POLYMERISIERBARES TETRAMETHYLGLYCOLID

Description

Field of the invention

[0001] The present invention relates to a novel process for producing polymerizable tetramethyl glycolide. In this process, a reaction mixture containing 2-hydroxyisobutyric acid is first reacted to obtain a product mixture containing tetramethyl glycolide. This product mixture is then crystallized and separated to obtain the polymerizable tetramethyl glycolide. Furthermore, the present invention relates to the polymerizable tetramethyl glycolide obtainable by the process according to the invention. State of the art

[0002] Poly(2-hydroxyisobutyric acid) (poly(2-HIBS)) was first synthesized in 1922 by Blaise and Montagne (Blaise, EE, Montagne M., A. Compte Rendu 174 (1922) 1553). In their preparation process, 2-hydroxyisobutyric acid anhydrosulfite was used as the monomer building block. A similar homopolymer is also disclosed in US Pat. No. 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). The polymerization in each case occurs by ring-opening polymerization of tetramethylglycolide (TMG), the cyclic dimer of 2-hydroxyisobutyric acid. The resulting poly(2-HIBS) is characterized by transparency, good mechanical properties, high chemical resistance, and a low tendency to crystallize. Furthermore, it can be used to produce permanently flexible films. Poly(2-HIBS) is also fully recyclable.

[0004] In contrast, Hall, HK, Schneider, AK, J. Am. Chem. Soc 80 (1958) 6409, Brandrup, J., Immergut, EH, Grulke, EA, (Editors) Polymer Handbook, 4th Edition, Volume 1 (1999) II / 404, Salyasombat, W., Molloy, R., Nicholson, TM, Johnson, AF, Ward, IM, Poshyachinda, B., Polymer, 39 (1998) 5581, and Kricheldorf, H., Lomadze, N., Schwarz, G., J. Polym. Sci. A 46 (2008) 6229 rule out a polymerization of 2-HIBS.

[0005] WO 2008 / 061821 relates to the preparation of TMG by heating 2-HIBS to a temperature of at least 100 °C. Likewise, EP 0 834 511 A describes a process for preparing cyclic esters by depolymerization of polymers or by cyclization of α-hydroxycarboxylic acids.

[0006] The cyclic esters described in the prior art, especially TMG, often have insufficient purity to produce polymers from them. Furthermore, the processes described in the prior art for polymerizing cyclic esters and for producing polymerizable cyclic esters are not reliably reproducible. US 2010 / 010276 A1 discloses a process for producing tetramethylglycolide, comprising heating a composition comprising at least 50 wt. % 2-hydroxyisobutyric acid and tetramethylglycolide to a temperature of at least 150°C at a pressure of 0.1 to 0.4 bar, wherein the reaction is carried out autocatalytically. WO 95 / 09142 A1 relates to a process for producing cyclic esters by converting hydroxycarboxylic acids and their derivatives into their respective cyclic esters and discloses the preparation of the cyclic diester of α-hydroxyisobutyric acid (TMG).

[0007] There is therefore a need for processes to produce cyclic esters, especially TMG, with particularly high purity. In particular, the cyclic esters should be sufficiently pure to produce polymers from them, and the polymerization of the cyclic esters should be reliably reproducible. Task

[0008] The object underlying the present invention was to provide a process that allows the production of tetramethyl glycolide with particularly high purity. In particular, the tetramethyl glycolide thus obtainable should be suitable for polymerization, especially for the production of poly(2-hydroxyisobutyric acid). Solution

[0009] This problem was solved by a process for the preparation of polymerizable tetramethylglycolide comprising the following steps a) to c): a) reacting a reaction mixture containing 2-hydroxyisobutyric acid in a first reactor at a first temperature in the range from 150°C to 200°C and a first pressure in the range from 100 mbar to 1013 mbar, the degree of conversion of the 2-hydroxyisobutyric acid in the reaction being at most 95%, to obtain a product mixture containing 5 to 50% by weight of 2-hydroxyisobutyric acid, 25 to 85% by weight of tetramethyl glycolide and 5 to 25% by weight of di-2-hydroxyisobutyric acid, in each case based on the total weight of the product mixture, b) crystallizing the tetramethyl glycolide contained in the product mixture to obtain crystallized tetramethyl glycolide and a product liquid phase, c) separating the tetramethyl glycolide crystallized in step b) from the product liquid phase to obtain the polymerizable tetramethyl glycolide.

[0010] This object was further achieved by polymerizable tetramethylglycolide obtainable by the process according to the invention.

[0011] The process according to the invention surprisingly allows the production of particularly pure tetramethylglycolide (TMG), which is also polymerizable to poly(2-hydroxyisobutyric acid).

[0012] The process according to the invention allows in particular the production of tetramethylglycolide starting from acetone cyanohydrin production processes, in particular from an acetone cyanohydrin production process in which acetone is reacted with hydrogen cyanide to form acetone cyanohydrin.

[0013] The poly(2-hydroxyisobutyric acid) obtainable by polymerizing the TMG produced by the process according to the invention is advantageously characterized by high transparency and complete biodegradability to CO 2 . Poly(2-hydroxyisobutyric acid) can also be processed by conventional methods such as injection molding, casting, extrusion, or fiber or film production and is cleavable into its starting materials, 2-hydroxyisobutyric acid and TMG.

