A method for preparing a cyclic ester of a hydroxy acid containing composition using polyalcohol

Abstract

The present invention relates to a method for preparing a cyclic ester of hydroxy acid containing composition comprising the steps a) of subjecting a starting mix- ture containing hydroxy acid and at least one polyalcohol comprising at least four 5 hydroxyl groups per molecule to an oligomerization reaction so as to obtain as reaction mixture an oligomer composition containing hydroxy acid oligomer and b) of subjecting the oligomer composition obtained in step a) or a modified oligomer composition, which is obtained by adding at least one polyalcohol comprising at least four hydroxyl groups per molecule to the oligomer composition obtained in 10 step a) and incubating the so obtained mixture so as to obtain the modified oligo- mer composition, to a cyclization reaction so as to obtain a cyclic ester of hydroxy acid containing composition, wherein the molar ratio of the hydroxy acid carboxylic groups divided by the polyalcohol hydroxy groups in the starting mixture is at least 2, and wherein during the oligomerization reaction at least a portion of the reaction 15 mixture, which is formed during the oligomerization reaction, is at least temporarily distilled so as to remove water from the reaction mixture.

Description

[0001] Sulzer Management AG S13784P

[0002] A method for preparing a cyclic ester of a hydroxy acid containing composition using polyalcohol

[0003] The present invention relates to a method for preparing a cyclic ester of a hydroxy acid containing composition, such as a lactide composition, using polyalcohol as well as to a plant, in which the method may be performed.

[0004] Polyhydroxy acid homo- or copolymers are of particular interest, because they can be obtained from renewable resources and are mostly compostable and / or biodegradable. Moreover, the technological properties of these polymers come quite close to the properties of those polymers derived from fossil-based resources, which explains why these polymers are regarded as highly promising substitutes for the latter. One example for a commercially important polyhydroxy acid homopolymer is polylactic acid, which is based on an a-hydroxy acid, namely on lactic acid, and which has a wide range of applications. For instance, polylactic acid is used in the biomedical field in chirurgical implants, in films, such as e.g. in packaging, in fibers, such as e.g. for garments, in hygienic articles, in carpets and in disposable plastic products, such as e.g. disposable cutlery or containers. Moreover, polylactic acid has found wide application in composite materials, such as in fiber- reinforced plastics. Another important example for a respective polyhydroxy acid homopolymer is polycaprolactone, which is a poly-co-hydroxy acid and is derived from a cyclic ester, caprolactone, originated from the intramolecular esterification of 6-hydroxyhexanoic acid. This polymer finds widespread applications in the production of speciality polyurethanes and is characterized by a good resistance to water, to oil and to solvent. Other examples are polyglycolic acid, polyvalerolactone and the like. Polyhydroxy acid copolymers are interesting alternatives, since by an appropriate selection of the comonomers and their relative amounts to each other and by adjusting an appropriate molecular weight, certain properties of the copolymers may be tailored to the intended use.

[0005] Generally, two alternative principal methods are known for synthesizing polyhydroxy acid homo- or copolymers. The first principal method is the direct polycondensation of one or more aliphatic hydroxy acid(s) to the respective homopolymer or copolymer, respectively, such as the direct polycondensation of lactic acid to polylactic acid. However, this principal method only leads to low molecular weight (co)polymers and is thus limited to specific (co)polymers. The second principal method is the ring-opening-polymerization of one or more cyclic esters of hydroxy acid(s), such as the ring-opening-polymerization of lactide (which is the cyclic diester of lactic acid), glycolide (which is the cyclic diester of glycolic acid), lactones (which are cyclic monoesters) or the like. This is the preferred method nowadays for the industrial production of polylactic acid and other polyhydroxy acid homo- or copolymers. On account thereof, there is an increasing demand for cyclic esters of hydroxy acid, such as lactide, and thus for efficient methods for preparing cyclic esters of hydroxy acid, such as lactide.

[0006] In principle, a cyclic ester of hydroxy acid, such as lactide, may be produced by condensation of two hydroxy acid molecules, such as lactic acid molecules, which is, however, quite an inefficient method. Therefore, cyclic esters of hydroxy acid are industrially predominantly produced by first oligomerizing the hydroxy acid and then subjecting the hydroxy acid oligomer to a cyclization reaction. For instance, in the case of lactic acid as the hydroxy acid, the cyclic ester, i.e. lactide, is prepared by fermentation of carbohydrates from biomass, such as starch, sugar or com resulting into an aqueous lactic acid solution, by then dewatering the aqueous lactic acid solution so as to obtain lactic acid, by then oligomerizing the lactic acid to oligomers of lactic acid and by afterwards subjecting the oligomers to the cyclization reaction so as to convert the oligomers of lactic acid to lactide. Explained for lactic acid as example of a hydroxy acid, the oligomerization of lactic acid is based on polycondensation reactions between the acid group and the hydroxyl group of different lactic acid molecules forming ester bonds and thereby combining different lactic acid molecules covalently together to lactic acid oligomers, while also forming water as reaction side-product. In turn, the cyclization reaction is based on back-biting and end-biting reactions. End-biting denotes the intramolecular esterification of a linear (i.e. non-cyclic) lactic acid dimer to lactide and water, whereas back-biting denotes the splitting off from lactide being formed by intramolecular esterification between the terminal hydroxy group of a terminal repeating unit and the ester group of an adjacent repeating unit from a lactic acid oligomer comprising three or more repeating units, thereby leading to lactide and a lactic acid oligomer containing two fewer repeating units than the lactic acid oligomer before the reaction. Thus, longer lactic acid oligomers first react during the cyclization reaction, which is usually performed in the presence of an appropriate catalyst, by back-biting to shorter oligomers and lactide, before finally the remaining linear lactic acid dimers react by end-biting to lactide and water. Because endbiting leads to the formation of water, which may hydrolyze the freshly formed lactide, it is preferred to minimize the end-biting reactions during the cyclization reaction as much as possible so as to keep the water formation as low as possible. On account of this reason, it is common practice to prepare comparable long lactic acid oligomers during the oligomerization step, because then the long lactic acid oligomers form comparable high amounts of lactide by back-biting in comparison to lactide formed by end-biting. The produced lactide is separated during the cyclization reaction from the reaction mixture through distillation under reduced pressure, whilst back-biting reactions are ongoing in the reaction mixture until the oligomers are largely depleted.

[0007] However, conducting the synthesis of a cyclic ester of hydroxy acid with comparably long hydroxy acid oligomers as described above for the lactide synthesis, i.e. conducting the lactide synthesis with comparably long lactic acid oligomers, is disadvantageous. Firstly, forming comparably long lactic acid oligomers by oli- gomerization of lactic acid requires harsh reaction conditions, which are a cause for the formation of undesired meso-lactide and thus for a reduced optical purity of the synthesized lactide. Furthermore, the use of comparably long lactic acid oligomers leads to a higher viscosity of the reaction mixture during the cyclization reaction and the increased viscosity encumbers the retrieval of lactide through distillation from the reaction mixture of the cyclization reaction, which in turn leads to a significantly slowing down of the productivity and increased operational costs of the method. Again, the use of shorter lactic acid oligomers, which may be prepared during the oligomerization of lactic acid under milder conditions and which lead during the cyclization reaction to less viscous reaction mixtures, are not a suitable alternative, because the shorter oligomers are prone to end-biting reactions during the cyclization reaction leading to the formation of high amounts of water during the oligomerization reaction, which may hydrolyze the freshly formed lactide.

