Polymerizable tetramethyl glycolide

Abstract

The present invention relates to a novel process for the manufacture of polymerizable tetramethyl glycolide. In the method, a reaction mixture containing 2-hydroxyisobutyric acid is first reacted to obtain a product mixture containing tetramethyl glycolide. The tetramethyl glycolide is then crystallized and isolated to obtain a polymerizable tetramethyl glycolide. The invention also relates to polymerizable tetramethyl glycolide obtainable by the process according to the invention.

Description

Technical Field

[0001] The present invention relates to a new method for preparing polymerizable tetramethylethyl glycolate. In this method, a reaction mixture containing 2-hydroxyisobutyric acid is first reacted to obtain a product mixture containing tetramethylethyl glycolate. Then it is crystallized and separated to obtain polymerizable tetramethylethyl glycolate. In addition, the present invention relates to polymerizable tetramethylethyl glycolate obtainable by the method of the present invention. Background Art

[0002] Poly(2-hydroxyisobutyric acid) (poly(2-HIBS)) was first synthesized by Blaise and Montagne in 1922 (Blaise, E.E., Montagne M., A. Compte Rendu 174 (1922) 1553). In its preparation method, 2-hydroxyisobutyric acid anhydrosulfite is used as a monomer building unit. Such homopolymers are also disclosed in US 2,811,511.

[0003] Deibig, H., Geiger, J., Sander, M., Makromol. Chem. 145 (1971) 121, Deibig, H., Geiger, J., Sander, M., Makromol. Chem. 145 (1971) 133, Nishida, H., Andou, Y., Watanabe, K, Arazoe, Y., Ide, S., Shirai, Y., Macromol. 44 (2011) 12 and Watanabe, K., Andou, Y., Shirai, Y., Nishida, H., Chem. Lett. 42 (2013) 159 each describe the preparation of poly(2-HIBS) starting from 2-hydroxyisobutyric acid (2-HIBS). In each case, polymerization is carried out by ring-opening polymerization of the cyclic dimer of 2-hydroxyisobutyric acid, tetramethylethyl glycolate (TMG). The resulting poly(2-HIBS) is outstanding in terms of transparency, good mechanical properties, high chemical resistance and low crystallization tendency. In addition, durable flexible films can be produced from it. Poly(2-HIBS) is also completely recyclable.

[0004] In contrast, Hall, H.K., Schneider, A.K., J. Am. Chem. Soc. 80 (1958) 6409, Brandrup, J., Immergut, E.H., Grulke, E.A. (eds.), Polymer Handbook, 4th Edition, Volume 1 (1999) II / 404, Salyasombat, W., Molloy, R., Nicholson, T.M., Johnson, A.F., Ward, I.M., Poshyachinda, B., Polymer, 39 (1998) 5581 and Kricheldorf, H., Lomadze, N., Schwarz, G., J. Polym. Sci. A 46 (2008) 6229 excluded the polymerization of 2-HIBS.

[0005] WO 2008 / 061821 relates to the production of TMG by heating 2-HIBS to a temperature of at least 100 °C. EP 0 834511 A also describes methods for producing cyclic esters by depolymerization of polymers or by cyclization of α-hydroxycarboxylic acids.

[0006] The cyclic esters described in the prior art, in particular TMG, often have insufficient purity to produce polymers therefrom. In addition, the methods for polymerizing cyclic esters and producing polymerizable cyclic esters described in the prior art cannot be reliably reproduced.

[0007] Therefore, there is a need for methods for producing cyclic esters, in particular TMG, with particularly high purity. In particular, the cyclic esters should be pure enough to produce polymers therefrom and the polymerization of the cyclic esters should be reproducible reliably. Summary of the Invention

[0008] Object

[0009] The object on which the present invention is based is to provide methods that allow the preparation of tetramethyl glycolide with particularly high purity. In particular, the tetramethyl glycolide obtainable in this way should be suitable for polymerization, in particular for the production of poly(2-hydroxyisobutyric acid).

[0010] Solution

[0011] The object is achieved by a method for preparing polymerizable tetramethyl glycolide, which method comprises the following steps a) to c):

[0012] a) React a reaction mixture containing 2-hydroxyisobutyric acid in a first reactor at a first temperature of 150 °C to 200 °C and a first pressure of 100 mbar to 1013 mbar, wherein the conversion of 2-hydroxyisobutyric acid in said reaction does not exceed 95%, and obtain a product mixture which contains, in each case based on the total weight of the product mixture,

[0013] 5 to 50% by weight of 2-hydroxyisobutyric acid,

[0014] 25 to 85% by weight of tetramethyglycolide and

[0015] 5 to 25% by weight of di-2-hydroxyisobutyric acid,

[0016] b) Crystallize the tetramethyglycolide contained in the product mixture to obtain crystalline tetramethyglycolide and a product liquid phase,

[0017] c) Separate the crystalline tetramethyglycolide from the product liquid phase in step b) to obtain polymerizable tetramethyglycolide.

[0018] The object is furthermore achieved by polymerizable tetramethyglycolide obtainable by the process according to the invention.

