Process for preparing vinyl aromatic polymers

Abstract

The method comprises the steps of continuously feeding at least one vinyl aromatic monomer and at least one radical initiator to a mixing device to obtain a reaction mixture, feeding the reaction mixture to a continuous stirred tank reactor (CSTR), the continuous stirred tank reactor (CSTR) containing in the liquid phase reaction mixture a polymer fraction of between 45% and 60% by weight, preferably between 50% and 58% by weight, based on the total weight of the liquid phase reaction mixture, and feeding the liquid phase reaction mixture leaving the continuous stirred tank reactor (CSTR) to at least one plug flow reactor (PFR), the at least one plug flow reactor (PFR) containing in the liquid phase reaction mixture leaving the at least one plug flow reactor (PFR) a polymer fraction of at least 65% by weight, preferably between 65% and 58% by weight, based on the total weight of the liquid phase reaction mixture leaving the at least one plug flow reactor (PFR). A continuous bulk polymerization process for the preparation of vinyl aromatic polymers, comprising the steps of: recycling a fraction of the liquid phase reaction mixture leaving said at least one plug flow reactor (PFR) to said mixing device, said fraction being between 25% and 50% by weight, preferably between 25% and 40% by weight, based on the total weight of the liquid phase reaction mixture leaving said at least one plug flow reactor (PFR); feeding the remaining fraction of the liquid phase reaction mixture leaving said at least one plug flow reactor (PFR) to a devolatilization system, optionally feeding the polymer leaving the devolatilization system to an addition system, and feeding the polymer leaving the devolatilization system or leaving the addition system to a pelletization system to recover the polymer. The vinyl aromatic polymers thus obtained can be advantageously used for the production of compact articles of manufacture, foams and expandable beads.

Description

[Technical field]

[0001] The present invention relates to a process for the preparation of vinyl aromatic polymers.

[0002] More specifically, the present invention relates to a continuous bulk polymerization process for the preparation of vinyl aromatic polymers, comprising the steps of continuously feeding at least one vinyl aromatic monomer and at least one radical initiator to a mixing apparatus to obtain a reaction mixture, feeding said reaction mixture to a continuous stirred tank reactor (CSTR), feeding the liquid phase reaction mixture leaving said continuous stirred tank reactor (CSTR) to at least one plug flow reactor (PFR), and recycling a fraction of the liquid phase reaction mixture leaving said at least one plug flow reactor (PFR) to said mixing apparatus.

[0003] The vinyl aromatic polymers thus obtained can be advantageously used for the production of compact articles, foams and expandable beads. [Background technology]

[0004] Continuous radical bulk polymerization of vinyl aromatic monomers, such as styrene, is usually carried out in a plant that includes a feed section where monomer and solvent purification is possible, a reaction section containing one or more reactors connected in series, a product polymer devolatilization section to remove residual monomer and solvent, and a finishing section. Each of these sections consists of one or more pieces of equipment.

[0005] Processes for preparing vinyl aromatic monomers are known in the art.

[0006] For example, US Patent No. 3,903,202 relates to a process for the continuous bulk polymerization of polyalkenyl aromatic polymers containing a dispersed grafted diene rubber phase, wherein said continuous bulk polymerization is carried out in a plant comprising a continuous stirred bed reactor (CSTR) (reactor 1) and a continuous stirred staged isobaric reactor (SIRS) (reactor 2) with continuous evaporation, with a fraction of the polymer being present in the reaction mixture as early as possible in the first reaction stage. The polymer fraction in the reaction mixture in the first reactor (reactor 1) is in the range of 10% to 50%.

[0007] No. 4,777,210 relates to a polymerization process for producing high impact polystyrene (HIPS) in a continuous plant containing two continuous stirred tank reactors (CSTRs) and at least one plug flow reactor (PFR). The polymer fraction in the reaction mixture in the first reactor is less than 20%.

[0008] U.S. Pat. No. 2,769,804 relates to a continuous bulk polymerization process for obtaining polymers having a uniform molecular weight distribution curve (MWD) from monovinylaromatic monomers using a continuous stirred tank reactor (CSTR) or a plug flow reactor (PFR), in which a portion of the reaction mixture leaving the reactor is recycled to said reactor and a portion is devolatilized to separate unreacted mixture from the resulting polymer and recycled as feed to the reactor.

[0009] US Pat. No. 3,821,330 relates to a continuous bulk polymerization process for obtaining acrylic polymers using an adiabatic plug flow reactor (PFR), in which at least 55% of the effluent reaction mixture is recycled to the feed.

[0010] US Patent No. 3,954,722 relates to a continuous polymerization process of unsaturated olefinic monomers in bulk or in solution using a continuous stirred tank reactor (CSTR), in which a part of the reaction mixture in the reactor, the viscosity of which in the feed is more than 100 poise, is recycled to increase the viscosity of the feed mixture, which would be less than 10 poise without recycling, in order to increase the homogeneity of the mixture reaction. A mixture consisting of unsaturated olefinic monomers, solvent, initiator, evaporating condensate of the continuous stirred tank reactor (CSTR), condensate coming from the devolatilization section of the polymer mixture discharged from the reactor, and the recycled reaction mixture of the reactor is prepared in a special mixer that feeds said reactor.

[0011] US Patent No. 4,209,599 relates to a continuous bulk polymerization process for the production of vinyl polymers, in which a monomer mixture in the presence of a polymerization initiator is fed to at least one plug flow reactor (PFR), which has a static mixing element that converts the monomers at an output of at least 75% and recycles 50% to 95% of the outgoing reaction mixture when fed to said plug flow reactor (PFR). The recycle ratio (RR) between the flow rate of the recycled reaction mixture and the flow rate sent to the subsequent devolatilization section thus varies between 1 and 19, and the inlet conversion varies between 37.5% and 71.3%. The plant in which the aforementioned process is carried out can include several plug flow reactors (PFR) in series, each with a recycle.

[0012] US Patent No. 4,948,847 relates to a continuous bulk polymerization process of styrene and an optional vinyl comonomer, comprising at least one recyclable and at least one non-recyclable tubular reactor arranged in series, in which the piston flow in each tubular reactor and the heat exchange with the wall are promoted by static mixing elements. The monomer mixture fed comprises 2%-10% solvent, 98%-90% monomer, and an organic peroxide having a half-life of 10 hours between 75°C and 130°C, and reacts at least 70% in said at least one recyclable tubular reactor. The tubular reactors can be Sulzer type tubular mixers, Kenix type static mixers, Toray type tubular mixers, with or without recycle.

