KR20050105183A - Multiple stage solid state devolatilization of syndiotactic vinyl aromatic polymers - Google Patents

Abstract

A multistep polymer devolatilization process which comprises: a) heating a feed mixture comprising solid particulated syndiotactic vinyl aromatic polymer, residual vinyl aromatic monomer(s), and optionally residual process solvents and active catalyst residues in the presence of steam in a first contacting apparatus and separating the polymer product; b) contacting the polymer resulting from step a) with an inert fluid under devolatilizing conditions, and separating the fluid and polymer, and c) melting, extruding and pelletizing the resulting polymer product.

Description

Multiple stage solid state devolatilization of syndiotactic vinyl aromatic polymers

The process of the invention relates to a process for the preparation of syndiotactic vinyl aromatic polymers. In the production of syndiotactic vinyl aromatic polymers, such as syndiotactic polystyrene (SPS), typically a deolatilization step is performed to remove residual monomers, process solvents and other volatile components from the SPS. This method has the problem that residual vinyl aromatic monomers and other monomers can autopolymerize on heating to form atactic vinyl aromatic polymers and other polymers, such as atactic polystyrene, which are undesirable contaminants in the SPS polymer. Atactic vinyl aromatic polymers lower the SPS polymer properties, such as heat distortion temperature, and reduce the crystallization rate of the SPS homopolymer and copolymer resin.

In order to prevent discoloration of the SPS polymer, a deashing process is typically performed before extracting the active catalyst residue prior to performing the hernia process. In ash removal the polymer should be treated with ash removal agents such as hydrochloric acid, potassium hydroxide and the like. Alternatively, at low catalyst content, the active catalyst residue may simply be deactivated prior to hernia, whereby the active catalyst residue may remain in the final resin. Typically, hernias are performed by homogeneously mixing the polymer with an active nucleophile, preferably with a protic solvent, such as methanol.

Several hernia methods are known in the art and include, for example, melt hernia, which first melts the polymer and then degasses in fluid state and solid state hernia that heats the solid polymer and desorbs from the glass transition temperature to the melt temperature of the polymer. There is a law.

Yamamoto's JP 03056504 describes a melt hernia method in which SPS powder containing volatiles is fed to a twin screw extruder to melt and degrease. Although residual volatiles are reduced, catalyst deactivation and ash removal steps are required to prevent discoloration due to the presence of active catalyst residues.

Yamamoto's JP 03064303 describes a two-stage solid state hernia method, which first supplies an SPS powder containing volatiles to a dryer, where the powder is heated to a glass transition temperature to a melting point temperature of the SPS, Hermetically degassed by solvent degassing in a vacuum evacuated twin screw extruder as described above in JP 03056504. However, this method is very time consuming and takes 9 or 10 hours to terminate and also requires a catalyst deactivation or ash removal step.

Several methods of ashing and demineralization of active catalyst residues from SPS polymers are known. US Pat. No. 5,321,122 to Kuramoto et al. Describes a method for purifying styrene polymers by removing ash with an alcoholic alkaline solution and then washing with alcohol. US Pat. No. 5,426,176 to Teshima et al. Describes a method for purifying styrene polymers by removing ash with a ash remover, such as HCl, KOH, at temperatures above the glass transition temperature of the polymer. US Pat. No. 5,449,746 to Desima describes a process for purifying styrene polymers by treatment with swelling agents such as ethylbenzene and inactivating agents such as methanol or ethanol. U.S. Patent No. 5,612,452, issued to Deshima and Yamasaki, simultaneously inactivates and removes crystalline styrene polymers by treating the crystalline styrene polymer with a poor solvent containing 15 to 10,000 ppm water. Methods of treatment are described. However, these methods are additional processing steps that increase the complexity and cost in preparing SPS polymers.

U.S. Patent 5,877,271 describes a method of rapidly heating a syndiotactic polymer to hernia to form less colored body. In US Pat. No. 6,031,070 steam was used to remove residual catalyst and reduce residual monomer content in the polymer obtained. A multi-step process is illustrated in which one is using a steam and the other is drying in a double dryer using nitrogen, followed by melt extrusion of the final product.

