Preparation method of cyclic alkyl oxalate and ring-opening copolymerization of cyclic alkyl oxalate

Abstract

The invention provides a preparation method of an alkyl oxalate monomer and an alkyl oxalate copolymer. Compared with methods reported in the past, the method provided by the invention has the following beneficial effects: 1) the alkyl oxalate copolyester oligomer prepared in the condensation polymerization stage is low in viscosity, and mass and heat transfer of a system is facilitated; 2) in the depolymerization stage, since the alkyl oxalate copolyester is not crystallized or has a low melting point, the alkyl oxalate copolyester can be in full contact with a depolymerization catalyst at a lower temperature, the depolymerization temperature is milder, side reaction at a high temperature is avoided, and the monomer yield and purity are high; 3) the cyclic alkyl oxalate monomer mixture obtained by depolymerization is easy to separate; and 4) by designing and screening an efficient catalyst, controllable ring-opening polymerization of cyclic alkyl oxalate is realized, high-molecular-weight random and segmented copolymers are prepared, and the copolymers can be depolymerized back to monomers under certain conditions.

Description

Technical Field

[0001] The invention relates to the field of polymer synthesis, and in particular to a method for preparing a cyclic alkyl oxalate monomer and a method for preparing an alkyl oxalate copolymer through ring-opening copolymerization thereof. Background Art

[0002] With the continuous development of low-carbon and green economy, people are eager to seek recyclable polymer materials to get rid of dependence on petroleum resources and avoid the pollution of the environment by non-degradable and recyclable polymer materials. With the deepening of research, more and more recyclable polymer materials have been discovered, such as polyester, polycarbonate, polythioester, etc. Among them, aliphatic polyesters have attracted much attention as a type of recyclable polymer material with excellent performance, wide application, and can be synthesized using bio-based raw materials.

[0003] As a typical aliphatic polyester material, polyalkyl oxalate was first used in 1986. et al. have proved that it has the potential for recycling (Polym. Degrad. Stab. 1986, 15 (4), 345-356). Moreover, the special conjugated carbonyl structure of polyalkyl oxalate gives it special thermal and crystallization properties, showing high tensile strength, high modulus and high crystallinity (Materials Today Sustainability 2023, 22.). In addition, polyalkyl oxalate also exhibits excellent non-enzymatic hydrolysis ability and good biocompatibility, and can be used in medical fields such as absorbable sutures and drug carriers.

[0004] The domestic research group of Wei Zhiyong and Zhou Weidong and the research group of Miller and Shalaby abroad used dimethyl oxalate and aliphatic diols to prepare polyalkyl oxalates by condensation polymerization under high temperature bulk conditions, and made important progress (Macromolecules 2023, 56, 3149-3159. Angew Chem, 2021, 133 (33), 18483-18491. Polym. Chem., 2014, 5, 955-961. US4140678A). However, condensation polymerization requires a long reaction time, and can only obtain polyalkyl oxalates of several thousand molecular weight, with low yield, and it is difficult to prepare copolyesters with clear sequence structures. Ring-opening polymerization of cyclic monomers has the advantages of atom economy, mild polymerization conditions, and diverse catalyst selection, and can synthesize copolyesters with clear composition, chain structure, and sequence structure.

[0005] The key to preparing high molecular weight and end group structured polyalkyl oxalates by ring-opening polymerization of cyclic alkyl oxalate monomers is the synthesis and purification of cyclic alkyl oxalate monomers. For recyclable materials, the most important feature is the closed cycle life cycle of "monomer-polymer-monomer", and the monomer can be obtained by depolymerizing the polymer under bulk conditions. The polymer can be obtained by condensation polymerization, so the monomer can be synthesized on a large scale by constructing a two-step (condensation-depolymerization) method. Finally, the catalytic system is screened to achieve controllable ring-opening polymerization and depolymerization recovery, thereby constructing a closed cycle of "monomer-polymer-monomer". In condensation polymerization, oxalic acid monomers include oxalic acid, dimethyl oxalate, diethyl oxalate and oxalyl chloride, but considering the price and stability of the monomers, and with the industrialization of the carbon monoxide catalytic coupling method for preparing oxalate esters, the green chemical process for preparing polyalkyl oxalates using oxalate esters as raw materials is more meaningful.