[0019] Surprisingly, the process according to the invention allows the preparation of particularly pure tetramethyglycolide (TMG), which can furthermore be polymerized to poly(2-hydroxyisobutyric acid).

[0020] The process according to the invention in particular allows the preparation of tetramethyglycolide starting from a method for producing acetone cyanohydrin, in particular a method for producing acetone cyanohydrin in which acetone is reacted with hydrogen cyanide to obtain acetone cyanohydrin.

[0021] The poly(2-hydroxyisobutyric acid) obtainable by polymerization of the TMG produced by the process according to the invention is advantageously distinguished by high transparency and complete biodegradability to CO2. Poly(2-hydroxyisobutyric acid) can furthermore be processed by conventional methods such as injection molding, casting methods, extrusion methods or methods for producing fibers or films, and can be cleaved into its starting materials 2-hydroxyisobutyric acid and TMG.

[0022] Poly(2-hydroxyisobutyric acid) is furthermore distinguished by a higher hydrolysis resistance and glass transition temperature compared to poly(lactic acid), which enables processing and use in fields where poly(lactic acid) cannot be used.

[0023] The process according to the invention is explained in more detail below.

[0024] In step a) of the process according to the invention, the reaction mixture is reacted in a first reactor at a first temperature and a first pressure to obtain a product mixture.

[0025] The reaction mixture contains 2-hydroxyisobutyric acid. 2-Hydroxyisobutyric acid is known per se and has the CAS number 594-61-6. It is also referred to as 2-hydroxy-2-methylpropionic acid or 2-HIBS. Therefore, the terms "2-hydroxyisobutyric acid" and "2-HIBS" are used synonymously hereinafter and have the same meaning.

[0026] The reaction mixture may further contain at least one additional component. The at least one additional component is, for example, selected from tetramethyglycolide, di-2-hydroxyisobutyric acid, and oligomeric 2-hydroxyisobutyric acid.

[0027] Therefore, a method is also preferred, in which the reaction mixture additionally contains at least one additional component selected from tetramethyglycolide, di-2-hydroxyisobutyric acid, and oligomeric 2-hydroxyisobutyric acid.

[0028] For example, if in a preferred embodiment of the present invention, the first overhead stream obtained in step a1) is recycled to step a) and / or the product liquid phase obtained in step c) is recycled to step a), the reaction mixture contains the at least one additional component.

[0029] The reaction mixture is preferably obtainable by a method for preparing acetone cyanohydrin by reacting acetone with hydrocyanic acid.

[0030] Therefore, a method is also preferred, in which the reaction mixture for the reaction in step a) is obtainable by a method for preparing acetone cyanohydrin by reacting acetone with hydrocyanic acid.

[0031] The method for preparing acetone cyanohydrin by reacting acetone with hydrocyanic acid is known per se.

[0032] In this method, acetone typically reacts with hydrocyanic acid to obtain acetone cyanohydrin. It can further react with sulfuric acid to form sulfoxy-α-hydroxyisobutyramide. The latter is also referred to as SIBA. Then, for example, at a temperature of 90 °C to 120 °C and in the presence of water, SIBA can be converted into 2-hydroxyisobutyric acid and ammonium sulfate. This method is known per se.

[0033] The 2-hydroxyisobutyric acid thus obtained can be separated from ammonium sulfate, for example, by distillation and / or extraction, and then reacted as the reaction mixture in step a). The reaction mixture obtainable in this way typically also contains a residue of ammonium sulfate.

[0034] In particular, if the reaction mixture is obtainable by a method for preparing acetone cyanohydrin by reacting acetone with hydrocyanic acid, the reaction mixture thus additionally contains ammonium sulfate.

[0035] In the reaction, the conversion rate of 2-hydroxyisobutyric acid does not exceed 95%, preferably 50% to 90%, more preferably 60% to 85% and most preferably 70% to 80%. In the context of the present invention, the conversion rate of 2-hydroxyisobutyric acid is understood as the ratio of the amount of 2-hydroxyisobutyric acid that has reacted to the amount of 2-hydroxyisobutyric acid contained in the reaction mixture before the start of the reaction. It can be determined by methods known to those skilled in the art, such as by HPLC or GC.

[0036] Therefore, the product mixture contains 5 to 50% by weight of 2-hydroxyisobutyric acid, preferably 5 to 10% by weight, in each case based on the total weight of the product mixture.

[0037] In the reaction in step a), 2-HIBS dimerizes. First, di-2-hydroxyisobutyric acid described below is formed by condensation. Then, tetramethyl glycolide is formed from di-2-hydroxyisobutyric acid by further condensation.

[0038] Tetramethyl glycolide is itself known and is also called 3,3,6,6-tetramethyl-1,4-dioxane-2,5-dione. It is also known by the name TMG. Therefore, within the scope of the present invention, the terms "tetramethyl glycolide" and "TMG" are used synonymously and have the same meaning. TMG is the cyclic dimer of 2-HIBS.

[0039] The product mixture contains 25 to 85% by weight of TMG, preferably 80 to 85% by weight, in each case based on the total weight of the product mixture.