Despite the advances in the art provided by the above methods, there is still a need for a method of economically and consistently producing syndiotactic vinyl aromatic polymers having a low residual monomer content, especially for end uses involving contact with food. have. Uses include, for example, food packaging and food handling devices, as well as direct contact uses, such as firing trays and food storage and / or reheating containers. For this use, polymers having a residual monomer content of less than 500 ppm, preferably less than 100 ppm and even more preferably less than 50 ppm are very preferred. Therefore, there is a need for a method of demineralizing syndiotactic vinyl aromatic polymers that provides polymers having a reduced content of volatiles, particularly residual monomers.

Summary of the Invention

The present invention

(a) solid particulate syndiotactic vinyl aromatic polymer; Residual vinyl aromatic monomer (s) and optionally residual process solvents; And heating a feed mixture comprising active catalyst residues in the presence of steam in a first contacting device and separating the polymer product,

(b) contacting the polymer obtained from step (a) with an inert fluid under hernia conditions and separating the fluid and the polymer, and

(c) providing a multistage polymer hernia method comprising melting, extruding and pelletizing the polymer product obtained.

This improved hermetic method removes volatiles, in particular residual vinyl aromatic monomer (s), from solid syndiotactic vinyl aromatic polymers, while inactivating active catalyst residues to provide products suitable for food contact applications. In addition, no separate ash removal and / or catalyst deactivation steps are required. Surprisingly, by using the improved hernia method of the present invention, a polymer having a low content of residual monomers and other volatile components, a low content of atactic polystyrene, reduced discoloration and improved whiteness is obtained.

The Periodic Table of Elements herein refers to the Periodic Table published and copyrighted by CRC Press, Inc. (2001). Element family (s) also refers to group (s) as reflected in the periodic table using the IUPAC system for numbering groups. For purposes of US patent practice, the content of patents, patent applications or patent publications incorporated herein by reference is incorporated by reference in its entirety, particularly with respect to the description of analytical or synthetic techniques and general knowledge in the art.

The term “comprising” and derivatives thereof does not exclude the presence of any additional component, step or process, whether identical or not described herein. To avoid all doubt, all compositions claimed herein using the term “comprising” can include any additional additives, adjuvants or compounds that are or are not polymeric unless otherwise noted. In contrast, the term “consisting essentially of” excludes, from the following stated ranges, any other component, step or process except those not essential to workability. The term “consisting of” excludes any component, step or process not specifically described or described. Unless stated otherwise, the term “or” refers to the described members individually and in any combination.

The term "polymer" as used herein includes both homopolymers, ie polymers made from a single reactive compound and copolymers, ie polymers made by the reaction of two or more reactive monomer compounds forming a polymer. The term "crystalline" means a polymer exhibiting an X-ray diffraction pattern at 25 ° C. and having a first order transition or crystal melting point (T _m ). The term may be used interchangeably with the term "semicrystalline". The term "syndiotactic" refers to a polymer having a stereoregular structure of racemic triad, measured by ¹³ C nuclear magnetic resonance spectroscopy, with a syndiotactic degree of greater than 90%, preferably greater than 95%. .

Syndiotactic vinyl aromatic polymers are homopolymers and copolymers of vinyl aromatic monomers, ie monomers having chemical structures with both unsaturated and aromatic moieties. Preferred vinyl aromatic monomers are compounds of formula (I).

H ₂ C = CR-Ar

In Formula I,

R is hydrogen or an alkyl group having 1 to 4 carbon atoms,

Ar is an aromatic radical of 6 to 10 carbon atoms, including an alkyl- or halo-cyclic substituted aromatic radical.

Examples of such vinyl aromatic monomers are styrene, α-methylstyrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, vinyl toluene, pt-butylstyrene, vinyl naphthalene, divinylbenzene, chlorostyrene, bromostyrene And the like. Currently preferred syndiotactic vinyl aromatic polymers are syndiotactic polystyrenes. Typical polymerization methods and coordination catalyst systems for preparing syndiotactic vinyl aromatic polymers are well known in the art and described in the literature. See, for example, US Pat. Nos. 4,680,353, 5,066,741, 5,206,197, and 5,294,685. Etc].

During the polymerization of vinyl aromatic monomers, the polymerization reaction is typically not carried out completely, resulting in a mixture of syndiotactic vinyl aromatic polymers with volatiles such as residual monomers and process solvents. Typically such mixtures comprise from about 2 to about 99 weight percent, preferably from about 30 to about 95 weight percent, more preferably from about 40 to about 95 weight percent of the solid nonvolatile high molecular weight polymer, based on the total weight of the mixture. %, Most preferably about 70 to about 90% by weight. The bulk density of the feed is typically less than 400 kg / m ³ , preferably less than 350 kg / m ³ . The average particle size dp ₅₀ is generally less than 500 μm, preferably less than 400 μm. The polymer can then be recovered from the mixture using a treatment process, such as a hernia process, to obtain a resin useful for molding injection molded articles, films, fibers and the like. The process of the present invention is an improved process for hernias in syndiotactic vinyl aromatic polymer / volatile mixtures, hereinafter referred to as "feed" mixtures.