[0006] At present, there are relatively few studies on the ring-opening polymerization and recycling of cyclic alkyl oxalates. The earliest study can be traced back to 1933, when Carothers et al. found that linear polyesters can be depolymerized into cyclic lactone monomers (J.Am.Chem.Soc.1933,55(12),5031-5039). In 1979, Alksnis and Deme obtained hexavalent oxalate monomers by depolymerizing polyethylene oxalate (J.Polym.Sci.,Part A:Polym.Chem.1979,15(4),2701-2706), proving that oxalate has the potential to achieve closed-loop recycling. Going further et al. imitated the preparation method of lactide (condensation-depolymerization two-step method), first condensed ethylene glycol and oxalic acid into oligomers and then depolymerized them into oxalic acid ethylene glycol ester monomers, and achieved the ring-opening polymerization of the monomers (Polym. Degrad. Stab. 1986, 15 (4), 345-356). Sinopec Beijing Chemical Research Institute designed a continuous production process for oxalate cyclic monomers in 2012, which is conducive to large-scale industrial production, but the yield is low (CN201210425013.4).

[0007] The research on cyclic oxalic acid ethylene glycol ester has filled the gap in cyclic oxalic acid alkyl ester monomers, but there are still some unresolved problems. First, for polyalkyl oxalates with high crystallinity and melting point, the depolymerization temperature needs to be above the melting point. The higher depolymerization temperature will produce a large number of side reactions, affecting the monomer yield and purity. Second, polyethylene glycol oxalate has high strength but insufficient toughness, which limits the further application of polyethylene glycol oxalate. Third, the polyester prepared by ring-opening polymerization of alkyl oxalate has a low molecular weight and an unclear terminal structure, and there is a lack of design and development of catalysts for ring-opening polymerization and depolymerization recovery of cyclic oxalate.

[0008] Therefore, the present invention uses bulk commodity diol compounds and oxalic acid esters as raw materials, constructs a two-step method of "condensation-low molecular weight copolymer-depolymerization" to efficiently prepare cyclic alkyl oxalate monomers, and uses the difference in monomer solubility and boiling point to achieve monomer separation and purification; at the same time, the present invention develops efficient catalysts and uses a variety of polymerization methods such as solution, bulk, one-pot method and sequential feeding to prepare polyalkyl oxalate copolymers with diverse structures and excellent performance. The core idea of ​​the present invention is to prepare non-crystalline or low-melting-point alkyl oxalate copolyesters through copolymerization, thereby avoiding high polycondensation temperature and / or depolymerization temperature. Compared with the existing method of preparing cyclic alkyl oxalate by depolymerization of homopolymer, the method provided by the present invention has the following advantages: 1) the alkyl oxalate copolyester oligomer prepared in the polycondensation stage has low viscosity, which is beneficial to the mass transfer and heat transfer of the system; 2) in the depolymerization stage, since the alkyl oxalate copolyester does not crystallize or has a low melting point, it can fully contact with the depolymerization catalyst at a relatively low temperature, the depolymerization temperature is more moderate, the generation of side reactions at high temperature is avoided, and the monomer yield and purity are high; 3) the cyclic alkyl oxalate monomer mixture obtained by depolymerization is easy to separate; 4) by designing and screening high-efficiency catalysts, the controlled ring-opening polymerization of cyclic alkyl oxalate is realized, and high molecular weight random and block copolymers are prepared, and the copolymers can be depolymerized back to monomers under certain conditions. Summary of the invention

[0009] The purpose of the present invention is to provide: (1) the present invention uses bulk or bio-based dimethyl oxalate and aliphatic diol as raw materials, and obtains low molecular weight oxalate alkyl copolyester through condensation polymerization of the two diols and dimethyl oxalate. (2) under catalyst conditions, the oxalate alkyl copolyester is depolymerized into cyclic monomers, and the crude product can be used to obtain two cyclic oxalate alkyl ester monomers through simple separation and purification methods by utilizing the difference in monomer solubility or boiling point. (3) by screening catalysts to achieve controlled ring-opening copolymerization of monomers, oxalate alkyl copolyester is prepared, and the obtained oxalate alkyl copolyester can be recycled under certain conditions.

