A process for efficiently producing polylactone

Abstract

The present invention relates to a process of producing polylactone from lactone comprising a step a) of purifying a crude composition containing at least one lactone so as to obtain a purified lactone composition and a step b) of subjecting the purified lactone composition obtained in step a) to at least one ring-opening-polymerization step so as to obtain a polylactone composition, wherein the step a) of purifying the crude composition containing at least one lactone comprises at least one distillation step so as to obtain at least an overhead composition and a bottom composition and optionally at least one side composition as well as comprises at least one crystallization step of crystallizing the overhead composition, the bottom composition or the optional at least one side composition obtained in the at least one distillation step so as to obtain the purified lactone composition.

Description

[0001] A process for efficiently producing polylactone

[0002] The present invention relates to a process for producing polylactone, such as polycaprolactone.

[0003] Polylactones, i.e. homo- and copolymers based on lactone, are of particular interest, because they are completely biodegradable. Moreover, the technological properties of these polymers come quite close to the properties of non- biodegradable polymers, such as polyethylene, which explains why these polymers are regarded as highly promising substitutes for the latter. One example for a commercially important polylactone homopolymer is polycaprolactone, which is derived from a cyclic ester, namely from s-caprolactone, which in turn is originated from the intramolecular esterification of 6-hydroxycaproic 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. Polycaprolactone is for instance used as material for packings or for biomedical articles, such as drug delivery devices, adhesives, synthetic wound dressings and orthopedic imprints. Other examples for commercially interesting polylactones are polypropiolactone, polyvalerolactone, polybutyrolactone and the like. Polylactone copolymers are interesting alternatives to polylactone homopolymers, 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.

[0004] Generally, two alternative principal processes are known for synthesizing polylactones. The first principal process, which is still an important industrial process used for producing polylactones, is based on polycondensation, which is a batch process. However, this process has the disadvantages of requiring a long resi- dence time and of leading to quite impure polylactones having an undesired coloration as well as a comparable broad molecular weight distribution. The presence of unremoved residue caprolactone also hinder its application in the biomedical field. Therefore, more and more processes for producing polylactones are suggested, which based on the ring-opening-polymerization of lactones, i.e., of cyclic monoesters, usually in the presence of a catalyst and optionally an initiator to form polylactone. However, the ring-opening-polymerization of lactones to polylactones, such as of £-caprolactone to polycaprolactone, requires a good control of the purity of the £-caprolactone. In particular, lower quality £-caprolactone leads to polymer of low molecular weight and of high polydispersity even via ring-opening- polymerization. Furthermore, the ring-opening-polymerization step is performed in such processes for a long time to ensure a maximum yield of the polymer, which is, however, detrimental to the ultimate polymer properties. Also, the purity degree of the polylactones produced with common processes based on ring-opening- polymerization is in need of improvement, because the residual monomer content in the final polylactone compositions is at least 0.3% by weight, but not lower.

[0005] In view of this, the object of the present invention is to provide a process of producing polylactone from lactone by ring-opening-polymerization, wherein the process leads in high yield to particular pure polylactone having a residual lactone content, i.e. a residual monomer content, of less than 0.3% by weight and preferably of even less than 0.1% by weight, wherein the process requires only a short residence time, thus leading to polylactone having a narrow molecular weight distribution, and wherein the process is characterized by reduced capital expenditures as well as reduced operational costs.

[0006] In accordance with the present invention, this object is satisfied by providing a process of producing polylactone from lactone comprising a step a) of purifying a crude composition containing at least one lactone so as to obtain a purified lactone composition and a step b) of subjecting the purified lactone composition obtained in step a) to at least one ring-opening-polymerization step so as to obtain a polylactone composition, wherein the step a) of purifying the crude composition containing at least one lactone comprises at least one distillation step so as to obtain at least an overhead composition and a bottom composition and optionally at least one side composition as well as comprises at least one crystallization step of crystallizing the overhead composition, the bottom composition or the optional at least one side composition obtained in the at least one distillation step so as to obtain the purified lactone composition.

[0007] This solution bases on the surprising finding that by performing the ring-opening reaction with lactone having been purified first by distillation and then by crystallization, the residence time or reaction time, respectively, of the ring-opening- polymerization of lactone to polylactone with a given molecular weight can be significantly reduced, namely from about 9 hours to about 1 to 2 hours. This does not only lead to reduced capital expenditures as well as reduced operational costs required per ton product, but - due to the shorter residence time - also to polylactone having a particularly low polydispersity, i.e. a particularly narrow molecular weight distribution. In addition, the process in accordance with the present invention allows to produce very pure polylactone having a residual monomer content of as low as less than 0.1% by weight. Finally, the process in accordance with the present invention is characterized by a high yield. All in all, the process of producing polylactone from lactone in accordance with the present invention leads in high yield to particular pure polylactone having a residual lactone content of even less than 0.1% by weight, wherein the process requires only a short residence time, thus leading to polylactone having a narrow molecular weight distribution, and wherein the process is characterized by reduced capital expenditures as well as reduced operational costs required per ton product.

[0008] The term polymer as used herein means in accordance with the present invention any kind of polymer and thus covers homopolymers as well copolymers. In accordance with the present invention, polymers, such as in particular polylactones, are macromolecules being composed of at least 87 repeating subunits, whereas oligomers are macromolecules being composed of 2 to 86 repeating subunits.

[0009] The present invention is not particularly limited concerning the kind of the at least one lactone being contained in the crude composition. Good results are for instance obtained, when the at least one lactone is selected from the group consisting of propiolactone, caprolactone, valerolactone, butyrolactone, decanolactone and arbitrary combinations thereof. Specific examples are propiolactone, y-butyrolactone, 5-valerolactone, y-valerolactone, y-caprolactone, s-caprolactone, 5- decanolactone and y- decanolactone. Most preferably, the crude composition contains s-caprolactone as sole lactone or s-caprolactone in admixture with one or more other lactones.

[0010] In accordance with the present invention, step a) of purifying the crude composition containing at least one lactone comprises at least one distillation step. Preferably, the step a) of purifying a crude composition comprises one to ten subsequent distillation steps, more preferably one to five subsequent distillation steps, even more preferably one to three subsequent distillation steps, still more preferably one or two subsequent distillation steps and most preferably two distillation steps. Each of the distillation steps is performed in a distillation column, wherein the crude lactone composition is fed into the distillation column of the first distillation step, if more than one distillation steps are comprised, or into the distillation column of the one distillation step, if only one distillation step is comprised. During the at least one distillation step, lower boiling impurities, i.e. impurities having a lower boiling point than the lactone, such as - if the lactone is caprolactone - cyclohexanone, acetic acid and water are removed from the crude composition as overheads fraction from the distillation column, whereas the remaining crude composi- tion (such as 6-hydroxyhexanoic acid, 2-hydroxycyclohexanone, and other impurities) is withdrawn from the distillation column as bottom fraction, or lower boiling impurities are removed from the crude composition as overheads fraction and higher boiling impurities are removed from the crude composition as bottom fraction from the distillation column, whereas the remaining crude composition is withdrawn from the distillation column as side fraction. The remaining crude composition may then be fed into the at least one crystallization step or may then be fed into a second distillation column and further purified therein, before it is withdrawn from the second distillation column as bottom fraction, if further lower boiling impurities are removed, or as overheads fraction, if higher boiling impurities are removed, or as side fraction, if lower boiling impurities as well as higher boiling impurities are removed during the second distillation step. Particularly good results are obtained, when the step a) of purifying a crude composition comprises two distillation steps, wherein in the first distillation step light boilers, (e.g. acetic acid, cyclohexanone and other volatile organic compounds are removed as overhead fraction, whereas the remaining crude composition is withdrawn in the first distillation step from the distillation column as bottom fraction. The bottom fraction is then fed into the second distillation column used for the second distillation step, wherein the remaining crude composition is withdrawn from the second distillation column as overhead fraction, whereas heavy boilers, e.g. 6-hydroxyhexanoic acid, 2-hydroxycyclohexanone and caprolactone oligomers, is withdrawn from the second distillation column as bottom fraction. In order to prevent the degradation of caprolactone and its oligomerization, the preferred two distillation columns are preferably operated under sub-ambient pressure, such as the first distillation column at a pressure of 500 Pa to 65 kPa and the second distillation column at a pressure of 500 Pa to 4 kPa.