[0013] However, the processes reported above may have some drawbacks. For example, the process using a continuous stirred tank reactor (CSTR) allows the production of uniform polymers with narrow molecular weight distribution curves (MWD) with or without recycle, but does not allow the polymerization initiator to be fully reacted and used efficiently. In fact, it has been reported that a single continuous stirred tank reactor (CSTR) can be used to increase the fraction of polymer in the reaction mixture to more than 60%, improving the reaction rate and weight average molecular weight (M w Long residence times are necessary in the reactor to completely react radical initiators, such as the bifunctional radical initiators optionally provided to increase the molecular weight (MwD). Furthermore, with continuous stirred tank reactors (CSTRs), even when operated under good mixing conditions, it is not possible to obtain broad molecular weight distribution curves (MWD) suitable for applications in the field of foamed foams and granular expandable polymers.

[0014] In the case of a process using a series of continuous stirred tank reactors (CSTR), a broad molecular weight distribution curve (MWD) is obtained and there is a high possibility that the radical initiator, such as the bifunctional radical initiator, fed with the monomer mixture in the first CSTR, can be completely reacted, but if the polymer fraction in the first reactor is lower than 50%, a large amount of cyclic oligomers will be formed, and if the polymer fraction is higher than 70%, the viscosity will become high and difficult to control in case of evaporation in the last CSTR.

[0015] In the case of a process using one or a series of plug flow reactors (PFR), operated with good control of the reaction temperature, there is more flexibility in the production of polymers with the desired molecular weight distribution curve (MWD), the fraction of polymer leaving the series of reactors is high, any initiator such as a bifunctional initiator fed with the monomer mixture in the first plug flow reactor (PFR) is completely consumed, but on the other hand, the production of cyclic oligomers is high due to the high fraction of monomer in said first plug flow reactor (PFR) to which the reaction mixture is fed.

[0016] In the case of a process using one or a series of plug flow reactors (PFRs) where a part of the reaction mixture is recycled from the last PFR to the first, in order to obtain a high fraction of polymer entering the first PFR, i.e., more than 45%, a high recycle rate (RR) of the viscous polymer solution is required due to the high polymer fraction in the recycled reaction mixture.

[0017] Therefore, the applicant set himself the task of finding a process for preparing vinyl aromatic polymers that can overcome the above problems. Specifically, the applicant set himself the task of finding a process for preparing vinyl aromatic polymers that can suppress the formation of oligomers to obtain high productivity, can control the molecular weight distribution curve (MWD), can reduce energy consumption, and can keep equipment costs low.

[0018] The applicant has found a novel continuous bulk polymerization process for preparing vinyl aromatic polymers, comprising the steps of continuously feeding at least one vinyl aromatic monomer and at least one radical initiator into a mixing device to obtain a reaction mixture, feeding the reaction mixture into a continuous stirred tank reactor (CSTR), feeding the liquid phase reaction mixture leaving the continuous stirred tank reactor (CSTR) into at least one plug flow reactor (PFR), and recycling a fraction of the liquid phase reaction mixture leaving the at least one plug flow reactor (PFR) to the mixing device. The process allows high productivity with low oligomer formation, control of the molecular weight distribution curve (MWD), low energy consumption and low equipment costs. The vinyl aromatic polymers thus obtained can be advantageously used for the production of compact articles, foams and expandable beads.

[0019] The subject of the present invention is therefore continuously feeding at least one vinyl aromatic monomer and at least one radical initiator into a mixing device to obtain a reaction mixture; feeding the reaction mixture into a continuous stirred tank reactor (CSTR), the CSTR containing in the liquid phase reaction mixture a polymer fraction between 45% and 60% by weight, preferably between 50% and 58% by weight, based on the total weight of the liquid phase reaction mixture; feeding the liquid phase reaction mixture leaving the continuous stirred tank reactor (CSTR) to at least one plug flow reactor (PFR), the at least one plug flow reactor (PFR) containing in the liquid phase reaction mixture leaving the at least one plug flow reactor (PFR) a polymer fraction of at least 65% by weight, preferably between 70% and 80% by weight, based on the total weight of the liquid phase reaction mixture leaving the at least one plug flow reactor (PFR); recycling a fraction of the liquid phase reaction mixture leaving said at least one plug flow reactor (PFR) to said mixing device, said fraction being between 25% and 50% by weight, preferably between 25% and 40% by weight, relative to the total weight of the liquid phase reaction mixture leaving said at least one plug flow reactor (PFR); feeding the remaining fraction of the liquid phase reaction mixture exiting the at least one plug flow reactor (PFR) to a devolatilization system; Optionally, feeding the polymer exiting the devolatilization system to an addition system; feeding the polymer exiting the devolatilization system or exiting the addition system to a pelletizing system to recover the polymer; A continuous bulk polymerization process for the preparation of vinyl aromatic polymers comprising:

[0020] In this specification and the claims that follow, numerical range definitions are always inclusive unless expressly stated otherwise.

[0021] As used in this specification and the claims that follow, the term "comprising" also includes the terms "consisting essentially of" or "consisting of."

[0022] As used herein and in the claims that follow, the term "continuously stirred tank reactor" (CSTR) refers to a mixed reactor that, at steady state, has constant and equal mass flow rates at the feed and outlet (no accumulation) and in which the temperature and composition of the liquid-phase reaction mixture are substantially the same throughout the liquid reaction volume.

[0023] In this specification and in the claims that follow, the term "plug flow reactor" (PFR), also known as a "continuous tubular reactor", refers to a reactor which, at steady state, has constant and equal mass flow rates at the feed and outlet (no accumulation) and the extent of progression of the liquid phase reaction mixture from reactants to products increases along the axis of the reactor from the inlet to the outlet and is substantially constant radially.

[0024] In this specification and in the claims that follow, the term "liquid phase reaction mixture exiting said at least one plug flow reactor (PFR)" refers to the liquid phase reaction mixture exiting the last of said plug flow reactors (PFR), when there are several plug flow reactors (PFR) in series.