Typically the feed mixture is withdrawn from the polymerization reactor or polymer recovery system at temperatures below 100 ° C., typically from about 10 to about 90 ° C. The mixture is then subjected to a temperature between the glass transition temperature (typically about 100 ° C.) and the melting point (typically between 200 and 320 ° C.) of the syndiotactic vinyl aromatic polymer that is hernia stripped in the presence of steam and optionally one or more other catalyst deactivators. Hernia In order to reduce the time required to remove volatiles to the desired level, the feed mixture can be heated to at least 110 ° C, more preferably at least 125 ° C, before or at the same time as contacting the feed mixture with steam. In one embodiment of the present invention, the steam itself can be used as the heating medium to achieve the above-mentioned temperature limits.

Any feed mixture heating means can be used in the process of the invention. Examples of such heating means include indirect dryers, such as disks, drums, slow and fast paddle type, rotary, and screw conveyor dryers, in which the feed material contacts the heated metal surface by a suitable heat transfer fluid; Kinetic energy heaters using plow-type mixers / dryers reinforced by high speed shredders, batch operated mixers / homogenizers using a pneumatically conveyed hammer mill or a high speed stirrer; Direct dryers using a heated gas stream for heating, such as a flash dryer; All types of fluid bed dryers, conveyors, trays and direct heating rotary dryers; Conventional dryer / heating apparatuses, and combinations thereof, supplemented with auxiliary heating techniques such as radiant infrared, microwave heating, or similar techniques. Further suitable devices include insulated, gas purging mass flow hoppers and storage silos.

The first step of the degassing process is carried out in the presence of a vapor which inactivates the active catalyst residue contained in the feed mixture, making it inert to further polymerization reactions and helping to remove (purge) residual volatile components. Preferably, the steam is produced in a separate process and heated to a temperature above the boiling point of water at the hernia pressure (superheated steam). Alternatively, liquid water can be injected directly into the hernia apparatus or mixed with the feed mixture introduced to vaporize the entire mixture. The arrangement of the feed mixture inlet, purge gas (vapor) inlet and liquid inlet should be aligned in the hernia apparatus to maximize the contact time of the feed mixture with the vapor. It is desirable for the vapor to diffuse rapidly into the polymer mass and react with the active residual catalyst components to inactivate them, thereby preventing prolonged exposure to elevated temperatures and polymer discoloration or degradation.

Very preferably, the steam is at a mass ratio of steam to volatiles between 130 and 210 ° C., preferably between 150 and 210 ° C., in the first hernia in which the steam is injected in a direction opposite the feed flow direction. Contacting the feed mixture, preferably in an amount of from 1 to 5.

In addition to steam, other catalyst deactivators may also be used in step (a). Typically, in addition to steam, any active nucleophilic compound in vaporized form may be used that can inactivate residual active catalyst components. Such compounds include a wide variety of polar organic and inorganic compounds, such as those of Formula II.

C _i H _j O _k S _l N _m X _n

In Chemical Formula II,

X is fluorine, chlorine, bromine or iodine,

i is an integer from 0 to 6,

j is an integer from 0 to 14,

k is an integer from 0 to 3,

l and m are integers from 0 to 2,

n is an integer from 0 to 6, and these integers ensure that all suitable valences are satisfied.

Typically, when a suitable catalyst deactivator is used, it is characterized by a molecular weight of less than about 100D, limited solubility in the resulting polymer and miscibility with steam or water vapor. It does not need to be removed in a later step and is not decomposed by reaction with the steam used in the deactivation and deaeration process. In order to facilitate reuse and potential polymer discoloration of the monomers and other volatile components recovered from the hermetic process, any second catalytic deactivator may also be non-reactive with the vinyl aromatic monomer and process solvent under the conditions used in the hermetic process. Is preferably. Typical second inactivating agents used according to the present invention include carbon dioxide, carbon monoxide, hydrogen sulfide, sulfur dioxide, ammonia, polar organic compounds such as alcohols, aldehydes, ketones, and the like, and mixtures thereof.