[0011] In accordance with the present invention, step a) of purifying the crude composition containing at least one lactone comprises at least one crystallization step. In principle, the present invention is not particularly limited concerning the kind of crystallization used in step a). However, it is preferred that the at least one crystallization step is a melt crystallization step. Melt crystallization means in accordance with the present invention a crystallization, which is performed from the melt of the crude composition containing at least one lactone having been pre-purified in the at least one previous distillation step without addition of further components thereto, such as for examples solvents. The at least one crystallization step may be performed in any manner, for example the at least one crystallization step may comprise a static crystallization step or a dynamic crystallization step, for instance a falling film crystallization step or a suspension crystallization step. Alternatively, the at least one crystallization step may comprise a combination of a static crystallization step followed by a dynamic crystallization step or a dynamic crystallization step followed by a static crystallization step.

[0012] In accordance with a preferred embodiment of the present invention, the at least one crystallization step comprises at least one static crystallization step. Static crystallization has the advantage that it economically purifies compounds, which are contained in the crystallization liquid in comparable high amounts. During the static crystallization, the liquid phase is not moved and thus the crystals are formed and grown in a static liquid phase. More specifically, a typical static crystallizer comprises a plurality of walls, such as plates or tubes or finned tubes, which can be cooled and heated by circulating a heat transfer medium through the interior of the plates. At the beginning, the static crystallizer is filled with the crude composition containing at least one lactone having been pre-purified in the at least one previous distillation step so that the plates contact the crude composition having been pre-purified by distillation. Then, the plates of the static crystallization vessel are cooled to a temperature below the equilibrium freezing temperature (ca. -1°C for caprolactone) of the crude composition so that crystals enriched in lactone are formed and deposited on the cooled outer surfaces of the plates. After completion of the crystallization, the remaining liquid phase is completely removed from the static crystallization vessel, the cooling of the plates is terminated and the plates are heated so that the crystal layers formed on the outer surfaces of the plates melt, before the melt is removed from the crystallization vessel in order to obtain the purified lactone composition. In order to increase the purity of the lactone, the crystal layers may be sweated by gently heating them to a temperature close to the melting point of the lactone in order to partially melt the crystals, before the final (complete) melting of the crystal layers. During the sweating, impurities contained in the pores of the crystal layers are removed with traces of lactone from the crystal layers and are withdrawn from the crystallization vessel, before the plates are heated so that the crystal layers formed on the outer surfaces of the plates melt, before the melt is removed from the crystallization vessel in order to obtain the purified lactone composition.

[0013] In accordance with another preferred embodiment of the present invention, the at least one crystallization step comprises at least one dynamic crystallization step. During the dynamic crystallization, the liquid phase moved in the crystallizer. Prominent dynamic crystallization techniques are suspension crystallization and falling film crystallization. More specifically, during the suspension crystallization the crude composition containing at least one lactone having been pre-purified in the at least one previous distillation step is cooled in a vessel so that crystal particles being enriched in lactone are formed so as to form a suspension, in which the crystal particles are dispersed in the liquid phase, which is depleted during the crystallization more and more concerning the lactone so that the liquid phase is called after the start of the suspension crystallization mother liquid. After completion of the crystallization, the crystal particles are separated from the mother liquid, e.g. by filters, centrifuges or other equipment for solid / liquid separation, and are then molten so as to obtain the purified lactone composition. A prominent example for a solid / liquid separator for separating the crystal particles from the mother liquid is a wash column apparatus, which comprise a cylindrical vessel, wherein the cylindrical vessel comprises: i) a piston with a piston head and a piston rod, wherein the piston is arranged reciprocatingly movable in the cylindrical vessel, wherein the piston bounds below the piston head a wash chamber inside the cylindrical vessel and wherein the piston head comprises at least one filter means, ii) an inlet for supplying the crystal suspension mixture composed of the lactone crystals and mother liquid into the cylindrical vessel, iii) an outlet for discharging mother liquid from the cylindrical vessel, iv) an outlet for discharging the lactone crystals and / or lactone crystal melt from the cylindrical vessel, v) a circulation conduit for circulating melt arranged outside the cylindrical vessel, which is in communication with the wash chamber, and vi) a means arranged in the wash chamber for restricting the movement of the crystal bed that has been compacted in the wash chamber by the piston and for directing the wash liquid entering into the cylindrical vessel from the circulation conduit so as to homogeneously distribute it over the entire cross-section of the cylindrical vessel. The means may be for instance a rotating scraper or a static grid.

[0014] In turn, falling film crystallization is performed in a falling film crystallizer, which is a crystallization column, which comprises hollow tubes being arranged at least substantially vertically and extending from the upper part of the falling film crystallizer into the bottom area of the falling film crystallizer. Liquid crude composition containing at least one lactone is filled into the bottom area of the falling film crystallizer, before the crystallization process is started. During the crystallization process, a portion of this crude composition is pumped by means of one or more pumps continuously from the bottom area of the falling film crystallizer to the upper part of the falling film crystallizer and is introduced into the upper end of the hollow tubes and allowed to fall down as falling film on the inner surfaces of the hollow tubes back to the bottom area of the falling film crystallizer. Concurrently, the outer walls of the hollow tubes are cooled to a temperature below the equilibrium freezing temperature of the crude composition by allowing a cold heat transfer medium to flow as falling film down the outer surfaces of the hollow tubes so that crystal layers enriched in lactone are deposited on the cooled inner wall surfaces of the hollow tubes. As a consequence of the deposition of lactone crystal layers on the cooled inner wall surfaces of the hollow tubes leading to a depletion of this lactone in the crude composition, a mother liquid is formed, which has a lower lactone concentration than the crude composition. The circulation of the mother liquid is conducted as long as necessary to separate the desired amount of lactone from the mother liquid and to deposit it as crystals on the inner wall surfaces of the hollow tubes. After completion of the crystallization, the mother liquid is completely removed from the falling film crystallizer, the lactone crystal layers deposited on the inner wall surfaces of the hollow tubes are molten and then removed from the falling film crystallizer in order to obtain the purified lactone composition. Optionally, in order to increase the purity of the lactone, the crystal layers may be sweat- ened as set out before with regard to the static crystallization by gently heating them to a temperature being close to the melting point of the purified compound in order to partially melt the crystals and removing the so generated liquid phase, before the crystal layers are completely molten and withdrawn from the crystallizer as purified lactone composition.