[0025] According to a preferred embodiment of the present invention, the vinyl aromatic monomer is, for example, represented by the general formula (I): [ka] (In the formula, R is a hydrogen atom or a methyl group; n is 0 or 1, Y is a halogen atom such as chlorine, bromine, or hydroxyl, or a halogenated alkyl group having 1-2 carbon atoms such as chloromethyl, bromomethyl, 1-bromoethyl, 1-chloroethyl, or an alkyl or alkoxy group having 1-2 carbon atoms. The vinyl aromatic monomers may be selected from those having the formula:

[0026] According to a preferred embodiment of the present invention, said vinyl aromatic monomer having general formula (I) may be selected, for example, from styrene, α-methylstyrene, isomers of vinyltoluene, isomers of ethylstyrene, isomers of bromostyrene, isomers of chlorine styrene, isomers of methylbromostyrene, isomers of methylchlorostyrene, isomers of 1-bromoethylstyrene, isomers of 1-chloroethylstyrene, isomers of methoxystyrene, isomers of acetoxystyrene, isomers of hydroxystyrene, isomers of methylhydroxystyrene, or mixtures thereof, preferably styrene.

[0027] According to one embodiment of the present invention, at least one comonomer may be fed to said mixing device.

[0028] According to a preferred embodiment of the present invention, said comonomer may be selected from vinyl monomers such as, for example, C4-C8 alkyl esters derived from (meth)acrylic acid, glycidyl (meth)acrylate, or mixtures thereof; divinyl monomers such as isomers of divinylbenzene, esters of (meth)acrylic acid with diols, for example, ethylene glycol-dimethacrylate, butanediol-diacrylate, butanediol-dimethacrylate, hexanediol-diacrylate, hexanediol-dimethacrylate, or mixtures thereof; or mixtures thereof.

[0029] The vinyl aromatic polymers obtained according to the above process are preferably: a polymer comprising 100% by weight of a vinyl aromatic monomer having general formula (I) relative to the total weight of monomers in the reaction mixture, preferably a polymer comprising 100% by weight of styrene relative to the total weight of monomers in the reaction mixture (i.e. polystyrene); Copolymers comprising vinyl aromatic monomers having general formula (I), comprising at least 90% by weight of styrene, based on the total weight of monomers in the reaction mixture, and 10% by weight or less of a vinyl comonomer, such as styrene-alkyl acrylate copolymer, styrene-alkyl methacrylate copolymer, styrene-glycidyl methacrylate copolymer, based on the total weight of monomers in the reaction mixture; Copolymers comprising vinyl aromatic monomers having the general formula (I), comprising at least 90% by weight of styrene, based on the total weight of monomers in the reaction mixture, and not more than 10% by weight of vinyl comonomer, based on the total weight of monomers in the reaction mixture, and less than 500 ppm by mole of divinyl monomer, calculated on all monomers present in the reaction mixture (e.g., for example, styrene-divinylbenzene, styrene-n-butyl acrylate-divinylbenzene, styrene-2-ethylhexyl acrylate-divinylbenzene, styrene-n-butyl methacrylate-divinylbenzene, styrene-2-ethylhexyl methacrylate-divinylbenzene, styrene-n-butyl acrylate-ethylene glycol dimethacrylate, styrene-2-ethylhexyl acrylate-ethylene glycol dimethacrylate, styrene-n-butyl methacrylate-ethylene glycol dimethacrylate, styrene-2-ethylhexyl methacrylate-ethylene glycol dimethacrylate).

[0030] It should be noted that the vinyl aromatic polymer obtained according to the process of the present invention may contain impurities, such as substituted aromatic hydrocarbons, such as ethylbenzene, xylene, cumene, n-propyl-benzene, and / or linear and branched paraffins having 5 or more carbon atoms, in an amount of less than 500 ppm by weight, relative to the total weight of the vinyl aromatic polymer obtained; and / or dimers or trimers, linear or cyclic (waxes), formed by the reaction of vinyl aromatic monomers in parallel with the polymer, in an amount of less than 0.5% by weight, preferably less than 0.25% by weight, relative to the total weight of the vinyl aromatic polymer obtained.

[0031] According to a preferred embodiment of the present invention, the radical initiator may be selected from radical initiators having a half-life of 1 hour as determined by DSC (differential scanning calorimetry) at 105°C to 134°C in a monochlorobenzene solvent, and preferably a bifunctional radical initiator, such as 1,1-di(tert-butylperoxy)-3,3,5-trimethylcyclohexane, 1,1-di(tert-amylperoxy)-cyclohexane, 1,1-di(tert-butylperoxy)-cyclohexane, tert-amylperoxy 2-ethylhexyl carbonate, tert-amylperoxy acetate, tert-butylperoxy-3, 5,5-trimethylhexanoate, 2,2-di-tert-butyl-peroxybutane, tert-butylperoxyisopropylcarbonate, tert-butylperoxy2-ethylhexylcarbonate, tert-amylperoxybenzoate, tert-butylperoxyacetate, butyl-4,4-di(tert-butylperoxy)valerate, tert-butylperoxybenzoate, di-tert-amylperoxide, dicumylperoxide, di(tert-butylperoxyisopropyl)benzene, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, or mixtures thereof. 1,1-di(tert-butylperoxy)-cyclohexane is preferred.

[0032] The above radical initiators can be used individually or mixed together. w In order to obtain polymers with high polymerization activity, bifunctional radical initiators are preferred and are used under conditions of residence time and reaction temperature such that at least 99.5%, preferably 99.9%, more preferably 100% of the feed amount is reacted in the reaction mixture at the outlet from the last plug flow reactor (PFR); among these, 1,1-di(tert-butylperoxy)-cyclohexane is preferred.

[0033] According to a preferred embodiment of the present invention, the radical initiator may be present in the reaction mixture fed to the continuous stirred tank reactor (CSTR) in a concentration of between 0.2 and 2.5 mmoles of peroxide groups -[OO]- per kg of liquid phase reaction mixture, preferably in a concentration of between 0.4 and 2.0 mmoles of peroxide groups -[OO]- per kg of liquid phase reaction mixture, more preferably in a concentration of between 0.6 and 1.8 mmoles of peroxide groups -[OO]- per kg of liquid phase reaction mixture, calculated based on the weight flow rate of the liquid phase reaction mixture entering the devolatilization system.

[0034] It should be noted that the continuous bulk polymerization process for the preparation of the aforementioned vinyl aromatic polymers can be carried out in an open cycle, as reported in the examples (pilot plant) for illustrative, but not limiting purposes of the present invention, or in a closed cycle, in the case of an industrial plant.

[0035] According to one embodiment of the present invention, at least one solvent may be fed to said mixing device in an amount between 0% and 20% by weight, preferably between 2% and 10% by weight, relative to the total weight of the reaction mixture, said solvent being preferably chosen from optionally substituted aromatic hydrocarbons such as, for example, ethylbenzene, xylene, n-propylbenzene, cumene, ethyltoluene, etc.

