reaction, so the reaction rate at 180°C is faster than that at 150°C.

[0076] Example 5

[0077] The MALDI-TOF spectrum of polylactic acid was obtained under the condition of lactide:CHO:Zn=5000:1000:1. Figure 4 As shown in the figure, the interval of the corresponding peak of PLLA is 72.02uma, which corresponds to the molecular weight of half a lactide monomer, indicating that an ester exchange reaction occurred during the polymerization reaction. After calculation, it can be determined that there are two main peaks related to linear polymers in the spectrum, as well as a peak corresponding to cyclic products. These three obvious peaks can be attributed to hydrolysis byproducts or ester exchange products within and between polymer molecules. Another minor peak can be attributed to cyclic products without end groups, which is the result of a backbiting reaction when another CHO is inserted into the growing molecular chain. In addition, no interval of 98.15m / z of the CHO repeating unit was observed, which once again proves that CHO only works in the initiation step and will not open the ring to form polyethers by itself. Through the analysis of the characterization results, it is proved that the main polymerization product is linear polylactic acid.

[0078] Based on the MALDI-TOF results, a coordination insertion mechanism for the synthesis of polylactic acid was proposed, such as Figure 6 As shown, the coordination bond between the N-donor atom of the ligand and Zn is broken, thereby the initiation step occurs. Subsequently, the lactide ring-opening generates a new active species, in which the alkoxy bond connected to the carbonyl group in the lactide monomer is broken to form an eight-membered heterocyclic species. Finally, the first insertion product containing a five-membered zinc lactate heterocycle is generated, and the ligand is attached to the end of the newly generated polymer chain. This hypothesized structure is consistent with the previous analysis, and the subsequent chain growth step is achieved by the continuous insertion of lactide monomers into the metal-oxygen bond.

[0079] In summary, the present invention is based on the depolymerization and repolymerization of polylactic acid catalyzed by 2,4-pentanedione iron, 2,4-pentanedione iron has good thermal stability and is non-toxic and harmless to the human body, and the lactide prepared by methyl lactate or ethyl lactate has stable performance, high selectivity and high yield, and solves the problem of difficulty in purifying lactide synthesized from lactic acid in industry, and then adopts CHO as a co-initiator and Zn as a catalyst, so that it is well suitable for catalyzing the ring-opening polymerization of lactide, meets the needs of the staff, and is conducive to popularization and use.

[0080] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and descriptions only describe the principles of the present invention. The present invention may be subject to various changes and improvements without departing from the spirit and scope of the present invention. These changes and improvements fall within the scope of the present invention. The scope of protection claimed by the present invention is defined by the attached claims and their equivalents.

Claims

1. A method for chemically recovering polylactic acid with high monomer recovery rate, The invention comprises, wherein: S1. Under normal pressure, inert gas protection, high temperature and catalyst, alcohol compounds and 2,4-pentanedione iron are added to polylactic acid plastic to obtain lactic acid ester; S2, continue to add 2,4-pentanedione iron to the reaction system, react under high temperature and high pressure conditions to obtain an intermediate; S3, reacting the intermediate under high temperature and high pressure conditions to obtain a crude lactide product, and then purifying the crude lactide product to obtain a lactide monomer; S4, adding lactide monomer and 2,4-pentanedione iron into the reaction system, and preparing polylactic acid by bulk polymerization.

2. The method for chemically recovering polylactic acid with high monomer recovery rate according to claim 1, characterized in that: The alcohol compound is one or more of methanol, ethanol, isopropanol, ethylene glycol, butanediol, propylene glycol, phenol, phenyl-1,2-ethylene glycol, 3,3-dimethyl-1,2-butanediol, and cis-1,2-cyclopentanediol mixed in any proportion.

3. The method for chemically recovering polylactic acid with high monomer recovery rate according to claim 1, characterized in that: The high temperature condition in S1 is to react at 150-200° C. for 30-60 min.

4. The method for chemically recovering polylactic acid with high monomer recovery rate according to claim 1, characterized in that: The high temperature and high pressure conditions in S2 are 130-180° C. and 75-95 MPa for 6-12 hours.

5. The method for chemically recovering polylactic acid with high monomer recovery rate according to claim 1, characterized in that: The high temperature and high pressure conditions in S3 are 150-200° C. and 10-15 MPa for 2-5 hours.

6. The method for chemically recovering polylactic acid with high monomer recovery rate according to claim 1, characterized in that: The purification process in S3 is to recrystallize the crude lactide using ethyl acetate, acetone and petroleum ether.

7. The method for chemically recovering polylactic acid with high monomer recovery rate according to claim 1, characterized in that: The lactate in S1 is methyl lactate or ethyl lactate.

8. The method for chemically recovering polylactic acid with high monomer recovery rate according to claim 1, characterized in that: The molar ratio of the alcohol compound to 2,4-pentanedione iron in S1 is 5:

13.

9. The method for chemically recovering polylactic acid with high monomer recovery rate according to claim 1, characterized in that: The bulk polymerization method in S4 uses CHO as a co-initiator and Zn as a catalyst.

10. The method for chemically recovering polylactic acid with high monomer recovery rate according to claim 9, characterized in that: During the bulk polymerization in S4, the ratio of lactide monomer to Zn catalyst is 5000:1.

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