[0015] Each of the aforementioned crystallization steps may be performed in one crystallization stage or, if the purity of the lactone composition after one crystallization stage is not high enough, in multiple crystallization stages comprising two or more crystallization stages. If two crystallization stages are performed, the purified lactone composition obtained after the first crystallization stage is fed as feed into a second crystallization stage and is crystallized there again so as to obtain crystal layers of further purified lactone and mother liquid being depleted in lactone, whereas the mother liquid obtained in the second crystallization stage is led into the first crystallization stage. If three crystallization stages are performed, the purified lactone composition obtained after the second crystallization stage is fed as feed into a third crystallization stage and is crystallized there again so as to obtain crystal layers of further purified lactone and mother liquid being depleted in lactone and so forth and so forth, whereas the mother liquid obtained in the third crystal- lization stage is led into the second crystallization stage and the mother liquid obtained in the second crystallization stage is led into the first crystallization stage.

[0016] Good results are in particular obtained, when the at least one crystallization step comprises one to ten crystallization stages, more preferably one to five crystallization stages, still more preferably one to three crystallization stages and most preferably two or three crystallization stages.

[0017] In accordance with the present invention, the purified lactone composition obtained in step a) is subjected in step b) to at least one ring-opening-polymerization step so as to obtain a polylactone composition. Preferably, the ring-opening- polymerization step is performed at a temperature of 160 to 240°C, more preferably at a temperature of 170 to 220°C and most preferably at a temperature of 180 to 200°C.

[0018] Furthermore, it is preferred that the ring-opening-polymerization step is at ambient pressure.

[0019] A particular advantage of the present invention is that the reaction time of the ring- opening-polymerization step may be shorter than in the prior art processes, which does not only lead to reduced capital expenditures as well as reduced operational costs, but which leads in particular to polylactone having a narrow molecular weight distribution. In particular, it is sufficient that the ring-opening-polymerization step is performed for a reaction time of 0.5 to 3 hours and more preferably for a reaction time of 1 to 2 hours.

[0020] In a further development of the idea of the present invention, it is suggested that the purified lactone composition comprises at least one catalyst or at least one catalyst is added to the purified lactone composition before starting the ring- opening-polymerization step or at least one catalyst is added to the purified lac- tone during the ring-opening-polymerization step. Thereby, possible side-reactions are minimized and the yield is increased. Moreover, the presence of catalyst assists in the shortening of the required reaction time and of the required reaction temperature. Examples for suitable catalysts are metal oxides, metal carbonates, metal bicarbonates and organometallic compounds. Suitable examples for metal oxides are transition metal oxides and 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. Suitable examples for metal carbonates and metal bicarbonates are alkali metal carbonates, alkaline earth metal carbonates, alkali metal bicarbonates and alkaline earth metal bicarbonates. Particular suitable examples for catalysts are catalysts being selected from the group consisting of tin oxide, iron oxide, copper oxide, tin octanoate, butyl tin oxide, calcium carbonate, potassium carbonate and arbitrary combinations of two or more of the aforementioned catalysts. Most preferably, the catalyst is tin octanoate (Sn(Oct)2).

[0021] In addition, it is preferred that the catalyst content in the purified lactone composition being subjected to the ring-opening-polymerization step is 20 to 350 ppm and more preferably of 30 to 150 ppm.

[0022] In accordance with a further preferred embodiment of the present invention, the purified lactone composition comprises at least one initiator or at least one initiator is added to the purified lactone composition before starting the ring-opening- polymerization step or at least one initiator is added to the purified lactone composition during the ring-opening-polymerization step. The at least one initiator is preferably a hydroxy compound and more preferably a hydroxy compound being selected from the group consisting of monohydroxy compounds, dihydroxy com- pounds, trihydroxy compounds, tetrahydroxy compounds and any combination of two or more of the aforementioned compounds. By the functionality of the at least one hydroxy compound, the design of the resulting copolymer can be adjusted. If a monohydroxy compound is used, a linear copolymer will be produced, whereas branched copolymers may be produced by using one or more dihydroxy compounds, trihydroxy compounds and / or tetrahydroxy compounds.

[0023] Good results are in particular obtained, when the at least one initiator is selected from the group consisting of 2-ethyl hexanol, 1 -decanol, Cio-C2o-monohydroxy fatty alcohols, benzyl alcohol, p-phenylbenzyl alcohol, ethylene glycol, propylene glycol, butane-1 ,4-diol, poly(ethylene glycol) with a weight average molecular weight of 200 to 10,000 g / mol, 2-hydroxymethyl-1 ,3-propane, glycerol, polyglycerol with a weight average molecular weight of 100 to 1 ,000 g / mol, trihydroxybenzene (phloroglucinol), trimethylolpropane and its dimer, pentaerythritol and its dimers and any combination of two or more of the aforementioned compounds.

[0024] In a further development of the idea of the present invention, it is proposed that the molar ratio of lactone being contained in the purified lactone composition to initiator is more than 100 to 10,000. More preferably, the molar ratio of lactone being contained in the purified lactone composition to initiator is 300 to 10,000, even more preferably 500 to 10,000 and most preferably 500 to 3,000.

[0025] The total amount of the at least one initiator applied during the ring-opening- polymerization may be also expressed as less than 0.1 to 50 meq or less than 0.1 to 50 mmol / kg, respectively, preferably 0.5 to 40 meq or mmol / kg, respectively, more preferably 1 to 30 meq or mmol / kg, respectively, and most preferably 10 to 20 meq or mmol / kg, respectively.

[0026] As set out above, it is a particularly important advantage of the process in accordance with the present invention that polylactone with a narrow molecular weight distribution is obtained. In addition, the process in accordance with the present invention allows to obtain polylactone having a comparable high molecular weight.

[0027] More specifically, the polylactone contained in the polylactone composition preferably has a number average molecular weight of more than 10,000 g / mol and more preferably a number average molecular weight of 25,000 to 150,000 g / mol. In accordance with the present invention, the number- and weight-average molecular weight (Mnand Mw) of polymers are determined by gel permeation chromatography using a poly(methyl methacrylate) standard and a sample concentration of 1 to 5 mg / ml in 1ml HFIP depending on the sample’s molecular weight, wherein the column temperature is 40 °C, the temperature of the Rl-Detector (refractive index) is 40 °C and the flow rate 1 ml / min. As instrument, (GPC OMNISEC from Malvern Panalytical, UK equipped with a GPC / SEC detector module, a precolumn Tguard (10 mm length and 4.6 mm internal diameter), two columns (T6000M and T3000; 300 mm length and 8 mm internal diameter), and a triple detector (Rl, UV and viscosimeter) may be used. The calibration curve may be constructed using poly(methylmethacrylate) (PMMA) standard (Mn, max = 50,352 g / mol and D of 1.023).

[0028] In accordance with a particularly preferred embodiment of the present invention, the polydispersity i.e. the ratio of Mw / Mn, of the polylactone is at most 1.8. More preferably, the polydispersity of the polylactone is 1.6 or less and still more preferably 1.4 or less, such as 1 .0 to 1 .6 or 1 .1 to 1 .4.

[0029] The present invention is not particularly limited concerning the kind of reactor, in which the ring-opening-polymerization step is performed. Good results are in particular obtained, when the ring-opening-polymerization step is performed in a reactor system, which comprises at least one continuous stirred-tank reactor, at least one loop reactor or a, in series, a combination of at least one loop reactor and at least one plug flow reactor. In order to obtain a homogeneously mixing of the lactone with the preferred catalyst and optional initiator in the reaction mixture being subjected to the and / or distributing the heat the mixture flowing through the ring-opening-polymerization reaction, it is suggested in a further development of the idea of the present invention that the reactor, if one reactor is comprised in the reactor system, or at least one and more preferably each of the reactors, if two or more reactors are comprised in the reactor system, in which the ring-opening-polymerization step is performed, comprises at least one mixer and / or at least one heat transfer element. More preferably, the reactor, if one reactor is comprised in the reactor system, or at least one and more preferably each of the reactors, if two or more reactors are comprised in the reactor system, in which the ring-opening-polymerization step is performed, comprises at least one mixer and at least one heat transfer element.