[0010] The main structural features of the synthetic route are:

[0011] (1) The present invention uses bulk oxalate and aliphatic diol compounds as raw materials, and the added oxalic acid (ester) includes but is not limited to dimethyl oxalate, diethyl oxalate, di-n-propyl oxalate, diisopropyl oxalate, di-n-butyl oxalate, ditert-butyl oxalate, diphenylmethane oxalate, monomethyl oxalate or monoethyl oxalate. At least one of the added aliphatic diols is an aliphatic diol with a branched structure, including 1,2-propylene glycol, 2,3-butanediol, 2,2-butanediol, 1,2-butanediol, 1,2-pentanediol, 1,2-hexanediol, 1,2-cyclohexanediol, 1,2-cyclopentanediol, 1,2-cyclononanediol, 1,2-cyclodecanediol and phenyl-1,2-ethylene glycol.

[0012] (2) In the present invention, the polycondensation catalyst is selected from stannous octoate, tetrabutyl titanate, tetraisopropyl titanate, zinc acetate, magnesium oxide, magnesium hydroxide, magnesium carbonate, tin chloride, ferric chloride, titanium chloride, zinc chloride, aluminum chloride, zinc oxide, tin oxide, ferric oxide, bismuth trifluoromethanesulfonate, zinc trifluoromethanesulfonate, 4-dimethylaminopyridine, nitrogen heterocyclic carbene, p-toluenesulfonic acid, trifluoromethanesulfonic acid or diphenyl phosphate.

[0013] (3) In the present invention, the depolymerization temperature of the oxalic acid alkyl copolyester is 50-200°C and the pressure is 50-2000Pa.

[0014] (4) In the present invention, the amount of the polycondensation catalyst is 0.0001% to 1.0% of the mass of the oxalate compound; the amount of the depolymerization catalyst is 0.001% to 5.0% of the mass of the low molecular weight alkyl oxalate random copolymer.

[0015] (5) In the present invention, the depolymerization catalyst includes but is not limited to stannous octoate, tetrabutyl titanate, tin chloride, ferric chloride, titanium chloride, zinc chloride, aluminum chloride, zinc oxide, tin oxide, ferric oxide, 4-dimethylaminopyridine, p-toluenesulfonic acid, trifluoromethanesulfonic acid, diphenyl phosphate, ionic liquid, sodium glycolate, sodium acetate. The use of an efficient depolymerization catalyst can improve the specificity of the depolymerization reaction, ensuring a higher yield of the target product and a lower generation of by-products.

[0016] (6) In the present invention, according to the difference in monomer solubility, a solvent is added to achieve separation and purification of the monomer mixture, and the solvent can be selected from one or more of n-heptane, dichloromethane, chloroform, carbon tetrachloride, 1,2-dichloroethane, 1,2,2-trichloroethane, methanol, ethanol, isopropanol, ether, acetone, methyl ethyl ketone, acetic acid, ethyl acetate, isopropyl formate, butyl acetate, 1,4-dioxane, tetrahydrofuran, cyclohexanone, benzene, and dimethyl sulfoxide.

[0017] (7) The method for preparing a high molecular weight alkyl oxalate copolymer using the cyclic alkyl oxalate monomer obtained after separation and purification in the present invention comprises the following steps: mixing a ring-opening polymerization catalyst and an initiator, adding the cyclic alkyl oxalate in a mixed feeding or sequential feeding manner, and reacting at 50 to 250° C. for 10 min to 24 h to obtain a high molecular weight random or block structure alkyl oxalate copolymer.

[0018] The initiators involved include one of methanol, ethanol, isopropanol, benzyl alcohol, phenylpropanol, propylene alcohol, ethylene glycol, 1,4-butanediol, terephthalic acid, glycerol, trimethylolpropane, and pentaerythritol.

[0019] The catalyst used includes but is not limited to the following: stannous octoate, dibutyltin dilaurate, tin dioxide, diethylzinc, zinc dichloride, n-butyllithium, isobutyllithium, lithium diisopropylamide, sodium methoxide, sodium tert-butoxide, potassium tert-butoxide, methanesulfonic acid, trifluoromethanesulfonic acid, p-toluenesulfonic acid, boron trifluoride, tri(pentafluorophenyl)boron, triethylboron, triethylamine, pyridine, 4-dimethylaminopyridine, 1,8-diazabicyclo[5.4.0]undec-7-ene, 1,5,7-triazabicyclo[4.4.0]dec-5-ene, hexamethyldisilazide, and one of phosphazene bases.