[0014] Poly(2-hydroxyisobutyric acid) is also characterized by higher hydrolysis resistance and glass transition temperature compared to poly(lactic acid), which enables processing and use in areas where poly(lactic acid) cannot be used.

[0015] The method according to the invention is described in more detail below.

[0016] In step a) of the process according to the invention, a reaction mixture is reacted in a first reactor at a first temperature and a first pressure to obtain a product mixture.

[0017] The reaction mixture contains 2-hydroxyisobutyric acid. 2-Hydroxyisobutyric acid is known as such and has the CAS number 594-61-6. It is also referred to as 2-hydroxy-2-methylpropanoic acid or 2-HIBS. Therefore, the terms "2-hydroxyisobutyric acid" and "2-HIBS" are used synonymously below and have the same meaning.

[0018] The reaction mixture may further contain at least one further component. The at least one further component is selected, for example, from the group consisting of tetramethyl glycolide, di-2-hydroxyisobutyric acid, and oligomeric 2-hydroxyisobutyric acid.

[0019] Therefore, a process is also preferred in which the reaction mixture additionally contains at least one further component selected from the group consisting of tetramethylglycolide, di-2-hydroxyisobutyric acid and oligomeric 2-hydroxyisobutyric acid.

[0020] The at least one further component is present in the reaction mixture, for example, if, in a preferred embodiment of the 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).

[0021] The reaction mixture is preferably obtainable by a process for preparing acetone cyanohydrin by reacting acetone with hydrogen cyanide.

[0022] Therefore, a process is also preferred in which the reaction mixture reacted in step a) is obtainable by a process for preparing acetone cyanohydrin by reacting acetone with hydrogen cyanide.

[0023] Processes for the production of acetone cyanohydrin by reacting acetone with hydrogen cyanide are known as such.

[0024] In this process, acetone is typically reacted with hydrogen cyanide to yield acetone cyanohydrin. This can be further reacted with sulfuric acid to produce sulfoxy-alpha-hydroxyisobutyric acid amide, also known as SIBA. SIBA can then be reacted at temperatures ranging from 90 °C to 120 °C in the presence of water to produce 2-hydroxyisobutyric acid and ammonium sulfate. This process is known as such.

[0025] The resulting 2-hydroxyisobutyric acid can be separated from ammonium sulfate, for example by distillation and / or extraction, and then reacted as a reaction mixture in step a). A reaction mixture obtained in this way usually still contains residues of ammonium sulfate.

[0026] In particular, if the reaction mixture is obtainable by a process for producing acetone cyanohydrin by reacting acetone with hydrogen cyanide, the reaction mixture additionally contains ammonium sulfate.

[0027] The conversion of 2-hydroxyisobutyric acid during the reaction is a maximum of 95%, preferably in the range of 50% to 90%, more preferably in the range of 60% to 85%, and most preferably in the range of 70% to 80%. For the purposes of the present invention, the conversion of 2-hydroxyisobutyric acid is understood to mean the proportion of the converted amount of 2-hydroxyisobutyric acid relative to the amount of 2-hydroxyisobutyric acid present in the reaction mixture before the start of the reaction. The determination can be carried out by methods known to those skilled in the art, for example, by means of HPLC or GC.

[0028] The product mixture therefore contains in the range of 5 to 50 wt.% 2-hydroxyisobutyric acid, preferably in the range of 5 to 10 wt.%, in each case based on the total weight of the product mixture.

[0029] During the reaction in step a), 2-HIBS dimerizes. Di-2-hydroxyisobutyric acid, described below, is initially formed by condensation. Di-2-hydroxyisobutyric acid then forms tetramethylglycolide through further condensation.

[0030] Tetramethylglycolide is known as such and is also referred to as 3,3,6,6-tetramethyl-1,4-dioxane-2,5-dione. It is also known as TMG. The terms "tetramethylglycolide" and "TMG" are therefore used synonymously in the present invention and have the same meaning. TMG is the cyclic dimer of 2-HIBS.

[0031] The product mixture contains in the range of 25 to 85 wt.% TMG, preferably in the range of 80 to 85 wt.% TMG, in each case based on the total weight of the product mixture.

[0032] During the dimerization of 2-HIBS, di-2-hydroxyisobutyric acid is formed as described above. For the purposes of the present invention, di-2-hydroxyisobutyric acid is understood to mean 2-((2-hydroxy-2-methylpropanoyl)oxy)-2-methylpropanoic acid. Di-2-hydroxyisobutyric acid is known as such and has the CAS number 87422-65-9. Di-2-hydroxyisobutyric acid is also referred to as di-2-HIBS. For the purposes 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 therefore have the same meaning.

[0033] The product mixture contains in the range of 5 to 25 wt.% Di-2-HIBS, preferably in the range of 10 to 20 wt.% Di-2-HIBS, in each case based on the total weight of the product mixture.

[0034] During the reaction mixture, 2-HIBS also typically undergoes partial oligomerization. During the oligomerization of 2-HIBS, trimers, tetramers, and pentamers of 2-HIBS are formed, for example. Oligomerized 2-HIBS is also referred to as "oligomeric 2-hydroxyisobutyric acid" in the context of the present invention.

