Method and device for production of a polymer

Abstract

This invention relates to a method and device for producing high molecular weight polymers, including polylactide, polyglycolide, and polycaprolactone, with low polydispersity through a four-stage process: pre-polymerization, intermediate polymerization, final polymerization, and devolatilization. The process employs supercritical carbon dioxide, introduced either before or after the intermediate polymerization stage, in combination with a specially designed static mixing reactor and a phase separation vessel during final polymerization. Supercritical carbon dioxide improves heat and mass transfer while leveraging its solvating properties to effectively extract volatiles, such as monomers, oligomers, and catalysts, from the polymer matrix. In the phase separation vessel, the supercritical carbon dioxide-rich phase rises, extracting dissolved volatiles from the top of the vessel. During devolatilization, the dissolved supercritical carbon dioxide within the polymer serves as a stripping agent, ensuring the effective removal of residual impurities and resulting in high-quality, purified polymers.

Description

[0001] METHOD AND DEVICE FOR PRODUCTION OF A POLYMER BACKGROUND OF THE INVENTION

[0002] Polymers synthesized through ring-opening polymerization of cyclic ester monomers, such as lactide, glycolide, and caprolactone, and their corresponding copolymers, are crucial for the development of biodegradable and biocompatible materials. High molecular weight polymers, including polylactide, polyglycolide, polycaprolactone, and their copolymers, find extensive applications in the medical industry for products like surgical sutures, staples, and bone fixation devices. Their widespread use is attributed to their excellent mechanical properties, versatility, and ability to degrade into non-toxic byproducts in an environmentally friendly manner. Despite these advantages, achieving the industrial -scale production of high molecular weight polymers remains significant challenges.

[0003] One of the major challenges in producing high molecular weight polymers during polymerization is the rapid increase in viscosity as the reaction progresses, which impairs efficient heat and mass transfer To address this, production methods frequently utilize mechanical systems like extruders and kneader reactors that utilize mechanical shear to improve mixing and lower viscosity.

[0004] Despite their effectiveness, these approaches have inherent limitations.

[0005] These mechanical systems, such as extruders and kneader reactors, rely heavily on intense mechanical shear to achieve adequate mixing This approach demands significant energy input and often results in inefficient heat transfer. The limited surface contact between reactor components, such as blades and shafts, and the polymer melt can cause the formation of hot spots, increasing the risk of localized overheating and polymer degradation. Another limitation is non-uniform mixing, which disrupts polymerization kinetics and often results in polymers with a high poly dispersity index. For instance, kneader reactors and extruders typically produce polylactide with a polydispersity index greater than 2, reflecting a broad molecular weight distribution. This is undesirable for applications such as medical devices, which require precise control over polymer properties and material consistency.

[0006] Supercritical carbon dioxide has recently attracted significant interest for its use in polymerization and polymer devolatilization Its unique ability to behave as both a gas and a liquid under high pressure and temperature enables it to dissolve into the polymer matrix, effectively reducing viscosity and enhancing process efficiency. Additionally, it serves as an efficient medium for dissolving and removing unwanted substances from polymers.

[0007] For example, as detailed in U.S. Pat. No. 20,140,213,754, the integration of supercritical carbon dioxide with biaxial kneader reactors has been proposed as a method to reduce viscosity during polymerization. Following the polymerization process, the polymer melt is exposed to supercritical carbon dioxide, which penetrates the material to extract residual monomers, oligomers and catalysts. Once the extraction is complete, the supercritical carbon dioxide is

[0008]

[0009] depressurized to gaseous state, allowing the separation of the undesired components from the produced polymer.

[0010] While the use of supercritical carbon dioxide in kneader reactors has shown promise for reducing viscosity in lab-scale processes, it faces considerable challenges when scaled up for industrial applications.

[0011] As the size of kneader reactors or extruders increases, the efficiency of mechanical mixing decreases, making it more difficult to maintain uniform mass and heat transfer. Additionally, the method often requires a high carbon dioxide-to-polymer ratio during the extraction step, typically exceeding 0.4, necessitating the use of substantial volumes of carbon dioxide. To maintain the process, the carbon dioxide must be continuously recycled to its supercritical state through repeated cycles of compression, cooling, and heating, which is highly energy-intensive and incurs substantial operational costs.

[0012] Further complications arise during the depressurization process, where large quantities of carbon dioxide can carry polymer residues into the piping and depressurization systems. This can result in blockages, increased maintenance needs, and operational disruptions, adding to the challenges of implementing this approach on an industrial scale.

[0013] Static mixing reactors offer a superior alternative to traditional mechanical systems, delivering enhanced heat transfer and uniform mixing while eliminating the drawbacks associated with moving parts. Their innovative design not only reduces energy consumption but also minimizes maintenance requirements, making them more efficient and reliable. Additionally, by avoiding mechanical shear, static mixing reactors prevent localized overheating, a common issue in traditional systems like kneader reactors and extruders.

[0014] Nevertheless, despite these advantages, producing high molecular weight polymers remains a challenge for static mixing reactors. These polymers exhibit significantly higher viscosity compared to standard polymers, impairing the flow dynamics critical for efficient static mixing. This increased viscosity limits the reactor’s ability to achieve proper mixing and uniform distribution of monomers, catalysts, and intermediates, making the production of such polymers more difficult.

[0015] Static devolatilization, as outlined in U.S. PatentNo. 10,143,941 and U.S. PatentNo. 7,942,955, provides several benefits, including lower energy consumption and reduced maintenance requirements. However, processing high molecular weight polymer solutions introduces specific challenges. During depressurization, volatile substances (hereafter referred to as "volatiles," including monomers, oligomers, catalysts, and other low molecular weight compounds) evaporate, causing a significant drop in the polymer’s temperature This temperature decrease leads to a rapid rise in viscosity, which can cause the polymer to solidify within the distributors of the devolatilization system, disrupting the process.