[0008] In view of this, the object underlying the present invention is to provide an efficient method and a plant for preparing with minimal operational costs a cyclic ester of hydroxy acid containing composition, in particular a lactide composition, with excellent optical purity of the cyclic ester of the hydroxy acid, in particular of the lactide, in high yield.

[0009] In accordance with the present invention this object is satisfied by providing a method for preparing a cyclic ester of hydroxy acid containing composition comprising the steps a) of subjecting a starting mixture containing hydroxy acid and at least one polyalcohol comprising at least four hydroxyl groups per molecule to an oligomerization reaction so as to obtain as reaction mixture an oligomer composition containing hydroxy acid oligomer and b) of subjecting the oligomer composition obtained in step a) or a modified oligomer composition, which is obtained by adding at least one polyalcohol comprising at least four hydroxyl groups per molecule to the oligomer composition obtained in step a) and incubating the so ob- tained mixture so as to obtain the modified oligomer composition, to a cyclization reaction so as to obtain a cyclic ester of the hydroxy acid containing composition, wherein the molar ratio of the hydroxy acid carboxylic groups divided by the polyalcohol hydroxy groups in the starting mixture is at least 2, and wherein during the oligomerization reaction at least a portion of the reaction mixture, which is formed during the oligomerization reaction, is at least temporarily distilled so as to remove water from the reaction mixture.

[0010] This solution is based on the surprising finding that by performing the oligomerization reaction with a starting mixture containing in addition to hydroxy acid, such as in particular lactic acid, at least one polyalcohol comprising at least four hydroxyl groups per molecule, wherein the molar ratio of the hydroxy acid carboxylic groups divided by the polyalcohol hydroxy groups in the starting mixture is at least 2, and by at least temporarily distilling during the oligomerization reaction at least a portion of the reaction mixture, which is formed during the oligomerization reaction, so as to remove water from the reaction mixture, the method may be performed with comparably short oligomers. This is due to the fact that the oligomerization of hydroxy acid in the presence of a polyalcohol, wherein the molar ratio of the hydroxy acid carboxylic groups divided by the polyalcohol hydroxy groups in the starting mixture is at least 2, leads to hydroxy acid oligomers, which do not form water during the cyclization reaction, because the oligomers have - due to the reaction of hydroxy acid with the polyalcohol during the oligomerization reaction - no free carboxylic acid groups, but instead a terminal ester group, and thus cannot perform an end-biting reaction to cyclic ester of the hydroxy acid and water. Thereby, the disadvantages of the use of short oligomers formed by oligomerizing hydroxy acid without polyalcohol, as described above, do not occur in the method in accordance with the present invention, hence allowing to use short hydroxy acid oligomers in the cyclization step of the method in accordance with the present invention. In turn, the use of comparably short hydroxy acid oligomers allows to perform the oligomerization reaction under mild reaction conditions, thus leading to optically pure cyclic ester of the hydroxy acid by avoiding the formation of high amounts of undesired meso-lactide. Moreover, the use of comparably short hydroxy acid oligomers leads to a reaction mixture with comparably low viscosity during the cyclization reaction, thereby facilitating the retrieval of cyclic ester of the hydroxy acid through distillation from the reaction mixture of the cyclization reaction, which in turn leads to a significant increase of the productivity of the method at low operational costs. All in all, the method in accordance with the present invention allows to efficiently and in high yield synthesize cyclic ester of hydroxy acid, such as in particular lactide, with excellent optical purity and with minimal operational costs.

[0011] More specifically, the oligomerization of hydroxy acid in the presence of polyalcohol leads to the reaction between a hydroxyl group of the polyalcohol and the acid group of hydroxy acid, thereby forming an ester bond. Polycondensation between hydroxy acid molecules takes place simultaneously, but is strongly limited due to the competing esterification of hydroxy acid molecules with the polyalcohol. By adjusting the molar ratio between acid groups of hydroxy acid and hydroxyl groups of the polyalcohol so that the molar ratio of the hydroxy acid carboxylic groups divided by the polyalcohol hydroxy groups in the starting mixture is at least 2, the maximum average molecular weight of the resulting hydroxy acid oligomers can be minimized, without forming significant amounts of species that are not able to perform back-biting reactions during cyclization as they would be generated when the oligomerization reaction is performed with a molar ratio of the hydroxy acid carboxylic groups divided by the polyalcohol hydroxy groups in the starting mixture of less than 2. During the initial esterification and polycondensation during the oligomerization reaction, water is generated and the generated water is removed during the oligomerization step from the reaction mixture, which is formed during the oligomerization reaction, by distillation. It is an advantage of the method in accordance with the present invention that the addition of a polyalcohol provides an additional driving force for the oligomerization and that it limits the ultimate molar mass and viscosity, therefore driving the water formation and facilitating the removal of the complete amount of water being generated during the oligomerization step a), being that portion of water, which is generated when a molar ratio of the hydroxy acid carboxylic groups divided by the polyalcohol hydroxy groups in the starting mixture of at least 2 converts to oligomers. Therefore, the time and energy needed to remove water in the initial oligomerization stage can be greatly reduced.

[0012] In accordance with the present invention, the method comprises a step a) of forming an oligomer composition containing hydroxy acid oligomer. The term “oligomer” as used herein is defined broadly as any macromolecule being composed of 2 to 99 repeating units, whereas polymers are macromolecules being composed of at least 100 repeating units.

[0013] In turn, cyclic ester means in accordance with the present invention any cyclic molecule containing an ester group in the cyclic structure. In other words, a cyclic molecule containing an ester group only as attached side group, but not in the cyclic structure, is not considered in the present invention as cyclic ester. The cyclic ester may be a cyclic monoester, such as in the case of lactones, for instance s-capro lactone or y-valerolactone, or a cyclic diester, such as in the case of lactide and glycolide.

[0014] Moreover, in accordance with the present invention the starting mixture contains at least one polyalcohol comprising at least four hydroxyl groups per molecule. The term polyalcohol is defined in accordance with the present invention to be a compound comprising at least four hydroxyl groups per molecule. For the ease of formulation, above and subsequently the term polyalcohol is also used without mentioning the number of hydroxyl groups per molecule, but in fact then also a polyalcohol with at least four hydroxyl groups per molecule is meant. In accordance with the present invention, hydroxy acid being contained in a starting mixture containing the hydroxy acid is converted to a cyclic ester of the hydroxy acid. The present invention is not particularly limited concerning the kind of used hydroxy acid and of produced cyclic ester of hydroxy acid, as long as the kind of the hydroxy acid, from which the cyclic ester is formed, is the same as the hydroxy acid being contained in the starting mixture. Preferably, the hydroxy acid is a co-hydroxy acid or a a-hydroxy acid, wherein the hydroxy acid is preferably selected from the group consisting of lactic acid, glycolic acid, 6-hydroxyhexanoic acid, 5-hydroxypentanoic acid and arbitrary combinations of two or more of the aforementioned acids. Consequently, preferably the method of the present invention leads to a cyclic ester, which is selected from the group consisting of lactide, glycolide, caprolactone, valerolactone and arbitrary combinations of two or more of the aforementioned cyclic esters. Most preferably, the hydroxy acid is lactic acid, i.e. the method produces lactide from lactic acid.

