Process and apparatus for continuously manufacturing a homopolymer

Abstract

A process of continuously manufacturing a homopolymer with high viscosity, in particular poly(hydroxy acid) homopolymer is proposed. The process comprising the steps of continuously providing at least one monomer into a reactor system (10), the reactor system (10) comprising at least one reactor (12, 14, 14', 16) with a reactor inlet (I1, I2) through which the monomer is continuously provided and an reactor outlet (O1, O2) through which a reaction mixture is continuously discharged, and polymerizing the at least one monomer to the homopolymer in the reactor system (10). The reactor system (10) further comprises a monomer side feed (17) with at least one feeding point (18, 18', 18", 18"', 18'''') through which a monomer composition is fed into the at least one reactor (12, 14, 14', 16), wherein the at least one feeding point (18, 18', 18", 18"', 18'''') is provided on the at least one reactor (12, 14, 14', 16) between the reactor inlet (I1, I2) and the reactor outlet (O1, O2), such that the monomer composition is provided into the reaction mixture.

Description

[0001] P1850

[0002] Sulzer Management AG, CH-8401 Winterthur (Schweiz)

[0003] Process of continuously manufacturing a polymer

[0004] The present invention relates to a process of continuously manufacturing a homopolymer and a plant for continuously manufacturing a homo- or copolymer in accordance with the preamble of the independent claims.

[0005] A polymer is a molecule of high relative molecular weight, the structure of which essentially comprises the multiple repetition of units derived, actually or conceptually, from molecules of low relative molecular weight. Polymers, both natural and synthetic, are created via polymerization of many small molecules, known as monomers. A polymer may be defined as comprising more than 50 monomer units. A group of polymers are poly(hydroxy acids), which are polyesters that are obtained using hydroxy acids as monomer.

[0006] Poly(hydroxy acid) homo- or copolymers are of particular interest, because they can be synthesized from renewable resources and are mostly compostable, biodegradable, and / or recyclable. Moreover, the technological properties of these polymers come guite 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 poly(hydroxy acid) homopolymer is polylactic acid (PLA), which is a thermoplastic polyester based on the a-hydroxy acid lactic acid, a-hydroxy acids being carboxylic acids having a hydroxy group arranged one carbon atom away from the carboxyl group.

[0007] Polylactic acid has a wide range of applications, for instance, it 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 poly(hydroxy acid) homopolymer is polycaprolactone, which is derived from a cyclic ester, caprolactone, originated from the intramolecular esterification of hydroxy acid caproic acid. This polymer finds widespread applications in the production of specialty polyurethanes and is characterized by a good resistance to water, to oil and to solvent. Other examples are polyglycolic acid I poly (glycolide) (PGA), poly(G-valerolactone), poly(G-decalactone) and the like.

[0008] Poly(hydroxy acid) copolymers (i.e. polymers containing two or more different types of monomers, while homopolymers only contain a single type of monomer) are interesting alternatives, since by 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.

[0009] Generally, two alternative principal methods are known for synthesizing poly(hydroxy acid) homo- or copolymers. The first known 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 polymers and is thus limited to specific 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 or the like. This is the preferred method nowadays for the industrial production of polylactic acid and other poly(hydroxy acid) homo- or copolymers. The cyclic diesters may be produced by intramolecular esterification of an aliphatic hydroxy acid, such as in the case of a lactone, for instance caprolactone, or by condensation of two hydroxy acid molecules into a cyclic diester, such as condensation of lactic acid to lactide or condensation of glycolic acid to glycolide. Alternatively, the cyclic ester, such as in particular cyclic diester may be produced by first oligomerizing a hydroxy acid and then subjecting the oligomer to a depolymerization reaction in order to obtain the cyclic diester. For instance, lactide is often prepared by the latter method, for instance by fermentation of carbohydrates from biomass, such as starch, sugar or com, to lactic acid, by then oligomerizing the lactic acid, and by afterwards subjecting the oligomers to a depolymerization reaction to obtain lactide. After purification, the one or more cyclic esters of hydroxy acid as monomer(s) are then polymerized in the presence of a catalyst and optionally an initiator to form high molecular weight polymer. The unreacted cyclic ester should be removed after the polymerization to a final concentration of less than at least 0.5 % by weight, in order to obtain a product of marketable quality. Such a removal of unreacted cyclic ester may be achieved for instance in the case of lactide by means of at least one devolatilization step conducted at elevated temperature and reduced pressure. For example, a two-stage devolatilization process can be performed to obtain the required degree of lactide removal and thus to obtain a polymer having the required quality. To terminate the polymerization reaction, an inhibitor is usually added to the reaction mixture at the end of the polymerization and before or after the first devolatilization step. Alternatively, residual monomers can be removed by dissolving the monomer and polymer mixture in a solvent such as 1 ,1 ,1 ,3,3,3- hexafluoro-2-propanol (HFIP), dichloromethane (DCM) and chloroform and reprecipitate the monomer-free polymer with an anti-solvent such as methanol, acetone and hexane. Still, this latter method is less preferable in an industrial scale, considering the heavy solvent consumption and long dissolution time.

[0010] Thus, a typical process of preparing a polylactic acid comprises the steps of i) carrying out a ring-opening-polymerization of lactide in the presence of a catalyst and / or an initiator for ring-opening-polymerization of the lactide to polylactic acid, ii) adding a compound capable of deactivating the catalyst to the resulting reaction mixture and iii) reducing the pressure in a reactor containing the reaction mixture and / or allowing an inert gas to pass through the reactor to remove unreacted lactide from the polylactic acid by devolatilization. Usually, two subsequent devolatilization steps are performed and the vapor streams enriched in lactide are recycled to the polymerization reactor or upstream unit operations for purification.

[0011] For a better understanding of the present invention, a process of preparing polymers such as polylactic acid known from the prior art is described below with reference to Fig. 1 and 2.

[0012] To better distinguish the known prior art from the present invention, reference signs for features known from the prior art are provided with an “A” in the context of this application, whereas the reference signs of features according to the invention or their components do not bear “A”.

[0013] Polylactic acid I polylactide is produced from lactide through ring-opening polymerization in a plant 1A. Thereby, lactide is provided from a monomer feed 2A and mixed with a catalyst from catalyst feed 3A and co-catalyst (initiator) from co-catalyst feed 4A.

[0014] Then the lactide is pre-polymerized in a loop reactor 16A. A reaction mixture of pre-polymerized lactide and I or polylactide I lactide oligomers is further polymerized in a plug flow reactor 14A. After this post polymerization in the plug flow reactor 14A, the reaction ends at a conversion between about 85-95% of the lactide. To end the reaction and stabilize the reaction mixture, an inhibitor is provided from inhibitor feed 5A.

[0015] Afterwards the reaction mixture is devolatilized in a devolatilization unit 7A to separate the residue of lactide out of the polylactide.

[0016] While the polylactide is post processed, i.e. crystallized and I or pelletized in a post processing unit 8A, the lactide is cleaned up in a crystallizer 6A and provided to the monomer feed 2A and I or directly recycled to the loop reactor 16A.

[0017] In this process all lactide (about 5-15%) is dosed already at the beginning of the synthesis from the monomer feed 2A where the viscosity is still low.

[0018] However, in such a process with typically about 95% conversion of the monomer, the viscosity of the reaction mixture is very high. This has the impact, that when targeting for a higher molecular weight and I or superior grade of product, the technical limit of the reactor system, i.e. mechanical limit of equipment will be reached. However, a high molecular weight and I or superior grade of product is desirable, especially if the polymer is to be used for medical products.

[0019] Furthermore, it is difficult to stop the reaction with a particular and desired value of conversion, residual amount of monomer, and molecular weight, i.e. it is more difficult to control the reaction all due to a high viscosity in the plug flow reactor.