[0016] Given these challenges, there is a pressing need for a continuous, economically viable process to produce a high molecular weight polymer from cyclic ester monomers on an industrial scale. Such a process must address the inefficiencies of mechanical systems, minimize the carry-over

[0017]

[0018] of residual polymers during depressurization, prevent polymer solidification within the distributor, and achieve a low polydispersity index. These improvements are essential to meet the rigorous standards required for advanced applications, such as medical devices.

[0019] SUMMARY OF THE INVENTION

[0020] The present invention provides a novel method and device for the continuous, industrial-scale production of high molecular weight polymers, such as poly lactide, polyglycolide and polycaprolactone, through the ring-opening polymerization of cyclic ester monomers. This invention addresses critical challenges in polymer production, including high viscosity, inefficient heat transfer, and non-uniform mixing, by integrating advanced reactor designs with the application of supercritical carbon dioxide.

[0021] The method begins with a pre-polymerization stage to initiate polymer formation, followed by sequential polymerization stages carried out in static mixing reactors These stages include an intermediate polymerization stage and a final polymerization stage. In the final stage, supercritical carbon dioxide is introduced into a specially designed static mixing reactor, where it mixes thoroughly with the polymer This process reduces the polymer's viscosity, enhances heat and mass transfer, and ensures uniform mixing.

[0022] The static mixing reactor is uniquely designed with a perforated section that serves a dual purpose: facilitating polymerization as a reactor and channeling the polymer into a phase separation vessel as a distributor. In the phase separation vessel, the supercritical carbon dioxideenriched phase is efficiently separated, carrying dissolved volatiles away from the polymer. Another objective of the present invention is to provide a devolatilization stage that employs a specially designed static mixing reactor with a perforated section functioning as a distributor. This design ensures the efficient removal of residual volatiles from the final polymer product.

[0023] BRIEF DESCRIPTION OF THE DRAWING FIG. 1 illustrates a preferred polymer production system that employs cyclic ester monomers as the starting material, with the introduction of supercritical carbon dioxide positioned between the pre-polymerization and intermediate polymerization stage, in accordance with the present invention.

[0024] FIG. 2 illustrates a partial side view of the final polymerization stage shown in FIG. 1.

[0025] FIG. 3 illustrates a partial side view of the devolatilization stage shown in FIG. 1.

[0026] FIG. 4 illustrates a preferred polymer production system that employs cyclic ester monomers as the starting material, with the introduction of supercritical carbon dioxide positioned between the intermediate polymerization and final polymerization stage, in accordance with the present invention.

[0027]

[0028] FIG. 5 illustrates a partial sectional side view of a final polymerization stage featuring a vertical static mixing reactor equipped with a perforated plate at the bottom.

[0029] FIG. 6 illustrates a partial sectional side view of a final polymerization stage incorporating a horizontal static mixing reactor equipped with a sloped perforated plate.

[0030] DETAILED DESCRIPTION

[0031] Ring-opening polymerization of cyclic ester monomers, such as lactide, glycolide, or caprolactone, can be carried out using two primary methods: solution polymerization and bulk polymerization. In solution polymerization, the process is performed in the presence of organic solvents that are free of water to prevent hydrolysis and other undesirable side reactions This method provides advantages in controlling reaction conditions and heat dissipation but introduces complexities related to solvent removal and potential environmental concerns.

[0032] Bulk polymerization, on the other hand, is performed without the use of solvents and is particularly advantageous for these monomers. Cyclic ester monomers like lactide, glycolide, and caprolactone are highly reactive and tend to polymerize rapidly during the early stages of the reaction. Additionally, the absence of solvents simplifies the process, reduces environmental impact, and eliminates the need for solvent recovery and removal steps.

[0033] Bulk polymerization can be carried out in various reactors, including continuous stirred tank reactors, extruders, kneaders, and static mixing reactors. While continuous stirred tank reactors, extruders, and kneaders rely on mechanical mixing and shear, they often face challenges such as high energy consumption, poor heat transfer, and risks of localized overheating or polymer degradation. In contrast, static mixing reactors offer superior advantages: they require no moving parts, ensuring efficient mixing and heat transfer while minimizing energy use, maintenance, and the risk of hot spots.

[0034] A standard static mixing reactor is a tubular device designed for transforming cyclic ester monomers into a polymer under controlled thermal conditions. It consists of a shell equipped with an integrated heat transfer system comprising multiple layers of serpentine coils. These coils facilitate consistent and efficient heat exchange throughout the polymerization process. A stream of cyclic ester monomers is introduced into the reactor through a designated inlet, flow along the shell side, and undergo polymerization. The resulting polymers are then discharged through a polymer outlet, completing the transformation process.

[0035] The heat transfer system is central to the reactor’s efficiency in managing the exothermic nature of ring-opening polymerization. Thermal oil, used as the heat transfer medium, is introduced into the reactor through an oil inlet header, which distributes the oil evenly into the serpentine coils. These coils, which are actually small tubes, follow a continuous serpentine path and extend along the length of the reactor from the oil inlet header to the oil outlet header, creating an extended surface area for effective heat exchange. The monomer and produced polymer flow on the shell side of the reactor, while the thermal oil flows through the tube side. This arrangement allows the heat generated by the polymerization process to be efficiently removed by the oil flowing within

[0036]

[0037] the coils, preventing localized overheating and maintaining reaction stability. Within each layer, the straight segments of a coil intersect at angles of 60° to 120° with the straight segments of another coil, forming a single integrated layer of coils. Multiple such layers are stacked within the reactor, creating a bundle of coils that occupies its inner space and enhances uniform mixing by redistributing the reaction mixture across the layers.

[0038] Ring-opening polymerization of cyclic ester monomers is a dynamic process, involving a delicate balance between polymer formation and depolymerization. The reaction begins with the activation of cyclic ester monomers by catalysts, triggering a chain reaction that converts monomers into long-chain polymers. As the polymer chains grow, the viscosity of the system increases due to molecular entanglements. This rise in viscosity can hinder the efficient diffusion of reactants and limit heat transfer, posing challenges to the reaction process.