[0036] The solvent can be fed to the mixing device as it is or through recycling to the mixing device a fraction of the liquid phase reaction mixture leaving the at least one plug flow reactor (PFR). In fact, except for the initial start-up phase of a closed cycle plant (for example in the case of an industrial plant), in the liquid phase reaction mixture fed to the continuous stirred tank reactor (CSTR), non-polymerizable substances are present, which constitute the solvent due to the accumulation of impurities formed by the reaction of vinyl aromatic monomers in parallel with the polymerization reaction, such as substituted aromatic hydrocarbons such as ethylbenzene, xylene, n-propylbenzene, ethyltoluene, cumene, linear and branched paraffins with 5 or more carbon atoms, dimers, trimers, linear and cyclic (waxes). The impurities constitute the solvent by recycling to the mixing device a fraction of the liquid phase reaction mixture leaving the at least one plug flow reactor (PFR).

[0037] The amount of solvent fed to the mixing device is determined based on its purpose of diluting the monomers and its reactivity as a chain transfer agent and regulator, as well as the reaction rate and weight average molecular weight (M) of the vinyl aromatic polymer to be obtained through the process. w It should be noted that the solvent content can be adjusted for both the purpose of reducing the polymerization rate and the molecular weight of the vinyl aromatic polymer. As mentioned above, apart from expensive separation operations, in the solvent recycled to said mixing device by recycling the fraction of the liquid phase reaction mixture leaving the last plug flow reactor (PFR) and, optionally, the fraction of the condensed mixture of the evaporants leaving the devolatilization system, there is also a part of oligomers, such as linear and cyclic dimers and trimers (waxes), originating from the reaction of the vinyl aromatic monomers in parallel with the polymerization reaction. Linear oligomers are particularly important for achieving high polymerization rates and high weight average molecular weights (M) of the vinyl aromatic polymers, since they have an important effect both as chain transfer agents and as regulators of the reaction rate. wIts concentration must be reduced if it is intended to obtain a high molecular weight polymer. The fractions of linear and cyclic dimers and trimers (waxes) present in the final polymer also contribute to a decrease in the glass transition temperature (Tg), and some of the cyclic dimers can decompose to styrene during the subsequent transformation processes of the granules (extrusion and injection molding) to produce compact products.

[0038] In order to obtain a broad molecular weight distribution curve (MWD), the weight average molecular weight (M w When it is intended to lower the .alpha.-methylphenylacetate, chain transfer agents can be used.

[0039] Chain transfer agents with low and medium reactivity towards growing vinyl aromatic radical chains increase the weight average molecular weight (M w ) can be adjusted over a wide monomer conversion range and is preferably fed to a continuous stirred tank reactor (CSTR) as described above, such as by recycling to said mixing device a fraction of the liquid phase reaction mixture leaving the last plug flow reactor (PFR) and, optionally, a fraction of the condensed mixture of evaporants leaving the devolatilization system.

[0040] Chain transfer agents having a high reactivity towards the growing vinyl aromatic radical chains are used, in particular when it is intended to obtain vinyl aromatic polymers with a broad molecular weight distribution curve (MWD), and are preferably fed to a plug flow reactor (PFR) or a series of plug flow reactors (PFRs) operated under conditions of residence time and reaction temperature such that they react in an amount of more than 99.9% relative to the feed amount and are therefore present in an amount of less than 0.1% relative to the feed amount, or even absent, both in the fraction of the liquid phase reaction mixture leaving the last plug flow reactor (PFR) and recycled to the mixing device and in the fraction of the liquid phase reaction mixture fed to the devolatilization system.

[0041] According to one embodiment of the present invention, at least one chain transfer agent may be fed to said continuous stirred tank reactor (CSTR) or said at least one plug flow reactor (PFR).

[0042] According to one embodiment of the present invention, the chain transfer agent may be selected from low reactivity chain transfer agents such as 2,4-diphenyl-4-methyl-1-pentene (α-methylstyrene dimer), polyunsaturated organic substances of the hydrocarbon type, for example, vegetable oils, squalene, farnesene, limonene, terpinolene, or mixtures thereof; medium reactivity chain transfer agents such as tertiary mercaptans having 4 to 12 carbon atoms, for example, tert-butyl mercaptan, tert-dodecyl mercaptan, or mixtures thereof; high reactivity chain transfer agents such as primary mercaptans having 4 to 12 carbon atoms, for example, n-butyl mercaptan, n-dodecyl mercaptan, or mixtures thereof.

[0043] The weight average molecular weight (M w ), the preferred chain transfer agents are tert-dodecyl mercaptan and n-dodecyl mercaptan. tert-dodecyl mercaptan is preferably fed to the continuous stirred tank reactor (CSTR) at a concentration between 0.005 parts by weight and 1 part by weight per 100 parts by weight of the liquid phase reaction mixture, said concentration being calculated based on the weight flow rate of the liquid phase reaction mixture exiting the continuous stirred tank reactor (CSTR); n-Dodecyl mercaptan is preferably fed to the plug flow reactors (PFR) at a concentration between 0.01 parts by weight and 1 part by weight per 100 parts by weight of the liquid phase reaction mixture, said concentration being calculated based on the weight flow rate of the liquid phase reaction mixture exiting the last plug flow reactor (PFR).

[0044] According to a preferred embodiment of the present invention, in the continuous stirred tank reactor (CSTR), the reaction temperature may be between 120°C and 140°C, preferably between 122°C and 135°C.

[0045] According to a preferred embodiment of the present invention, in said at least one plug flow reactor (PFR), the reaction temperature may be between 130°C and 175°C, preferably between 135°C and 170°C.