[0030] The mixer may be a static mixer, a dynamic mixer or a combination of both. Preferably, any of the continuous stirred-tank reactors, if present, comprises a dynamic mixer and preferably also a heat transfer element, wherein any of the loop reactors, if present, and any of the plug flow reactors, if present, comprises at least one static mixer, and more preferably also at least one heat transfer element. The mixer(s) and heat transfer element(s) may be combined, namely so that the static mixer is made of hollow pipes, which are formed so that the reaction mixture is mixed, when it passes through the area formed between the hollow pipes. By pumping heat transfer medium through the hollow pipes, they also function as heat transfer elements. It is particularly preferred that the reactor system comprises, in series i) at the upstream end one continuous stirred-tank reactor comprising at least one dynamic mixer and / or a heat transfer element or one loop reactor comprising at least one static mixer and / or a heat transfer element and ii) downstream thereof at least one continuous stirred-tank reactor comprising at least one dynamic mixer and / or a heat transfer element and / or at least one loop reactor comprising at least one static mixer and / or a heat transfer element and / or at least one plug flow reactor comprising at least one static mixer and / or a heat transfer element. Most preferably, the reactor system comprises, in series seen from upstream to downstream i) three continuous stirred-tank reactors each comprising at least one dynamic mixer and optionally also a heat transfer element, ii) three loop reactors each comprising at least one static mixer and optionally also a heat transfer element, iii) a continuous stirred-tank reactor comprising at least one dynamic mixer and optionally also a heat transfer element and a plug flow section comprising at least one static mixer and optionally also a heat transfer element or iv) a loop reactor and a plug flow section each comprising at least one static mixer and optionally also a heat transfer element. Between any of two adjacent of the aforementioned reactors, a pump may be provided.

[0031] As set out above, the mixer used in a continuous stirred-tank reactor is preferably a dynamic mixer, i.e. a mixer comprising moving and in particular rotating parts. The dynamic mixer may be a dynamic mixer of the impeller-type, such as preferably a dynamic mixer comprising one or more paddle-type impellers, one or more anchor type-impellers, one or more gate-type impellers and / or one or more helicaltype impellers.

[0032] As further set out above, the mixer used in a loop reactor or in a plug flow reactor is preferably a static mixer, i.e. a mixer not comprising moving and in particular rotating parts. Static mixers usually produce a mixing effect by generating a turbulent flow due to static, i.e. non-moving elements, such as plates, bars, crossbars, baffles, helically formed deflection means, grids and the like. Suitable examples for static mixers, are x-type static mixers, spiral / helical-type static mixers, quattro-type static mixers, baffle plate-type static mixers, turbulator strips-type static mixers and any combination of two or more of the abovementioned mixer types. X-type static mixers comprise deflection means in the form of bars, crossbars, plates or the like having in a plan view and / or side view and / or cross-sectional view a x-like form. Such x-type static mixers are described for instance in WO 2010 / 066457 A1, EP 1 206 962 A1, EP 2 158 027 B1 and EP 0 655 275 B1 and are commercially available from Sulzer Chemtech Ltd, Winterthur, Switzerland under the tradenames SMX, SMXL and SMX plus as well as from Fluitec, Neftenbach, Switzerland under the tradename CSE-X. Spiral / helical-type static mixers have a helically formed deflection means and are described for instance in US 3,743,250 A, whereas quattro-type static mixers comprise deflection means forming chamber-like mixing sections and are described for instance in EP 2 548 634 B1 and in EP 0 815 929 B1 . While baffle plate-type static mixers comprise usually longitudinal deflection means and are described for instance in EP 1 510 247 B1 and in US 4,093,188 A, turbulator strip-type static mixers comprise in a tube a plurality of elongated strips, each of which being formed by a series of alternating deflection panels successively joined together by for example substantially triangular bridging portions with the strips being held together and anchored substantially on the axis of the tube by alternate ones of the bridging portions and the other bridging sections being disposed adjacent the inner wall of the tube and are described for instance in

[0033] US 4,296,779 A. Other suitable static mixers are distributed from Sulzer Chemtech AG under the tradenames CompaX, SMI, KVM, SMV and GVM and from Stamixco AG, Wollerau, Switzerland under the tradename GVM.

[0034] In a further development of the idea of the present invention it is suggested to use as heat transfer element a tube bundle heat exchanger. Preferably, the tube bundles of the heat transfer element are formed so that they simultaneously function as static mixing element. Such heat transfer elements, which are also static mixers, are for instance described in EP 1 967 806 B1 and in EP 2 052 199 B1 and are commercially available form Sulzer Chemtech AG under the tradename SMR and from Fluitec under the tradename CSE-XR. For example, such a combined heat transfer element and static mixer comprises a housing disposed on a longitudinal axis and a plurality of installations in the housing, each of the installations including at least a first hollow structure and at least a second hollow structure for the passage of a first fluid therethrough and the passage of a second fluid there- over, the first hollow structure and the second hollow structure being arranged cross-wise with respect to one another, each the hollow structure having a flow cross-section with a first width B1 and a second width B2 perpendicular to the first width B1 , with the ratio B1 / B2 being larger than one and B1 being oriented normally to a plane contains the longitudinal axis of the housing. Moreover, such a combined heat transfer element and static mixer may comprise fittings being arranged in a casing of extending longitudinally between a head end and a base end so that the fittings form a heat-transferring and mixing structure, whereby the fittings comprise tubes so that the heat-transferring medium is transportable as inner stream inside the tubes of the fittings from the base end to the head end and the liquid is transportable as outer stream outside the tubes from the head end to the base end, whereby the tubes of the fittings preferably form plane layers arranged in parallel, in which one respective tube extend from one entry end to an outlet end in a serpentine configuration comprising arcs and parallel partial tubular pieces. Reinforcement elements may be provided, which stabilize the tubes of the fittings in longitudinal direction against the pressure gradients generated by the liquid, whereby the reinforcement elements connect the parallel partial tubular pieces in a main area to form a non-extensible partial structure, and whereby the fittings remain partially unreinforced in an auxiliary area as a longitudinally extensible partial structure that complements the main area.

[0035] In accordance with a further particular embodiment of the present invention, the process further comprises a step c) of adding at least one catalyst inhibitor to the polylactone composition. By adding catalyst inhibitor to the polylactone composition, a degradation of the polylactone by a back-biting reaction, i.e. the splitting off from lactone being formed by intramolecular esterification between the terminal carboxylic acid group of a terminal repeating unit and the ester group of an adjacent repeating unit from a polylactone molecule, thus leading to degraded polylactone with reduced molecular weight, is reliably avoided. The addition of a catalyst inhibitor to the polylactone composition is in particular advantageous, when the polylactone composition is afterwards subjected to one or more devolatilization steps for removing volatile impurities, such as in particular unreacted lactone, because the high temperatures of a devolatilization step favors back-biting reactions. Good results are in particular obtained, when the at least one catalyst inhibitor is a phosphate ester or an alkyl phosphite. More preferably, the phosphate ester or alkyl phosphite has a total carbon number per molecule of 8 to 18. A sufficiently long carbon chain in the catalyst inhibitor leads to a low vaporization under devolatilization conditions, whereas a not too long carbon chain in the catalyst inhibitor assures that the melting point of the catalyst inhibitor is not too high, so that the catalyst inhibitor is a viscous liquid at room temperature. Suitable examples for phosphate esters are phosphated alcohols, phosphated alcohol ethoxylates and phosphated phenol ethoxylates, whereas suitable examples for alkyl phosphites are trioctyl phosphite, tri-isooctyl phosphite, tri-2-ethyl hexyl phosphite, trinonyl phosphite, triisodecyl phosphite, tri-lauryl phosphite, tricetyl phosphite and tristearyl phosphite.