[0039] The standard static mixing reactors address some of these challenges by enhancing the mixing of the polymerizing system and facilitating heat exchange through thermal oil flowing around bundles of coils. During the early stages of polymerization, when the viscosity of the reaction mixture remains low, diffusion is efficient, and the polymerization rate exceeds the depolymerization rate, allowing polymer chains to grow.

[0040] However, as the polymer chains lengthen, the viscosity of the mixture increases significantly, reducing diffusion efficiency. At the point where the polymerization rate equals the depolymerization rate, the system reaches equilibrium, causing the polymer chains to stop growing, even if the residence time in the reactor is extended.

[0041] While standard static mixing reactors improve reaction efficiency during the early and intermediate stages of polymerization, their capacity to produce high molecular weight polymers is limited. For instance, producing polylactide with a number-average molecular weight exceeding 150,000 g / mol is challenging when relying solely on static mixing reactors. This highlights the need for additional strategies or reactor designs to achieve higher molecular weights in such polymerization processes.

[0042] Supercritical carbon dioxide is a state of carbon dioxide achieved when it is subjected to conditions beyond its critical temperature (31.1° C) and critical pressure (73.8 bar). In this supercritical state, carbon dioxide exhibits properties that are intermediate between those of gases and liquids, combining gas-like diffusivity with liquid-like density. These unique characteristics make supercritical carbon dioxide an attractive medium for various industrial and scientific applications, including its growing use in polymerization processes.

[0043] One unique feature of supercritical carbon dioxide is its ability to dissolve in small amounts into the produced polymer during polymerization, with the extent of dissolution depending on factors such as pressure, temperature, and the properties of the polymer itself. Even this limited dissolution significantly reduces the viscosity of the polymer, which is particularly beneficial in systems where high viscosity can impede efficient mixing, heat transfer, and reactant diffusion. By lowering viscosity, supercritical carbon dioxide in reduced quantities enhances uniform mixing and heat transfer, enabling the processing of high molecular weight polymers that would otherwise be difficult to manage in reactors alone.

[0044]

[0045] Another important role of supercritical carbon dioxide is its ability to act as a solvent or reaction medium, effectively dissolving a wide range of substances, including monomers, oligomers, and catalysts. One of its key advantages is its selective solubility: it readily dissolves monomers, oligomers and catalysts while showing limited solubility for high molecular weight polymers. During the pre-polymerization stage, monomers react to initiate the growth of polymer chains. At this stage, the reaction mixture maintains a relatively low viscosity to ensure smooth processing. This process can be conducted in various types of reactors, such as continuous stirred-tank reactors, loop reactors, and plug flow reactors, which can be tailored to meet specific requirements. A combination of these systems may also be used to enhance efficiency and control.

[0046] Following the pre-polymerization stage, the partially polymerized product, known as the prepolymer, is transferred to an intermediate polymerization stage for further reaction. In this stage, the pre-polymer undergoes additional reactions, resulting in the formation of an intermediate polymer. This intermediate polymer is then processed in the final polymerization stage, where the polymerization reaction is completed, producing a final polymer characterized by a high molecular weight.

[0047] To address these challenges, a method has been developed that integrates a specially designed static mixing reactor with supercritical carbon dioxide. Supercritical carbon dioxide can be introduced at various points during the polymerization process. For example, it may be injected into a static mixer prior to the pre-polymerization stage, ahead of the intermediate polymerization stage, or after the intermediate polymerization stage. In each case, it is thoroughly blended with the reaction mixture, effectively reducing its viscosity and enabling smoother and more efficient processing.

[0048] The optimal amount of supercritical carbon dioxide added typically ranges from 3% to 25% by weight, with a preferred range of 5% to 20%, and an even more preferred range of 8% to 15% by weight. Given the limited solubility of supercritical carbon dioxide in polymers, an effective approach is to add it in quantities that exceed its dissolution capacity. For instance, excess supercritical carbon dioxide can be introduced between the pre-polymerization and intermediate polymerization stages. A portion of it dissolves into the pre-polymer, thereby reducing the viscosity of the reaction mixture and enhancing both mixing and heat transfer efficiency. The excess supercritical carbon dioxide, which does not dissolve into the polymer, dissolves volatiles. This action is further enhanced by the unique geometry of the serpentine coil bundle, with its interconnected straight segments ensuring uniform dispersion of the excess supercritical carbon dioxide throughout the reaction mixture within the standard static mixing reactor. This even dispersion maximizes the solvating power of the supercritical carbon dioxide, enabling it to effectively extract monomers and other volatiles from the polymer matrix.

[0049] The reaction mixture from the intermediate polymerization stage is transferred to a specially designed static mixing reactor for the final polymerization. This modified reactor retains the core components of a standard static mixing reactor, including a shell and an integrated heat transfer system. However, instead of a traditional polymer outlet, it features perforated sections. These

[0050]

[0051] modifications enable the reactor to perform two critical functions: completing the polymerization process and effectively distributing the processed polymer into an integrated phase separation vessel.

[0052] The reactor comprises two primary sections. The exterior portion, located outside the vessel, acts as the entry point for the reaction mixture. The interior portion, situated within the vessel, features a closed rear end, while a perforated section is located along the lower length of the reactor shell. The polymer mixture, containing supercritical carbon dioxide and volatiles, enters the reactor, where polymerization proceeds under controlled conditions. An integrated heat transfer system within the reactor shell ensures efficient and precise temperature regulation, enabling consistent heat transfer throughout the process.

[0053] As the polymer mixture flows through the reactor, it reaches the perforated section, where it is extruded through a series of openings distributed along the reactor’s length into the phase separation vessel. These openings are designed to create fine polymer strands with a high specific surface area, greatly improving the efficiency of phase separation.

[0054] Within the phase separation vessel, volatiles dissolved in a supercritical carbon dioxide-rich phase are effectively separated from the polymer-rich phase. This separation is driven by the density differences between the two phases The less dense supercritical carbon dioxide-enriched phase, containing the extracted volatiles, naturally rises to the top of the vessel, while the denser polymer-enriched phase settles at the bottom. The polymer-enriched phase is subsequently collected and transferred for further processing.