[0020] In a word, the present invention selects bulk low-priced chemicals and bio-based products as raw materials, and by optimizing the monomer synthesis process, improves the productivity and purity of monomers, and promotes the industrialization of polyoxalate. By introducing copolymerization into (the two-step method of condensation-depolymerization), not only can the depolymerization of oxalic acid alkyl copolyester be realized under more mild conditions, the yield of monomer is improved, and the monomer obtained is easy to realize the separation and purification of mixed monomers according to the difference of solubility, and lays the foundation for the recycling of copolymer. Finally, by designing and screening efficient catalysts, oxalic acid alkyl ester monomers can be open-loop copolymerized under mild conditions, and the preparation of high molecular weight, recyclable oxalic acid alkyl ester copolymers is realized. This not only makes up for the defects of homopolymers, enriches the structure of oxalic acid-based polyesters, provides a wide space for designing high-performance, highly functional new materials, and avoids environmental pollution and waste of resources caused by discarding polymer materials, and promotes the green sustainable development of polymer materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 is the NMR spectrum of propylene glycol oxalate lactone obtained in Example 1;

[0022] Figure 2 is the nuclear magnetic spectrum of 1,2-oxalic acid hexanediol lactone obtained in Example 1;

[0023] Figure 3 is the NMR spectrum of oxalic acid glycol lactone obtained in Example 2;

[0024] Figure 4 is the NMR spectrum of 1,2-oxalyl propanediol lactone obtained in Example 2;

[0025] Figure 5 is the NMR spectrum of 1,2-oxalylbutanediol lactone obtained in Example 3;

[0026] Figure 6 is the GPC chart of poly(ethylene oxalate lactone-co-propylene oxalate lactone) obtained in Example 4;

[0027] Figure 7is a GPC chart of poly(ethylene glycol oxalate lactone-b-hexanediol oxalate lactone-b-ethylene glycol oxalate lactone) obtained in Example 5;

[0028] Figure 8 is a GPC chart of poly(propylene glycol oxalate lactone-b-butylene glycol oxalate lactone-b-propylene glycol oxalate lactone) obtained in Example 6; DETAILED DESCRIPTION

[0029] In order to better understand the content of the present invention, further description is given below in conjunction with specific embodiments. These embodiments are only used to further illustrate the present invention, and do not limit the scope of the present invention. In addition, after reading the content described in the present invention, those skilled in the art make some non-essential changes or adjustments to the present invention, which still belong to the protection scope of the present invention.

[0030] Example 1. Synthesis of 1,2-oxalyl propanediol lactone and 1,2-oxalyl hexanediol lactone

[0031] Add dimethyl oxalate (60g, 0.51mol), 1,3-propylene glycol (31.2g, 0.41mol), 1,2-hexanediol (11.8g, 0.10mol) and trifluoromethanesulfonic acid (0.076g) to a 250mL reaction bottle. React at 80°C under normal pressure for 1.5h; heat to 110°C for 1.5h; heat to 140°C for 3h; heat to 150°C for 1h. At 150°C, gradually reduce the pressure to 10000Pa for 0.5h, and gradually reduce the pressure to 50Pa for 1-2h. Add stannous octoate (0.5wt%), maintain the pressure at 50Pa, heat to 180°C for 2h to obtain a mixture of propylene glycol oxalate lactone monomer and 1,2-butylene glycol oxalate lactone monomer. The monomer mixture was added dropwise into 100 mL of cold ethyl acetate, and propylene glycol oxalate lactone was crystallized in ethyl acetate. After filtration, washing and drying, pure propylene glycol oxalate lactone monomer was obtained. The ethyl acetate phase was concentrated to obtain 1,2-oxalyl hexanediol lactone with yields of 86% and 83%, respectively.