[0046] The subject of the process of the invention is the control of the weight average molecular weight (M) of the volatiles from the devolatilization system by adjusting the feed to the mixer and the continuous stirred tank reactor (CSTR) and between the continuous stirred tank reactor (CSTR) and the plug flow reactor (PFR) or the series of plug flow reactors (PFRs) containing the solvent from the devolatilization system and optionally by adjusting the feed from the evaporation of the reaction mixture in said two reactors, by adjusting the reaction temperature in the continuous stirred tank reactor (CSTR) and the plug flow reactor (PFR) or the series of plug flow reactors (PFRs), by adjusting the concentration of the radical initiator, by adjusting the concentration of the comonomer optionally present in the feed to the mixer and the continuous stirred tank reactor (CSTR), or by optionally completely reacting said chain transfer agent at the outlet from the last plug flow reactor (PFR) if said chain transfer agent is highly reactive, between the continuous stirred tank reactor (CSTR) and the plug flow reactor (PFR) or in a series of reactors including the plug flow reactor (PFR), w ) and number average molecular weight (M n ) between (M w / M n It is possible to obtain vinyl aromatic polymers with a polydispersity index, given by

[0047] The glass transition temperature (T) of the vinyl aromatic polymer can also be increased by feeding a comonomer that lowers the glass transition temperature, e.g., n-butyl acrylate, 2-ethylhexyl acrylate, or a comonomer that increases the glass transition temperature, e.g., α-methylstyrene, to the mixer and the continuous stirred tank reactor (CSTR). g ) can also be easily adjusted, which can improve processability, in particular for the production of expandable granules, as described, for example, in European patents EP 3056534 and EP 1485429.

[0048] For the purposes of the present invention, a plant for the continuous bulk polymerization of vinyl aromatic polymers can be advantageously used, which plant comprises: at least one mixing device, such as a static mixer or a dynamic mixer, at least one feed line to a continuous stirred tank reactor (CSTR), and at least one line for recycling to said mixing device a fraction of the liquid phase reaction mixture leaving a plug flow reactor (PFR) at the end of the reaction section, and optionally Purification section of the monomer from inhibitors, oxygen and any other unwanted impurities that may be present; A section for thermostating the reaction mixture that feeds the continuous stirred tank reactor (CSTR); Including, Supply Section; a reaction section comprising a continuous stirred tank reactor (CSTR) with temperature control of the reaction mixture, said control being preferably carried out by regulating the feed temperature of said reaction mixture, by evaporation of a portion of said reaction mixture and by regulating the wall temperature of said CSTR due to the presence of a thermostatic jacket, said continuous stirred tank reactor (CSTR) being provided with a suitable stirring system allowing both an adjustable level of the liquid phase and a uniform temperature and compositional homogeneity of the reaction mixture in the liquid phase, and at least one plug flow reactor (PFR) in series, preferably without evaporation and having at least two thermostatic zones, or only one thermostatic zone, if one evaporating plug flow reactor (PFR) is used; a mixing system for optionally adding the chain transfer agent and the condensed monomer mixture exiting as vapor from the devolatilization section, if vaporized, and optionally from said reactor, or a portion thereof, on the feed line to the continuous stirred tank reactor (CSTR), on the inlet lines to the plug flow reactors (PFR) in series, and optionally at the inlet of each plug flow reactor (PFR) following the first reactor; a devolatilization section where the polymer is separated from the solvent and unpolymerized monomers; Finishing section.

[0049] Optionally, at the inlet and / or outlet of said devolatilization section, which can be of any type known in the art based on the chemical-physical properties of the type of additive, there may be a feeding and mixing system for adding suitable and different additives depending on the application of the resulting vinyl aromatic polymer, such as antioxidants, plasticizers, release agents, heat resistant agents, flame retardants, blowing agents, antistatic agents, dyes, stabilizers, etc.

[0050] The aforementioned finishing section can be of any type known in the art suitable for producing granules, expanded sheets and expandable granules.

[0051] The present invention will now be described in more detail through embodiments with reference to FIG. 1 shown below.

[0052] Figure 1 shows an overview of one embodiment of the pilot plant used in the following examples (polymerizations are carried out in an open cycle). In particular, it shows a mixing device (e.g., dynamic mixer) (1) to which one or more monomers (M) (e.g., styrene), a solvent (S) (e.g., ethylbenzene), a polymerization initiator (preferably bifunctional, e.g., 1,1-di-tert-butylperoxycyclohexane at 50% by weight in mineral oil) (I) and any other additives and reagents (designated by dashed arrows in Figure 1), a pump with back pressure valve (2a), a gear pump (2b) and a flow meter (3) for the reaction mixture sent to said pump with back pressure valve (2a) for low-viscosity liquids (no recycle, used in Examples 1, 2, 4 and 5 reported below) and to said gear pump (2b) for viscous liquids (no recycle, used in Examples 3, 6 and 7 reported below), which adjusts the water flow rate depending on the internal temperature of the reaction mixture. The plant includes a flowmeter (3) connected to a jacketed evaporative continuous stirred tank reactor (CSTR) (4) equipped with a thermostatic valve (not shown in Fig. 1) with adjustable temperature and stirred by a double belt agitator for viscous mixtures (not shown in Fig. 1), a discharge nozzle at the bottom (not shown in Fig. 1) connected to a gear pump (4a) that feeds the reaction mixture to a plug flow reactor (PFR) (5), a level meter (not shown in Fig. 1) for the pressure difference, a discharge nozzle at the bottom (not shown in Fig. 1) connected to a gear pump (4a) that feeds the reaction mixture to a plug flow reactor (PFR) (5), and a nozzle on the lid (not shown in Fig. 1) connected to a liquid recovery tank (4c) connected to a condenser (4b) and then to an outlet flowmeter (4d). A sampling valve (4e) installed between the gear pump (4a) and the plug flow reactor (PFR) (5) allows a sample of the liquid phase reaction mixture at the outlet from the continuous stirred tank reactor (CSTR) (4) to determine its composition. A mixer (4f) installed between the CSTR (4) (4) and the plug flow reactor (PFR) (5) after the sampling valve (4e) allows for the supply of liquids (e.g., solvents, chain transfer agents) to the liquid phase reaction mixture leaving the CSTR (4) (dashed line in Figure 1).

[0053] The plug flow reactor (PFR) (5) is divided into three constant temperature zones, has pipes containing circulating oil to regulate the separate reaction temperatures of the three zones (not shown in FIG. 1), has an inlet for the reaction mixture leaving the continuous stirred tank reactor (CSTR) (4) and two outlets, one of which has a gear pump (5a) feeding the mixing device (1) and the other feeding a devolatilization system (6) equipped with a heat exchanger (6a) and a vacuum vessel (6b) to separate the polymer leaving the plug flow reactor (PFR) (5) from the non-polymerized components of the liquid phase reaction mixture. A sampling valve (5b) installed between the plug flow reactor (PFR) (5) and the heat exchanger (6a) allows a sample of the liquid phase reaction mixture to be taken at the outlet from the plug flow reactor (PFR) (5) to determine its composition. The resulting polymer leaves the bottom of the vacuum vessel (6b) and is sent to the granulation system (7) by a gear pump (6c). The solvent vapor and unreacted monomer present in the vacuum vessel (6b) are sent to a condenser (6d), which is connected to a liquid recovery tank (6e) and a sampling valve (6f), from which a liquid sample is taken and its composition is determined. EXAMPLES

[0054] In order to better understand the present invention and to practice it, some illustrative and non-limiting examples of the present invention are given below.