[0055] The openings in the perforated section may vary in shape, including holes, slits, squares or rectangles, with holes being the preferred design. The diameters of the holes in the perforated section typically range from 1 to 50 mm, with a preferable range of 3 to 10 mm. To address varying flow conditions, the holes can be distributed irregularly to create variable densities. For instance, larger holes with lower densities near the enclosed rear end of the reactor are particularly advantageous for processing high-viscosity polymers, as they help minimize pressure drop during extrusion.

[0056] As polymer strands are extruded through openings in the lower portion of the shell of the specially designed static mixing reactor, a pressure drop occurs due to the resistance generated during extrusion. This results in a lower pressure at the exit of the openings compared to the internal pressure of the reactor. The extent of the pressure drop can vary, typically ranging from 2 to 100 bar, with a preferred range of 10 to 80 bar, and an even more preferred range of 20 to 60 bar. The size and number of the openings are key factors influencing the magnitude of this pressure drop. This pressure differential ensures controlled strand formation and optimal exposure to the phase separation environment.

[0057] To manage excess supercritical carbon dioxide and extracted volatiles, an outlet pipe is installed on the top of the phase separation vessel. The pressure within the vessel is regulated by a backpressure valve connected to this outlet pipe. The valve's inlet pressure is maintained above 75 bar to ensure supercritical conditions inside the vessel. The preferred inlet pressure range is 75 to 150 bar, with a more preferable range of 90 to 120 bar. The valve's outlet pressure is set below

[0058]

[0059] carbon dioxide's supercritical pressure threshold of 73.8 bar, such as at atmospheric pressure, to allow for the controlled release of volatiles and excess carbon dioxide.

[0060] The high inlet pressure of the back-pressure valve indicates that the pressure within the specially designed static mixing reactor is at least 75 bar. This elevated pressure enhances the mixing of the supercritical carbon dioxide-enriched phase with the polymer-enriched phase in the confined space of the reactor. This intense mixing maximizes the extraction efficiency of volatiles.

[0061] The specially designed static mixing reactor offers a significant improvement over conventional devolatilization methods described in U.S. Patent No. 10,143,941 and U.S. Patent No. 7,942,955, which use distributors within vacuum vessels to evaporate and separate volatiles. However, these approaches face challenges with high molecular weight polymers. During the evaporation of volatiles and supercritical carbon dioxide, the viscosity of the polymer increases sharply, which can result in blockages or even solidification due to heat loss and localized cooling.

[0062] To address these issues, one proposed solution in conventional systems is to elevate the inlet temperature of the polymer before it enters the distributor. Maintaining a higher temperature after volatile evaporation reduces the viscosity of the devolatilized polymer. However, this approach carries the risk of polymer degradation due to prolonged exposure to elevated temperatures. In contrast, the specially designed static mixing reactor and phase separation vessel prevent blockages by maintaining an almost uniform temperature throughout the final polymerization and phase separation process. This is achieved by sustaining high pressure above carbon dioxide's supercritical pressure threshold, effectively preventing volatile evaporation within both the reactor and the vessel. Additionally, the reactor’s specialized design ensures continuous and efficient mixing of the polymer-enriched and supercritical carbon dioxide-enriched phases, effectively eliminating localized cooling and viscosity spikes. The reaction mixture is guided through the reactor's openings, forming small, consistent polymer strands and facilitating the efficient separation of the supercritical carbon dioxide-enriched phase from the polymer-enriched phase.

[0063] When supercritical carbon dioxide carrying dissolved volatiles exits the separation vessel through the top outlet pipe, it flows through a back-pressure valve where the outlet pressure drops below the supercritical pressure threshold. This pressure drop causes the carbon dioxide to transition into a gaseous state while carrying the volatiles. The volatile- laden carbon dioxide is then directed to a scrubber, where a hot stream enriched with cyclic ester monomer is introduced at the top to absorb the volatiles, preventing their accumulation in downstream equipment and piping.

[0064] The purified carbon dioxide vapors, now free of volatiles, exit the scrubber and are routed to a recycling system for reuse. Meanwhile, the liquid mixture collected at the bottom of the scrubber, containing cyclic ester monomer and absorbed volatiles, is sent to a crystallization system to recover high-purity cyclic ester monomers. The remaining mother liquor from the crystallization system, containing cyclic ester monomers, oligomers, and catalysts, is purged for further treatment.

[0065]

[0066] The devolatilization stage is specifically designed to efficiently eliminate residual volatiles remaining after the final polymerization stage. The process starts with the polymer-enriched phase being transferred from the phase separation vessel to a static mixer using a gear pump. In the static mixer, a selected mixture of inhibitors is introduced and thoroughly blended into the polymer. These inhibitors prevent undesirable reactions, such as thermal degradation or chain scission, which could negatively impact the polymer's molecular weight and overall quality. The stabilized polymer mixture is then directed to the devolatilization stage, which incorporates a specially designed static mixing reactor that serves a dual purpose: preheating the polymer mixture and distributing it effectively. This reactor is horizontally positioned within a devolatilization vessel and consists of two main sections: an exterior section for the entry of the polymer mixture and an interior section housed within the vessel. The interior section is designed with a closed rear end and includes a perforated section that runs along the length of the reactor shell. Within the reactor, the polymer mixture is heated above its melting point through efficient heat exchange with thermal oil circulating through an integrated coil bundle.

[0067] As the polymer mixture reaches the perforated section of the reactor shell, it is extruded through multiple openings located in the lower portion of the shell, forming polymer strands. These openings in the perforated section can vary in shape, such as holes or slits, though holes are the preferred design. During this extrusion process, a pressure drop is induced, typically ranging from 5 to 120 bar, with an optimal range of 20 to 100 bar and a preferred range of 40 to 80 bar. This pressure configuration ensures that a portion of the reactor operates at pressures above the supercritical pressure threshold, allowing for efficient mixing of supercritical carbon dioxide with the polymer to facilitate devolatilization.