[0035] The product mixture can therefore contain, for example, in the range of 0.1 to 10 wt.%, preferably in the range of 0.5 to 5 wt.% of oligomeric 2-hydroxyisobutyric acid, in each case based on the total weight of the product mixture.

[0036] The dimerization and oligomerization of 2-HIBS are condensation reactions. Therefore, water is also formed during the dimerization and oligomerization of 2-HIBS. Therefore, the product mixture usually contains additional water.

[0037] 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 a preferred embodiment of the invention. The removal of the water formed is advantageous because it shifts the reaction equilibrium from 2-HIBS toward TMG.

[0038] For example, the product mixture obtained in step a) additionally contains 0.001 to 0.5 wt.% water, preferably 0.005 to 0.2 wt.% water, based on the total weight of the product mixture.

[0039] If the reaction mixture additionally contains ammonium sulfate and / or other nitrogen-containing compounds, the reaction mixture in step a) may additionally form, for example, 1,3-substituted diketones of morpholine substituted by four methyl groups and / or 1,4-substituted diketones of morpholine substituted by four methyl groups. Therefore, the product mixture may additionally contain 1,3-substituted diketones of morpholine substituted by four methyl groups and / or 1,4-substituted diketones of morpholine substituted by four methyl groups.

[0040] During the reaction of the reaction mixture in step a), any tetramethylglycolide present in the reaction mixture and / or tetramethylglycolide formed during the reaction can also be at least partially cleaved. This produces methacrylic acid, acetone, and carbon monoxide. Carbon monoxide is typically removed as a gas from the reaction in step a). Therefore, the product mixture may also contain methacrylic acid and / or acetone.

[0041] The reaction in step a) can take place over any desired period of time. The reaction time in step a) is preferably in the range of 1 hour to 24 hours, particularly preferably in the range of 6 hours to 9 hours.

[0042] Therefore, a process is also preferred in which the reaction time of the reaction in step a) is in the range of 1 hour to 24 hours.

[0043] The "reaction time" is understood to be the period from reaching the first temperature until the degree of conversion of 2-hydroxyisobutyric acid is reached.

[0044] The reaction in step a) can be carried out in batch mode, but it is also possible to proceed continuously. If the reaction in step a) takes place continuously, the reaction time is understood to be the average residence time of the reaction mixture in the first reactor.

[0045] The reaction can take place in the presence of a catalyst, preferably in the presence of an acidic catalyst. Suitable acidic catalysts for catalyzing the reaction in step a) are known per se and are selected, for example, from the group consisting of inorganic acids, organic acids, and acidic cation exchangers. Suitable inorganic acids include, for example, sulfuric acid or hydrochloric acid. Suitable organic acids include, for example, sulfonic acids such as p-toluenesulfonic acid. Furthermore, sulfonated aromatics embedded in a polymer matrix can be used as strongly acidic ion exchangers. One such sulfonated aromatic embedded in a polymer matrix is, for example, sulfonated polystyrene in a polystyrene matrix.

[0046] Particularly preferred is 2-hydroxyisobutyric acid itself as the acid catalyst in the reaction. Therefore, the reaction preferably proceeds autocatalytically.

[0047] A process in which the reaction in step a) takes place autocatalytically is therefore also preferred.

[0048] The reaction takes place at a first temperature. The first temperature is in the range of 150 °C to 200 °C, preferably in the range of 170 °C to 190 °C.

[0049] The first temperature refers to the temperature inside the first reactor. It will be clear to those skilled in the art that if the first reactor comprises a column in a preferred embodiment, the temperature in the column may be lower than the first temperature.

[0050] The reaction takes place at an initial pressure. The initial pressure is in the range of 100 mbar to 1013 mbar, preferably in the range of 200 mbar to 400 mbar.

[0051] The reaction in step a) takes place in a first reactor. Suitable first reactors are reactors known to those skilled in the art for cyclizations, such as stirred tank reactors. A stirred tank cascade can also be used as the first reactor. This is particularly advantageous if the process according to the invention is to be carried out continuously. Stainless steel or enamel is preferably used as the reactor material.

[0052] Preferably, the first reactor comprises at least one column, particularly preferably at least one distillation column and / or at least one rectification column.

[0053] A process in which the first reactor comprises at least one column is therefore also preferred.

[0054] The column preferably removes at least some of the water formed during the reaction. Water can be removed at least partially as an azeotrope with 2-HIBS. Acetone formed, for example, during the cleavage of tetramethylglycolide can also be removed via the column.

[0055] In step b) of the process according to the invention, the tetramethylglycolide contained in the product mixture is crystallized to obtain crystallized tetramethylglycolide and a product liquid phase.

[0056] The TMG contained in the product mixture can be crystallized using methods known to those skilled in the art. To crystallize the tetramethyl glycolide, the product mixture is preferably cooled to a second temperature. The second temperature is, for example, in the range from 50°C to 80°C. Thus, crystallization in step b) preferably takes place at a second temperature in the range from 50°C to 80°C.

[0057] A process is therefore also preferred in which the crystallization in step b) takes place at a second temperature in the range from 50 °C to 80 °C.

[0058] It is possible for crystallization to occur with a first solvent. For example, a first solvent can be added to the product mixture to crystallize TMG. If crystallization occurs with a first solvent, the product mixture can be additionally cooled for crystallization. A suitable first solvent, for example, is a solvent selected from the group consisting of isopropanol, methyl isobutyl ketone, toluene, pentane, hexane, substituted hexane derivatives, and cyclized hexane derivatives.