The amount of steam or mixture of steam and other catalyst deactivator (s) necessary to achieve the desired deactivation depends on the residual amount of the total active catalyst component in the feed mixture, but typically is used in a significant excess to provide complete deactivation. To ensure. The mass flow rate of the vapor / catalyst deactivating gas mixture used is typically in the range of 0.1 to 80% of the feed mixture flow rate. Very preferably, the steam or steam mixture is passed through the apparatus in a direction opposite to the flow direction of the feed mixture. Preferred steam temperatures are 150 to 270 ° C, preferably 150 to 210 ° C.

In addition to steam or mixtures of steam with other catalyst deactivators, other inert gases may also be present that do not absorb active catalyst residues or volatile components generated and are not absorbed into the polymer to an appreciable degree. Typical inert gases include nitrogen, rare gases such as argon and helium, alkanes such as methane and ethane, hydrogen, and mixtures thereof. These components act as diluents and may help to transport volatiles from the hernia apparatus to the outside and reduce the residual volatiles content in the dried product. In order to best achieve the desired level of inactivation, the molar ratio of inert gas to vapor or mixture of vapor and other catalytic deactivator should typically not exceed 99/1.

The first step of the improved hernia method of the present invention can be carried out at various working pressures in the hernia apparatus, provided that the steam is kept in steam at the temperature and pressure used in the hernia method. The hernia process can be carried out at or near atmospheric pressure, or in the case of a suitable hernia apparatus at elevated or reduced pressure. Operation below subatmospheric pressure can also be performed by applying a vacuum to degas the generated volatiles and excess steam or other deactivator from the hernia apparatus. Very preferably, the first hernia step is carried out at or near atmospheric using superheated steam, in particular steam at 130 to 210 ° C.

As a result of the heating of the feed mixture, the volatile components comprising residual vinyl aromatic monomers are released from the polymer and vaporized and transported out of the device together with the flowing exhaust steam. The residence time of the polymer in the hernia device is based on the residual vinyl aromatic monomer content in the hernia polymer, based on the hernia polymer, the initial content of the residual vinyl aromatic monomer in the feed mixture, typically from 5 to 60% to less than 3%, It should preferably be sufficient to reduce it to less than 1%. The residence time in the hernia apparatus required to achieve such a reduction in the content of volatiles depends on the initial volatile content of the feed mixture, the temperature in the hernia apparatus, the catalyst inactivation and the total flow rate of the inert gas, the absolute pressure in the hernia apparatus and the feed mixture It depends on the physical properties. In general, hernias have a residence time required to achieve the above-mentioned residual vinyl aromatic monomer content of 24 hours or less, typically 12 hours or less, preferably 4 hours or less, more preferably 1 hour or less, most preferably It is carried out under conditions such that it is 30 minutes or less.

Preferably, the feed mixture is rapidly heated to about 110 ° C. to the melting point of the syndiotactic vinyl aromatic polymer. Preferably, the feed mixture is heated to a temperature about 20-100 ° C. below the melting point of the sufficiently dried polymer. In general, rapid heating results in a temperature of the feed mixture of at least 10 ° C./min, typically 10 to 1000 ° C./min, preferably at least 20 ° C./min, more preferably at least 30 ° C./min, most preferably It can be carried out in an apparatus which can be increased above 40 ° C / min. By heating at a higher rate, the residual monomer is more likely to volatilize than polymerize, thereby reducing the amount of atactic vinyl aromatic polymer produced.

After step (a), the polymer product is further degreased by contacting a second hernia device, for example, a hernia agent with opposite flow direction, in the device described above in connection with step (a). . Suitable hernias include steam, the second hernias and inert gases described in step (a), and mixtures thereof. Particularly preferred hernias used in step (b) are steam or a mixture of steam and a second hernia. Suitable examples of hernia devices suitable for use in step (b) include direct and indirect heat dryers. Most preferably, the apparatus for step (b) is a high temperature rotary dryer infused with a degreaser and equipped with a gas vent, opposite the flow direction. In the second contacting step, the hernia agent used is preferably above the temperature of the vapor or steam mixture used in the first hernia step. The preferred temperature of the hernia in step (b) is 150 to 270 ° C, preferably 170 to 230 ° C.