[0068] The extruded polymer strands are subjected to a vacuum environment within the devolatilization vessel, which is maintained at a pressure of 1-10 mbar. In this low-pressure environment, supercritical carbon dioxide, transitioning to its gaseous state, acts as an efficient stripping agent by capturing volatiles from within the polymer strands. Simultaneously, the vacuum conditions facilitate the evaporation of these volatiles The volatile-laden carbon dioxide exits the vessel through an outlet pipe located at the top, ensuring the effective removal of residual volatiles. The devolatilized polymer strands, now free of residual impurities, descend to the bottom of the vessel, where they are transported via a gear pump to a pelletizer for further processing.

[0069] During the final polymerization process, the majority of the volatiles and supercritical carbon dioxide from the specially designed static mixing reactor are directed to the top of the phase separation vessel. Consequently, the devolatilization process is required to manage only a small amount of supercritical carbon dioxide and residual volatiles that remain dissolved in the polymer. As these substances evaporate, the polymer’s temperature decreases slightly. Therefore, the polymer mixture in the specially designed static mixing reactor is preheated to a temperature that is not excessively high To ensure smooth operation of the devolatilization system and to prevent solidification, the temperature of the devolatilized polymer is maintained above its melting point.

[0070]

[0071] The extrusion process in this invention’s devolatilization stage, driven by an upstream gear pump, enables smooth and controlled polymer flow through perforated sections, offering a clear improvement over conventional static devolatilization methods, as outlined in U.S. Patent Nos.

[0072] 10,143,941 and 7,942,955. These static devolatilization systems rely exclusively on vacuum pressure and gravity to transport polymer through a perforated plate, necessitating larger holes to minimize pressure drop. However, the use of these larger holes results in polymer strands with a significantly lower specific surface area, thereby limiting their exposure to the vacuum environment and decreasing the efficiency of volatile removal.

[0073] In contrast, the gear pump-driven system allows for the use of smaller, precisely optimized perforations, resulting in polymer strands with much higher specific surface area. This enhanced surface area increases the strands’ exposure to the vacuum environment, thereby improving the removal of residual volatiles and ensuring more efficient devolatilization.

[0074] At the outlet pipe located on the top of the devolatilization vessel, gaseous carbon dioxide containing volatiles is channeled into a scrubber. In the scrubber, a hot stream enriched with cyclic ester monomer is introduced at its top to effectively absorb the volatiles, thereby preventing blockages in the vacuum system and associated piping. The carbon dioxide vapors, now stripped of volatiles, exit the scrubber and are directed either to a recycling system for reuse or safely disposed of in an appropriate manner.

[0075] The liquid mixture collected at the bottom of the scrubber, enriched with cyclic ester monomer and absorbed volatiles, is directed to a crystallization system. Within this system, purified cyclic ester monomer is recovered, while the remaining mother liquor, which contains cyclic ester monomers and other volatiles, is purged for further processing.

[0076] In addition to the specially designed static mixing reactors with perforated sections extending along their length, two distinct variants with unique configurations and perforation placements are available. The first variant is vertically installed at the top of the phase separation or devolatilization vessel and features an exterior portion, located outside the vessel, that serves as the entry point for the polymer mixture, along with an interior portion positioned inside the vessel, where a perforated plate is located at the bottom. This arrangement enables the polymer mixture to flow vertically through the perforations, forming multiple strands, as illustrated in FIG. 5. The second variant is horizontally installed within the vessels and includes an exterior portion acting as the entry point for the polymer mixture, while the interior portion is equipped with a sloped perforated plate at the rear end of the reactor. This configuration allows the polymer mixture to flow horizontally along the length of the reactor toward the perforated plate, where it passes through the perforations to form multiple strands, as illustrated in FIG. 6. Both designs rely on the gear pump’s driving force to effectively propel the polymer mixture through the perforations.

[0077] In both configurations, the horizontal design with a sloped perforated plate and the vertical design with a perforated plate at the bottom, the placement of the perforations at the far end of the reactor makes it impossible to position the oil outlet header at that location. Therefore, each coil begins at the oil inlet header, runs along the length of the reactor to the far end, and then

[0078]

[0079] loops back to connect with the oil outlet header. This arrangement ensures that both the oil inlet and outlet headers are positioned on the same side of the reactor. The straight segments of the coil extending from the oil inlet header to the far end of the reactor intersect with the straight segments of the same coil returning from the far end to the oil outlet header at angles ranging from 60° to 120°, forming a single integrated layer of coil. Multiple such layers are arranged in a stacked configuration within the reactor, creating a dense bundle of coils that fills the reactor's inner space.

[0080] The static mixing reactor, featuring a perforated section running along its length, allows for two possible configurations for the placement of the oil mlet and outlet headers. In the first configuration, the oil inlet and outlet headers are positioned at opposite ends of the reactor. In this setup, the straight segments of one coil intersect with the straight segments of another coil at angles between 60° and 120°, forming a single integrated layer of coils. In the second configuration, the oil inlet and outlet headers are located on the same side of the reactor The straight segments of the coil extend from the oil inlet header to the far end of the reactor and loop back, intersecting with the returning straight segments at angles ranging from 60° to 120°, also forming a single integrated layer of coils These two configurations for integrating the coil bundle within the reactor shell provide flexibility to meet various design and operational requirements.

[0081] The static mixing reactor with a perforated section extending along its length offers an advantage over the other two variants in terms of perforated surface area. With the same reactor diameter, the extended perforated section provides a significantly larger processing surface compared to the other designs. This increased surface area allows the reactor to handle a greater feed volume, resulting in more efficient material processing and higher throughput.