[0032] Example 2. Synthesis of oxalic acid ethylene glycol lactone and 1,2-oxalic acid propylene glycol lactone

[0033] Add dimethyl oxalate (600g, 5.08mol), ethylene glycol (189g, 3.05mol), 1,2-propylene glycol (232g, 3.05mol) and tetrabutyl titanate (0.18g) to a 2L reaction bottle. React at 80°C under normal pressure for 1.5h; heat to 110°C for 1.5h; heat to 140°C for 3h; heat to 150°C for 1h. At 150°C, gradually reduce the pressure to 10000Pa for 0.5h, and gradually reduce the pressure to 50Pa for 1-2h. Add sodium glycolate (1wt%), maintain the pressure at 50Pa, heat to 180°C for 6h to obtain a mixture of oxalic acid ethylene glycol lactone monomer and 1,2-oxalic acid propylene glycol lactone monomer. The monomer mixture was added dropwise into 1.5 L of cold dichloromethane, and oxalic acid glycol lactone was crystallized in dichloromethane. After filtration, washing and drying, pure oxalic acid glycol lactone monomer was obtained. The dichloromethane phase was concentrated to obtain 1,2-oxalic acid propiolactone, with yields of 84% and 85%, respectively.

[0034] Example 3. Synthesis of oxalic acid ethylene glycol lactone and 1,2-oxalic acid butylene glycol lactone

[0035] Add dimethyl oxalate (295g, 2.50mol), ethylene glycol (93g, 1.50mol), 1,2-butanediol (103g, 1.12mol) and stannous octoate (0.3g) to a 1L reaction bottle. React at 80°C under normal pressure for 1.5h; heat to 110°C for 1.5h; heat to 140°C for 3h; heat to 150°C for 1h. At 150°C, gradually reduce the pressure to 10000Pa for 0.5h, and gradually reduce the pressure to 50Pa for 1-2h. Add sodium glycolate (3wt%), maintain the pressure at 50Pa, heat to 160°C for 4h to obtain a mixture of oxalic acid ethylene glycol lactone monomer and 1,2-oxalic acid propylene glycol lactone monomer. The monomer mixture was added dropwise into 1.0 L of cold methanol, and oxalic acid glycol lactone was crystallized in methanol. After filtration, washing and drying, pure oxalic acid glycol lactone monomer was obtained. The methanol phase was concentrated to obtain 1,2-oxalic acid butanediol lactone, with yields of 87% and 88%, respectively.

[0036] Example 4. Preparation of random copolymer of alkyl oxalate

[0037] In a glove box under nitrogen atmosphere, stannous octoate (0.041 g, 0.1 mmol), benzyl alcohol (0.011 g, 0.1 mmol), oxalic acid ethylene glycol lactone (9.28 g, 80 mmol), and oxalic acid propylene glycol lactone (2.60 g, 20 mmol) were added to a dry Schlenk bottle and polymerized at 100°C for 2 h. After the polymerization, the reactant was poured into excess cold methanol and washed three times, and then vacuum dried to obtain poly (oxalic acid ethylene glycol lactone-co-oxalic acid propylene glycol lactone).

[0038] Example 5. Preparation of alkyl oxalate block copolymer

[0039] In a glove box under nitrogen atmosphere, 1,8-diazabicyclo[5.4.0]undec-7-ene (0.030 g, 0.2 mmol), p-phenylenediol (0.028 g, 0.2 mmol) and oxalic acid hexanediol lactone (10.33 g, 60 mmol) were added to a dry Schlenk bottle and polymerized at 25°C for 10 min. Then, a 2M solution of oxalic acid ethylene glycol lactone (9.28 g, 80 mmol) in N-methylpyrrolidone was added and polymerized at 25°C for 15 min. After the polymerization, the reactant was poured into excess cold methanol and washed three times. After vacuum drying, a triblock copolymer was obtained: poly(oxalic acid ethylene glycol lactone-b-oxalic acid ethylene glycol lactone-b-oxalic acid ethylene glycol lactone).

[0040] Example 6. Preparation of alkyl oxalate block copolymer

[0041] In a glove box under nitrogen atmosphere, stannous octoate (0.41 g, 1.0 mmol), pentaerythritol (0.136 g, 1.0 mmol) and oxalic acid butanediol lactone (57.6 g, 0.4 mol) were added to a dry Schlenk bottle and polymerized at 130°C for 0.5 h, and then a 4M solution of 1,3-oxalic acid propanediol lactone (65.0 g, 0.5 mol) in N-methylpyrrolidone was added and polymerized at 150°C for 2 h. After the polymerization, the reactant was poured into excess cold methanol and washed three times, and then vacuum dried to obtain an oxalic acid alkyl ester block copolymer: poly (oxalic acid propanediol lactone-b-oxalic acid butanediol lactone-b-oxalic acid propanediol lactone).