[0036] In a further development of the idea of the present invention, it is suggested to add the at least one catalyst inhibitor to the polylactone composition and to mix the so obtained mixture using one or more mixers. All of the mixers mentioned above as suitable for the reactor system are also suitable as mixer for the mixing of the mixture containing polylactone composition and the at least one catalyst inhibitor.

[0037] Preferably, 0.01 to 1% by weight and more preferably 0.05 to less than 0.1% by weight of catalyst inhibitor are added to 100% by weight of polylactone composition. If at least one devolatilization step is performed, the catalyst inhibitor is preferably added before the first of the at least one devolatilization step.

[0038] In accordance with a further particular preferred embodiment of the present invention, the process further comprises a step d) of devolatilizing the polylactone composition at a temperature of at least 180°C and at a pressure of 1,000 Pa or less in at least one devolatilization stage so as to produce a purified polylactone composition and a gaseous composition containing unreacted lactone. Preferably, the polylactone composition being subjected to the devolatilization step d) comprises at least one catalyst inhibitor.

[0039] Good results are in particular obtained, when the at least one devolatilization stage of the devolatilization step at a temperature of 180 to 250°C and at a pressure of 100 to 500 Pa. More preferably, the at least one devolatilization stage of the devolatilization step at a temperature of 210 to 230°C and still more preferably at a temperature of 220 to 225°C. Furthermore, it is particularly preferred that the at least one devolatilization stage of the devolatilization step at a pressure of less than 200 Pa.

[0040] The devolatilization may be performed in one devolatilization stage or in two or more subsequent devolatilization stages. Preferably, the devolatilization is performed in one to five, more preferably in one to four, still more preferably in one to three and most preferably in one or two devolatilization stages. If two or more devolatilization stages are performed, the polylactone being devolatilized in the first devolatilization stage is fed into the second devolatilization stage and is devolatilized therein and is, if one or more further devolatilization stages are performed, fed into the next devolatilization stage.

[0041] Moreover, it is preferred that the gaseous composition containing unreacted lactone obtained in the one or more devolatilization stages is at least partially recycled to the crude composition containing at least one lactone or to the crude composition having been subjected to the one or more distillation steps before it is fed into the at least one crystallization step or to the purified lactone composition. Preferably, all of the gaseous composition or at least 50 to 95% of the gaseous composition are recycled to the crude composition containing at least one lactone or to the crude composition having been subjected to the one or more distillation steps before it is fed into the at least one crystallization step or to the purified lactone composition. More preferably, the gaseous composition containing unreacted lactone obtained in the one or more devolatilization stages is first condensed, before it is at least partially recycled so that the process preferably further comprises a step e) of condensing the gaseous composition containing unreacted lactone to a liquid composition containing unreacted lactone, before the liquid composition containing unreacted lactone is at least partially recycled to the crude composition containing at least one lactone or to the crude composition having been subjected to the one or more distillation steps before it is fed into the at least one crystallization step or to the purified lactone composition.

[0042] In a further development of the idea of the present invention, it is proposed that the process further comprises a step f) of cooling and pelletizing the purified polylactone composition to polylactone pellets and drying them, for instance in vacuum, in inert atmosphere or hot air. For instance, the purified polylactone composition may be led through a melt-cooler in order to facilitate the pelletization, for example as strand pelletization or as underwater pelletization. Alternatively, additives may be introduced to the purified polylactone composition, which is led through a side feeder or extruder before or after the melt-cooler and mixed with the polymer stream by a static or dynamic mixer.

[0043] In accordance with a particular preferred embodiment of the present invention, the process comprises the following steps: a) purifying a crude composition containing at least one lactone so as to obtain a purified lactone composition, wherein the step of purifying the crude composition containing at least one lactone comprises at least one distillation step so as to obtain at least an overhead composition and a bottom composition and optionally at least one side composition as well as at least one crystallization step of crystallizing the overhead composition, the bottom composition or the optional at least one side composition obtained in the at least one distillation step so as to obtain the purified lactone composition, b) adding at least one catalyst and optionally at least one initiator and optionally at least one further additive into the purified lactone composition obtained in step a) so that the molar ratio of catalyst to lactone being contained in the purified lactone composition is more than 10,000 and, if initiator is added, the molar ratio of lactone being contained in the purified lactone composition to initiator is more than 100 to 10,000 and subjecting the so obtained mixture to at least one ring-opening-polymerization step in a reactor system, which comprises at least one continuous stirred-tank reactor, at least one loop reactor or, in series, a combination of at least one loop reactor and at least one plug flow reactor, wherein preferably at least one of these reactors of the reactor system comprises at least one mixer and / or at least one heat transfer element, so as to obtain a polylactone composition, c) adding at least one catalyst inhibitor to the polylactone composition obtained in step b), wherein the at least one catalyst inhibitor is a phosphate ester or an alkyl phosphite having a total carbon number per molecule of 8 to 18, d) devolatilizing the polylactone composition obtained in step c), at a temperature of at least 200°C and at a pressure of less than 500 Pa so as to produce a purified polylactone composition and a gaseous composition containing unreacted lactone and e) condensing the gaseous composition containing unreacted lactone obtained in step d) to a liquid composition containing unreacted lactone, wherein the liquid composition containing unreacted lactone is at least partially recycled to the crude composition used in step a) and / or to the purified lactone composition used in step b) and f) preferably cooling and pelletizing the polylactone composition obtained in step e) into polylactone pellets using a water bath followed by a strand pel- letizer or directly an underwater pelletizer, and drying the pellets in under vacuum or inert atmosphere or hot air.

[0044] Preferably, the purified polylactone composition contains, based on 100% by weight of the purified polylactone composition, 99.7% by weight or more, preferably 99.8% by weight or more, more preferably 99.9% by weight or more and most preferably at least 99.95% by weight of polylactone.

[0045] Furthermore, the single pass conversion of the process is preferably at least 95% and more preferably at least 98%.

[0046] Subsequently, the present invention is described by means of illustrative, but not limiting figures, in which:

[0047] Fig. 1 shows a schematic view of a plant allowing to perform the process of producing polylactone in accordance with one embodiment of the present invention.

[0048] Fig. 2a to c show detailed schematic views of three embodiments for a crystallizer for performing the crystallization step of the process according to the present invention.

[0049] Fig. 3a to d show four specific reactor systems suitable for performing the process in accordance with the present invention.

[0050] Fig. 4a to e show four different types of static mixers one of which being also a heat transfer element useable in the process in accordance with the present invention. Fig. 5a to b show detailed schematic views of two embodiments for a devolatilization unit suitable for performing the process in accordance with the present invention.

[0051] Fig. 6a to b show a comparative example of a caprolactone derived from a method in accordance with present invention and a distilled caprolactone.