[0055] Examples 1 to 5 (Comparative Examples) and Examples 6 to 7 (Inventive Examples) An example was carried out in a pilot open cycle plant according to Figure 1.

[0056] For this purpose, a 1000 liter jacketed dynamic mixer (1) equipped with agitator, temperature control and level control (not shown in FIG. 1) was continuously fed at 70° C. with styrene (99.8% purity - Versalis) and ethylbenzene (Versalis) in a mass ratio of 94 / 6, 1,1-di(tert-butylperoxy)-cyclohexane (Akzo Nobel) at 50% by mass in mineral oil (Examples 4 to 7, in the amounts shown in Table 1), and also with a portion of the liquid phase reaction mixture leaving the plug flow reactor (PFR) (5) (Examples 3, 6 to 7 in Table 1).

[0057] The resulting reaction mixture is pumped at 100% level (224 kg of reaction mixture in the liquid phase, equal to a filling rate of 75%) through a pump (2a) with a back pressure valve for low viscosity liquids (used in examples 1, 2, 4 and 5 without recycle as shown in Table 1) or a pump with gears (2b) for viscous liquids (used in examples 3, 6 and 7 with recycle as shown in Table 1) and a flow meter (3) to a 300 liter jacketed evaporative continuous stirred tank reactor (CSTR) (4) with a thermostatic valve (not shown in Figure 1) for the temperature of the circulating water in the jacket, which is adjustable based on the internal temperature of the reaction mixture. A nozzle bottom drain (not shown in Fig. 1) connected to a gear pump (4a) that supplies the reaction mixture to a vertical stirred 120 liter plug flow reactor (PFR) (5) equipped with a pressure gauge (4b) and a double belt agitator (not shown in Fig. 1) for viscous mixtures, a level meter for pressure difference (not shown in Fig. 1), a pipe containing circulating oil for adjusting different reaction temperatures in the three zones, and a nozzle on the lid (not shown in Fig. 1) connected to a liquid recovery tank (4c) connected to a condenser (4b) and then to an outlet flow meter (4d). A sampling valve (4e) installed between the gear pump (4a) and the plug flow reactor (PFR) (5) allowed a sample of the liquid phase reaction mixture to be taken at the outlet from the continuous stirred tank reactor (CSTR) (4) to determine its composition.

[0058] In Examples 3 and 6-7, a fraction (33.3%) of the liquid phase reaction mixture leaving the plug flow reactor (PFR) (5) was recycled via the pump gear (5a) to the dynamic mixer (1) operated at the recycle ratio (RR) shown in Table 1.

[0059] The remaining fraction of the reaction mixture leaving the plug flow reactor (PFR) (5) was fed to a devolatilization system (6) where it was heated to a temperature of 250° C. in a heat exchanger (6a) and maintained at a residual pressure of 11 mmHg in a vacuum vessel (6b) to remove the solvent and unreacted monomers.

[0060] The polymer leaving the devolatilization system was sent to the granulator (7) at a nearly constant flow rate (equal to about 60.7 kg / h) for all examples shown in Table 1.

[0061] The weight average molecular weight (Mw) of the polymer present in the reaction mixture withdrawn from the sampling valve (5b) was measured using a GPC (gel permeation chromatography) apparatus consisting of: Waters Alliance E2695 Pump-Injector Module with Degasser; Waters oven equipped with a precolumn and four Phenogel (Phenomenex) columns, dimensions 300 × 7.8 mm, particle size 5μ, porosity 106 Å, 105 Å, 104 Å, 103 Å; RI Waters 410 refractive index detector.

[0062] The operating conditions used were as follows: Solvent: tetrahydrofuran (THF) (Merck); Column temperature: 30°C; Flow: 1ml / min; Internal standard: toluene; Injection volume: 200 microliters.

[0063] The (polydisperse) samples were injected at a concentration of 1 mg / ml.

[0064] A universal calibration curve was constructed by injecting 20 monodisperse polystyrene standards with molecular weights (Mp) ranging from 2170 Da to 4340000 Da and recording the intrinsic viscosity and elution volume for each molecular weight.

[0065] Data acquisition and processing were performed using Empower2 software (Waters).

[0066] Table 1 shows the mass average molecular weight (Mw) of the obtained polymer.

[0067] The amount of polymer in the reaction mixture withdrawn from the sampling valve (4e) and the sampling valve (5b) was determined gravimetrically by dissolving 0.5 g of the reaction mixture (weighed on a balance to the nearest thousandth of a gram) in 20 ml of chloroform (Carlo Erba) and precipitating the resulting solution dropwise into a glass containing 150 ml of ethanol (Carlo Erba) under stirring. Finally, the whole was left to stand until the polymer had settled to the bottom and the liquid was clear. The polymer was then recovered by filtration through a VitraPOR™ 20304 (Robu Glass Filter) filter crucible filter and dried to constant weight in an oven at 110° C. under vacuum.

[0068] The amount of linear and cyclic dimers and trimers of styrene (wax) present in the reaction mixture withdrawn through the sampling valve (5b) was determined by gas chromatography by the following procedure: a gas chromatograph (Trace 1300) equipped with an “on-column” injector, autosampler (Triplus) and electronic carrier flow control; a chromatography column with a methylsilicone phase (Agilent HP1) of 25 m length, 0.52 mm stationary phase thickness, and 0.32 mm diameter; Flame ionization detector (FID); Chromatography acquisition software. The gas chromatograph was set as follows: Carrier: Hydrogen; Flow ramp: 2ml / min for 1 min, increasing by 0.2ml / min to 4.2ml / min, then constant at 4.2ml / min until end of stroke; Detector temperature: 330℃; Temperature program: 60°C to 160°C at 40°C / min, isothermal at 160°C for 5 min, ramp to 325°C at 8°C / min, isothermal for 5 min.