[0059] Preferably, the crystallization takes place without adding a first solvent to the product mixture.

[0060] Tetramethylglycolide is therefore preferably crystallized from a melt of the product mixture.

[0061] Tetramethylglycolide is particularly preferably fractionally crystallized in step b). Processes for fractional crystallization are known per se.

[0062] In step c) of the process according to the invention, the tetramethylglycolide crystallized in step b) is separated from the product liquid phase to obtain the polymerizable tetramethylglycolide.

[0063] The tetramethyl glycolide crystallized in step b) can be separated from the product liquid phase using methods known to those skilled in the art, for example, by filtration or centrifugation. The crystallized tetramethyl glycolide can be additionally washed to separate it from the product liquid phase. For example, the crystallized tetramethyl glycolide can first be filtered and then washed. Second solvents known to those skilled in the art, in which tetramethyl glycolide is poorly soluble, are suitable for washing. Such solvents include, for example, 2-propanol, methyl isobutyl ketone, and / or ethanol.

[0064] Particularly when tetramethyl glycolide is fractionally crystallized in step b), it is preferred that, after separation of the product liquid phase in step c), the crystallized tetramethyl glycolide is partially melted. This removes impurities from the crystallized tetramethyl glycolide, thus yielding polymerizable tetramethyl glycolide.

[0065] Separating the crystallized tetramethyl glycolide from the product liquid phase yields the polymerizable tetramethyl glycolide. The product liquid phase is also obtained. The product liquid phase typically contains the components that were present in the product mixture and that were not crystallized with tetramethyl glycolide in step b).

[0066] For example, the product liquid phase contains at least one component selected from the group consisting of 2-hydroxyisobutyric acid, tetramethylglycolide, di-2-hydroxyisobutyric acid, oligomeric 2-hydroxyisobutyric acid and water.

[0067] If tetramethylglycolide was crystallized in step b) by adding a first solvent, the product liquid phase usually also contains this first solvent.

[0068] Preferably, the product liquid phase obtained in step c) is recycled into the reaction mixture in step a).

[0069] A process is therefore also preferred in which the product liquid phase obtained in step c) is recycled to step a).

[0070] If the product liquid phase additionally contains a first solvent, this is preferably separated before the product liquid phase is returned to the reaction mixture.

[0071] Processes for separating the first solvent from the product liquid phase are known. For example, the first solvent can be separated by distillation, and separation by phase separation is also possible. The first solvent is preferably separated by distillation.

[0072] If the product mixture contains oligomeric 2-hydroxyisobutyric acid in an embodiment described above, the product liquid phase obtained in step c) typically also contains oligomeric 2-hydroxyisobutyric acid. The oligomeric 2-hydroxyisobutyric acid is preferably separated from the product liquid phase before recirculation of the product liquid phase in step a).

[0073] 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 a tin(IV) oxide catalyst. Dioctyltin oxide is known as such and has the CAS number 870-08-6. It is also referred to as DOTO.

[0074] The catalyst cleaves oligomeric 2-hydroxyisobutyric acid into shorter oligomers of 2-hydroxyisobutyric acid, 2-hydroxyisobutyric acid, di-2-hydroxyisobutyric acid, and / or tetramethyl glycolide. The shorter oligomers of 2-hydroxyisobutyric acid formed during the cleavage are typically further cleaved by the catalyst to 2-hydroxyisobutyric acid, di-2-hydroxyisobutyric acid, and / or tetramethyl glycolide. TMG formed during the cleavage can be distilled from the product liquid phase, and the product liquid phase, from which oligomeric 2-hydroxyisobutyric acid has been separated, can then be recycled to step a).

[0075] Therefore, a process is also preferred in which the product mixture obtained in step a) additionally contains oligomeric 2-hydroxyisobutyric acid and this is cleaved in the presence of a tin(IV) oxide catalyst and recycled to step a).

[0076] Preferably, in the process according to the invention, the following step a1) is carried out after step a) and before step b): a1) distillation of the product mixture obtained in step a) at a second pressure which is in the range from 100 mbar to 500 mbar, to obtain a first bottom stream and a first top stream, wherein the first bottom stream at least 80 wt.% TMG, at most 1 wt.% 2-HIBS, at most 2 wt.% di-2-hydroxyisobutyric acid and at least 5 wt.% oligomeric 2-hydroxyisobutyric acid, in each case based on the total weight of the first bottom stream, and wherein the first top stream contains 0.1 to 5 wt.% di-2-hydroxyisobutyric acid, 10 to 50 wt.% 2-hydroxyisobutyric acid and 40 to 85 wt.% tetramethylglycolide, in each case based on the total weight of the first top stream, and in step b) the tetramethylglycolide contained in the first bottom stream is then crystallized.

[0077] Therefore, a process is also preferred in which, after step a) and before step b), the following step a1) is carried out: a1) distillation of the product mixture obtained in step a) at a second pressure which is in the range from 100 mbar to 500 mbar, to obtain a first bottom stream and a first top stream, wherein the first bottom stream at least 80% by weight of tetramethyl glycolide, 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, in each case based on the total weight of the first bottom stream, and wherein the first top stream contains 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 tetramethyl glycolide, in each case based on the total weight of the first top stream, and in step b) the tetramethyl glycolide contained in the first bottom stream is then crystallized.