[0020] Fig. 2 shows embodiments a) and b) of a reactor system 10A known from the prior art EP4288480A1 , which is hereby incorporated by reference. Fig. 2 embodiment a) shows a reactor system 10A comprising a continuous stirred-tank reactor 12A and downstream thereof two plug flow reactors 14A, 14A', each of which comprise two static mixers embodied also as heat transfer element 15A. In addition, the continuous stirred-tank reactor 12A comprises an agitated, i.e. dynamic mixer 13A which is driven by a motor 20A.

[0021] Fig. 2 embodiments b) shows the reactor system 10A comprising a loop reactor 16A and downstream thereof two plug flow reactors 14A, 14A', each of which comprise two static mixers embodied also as heat transfer element 15A.

[0022] The reactors of embodiments a) and b) are arranged in series. In addition, a pump 26A is provided and a final product stream is withdrawn from the most downstream reactor via line 22A. Furthermore, there are two feeding points 18A, 18A’ for adding monomer, catalyst and initiator, which are provided between the reactors at connecting lines 24A, 24A’. This setup is in particular used to provide copolymers.

[0023] As indicated above a drawback of these current processes of preparing poly(hydroxy acid) polymers, regardless of ring-opening polymerization or polycondensation is, that there are limits due to the handling of the viscosity of the reaction mixture. Due to the high viscosity, inside the reactors, in particular inside the plug flow reactors 14A, 14A’, these reactors will reach their technical limit when targeting high molecular weights. The only option to prevent the reactors from reaching their technical limit is to stop the reaction at a lower conversion of the monomer (e.g., at 80% conversion). However, such an approach will require an excess of monomer as well as a higher amount of catalyst, which remains in the product.

[0024] Starting from this state of the art, the object of the present invention is to provide a plant and a process of continuously manufacturing a homopolymer, which avoids the adverse effects of the state of the art. In particular, a plant and a process are to be provided, which allow the production of polymers having a tunable and I or comparably high molecular weight. Especially, the process and plant shall allow low use of resources such as catalyst.

[0025] The subject matter of the invention satisfying this object is characterized by the features of the independent claims. Thus, according to the invention a process of continuously manufacturing a homopolymer, in particular with high viscosity, preferably a poly(hydroxy acid) homopolymer, is proposed. The process comprising the steps of continuously providing at least one monomer, in particular one or more cyclic esters of hydroxy acid, into a reactor system and polymerizing the at least one monomer, in particular one or more cyclic esters of hydroxy acid by ring-opening- polymerization, to the homopolymer in the reactor system.

[0026] The reactor system comprises at least one reactor with a reactor inlet through which the monomer can be continuously provided into the reactor and a reactor outlet through which a reaction mixture can be continuously discharged from the reactor.

[0027] Thereby, the reactor system further comprises a monomer side feed with at least one feeding point through which a monomer composition can be fed into the at least one reactor. The at least one feeding point is provided on the at least one reactor between the reactor inlet and the reactor outlet, such that the monomer composition can be fed into the reaction mixture, in particular can be provided directly into the reaction mixture inside of the reactor. Hence, the monomer side feed can be arranged at the reactor system, in particular at the at least one reactor.

[0028] In the framework of this application, the term “side feed” is used to refer to the monomer side feed defined above.

[0029] In addition, according to the invention a plant for manufacturing a homopolymer is proposed, the plant comprising the above-defined reactor system. The plant according to the invention can be used for the process according to the invention. Most preferably the poly(hydroxy acid) homopolymer is produced by ring- opening-polymerization, i.e. there is a chain growth due to a ring opening of a cyclic monomer.

[0030] In contrast to the state of the art the feeding point is not (at least not only) provided at connecting lines leading to the reactor and I or connecting different reactors. Hence, the feeding point according to the invention can provide additional monomer in form of the monomer composition thereby influencing a viscosity of the reaction mixture inside of the respective reactor at a determinable point.

[0031] Viscosity is used to describe the thickness of a liquid and quantifies the internal frictional force of the liquid. The process and plant described herein allow a flexible and versatile handling of the viscosity. In particular, the plant and process enable the production of polymers with high molecular weight, in particular poly(hydroxy acid) homopolymers. By having a monomer side feed, the monomer composition, in particular comprising the monomer, can be used as a dilutant for the polymer and reduce the viscosity of the reaction mixture. Hence, the process according to the invention can be operated, in particular by adjusting the amount of monomer provided by the side feed, so that a melt viscosity of the reaction mixture at the reactor outlet of a last reactor of the reactor system is between 1000 Paxs and 4000 Paxs, in particular below 4000 Paxs, preferably below 2500 Paxs, more preferably below 1500 Paxs, in particular as determined by a rheometer using a cone plate measuring setup in oscillation mode under various temperatures and shear rates. Further information regarding oscillatory rheometry is derivable from ISO 3219-2:2021.

[0032] Further advantages that can be achieved include use of fewer resources, such as initiator, and catalyst thereby also resulting in lower contamination in the final polymer. In particular, without the intention to be bound to any theory, it is considered that due to the possibility to use a high molar ratio of monomers to the catalyst, i.e. due to the possibility of a comparable low concentration of catalyst in the reaction mixture, the process can be designed more flexible, since for example an increase in the reaction temperature does not lead to an uncontrollable process.

[0033] Due to the side feed according to the invention, an excess of the monomer composition I the monomer can be used only where it is necessary to reduce the viscosity. In the state of the art, which is described with reference to Fig. 2, the side feed is used upstream or between reactors. However, this is not where viscosity adjustment is crucial. Furthermore, the side feed according to Fig. 2 is used to provide a comonomer to produce copolymers. However, the process according to the invention, is most preferably used for manufacturing homopolymers, in particular with adjustable I high molecular weight, by adjusting the viscosity inside the reactor.

[0034] Within a single reactor, the viscosity increases in the direction of a flow of the reaction mixture (i.e. from reactor inlet to reactor outlet), as more and more monomer is converted to polymer and the chain length of the polymer molecules increases. Inside the reactors there are usually structures which are used, for example, to control the flow rate, temperature and mixing. Due to the aforementioned structures and also generally due to the size, it is possible that the viscosity of the reaction mixture inside the reactor(s) is too high, which means that flow I movement of the reaction mixture is no longer possible or very severely restricted. Therefore, and to avoid the aforementioned condition, it is only possible to produce polymers that do not have a high molecular weight, e.g. with absolute number average molecular weight (Mn) up to 75 kg / mol, and weight average molecular weight (Mw) up to 140 kg / mol and a reliable control of the molecular weight is not possible.

[0035] By the side feed according to the invention the viscosity inside the reactor can be altered. Hence it is possible to provide polymer with high molecular weight. In the framework of this application, the term “high molecular weight” can be used for an absolute number average molecular weight (Mn) of up to I about 200 kg / mol, and weight average molecular weight (Mw) of up to I about 200 kg / mol. Preferably an absolute number average molecular weight (Mn) of 30kg / mol to 150kg / mol, more preferably between 50kg / mol and 130kg / mol, still more preferably between 80kg / mol and 100kg / mol and / or a weight average molecular weight (Mw) of 80kg / mol to 250kg / mol, more preferably between 100kg / mol and 230kg / mol, still more preferably between 150kg / mol and 200kg / mol. Preferably, the poly(hydroxy acid) homopolymer produced during ring-opening-polymerization have the above- mentioned molecular weight.

[0036] In accordance with the present invention, the number- and weight-average molecular weight (Mnand Mw) of polymers is determined by gel permeation chromatography using a poly(methyl methacrylate) standard and a sample concentration of 1 to 5 mg / ml in 1 ml 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 RESOLVE from Malvern Panalytical, UK, a precolumn guard (Tguard, Org Guard Col 10 mm length and 4.6 mm internal diameter from Malvern Panalytical, UK), two columns (T6000M, General Mixed Org 300 length and 8mm internal diameter in series with T4000, Org GPC / SEC col 300 length and 8mm internal diameter from Malvern Panalytical, UK), and a quadruple detector Omnisec RESOLVE (Rl, UV, light scattering and viscosimeter) may be used. The calibration curve may be constructed using poly(methylmethacrylate) (PMMA) standard (Mn,max = 50,352 g / mol and i3?]The polydispersity index, i.e. the ratio of Mw / Mn, of the polymer produced during the ring-opening- polymerization may be 1 to 2, preferably 1 to 1.6, more preferably 1 to 1.4.