[0069] The polymer samples to be analyzed were prepared by dissolving 0.5 g of sample in 3 ml of dichloromethane (Merck) containing 50 ppm of n-hexadecane (Merck) as an internal standard, followed by precipitation of the polymer with 8 ml of ethanol (Carlo Erba): 1 ml of the liquid obtained from said precipitation was injected into the gas chromatograph as described above. A response factor of 1 was assigned to all oligomers: the values ​​obtained are shown in Table 1.

[0070] Table 1 also shows the reaction conditions used.

[0071] [Table 1]

[0072] Q in CSTR: total flow rate (kg / h) of the reaction mixture fed to the CSTR (4) determined by the flow meter (3); Q c CSTR: At steady state, the flow rate (kg / h) of condensed vapours leaving the CSTR (4) determined by measuring the hourly level difference in the liquid recovery tank (4c) and the density of the liquid in a sample taken from (4d); F. init.: Mass fraction of the radical initiator [50% 1,1-di(tert-butylperoxy)-cyclohexane in mineral oil] [g / (g×10 6 )]; T CSTR: temperature of the reaction mixture and the CSTR jacket (4) (°C); F. Polout CSTR: The fraction (%) of polymer in the liquid phase reaction mixture leaving the CSTR (4) as determined by gravimetric analysis of reaction mixture samples taken from the sampling valve (4e); T PFR: the temperature rise range of the liquid phase reaction mixture in the three zones of the PFR(5) (℃); Q out PFR: The total flow rate (kg / h) of the liquid phase reaction mixture leaving the PFR (5). It is equal to the flow rate entering the PFR (5) and the flow rate of the liquid phase leaving the CSTR (4), and is calculated based on the flow rate measured by the flowmeter flow (3) minus the flow rate of the condensed vapor in the liquid recovery tank (4c); F. Pol out PFR: The fraction (%) of polymer in the liquid phase reaction mixture leaving the PFR (5). It is determined by gravimetric analysis of the reaction mixture sample taken through the sampling valve (5b) and is calculated based on the amount of polymer leaving the granulator (7), the flow rate of condensed vapors measured by the hourly level difference in the liquid recovery tank (6e) and the density of the liquid in the sample taken from the sampling valve (6f); RR: the recycle ratio between the flow rate of the liquid phase reaction mixture leaving the PFR (5) and recycled to the dynamic mixer (1) by the gear pump (5a) and the flow rate of the reaction mixture leaving the PFR and sent to the devolatilization system (6). The ratio of the rotation speeds of the gear pumps (4a) and (5a) and the flow rate Q out Calculated based on the PFR and the flow rate of granules exiting the granulator (7); Q wax : the flow rate (g / h) of linear and cyclic dimers and trimers (wax) formed in the reaction and measured in the liquid phase reaction mixture sampled by the sampling valve (5b) feeding the devolatilization system (6); Q wax / Q pol : the specific amount of linear and cyclic dimers and trimers (wax) formed in the reaction per kg of polymer fed to the devolatilization system (6) (g / kg); M w: Weight average molecular weight (kDa) of the polymer leaving the PFR (5) as determined from a sample taken from the sampling valve (5b).

[0073] From the data presented in Table 1, the following can be inferred: In Example 1 (comparative example), where no radical initiator was fed and no recycle from the PFR (5) to the dynamic mixer (1) was performed, a high wax production (655 g / h, Q wax / Q pol ratio of 10.8 g / kg) and a weight-average molecular weight value (M w ) was shown; In Example 2 (Comparative Example), in which no radical initiator was fed as in Example 1 (Comparative Example), no recycle from the PFR (5) to the dynamic mixer (1) was performed, the flow rate to the CSTR (4) was lowered, and the reaction temperatures in the CSTR (4) and the PFR (5) were increased, the fraction of polymer leaving the CSTR (4) and the PFR (5) increased, and the wax produced was slightly reduced (643 g / h, Q wax / Q pol ratio is 10.6g / kg), weight average molecular weight (M w ) was reduced to 252 kDa; In Example 3 (Comparative Example), where no radical initiator was fed as in Example 1 (Comparative Example), but a recycle equal to 33.3% of the reaction mixture leaving the PFR (5) was inserted, a slight reduction in the wax produced (617 g / h, Q wax / Q pol 10.2g / kg), weight average molecular weight (M w ) was further reduced to 236 kDa; In Example 4 (Comparative) and Example 5 (Comparative), it was possible to reduce the reaction temperature, especially in the CSTR (4), at the same polymer production rate by not recycling from the PFR (5) to the dynamic mixer (1) but rather by increasing the amount of radical initiator fed: in Example 5 (Comparative), the wax production was halved (329 g / h, Q wax / Q pol Ratio is 5.4g / kg), weight average molecular weight (Mw ) increased to 288 kDa; In Example 6 (Example), a recycle equal to 33.3% of the reaction mixture leaving the PFR (5) was inserted in the feed to the dynamic mixer (1) (RR=0.5), so that it was possible to maintain the flow rate of the product polymer by lowering the reaction temperature, especially in the CSTR (4), and to reduce the production of wax (235 g / h, Q wax / Q pol ratio of 3.9 g / kg) and weight average molecular weight (M w ) to 291 kDa was observed; In Example 7 (Example), where the recycle ratio (RR) equal to 0.5 was kept as in Example 6 (Example), but the flow rate of the radical initiator was increased, the wax production was further reduced (182 g / h, Q wax / Q pol Ratio is 3.0g / kg), weight average molecular weight (M w ) further increased to 300 kDa.

Claims

1. continuously feeding at least one vinyl aromatic monomer and at least one radical initiator into a mixing device to obtain a reaction mixture; feeding the reaction mixture into a continuous stirred tank reactor (CSTR), the CSTR containing in the liquid phase reaction mixture a polymer fraction between 45% and 60% by weight, preferably between 50% and 58% by weight, based on the total weight of the liquid phase reaction mixture; feeding the liquid phase reaction mixture leaving said continuous stirred tank reactor (CSTR) to at least one plug flow reactor (PFR), said at least one plug flow reactor (PFR) containing in the liquid phase reaction mixture leaving said at least one plug flow reactor (PFR) a polymer fraction of at least 65% by weight, preferably between 70% and 80% by weight, based on the total weight of the liquid phase reaction mixture leaving said at least one plug flow reactor (PFR); recycling a fraction of the liquid phase reaction mixture leaving said at least one plug flow reactor (PFR) to said mixing device, said fraction being between 25% and 50% by weight, preferably between 25% and 40% by weight, relative to the total weight of the liquid phase reaction mixture leaving said at least one plug flow reactor (PFR); feeding the remaining fraction of the liquid phase reaction mixture exiting the at least one plug flow reactor (PFR) to a devolatilization system; Optionally, feeding the polymer exiting the devolatilization system to an addition system; feeding the polymer exiting the devolatilization system or exiting the addition system to a pelletizing system to recover the polymer; 1. A continuous bulk polymerization process for the preparation of vinyl aromatic polymers comprising:

2. The vinyl aromatic monomer has the general formula (I): 【Chemistry 1】 (In the formula, R is a hydrogen atom or a methyl group; n is 0 or 1; Y is a halogen atom such as chlorine, bromine, or hydroxyl, or a halogenated alkyl group having 1 to 2 carbon atoms such as chloromethyl, bromomethyl, 1-bromoethyl, 1-chloroethyl, or an alkyl or alkoxy group having 1 to 2 carbon atoms.