[0015] In accordance with the present invention, the starting mixture comprises at least one polyalcohol comprising at least four hydroxyl groups per molecule. The present invention is not particularly restricted concerning the chemical nature of the at least one polyalcohol used in the oligomerization step a). However, it is preferred that the volatility of the cyclic ester of hydroxy acid, which is prepared with the process according to the present invention, is at least 1 .2 times higher than the volatility of the at least one polyalcohol. This allows to comparably easily separate the cyclic ester of hydroxy acid from the reaction mixture of the cyclization step b). More preferably, the volatility of the cyclic ester of hydroxy acid is more than 6.2 times higher and most preferably more than 20 times higher than the volatility of the at least one polyalcohol. For instance, the at least one polyalcohol has a volatility at 200°C of 5.2 kPa or less, more preferably of 1 kPa or less, still more preferably of 0.1 kPa or less and most preferably of 0.01 kPa or less, wherein the volatility is measured in accordance with ASTM D2879-18 or ASTM E1194-17, depending on the volatility of the alcohol. Suitable examples of polyalcohols are those being selected from the group consisting of alkane polyols, polyether polyols, polyester polyols, polyamide polyols, polyphenols, saccharides and arbitrary combinations thereof.

[0016] Good results are in particular obtained, when the at least one polyalcohol being contained in the starting mixture is selected from the group consisting of pentaerythritol, dipentaerythritol, tripentaerythritol, diglycerol, triglycerol, multi-armed oligo(ethylene glycol), multi-armed oligo(lactic acid), monosaccharides, disaccharides, cyclitols and arbitrary combinations thereof.

[0017] Most preferably, the at least one polyalcohol being contained in the starting mixture is tripentaerythritol, triglycerol or trehalose.

[0018] In the cyclization step b), either the oligomer composition being produced in the oligomerization step a) or a modified oligomer composition, which is obtained by adding at least one polyalcohol comprising at least four hydroxyl groups per molecule to the oligomer composition being produced in the oligomerization step a), is used. The polyalcohol being used for preparing the modified oligomer composition may be any of the aforementioned polyalcohols. The at least one polyalcohol being contained in the starting mixture of the oligomerization step a) may be the same or may be different to the at least one polyalcohol being used for preparing the modified oligomer composition. However, it is preferred that in the embodiment of using for the cyclization step b) a modified oligomer composition, the at least one polyalcohol being contained in the starting mixture of the oligomerization step a) is the same as the at least one polyalcohol being used for preparing the modified oligomer composition.

[0019] In a further development of the idea of the present invention, it is suggested that the starting mixture contains, based on 100% by mole of the starting mixture, 5 to 99.99% by mole of hydroxy acid, 0.01 to 11 % by mole of polyalcohol and 0 to 95% by mole of water. Even more preferably, the starting mixture contains, based on 100% by mole of the starting mixture, 30 to 90% by mole of hydroxy acid, 0.05 to 10% by mole of polyalcohol and 10 to 70% by mole of water. Most preferably, the starting mixture contains, based on 100% by mole of the starting mixture, 50 to 80% by mole of hydroxy acid, 0.5 to 9% by mole of polyalcohol and 20 to 45% by mole of water.

[0020] The oligomerization step a) may be performed in the presence or in the absence of catalyst. If the oligomerization step a) is performed in the presence of a catalyst, it is preferred that the starting mixture contains at least one catalyst being selected from the group consisting of metal oxides, metal alkoxides, metal carbonates, metal bicarbonates, organometallic compounds and arbitrary mixtures of two or more thereof. The catalyst may be a homogenous catalyst or a heterogeneous catalyst. While homogeneous catalyst means a catalyst that is present in the same phase as the reaction mixture of the oligomerization reaction, a heterogeneous catalysts is in a different phase.

[0021] Suitable examples for a metal alkoxide catalyst are tin alkoxides, zinc alkoxides, aluminum alkoxides and titanium alkoxides, whereas suitable examples for a metal carboxylate catalyst are tin carboxylates, zinc carboxylates, aluminum carboxylates and titanium carboxylates. Preferably, the metal carboxylate catalyst comprises tin carboxylate comprising one or more C2-i4-carboxylate groups, more preferably one or more Ce-u-carboxylate groups and most preferably tin octoate. In turn, suitable examples for a metal oxide catalyst are transition metal oxides, such as tin oxide, iron oxide and copper oxide, whereas suitable examples for organometallic compounds are those comprising a metal selected from the group consisting of magnesium, titanium, zinc, aluminum, indium, yttrium, tin, lead, antimony, bismuth and any combination of two or more of the aforementioned metals and an organic residue being selected from the group consisting of alkyl groups, aryl groups, halides, oxides, alkanoates, alkoxides and any combination of two or more of the aforementioned groups. In addition, suitable examples for metal bicarbonates are alkali metal bicarbonates and alkaline earth metal bicarbonates.

[0022] Most preferably, the starting mixture contains at least one catalyst being selected from the group consisting of tin oxide, iron oxide, copper oxide, tin octanoate, butyl tin oxide, calcium carbonate, potassium carbonate, tin alkoxides, zinc alkoxides, aluminum alkoxides, titanium alkoxides, tin carboxylates, zinc carboxylates, aluminum carboxylates, titanium carboxylates and arbitrary mixtures of two or more of these compounds.

[0023] Good results are in particular obtained, when the starting mixture contains, based on 100% by mole of the starting mixture, 0 to 50,000 ppm and more preferably 500 to 5,000 ppm of catalyst.

[0024] In accordance with the present invention, the molar ratio of the hydroxy acid carboxylic groups divided by the polyalcohol hydroxy groups in the starting mixture is at least 2. The molar ratio of the hydroxy acid carboxylic groups divided by the polyalcohol hydroxy groups in the starting mixture corresponds to the quotient of (mole content of hydroxy acid in the starting mixture) I (mole content of polyalcohol in the starting mixture multiplied by the number of hydroxyl groups per polyalcohol molecule).

[0025] Good results are in particular obtained, when the molar ratio of the hydroxy acid carboxylic groups divided by the polyalcohol hydroxy groups in the starting mixture is more than 2 to 50 and most preferably 2 to 10.

[0026] In accordance with another preferred embodiment of the present invention, the oligomerization reaction is performed in step a) at a temperature of 80 to 250°C. Good results are in particular obtained, when the oligomerization reaction is performed at a temperature of 120 to 200°C.

[0027] In a further development of the idea of the present invention, it is proposed that the oligomerization reaction is performed in step a) at a pressure of 0.1 to 90 kPa. Good results are in particular obtained, when the oligomerization reaction is performed at a pressure of 1 to 35 kPa.

[0028] Preferably, the reaction time in step a) is 1 minute to 48 hours and more preferably 1 to 8 hours.

[0029] The distillation during the oligomerization step a) may be performed with one distillation column or with a combination of two or more distillation columns being arranged in series to each other or being arranged parallel to each other. Preferably, the one or more distillation columns are arranged on top of one or more reactors, in which the oligomerization step a) is performed. For instance, the oligomerization step a) is performed in one reactor on top of which a distillation column is arranged.

[0030] The present invention is not particularly limited concerning the number and kind of reactor(s), in which the oligomerization step a) is performed. For instance, the oligomerization step a) may be performed in a reactor system, which comprises one or more continuous stirred-tank reactors, one or more loop reactors, one or more plug flow reactors and arbitrary combinations of one or more of the above mentioned reactors. For instance, the oligomerization step a) is performed in a continuous stirred-tank reactor or in a plug flow reactor.