[0037] Furthermore, it is also possible to reduce the residence time of the reaction mixture in the reactor (for example due to higher flow velocity), wherein the reaction mixture will be less thermally exposed. Thereby, damages to the polymer and residual monomer can be reduced and more monomer can be recycled.

[0038] In some preferred embodiments the process is a continuous melt process, and I or comprises steps such as continuously providing the monomer, catalyst and optionally initiator as a monomer feed, to the reactor system for a polymerization and I or continuously removing the reaction mixture from a first reactor of the reactor system and continuously providing the reaction mixture to a second reactor, in particular further reactors of the reactor system. In addition, the reaction mixture prior to or after being discharged from the reactor system may be subjected to a stabilization step, which comprises treatment of the composition as obtained from the reactor system with compounds that increase the stability against further polymerization and I or depolymerization such as the inhibitor. Preferably a volume percent or weight percent of the monomer in the monomer feed, the reaction mixture and I or the monomer composition is between 40% and 100%, more preferably between 50% and 100%, still more preferably between 50% and 95%, %, still more preferably between 50% and 75%.

[0039] The reaction mixture, the monomer feed initially provided into the plant and I or the monomer composition can in addition to the monomer comprise a solvent such as water and / or a hydrocarbon solvent. In accordance with the present invention, the reactor system can be understood as the combination of all reactors and lines connecting them (also referred to as connecting lines) with each other used for the process, i.e. for instance the combination of at least one continuous stirred-tank reactor and / or at least one loop reactor and / or at least one plug flow reactor, in which the process is conducted.

[0040] In a preferred configuration the feeding point being between the reactor inlet and the reactor outlet means that the feeding point is or feeding points are arranged at the location of the reactor, where the aforementioned structures are arranged inside the reactor. The aforementioned structures can include at least one mixer and / or at least one heat transfer element and can be arranged inside of the respective reactor, between the reactor inlet and the reactor outlet. Hence the at least one reactor can comprise the mixer, in particular a plurality of mixers and I or the heat transfer element, in particular a plurality of heat transfer elements. The mixer can be a static mixer and / or a dynamic mixer. Static mixers can be combined with heat transfer elements. Furthermore, the static mixer and I or dynamic mixer can be combined with the feeding point meaning that the monomer composition can be feed directly into the reaction mixture via the mixer. If the feeding point is positioned at the mixer, this has the advantage that the newly introduced monomer composition is directly and evenly distributed in the reaction mixture. For this purpose, output channels can be arranged on the static mixer and I or the dynamic mixer. One preferred example of such an arrangement is that at least one reactor is a plug flow reactor, the plug flow reactor comprising a distributor element arranged between the reactor inlet and the reactor outlet, the distributor element comprising the at least one feeding point and a static mixing element, such that the monomer composition can be mixed with the reaction mixture when being fed into the plug flow reactor. A similar arrangement is of course possible for a loop reactor. The mixer and / or heat transfer element allow for homogeneously mixing and / or distributing the heat and the reaction mixture flowing through the respective reactor and thereby allow to reduce the total residence time in the system. More specifically, the use of one or more mixers and / or heat transfer elements allows to assure a homogeneous mixture within the reactor system without significant heat and concentration gradients within the reactor system except the decrement of monomer concentration throughout the reactor system due to the polymerization and the concentration increment of components after a feeding point. Thereby, an effective mixing of reactants as well as an efficient heat removal from the highly viscous reaction mixture are assured, so that the process remains reliably controllable and stable without undesired degradation of components of the reaction mixture or the creation of undesired by-products.

[0041] As indicated above the monomer side feed can comprise a plurality of feeding points through which the monomer composition can be fed into the at least one reactor. Furthermore, the reactor system can have a plurality of monomer side feeds each having at least one feeding point, preferably a plurality of feeding points. Measures mentioned regarding a single feeding point can be applied to all or only some of the feeding points. The different feeding points of the plurality of feeding points are preferably provided at different locations on the at least one reactor between the reactor inlet and the reactor outlet, such that the monomer composition is provided at different locations into the reaction mixture. This allows viscosity and thus chain length I molecular weight to be adjusted at different points. In addition, other reaction parameters can also be adjusted, which further increases the flexibility of the process. For example, at different feeding points of the plurality of feeding points the monomer composition can be fed into the at least one reactor at different temperatures. A temperature difference of the monomer composition between the different feeding points can be 1-50°C, more preferably 1-25°C, yet more preferably 1-15°C, even more preferably 1-10°C, still more preferably 1-5°C, whereby the temperature of the monomer composition supplied preferably decreases in the direction of the reactor outlet. Alternatively, or in addition, the temperature difference between the monomer composition and the reaction mixture may exhibit the aforementioned differences, with the monomer composition preferably being cooler. Alternatively, or in addition, the monomer composition fed into the at least one reactor has different compositions. Thereby, the at least one monomer can be continuously provided from the monomer side feed in the monomer composition in the presence of at least one catalyst, and I or co-catalyst, and I or initiator, and I or solvent, and I or inhibitor into the reactor system. Near the reactor inlet the monomer composition can include catalyst and I or initiator to promote the polymerization, near the reactor outlet the monomer composition can include inhibitor to terminate the polymerization. Of course, the monomer composition can comprise solely the monomer without any extra additive, so that the monomer composition consists of monomer. Alternatively, or in addition, a different volume of the monomer composition can be fed into the at least one reactor at different feeding points. The more advanced the polymerization, the more monomer composition can be added via the side feed, preferably, more monomer composition is added at the feeding points which are closer to the reactor outlet. The volume of monomer composition per unit of time added at different feeding points can differ by 1-20%, more preferably 1-15 %, yet more preferably 1-10 %, even more preferably 1-5%. By such measures, better control of the reaction and process are enabled.

[0042] In accordance with another preferred embodiment of the present invention, the proportion of monomer supplied by the side feed in relation to the total proportion of monomer supplied for the polymerization, in particular ring-opening- polymerization, is between 0% and 50%, preferably between 5% and 40%, more preferably between 10% and 35%, still more preferably between 15% and 30%, most preferably about 20% and 25%.

[0043] In principle, the process and devices according to the invention can correspond to the known processes and devices, whereby the at least one reactor is modified with the side feed according to the invention.

[0044] The at least one reactor, for example, can be any of the continuous stirred-tank reactor, the loop reactor, an extruder, and the plug flow reactor. In the most preferable embodiment, the at least one reactor is a plug flow reactor. Furthermore, the plant can have a plurality of reactors, which are preferably arranged in series i.e. the reaction mixture enters from one reactor to another, and the reactors are connected via connecting lines. Hence, a plurality of plug flow reactors with a side feed according to the invention can be used. Alternatively, or in addition, the continuous stirred-tank reactor and / or the loop reactor (or plurality of) can be part of the plant. These additional reactors can have a side feed according to the invention as well. This can contribute to a more precise controllability of the process but is not necessary. Hence, not all reactors need a side feed according to the invention, but only the at least one reactor. All measures described with reference to the side feed can therefore be applied to all or only some of the reactors. Preferably, they are applied at least to the plug flow reactors. By applying the side feed at least to the plug flow reactor, it is possible to reduce the extent of reaction and thus the viscosity and reaction volume in the previous reactors such as the loop reactor, because the polymerization in these preceding reactors is no longer required to be so advanced for the same ultimate molecular weight.

[0045] In a preferred embodiment of the plant, the at least one reactor is the plug flow reactor, and the reactor system additionally comprises one reactor, which is a continuous stirred-tank reactor or a loop reactor. The reactors are thereby arranged in series to each other, such that the first reactor into which a feed line leads, is the continuous stirred-tank reactor or the loop reactor, the second reactor downstream of the first reactor is the plug flow reactor, wherein both reactors are connected via the connecting line.