[0052] The plant 10 allowing to perform the process of producing polylactone in accordance with one embodiment of the present invention shown in figure 1 comprises, in series, a distillation column 20, a crystallization unit 30, a polymerization reactor system 40, devolatilization unit 50 and a pelletizing unit 60. More specifically, the distillation column 20 comprises an inlet line 22 for crude composition containing at least one lactone, an overhead outline line 24, a side outlet line 32 and a bottom outlet line 26. The side outlet line 32 of the distillation column 20 is combined with a recycle line 58 and leads into an inlet of the crystallization unit 30, which in turn has an outlet line 42 for purified lactone composition and an outlet line 34 for mother liquid. The outlet line 42 for purified lactone composition of the crystallization unit 30 is combined with an inlet line 44 for catalyst, with an inlet line 46 for initiator and with an inlet line 48 for further additive and leads into an inlet of the polymerization reactor system 40, which comprises a mixer (not shown) and a heat transfer element (not shown) and which comprises an outlet line 52 for polylactone composition. Furthermore, the outlet line 52 for polylactone composition of the reactor system 40 is combined with an inlet line 54 for catalyst inhibitor and with an inlet line 54’ for further additive and leads into an inlet of the devolatilization unit 50. The devolatilization unit 50 comprises an outlet and recycle line 58 for unreacted lactone which is connected with a condenser (not shown) and leads into the outlet line 32 of the distillation column 20 leading into the inlet of the crystallization unit 30. In addition, the devolatilization unit 50 comprises an outlet line 56 for waste as well as an outlet line 62 for devolatilized purified polylactone composi- tion. Finally, the outlet line 62 for devolatilized purified polylactone composition of the devolatilization unit 50 is combined with an inlet line 64 for additive and leads into the pelletizing unit 60, which comprises an outlet line 66 for polylactone pellets.

[0053] During the operation of the plant 10, crude composition containing at least one lactone is fed through line 22 into the distillation column 20, in which it is distilled. While lower boiling impurities are withdrawn from the distillation column 20 via the overhead outlet line 24 and higher boiling impurities are withdrawn from the distillation column 20 via the bottom outlet line 26, the pre-purified crude composition is withdrawn from the distillation column 20 via the side outlet line 32. The prepurified crude composition is then mixed in the side outlet line 32 with the recycled condensed unreacted lactone and is led into the crystallization unit 30, in which it is crystallized. While mother liquid is withdrawn from the crystallization unit 30 via the outlet line 34, the purified lactone composition is withdrawn from the crystallization unit 30 via the outlet line 42, wherein catalyst, initiator and further additive are added thereto via the inlet lines 44, 46, 48, before the so obtained mixture is led into the polymerization reactor system 40, in which it is mixed to a homogenous mixture and subjected to a ring-opening-polymerization reaction so as to obtain polylactone. The polylactone composition is then led via line 52, while catalyst inhibitor and further additive are added thereto via the inlet lines 54, 54’, into the devolatilization unit 50, in which the composition is separated into waste being withdrawn via the outlet line 56, into unreacted lactone being withdrawn via the outlet and recycle line 58, being condensed and recycled into line 32, as well as into purified devolatilized polylactone composition, which is withdrawn from the devolatilization unit 50 via the outlet line 62. Further additive is added to the purified devolatilized polylactone composition, before the so obtained mixture is led into the pelletizing unit 60, in which the purified devolatilized polylactone composition is pelletized into pellets, which are withdrawn from the plant 10 via the outlet line 66. Figures 2a to 2c show three alternate embodiments for the crystallization unit 30 of the plant shown in figure 1 . The crystallization unit 30 of figure 2a comprises one crystallization stage 31. The lactone composition having been pre-purified in the distillation column 20 is fed via line 32 into the crystallization stage 31 and is crystallized therein, wherein the mother liquor depleted in lactone is withdrawn from the crystallization stage 31 via line 34, whereas the purified lactone composition is withdrawn from the crystallization stage 31 via line 42 and is led into the polymerization reactor system 40. However, the crystallization unit 30 of figure 2b comprises two crystallization stages 31, 3T. The lactone composition having been prepurified in the distillation column 20 is fed via line 32 into the first crystallization stage 31 and is crystallized therein, wherein the mother liquor depleted in lactone is withdrawn from the first crystallization stage 31 via line 34, whereas the purified lactone composition is withdrawn from the crystallization stage 31 via line 36 and is led into the second crystallization stage 3T and is crystallized therein. While the purified lactone composition is withdrawn from the second crystallization stage 3T via line 42 and is led into the polymerization reactor system 40, the mother liquid is withdrawn from the second crystallization stage 3T via line 38 and is led back into the first crystallization stage 31 . In turn, the crystallization unit 30 of figure 2c comprises three crystallization stages 31 , 3T, 31”. The lactone composition having been pre-purified in the distillation column 20 is fed via line 32 into the first crystallization stage 31 and is crystallized therein, wherein the mother liquor depleted in lactone is withdrawn from the first crystallization stage 31 via line 34, whereas the purified lactone composition is withdrawn from the crystallization stage 31 via line 36 and is led into the second crystallization stage 3T and is crystallized therein. While the purified lactone composition is withdrawn from the second crystallization stage 3T via line 36’ and is led into the third crystallization stage 31” and is crystallized therein, the mother liquid is withdrawn from the second crystallization stage 3T via line 38 and is led back into the first crystallization stage 31. The purified lactone composition is withdrawn from the third crystallization stage 31” via line 42 and is fed into the polymerization reactor system 40, whereas the mother liquid is withdrawn from the third crystallization stage 31” via line 38’ and is fed back into the second crystallization stage 31’.

[0054] Figures 3a to 3d show four specific reactor systems suitable for performing the process in accordance with the present invention. More specifically, figure 3a shows a polymerization reactor system 40 suitable for performing the method in accordance with the present invention according to a first embodiment, which comprises in series three continuous stirred-tank reactors 68, 68’, 68”. The inlet line 42 and two feeding points 70, 70’ for adding catalyst and initiator is provided upstream of the most upstream continuous stirred-tank reactors 68, whereas the two feeding points 70, 70’ for adding catalyst and initiator are provided between the first and second continuous stirred-tank reactors 68, 68’ and between the second and most downstream continuous stirred-tank reactors 68’, 68”. Between each two of the continuous stirred-tank reactors 68, 68’ and 68’, 68” a melt pump 71 , 7T is provided. Each of the continuous stirred-tank reactors 68, 68’, 68” comprises an agitated, i.e. dynamic mixer 72, 72’, 72”, each of which is driven by a motor 74, 74’, 74”. The product stream is withdrawn from the most downstream continuous stirred-tank reactors 68” via outlet line 52.

[0055] Figure 3b shows a reactor system 40 being similar to that of figure 3a, except that the continuous stirred-tank reactors 68, 68’, 68” are replaced by three loop reactors 76, 76’, 76”, each of which comprising two static mixers embodied also as heat transfer element 78, 78’, 78” and each of which comprising a melt pump 71 , 7T, 71”.

[0056] Figure 3c shows a reactor system 40 comprising a continuous stirred-tank reactor 68 and downstream thereof two plug flow reactors 80, 80’, each of which comprising two static mixers embodied also as heat transfer element 78, whereas figure 3d shows a reactor system 40 comprising a loop reactor 76 and downstream thereof two plug flow reactors 80 and 80’, each of which comprising two static mixers embodied also as heat transfer element 78’.

[0057] Figure 4 shows five different types of static mixers useable in the method in accordance with the present invention, namely in figure 4a a static mixer 82 of the x- type comprising deflection means 84 in the form of crossbars having in a plan view as well as in side view a x-like form. Figure 4b shows a static mixer 82 of the baffle plate-type comprising longitudinal deflection means 84, whereas figure 4c and 4d show static mixers 82 with curved deflection means 84. Figure 4e shows a combined static mixer and heat transfer element 26 with tube-like deflection means 84 being formed so that they function as heat transfer element by transporting heat transfer medium within the tubes and simultaneously as static mixer for liquid being transported outside of the tube-like deflection means 84, such as it is commercially distributed by Sulzer Chemtech Ltd under the tradename SMR.