[0031] In a further preferred embodiment of the present invention, the distillation in step b) is performed in a distillation column, which is preferably arranged on top of the reactor which is more preferably a continuous stirred-tank reactor. In accordance with the present invention, at least a portion of the reaction mixture, which is formed during the oligomerization reaction, is at least temporarily distilled so as to remove water from the reaction mixture of the oligomerization step a). This comprises that a portion of for instance 10 to 90% by weight or 30 to 60% by weight of the reaction mixture is temporarily, such as for example for 10 to 90% or 30 to 60% of the reaction time of the oligomerization step a), distilled. Alternatively, the complete reaction mixture is temporarily, such as for example for 10 to 90% or 30 to 60% of the reaction time of the oligomerization step a), distilled. Still alternatively, a portion of for instance 10 to 90% by weight or 30 to 60% by weight of the reaction mixture is distilled over the complete reaction time of the oligomerization step a). Still alternatively, the complete reaction mixture is distilled over the complete reaction time of the oligomerization step a).

[0032] In accordance with a particular preferred embodiment of the present invention, the oligomerization reaction is performed for a first time period at a first temperature and at a first pressure, before the temperature is increased from the first temperature to a second temperature and / or the pressure is increased from the first pressure to a second pressure and the oligomerization reaction is continued at the second temperature and at the second pressure for a second time period. Likewise, the aforementioned embodiment may be continued so that afterwards the temperature is increased from the second temperature to a third temperature and / or the pressure is increased from the second pressure to a third pressure and the oligomerization reaction is continued at the third temperature and at the third pressure for a third time period. Also, the aforementioned embodiment may be continued so that afterwards the temperature is increased from the third temperature to a fourth temperature and / or the pressure is increased from the third pressure to a fourth pressure and the oligomerization reaction is continued at the fourth temperature and at the fourth pressure for a fourth time period. Further temperature and / or pressure changes may follow. In these embodiments, no distillation may be performed during the first time period, but only during the second time period so that during the first time period less operational costs are necessary as for the second time period and thus the operational costs are lower than when the complete oligomerization step b) is performed at the second temperature and pressure. For instance, the first time period is 1 minute to 24 hours and preferably 30 minutes to 4 hours, whereas the first temperature is 100 to 140°C and the first pressure is 3 to 15 kPa. Moreover, the second time period may be 1 minute to 24 hours and preferably 30 minutes to 4 hours, whereas the second temperature is more than 140 to 180°C and the second pressure is 5 to 90 kPa. Good results are in particular achieved, when the difference between the second temperature and the first temperature is 20 to 80°C and / or the difference between the second pressure and the first pressure is 15 to 80 kPa.

[0033] As set out above, it is an advantage of the method in accordance with the present invention that the addition of a polyalcohol provides an additional driving force for the oligomerization and that it limits the ultimate molar mass and viscosity, therefore driving the water formation and facilitating the removal of the complete amount of water being generated during the oligomerization step a), being that portion of water, which is generated when a molar ratio of the hydroxy acid carboxylic groups divided by the polyalcohol hydroxy groups in the starting mixture of at least 2 converts to oligomers, thereby reducing the time and energy needed to remove water in the oligomerization step a). Likewise, the water content of the crude hydroxy acid composition used for preparing the starting mixture for the oligomerization step a) is preferably reduced, but the water is preferably not completely removed from the crude hydroxy acid. Usually, lactic acid as example for hydroxy acid is obtained, for instance by fermentation, as aqueous lactic acid solution having a water content of 10 to 30% by weight.

[0034] In view thereof, it is suggested in a further development of the idea of the present patent application that the water contained in the crude hydroxy acid composition or aqueous hydroxy acid solution, respectively, is reduced, either before the aqueous hydroxy acid solution is mixed with the polyalcohol so as to form the starting mixture or, less preferred, after the aqueous hydroxy acid solution has been mixed with the polyalcohol, but before the oligomerization reaction is started. Accordingly, the starting mixture being subjected to the oligomerization reaction may be prepared according to a first variant of this embodiment by mixing an aqueous solution of hydroxy acid with at least one polyalcohol and optionally with at least one catalyst and by dewatering the so obtained blend, wherein the blend is preferably dewatered so that 90 to 98% by weight of the water being contained in the aqueous solution of hydroxy acid before having mixed it with at least one polyalcohol and optionally with at least one catalyst is, removed during the dewatering from the blend. According to a second variant, the starting mixture being subjected to the oligomerization reaction is prepared by dewatering an aqueous solution of hydroxy acid in water, wherein during the dewatering preferably 90 to 98% by weight of the water are removed from the aqueous solution, and by then mixing the dewatered aqueous solution with at least one polyalcohol and optionally with at least one catalyst. The dewatering is preferably performed by distillation in one or two or more distillation columns and preferably in one or two distillation columns being arranged in series to each other.

[0035] In accordance with a further particularly preferred embodiment of the present invention, during the oligomerization reaction water in an amount preferably corresponding to at least 60% of the maximum water that can be produced from the oligomerization is removed from the mixture by distillation. More preferably at least 70% of the maximum water that can be produced from the oligomerization is removed from the mixture by distillation. And most preferably at least 75% of the maximum water that can be produced from the oligomerization is removed from the mixture by distillation. The maximum being defined by the weight percentage of the hydroxy acid mass being fed into the method that may be liberated in the form of reaction water via esterification and is calculated as (molar mass of wa- ter) / (molar mass of hydroxy acid)*100%. Hydroxy acid mass being fed into the method means the weight of hydroxy acid (without water content) being fed into the starting mixture.

[0036] In a further development of the idea of the present invention, it is proposed that the oligomer composition or the modified oligomer composition, if a modified oligomer composition is prepared, contains hydroxy acid oligomer arms having in average 2 to 50 and preferably 2 to 10 average repeating units.

[0037] In accordance with another preferred embodiment of the present invention, the cyclization reaction is performed in step b) at a temperature of 180 to 230°C. Good results are in particular obtained, when the cyclization reaction is performed at a temperature of 190 to 210°C.

[0038] In a further development of the idea of the present invention, it is proposed that the cyclization reaction is performed in step b) at a pressure of 100 Pa to 5 kPa. Good results are in particular obtained, when the cyclization reaction is performed at a pressure of 100 to 800 Pa.

[0039] Preferably, the reaction time in step b) is 1 minute to 24 hours and more preferably 30 minutes to 4 hours.

[0040] The present invention is not particularly restricted regarding the kind of reactor used for the cyclization step b) and the same reactor(s) mentioned above for the oligomerization step a) may be used for the cyclization step b).

[0041] The cyclization step b) may be performed with or without catalyst. Preferably, the cyclization step b) is performed with catalyst, i.e. at least one catalyst is already present in the oligomer composition because it has been added to the starting mixture, and / or at least one catalyst is added to the oligomer composition or modi- fied oligomer composition, respectively. If at least one catalyst is added to the starting mixture and at least one catalyst is added to the oligomer composition or modified oligomer composition, respectively, the catalyst may be any of the catalysts mentioned above as suitable for performing the oligomerization step a) and it may be used in the same amount as described above for the catalyst content during the oligomerization step a). Moreover, the at least one catalyst used in the cyclization step b) may be the same or a different one as that used in the oligomerization step a).