[0082] The polymer production system in FIG. 1 outlines a method for producing a polymer from cyclic ester monomers, incorporating supercritical carbon dioxide between the pre-polymerization and intermediate polymerization stages. A mixture of cyclic ester monomers, initiator, and catalyst is introduced continuously via stream 1 into the pre-polymerization stage 2, where prepolymerization occurs. The resulting pre-polymer is transferred through stream 3 by pump 4 to a static mixer 7 via stream 6, where it combines with supercritical carbon dioxide introduced through stream 5, forming a mixture of pre-polymer and carbon dioxide. This mixture flows via stream 8 to a standard static mixing reactor 9 for an intermediate polymerization and subsequently through stream 10 into a specially designed static mixing reactor 11 for a final polymerization. The output mixture enters a phase separation vessel 12, where the supercritical carbon dioxide-enriched phase, containing carbon dioxide and volatiles, exits through stream 13 and passes through a back-pressure valve 14 before entering scrubber 16 via stream 15. A cyclic ester monomer- enriched stream 17 is introduced at the top to absorb monomers, oligomers, and catalyst, resulting in a liquid stream 18 sent to the crystallization system 36, which produces purified monomers in stream 38 and a purge stream 37 for further treatment. Uncondensed vapors, primarily carbon dioxide, exit through stream 19. The partly devolatilized polymer flows from the sump of the phase separation vessel 12 via stream 20 to pump 21, then through stream 23 to a static mixer 24, where inhibitors are introduced via stream 22. The resulting mixture

[0083]

[0084] flows through stream 25 into a specially designed static mixing reactor 26 for devolatilization. Rising vapors, containing carbon dioxide, monomers, oligomers, and catalyst, are directed via stream 28 to scrubber 29, where another monomer-enriched stream 30 is introduced at the top to absorb these components, producing a liquid stream 31 that is combined with stream 18 in the crystallization system 36 to yield purified monomers in stream 38. Uncondensed vapors are removed via stream 32 by a vacuum system. The devolatilized polymer flows from the devolatilization vessel 27 via stream 33 to pump 34 and is directed through stream 35 to a pelletizer for final processing.

[0085] FIG. 2 shows a partial side view of the final polymerization stage, highlighting the perforated section of the specially designed static mixing reactor 11. Positioned in the lower portion of the horizontally installed reactor, this perforated section allows the polymer mixture to discharge into the phase separation vessel 12 for phase separation. It consists of a series of holes along the length of the reactor 11, with the interior portion extending into the vessel 12 and terminating in an enclosed rear end.

[0086] FIG. 3 illustrates a partial side view of the devolatilization system, highlighting the perforated section of the specially designed static mixing reactor 26. This perforated section, positioned in the lower portion of the horizontally installed reactor, is designed to discharge the polymer into the devolatilization vessel 27 for devolatilization. It features a series of holes running along the length of the reactor 26, with its interior portion located inside the vessel 27 and terminating in an enclosed rear end.

[0087] FIG. 4 illustrates a polymer production system using cyclic ester monomers as starting materials, with a key difference from FIG. 1 : supercritical carbon dioxide is introduced between the intermediate and final polymerization stages. In this process, a mixture of cyclic ester monomers, initiator, and catalyst is continuously fed into the pre-polymerization stage 2 via stream 1, where pre-polymerization occurs. The pre-polymer is then pumped through stream 3 by pump 4 into a standard static mixing reactor 7’ via stream 6 for intermediate polymerization. The resulting intermediate polymer flows through stream 8’ into a static mixer 9’, where it combines with supercritical carbon dioxide introduced via stream 5’, forming a mixture of intermediate polymer and carbon dioxide. This mixture is subsequently sent through stream 10 into a specially designed static mixing reactor 11 for final polymerization, with the rest of the process remaining the same as in FIG. 1.

[0088] FIG. 5 presents a partial sectional side view of the final polymerization stage, featuring the specially designed static mixing reactor 11’, which is vertically installed into the phase separation vessel 12’ from the top. At its bottom, the reactor includes a perforated plate with a series of holes that enable the discharge of the polymer mixture into the phase separation vessel 12’ for phase separation. The reactor's lower portion extends inside the vessel 12’, while the upper portion remains outside, where the feed inlet 10’ introduces the polymer mixture into the reactor.

[0089] FIG. 6 depicts a partial sectional side view of the final polymerization stage, featuring a horizontally installed specially designed static mixing reactor 11” equipped with a sloped

[0090]

[0091] perforated plate at its rear end. This sloped perforated plate, located at the reactor's rear, includes a series of holes that facilitate the discharge of the polymer mixture into the phase separation vessel 12” for separation. The reactor's interior portion extends into the vessel 12”, while the exterior portion remains outside, with the feed inlet 10” supplying the polymer mixture to the reactor.

[0092] Subsequently, the present invention is illustrated in more details below with reference to the drawings and the examples.

[0093] EXAMPLES

[0094] Example 1

[0095] The polymerization of L-lactide was performed following the process outlined in FIG. 1. A purified liquid L-lactide stream, with a mass flow rate of 450 kg / h, was combined with a polymerization catalyst and an initiator to form stream 1. This stream was then introduced into a pre-polymerization stage 2, where the pre-polymerization was conducted at a temperature of 180° C, resulting in the formation of a pre-polymer. The pre-polymer, designated as stream 3, was transferred by gear pump 4 to static mixer 7 via stream 6, where it was combined with a supercritical carbon dioxide stream 5 (60 kg / h, 180° C, 240 bar). The resulting mixture was then directed via stream 8 into a standard static mixing reactor 9 for intermediate polymerization, yielding an intermediate polymer mixture.

[0096] This mixture was then fed via stream 10 into the specially designed static mixing reactor 11, located within the phase separation vessel 12, for final polymerization. Operating at 185° C, this process resulted in the formation of a high molecular weight polylactide. The polymer was extruded through a perforated section with 220 holes (4-6 mm in diameter) into the phase separation vessel 12, maintained at 90 bar. The supercritical carbon dioxide-enriched phase rose and exited through stream 13, passed a back-pressure valve 14 which maintained an inlet pressure of 90 bar, and entered scrubber 16 (1.03 bar) via stream 15.

[0097] In scrubber 16, vapors were contacted with a hot stream 17, enriched with L-lactide, introduced at the top. This stream absorbed monomers, oligomers, and catalysts from the vapors, forming a liquid at the scrubber’s base. The liquid was routed via stream 18 to the crystallization system 36, where purified L-lactide stream 38 was recovered, and the purge stream 37 was removed for further treatment. Uncondensed vapors exited via stream 19 to safety facilities.