[0040] In the dimerization of 2-HIBS, di-2-hydroxyisobutyric acid described above is formed. Within the scope of the present invention, di-2-hydroxyisobutyric acid is understood as 2-((2-hydroxy-2-methylpropanoyl)oxy)-2-methylpropanoic acid. Di-2-hydroxyisobutyric acid is itself known and has the CAS number 87422-65-9. Di-2-hydroxyisobutyric acid is also called di-2-HIBS. Therefore, within the scope of the present invention, the terms "di-2-hydroxyisobutyric acid", "2-((2-hydroxy-2-methylpropanoyl)oxy)-2-methylpropanoic acid" and "di-2-HIBS" are used synonymously and thus have the same meaning.

[0041] The product mixture contains 5 to 25% by weight of di-2-HIBS, preferably 10 to 20% by weight, in each case based on the total weight of the product mixture.

[0042] Furthermore, during the reaction of the reaction mixture, 2-HIBS is usually partially oligomerized. In the oligomerization of 2-HIBS, trimers, tetramers, and pentamers of 2-HIBS are formed, for example. Oligomerized 2-HIBS within the scope of the present invention is also referred to as "oligomeric 2-hydroxyisobutyric acid".

[0043] Thus, the product mixture may contain, for example, 0.1 to 10% by weight, preferably 0.5 to 5% by weight, of oligomeric 2-hydroxyisobutyric acid in each case based on the total weight of the product mixture.

[0044] The dimerization and oligomerization of 2-HIBS are condensation reactions. Thus, water is additionally formed during the dimerization and oligomerization of 2-HIBS. Therefore, the product mixture usually additionally contains water.

[0045] The water formed during the reaction can be at least partially removed from the first reactor during the reaction, for example, by the column included in the first reactor in the preferred embodiment of the present invention. Removing the formed water is advantageous because it shifts the reaction equilibrium from 2-HIBS towards TMG.

[0046] For example, the product mixture obtained in step a) additionally contains 0.001 to 0.5% by weight of water, preferably 0.005 to 0.2% by weight of water, based on the total weight of the product mixture.

[0047] If the reaction mixture additionally contains ammonium sulfate and / or other nitrogen-containing compounds, then, for example, 1,3-disubstituted diketones of morpholine substituted by four methyl groups, and / or 1,4-disubstituted diketones of morpholine substituted by four methyl groups are additionally formed during the reaction of the reaction mixture in step a). Thus, the product mixture may additionally contain 1,3-disubstituted diketones of morpholine substituted by four methyl groups, and / or 1,4-disubstituted diketones of morpholine substituted by four methyl groups.

[0048] During the reaction of the reaction mixture in step a), tetramethyl glycolate that may be present in the reaction mixture and / or tetramethyl glycolate formed during the reaction may also be at least partially cleaved. Methacrylic acid, acetone, and carbon monoxide are formed here. Carbon monoxide is usually removed as a gas from the reaction in step a). Thus, the product mixture may additionally contain methacrylic acid and / or acetone.

[0049] The reaction in step a) can occur for any period of time. Preferably, the reaction time of the reaction in step a) is 1 hour to 24 hours, particularly preferably 6 hours to 9 hours.

[0050] Thus, a method is also preferred, wherein the reaction time of the reaction in step a) is 1 hour to 24 hours.

[0051] The "reaction time" is understood as the time period from reaching the first temperature until the conversion rate of 2-hydroxyisobutyric acid is reached.

[0052] The reaction in step a) can occur in batch mode, and its continuous occurrence is also feasible. In the case where the reaction in step a) occurs continuously, the reaction time is understood as the average residence time of the reaction mixture in the first reactor.

[0053] The reaction can occur in the presence of a catalyst, preferably in the presence of an acidic catalyst. Acidic catalysts suitable for catalyzing the reaction in step a) are known per se and are selected, for example, from inorganic acids, organic acids, and acidic cation exchangers. Suitable inorganic acids are, for example, sulfuric acid or hydrochloric acid. Suitable organic acids are, for example, sulfonic acids, such as p-toluenesulfonic acid. In addition, for example, sulfonated aromatic compounds embedded in a polymer matrix can be used as strongly acidic ion exchangers. An example of such a sulfonated aromatic compound embedded in a polymer matrix is sulfonated polystyrene in a polystyrene matrix.

[0054] Particularly preferably, 2-hydroxyisobutyric acid itself acts as an acidic catalyst in the reaction. Preferably, the reaction proceeds autocatalytically in this way.

[0055] Therefore, a method is also preferred in which the reaction in step a) proceeds autocatalytically.

[0056] The reaction occurs at a first temperature. The first temperature is from 150 °C to 200 °C, preferably from 170 °C to 190 °C.

[0057] The first temperature refers to the temperature inside the first reactor. It is clear to those skilled in the art that if the first reactor in the preferred embodiment includes a column, the temperature in the column can be lower than the first temperature.

[0058] The reaction occurs at a first pressure. The first pressure is from 100 mbar to 1013 mbar, preferably from 200 mbar to 400 mbar.

[0059] The reaction in step a) occurs in a first reactor. Reactors suitable as the first reactor are those known to those skilled in the art for cyclization, such as stirred tank reactors. A stirred tank cascade can also be used as the first reactor. This is particularly advantageous if the method of the present invention is to be carried out continuously. Preferably, stainless steel or enamel is used as the reactor material.