[0042] In a further development of the idea of the present invention, it is suggested that during and / or after the cyclization reaction at least a portion of the reaction mixture being formed during the cyclization reaction is at least temporarily distilled so as to separate the cyclic ester of hydroxy acid containing composition from the reaction mixture. This comprises that a portion of for instance 10 to 90% by weight or 30 to 60% by weight of the reaction mixture is temporarily, such as for example for 10 to 90% or 30 to 60% of the reaction time of the cyclization step b), distilled. Alternatively, the complete reaction mixture is temporarily, such as for example for 10 to 90% or 30 to 60% of the reaction time of the cyclization step b), distilled. Still alternatively, a portion of for instance 10 to 90% by weight or 30 to 60% by weight of the reaction mixture is distilled over the complete reaction time of the cyclization step b). Still alternatively and in fact preferably, the complete reaction mixture is distilled over the complete reaction time of the cyclization step b).

[0043] In accordance with a particular preferred embodiment of the present invention, all or a portion of the reaction mixture of the cyclization step b) is recycled to the starting mixture being subjected to the oligomerization reaction in step a) or to an optional dewatering step used to prepare the starting mixture being subjected to the oligomerization reaction in step a) after dewatering. If not all of the reaction mixture of the cyclization step b) is recycled, it is preferred that 5 to 95% by weight and more preferably 25 to 75% by weight of the reaction mixture of the cyclization step b) is recycled to the starting mixture being subjected to the oligomerization reaction in step a) or to an optional dewatering step used to prepare the starting mixture being subjected to the oligomerization reaction in step a).

[0044] In accordance with a further particular preferred embodiment of the present invention, during the oligomerization step a) oligomer having a slightly higher average molecular weight as intended for the cyclization reaction is prepared, before this oligomer is subjected to an alcoholysis step for redistributing or lowering the average molecular weight of the oligomer, before the so obtained modified oligomer composition is subjected to the cyclization reaction. This embodiment has the advantage that an even better control of the average molecular weight of the oligomer composition being subjected to cyclization is possible and the generation of undesired, unreacted hydroxy acid species without back-biting capability is reduced in the oligomer composition being subjected to cyclization to a minimum. The alcoholysis step is performed by adding at least one polyalcohol comprising at least four hydroxyl groups per molecule to the oligomer composition obtained in the oligomerization step a) and by incubating the so obtained mixture so as to obtain a modified oligomer composition, which is then subjected to the cyclization reaction b) so as to obtain the cyclic ester of hydroxy acid containing composition. Incubation means in this connection that the mixture is maintained for a certain period at a certain temperature. Preferably, the modified oligomer composition is prepared by incubating the mixture being obtained after adding the at least one polyalcohol to the oligomer composition at a temperature of 120 to 220°C, more preferably at a temperature of 160 to 200°C and most preferably at a temperature of 170-190 °C. A pressure of 1 to 3000 kPa for 5 minutes to 24 hours is preferably applied. Exceptionally good results are obtained if this alcoholysis step is performed in static mixing equipment operated at a pressure of at least 100 kPa and up to 3000 kPa, allowing for notably short reaction times of preferably between 15 and 60 minutes. Good results are in particular obtained in this embodiment, when the molar ratio of the hydroxy acid carboxylic groups divided by the polyalcohol hydroxy groups in the starting mixture is at least 4 and when the molar ratio of the resulting oligomer’s ester groups divided by the polyalcohol hydroxy groups in the modified oligomer composition being obtained after adding the at least one polyalcohol to the oligomer composition during incubation is at least 2. I.e. the molar ratio of hydroxy acid carboxylic groups fed into the process divided by the total number of polyalcohol hydroxy groups comprising those in the starting mixture and those added into the incubation step, corresponds to the quotient of (mole content of hydroxy acid in the starting mixture) I ([mole content of polyalcohol in the starting mixture multiplied by the number of hydroxyl groups per polyalcohol molecule] + [mole content of polyalcohol added to the incubation step multiplied by the number of hydroxy groups per polyalcohol molecule]), which is preferably at least 2.

[0045] Furthermore, it is preferred that at least one catalyst is present during the alcoholysis step, i.e. that in addition to at least one polyalcohol also at least one catalyst is added the oligomer composition, before this is incubated so as to obtain the modified oligomer composition. The at least one catalyst may be added to the oligomer composition simultaneously, before or after the addition of the polyalcohol. Alternatively, the at least one catalyst may be already added to the starting mixture of step a) so that it is still present in the oligomer composition during the alcoholysis step. Still alternatively, at least one catalyst may be already added to the starting mixture of step a) so that it is still present in the oligomer composition during the alcoholysis step, and simultaneously, before or after the addition of the polyalcohol at least one catalyst, which may be the same or different as that contained in the starting mixture, is added to the oligomer composition. The at least one catalyst added simultaneously, before or after the addition of the polyalcohol to the oligomer composition may be any of the catalysts mentioned above. In a further development of the idea of the present invention, it is proposed that the cyclic ester of hydroxy acid containing composition comprises, based on 100% by mole of the cyclic ester of hydroxy acid containing composition, at least 40% by mole, preferably at least 60% by mole and more preferably at least 75% by mole of cyclic ester of hydroxy acid.

[0046] Moreover, in case that the cyclic ester of hydroxy acid is lactide, it is preferred that at least 60% by mole, preferably at least 75% by mole and more preferably at least 90% by mole of the lactide contained in the lactide composition is L-lactide or D- lactide.

[0047] In accordance with another aspect, the present invention relates to a plant for preparing a cyclic ester of hydroxy acid containing composition comprising: i) an oligomerization reactor system comprising at least one oligomerization reactor and at least one distillation column, wherein the oligomerization reactor comprises at least one inlet for starting mixture and an outlet for oligomer composition, ii) optionally at least one alcoholysis reactor or static mixer comprising an inlet being connected with the outlet for oligomer composition of the at least one oligomerization reactor, an inlet for at least one polyalcohol, an optional inlet being connected with the outlet for reaction mixture from a cyclization reactor and an outlet for modified oligomer composition and iii) a cyclization reactor system comprising at least one cyclization reactor and at least one distillation column, wherein the cyclization reactor comprises an inlet being connected with the outlet for modified oligomer composition of the at least one alcoholysis reactor, an outlet for cyclic ester of hydroxy acid containing composition and an outlet for reaction mixture.

[0048] Good results are in particular obtained, when the plant further comprises a dewatering system comprising at least one dewatering reactor, which comprises an inlet for aqueous solution of hydroxy acid, an inlet for at least one polyalcohol, an outlet for water and an outlet for starting mixture, wherein the outlet for starting mixture is connected with at least one inlet of the at least one oligomerization reactor. Preferably, the dewatering system comprises one or two distillation columns being arranged in series to each other.

[0049] In accordance with a further preferred embodiment of the present invention, the outlet for reaction mixture of the at least one cyclization reactor is connected with an inlet of the at least one oligomerization reactor and / or with an inlet of the at least one dewatering reactor. In an additionally preferred embodiment of the current invention, the optional at least one alcoholysis reactor is a pressurized static mixer comprising an inlet being connected with the outlet for oligomer composition of the at least one oligomerization reactor, an inlet for at least one polyalcohol and an outlet for modified oligomer composition, and an additional inlet connecting the outlet for reaction mixture from the at least one cyclization reactor to the static mixer.

[0050] Subsequently, the present invention is described by means of illustrative, but not limiting figures, in which:

[0051] Fig. 1 shows a schematic view of a plant for conducting a method for preparing a lactide composition in accordance with one embodiment of the present invention.

[0052] Fig. 2 shows a schematic view of a plant for conducting a method for preparing a lactide composition in accordance with another embodiment of the present invention.