[0046] In accordance with the present invention, the reactor system may comprise one or more continuous stirred-tank reactors. Alternatively, the reactor system may comprise one or more loop reactors. Still alternatively, the reactor system may comprise at least one plug flow reactor. Still alternatively, the reactor system may comprise any possible combination of at least one continuous stirred-tank reactor and / or at least one loop reactor and / or at least one plug flow reactor. Preferably, the reactor system comprises, in series i) at the upstream end one continuous stirred-tank reactor or one loop reactor and ii) downstream thereof at least one continuous stirred-tank reactor and / or at least one loop reactor and / or at least one plug flow reactor. For instance, the reactor system comprises, in series seen from upstream to downstream i) one loop reactors or one continuous stirred-tank reactor and ii) one plug flow reactor with a side feed having at least three feeding points. Between any of two adjacent of the aforementioned reactors, a pump and connecting lines may be provided. Furthermore, the monomer side feed can be provided for any of the reactors.

[0047] In a preferred embodiment the at least one reactor is a plug flow reactor and the reactor system additionally comprises at least one additional reactor, which is a plug flow reactor (or a plurality of additional plug flow reactors), the reactors being arranged in series to each other, such that a first reactor into which the monomer is provided, is the at least one reactor, and a second reactor downstream of the first reactor is the at least one additional reactor. A diameter and I or length and I or volume of the at least one additional reactor is larger than the diameter and I or length and I or volume of the at least one reactor (if there are several, the reactors become larger in the direction of flow), such that a flow rate of the reaction mixture in these reactors remains essentially constant. If there is no varied size of such a series of plug flow reactors, by adding additional monomer via the side feed into all the plug flow reactors, the overall flow rate of reaction mixture will rise. This will lead to a reduction of residence time in all plug flow reactors, thus influencing the reaction. However, above indicated increase of the size of the plug flow reactors can accommodate the residence time. Alternatively, the flow rate in these reactors can also be influenced by adequately adjusting the diameter and / or length and / or volume.

[0048] If the reactor system comprises more than one reactor, preferably at least 50%, more preferably at least 75% and most preferably all of the reactors of the reactor system comprise at least one mixer or at least one heat transfer element and more preferably at least one mixer as well as at least one heat transfer element. More 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.

[0049] As set out above, the mixer used in the 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 helical-type impellers.

[0050] As further set out above, the mixer used in the loop reactor or in the 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, spi ral / hel ical- 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 0655 275 B1 , which are hereby incorporated by reference, 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, which is hereby incorporated by reference, 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 , which are hereby incorporated by reference. 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, which are hereby incorporated by reference, 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 US 4,296,779 A, which is hereby incorporated by reference. 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.

[0051] In a further development of the idea of the present invention it is suggested to use a tube bundle heat exchanger as the heat transfer element. Preferably, 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 , which are hereby incorporated by reference and are commercially available form Sulzer Chemtech AG under the tradename SMR and from Fluitec under the tradename CSE-XR.

[0052] In a further development of the idea of the present invention, it is proposed that the residence time - or reaction time, respectively - in the reactor system is adjusted to be 0.1 to 5.0 hours, more preferably 0.2 to 4.0 hours, yet more preferably 0.3 to 3.0 hours, still more preferably 0.4 to 2.0 hours and most preferably 0.5 to 1.5 hours. The residence time - or reaction time, respectively - may be adjusted, among others, by appropriately selecting the volume of the continuous stirred-tank reactors, the length of the loop reactors and the plug flow reactors and the flow rate the reaction mixture.

[0053] In addition, or alternatively, for reactions requiring a short residence time a bypass can be installed such that the reaction mixture is not directed through all reactors or for reactions requiring a long residence time fluid from one reactor can be recycled back to a previous reactor. This enables a flexible operation with the same reactor system and these options can be realized by having valves before each reactor section to direct the fluid flow and having bypass and I or recycling lines.

[0054] There are various options for the structure or setup of the monomer side feed, which can essentially be adapted to the desired applications. The monomer side feed can comprise a pump and / or a valve, in particular a plurality of pumps and I or valves to regulate the supply of the monomer composition. Alternatively, or in addition the monomer side feed can comprise a side feed inlet line or a plurality of inlet lines (for the plurality of feeding points). The side feed inlet line can be connected to a main supply of the monomer (monomer feed) or to a devolatilization unit. However, preferably the monomer side feed comprises a reservoir (or tank I container) in which the monomer composition is provided, and which is connected to the reactor via the side feed inlet line. If monomer compositions of different compositions are used, different reservoirs can be used either for different monomer compositions or to provide different components of the monomer composition (also in different compositions). If the monomer composition is to be fed at a specific temperature in order to adjust the temperature of the reaction mixture, the side feed can have a cooling element and / or heating element (or several). The temperature of the polymerization can therefore be controlled by providing the monomer composition from the monomer side feed with a predeterminable temperature. Furthermore, the side feed can include a side feed outlet line. The outlet line allows the reaction mixture to be discharged from the reactor at the feeding point. This can be used to regulate the flow rate in the reactor or to maintain a constant volume of reaction mixture in the reactor. The reaction mixture discharged through the outlet line can be fed in again further upstream or downstream in the process I plant. Either by adding monomer composition and providing it via the feeding point downstream to adjust the viscosity or directly upstream into a connecting line.

[0055] The side feed according to the invention, in particular if used for continuous ring- opening-polymerization, allows to easily adapt - with the same reactor system - the molecular weight of the polymer. In other words, the process and plant in accordance with the present invention allows to produce - with the same reactor system - polymers of high and I or different molecular weights (grades), for instance by varying the temperature of monomer composition, the initiator contents and / or catalyst contents, and / or the number and / or location of feeding points, at which the monomer composition is added into the reactor.

[0056] Furthermore, the monomer side feed can comprise an additional feeding point through which the monomer composition can be fed into an inflow of the at least one reactor and I or into the connecting line, such that the monomer composition is additionally provided at the reactor inlet into the reactor. Since the polymerization is not limited to the reactors this enables additional control of the process, even in the connecting lines. In addition, the viscosity can also be adjusted shortly before the reaction mixture enters the reactor.

[0057] By tuning the residence time of the monomer, catalyst and optional initiator between the single feeding points and by appropriately adjusting the amounts of catalyst and optional initiator, the chain length of the blocks and thus the molecular weight of the polymer may be controlled. Moreover, by adjusting an initial feed composition and the monomer composition, the composition and structure of the polymer chain and the reaction rate may be controlled. Furthermore, by using an appropriate type of initiator, it may be controlled whether the polymer is linear or branched. On account of the aforementioned reasons, the process in accordance with the present invention is well suited to produce branched poly(hydroxy acid) homopolymers, which are superior compared to linear polylactic acid, since such branched polymers are usually less brittle and have a higher toughness and impact resistance than linear polylactic acid.

[0058] In accordance with a further preferred embodiment of the present invention at least a part of the polymerization, in particular ring-opening-polymerization, is carried out at a temperature of 160°C or more. For instance, the temperature of the polymerization within a part of the at least one continuous stirred-tank reactor and / or of the at least one loop reactor and / or of the at least one plug flow reactor may be 160°C or more, whereas the temperature in another part of the at least one continuous stirred-tank reactor and / or at least one loop reactor and / or of the at least one plug flow reactor may be less than 160°C. Alternatively, the temperature of the polymerization within the whole of the at least one continuous stirred-tank reactor and / or at least one loop reactor and / or of the at least one plug flow reactor may be 160°C or more. However, it is most preferred that the temperature throughout all of the reactors of the reactor system, i.e. in all of the continuous stirred-tank reactors, loop reactors, plug flow reactors and optional any further reactor, is 160°C or more. The adjustment of this temperature range, which is slightly higher in comparison to the temperature range used in commercial production processes, allows to enhance the reaction kinetic and thus shorten the reaction time, i.e. to decrease the residence time of the reaction mixture within the reactor system, however, surprisingly without making the process uncontrollable, which would lead to a degradation of components of the mixture within the reactor system and in particular of the produced polymer, to the formation of colored by-products and the like. In a preferred embodiment the polymerization is at least partially and more preferably completely carried out at a temperature of at least 160°C, preferably at a temperature of 160°C to 2500°C, more preferably at a temperature of 170°C or more to 230°C, even more preferably at a temperature of 175°C or more to 220°C and most preferably at a temperature of 180°C or more to 210°C. In accordance with another preferred embodiment of the present invention, the content of the at least one catalyst applied during the polymerization, in particular ring-opening-polymerization, is between more than 0 ppm and 200 ppm or less based on the reaction mixture. If there are two or more feeding points for adding the monomer composition including monomer and / or catalyst into the reaction system so that the concentration of catalyst in the reaction mixture may be higher downstream of a first feeding point, the maximum amount of the at least one catalyst applied during the polymerization in the reaction mixture in sum at any location in the reaction system is between more than 0 ppm and 200 ppm or less based on the reaction mixture. Good results are in particular obtained, when the content of the at least one catalyst applied during the polymerization is between 1 ppm and 180 ppm, more preferably between 10 ppm and 150 ppm, still more preferably between 20 ppm and 100 ppm and most preferably between 50 ppm and 100 ppm.