[0060] Preferably, the first reactor includes at least one column, particularly preferably at least one distillation column and / or at least one rectification column.

[0061] Therefore, a method is also preferred in which the first reactor includes at least one column.

[0062] Preferably, water formed in the reaction is at least partially removed via a column. The water can be at least partially removed as an azeotrope with 2-HIBS. Similarly, for example, acetone formed in the cleavage of tetramethyl glycolate can be removed via a column.

[0063] In step b) of the process according to the invention, the tetramethyl glycolate contained in the product mixture is crystallized to obtain crystalline tetramethyl glycolate and a product liquid phase.

[0064] The TMG contained in the product mixture can be crystallized according to methods known to those skilled in the art. Preferably, the product mixture is cooled to a second temperature to crystallize the tetramethyl glycolate. The second temperature is, for example, 50 °C to 80 °C. Preferably, the crystallization in step b) takes place at the second temperature of 50 °C to 80 °C.

[0065] Therefore, a method is also preferred in which the crystallization in step b) takes place at a second temperature of 50 °C to 80 °C.

[0066] It is possible to effect the crystallization using a first solvent. For example, a first solvent can be added to the product mixture and thereby the TMG can be crystallized. If a first solvent is used to effect the crystallization, the product mixture can be additionally cooled to crystallize. Suitable first solvents are, for example, solvents selected from isopropanol, methyl isobutyl ketone, toluene, pentane, hexane, substituted hexane derivatives and cyclohexane derivatives.

[0067] Preferably, the crystallization is carried out without adding a first solvent to the product mixture.

[0068] Therefore, it is preferred to crystallize the tetramethyl glycolate from the melt of the product mixture.

[0069] Particularly preferably, the tetramethyl glycolate is fractionally crystallized in step b). The method of fractional crystallization is known per se.

[0070] In step c) of the process according to the invention, the tetramethyl glycolate crystallized in step b) is separated from the product liquid phase to obtain polymerizable tetramethyl glycolate.

[0071] The tetramethyl glycolate crystallized in step b) can be separated from the product liquid phase by methods known to those skilled in the art, for example by filtration or centrifugation. The crystallized tetramethyl glycolate can be additionally washed to separate it from the product liquid phase. For example, the crystallized tetramethyl glycolate can first be filtered and then washed. Suitable for washing is a second solvent in which tetramethyl glycolate is sparingly soluble and known to those skilled in the art. Such solvents are, for example, 2-propanol, methyl isobutyl ketone and / or ethanol.

[0072] In particular, if in step b) tetramethyl glycolide is fractionally crystallized, it is preferred that after separating the product liquid phase in step c), the crystallized tetramethyl glycolide is partially melted. Thereby impurities are removed from the crystallized tetramethyl glycolide, such that polymerizable tetramethyl glycolide is then obtained.

[0073] When the crystallized tetramethyl glycolide is separated from the product liquid phase, polymerizable tetramethyl glycolide is obtained. In addition, the product liquid phase is obtained. The product liquid phase generally contains the components that are contained in the product mixture and do not crystallize with tetramethyl glycolide in step b).

[0074] For example, the product liquid phase contains at least one component selected from 2-hydroxyisobutyric acid, tetramethyl glycolide, di-2-hydroxyisobutyric acid, oligomeric 2-hydroxyisobutyric acid, and water.

[0075] If tetramethyl glycolide is crystallized in step b) by adding a first solvent, the product liquid phase generally additionally contains this first solvent.

[0076] Preferably, the product liquid phase obtained in step c) is recycled to the reaction mixture in step a).

[0077] Therefore, a method is also preferred, in which the product liquid phase obtained in step c) is recycled to step a).

[0078] If the product liquid phase additionally contains the first solvent, it is preferred to separate and remove this solvent before recycling the product liquid phase to the reaction mixture.

[0079] The method for separating and removing the first solvent from the product liquid phase is known per se. For example, the first solvent can be separated and removed by distillation, and separation by phase separation is also feasible. Preferably, the first solvent is separated and removed by distillation.

[0080] If the product mixture in the above-described embodiments contains oligomeric 2-hydroxyisobutyric acid, the product liquid phase obtained in step c) also generally contains oligomeric 2-hydroxyisobutyric acid. It is preferred to separate oligomeric 2-hydroxyisobutyric acid from the product liquid phase before recycling the product liquid phase to step a).

[0081] For example, oligomeric 2-hydroxyisobutyric acid is separated from the product liquid phase by adding a catalyst to the product liquid phase. The catalyst is preferably a tin(IV) oxide catalyst. Dioctyltin oxide is particularly suitable as the tin(IV) oxide catalyst. Dioctyltin oxide is known per se and has the CAS number 870-08-6. It is also referred to as DOTO.