2. A continuous bulk polymerization process for the preparation of the vinyl aromatic polymer of claim 1, wherein the vinyl aromatic monomer is selected from those having the formula:

3. 2. The continuous bulk polymerization process for the preparation of vinyl aromatic polymers according to claim 1, wherein said vinyl aromatic monomer having general formula (I) is selected from styrene, α-methylstyrene, isomers of vinyltoluene, isomers of ethylstyrene, isomers of bromostyrene, isomers of chlorine styrene, isomers of methylbromostyrene, isomers of methylchlorostyrene, isomers of 1-bromoethylstyrene, isomers of 1-chloroethylstyrene, isomers of methoxystyrene, isomers of acetoxystyrene, isomers of hydroxystyrene, isomers of methylhydroxystyrene, or mixtures thereof, preferably styrene.

4. 10. The continuous bulk polymerization process for the preparation of vinyl aromatic polymers according to claim 1, wherein at least one comonomer is fed to said mixing device.

5. The comonomer may be a C 16 copolymer derived from (meth)acrylic acid, glycidyl (meth)acrylate, or a mixture thereof. ４ -C ８ 5. A continuous bulk polymerization process for the preparation of vinyl aromatic polymers according to claim 4, wherein the divinyl monomers are selected from vinyl monomers such as alkyl esters; divinyl monomers such as isomers of divinylbenzene, esters of (meth)acrylic acid with diols, e.g. ethylene glycol-dimethacrylate, butanediol-diacrylate, butanediol-dimethacrylate, hexanediol-diacrylate, hexanediol-dimethacrylate, or mixtures thereof; or mixtures thereof.

6. The radical initiator is selected from radical initiators having a half-life of 1 hour as determined by DSC (differential scanning calorimetry) in monochlorobenzene solvent between 105° C. and 134° C., preferably a bifunctional radical initiator, such as 1,1-di(tert-butylperoxy)-3,3,5-trimethylcyclohexane, 1,1-di(tert-amylperoxy)-cyclohexane, 1,1-di(tert-butylperoxy)-cyclohexane, tert-amylperoxy 2-ethylhexyl carbonate, tert-amylperoxy acetate, tert-butyl-peroxy-3,5,5-trimethylhexanoate, 2,2-di-tert-butyl-peroxybutane, tert-butylperoxy-2-ethylhexylcarbonate ...

2. A continuous bulk polymerization process for the preparation of vinyl aromatic polymers according to claim 1, wherein the peroxy group is selected from the group consisting of butyl peroxyisopropyl carbonate, tert-butylperoxy 2-ethylhexyl carbonate, tert-amyl peroxybenzoate, tert-butylperoxyacetate, butyl-4,4-di(tert-butylperoxy)valerate, tert-butylperoxybenzoate, di-tert-amyl peroxide, dicumyl peroxide, di(tert-butylperoxy-isopropyl)benzene, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, or mixtures thereof, preferably 1,1-di(tert-butyl-peroxy)-cyclohexane.

7. 2. The continuous bulk polymerization process for the preparation of vinyl aromatic polymers according to claim 1, wherein the radical initiator is present in the reaction mixture fed to the continuous stirred tank reactor (CSTR) at a concentration of between 0.2 and 2.5 mmoles of peroxide groups -[OO]- per kg of liquid phase reaction mixture, preferably at a concentration of between 0.4 and 2.0 mmoles of peroxide groups -[OO]- per kg of liquid phase reaction mixture, more preferably at a concentration of between 0.6 and 1.8 mmoles of peroxide groups -[OO]- per kg of liquid phase reaction mixture, calculated based on the weight flow rate of the liquid phase reaction mixture entering the devolatilization system.

8. 2. The continuous bulk polymerization process for the preparation of vinyl aromatic polymers according to claim 1, wherein at least one solvent is fed to said mixing device in an amount between 0% and 20% by weight, preferably between 2% and 10% by weight, relative to the total weight of the reaction mixture, said solvent being preferably selected from optionally substituted aromatic hydrocarbons such as ethylbenzene, xylene, n-propylbenzene, cumene, ethyltoluene, etc.

9. 2. The continuous bulk polymerization process for the preparation of vinyl aromatic polymers according to claim 1, wherein at least one chain transfer is fed to said continuous stirred tank reactor (CSTR) or said at least one plug flow reactor (PFR).

10. 10. The continuous bulk polymerization process for the preparation of vinyl aromatic polymers according to claim 9, wherein the chain transfer agent is selected from low reactivity chain transfer agents such as 2,4-diphenyl-4-methyl-1-pentene (α-methylstyrene dimer), polyunsaturated organic substances of the hydrocarbon type, for example vegetable oils, squalene, farnesene, limonene, terpinolene, or mixtures thereof; medium reactivity chain transfer agents such as tertiary mercaptans having from 4 to 12 carbon atoms, for example tert-butyl mercaptan, tert-dodecyl mercaptan, or mixtures thereof; high reactivity chain transfer agents such as primary mercaptans having from 4 to 12 carbon atoms, for example n-butyl mercaptan, n-dodecyl mercaptan, or mixtures thereof.

11. 2. A continuous bulk polymerization process for the preparation of vinyl aromatic polymers according to claim 1, wherein in said continuous stirred tank reactor (CSTR) the reaction temperature is between 120°C and 140°C, preferably between 122°C and 135°C.

12. 12. A continuous bulk polymerization process for the preparation of vinyl aromatic polymers according to any of claims 1 to 11, wherein in said at least one plug flow reactor (PFR), the reaction temperature is between 130°C and 175°C, preferably between 135°C and 170°C.

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