[0059] In principle, the present invention is not particularly limited concerning the chemical nature of the used catalyst. In particular, good results are obtained, if the catalyst is at least one organometallic compound. Good results are in particular obtained, when the catalyst is at least one organometallic compound 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. The at least one organometallic compound preferably comprises as organic residue a residue 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. More preferably, the catalyst is at least one organometallic compound comprising as metal aluminum and / or tin. Still more preferably, the catalyst is at least one organometallic compound being selected from the group consisting of tin octoate, tetraphenyl tin, tributyltin methoxide, dibutyltin oxide, aluminum isopropoxide, tetraphenylporphinato aluminum compound and any combination of two or more of the aforementioned compounds. Particularly suitable as catalyst is tin octoate, such as tin(ll) 2-ethylhexanoate.

[0060] The present invention is not particularly limited concerning the type of monomers used. Preferably, the monomer for ring-opening-polymerization is the at least one cyclic ester selected from the group consisting of lactide, glycolide, caprolactone, valerolactone, decalactone, butyrolactone, dodecalactone, octanolactone and any combination of two or more of the aforementioned compounds. More preferably the at least one cyclic ester is selected from the group consisting of L- lactide, D-lactide, meso-lactide, lactide racemic mixture, glycolide, E- caprolactone, y-caprolactone, 5-valerolactone, y-valerolactone, 5-decalactone, 5- decalactone, 5-butyrolactone, 6-dodecalactone, 5-dodecalactone, 5- octanolactone and any combination of two or more of the aforementioned compounds. However, monomers could also include other compounds which can be polymerized by a polycondensation. For example, polylactides can also be produced by a direct condensation reaction of lactic acid molecules.

[0061] Preferably, polyesters such as polylactic acid, polycaprolactone polyglycolide, poly(G-valerolactone), and poly(G-decalactone) are synthesized by the process according to the invention. Even more preferably, during the ring-opening- polymerization i) a lactide is polymerized so as to manufacture a polylactide or ii) a caprolactone is polymerized so as to manufacture a polycaprolactone or iii) a glycolide is polymerized so as to manufacture a polyglycolide or iv) a lactide and a caprolactone are polymerized so as to manufacture a poly(lactide-co- caprolactone) or v) a lactide and a glycolide are polymerized so as to manufacture a poly(lactide-co-glycolide). Still more preferably, the at least one cyclic ester i) is selected from L-lactide, D-lactide, meso-lactide, lactide racemic mixture and any combination of two or more of the aforementioned compounds, or ii) is s-caprolactone and / or y-caprolactone or iii) is glycolide or iv) is a mixture of at least one compound being selected from L-lactide, D-lactide, meso-lactide, lactide racemic mixture and any combination of two or more of the aforementioned compounds and s-caprolactone and / or y-caprolactone or is a mixture of at least one compound being selected from L-lactide, D-lactide, mesolactide, lactide racemic mixture and any combination of two or more of the aforementioned compounds and glycolide. However, most preferably polylactic acid and I or polycaprolactone are synthesized by the process according to the invention.

[0062] In accordance with a further particularly preferred embodiment of the present invention, during the polymerization, in particular ring-opening-polymerization, at least one initiator is present so that the step of polymerization comprises that the one or more cyclic esters is / are polymerized in the reactor system in the presence of at least one catalyst and at least one initiator. 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 compounds, 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 polymer can be adjusted. If a monohydroxy compound is used, a linear polymer will be produced, whereas branched polymers may be produced by using one or more dihydroxy compounds, trihydroxy compounds and / or tetrahydroxy compounds. Good results are in particular obtained, when the at least one initiator is selected from the group consisting of 2-ethyl hexanol, 1 -decanol, Cw- C20-monohydroxy fatty alcohols, benzyl alcohol, phenylbenzyl alcohol, ethylene glycol, propylene glycol, butane-1 ,4-diol, polyethylene 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.

[0063] In a further development of the idea of the present invention, it is suggested that the molar ratio of the total amount of the one or more cyclic esters to the total amount of the at least one initiator applied during the ring-opening-polymerization is 100 to 10,000. More preferably, the molar ratio of the total amount of the one or more cyclic esters to the total amount of the at least one initiator applied during the ring-opening-polymerization is 300 to 10,000, even more preferably 500 to 10,000 and most preferably 500 to 3,000.

[0064] In a further development of the idea of the present invention it is suggested that the process is operated so that the overall conversion of the monomer (also known as degree of polymerization) to the polymer, in particular of the one or more cyclic esters to the poly(hydroxy acid) homopolymer during the ring- opening-polymerization is at least 40%, preferably between 45% and 90%, more preferably between 45% and 75%, still more preferably between 50% and 65%. Having a higher amount of monomer, i.e. adding in the feeding points additional monomer results in a dilution of the reaction mixture, whereby the viscosity is reduced. The change in conversion of monomers with time may be followed according to the present invention by gas chromatography (GC) for instance with the following procedure. Approximately 100 mg of sample are weighed, dissolved in 10 mL of DCM containing 30 mg of 1 -octanol as internal standard. 1 ml of this solution is precipitated in 10 ml of 95:5 (v / v) hexane / acetone mixture. Afterwards, 1.5 ml of the mixed suspension was filtered through a 0.45 pm polytetrafluoroethylene (PTFE) filter for measurement. For polymers produced (for example using glycolide as one of the monomers), 100 mg of sample are weighed, dissolved in 20 ml of tetrahydrofuran (THF) containing 30 mg of 1- octanol as internal standard. 2 ml of this solution is precipitated in 10 ml of methanol. Afterwards, 1.5 ml of the mixed suspension is filtered through a 0.45 pm polytetrafluoroethylene (PTFE) filter for measurement. Measurements are performed using a GC Clarus 580 (from Perkin Elmer, UK) equipped with an auto-sampler, an injector channel, an oven and a flame ionization detector (FID). The GC column is J&W DB-17MS (Agilent). The carrier gas is Helium (99.999%, 1 ml / min of flow), while the fuel gases are purified air after filter (Drypoint M from BEKO Technologies, Germany, flow of 450 ml / min) and hydrogen (from water electrolysis, 45 ml / min flow). The injector temperature is fixed at 180°C, while the detector is set to 350 °C.