[0098] The polymer strands in the phase separation vessel 12 were collected as partly devolatilized polylactide in its sump and transferred through stream 20 to gear pump 21, then through stream 23 to static mixer 24, where inhibitors were added via stream 22. The resulting mixture, stream 25, entered the specially designed static mixing reactor 26 for devolatilization. The reactor, heated to 205° C, extruded polymer through a perforated section with 144 holes (5-7 mm in diameter). The extruded polymer entered the sump of the devolatilization vessel 27, operating at

[0099]

[0100] 2 mbar, where residual carbon dioxide, lactide, oligomers, and catalysts evaporated and exited through stream 28.

[0101] The vapors from vessel 27 entered scrubber 29, where a hot, L-lactide-enriched stream 30 absorbed residual lactide, oligomers, and catalysts. The liquid collected at the bottom of scrubber 29 was routed through stream 31, combined with stream 18, and sent to the crystallization system 36 for further L-lactide recovery. The final devolatilized polylactide from vessel 27 was transferred via stream 33 to gear pump 34 and through stream 35 to a pelletizer for processing. Molecular weight analysis using gel permeation chromatography revealed a number-average molecular weight of 185,000, a weight-average molecular weight of 274,000 g / mol, and a poly dispersity index of 1.5.

[0102] Example 2

[0103] The polymerization of glycolide was conducted following the process depicted in FIG. 1. A purified liquid glycolide stream with a mass flow rate of 450 kg / h was mixed with a polymerization catalyst and initiator to form stream 1, which was fed into the pre-polymerization stage 2. Pre-polymerization was carried out at 200° C, producing a pre-polymer. The prepolymer, referred to as stream 3, was transferred by a gear pump 4 to a static mixer 7 via stream 6, where it was combined with a supercritical carbon dioxide stream 5, at a flow rate of 70 kg / h, 200° C, and 240 bar. The resulting mixture was introduced into a standard static mixing reactor 9 via stream 8, maintained at 220° C, where the intermediate polymerization occurred. This step produced an intermediate polymer mixture.

[0104] This mixture was fed via stream 10 into the specially designed static mixing reactor 11, located inside the phase separation vessel 12, for the final polymerization. At 220° C, further polymerization resulted in a high molecular weight polyglycolide. The polymer was extruded through a perforated section with 220 holes (4-6 mm in diameter) into the phase separation vessel 12, maintained at 100 bar. The supercritical carbon dioxide-enriched phase rose to the top and exited via stream 13 to a back-pressure valve 14, which maintained an inlet pressure of 100 bar. The outlet from valve 14, stream 15, was directed to the bottom of scrubber 16, maintained at a pressure of 1.03 bar.

[0105] In scrubber 16, the vapors were contacted with a hot stream 17, enriched with glycolide, introduced at the top. This stream absorbed monomers, oligomers, and catalysts from the vapors, forming a liquid at the scrubber’s base. The liquid was routed via stream 18 to the crystallization system 36, where purified glycolide stream 38 was recovered, and the purge stream 37 was removed for further treatment. Uncondensed vapors exited via stream 19 to safety facilities. The polymer strands in the phase separation vessel 12 were collected as partly devolatilized poly lycolide in its sump and transferred through stream 20 to gear pump 21, then through stream 23 to static mixer 24, where inhibitors were added via stream 22. The resulting mixture, stream 25, entered the specially designed static mixing reactor 26 for devolatilization. The reactor, heated to 235° C, extruded polymer through a perforated section with 144 holes (5-7

[0106]

[0107] mm in diameter). The extruded polymer entered the sump of the devolatilization vessel 27, operating at 2 mbar. Residual carbon dioxide, glycolide, oligomers, and catalysts evaporated from the polymer strands and exited via stream 28.

[0108] The vapors from vessel 27 entered scrubber 29, where a hot, glycolide-enriched stream 30 absorbed residual glycolide, oligomers, and catalysts. The absorbed materials flowed downward and accumulated as liquid at the bottom of scrubber 29. This liquid was routed through stream 31, combined with stream 18, and sent to the crystallization system 36 for further glycolide recovery. The final devolatilized polyglycolide from vessel 27 was transferred via stream 33 to gear pump 34 and through stream 35 to a pelletizer for processing.

[0109] Molecular weight analysis using gel permeation chromatography revealed a number-average molecular weight of 147,000, a weight-average molecular weight of 235,000 g / mol, and a poly dispersity index of 1.6.

[0110] Example 3

[0111] The polymerization of caprolactone was conducted as illustrated in FIG. 4. A purified liquid caprolactone stream, with a mass flow rate of 450 kg / h, was combined with a polymerization catalyst and an initiator to form stream 1. This stream was then introduced into the prepolymerization stage 2, where pre-polymerization occurred at a temperature of 155° C, resulting in a pre-polymer product. The pre-polymer, designated as stream 3, was conveyed by a gear pump 4 to a standard static mixing reactor 7’, maintained at 155° C, for intermediate polymerization. The intermediate polymer was subsequently routed via stream 8’ to a static mixer 9’, where it was combined with supercritical carbon dioxide delivered through stream 5’.

[0112] The supercritical carbon dioxide was supplied at a flow rate of 40 kg / h, a temperature of 155° C, and a pressure of 220 bar. This process yielded a mixture of the intermediate polymer and supercritical carbon dioxide.

[0113] This mixture was fed via stream 10 into the specially designed static mixing reactor 11, located inside the phase separation vessel 12, for the final polymerization. At 155° C, further polymerization resulted in a high molecular weight polycaprolactone. The polymer was extruded through a perforated section with 220 holes (4-6 mm in diameter) into the phase separation vessel 12, maintained at 85 bar. The supercritical carbon dioxide-enriched phase rose to the top and exited via stream 13 to a back-pressure valve 14, which maintained an inlet pressure of 85 bar. The outlet from valve 14, stream 15, was directed to the bottom of scrubber 16, maintained at a pressure of 1.03 bar.