[0082] The oligomeric 2-hydroxyisobutyric acid is split into shorter oligomers of 2-hydroxyisobutyric acid, 2-hydroxyisobutyric acid, di-2-hydroxyisobutyric acid and / or tetramethyl glycolide by means of a catalyst. The shorter oligomers of 2-hydroxyisobutyric acid formed in the splitting are generally further split by means of a catalyst to form 2-hydroxyisobutyric acid, di-2-hydroxyisobutyric acid and / or tetramethyl glycolide. The TMG formed in the splitting can be distilled off from the product liquid phase, and then the product liquid phase from which the oligomeric 2-hydroxyisobutyric acid has been separated can be recycled to step a).

[0083] Accordingly, preference is also given to a process in which the product mixture obtained in step a) additionally contains oligomeric 2-hydroxyisobutyric acid and is split in the presence of a tin(IV) oxide catalyst and recycled to step a).

[0084] Preferably, in the process according to the invention, the following step a1) is carried out after step a) and before step b):

[0085] a1) Distilling the product mixture obtained in step a) at a second pressure of from 100 mbar to 500 mbar to obtain a first bottoms stream and a first tops stream, where the first bottoms stream contains, in each case based on the total weight of the first bottoms stream,

[0086] at least 80% by weight of TMG,

[0087] at most 1% by weight of 2-HIBS,

[0088] at most 2% by weight of di-2-hydroxyisobutyric acid and

[0089] at least 5% by weight of oligomeric 2-hydroxyisobutyric acid,

[0090] and the first tops stream contains, in each case based on the total weight of the first tops stream,

[0091] 0.1 to 5% by weight of di-2-hydroxyisobutyric acid,

[0092] 10 to 50% by weight of 2-hydroxyisobutyric acid and

[0093] 40 to 85% by weight of tetramethyl glycolide,

[0094] and then crystallizing the tetramethyl glycolide contained in the first bottoms stream in step b).

[0095] Accordingly, preference is also given to a process in which the following step a1) is carried out after step a) and before step b)

[0096] a1) Distill the product mixture obtained in step a) at a second pressure of 100 mbar to 500 mbar to obtain a first bottoms stream and a first tops stream, wherein the first bottoms stream contains, in each case based on the total weight of the first bottoms stream,

[0097] at least 80% by weight of tetramethyl glycolide,

[0098] at most 1% by weight of 2-hydroxyisobutyric acid,

[0099] at most 2% by weight of di-2-hydroxyisobutyric acid and

[0100] at least 5% by weight of oligomeric 2-hydroxyisobutyric acid,

[0101] and wherein the first tops stream contains, in each case based on the total weight of the first tops stream,

[0102] 0.1 to 5% by weight of di-2-hydroxyisobutyric acid,

[0103] 10 to 50% by weight of 2-hydroxyisobutyric acid and

[0104] 40 to 85% by weight of tetramethyl glycolide,

[0105] and then crystallize the tetramethyl glycolide contained in the first bottoms stream in step b).

[0106] If step a1) is carried out, then crystallize the tetramethyl glycolide contained in the first bottoms stream in step b). Thus, step b) of the process of the invention is:

[0107] b) Crystallize the tetramethyl glycolide contained in the first bottoms stream to obtain crystalline tetramethyl glycolide and a product liquid phase.

[0108] Step a1) is preferably carried out in a second column. Preferably, the second column is different from the column preferably included in the first reactor in step a). Preferably, the product mixture is continuously fed to the second column and the first bottoms stream and the first tops stream are continuously withdrawn.

[0109] The distillation in step a1) is preferably carried out at a second temperature. The second temperature refers to the bottom temperature during distillation and is preferably 150 °C to 200 °C, particularly preferably 160 °C to 190 °C.

[0110] During the distillation in step a1), TMG is usually partially split, and acetone, methacrylic acid and carbon monoxide are formed hereby. Thus, the first tops stream obtained in the distillation usually additionally contains methacrylic acid and / or acetone.

[0111] If the product mixture additionally contains methacrylic acid and / or acetone, the first overhead stream obtained in the distillation generally also additionally contains methacrylic acid and / or acetone.

[0112] The first overhead stream can be at least partially recycled to the reaction mixture. Preferably, the first overhead stream obtained in step a1) is recycled to step a).

[0113] Accordingly, a method is also preferred in which the first overhead stream obtained in step a1) is recycled to step a).

[0114] In step c), polymerizable tetramethylethyl glycolate is obtained.

[0115] Accordingly, the subject of the present invention is also polymerizable tetramethylethyl glycolate obtainable by the process of the present invention.

[0116] Generally, the polymerizable tetramethylethyl glycolate obtained in the process of the present invention additionally contains by-products.

[0117] The by-products preferably comprise, in each case based on the total weight of the polymerizable tetramethylethyl glycolate,

[0118] from 10 ppm by weight to 1000 ppm by weight of 2-hydroxyisobutyric acid,

[0119] from 10 ppm by weight to 1000 ppm by weight of water and

[0120] from 10 ppm by weight to 5000 ppm by weight of di-2-hydroxyisobutyric acid.

[0121] If the product mixture and / or the first bottom stream (from which the polymerizable tetramethylethyl glycolate is crystallized in step b) additionally contains methacrylic acid, the by-products contained in the polymerizable tetramethylethyl glycolate generally also contain methacrylic acid.

[0122] For example, the by-products additionally contain from 1 ppm by weight to 1000 ppm by weight of methacrylic acid based on the total weight of the polymerizable tetramethylethyl glycolate.