[0078] If step a1) is carried out, the tetramethyl glycolide contained in the first bottom stream is crystallized in step b). Step b) of the process according to the invention then comprises: b) crystallizing the tetramethyl glycolide obtained in the first bottom stream to obtain crystallized tetramethyl glycolide and a product liquid phase.

[0079] Step a1) is preferably carried out in a second column. The second column is preferably different from the column in step a) preferably comprised by the first reactor. Product mixture is preferably continuously fed into this second column, and the first bottom stream and the first overhead stream are continuously removed.

[0080] The distillation in step a1) is preferably carried out at a second temperature. The second temperature refers to the bottom temperature during the distillation and is preferably in the range from 150°C to 200°C, particularly preferably in the range from 160°C to 190°C.

[0081] During the distillation in step a1), TMG is typically partially cleaved, forming acetone, methacrylic acid, and carbon monoxide. Therefore, the first overhead stream obtained during distillation usually also contains methacrylic acid and / or acetone.

[0082] If the product mixture additionally contains methacrylic acid and / or acetone, the first overhead stream obtained during distillation usually also contains methacrylic acid and / or acetone.

[0083] The first overhead stream can be at least partially recycled into the reaction mixture. Preferably, the first overhead stream obtained in step a1) is recycled to step a).

[0084] A process is therefore also preferred in which the first head stream obtained in step a1) is recycled to step a).

[0085] In step c) polymerizable tetramethylglycolide is obtained.

[0086] The present invention therefore also relates to polymerizable tetramethylglycolide obtainable by the process according to the invention.

[0087] Typically, the polymerizable tetramethylglycolide obtained in the process according to the invention additionally contains by-products.

[0088] Preferably, the by-products comprise 10 ppm by weight to 1000 ppm by weight of 2-hydroxyisobutyric acid, 10 ppm by weight to 1000 ppm by weight of water and 10 ppm by weight to 5000 ppm by weight of di-2-hydroxyisobutyric acid, each based on the total weight of the polymerizable tetramethyl glycolide.

[0089] If the product mixture and / or the first bottom stream from which the polymerizable tetramethylglycolide is crystallized in step b) additionally contains methacrylic acid, the by-products containing the polymerizable tetramethylglycolide usually also comprise methacrylic acid.

[0090] For example, the by-products then additionally comprise 1 ppm by weight to 1000 ppm by weight of methacrylic acid, based on the total weight of the polymerizable tetramethylglycolide.

[0091] Therefore, polymerizable tetramethylglycolide is also preferred, wherein the by-products additionally comprise 1 ppm by weight to 1000 ppm by weight of methacrylic acid, based on the total weight of the polymerizable tetramethylglycolide.

[0092] If the product mixture and / or the first bottom stream from which the polymerizable tetramethyl glycolide is crystallized in step b) additionally contains oligomeric 2-hydroxyisobutyric acid, the by-products containing the polymerizable tetramethyl glycolide usually also comprise oligomeric 2-hydroxyisobutyric acid.

[0093] For example, the by-products additionally comprise 1 ppm by weight to 5000 ppm by weight of oligomeric 2-hydroxyisobutyric acid, based on the total weight of the polymerizable tetramethylglycolide.

[0094] Therefore, polymerizable tetramethylglycolide is also preferred, wherein the by-products additionally comprise 1 ppm by weight to 5000 ppm by weight of oligomeric 2-hydroxyisobutyric acid, based on the total weight of the polymerizable tetramethylglycolide.

[0095] If the product mixture and / or the first bottom stream from which the polymerizable tetramethylglycolide is crystallized in step b) additionally contains at least one diketone of morpholine substituted by four methyl groups, the by-products containing the polymerizable tetramethylglycolide usually also additionally comprise at least one diketone of morpholine substituted by four methyl groups.

[0096] For example, the by-products additionally contain from 1 ppm by weight to 500 ppm by weight of at least one diketone of morpholine substituted with four methyl groups, based on the total weight of the polymerizable tetramethylglycolide.

[0097] Therefore, polymerizable tetramethylglycolide is also preferred, wherein the by-products additionally comprise 1 ppm by weight to 500 ppm by weight of at least one diketone of morpholine substituted with four methyl groups, based on the total weight of the polymerizable tetramethylglycolide.

[0098] It goes without saying that the ppm by weight of the by-products refers to the total weight of the polymerizable tetramethylglycolide including the additional by-products contained.

[0099] The ratio of the molar sums of all acidic protons of the by-products to the molar amount of pure polymerizable tetramethylglycolide is preferably in the range of 0.05 to 0.0001.

[0100] Therefore, polymerizable tetramethylglycolide is also preferred, wherein the polymerizable tetramethylglycolide additionally contains by-products, wherein the by-products 10 ppm by weight to 1000 ppm by weight of 2-hydroxyisobutyric acid, 10 ppm by weight to 1000 ppm by weight of water and 10 ppm by weight to 5000 ppm by weight of di-2-hydroxyisobutyric acid, based on the total weight of the polymerizable tetramethyl glycolide and wherein the ratio of the molar sums of all acidic protons of the by-products to the molar amount of the polymerizable tetramethyl glycolide is in the range of 0.05 to 0.0001.