[0053] Fig. 3 shows a schematic view of a plant for conducting a method for preparing a lactide composition in accordance with still another embodiment of the present invention. Fig. 4 shows a schematic view of a plant for conducting a method for preparing a lactide composition in accordance with still another embodiment of the present invention.

[0054] Fig. 5 shows a schematic view of a plant for conducting a method for preparing a lactide composition in accordance with still another embodiment of the present invention.

[0055] Fig. 6 shows a schematic view of a plant for conducting a method for preparing a lactide composition in accordance with still another embodiment of the present invention.

[0056] The plant 10 shown in figure 1 comprises, in series, a dewatering distillation column 12, an oligomerization reactor 14 and a cyclization reactor 16. While the dewatering distillation column 12 comprises an inlet line 17 for aqueous lactic acid, an inlet line 18 for polyalcohol, an outlet line 20 for starting mixture and an outlet line 22 for water, the oligomerization reactor 14 comprises an inlet being connected with the outlet line 20 for starting mixture of the dewatering distillation column 12, an outlet line 24 for oligomer composition and an outlet line 26 for water. In turn, the cyclization reactor 16 comprises an inlet being connected with the outlet line 24 for oligomer composition of the cyclization reactor 16, an inlet line 28 for catalyst, an outlet line 30 for reaction mixture and an outlet line 32 for lactide.

[0057] During the operation of the plant 10, aqueous lactic acid with a water content of 10 to 30% by weight and polyalcohol comprising at least 4 hydroxyl groups per molecule are fed via inlet lines 17 and 18 into the dewatering distillation column 12, where the mixture is distilled so as to separate it into water being withdrawn from the dewatering distillation column 12 via outlet line 22 and into starting composition. The starting composition is then fed via line 20 into the oligomerization reac- tor 14, where it is reacted at an appropriate temperature and at an appropriate pressure for an appropriate reaction time to lactic acid oligomer and water. While the water is separated from the reaction mixture through distillation and is removed from the oligomerization reactor 14 via outlet line 26, oligomer composition is withdrawn from the oligomerization reactor 14 via outlet line 24 and is fed into the cyclization reactor 16. In addition, catalyst is fed into the cyclization reactor 16 via inlet line 28 and the lactic acid oligomer reacts in the presence of the catalyst in the cyclization reactor 16 at an appropriate temperature and at an appropriate pressure for an appropriate reaction time to lactide and low amounts of polyalcohol. While the lactide is separated from the reaction mixture through distillation and is removed from the cyclization reactor 16 via outlet line 32, remaining reaction mixture including catalyst, rest of lactic acid oligomers and polyalcohol is withdrawn from the cyclization reactor 16 via outlet line 30.

[0058] The plant 10 shown in figure 2 corresponds to that shown in figure 1 except that the inlet line 28 for catalyst does not lead into the cyclization reactor 16, but into the dewatering distillation column 12.

[0059] In turn, the plant 10 shown in figure 3 corresponds to that shown in figure 1 except that it further comprises a recycle line 34 connecting the outlet line 30 for reaction mixture and an inlet of the dewatering distillation column 12. In addition, a purge line 36 branches off from the outlet line 30.

[0060] Furthermore, the plant 10 shown in figure 4 corresponds to that shown in figure 1 except that the inlet line 18 for polyalcohol does not lead into the dewatering distillation column 12, but into the oligomerization reactor 14.

[0061] In addition, the plant 10 shown in figure 5 corresponds to that shown in figure 1 except that an alcoholysis reactor 38 is arranged between the oligomerization reactor 14 and the cyclization reactor 16. Thus, the outlet line 24 for oligomer composition of the oligomerization reactor 14 is connected with an inlet of the alcoholysis reactor 38, which further comprises an inlet line 40 for polyalcohol and an outlet line 42 for modified oligomer composition. The outlet line 42 for modified oligomer composition of the alcoholysis reactor 38 is connected with an inlet of the cyclization reactor 16. Optionally, the alcoholysis reactor 38 consists of a pressurized static mixer equipment.

[0062] Finally, the plant 10 in figure 6 corresponds to that in figure 5 except that the alcoholysis reactor is a static mixer 39 and further comprises a recycle line 34 connecting the outlet line for reaction mixture 30 and an inlet line of the static mixer 39. In addition, a purge line 36 branches of from the outlet line 30.

[0063] Subsequently, the present invention is described by means of illustrative, but not limiting examples.

[0064] Example 1

[0065] (Lactide production from alcohol-capped lactic acid oligomers)

[0066] In a 2-necked round-bottom flask equipped with a vigreux column, a water-cooled condenser and a collection flask, 68.6 g (0.657 mol lactic acid) of aqueous lactic acid and 15.3 g (41.06 mmol) of tripentaerythritol were mixed. Molar mass aim was 1525.4 g / mol, 144.12 g / mol per oligomer arm for this composition. Under stirring, the mixture was heated to 130°C and a pressure of 82 mbara was applied to distill out water. The reaction mixture was left under these conditions for 2 hours, after which a temperature of 160°C was set and pressure was increased to 310 mbara. The reaction mixture was left to stir for 2 hours under these conditions, whilst removing water. Thereafter, the vigreux column and water-cooled condenser were exchanged to a heated condenser with collection flask, which was set to 103°C. The reaction flask was heated to 195°C and 90 mg of stannous octoate was added into the reaction mixture, after which a pressure of 6 mbara was ap- plied. Crude lactide was collected under these conditions over 2 hours, with a water content of 0.2 wt% and a chiral purity of 98.5 wt% L-lactide.

[0067] Example 2

[0068] (Lactide production from alcohol-capped lactic acid oligomers with early catalyst addition, 1525.4 g / mol, 144.12 g / mol per hydroxyl group)

[0069] In a 2-necked round-bottom flask equipped with a vigreux column, a water-cooled condenser and a collection flask, 73.3 g (0.700 mol lactic acid) of aqueous lactic acid, 15.7 g (42.16 mmol) of tripentaerythritol and 92 mg of stannous octoate were mixed at 130°C. Molar mass aim was 1525.4 g / mol, 144.12 g / mol per oligomer arm for this composition. A pressure of 82 mbara was applied to distill out water. The reaction mixture was left under these conditions for 2 hours, after which a temperature of 160°C was set and pressure was increased to 310 mbara. The reaction mixture was left to stir for 2 hours under these conditions, whilst removing water. Thereafter, the vigreux column and water-cooled condenser were exchanged to a heated condenser with collection flask, which was set to 103°C. The reaction flask was heated to 195°C and a pressure of 6 mbara was applied. Crude lactide was collected under these conditions over 2 hours, with a water content of 0.9 wt% and a chiral purity of 97.9 wt% L-lactide, demonstrating the catalyst can be added at earlier time points in the process.

[0070] Example 3

[0071] (Recycling of cyclization residues for application as a polyalcohol and subsequent lactide production)

[0072] In a 2-necked round-bottom flask equipped with a vigreux column, a water-cooled condenser and a collection flask, 33.84 g (0.324 mol lactic acid) of aqueous lactic acid and 44.3 g of cyclization residue from example 2 (estimated 42 mmol of oligomers and 92 mg of stannous octoate) were mixed at 130°C. Molar mass aim was 1610.7 g / mol, 154.8 g / mol per oligomer arm for this composition. A pressure of 82 mbara was applied to distill out water. The reaction mixture was left under these conditions for 2 hours, after which a temperature of 160°C was set and pressure was increased to 310 mbara. The reaction mixture was left to stir for 2 hours under these conditions, whilst removing water. Thereafter, the vigreux column and water-cooled condenser were exchanged to a heated condenser with collection flask, which was set to 103°C. The reaction flask was heated to 195°C and a pressure of 6 mbara was applied. Crude lactide was collected under these conditions over 2 hours, with a water content of 0.5 wt% and a chiral purity of 93 wt% L-lactide, demonstrating that residues from the process may be re-used, although at a finite cost of chiral purity.