[0065] It is preferred that after the polymerization the non-reacted monomer remaining in the polymerization product is removed to a final concentration of preferably less than at least 0.5 % by weight, in order to obtain a product of marketable quality. Such a removal of unreacted monomer may be achieved by means of at least one devolatilization step conducted at elevated temperature e.g. between 190 and 230°C and at a reduced pressure of e.g. below 5 mbar(absolute), preferably in the devolatilization unit. For example, a two-stage devolatilization process may be performed to obtain the required degree of lactide removal and thus to obtain a polymer having the required quality. To stop the polymerization reaction, an inhibitor is preferably added to the polymer product at the end of the polymerization and before or after the first devolatilization step. To maximize the yield of polymer product per amount of monomer feed, it is further preferred that the unreacted monomer is recovered after the devolatilization e.g. by condensation, then optionally the condensed product is purified and thereafter the condensed product is recycled into the polymerization reaction. In particular, the addition of one or more efficient inhibitor additives at the end of the polymerization reaction is preferred for an efficient devolatilization. Before the first devolatilization step, between the first and second devolatilization step or after the second devolatilization step additives and / or other polymers may be mixed and / or blended in one or more units for mixing and / or blending additives into the product stream to improve the mechanical, rheological and / or thermal properties of the final polymer product. Examples of devolatilization units include extruders, especially twin screw extruders, wiped film evaporators, falling film evaporators, rotary devolatilization units, rotary disk devolatilization units, centrifugal devolatilization units, flat plate devolatilization units, and static expansion chambers, such as those involving special distributors, e.g., Sulzer devolatilization technology as described in EP1800724A1 , which is hereby incorporated by reference.

[0066] The final polymer product stream can then be directly further processed to enduse by extrusion, blow-molding, film casting, film blowing, thermoforming, foaming, or fiber-spinning at elevated temperatures to form useful articles. Moreover, the final polymer product stream may be cooled in a cooler and then pressed through a granulator or pelletizer, respectively, or through another forming unit. If so desired, the final polymer product stream may be compounded with additives such as anti-oxidants, nucleating agents, mineral fillers, glass or natural fibers, processing aids, UV-stabilizers, or other polymer-additives known to the skilled person.

[0067] The invention will be explained in more detail hereinafter with reference to embodiments of the invention and with reference to the drawings. There are shown in a schematic representation:

[0068] Fig. 1 : a schematic representation of a plant known from the prior art; and

[0069] Fig. 2: a schematic representation of reactor systems known from the prior art; and

[0070] Fig. 3: a schematic representation of a plant according to the invention, and

[0071] Fig. 4: a schematic representation of reactor systems according to the invention; and Fig. 5 a schematic representation of further reactor systems according to the invention; and

[0072] Fig. 6 a schematic representation of a plug flow reactor according to the invention; and

[0073] Fig. 7 four different types of dynamic mixers useable in the method in accordance with the present invention; and

[0074] Fig. 8 five different types of static mixers useable in the method in accordance with the present invention.

[0075] Fig. 1 and 2 are described above with reference to the prior art.

[0076] Fig. 3 shows a schematic representation of a plant 1 according to the invention, which essentially corresponds to the structure of Fig. 1 .

[0077] In this plant 1 polylactic acid (i.e. polylactide) is produced from lactide through ring-opening polymerization. Thereby, lactide is provided from a monomer feed 2 and mixed with a catalyst from catalyst feed 3 and co-catalyst from co-catalyst feed 4.

[0078] Then the lactide is pre-polymerized in a loop reactor 16. A reaction mixture of pre-polymerized lactide and polylactide I lactide oligomers is the further polymerized in a plug flow reactor 14.

[0079] Furthermore, there is a monomer side feed 17 with a plurality of feeding points through which a monomer composition is fed into the plug flow reactor 14. The plurality of feeding points is provided on the plug flow reactor 14 between a reactor inlet and a reactor outlet, such that the monomer composition is provided into the reaction mixture inside of the plug flow reactor 14. Furthermore, the monomer composition is fed into a connecting line prior to the plug flow reactor 14.

[0080] After this post polymerization in the plug flow reactor 14, the reaction ends at a conversion between about 40-75% of the lactide, due to the lactide provided as the monomer composition. To end the reaction and stabilize the reaction mixture, an inhibitor is provided from inhibitor feed 5. Afterwards the reaction mixture is devolatilized in a devolatilization unit 7 to separate the residue of lactide out of polylactide.

[0081] While the polylactide is post processed, i.e. crystallized and I or pelletized in a post processing unit 8, the lactide is cleaned up in a crystallizer 6 and provided to the monomer feed 2 or directly recycled to the loop reactor 16.

[0082] In this process the lactide is dosed at the beginning of the synthesis from the monomer feed 2 where the viscosity is still low and in the plug flow reactor 14 via the monomer side feed 17 to lower viscosity.

[0083] A major advantage of this process of preparing polylactide via ring-opening polymerization is, that it is possible to provide polymers with comparably high molecular weight. Due to lowering the viscosity in the plug flow reactor 14 it is possible to produce polylactide with absolute number average molecular weight (Mn) of about 100 kg / mol, and weight average molecular weight (Mw) of about 200 kg / mol.

[0084] The multiple side-feed 17 as shown, is even more efficient than a single side feed since along the plug flow reactor 14 the viscosity will increase due to the increase of the molecular weight, while the addition of monomer along the plug flow reactor 14 will support to reduce the viscosity progressively compensating the effect. Furthermore, by continuously adding fresh monomer, the reaction of polymerization will be continuously activated maintaining a living-polymerization. Due to this effect, the conversion will be kept low with lower polydispersity.

[0085] The monomer side feed 17 with multiple feed points brings the advantage to have a well-controlled polymerization reaction with the possibility to tune the desired conversion, viscosity, residual monomer, molecular weight and residence time.

[0086] All these considerations are also valid if the side-feed approach is used in another reactor such as the loop reactor.

[0087] In the plant 1 it can be possible to tune for example the following parameters: 1) feed ratio of polymer (from the loop reactor 16) and the fresh monomer entering the plug flow reactor 14, 2) the side streams of monomer from the monomer side feed 17.

[0088] For the following descriptions, the reactors on the left are upstream from the reactors arranged on the right. Fig. 4 shows a schematic representation of reactor systems 10 according to the invention.

[0089] Fig. 4a) shows a reactor system 10 suitable for performing the method in accordance with the present invention according to a first embodiment, which comprises a continuous stirred-tank reactor 12 with a dynamic mixer 13 which is driven by a motor 20. The product stream is withdrawn from the continuous stirred-tank reactors 12 via reactor outlet 01 , while the reaction mixture enters the reactor at reactor inlet 11 .

[0090] Two feeding points 18, 18' for providing the monomer composition are provided as part of the side feed 17 between the inlet 11 and the outlet 01. The monomer composition can include besides the monomer, catalyst and initiator, and is provided from a reservoir of the monomer side feed 17.

[0091] Fig. 4b) shows a reactor system 10 comprising a plug flow reactor 14, comprising two static mixers embodied also as heat transfer element 15.

[0092] Two feeding points 18, 18' for providing the monomer composition are provided as part of the side feed 17 between the inlet 11 and the outlet 01. The monomer composition can include besides the monomer, catalyst and initiator, and is provided from a reservoir of the monomer side feed 17.

[0093] Fig. 4c) shows a reactor system 10 comprising a continuous stirred-tank reactor 12 and downstream thereof plug flow reactors 14, 14', each of which comprise two static mixers embodied also as heat transfer element 15.

[0094] The reactors are connected via connecting lines 24, 24’ and a melt pump 26 is provided.

[0095] Four feeding points 18, 18', 18”, 18”’ for providing the monomer composition are provided as part of the side feed 17. Two feeding points 18’, 18” between the inlet 11 and the outlet 01 of the first plug flow reactor 14, one feeding point 18 in the connecting line 24 behind pump 26 and prior to reactor inlet 11 , and one feeding point 18’” between reactor outlet 01 of the first plug flow reactor 14 and reactor inlet I2 of the second plug flow reactor in connecting line 24’. After the reaction, the reaction mixture I polymer exit the second plug flow reactor 14’ at reactor outlet 02 into final outlet 22.

[0096] Fig. 5 shows a schematic representation of further reactor systems 10 according to the invention.

[0097] Fig. 5a) shows a reactor system 10 being similar to that of Fig. 4c), except that additional feeding points are located at the plug flow reactor 14 and the plug flow reactor is U-shaped by providing an additional reactor element with a static mixer 15” connecting two plug flow sections. Some feeding points 18’, 18’” are located at the static mixers embodied also as heat transfer element 15 and some feeding points 18”, 18”” are located at static mixers 15’.