[0058] Figures 5a to 5b show two alternate embodiments for the devolatilization unit 50 of the plant shown in figure 1 . The devolatilization unit 50 of figure 5a comprises one devolatilization stage 51. The purified polylactone composition having been produced in the polymerization reactor system 40 is fed via line 52 into the devolatilization stage 51 and is devolatilized therein, wherein the resulting vapor phase containing unreacted lactone is withdrawn from the devolatilization stage 51 via the recycle line 58, whereas the waste is withdrawn via the outlet line 56 and the purified devolatilized polylactone composition is withdrawn from the devolatilization unit 50 via the outlet line 62 and is fed into the pelletizing unit 60. In turn, the devolatilization unit 50 of figure 5b comprises two crystallization stages 51 , 51”. The purified polylactone composition having been produced in the polymerization reactor system 40 is fed via line 52 into the first devolatilization stage 51 and is devolatilized therein, wherein the resulting vapor phase containing unreacted lactone being obtained during the devolatilization is withdrawn from the first devolatilization stage 51 via the recycle line 58, whereas the waste being obtained during the devolatilization is withdrawn via the outlet line 56’ and the purified devolatilized polylactone composition is withdrawn from the first devolatilization stage 51 via the outlet line 62’ and is fed into the second devolatilization stage 51’ and is devolatilized therein. The resulting vapor phase containing unreacted lactone being obtained during the devolatilization is withdrawn from the second devolatilization stage 5T via the recycle line 58’, which leads into the recycle line 58, whereas the waste being obtained during the devolatilization is withdrawn via the outlet line 56 and the purified devolatilized polylactone composition is withdrawn from the second devolatilization stage 5T via the outlet line 62 and is fed into the pelletizing unit 60.

[0059] Fig. 6a and b show results of a comparative example of a caprolactone derived from a method in accordance with present invention and a distilled caprolactone and the polymerization of distilled caprolactone (only distilled) and (additionally) crystallized caprolactone (i.e. derived from the method in accordance with present invention).

[0060] The measured impurities of two caprolactones are shown in Fig. 6a, with the caprolactone derived from the method according to the invention having significantly less impurities.

[0061] The polymerization shown in Fig. 6b was carried out at 195°C with catalyst concentration of 120ppm and co-catalyst concentration of 15mmol / kg. As the reaction temperature, the catalyst and co-catalyst concentration are kept as the same level, the reaction rate is only impacted by the impurity level resulted from different process unit, namely, distillation and crystallization. The reaction rates of two different raw materials have been compared in Fig. 6b. It can be seen clearly, with only distillation as purification, the impurities cannot be removed completely. With additional treatment by the crystallization unit, the reaction rate can be dramatical- ly increased. Consequently, the reaction time has been shortened which leads to a much smaller design of industrial equipment and cost saving on the CAPEX level.

[0062] Subsequently, the present invention is described by means of an illustrative, but not limiting example.

[0063] Example 1

[0064] This example has been performed with a 2 to 4 kg / h melt polymerization reactor system, which comprised at the upstream end a continuous stirred thank reactor and at the downstream end a double-jacketed plug flow reactor encompassing static mixer internals.

[0065] In order to produce polycaprolactone (PCL) from s-caprolactone (CL), a fixed amount of 40 kg of CL was weighed, loaded to a feed tank under nitrogen atmosphere at 40°C and fed at a throughput of 3 kg / h into the 2.0 L continuous stirred- tank reactor, which was heated by an oil heat transfer unit operated at 175°C under steady state (reaction medium at 180°C). Separately from this feed, tin octoate / toluene (40 mg / ml) catalyst was introduced and mixed with the monomer stream via a static mixer prior to its entry to the continuous stirred-tank reactor. The total molar monomer / catalyst ratio was kept at 100 ppm. Separately from these two feeds, 2-ethyl hexanol was used as initiator and was dosed prior to their entry to the continuous stirred-tank reactor at a rate to achieve a concentration of 20 meq.

[0066] With an average residence time of 30 min (equivalent to 1 .5 L continuous stirred tank reactor (CSTR) volume), the mixture was then carried by a gear pump at the same throughput to a double-jacketed static mixer-based plug flow reactor operated within the same temperature range. The total residence time was kept at 90 min. When necessary, 0.1 wt.% Adeka Stab AX-71 was added to the polymer products and mixed through a short static mixing to terminate the polymerization. The residual monomer was removed via a devolatilization unit, pelletized, crystallized and dried. In this case, where CL of different monomer free acid contents (from 2.45 to 8.0 meq) and water contents (300 to 1500 ppm), were applied, the above experimental procedures were repeated for each condition. In order to enable a meaningful comparison, the monomer conversion and polymer properties were evaluated by keeping the feed flow rate (3 kg / h) and residence time (30 min). All measured samples were taken immediately after the CSTR and before the gear pump. Monomer conversion was evaluated via gas chromatography (GC) and number average molecular weight as well as polydispersity were determined by gel permeation chromatography (GPC), respectively. The results were summarized in the below table.

[0067] In order to obtain CL of different monomer free acid contents and water contents, a considerable pure CL with water content of 300 ppm (determined by Karl Fisher titration) and free acid content of 2.45 meq (determined by non-aqueous titration) was used as a base. Specifically, a fixed amount of capric acid and water were introduced to this pure CL so as to stimulate the impact of contamination. As an example, CL of 1500 ppm water and 2.45 meq free acid was obtained by adding 48 g of water (eq. to 1200 ppm) into 40 kg pure CL (300 ppm) and no additional capric acid.

[0068] The results are shown in the below table.

[0069] a all reactions were conducted using 20 meq 2-ethyl hexanol as initiator and 100 ppm Sn(Oct)2as catalyst at 180°C.

[0070] BXCL, 30min , Mn,3o min denote the monomer conversion and the number average molecular weight after 30 minutes of reaction time, respectively. Polydispersity refers to the Mw / Mnvalue at 30 min reaction.

[0071] As it derive from the above table, both the addition of water and of free acid to the system has detrimental impact on the reaction kinetics as well as the molecular weights after 30 min. This indicated the necessity to purify the caprolactone stream, removing undesirable water and free acid whenever possible.

[0072] Reference numerals

[0073] Plant

[0074] Distillation column

[0075] Inlet line for crude composition

[0076] Overhead outline line of the distillation column

[0077] Bottom outlet line of the distillation column

[0078] Crystallization unit , 31’, 31” Crystallization stage

[0079] Side outlet line of the distillation column

[0080] Outlet line for mother liquid , 36’ Lactone line , 38’ Mother liquid line

[0081] Polymerization reactor system

[0082] Outlet line for purified lactone composition

[0083] Inlet line for catalyst

[0084] Inlet line for initiator

[0085] Inlet line for further additive

[0086] Devolatilization unit , 51’ Devolatilization stage

[0087] Outlet line for polylactone composition

[0088] Inlet line for catalyst inhibitor ’ Inlet line for further additive , 56’ Outlet line for waste , 58’ Outlet and recycle line for unreacted lactone

[0089] Pelletizing unit , 62’ Outlet line for devolatilized purified polylactone composition

[0090] Inlet line for additive Outlet line for polylactone pellets , 68’, 68” Continuous stirred-tank reactor , 70’ Feeding point , 71’, 71” Melt pump , 72’, 72” Dynamic mixer , 74’, 74” Motor , 76’, 76” Loop reactor , 78’, 78” Static mixer and heat transfer elements, 80’ Plug flow reactor

[0091] Static mixer

[0092] Deflection means

Claims

Claims:1 . A process of producing polylactone from lactone comprising a step a) of purifying a crude composition containing at least one lactone so as to obtain a purified lactone composition and a step b) of subjecting the purified lactone composition obtained in step a) to at least one ring-opening- polymerization step so as to obtain a polylactone composition, wherein the step a) of purifying the crude composition containing at least one lactone comprises at least one distillation step so as to obtain at least an overhead composition and a bottom composition and optionally at least one side composition as well as comprises at least one crystallization step of crystallizing the overhead composition, the bottom composition or the optional at least one side composition obtained in the at least one distillation step so as to obtain the purified lactone composition.