Claims

1. A method for preparing a cyclic oxalic acid alkyl ester, comprising the following steps: (1) polycondensing an oxalate compound with two or more aliphatic diols under the catalysis of a polycondensation catalyst to obtain a low molecular weight oxalate alkyl ester random copolymer; (2) Under certain temperature and pressure, the alkyl oxalate random copolymer is depolymerized in the presence of a depolymerization catalyst to obtain a cyclic alkyl oxalate monomer mixture, which is then separated and purified to obtain a cyclic alkyl oxalate monomer.

2. The method according to claim 1, characterized in that The oxalate compound described in step (1) is dimethyl oxalate, diethyl oxalate, di-n-propyl oxalate, diisopropyl oxalate, di-n-butyl oxalate, di-tert-butyl oxalate, diphenylmethyl oxalate, monomethyl oxalate or monoethyl oxalate.

3. The method according to claim 1, characterized in that At least one of the aliphatic diols described in step (1) is an aliphatic diol with a branched structure.

4. The method according to claim 3, characterized in that The aliphatic diol with a branched structure is 1,2-propylene glycol, 2,3-butanediol, 2,2-butanediol, 1,2-butanediol, 1,2-pentanediol, 1,2-hexanediol, 1,2-cyclohexanediol, 1,2-cyclopentanediol, 1,2-cyclononanediol, 1,2-cyclodecanediol or phenyl-1,2-ethanediol.

5. The method according to claim 1, characterized in that The polycondensation catalyst described in step (1) is selected from stannous octoate, tetrabutyl titanate, tetraisopropyl titanate, zinc acetate, magnesium oxide, magnesium hydroxide, magnesium carbonate, tin chloride, ferric chloride, titanium chloride, zinc chloride, aluminum chloride, zinc oxide, tin oxide, ferric oxide, bismuth trifluoromethanesulfonate, zinc trifluoromethanesulfonate, 4-dimethylaminopyridine, nitrogen heterocyclic carbene, p-toluenesulfonic acid, trifluoromethanesulfonic acid or diphenyl phosphate.

6. The method according to claim 1, characterized in that The depolymerization catalyst described in step (2) is selected from stannous octoate, tin chloride, ferric chloride, titanium chloride, zinc chloride, aluminum chloride, zinc oxide, tin oxide, ferric oxide, zinc acetate, 4-dimethylaminopyridine, p-toluenesulfonic acid, trifluoromethanesulfonic acid, diphenyl phosphate, ionic liquid, sodium glycolate or sodium acetate.

7. The method according to claim 1, characterized in that The temperature in step (2) is 50-200° C., and the pressure is 50-2000 Pa.

8. The method according to claim 1, characterized in that: The dosage of the polycondensation catalyst is 0.0001% to 1.0% of the mass of the oxalate compound; the dosage of the depolymerization catalyst is 0.001% to 5.0% of the mass of the alkyl oxalate random copolymer.

9. The method according to claim 1, wherein the method comprises the following steps: mixing a ring-opening polymerization catalyst and an initiator, adding the cyclic oxalate alkyl ester in a mixed feeding or sequential feeding manner, and reacting at 50 to 250° C. for 10 min to 24 h to obtain a high molecular weight random or block structured alkyl oxalate copolymer.

10. The method according to claim 9, characterized in that The ring-opening polymerization catalyst is selected from one of stannous octoate, dibutyltin dilaurate, tin dioxide, diethylzinc, zinc dichloride, n-butyllithium, isobutyllithium, lithium diisopropylamide, sodium methoxide, sodium tert-butoxide, potassium tert-butoxide, methanesulfonic acid, trifluoromethanesulfonic acid, p-toluenesulfonic acid, boron trifluoride, tris(pentafluorophenyl)boron, triethylboron, triethylamine, pyridine, 4-dimethylaminopyridine, 1,8-diazabicyclo[5.4.0]undec-7-ene, 1,5,7-triazabicyclo[4.4.0]dec-5-ene, hexamethyldisilazane, and phosphazene base; the initiator is one of methanol, ethanol, isopropanol, benzyl alcohol, phenylpropanol, propylene alcohol, ethylene glycol, 1,4-butanediol, terephthalic acid, glycerol, trimethylolpropane, and pentaerythritol.

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