[0123] Accordingly, it is also preferred that such polymerizable tetramethylethyl glycolate, in which the by-products additionally contain from 1 ppm by weight to 1000 ppm by weight of methacrylic acid based on the total weight of the polymerizable tetramethylethyl glycolate.

[0124] If the product mixture and / or the first bottom stream (from which the polymerizable tetramethylethyl glycolate is crystallized in step b) additionally contains oligomeric 2-hydroxyisobutyric acid, the by-products contained in the polymerizable tetramethylethyl glycolate generally also contain oligomeric 2-hydroxyisobutyric acid.

[0125] For example, the by - product additionally contains from 1 ppm by weight to 5000 ppm by weight of oligomeric 2 - hydroxyisobutyric acid based on the total weight of the polymerizable tetramethylethyl glycolate.

[0126] Therefore, it is also preferred that such a polymerizable tetramethylethyl glycolate, wherein the by - product additionally contains from 1 ppm by weight to 5000 ppm by weight of oligomeric 2 - hydroxyisobutyric acid based on the total weight of the polymerizable tetramethylethyl glycolate.

[0127] If the product mixture and / or the first bottoms stream (from which the polymerizable tetramethylethyl glycolate is crystallized in step b) additionally contains at least one morpholine dione substituted by four methyl groups, the by - products contained in the polymerizable tetramethylethyl glycolate generally also additionally contain at least one morpholine dione substituted by four methyl groups.

[0128] For example, the by - product additionally contains from 1 ppm by weight to 500 ppm by weight of at least one morpholine dione substituted by four methyl groups based on the total weight of the polymerizable tetramethylethyl glycolate.

[0129] Therefore, it is also preferred that such a polymerizable tetramethylethyl glycolate, wherein the by - product additionally contains from 1 ppm by weight to 500 ppm by weight of at least one morpholine dione substituted by four methyl groups based on the total weight of the polymerizable tetramethylethyl glycolate.

[0130] It is understood that the ppm by weight of the by - product is based on the total weight of the polymerizable tetramethylethyl glycolate, including the additionally contained by - products.

[0131] The ratio of the total molar amount of all acidic (azid) protons of the by - product to the molar amount of the pure polymerizable tetramethylethyl glycolate is preferably from 0.05 to 0.0001.

[0132] Therefore, it is also preferred that such a polymerizable tetramethylethyl glycolate, wherein the polymerizable tetramethylethyl glycolate additionally contains a by - product, wherein the by - product contains

[0133] from 10 ppm by weight to 1000 ppm by weight of 2 - hydroxyisobutyric acid,

[0134] from 10 ppm by weight to 1000 ppm by weight of water and

[0135] from 10 ppm by weight to 5000 ppm by weight of di - 2 - hydroxyisobutyric acid, and

[0136] The ratio of the total molar amount of all acidic protons of the by-products to the molar amount of polymerizable tetramethyl glycolide is from 0.05 to 0.0001.

[0137] The acidic protons of the by-products are understood to be all OH-, NH-, PH- and SH-acidic protons of the by-products, preferably all OH- and NH-acidic protons of the by-products.

[0138] In an alternative embodiment of the process according to the invention, in addition to or instead of steps b) and c), preferably instead of them, the following steps b1) and b2) are carried out:

[0139] b1) Distilling the product mixture obtained in step a) to obtain a distillate containing tetramethyl glycolide,

[0140] b2) Washing the distillate containing tetramethyl glycolide obtained in step b1) to obtain polymerizable tetramethyl glycolide.

[0141] The distillation in step b1) can be carried out by methods known to those skilled in the art. It is preferably carried out in at least two stages. Generally, in this case, first, components having a lower boiling point than tetramethyl glycolide are separated off from the product mixture in a first distillation step. In a second distillation step, the tetramethyl glycolide is then separated off as an overhead stream to obtain a distillate containing tetramethyl glycolide.

[0142] The distillate containing tetramethyl glycolide contains tetramethyl glycolide. In addition, it generally also contains by-products formed from tetramethyl glycolide during distillation, such as acetone and / or methacrylic acid. In addition, it can contain, for example, residues of components contained in the product mixture, in particular a dioxo-morpholine substituted by four methyl groups.

[0143] Therefore, in step b2), the distillate containing tetramethyl glycolide is washed. The washing can be carried out by methods known to those skilled in the art, for example using water, preferably using alkaline water. This allows the separation of the by-products to obtain polymerizable tetramethyl glycolide. It is possible that in step b2), the distillate containing tetramethyl glycolide is first washed as described above and then purified by adsorption and / or absorption to obtain polymerizable tetramethyl glycolide. The adsorption and / or absorption can be carried out, for example, on silica, alumina and / or ion exchangers. These methods are known per se to those skilled in the art. Adsorption and / or absorption is particularly advantageous for removing at least one dioxo-morpholine substituted by four methyl groups that may be contained in the distillate containing tetramethyl glycolide. Detailed description

[0144] Examples

[0145] Polymerization of different TMG batches

[0146] The polymerization carried out here is suitable for testing the TMG batches produced. It determines whether it is possible in principle to use TMG for polymerization.