[0073] Example 4

[0074] (Lactide production from alcohol-capped lactic acid oligomers using an alternative alcohol)

[0075] In a 2-necked round-bottom flask equipped with a vigreux column, a water-cooled condenser and a collection flask, 70 g (0.670 mol lactic acid) of aqueous lactic acid and 16.10 g (67.0 mmol) of triglycerol were mixed. Molar mass aim was 1393.2 g / mol, 144.12 g / mol per oligomer arm for this composition. Under stirring, the mixture was heated to 130°C and a pressure of 82 mbara was applied to distill out water. The reaction mixture was left under these conditions for 2 hours, after which a temperature of 160°C was set and pressure was increased to 310 mbara. The reaction mixture was left to stir for 2 hours under these conditions, whilst removing water. Thereafter, the vigreux column and water-cooled condenser were exchanged to a heated condenser with collection flask, which was set to 103°C. The reaction flask was heated to 195°C and 60 mg of stannous octoate was added into the reaction mixture, after which a pressure of 6 mbara was applied. Crude lactide was collected under these conditions over 2 hours, with a water content of 0.3 wt% and a chiral purity of 98.0 wt% L-lactide. Example 5

[0076] (Lactide production from intermediate length, alcohol-capped oligomers)

[0077] In a 2-necked round-bottom flask equipped with a vigreux column, a water-cooled condenser and a collection flask, 62.48 g (0.590 mol lactic acid) of aqueous lactic acid and 4.57 g (12.30 mmol) of tripentaerythritol were mixed. Molar mass aim was 3831 .4 g / mol, 432.37 g / mol per oligomer arm for this composition. Under stirring, the mixture was heated to 130°C and a pressure of 82 mbara was applied to distill out water. The reaction mixture was left under these conditions for 2 hours, after which a temperature of 180°C was set and pressure was decreased to 40 mbara. The reaction mixture was left to stir for 2 hours under these conditions, whilst removing water. Thereafter, the vigreux column and water-cooled condenser were exchanged to a heated condenser with collection flask, which was set to 103°C. The reaction flask was heated to 195°C and 53 mg of stannous octoate was added into the reaction mixture, after which a pressure of 6 mbara was applied. Crude lactide was collected under these conditions over 2 hours, with a water content of 0.4 wt% and a chiral purity of 98.1 wt% L-lactide.

[0078] Comparative Example 1

[0079] (Comparative experiment without addition of alcohol, short oligomers with free acid ends)

[0080] In a 2-necked round-bottom flask equipped with a vigreux column, a water-cooled condenser and a collection flask, 74.22 g (0.700 mol lactic acid) of aqueous lactic acid was added. Under stirring, the lactic acid was heated to 130°C and a pressure of 82 mbara was applied to distill out water. The reaction mixture was left under these conditions for 2 hours, after which a temperature of 160°C was set and pressure was decreased to 310 mbara. The reaction mixture was left to stir for 2 hours under these conditions, whilst removing water. Final molar mass aim un- der these conditions was 260.4 g / mol. Thereafter, the vigreux column and water- cooled condenser were exchanged to a heated condenser with collection flask, which was set to 103°C. The reaction flask was heated to 195°C and 112 mg of stannous octoate was added into the reaction mixture, after which a pressure of 6 mbara was applied. Crude lactide was collected under these conditions over 2 hours, with a water content of 4.5 wt% and a chiral purity of 98.7 wt% L-lactide.

[0081] Comparative Example 2

[0082] (Comparative experiment without addition of alcohol, long oligomers with free acid ends, 647.0 g / mol)

[0083] In a 2-necked round-bottom flask equipped with a vigreux column, a water-cooled condenser and a collection flask, 70.24 g (0.663 mol lactic acid) of aqueous lactic acid was added. Under stirring, the lactic acid was heated to 130°C and a pressure of 82 mbara was applied to distill out water. The reaction mixture was left under these conditions for 2 hours, after which temperature and pressure was adjusted to 160 °C and 310 mbara respectively, and kept for 2 hours. Temperature and pressure were subsequently adjusted to 180 °C and 80 mbara respectively, which was kept for 2 hours. Lastly, temperature and pressure were adjusted at 190 °C and 20 mbara respectively, and kept for another 2 hours. Water was removed from the reaction flask during all previous stages. Final molar mass aim under these conditions was 647.0 g / mol. Thereafter, the vigreux column and water-cooled condenser were exchanged to a heated condenser with collection flask, which was set to 103°C. The reaction flask was heated to 195°C and 100.4 mg of stannous octoate was added into the reaction mixture, after which a pressure of 6 mbara was applied. Crude lactide was collected under these conditions over 2 hours, with a water content of 1 .0 wt% and a chiral purity of 96 wt% L-lactide.

[0084] The foregoing Examples demonstrate that the present invention provides a method for preparing a cyclic ester of hydroxy acid that achieves both low water content and high chiral purity, thereby combining the advantages of short- and long- oligomer routes while avoiding their respective drawbacks.

[0085] It should be noted that the comparatively higher apparent molar masses observed in Examples 1-5, relative to Comparative Examples 1-2, arise from the fact that multiple lactic-acid oligomers become bound to the employed polyalcohols. Importantly, each individual oligomeric arm is not necessarily long, as reflected by the measured molar masses of the oligomer arms themselves. This structural characteristic explains the difference in overall molar mass without indicating the formation of excessively long oligomers.

[0086] The Examples further show that both water content and chiral composition were explicitly measured. All Examples 1-5 yield lactide with water contents below 1 wt%, markedly lower than the value obtained in the mild, short-oligomer Comparative Example 1 (4.5 wt%). Even when compared with the intentionally optimized long-oligomer route (Comparative Example 2), designed to suppress water, the water levels achieved in Examples 1-5 remain consistently and significantly lower.

[0087] With respect to chiral purity, Examples 1 , 2, 4, and 5 exhibit L-lactide purities of about 98%. These values are comparable to those observed in the mild shortoligomer process (Comparative Example 1 ) and are superior to the chiral purities obtained via the long-oligomer process (Comparative Example 2). Thus, the inventive process achieves the desired chiral stability without relying on harsh processing conditions or excessive oligomer growth.

[0088] Taken together, these findings confirm that the invention delivers the best attributes of both conventional approaches. A low water content, surpassing both short- and long-oligomer comparative processes, and high chiral purity, matching or exceeding the values seen in established mild short-oligomer methods. According- ly, the inventive process provides a robust and advantageous route for producing high-quality lactide from alcohol-capped lactic acid oligomers.