In order to reduce discoloration of the polymers produced according to the process of the invention, it is important to contact the feed stream with air or oxygen after at least step (a), preferably after both step (a) and step (b) has been completed . Therefore, it is important to contact the feed stream and the resulting polymer with a catalyst deactivator or inert gas as defined above until the desired volatile content is achieved. In addition, prior to injecting the steam or steam mixture into the hernia apparatus, it must be filtered to remove contaminants and particles that can cause colorant formation.

The final hernia step of the polymer is a melting, extruding and pelletizing step using an extruder or discharge extruder which is preferably operated under reduced pressure. Examples of such melt extrusion apparatus used in step (c) include discharge or vacuum apparatuses equipped with single and twin screw extruders, water baths or similar coolers and shredders or similar pelletizers. If desired, filtration of the molten hernia product may also be carried out. In addition, these units incorporate additives such as antioxidants, processing aids, impact modifiers, flame retardants, fillers, such as fiberglass, minerals, or other polymeric materials, to form blends or alloys with the resulting polymer. By forming, it can also be used to prepare formulated products. By introducing a pelletizer, for example a water bath cooler and a shredder, in a very efficient production process, the obtained hernia extrudate can be molded into uniformly shaped pellets. Very preferably, before cooling and pelletizing, the degree of crystallinity of the extruded product is sufficiently increased to form opaque solid pellets having blocking resistance and agglomeration resistance.

Preferably, the feed mixture is maintained at elevated temperature between step (a) and step (b) as well as between step (b) and step (c). Typical product temperatures are 120 to 200 ° C. between step (a) and step (b) and 140 to 200 ° C. between step (b) and step (c). Preferably, the melting and extruding step is carried out continuously immediately after steps (a) and (b). The melt extrusion step is carried out at elevated temperatures up to the melting point of the polymer, preferably up to 10 ° C. or more above the crystalline melting point of the polymer. The maximum extrusion temperature of the polymer melt is preferably below 50 ° C. above the crystal melting point of the polymer. In the case of syndiotactic polystyrene, the final stage of the melt extruder is preferably 240 to 320 ° C. Preferably, vacuum is used during melting and extrusion to further reduce the volatile components of the melt. A pressure of generally 1000 to 5000 Pa, preferably 1000 to 1500 Pa is used during extrusion.

In the case of crystallized opaque pellets prepared using the hernia method of the present invention, the reduction in discoloration of the syndiotactic vinyl aromatic polymer can be measured according to ASTM E313, which measures the Yellowness Index (YIE). Typically, the polymers produced by the process of the present invention have a YIE of less than 10. It is also possible to use ASTM D1925, which compares the yellowing of an almost transparent extruded film of the same thickness using light transmission techniques.

Residual vinyl aromatic monomer content can be determined by headspace gas chromatography using a suitable solvent, such as orthodichlorobenzene, based on a sample having a known composition. The atactic polymer content can be measured by Soxhlet extraction using methyl ethyl ketone, which acts as a solvent for atactic vinyl aromatic polymers and a nonsolvent for crystalline syndiotactic vinyl aromatic homopolymers and copolymers.

Typically, the syndiotactic vinyl aromatic polymers prepared according to the present invention have a weight average molecular weight (M _w ) of at least 15,000, preferably at least 50,000 and most preferably from 150,000 to 500,000. The polymer crystal melting point (T _m ) is preferably above 240 ° C., more preferably above 245 ° C. Further, preferably, the polymer before hernia has a bulk density of 175 kg / m ³ (preferably 200 kg / m ³ ) to 400 kg / m ³ (preferably r350 kg / m ³ ) and an average particle size (dp ₅₀ ) of 75 M (preferably 100 m) to 450 m (preferably 400 m).

The improved hernia process of the present invention results in a reduced content of residual vinyl aromatic monomers and other volatile components (preferably less than 500 ppm, more preferably less than 400 ppm), compared to conventional catalyst deactivation techniques. It provides a syndiotactic vinyl aromatic polymer with reduced color. In addition, the vapors used as catalyst deactivators and degreasers can be condensed and reused to reduce potential emissions to the environment.

The following examples illustrate the invention. These examples are not intended to limit the scope of the invention and should not be construed as such. Unless stated otherwise, amounts refer to parts by weight or weight percent.