[0147] First, the amount of TMG to be polymerized (usually 30 g) is weighed into a conical flask with a magnetic stir bar. Then the conical flask is conditioned in a vacuum drying oven at 0 mbar to 130 °C for one hour. Here, the TMG melts and residual moisture from the TMG or the glass surface of the flask is removed. After one hour, the drying oven is vented, the flask is sealed with a rubber stopper, the flask is suspended in an oil bath preheated to 130 °C, and mixed with a magnetic stirrer. Then a small tube is pierced through the rubber stopper and the flask is flushed with argon. Then a catalyst of 0.15 mol% of lithium tert-butoxide (as t-BuOLi dissolved in THF, pure substance t-BuOLi based on the initial weight of TMG) is added via the small tube using a syringe (the amount required for the desired molar mass). If the material is suitable for polymerization, a viscosity at which the magnetic stir bar stops is reached after about 5 to 15 min. Then the polymerization is continued for another 6 - 8 h.

[0148] The purity and polymerizability of TMG batches produced similar to Example 17 and with a conversion of 2-hydroxyisobutyric acid less than 95% are reported in Table 1.

[0149] Table 1

[0150]

[0151] Example B17: Preparation of TMG from 2-HIBS:

[0152] In a stirred 5 L jacketed glass vessel (first reactor), a reaction mixture of the following composition (the deficit up to 100 wt% is caused on the one hand by the measurement accuracy of the HPLC and / or GC used and on the other hand by undetected secondary components) is introduced in advance:

[0153] 2-HIBS 79.3 wt%

[0154] 2-DiHIBS 16.2 wt%

[0155] TMG 1.64 wt%

[0156] The container is equipped with a column having three SULZER EX packings made of Hastelloy at the lower part (with glass Raschig rings above it). The container is heated with hot oil. The container and the column are evacuated to 300 mbar (absolute) and heated to an internal temperature of 165 to 170 °C. Over a period of 9 h (hours), water is drawn off via the top and samples are taken from both the first reactor and the distillate at 0 h (hours), 1 h, 2 h, 4 h, 6 h, 8 h, and 9 h. After 9 h, the reactor contains 2619 g of a product mixture (hereinafter referred to as crude TMG), 442.7 g is collected in the distillate, and at the end of the experiment, 17.8 g is taken out at once from the upstream cold trap of the vacuum. The composition of the reaction mixture and the samples during the reaction in the first reactor are shown in Table 2 (the deficiency up to 100 wt% is caused by the measurement accuracy of the analytical method used; KF: Karl Fischer titration).

[0157] Table 2

[0158]

[0159] The composition of the distillate and the cold trap at the end of the experiment are shown in Table 3 (the deficiency up to 100 wt% is caused by the measurement accuracy of the analytical method used; KF: Karl Fischer titration).

[0160] Table 3

[0161]

[0162] Although the conversion of the reaction continuously increases, the selectivity (by means of the distillate, which only still contains 89% water and MAS instead of >95%) starts to decline after 8 h or at about 75% conversion (based on 2-HIBS). Therefore, it is appropriate not to push the reaction to complete conversion but to terminate it earlier.

[0163] The following Comparative Examples V18, V19, and V20 give an overall evaluation for the crystallization of TMG from the mother liquor (Comparative Example 18), distillation of TMG (Comparative Example 19), and extraction of TMG (Comparative Example 20) (as also done above for the respective TMG batches). The deficiency up to 100 wt% in the following comparative examples is caused by the measurement accuracy of the analytical method used.

[0164] Comparative Example V18: Crystallization of TMG from mother liquor

[0165] After the synthesis based on Example B17 was completed, the vacuum was removed and the crude TMG was cooled overnight to about 58 °C. Pure TMG crystallized out, leaving a product liquid phase consisting of TMG, 2-HIBS, 2-DiHIBS, and MAS. This product liquid phase can be returned to the reaction. Then, the contents of the vessel were suction filtered through a filter heated to about 58 °C using a vacuum pump and divided into two fractions. The composition of the filter cake was as follows:

[0166]

[0167] The filtrate (product liquid phase) removed by suction filtration had the following composition:

[0168]

[0169] The filter cake was then washed with 300 g of 2-propanol: The composition after washing was:

[0170]

[0171]

[0172] The crystals were then dried under vacuum (<50 mbar) at room temperature for 48 h until a constant mass was obtained. After drying, a total of 517 g of TMG with the following composition was obtained:

[0173]

[0174] Comparative Example V19: Distillation of TMG

[0175] The distillation purification of the crude TMG based on Example B17 was carried out on a DN50 column 2 m in length, equipped with SULZER EX packing made of Hastelloy. The bottom material of the column was heated to an internal temperature of 189 °C; the column was heated to about 183 to 185 °C. The distillation was run at a reflux ratio of 3:1. The compositions of the input and output streams were:

[0176] Feed: 480 g / h

[0177]

[0178] Bottom material: 195 g / h

[0179]

[0180] Residue (oligomer)

[0181] Top material: 285 g / h

[0182]

[0183] The top product can be returned to the synthesis.