2. The process in accordance with claim 1 , wherein the at least one lactone is selected from the group consisting of propiolactone, caprolactone, valerolactone, butyrolactone, decanolactone and arbitrary combinations thereof and, wherein the at least one lactone is preferably s-caprolactone.

3. The process in accordance with claim 1 or 2, wherein the step of purifying a crude composition comprises one to ten subsequent distillation steps, preferably one to five subsequent distillation steps, more preferably one to three subsequent distillation steps, still more preferably one or two subsequent distillation steps and most preferably two distillation steps.

4. The process in accordance with any of the preceding claims, wherein the step of purifying a crude composition comprises a static crystallization step, a dynamic crystallization step, a combination of a static crystallization stepfollowed by a dynamic crystallization step or a dynamic crystallization step followed by a static crystallization step.

5. The process in accordance with any of the preceding claims, wherein the at least one crystallization step comprises one to ten crystallization stages, preferably one to five crystallization stages, more preferably one to three crystallization stages and most preferably two or three crystallization stages.

6. The process in accordance with any of the preceding claims, wherein the ring-opening-polymerization step is performed at a temperature of 160 to 240°C and the purified lactone composition comprises at least one catalyst or at least one catalyst is added to the purified lactone composition before starting the ring-opening-polymerization step or at least one catalyst is added to the purified lactone during the ring-opening-polymerization step, wherein the molar ratio of catalyst to lactone being contained in the purified lactone composition is more than 10,000, and wherein the at least one catalyst is preferably selected from the group consisting of tin oxide, iron oxide, copper oxide, tin octanoate, butyl tin oxide, calcium carbonate, potassium carbonate and arbitrary combinations of two or more of the aforementioned catalysts.

7. The process in accordance with claim 6, wherein the purified lactone composition comprises at least one initiator or at least one initiator is added to the purified lactone composition before starting the ring-opening- polymerization step or at least one initiator is added to the purified lactone composition during the ring-opening-polymerization step, wherein the molar ratio of lactone being contained in the purified lactone composition to initiator is more than 100 to 10,000, and wherein the at least one initiator is preferably selected from the group consisting of monohydroxy compounds,dihydroxy compounds, trihydroxy compounds, tetrahydroxy compounds and arbitrary combinations of two or more of the aforementioned initiators.

8. The process in accordance with any of the preceding claims, wherein the polylactone contained in the polylactone composition has a number average molecular weight of more than 10,000 g / mol as well as a polydispersity of at most 1 .8, preferably of 1 .6 or less and more preferably of 1 .4 or less, the polydispersity being the ratio of a weight-average molecular weight and the number average molecular weight, wherein the weight-average molecular weight and the number average molecular weight are determined by gel permeation chromatography.

9. The process in accordance with any of the preceding claims, wherein the ring-opening-polymerization step is performed in a reactor system, which comprises at least one continuous stirred-tank reactor, at least one loop reactor or a, in series, a combination of at least one loop reactor and at least one plug flow reactor, wherein preferably at least one of these reactors of the reactor system comprises at least one mixer and / or at least one heat transfer element.

10. The process in accordance with any of the preceding claims, wherein the process further comprises a step c) of adding at least one catalyst inhibitor to the polylactone composition, wherein the at least one catalyst inhibitor is preferably a phosphate ester or an alkyl phosphite having a total carbon number per molecule of 8 to 18, wherein preferably 0.01 to 1% by weight and more preferably 0.5 to less than 0.1% by weight of catalyst inhibitor are added to 100% by weight of polylactone composition and the so obtained mixture is mixed using one or more mixers.11 . The process in accordance with any of the preceding claims, wherein the process further comprises a step d) of devolatilizing the polylactone composition, which preferably comprises at least one catalyst inhibitor, at a temperature of at least 180°C, preferably of 180 to 250°C and more preferably of 210 to 230°C, and at a pressure of 1 ,000 Pa or less, preferably of 100 to 500 Pa and more preferably of less than 200 Pa in at least one devolatilization stage so as to produce a purified polylactone composition and a gaseous composition containing unreacted lactone.

12. The process in accordance with claim 11 , wherein the process further comprises a step e) of condensing the gaseous composition containing unreacted lactone to a liquid composition containing unreacted lactone, wherein the liquid composition containing unreacted lactone is at least partially recycled to the crude composition containing at least one lactone or to the crude composition having been subjected to the one or more distillation steps before it is fed into the at least one crystallization step or to the purified lactone composition.

13. The process in accordance with any of the preceding claims, wherein the process further comprises a step f) of cooling and pelletizing the purified polylactone composition to polylactone pellets and drying them in vacuum, in inert atmosphere or hot air.

14. The process in accordance with any of the preceding claims, wherein the process comprises the following steps: a) purifying a crude composition containing at least one lactone so as to obtain a purified lactone composition, wherein the step of purifying the crude composition containing at least one lactone comprises at least one distillation step so as to obtain at least an overhead composition and a bottom composition and optionally at least one side compositionas well as at least one crystallization step of crystallizing the overhead composition, the bottom composition or the optional at least one side composition obtained in the at least one distillation step so as to obtain the purified lactone composition, b) adding at least one catalyst and optionally at least one initiator and optionally at least one further additive into the purified lactone composition obtained in step a) so that the molar ratio of catalyst to lactone being contained in the purified lactone composition is more than 10,000 and, if initiator is added, the molar ratio of lactone being contained in the purified lactone composition to initiator is more than 100 to 10,000 and subjecting the so obtained mixture to at least one ring- opening-polymerization step in a reactor system, which comprises at least one continuous stirred-tank reactor, at least one loop reactor or a, in series, a combination of at least one loop reactor and at least one plug flow reactor, wherein preferably at least one of these reactors of the reactor system comprises at least one mixer and / or at least one heat transfer element, so as to obtain a polylactone composition, c) adding at least one catalyst inhibitor to the polylactone composition obtained in step b), wherein the at least one catalyst inhibitor is a phosphate ester or an alkyl phosphite having a total carbon number per molecule of 8 to 18, d) devolatilizing the polylactone composition obtained in step c), at a temperature of at least 200°C and at a pressure of less than 500 Pa so as to produce a purified polylactone composition and a gaseous composition containing unreacted lactone, e) condensing the gaseous composition containing unreacted lactone obtained in step d) to a liquid composition containing unreacted lactone, wherein the liquid composition containing unreacted lactone is at least partially recycled to the crude composition used in step a) and / or to the purified lactone composition used in step b) andf) preferably cooling and pelletizing the polylactone composition obtained in step e) into polylactone pellets using a water bath followed by a strand pelletizer or directly an underwater pelletizer, and drying the pellets in under vacuum or inert atmosphere or hot air.

15. The process in accordance with any of the preceding claims, wherein the purified polylactone composition contains, based on 100% by weight of the purified polylactone composition, 99.7% by weight or more, preferably 99.8% by weight or more, more preferably 99.9% by weight or more and most preferably at least 99.95% by weight of polylactone.

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