[0089] Reference numerals

[0090] 10 Plant

[0091] 12 Dewatering distillation column

[0092] 14 Oligomerization reactor

[0093] 16 Cyclization reactor

[0094] 17 Inlet line for hydroxy acid (lactic acid)

[0095] 18 Inlet line for polyalcohol

[0096] 20 Outlet line for starting mixture

[0097] 22 Outlet line for water

[0098] 24 Outlet line for oligomer composition

[0099] 26 Outlet line for water

[0100] 28 Inlet line for catalyst

[0101] 30 Outlet line for reaction mixture

[0102] 32 Outlet line for cyclic ester of hydroxy acid (lactide)

[0103] 34 Recycle line

[0104] 36 Purge line

[0105] 38 Alcoholysis reactor

[0106] 39 Static mixer

[0107] 40 Inlet line for polyalcohol

[0108] 42 Outlet line for modified oligomer composition

Claims

Claims:1 . A method for preparing a cyclic ester of hydroxy acid containing composition comprising the steps a) of subjecting a starting mixture containing hydroxy acid and at least one polyalcohol comprising at least four hydroxyl groups per molecule to an oligomerization reaction so as to obtain as reaction mixture an oligomer composition containing hydroxy acid oligomer and b) of subjecting the oligomer composition obtained in step a) or a modified oligomer composition, which is obtained by adding at least one polyalcohol comprising at least four hydroxyl groups per molecule to the oligomer composition obtained in step a) and incubating the so obtained mixture so as to obtain the modified oligomer composition, to a cyclization reaction so as to obtain a cyclic ester of hydroxy acid containing composition, wherein the molar ratio of the hydroxy acid carboxylic groups divided by the polyalcohol hydroxy groups in the starting mixture is at least 2, and wherein during the oligomerization reaction at least a portion of the reaction mixture, which is formed during the oligomerization reaction, is at least temporarily distilled so as to remove water from the reaction mixture.

2. The method in accordance with claim 1 , wherein the hydroxy acid is a cohydroxy acid or a a-hydroxy acid, which is preferably selected from the group consisting of lactic acid, glycolic acid, 6-hydroxyhexanoic acid, 5- hydroxypentanoic acid and arbitrary combinations of two or more of the aforementioned acids, wherein the hydroxy acid is most preferably lactic acid.

3. The method in accordance with claim 1 or 2, wherein the volatility of the cyclic ester of hydroxy acid is at least 1 .2 times higher, preferably more than6.2 times higher and most preferably more than 20 times higher than the volatility of the at least one polyalcohol.

4. The method in accordance with any of the preceding claims, wherein the at least one polyalcohol is selected from the group consisting of alkane polyols, polyether polyols, polyester polyols, polyamide polyols, polyphenols, saccharides and arbitrary combinations thereof, wherein preferably the at least one polyalcohol is selected from the group consisting of pentaerythritol, dipentaerythritol, tripentaerythritol, diglycerol, triglycerol, multi-armed ol- igo(ethylene glycol), multi-armed oligo(lactic acid), monosaccharides, disaccharides, cyclitols and arbitrary combinations thereof, wherein the at least one polyalcohol is preferably tripentaerythritol , triglycerol or trehalose.

5. The method in accordance with any of the preceding claims, wherein the starting mixture contains, based on 100% by mole of the starting mixture, 5 to 99.99% by mole of hydroxy acid, 0.01 to 11 % by mole of polyalcohol and 0 to 95% by mole of water.

6. The method in accordance with any of the preceding claims, wherein the molar ratio of the hydroxy acid carboxylic groups divided by the polyalcohol hydroxy groups in the starting mixture is more than 2 to 50 and preferably more than 2 to 10.

7. The method in accordance with any of the preceding claims, wherein the oligomerization reaction is performed at a temperature of 80 to 250°C and at a pressure of 0.1 to 90 kPa and preferably at a temperature of 120 to 200°C and at a pressure of 1 to 35 kPa.

8. The method in accordance with any of the preceding claims, wherein the oligomerization reaction is performed in one reactor or in two or more reac-tors being arranged in series to each other, wherein a distillation column is arranged on top of each of the one or more reactors.

9. The method in accordance with any of the preceding claims, wherein the oligomerization reaction is performed for a first time period at a first temperature and at a first pressure, before the temperature is increased from the first temperature to a second temperature and / or the pressure is increased from the first pressure to a second pressure and the oligomerization reaction is continued at the second temperature and at the second pressure for a second time period.

10. The method in accordance with any of the preceding claims, wherein the starting mixture being subjected to the oligomerization reaction is prepared i) by mixing an aqueous solution of hydroxy acid with at least one polyalcohol and optionally with at least one catalyst and by dewatering the so obtained blend, wherein the blend is preferably dewatered so that 90 to 98% by weight of the water being contained in the aqueous solution of hydroxy acid before having mixed it with at least one polyalcohol and optionally with at least one catalyst is removed during the dewatering from the blend, or ii) by dewatering an aqueous solution of hydroxy acid in water, wherein during the dewatering preferably 90 to 98% by weight of the water are removed from the aqueous solution, and by then mixing the dewatered aqueous solution with at least one polyalcohol and optionally with at least one catalyst.11 . The method in accordance with any of the preceding claims, wherein during the oligomerization reaction water in an amount corresponding to preferably at least 60%, more preferably at least 70% and even more preferably at least 75% of the maximum that can be produced by esterification is removed from the mixture by distillation, the maximum being defined as (mo-lar mass of water) / (molar mass of hydroxy acid)*100% of the weight of the hydroxy acid mass being fed into the method.

12. The method in accordance with any of the preceding claims, wherein the molar ratio of the hydroxy acid carboxylic groups divided by the polyalcohol hydroxy groups in the starting mixture is at least 4 and a modified oligomer composition is prepared by adding an amount of at least one polyalcohol to the oligomer composition so that the molar ratio of the oligomer ester groups divided by the polyalcohol hydroxy groups in the mixture being obtained after adding the at least one polyalcohol to the oligomer composition is at least 2.

13. The method in accordance with any of the preceding claims, wherein during and / or after the cyclization reaction at least a portion of the reaction mixture being formed during the cyclization reaction is at least temporarily distilled so as to separate the cyclic ester of hydroxy acid containing composition from the reaction mixture, wherein preferably all or a portion of the reaction mixture is recycled to the starting mixture being subjected to the oligomerization reaction, to an optional dewatering step used to prepare the starting mixture being subjected to the oligomerization reaction after dewatering or to an optional static mixing equipment used to alcoholyse the oligomer composition and the reaction mixture.

14. A plant for preparing a cyclic ester of hydroxy acid containing composition comprising: i) an oligomerization reactor system comprising at least one oligomerization reactor and at least one distillation column, wherein the oligomerization reactor comprises at least one inlet for starting mixture and an outlet for oligomer composition,ii) optionally at least one alcoholysis reactor or static mixer comprising an inlet being connected with the outlet for oligomer composition of the at least one oligomerization reactor, an inlet for at least one polyalcohol, an optional inlet being connected with the outlet for reaction mixture from a cyclization reactor and an outlet for modified oligomer composition and iii) a cyclization reactor system comprising at least one cyclization reactor and at least one distillation column, wherein the cyclization reactor comprises an inlet being connected with the outlet for modified oligomer composition of the at least one alcoholysis reactor, an outlet for cyclic ester of hydroxy acid containing composition and an outlet for reaction mixture.

15. The plant in accordance with claim 14, which further comprises a dewatering system comprising at least one dewatering reactor, which comprises an inlet for aqueous solution of hydroxy acid, an inlet for at least one polyalcohol, an outlet for water and an outlet for starting mixture, wherein the outlet for starting mixture is connected with at least one inlet of the at least one oligomerization reactor, wherein the outlet for reaction mixture of the at least one cyclization reactor is connected with an inlet of the at least one oligomerization reactor and / or with an inlet of the at least one dewatering reactor and / or with an inlet of the optional at least one alcoholysis reactor or static mixer.

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