[0184] Comparative Example V20: Extraction of TMG

[0185] Initially, the crude TMG from the previous example with the following composition was dissolved in toluene at a mass ratio of 1:1.75:

[0186] 2-HIBS 0.4 wt%

[0187] Di-2-HIBS 7.4 wt%

[0188] TMG 92.4 wt%

[0189] Any proportion of insoluble solids was filtered off. Then the toluene solution was dried on a rotary evaporator. The resulting crystals were separated and a portion was washed again with a small amount of petroleum ether to remove the remaining toluene. Then the crystals were dried in a vacuum drying oven at 50 °C and 0 mbar until the mass was constant.

Claims

1. A process for preparing polymerizable tetramethyglycolide, comprising the following steps a) to c): a) Reacting a reaction mixture containing 2-hydroxyisobutyric acid in a first reactor at a first temperature of 150 °C to 200 °C and a first pressure of 100 mbar to 1013 mbar, wherein the conversion of 2-hydroxyisobutyric acid in the reaction does not exceed 95%, and obtaining a product mixture which contains, in each case based on the total weight of the product mixture, 5 to 50% by weight of 2-hydroxyisobutyric acid, 25 to 85% by weight of tetramethyglycolide and 5 to 25% by weight of di-2-hydroxyisobutyric acid, b) Crystallizing the tetramethyglycolide contained in the product mixture to obtain crystalline tetramethyglycolide and a product liquid phase, c) Separating the tetramethyglycolide crystallized in step b) from the product liquid phase to obtain polymerizable tetramethyglycolide.

2. The method according to claim 1, wherein The reaction time of the reaction in step a) is 1 hour to 24 hours.

3. The method according to claim 1 or 2, characterized in that The first reactor comprises at least one column.

4. The method according to any one of claims 1 to 3, characterized in that The following step a1) is carried out after step a) and before step b): a1) Distilling the product mixture obtained in step a) at a second pressure of 100 mbar to 500 mbar to obtain a first bottom stream and a first top stream, wherein the first bottom stream contains, in each case based on the total weight of the first bottom stream, at least 80% by weight of tetramethyglycolide, at most 1% by weight of 2-hydroxyisobutyric acid, at most 2% by weight of di-2-hydroxyisobutyric acid and at least 5% by weight of oligomeric 2-hydroxyisobutyric acid, and wherein the first top stream contains, in each case based on the total weight of the first top stream, 0.1 to 5% by weight of di-2-hydroxyisobutyric acid, 10 to 50% by weight of 2-hydroxyisobutyric acid and 40 to 85% by weight of tetramethyglycolide, and then crystallizing the tetramethyglycolide contained in the first bottom stream in step b).

5. The method according to any one of claims 1 to 4, characterized in that The reaction in step a) proceeds autocatalytically.

6. The method according to any one of claims 1 to 5, characterized in that The reaction mixture additionally contains at least one further component selected from tetramethyglycolide, di-2-hydroxyisobutyric acid and oligomeric 2-hydroxyisobutyric acid.

7. The method according to any one of claims 1 to 6, characterized in that The crystallization in step b) takes place at a second temperature of 50 °C to 80 °C.

8. The method according to any one of claims 1 to 7, characterized in that The product liquid phase obtained in step c) is recycled to step a).

9. The method according to any one of claims 1 to 8, characterized in that The product mixture obtained in step a) additionally contains oligomeric 2-hydroxyisobutyric acid and it is cleaved in the presence of a tin(IV) oxide catalyst and recycled to step a).

10. The method according to any one of claims 1 to 9, characterized in that The reaction mixture in step a) can be obtained by a process for preparing acetone cyanohydrin by reacting acetone with hydrocyanic acid.

11. Polymerizable tetramethyglycolide obtainable by the process according to any one of claims 1 to 10.

12. The polymerizable tetramethyl glycolide according to claim 11, characterized in that The polymerizable tetramethyglycolide additionally contains by-products, wherein the by-products contain, based on the total weight of the polymerizable tetramethyglycolide, from 10 ppm by weight to 1000 ppm by weight of 2-hydroxyisobutyric acid, from 10 ppm by weight to 1000 ppm by weight of water and from 10 ppm by weight to 5000 ppm by weight of di-2-hydroxyisobutyric acid, and The ratio of the molar sum of all acidic protons of the by-product to the molar amount of the polymerizable tetramethylethyl glycolate is from 0.05 to 0.0001.

13. The polymerizable tetramethyglycolide according to claim 12, characterized in that The by-product additionally contains from 1 ppm by weight to 1000 ppm by weight of methacrylic acid, based on the total weight of the polymerizable tetramethylethyl glycolate.

14. The polymerizable tetramethyglycolide according to claim 12 or 13, characterized in that The by-product additionally contains from 1 ppm by weight to 5000 ppm by weight of oligomeric 2-hydroxyisobutyric acid, based on the total weight of the polymerizable tetramethylethyl glycolate.

15. The polymerizable tetramethyglycolide according to any one of claims 12 to 14, characterized in that The by-product additionally contains from 1 ppm by weight to 500 ppm by weight of at least one morpholine dione substituted with four methyl groups, based on the total weight of the polymerizable tetramethylethyl glycolate.

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