[0098] Fig. 5b) shows a reactor system 10 being similar to that of Fig. 5a), except that instead of a continuous stirred-tank reactor, a loop reactor 16 with a plurality of feed points 18’”, 18””, i.e. seven feeding points is provided upstream of the plug flow reactor 14.

[0099] Fig. 6 shows a schematic representation of a further embodiment of the plug flow reactor 14 according to the invention, comprising a distributor element 19 arranged between the reactor inlet and the reactor outlet. The distributor element 19 comprises the feeding point 18 and a ring-shaped static mixer, such that the monomer composition can be mixed with the reaction mixture when being fed into the plug flow reactor 14. Furthermore, there are the static mixers 15, 15’ arranged inside the plug flow reactor 14. The static mixers 15 include the heat transfer element and are connected to a coolant system 11 , 1 T.

[0100] The distributor element 19 is also shown separately in front of the plug flow reactor 14.

[0101] Fig. 7 shows four different types of dynamic mixers useable in the method in accordance with the present invention, namely in Fig. 7a a dynamic mixer with a paddle-type impeller 30, in Fig. 7b a dynamic mixer with an anchor-type impeller 32, in Fig. 7c a dynamic mixer with a gate-type impeller 34 and in Fig. 7d a dynamic mixer with a helical-type impeller 36.

[0102] Fig. 8 shows five different types of static mixers useable in the method in accordance with the present invention, namely in Fig. 8a a static mixer 38 of the x-type comprising deflection means 40 in the form of crossbars having in a plan view as well as in side view a x-like form. Fig. 8b shows a static mixer 38 of the baffle plate-type comprising longitudinal deflection means 40, whereas Fig. 8c and 8d show static mixers 38 with curved deflection means 40. Fig. 8e shows a plug flow reactor 14 with a combined static mixer and heat transfer element 40 with tube-like deflection means 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 40, such as it is commercially distributed by Sulzer Chemtech Ltd under the tradename SMR.

[0103] Reference signs of the embodiments according to the invention

[0104] 1 Plant

[0105] 2 Monomer feed

[0106] 3 Catalyst feed

[0107] 4 Co-catalyst feed

[0108] 5 Inhibitor feed

[0109] 6 Crystallizer

[0110] 7 Devolatilization unit

[0111] 8 Post processing unit

[0112] 10 Reactor system

[0113] 11 , 11’ Coolant system

[0114] 12 Continuous stirred-tank reactor

[0115] 13 Dynamic mixer

[0116] 14, 14' Plug flow reactor

[0117] 15, 15’, 15”, 15’” Static mixer and I or heat transfer elements

[0118] 16 Loop reactor

[0119] 17 Side feed

[0120] 18, 18', 18", 18'", 18”” Feeding point

[0121] 19 Distributor element

[0122] 20 Motor

[0123] 22 Product removal line

[0124] 24, 24’ Connecting line

[0125] 26 Pump

[0126] 30 Dynamic mixer with a paddle-type impeller

[0127] 32 Dynamic mixer with an anchor-type impeller

[0128] 34 Dynamic mixer with a gate-type impeller

[0129] 36 Dynamic mixer with a helical-type impeller

[0130] 38 Static mixer

[0131] 40 Deflection means

[0132] 01 , 02 Reactor outlet

[0133] 11 , I2 Reactor inlet Subsequently, the present invention is described by means of illustrative, but not limiting example.

[0134] Example

[0135] Continuous ring-opening polymerization to produce polylactide using plant according to Fig. 3.

[0136] This example has been performed in a plant according to Fig. 3 with a 100 kg / h polymerization reactor system (i.e. about 50% monomer conversion), which comprised at the upstream end a loop reactor and at the downstream end a double-jacketed plug flow reactor encompassing static mixer internals.

[0137] L-lactide was heated up to 180°C and pumped at a throughput of 100kg / h into the 42L loop reactor. Separately from this feed, tin octoate / toluene (40 mg / ml) catalyst and 2-ethyl hexanol were provided at 250 g / h and 56 g / h, respectively. Before entering the loop reactor, the L-lactide was mixed with the catalyst and cocatalyst in a static mixer. The mixed reactants entered the loop reactor and are further mixed with recirculated reactants at 180°C. The circulation rate of the loop reactor was 400 kg / h to achieve proper mixing.

[0138] With a residence time of 20 to 30 min in the loop reactor, the polymer was withdrawn from the reactor, and fed to the double-jacketed static mixer-based plug flow reactor operated at the same temperature. Additionally, L-lactide was pumped at a throughput of 20 kg / h into the 83 L plug flow reactor via the monomer side feed having multiple feed points. The same amount of L-lactide was supplied at all feeding points.

[0139] With a total residence time within 15 to 120 min, preferably 30 to 90 min, the final product was passed through the devolatilization unit, pelletizer, crystallizer and dryer. The number average molecular weight of the polymer was at least 120 kg / mol and reached 200 kg / mol.

[0140] Comparative Example

[0141] Continuous ring-opening polymerization to produce polylactide using plant of state of the art according to Fig. 1 .

[0142] This example has been performed in a plant according to Fig. 1 with a 100 kg / h polymerization reactor system (i.e. about 95% monomer conversion), which comprised at the upstream end a loop reactor and at the downstream end a double-jacketed plug flow reactor encompassing static mixer internals.

[0143] L-lactide was molten / heated under nitrogen atmosphere at 120°C in the lactide feed. L-lactide was pumped at a throughput of 105 kg / h into the 44 L loop reactor, which was heated by an oil heat transfer unit operated at 185°C (reaction medium at 180°C). The circulation rate of the loop reactor was 420 kg / h to achieve proper mixing.

[0144] Separately from this feed, tin octoate / toluene (40 mg / ml) catalyst and 2-ethyl hexanol were introduced at 262.5 g / h and 184 g / h, respectively. With a residence time of 20 to 60 min in the loop reactor, the polymer was withdrawn from the reactor, and fed to the double-jacketed static mixer-based plug flow reactor operated at the same temperature.

[0145] With a total residence time within 30 to 90 min, the final product was passed through the devolatilization unit, pelletizer, crystallizer and dryer. The absolute number average molecular weight of the polymer was at least 30 kg / mol and reached 75 kg / mol.

[0146] Hence, comparing the results of the example according to the invention and the comparative example, with the process and the plant according to the invention the production of polymers with high molecular weight is possible. By having the monomer side feed, the monomer can be used as diluent for the polymer and reduce the viscosity of the reaction mixture, wherein polymers with higher molecular weight can be provided.

[0147] The invention is not limited to the disclosed embodiments. Other variations of the disclosed embodiments may be understood and effected by those skilled in the art when practicing the claimed invention from a study of the drawings, the disclosure and the dependent claims. Each aspect defined may be combined with any other aspect or aspects unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous. In the claims, the word “comprising” does not exclude other elements or steps, and the singular forms “a”, “an,” and “the” include plural referents unless the context clearly dictates otherwise. By way of example, “a step” means one step or more than one step. The mere fact that certain measures are repeated in different dependent claims does not mean that a combination of these measures cannot be advantageously used. Any reference signs in the claims should not be construed as limiting the scope.

Claims

Patent claims1 . A process of continuously manufacturing a homopolymer with high viscosity, in particular poly(hydroxy acid) homopolymer comprising the steps of continuously providing at least one monomer into a reactor system (10), the reactor system (10) comprising at least one reactor (12, 14, 14’, 16) with a reactor inlet (11 , I2) through which the monomer is continuously provided and a reactor outlet (01 , 02) through which a reaction mixture is continuously discharged, and polymerizing the at least one monomer to the homopolymer in the reactor system (10), characterized in that, the reactor system (10) further comprises a monomer side feed (17) with at least one feeding point (18, 18’, 18”, 18’”, 18””) through which a monomer composition is fed into the at least one reactor (12, 14, 14’, 16), wherein the at least one feeding point (18, 18’, 18”, 18’”, 18””) is provided on the at least one reactor (12, 14, 14’, 16) between the reactor inlet (11 , I2) and the reactor outlet (01 , 02), such that the monomer composition is provided into the reaction mixture.