[0101] Acidic protons of the by-products are understood to mean all OH-, NH-, PH- and SH-acidic protons of the by-products, preferably all OH- and NH-acidic protons of the by-products.

[0102] In an alternative embodiment of the method according to the invention, in addition or alternatively, preferably alternatively, to steps b) and c), the following steps b1) and b2) are carried out: b1) distillation of the product mixture obtained in step a) to obtain a tetramethylglycolide-containing distillate, b2) washing the tetramethylglycolide-containing distillate obtained in step b1) to obtain the polymerizable tetramethylglycolide.

[0103] The distillation in step b1) can be carried out by methods known to those skilled in the art. It preferably takes place in at least two stages. Typically, in this case, components with a lower boiling point than tetramethyl glycolide are separated from the product mixture in a first distillation step. In a second distillation step, tetramethyl glycolide is then separated off as the overhead stream to obtain the tetramethyl glycolide-containing distillate.

[0104] The tetramethylglycolide-containing distillate contains tetramethylglycolide. Furthermore, it typically contains byproducts formed during distillation from tetramethylglycolide, such as acetone and / or methacrylic acid. Furthermore, it may contain, for example, residues of the components present in the product mixture, in particular at least one morpholine diketone substituted with four methyl groups.

[0105] In step b2), the tetramethylglycolide-containing distillate is therefore washed. The washing can be carried out by methods known to those skilled in the art, for example with water, preferably with basic water. This allows by-products to be separated off to obtain polymerizable tetramethylglycolide. It is possible that in step b2), the tetramethylglycolide-containing distillate is first washed as described above and then purified by adsorption and / or absorption to obtain the polymerizable tetramethylglycolide. The adsorption and / or absorption can be carried out, for example, on silica, aluminum oxide, and / or an ion exchanger. These processes are known to those skilled in the art. The adsorption and / or absorption is particularly advantageous for removing the at least one diketone of morpholine substituted with four methyl groups that may be present in the tetramethylglycolide-containing distillate. Examples Polymerization of different TMG batches

[0106] The polymerization performed here is suitable for testing the produced TMG batches. It determines whether polymerization is fundamentally feasible with a TMG.

[0107] The amount of TMG to be polymerized (typically 30 g) was first weighed into an Erlenmeyer flask fitted with a magnetic stirrer bar. The Erlenmeyer flask was then heated to 130 °C in a vacuum drying oven at 0 mbar for one hour. This melted the TMG, and any residual moisture from the TMG and the glass surface of the flask was removed. After one hour, the drying oven was vented, the flask was sealed with a rubber stopper, placed in an oil bath preheated to 130 °C, and mixed thoroughly using a magnetic stirrer. A cannula was then inserted through the stopper, and the flask was purged with argon. 0.15 mol% lithium tert-butylate (as t-BuOLi dissolved in THF, pure t-BuOLi based on the TMG weight) of catalyst was then added via the cannula using a syringe (the amount required for the desired molar mass). If the material was suitable for polymerization, a viscosity was reached after approximately 5 to 15 minutes at which the magnetic stir bar stopped.The polymerization was then continued for another 6-8 hours.

[0108] The purities of the TMG batches, which were prepared analogously to Example 17 and a conversion of 2-hydroxyisobutyric acid of less than 95%, as well as their polymerization capacity are given in Table 1. Table 1 Example TMG [wt.%] H 2 O [wt.%] 2-HIBS [wt%] Di-2-HIBS [wt.%] MAS [wt.%] Successful polymerization V1 (raw product) 98,9 0,003 0,1 0,989 0,005 No B2 (pure product) 99,5 0,004 0,01 0,313 0,003 Yes V3 (raw product) 92,1 0,012 5,4 2,4 0,009 no V4 (distilled) 90,4 0,098 0,36 1,3 0,002 no V5 (melt crystallization) 99,2 0,023 0,3 0,41 0,004 no B6 (melt crystallization) 99,5 0,018 0,08 0,32 0,003 Yes B7 (melt crystallization) 99,7 0,012 0,05 0,22 0,002 Yes V8 (crystallized i-PrOH + washed) 97,1 0,085 0,07 0,81 0,006 no V9 (crystallized i-PrOH + washed + dried) 99,1 0,009 0,08 0,71 0,002 no V10 (2x crystallized i-PrOH + washed 98,3 0,031 0,039 0,19 0,006 no B11 (2x crystallized i-PrOH + washed + dried) 99,71 0,0089 0,078 0,18 0,003 Yes V12 (2x crystallized i-PrOH + washed + dried + stored openly) 99,59 0,12 0,078 0,18 0,003 no V13 (crystallized toluene) 90,2 0,07 0,015 0,7 0,053 no B14 (crystallized twice from toluene) 90,5 0,03 0,01 0,25 0,027 Yes B15 (crystallized pentane) 98 0,062 0,089 0,4 0,025 Yes B16 (extracted toluene, crystallized pentane) 96,4 0,034 0,068 0,3 0,015 Yes Example B17: Production of TMG from 2-HIBS:

[0109] In a stirred 5L double-walled glass vessel (first reactor), 3112.6 g of reaction mixture with the following composition were placed (the amounts missing to reach 100 wt.% result, on the one hand, from the measurement accuracy of the HPLC and / or GC used, and, on the other hand, from undetected minor components): 2-HIBS 79.3 wt% 2-DiHIBS 16.2 wt.% TMG 1.64 wt%

[0110] The vessel was equipped with a column filled with three SULZER EX Hastelloy packings at the bottom and glass Raschig rings above. The vessel was heated with thermal oil. The vessel and column were evacuated to 300 mbara and heated to an internal temperature of 165 to 170 °C. Over a period of 9 h, water was withdrawn overhead, and both the first reactor and the distillate were sampled at 0 hour, 1 h, 2 h, 4 h, 6 h, 8 h, and 9 h. After 9 h, the reactor contained 2619 g of product mixture (hereinafter referred to as crude TMG), 442.7 g were collected in the distillate, and after completion of the experiment, 17.8 g were withdrawn once from the upstream cold trap of the vacuum. The composition of the reaction mixture samples and during the reaction in the first reactor is shown in Table 2 (the amounts missing to 100 wt% result from the measurement accuracy of the analytical methods used; KF: Karl Fischer titration). Table 2 Reaction mixture first reactor Test duration / h 0,0 1,0 2,0 4,0 6,0 8,0 9,0 Conversion grade 2-HiBS / % 15,5 31,4 52,6 65,0 73,7 78,2 Analytics HPLC 2-HIBS % by weight 79,3 69,4 58,1 41,8 31,8 24,3 20,4 HPLC MAS % by weight 0,0 0,1 0,1 0,1 0,1 0,0 0,040 HPLC Di-2-HIBS % by weight 16,2 8,5 9,7 7,9 6,8 5,2 4,7 HPLC TMG % by weight 1,64 18,6 28,5 46,7 58,8 67,6 71,6 GC acetone % by weight KF H2O % by weight 0,9 0,64 0,6 Masses m total g 3112,60 3006 2913 2799 2719 2668 2619

[0111] The composition of the distillate and the cold trap at the end of the experiment is shown in Table 3 (the amounts missing to 100 wt% result from the measurement accuracy of the analytical methods used; KF: Karl Fischer titration). Table 3 distillate cold trap Test duration / h 1,0 2,0 4,0 6,0 8,0 9,0 9,0 Analytics HPLC 2-HIBS % by weight 0,0 0,0 0,0 0,0 0,0 0,0 0,0 HPLC MAS % by weight 3,1 8,7 12,1 12,3 13,3 10,8 4,7 HPLC Di-2-HIBS % by weight 0,0 0,0 0,0 0,0 0,0 0,0 0,0 HPLC TMG % by weight 0,0 0,0 0,0 0,0 0,0 0,0 0,0 GC acetone % by weight 0,1 0,2 0,1 0,1 0,1 0,2 44,4 KF H2O % by weight 86,8 82,1 83,7 82,4 82,3 78,6 45,1 Masses m total g 102,2 89,5 109,6 71,4 48,9 21,1 17,8

[0112] Although the reaction conversion increases continuously, the selectivity (based on the distillate, which contains only 89% water and MAA instead of >95%) begins to decline after 8 hours or approximately 75% conversion (based on 2-HIBS). It is therefore advisable not to drive the reaction to full conversion and to terminate it prematurely.

[0113] The following Comparative Examples V18, V19, and V20 provide a general overview of the crystallization of TMG from the mother liquor (Comparative Example 18), the distillation of TMG (Comparative Example 19), and the extraction of TMG (Comparative Example 20), as also carried out above for the various TMG batches. The amounts missing from 100 wt.% in the following Comparative Examples result from the measurement accuracy of the analytical methods used. Comparative example V18 Crystallization of TMG from the mother liquor:

[0114] After completing a synthesis starting from Example B17, the vacuum was released and the crude TMG was cooled overnight to approximately 58°C. Pure TMG crystallized, leaving a product liquid phase consisting of TMG, 2-HIBS, 2-DiHIBS, and MAS. This product liquid phase can be recycled to the reaction. The contents of the vessel are then drawn in two fractions through a filter heated to approximately 58°C using a vacuum pump. The filter cake had the following composition: 2-HIBS 5.4 wt% MAS 0.009 wt.% Di-2-HIBS 2.4 wt% TMG 92.1% by weight

[0115] The extracted filtrate (product liquid phase) had the following composition: 2-HIBS 30.3 wt% MAS 0.1 wt.% Di-2-HIBS 9.9 wt.% TMG 55.1 wt%

[0116] The filter cake was then washed with 300 g of 2-propanol: The composition after washing was: 2-HIBS 0.07 wt.% MAS 0.006 wt.% Di-2-HIBS 0.81 wt.% TMG 97.1 wt.%

[0117] The crystals were then dried for 48 hours at room temperature under vacuum (< 50 mbar) until constant mass was reached. After drying, a total of 517 g of TMG with the following composition was obtained: 2-HIBS 0.08 wt.% MAS 0.002 wt.% Di-2-HIBS 0.71 wt.% TMG 99.1 wt.% Comparative example V19 Distillation of TMG