Example  One

A feed of syndiotactic polystyrene homopolymer containing 20% volatile components, less than 1% atactic polymer and active metallocene catalyst residues was inertly contacted from the polymerization reactor system at a rate of 1800 kg / hour without contacting air It is fed to a treatment process consisting of two heated rotary dryers equipped with a reverse steam inlet and then an extruder. The first dryer is a model SJS 48-30 dryer (manufacturer: Hosokawa Bepex Corp.) and the second dryer is a model CRJS 60-23 dryer (manufacturer: Hosokawa Bepex Corp.). The extruder is a Berstorff ZE 130A twin screw extruder (L / D = 25) with one vacuum (1500 Pa) outlet. The first dryer is operated at a feed zone jacket temperature of 185 ° C., a discharge zone jacket temperature of 210 ° C. and a rotor speed of 100 rpm. The steam at approximately atmospheric pressure is filtered to remove contaminants, heated to 120 ° C., and then fed to the first dryer at a rate of 800 kg / hour in a direction opposite to the solid flow, so that the steam / volatile mass ratio is 2.22: 1. The powder product discharge temperature is 196 ° C.

Similarly, the second dryer was fed with steam preheated to 210 ° C. at a feed rate of 310 kg / hour, giving a vapor / polymer mass ratio of 0.251. The second dryer is operated at a constant jacket temperature of 210 ° C. The temperature of the powder exiting the dryer is 143 ° C. and is filled directly into the feed port of the extruder. The extruder barrel fixed point temperature is 150 to 275 ° C. and the extruder screw speed is 150 rpm. The hernia extrudate is shaped into strands, cooled in a water bath, and cut into pellets.

The residual styrene content of the obtained polymer pellets was measured by headspace gas chromatography and found to be 350 ppm. The atactic polystyrene content in the polymer is 0.52%. The residual organic decomposition product of the polymerization catalyst is less than 12 ppb, which is detected by gas chromatography-mass spectrometry (GC-MS).

Example  2

Synthetic syndiotactic styrene / p-methylstyrene copolymer comprising 92.3% styrene and 7.7% p-methylstyrene, containing 20% volatile components, less than 1% atactic polymer and active metallocene catalyst residues Feed directly from the reactor system to the treatment process as used in Example 1 at a rate of 1350 kg / hour. The feed zone jacket temperature of the first dryer is maintained at 120 ° C., the discharge zone jacket temperature is maintained at 130 ° C., and the rotor speed is 100 rpm. Steam at approximately atmospheric pressure and 175 ° C. is fed to the first dryer at a rate of 600 kg / hour in a direction opposite to the flow direction such that the steam / volatile ratio is 2.222. The hernia powder discharged from the dryer has a temperature of 129 ° C. and is fed to a second dryer, where the powder is continuously contacted again with the same source of steam at a flow rate of 300 kg / hour to have a mass ratio (vapor: powder) of 0.278. The second dryer is operated at a constant jacket temperature of 175 ° C. The temperature of the hot powder exiting the second dryer is 146 ° C. and is filled with the feed port of the extruder, and then extruded and pelletized with a water-cooled pelletizer. The extruder barrel anchor point is 150 to 260 ° C. The final monomer content of the pellets is 377 ppm. Atactic polystyrene content in the polymer is 1.1%. The residual organic decomposition product of the polymerization catalyst is less than 12 ppb, which is detected by gas chromatography-mass spectrometry (GC-MS).

Description of Cross References

This application claims the benefit of US Provisional Application No. 60 / 443,313, filed January 28, 2003.

Claims (6)

(a) solid particulate syndiotactic vinyl aromatic polymer; Residual vinyl aromatic monomer (s) and optionally residual process solvents; And heating a feed mixture comprising active catalyst residues in the presence of steam in a first contacting device and separating the polymer product, (b) contacting the polymer obtained from step (a) with an inert fluid under devolatilization conditions and separating the fluid and the polymer, and (c) melting, extruding and pelletizing the obtained polymer product, provided that the feed mixture is contacted with air or oxygen after at least step (a) is completed. The method of claim 1 wherein steps (a), (b) and (c) are performed continuously. The method of claim 1 wherein the total residual monomer content of the hernia syndiotactic vinyl aromatic polymer is less than 500 ppm based on the total weight of hernia syndiotactic vinyl aromatic polymer. The method of claim 1 wherein the inert fluid used in step (b) is steam. The method of claim 4 wherein steam having a temperature of 150 to 270 ° C. is used in both steps (a) and (b). 5. The method of claim 1, wherein the polymer and inert fluid of step (b) are contacted with air or oxygen after step (b) is completed. 6.