2. The process in accordance with claim 1 , wherein the at least one reactor (12, 14, 14’, 16) is one of a continuous stirred-tank reactor (12), a loop reactor (16) and / or a plug flow reactor (14, 14’).

3. The process in accordance with claim 1 or 2, wherein the monomer side feed (17) comprises a plurality of feeding points (18, 18’, 18”, 18’”, 18””) through which the monomer composition is fed into the at least one reactor (12, 14, 14’, 16), wherein the plurality of feeding points (18, 18’, 18”, 18’”, 18””) are provided at different locations on the at least one reactor (12, 14, 14’, 16) between the reactor inlet (11 , I2) and the reactor outlet (01 , 02), such that the monomer composition is provided at different locations into the reaction mixture.

4. The process in accordance with claim 3, wherein at different feeding points of the plurality of feeding points (18, 18’, 18”, 18’”, 18””) themonomer composition is fed into the at least one reactor (12, 14, 14’, 16) at different temperatures, and I or the monomer composition fed into the at least one reactor (12, 14, 14’, 16) has different compositions, and / or a different volume of the monomer composition is fed into the at least one reactor (12, 14, 14’, 16).

5. The process in accordance with any of the preceding claims, wherein monomer side feed (17) comprises an additional feeding point (18) through which the monomer composition is fed into an inflow of the at least one reactor (12, 14, 14’, 16), such that the monomer composition is additionally provided at the inlet into the at least one reactor (12, 14, 14’, 16).

6. The process in accordance with any of the preceding claims, wherein the at least one reactor (12, 14, 14’, 16) of the reactor system (10) comprises at least one mixer (15, 15’, 15”, 15’”, 30, 32, 34, 36, 38, 40) and / or at least one heat transfer element (15, 15’, 15”, 15’”, 40) and the at least one feeding point (18, 18’, 18”, 18’”, 18””) is arranged at the at least one mixer (15, 15’, 15”, 15’”, 30, 32, 34, 36, 38, 40) and / or at the least one heat transfer element (15, 15’, 15”, 15’”, 40).

7. The process in accordance with any of the preceding claims, wherein the at least one monomer is continuously provided from the monomer side feed (17) in the monomer composition in the presence of at least one catalyst, and I or co-catalyst, and I or initiator, and I or solvent, and I or inhibitor into the reactor system (10).

8. The process in accordance with any of the preceding claims, wherein the process is operated so that the overall conversion of the monomer to the polymer, in particular of the one or more cyclic esters to the poly(hydroxy acid) homopolymer during the ring-opening-polymerization, is at least 40%, preferably between 45% and 75%, more preferably between 45% and 65%, still more preferably between 50% and 65%.

9. The process in accordance with any of the preceding claims, wherein a temperature of the polymerization is controlled by providing the monomer composition from the monomer side feed (17) with a predeterminable temperature.

10. The process in accordance with any of the preceding claims, wherein the content of a catalyst applied during the polymerization based on the reaction mixture is between more than 0 ppm and 200 ppm or less, preferably between 1 ppm and 180 ppm, more preferably between 10 ppm and 150 ppm, still more preferably between 10 ppm and 100 ppm and most preferably between 50 ppm and 100 ppm.11 . The process in accordance with any of the preceding claims, wherein the polymerization is a ring-opening-polymerization and the at least one monomer is a cyclic ester selected from the group consisting of lactide, glycolide, caprolactone, valerolactone, decalactone, butyrolactone, dodecalactone, octanolactone and any combination of two or more of the aforementioned compounds or is preferably selected from the group consisting of L-lactide, D-lactide, meso-lactide, lactide racemic mixture, glycolide, s-caprolactone, y-caprolactone, 5-valerolactone, y- valerolactone, 5-decalactone, 5-decalactone, 5-butyrolactone, 5- dodecalactone, 5-dodecalactone, 5-octanolactone and any combination of two or more of the aforementioned compounds, wherein preferably during the ring-opening-polymerization i) a lactide is polymerized so as to manufacture a polylactide or ii) a caprolactone is polymerized so as to manufacture a polycaprolactone or iii) a glycolide is polymerized so as to manufacture a polyglycolide or iv) a lactide and a caprolactone are polymerized so as to manufacture a poly(lactide-co-caprolactone) or v) a lactide and a glycolide are polymerized so as to manufacture a poly(lactide-co-glycolide).

12. The process in accordance with any of the preceding claims, wherein at least one and preferably at least 50%, more preferably at least 75% and most preferably all of the reactors in the reactor system (10) comprise at least one of the monomer side feed (17), a static mixer (15, 15’, 15”, 15”’,38), a dynamic mixer (18, 18', 18", 18"') or a combination of a static mixer and a heat transfer element (15, 15’, 15”, 15”’).

13. The process in accordance with any of the preceding claims, wherein the process is operated so that a melt viscosity of the reaction mixture at the reactor outlet (01 , 02) of a last reactor (14, 14’) of the reactor system (10) is below 4000 Paxs, preferably below 2500 Paxs, more preferably below 1500 Paxs, as determined by a rheometer using a cone plate measuring setup in oscillation mode.

14. A plant for continuously manufacturing a homopolymer, in particular a poly(hydroxy acid) homopolymer, from a monomer comprising a reactor system (10), the reactor system (10) comprising at least one reactor (12, 14, 14’, 16) with a reactor inlet (11 , I2) through which the monomer can be continuously provided and a reactor outlet (01 , 02) through which a reaction mixture can be continuously discharged, characterized in that, the reactor system (10) further comprises a monomer side feed (17) with at least one feeding point (18, 18', 18", 18"', 18””) through which a monomer composition can be fed into the at least one reactor (12, 14, 14’, 16), wherein the at least one feeding point (18, 18', 18", 18'", 18””) is provided on the at least one reactor (12, 14, 14’, 16) between the reactor inlet (11 , I2) and the reactor outlet (01 , 02), such that the monomer composition can be provided into the reaction mixture.

15. The plant in accordance with claim 14, wherein the at least one reactor (12, 14, 14’, 16) is the plug flow reactor (14, 14’) and I or loop reactor (16) comprising a distributor element (19) arranged between the reactor inlet (11 , I2) and the reactor outlet (01 , 02), the distributor element (19) comprising the at least one feeding point (18, 18', 18", 18'", 18””) and a static mixer (15, 15’, 15”, 15’”, 38), such that the monomer composition can be mixed with the reaction mixture when being fed into the plug flow reactor (14, 14’) and / or loop reactor (16).

16. The plant (10) of claim 14, wherein the at least one reactor (12, 14, 14’, 16) is the plug flow reactor (14, 14’) and the reactor system (10) additionally comprises one reactor, which is a continuous stirred-tank reactor (12) or a loop reactor (16), the reactors being arranged in series to each other, such that a first reactor (12, 16) into which the monomer is provided, is the continuous stirred-tank reactor (12) or the loop reactor (16), and a second reactor (14, 14’) downstream of the first reactor (12, 16) is the plug flow reactor (14, 14’), wherein both reactors (12, 14, 14’, 16) are connected via a connecting line (24, 24’).

17. The plant (10) of claim 14, wherein the at least one reactor (12, 14, 14’, 16) is a plug flow reactor (14, 14’) and the reactor system (10) additionally comprises at least one additional reactor (14, 14’), which is a plug flow reactor (14, 14’), the reactors being arranged in series to each other, such that a first reactor (12, 16) into which the monomer is provided, is the at least one reactor (12, 14, 14’, 16), and a second reactor (14, 14’) downstream of the first reactor (12, 16) is the at least one additional reactor (14, 14’), wherein a diameter and I or length and I or volume of the at least one additional reactor (14, 14’) is larger than the diameter and I or length and / or volume of the at least one reactor (12, 14, 14', 16), such that a flow rate of the reaction mixture in these reactors (12, 14, 14', 16) remains essentially constant.

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