[0118] The distillative purification of crude TMG starting from Example B17 was carried out on a 2-meter-long DN50 column filled with SULZER EX Hastelloy packing. The column bottom was heated to an internal temperature of 189°C, and the column was trace-heated to approximately 183 to 185°C. The distillation was operated with a reflux ratio of 3:1. The composition of the input and output streams was as follows: Feed: 480 g / h 2-HIBS 13.4 wt% MAS 0.06 wt.% Di-2-HIBS 6.2 wt.% TMG 77.2 wt% Sump: 195 g / h 2-HIBS 0.36 wt.% MAS 0 wt.% Di-2-HIBS 1.3 wt.% TMG 90.4% by weight Rest (oligomers) Head: 285 g / h 2-HIBS 25.8 wt% MAS 0.12 wt.% Di-2-HIBS 1.0 wt.% TMG 73% by weight

[0119] The top product can be returned to the synthesis. Comparative example V20 extraction of TMG

[0120] Raw TMG from a previous example with the following composition was first dissolved in toluene in a mass ratio of 1 : 1.75: 2-HIBS 0.4 wt.% Di-2-HIBS 7.4 wt% TMG 92.4% by weight

[0121] Any insoluble solid was filtered off. The toluene solution was then dried in a rotary evaporator. The resulting crystals were separated, and a portion was washed again with a small amount of petroleum ether to remove any remaining toluene. The crystals were then dried to constant mass in a vacuum oven at 50°C and 0 mbar.

Claims

1. Process for preparing polymerizable tetramethylglycolide comprising the following steps a) to c): a) reaction of a reaction mixture containing 2-hydroxyisobutyric acid in a first reactor at a first temperature in the range from 150°C to 200°C and a first pressure in the range from 100 mbar to 1013 mbar, where the degree of conversion of the 2-hydroxyisobutyric acid in the reaction is at most 95%, to obtain a product mixture containing 5% to 50% by weight of 2-hydroxyisobutyric acid, 25% to 85% by weight of tetramethylglycolide and 5% to 25% by weight of di-2-hydroxyisobutyric acid, in each case based on the total weight of the product mixture, b) crystallization of the tetramethylglycolide present in the product mixture to obtain crystallized tetramethylglycolide and a product liquid phase, c) separation of the tetramethylglycolide crystallized in step b) from the product liquid phase to obtain the polymerizable tetramethylglycolide.

2. Process according to Claim 1, characterized in that the reaction duration of the reaction in step a) is in the range from 1 hour to 24 hours.

3. Process according to Claim 1 or 2, characterized in that the first reactor comprises at least one column.

4. Process according to any of Claims 1 to 3, characterized in that after step a) and before step b) the following step a1) is carried out a1) distillation of the product mixture obtained in step a) at a second pressure in the range from 100 mbar to 500 mbar, to obtain a first bottom stream and a first overhead stream, where the first bottom stream contains at least 80% by weight of tetramethylglycolide, 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, in each case based on the total weight of the first bottom stream, and where the first overhead stream contains 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 tetramethylglycolide, in each case based on the total weight of the first overhead stream, and in step b) the tetramethylglycolide present in the first bottom stream is then crystallized.

5. Process according to any of Claims 1 to 4, characterized in that the reaction in step a) proceeds autocatalytically.

6. Process according to any of Claims 1 to 5, characterized in that the reaction mixture additionally contains at least one further component selected from the group consisting of tetramethylglycolide, di-2-hydroxyisobutyric acid and oligomeric 2-hydroxyisobutyric acid.

7. Process according to any of Claims 1 to 6, characterized in that the crystallization in step b) takes place at a second temperature in the range from 50°C to 80°C.

8. Process according to any of Claims 1 to 7, characterized in that the product liquid phase obtained in step c) is recycled into step a).

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

10. Process according to any of Claims 1 to 9, characterized in that the reaction mixture reacted in step a) is obtainable by a process for preparing acetone cyanohydrin by reacting acetone with hydrocyanic acid.

11. Polymerizable tetramethylglycolide obtainable by a process according to any of Claims 1 to 10.

12. Polymerizable tetramethylglycolide according to Claim 11, characterized in that the polymerizable tetramethylglycolide additionally contains byproducts, where the byproducts comprise 10 ppm by weight to 1000 ppm by weight of 2-hydroxyisobutyric acid, 10 ppm by weight to 1000 ppm by weight of water and 10 ppm by weight to 5000 ppm by weight of di-2-hydroxyisobutyric acid, based on the total weight of the polymerizable tetramethylglycolide, and where the ratio of the molar sum totals of all acidic protons of the byproducts to the molar amount of the polymerizable tetramethylglycolide is in the range from 0.05 to 0.0001.

13. Polymerizable tetramethylglycolide according to Claim 12, characterized in that the byproducts additionally comprise 1 ppm by weight to 1000 ppm by weight of methacrylic acid, based on the total weight of the polymerizable tetramethylglycolide.

14. Polymerizable tetramethylglycolide according to Claim 12 or 13, characterized in that the byproducts additionally comprise 1 ppm by weight to 5000 ppm by weight of oligomeric 2-hydroxyisobutyric acid, based on the total weight of the polymerizable tetramethylglycolide.

15. Polymerizable tetramethylglycolide according to any of Claims 12 to 14, characterized in that the byproducts additionally comprise 1 ppm by weight to 500 ppm by weight of at least one diketone of morpholine that is substituted by four methyl groups, based on the total weight of the polymerizable tetramethylglycolide.