[0114] In scrubber 16, the vapors were contacted with a hot stream 17, enriched with caprolactone, introduced at the top. This stream absorbed monomers, oligomers, and catalysts from the vapors, forming a liquid at the scrubber’s base. The liquid was routed via stream 18 to the crystallization system 36, where purified caprolactone stream 38 was recovered, and the purge stream 37 was removed for further treatment. Uncondensed vapors exited via stream 19 to safety facilities.

[0115]

[0116] The polymer strands in the phase separation vessel 12 were collected as partly devolatilized polycaprolactone in its sump and transferred through stream 20 to gear pump 21, then through stream 23 to static mixer 24, where inhibitors were added via stream 22. The resulting mixture, stream 25, entered the specially designed static mixing reactor 26 for devolatilization. The reactor, heated to 175° C, extruded polymer through a perforated section with 144 holes (5-7 mm in diameter). The extruded polymer entered the sump of the devolatilization vessel 27, operating at 2 mbar. Residual carbon dioxide, caprolactone, oligomers, and catalysts evaporated from the polymer strands and exited via stream 28.

[0117] The vapors from vessel 27 entered scrubber 29, where a hot, caprolactone-enriched stream 30 absorbed residual caprolactone, oligomers, and catalysts. The absorbed materials flowed downward and accumulated as liquid at the bottom of scrubber 29. This liquid was routed through stream 31, combined with stream 18, and sent to the crystallization system 36 for further caprolactone recovery The final devolatilized polycaprolactone from vessel 27 was transferred via stream 33 to gear pump 34 and through stream 35 to a pelletizer for processing.

[0118] Molecular weight analysis using gel permeation chromatography revealed a number-average molecular weight of 152,000, a weight-average molecular weight of 243,000 g / mol, and a polydispersity index of 1.6.

[0119]

Claims

The invention claimed is:

1. A method for producing a high molecular weight polymer through ring-opening polymerization of a cyclic ester monomer comprising:(a) pre-polymerization of the cyclic ester monomer to form a pre-polymer in a prepolymerization stage;(b) further polymerization of the pre-polymer to form an intermediate polymer in an intermediate polymerization stage;(c) further polymerization of the intermediate polymer to form a final polymer in a final polymerization stage, characterized in that a specially designed static mixing reactor integrated with a phase separation vessel is used, with the process aided by supercritical carbon dioxide; (d) devolatilization of the final polymer in a devolatilization stage to obtain a devolatilized polymer.

2. The method of claim 1, wherein the cyclic ester monomer is selected from the group consisting of lactide, glycolide, caprolactone or any combinations thereof.

3. The method of claim 1, wherein the specially designed static mixing reactor comprises:(a) an inlet for feeding the intermediate polymer. ;(b) a tubular shell and an integrated heat transfer system,wherein the tubular shell houses the integrated heat transfer system comprising multiple layers of serpentine coils,wherein each coil extends in a continuous serpentine path along the reactor’s length, with straight segments intersecting at angles between 60° and 120°, forming integrated layers of coils, and wherein each coil is connected between an oil inlet header and an oil outlet header, allowing continuous thermal oil flow for uniform heat exchange;(c) a perforated section for polymer extrusion, wherein the perforated section is selected from the group consisting of perforations distributed along the reactor shell, perforations in a perforated plate positioned at the bottom of the reactor, perforations in a sloped perforated plate at the rear end of the reactor, or any combination thereof.

4. The method of claim 1, wherein the supercritical carbon dioxide is provided in an amount ranging from 3% to 25% by weight of the reaction mixture, preferably from 5% to 20%, and more preferably from 8% to 15%.

5. The method of claim 3, wherein the perforated section comprises openings selected from the group consisting of holes, slits, squares, rectangles, or any combination thereof.

6. The method of claim 5, wherein diameters of the holes in the perforated section range from 1 to 50 mm, with a preferable range of 3 to 10 mm.

7. The method of claim 3 and 5, wherein the polymer extrusion through the openings induces a pressure drop ranging from 2 to 100 bar, with a preferred range of 10 to 80 bar, and an even more preferred range of 20 to 60 bar.

8. The method of claim 1, wherein the phase separation vessel includes a supercritical carbon dioxide-enriched phase outlet situated on the top, a polymer-enriched phase discharge outlet positioned at the bottom, and a connection point allowing for the incorporation of the specially designed static mixing reactor that extends through the vessel.

9. The method of claim 8, wherein the supercritical carbon dioxide-enriched phase primarily comprises supercritical carbon dioxide, along with dissolved volatiles, including monomers, oligomers, catalysts, and other low molecular weight compounds.

10. The method of claim 8, wherein the polymer-enriched phase primarily comprises a polymer, along with a limited amount of dissolved supercritical carbon dioxide and residual dissolved volatiles, including monomers, oligomers, catalysts, and other low-molecular-weight compounds.11 The method of claim 1 , wherein the specially designed static mixing reactor is horizontally incorporated into the phase separation vessel.

12. The method of claim 1, wherein the specially designed static mixing reactor is vertically incorporated into the phase separation vessel.

13. The method of claim 1, 11 and 12, wherein the specially designed static mixing reactor includes an exterior portion located outside the phase separation vessel and an interior portion featuring the perforated section positioned within the phase separation vessel.

14. The method of claim 1 and 8, wherein the outlet installed on top of the phase separation vessel is connected to a back-pressure valve.

15. The method of claim 1 and 14, wherein the pressure of the phase separation vessel is regulated by using the back-pressure valve, with a preferred inlet pressure range of 75 to 150 bar and a more preferable range of 90 to 120 bar.

16. The method of claim 1 and 14, wherein the back-pressure valve's outlet pressure is set below carbon dioxide's supercritical pressure threshold of 73.8 bar.

17. The method of claim 1 , wherein the specially designed static mixing reactor is maintained at a temperature ranging from 100° C to 260° C, with a more preferable temperature range of 150° C to 240° C during polymerization.

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