WO2013135701A1

WO2013135701A1 - Synthesis of polyphenol disulphides

Info

Publication number: WO2013135701A1
Application number: PCT/EP2013/055006
Authority: WO
Inventors: Christoph Fleckenstein; Hartmut Denecke; Sabine Fuchs; Matthias Mueller; Manfred Doering; Michael Cisielski; Jochen Wagner; Peter Deglmann; Sameer Nalawade; Peter Gutmann; Klaus Hahn; Klemens Massonne; Andreas Kleinke; Roland Helmut Kraemer
Original assignee: Basf Se
Priority date: 2012-03-13
Filing date: 2013-03-12
Publication date: 2013-09-19

Abstract

The invention relates to a process for producing a polyphenol disulphide of formula (I) by reacting one or more mercaptophenols of formula (II) with one or more oxidizing agents, optionally in the presence of one or more bases, where the symbols R¹, R², R³, R⁴ and n have the meanings given in the description.

Description

Synthesis of Polyphenol Disulfides The invention relates to a process for the preparation of polyphenol disulfides. Further, the invention relates to polyphenol disulfides obtainable by the process, a flame retardant system containing the polyphenol disulfide and a halogen-free organic phosphorus compound, and a polymer composition containing the flame retardant system. The equipment of polymers, especially foams, with flame retardants is important for a variety of applications, such as polystyrene foam expandable polystyrene foam (EPS) or expanded polystyrene foam boards (XPS) for building insulation. Currently, as flame retardants in plastics mainly polyhalogenated hydrocarbons, optionally together with suitable synergists used. A typical representative of these classic flame retardants is hexabromocyclododecane. Due to the bioaccumulation and persistence of some polyhalogenated hydrocarbons, it is a major effort in the plastics industry to replace halogenated flame retardants with halogen-free ones.

A promising alternative to halogen-containing systems is offered by halogen-free phosphorus-containing flame retardants in combination with halogen-free sulfur-containing synergists. The use of polyphenol disulfides as synergists to halogen-free phosphorus-containing

Flame retardants are already described in AT 508 304 and WO 201 1/000019 and in the non-prepublished international patent application PCT / EP201 1/073958.

Poly (tert-butylphenol disulfide) and poly (tert-pentylphenol disulfide) are important representatives of this class of compounds and can be obtained commercially, for example, as VULTAC® TB7 or VULTAC® 2 and 3, respectively. According to US 3,968,062 it is assumed that these are complex mixtures of polysulfides.

According to US Pat. No. 3,968,062, poly (tert-butylphenol disulfide) is prepared by heating para-tert-butylphenol with sulfur monochloride using trichlorethylene as solvent, the temperature of the heating mantle being between 15 and 155 ° C. The use of a low-boiling solvent such as trichlorethylene is recommended according to the above-mentioned US patent application to sublimated para-tert. Butylphenol attributed to the reaction vessel.

Although the polyphenol disulfides of the prior art are efficient synergists to halogen-free phosphorus-containing flame retardants, there is still room for improvement. conditions, in particular as regards their production and their performance properties.

A disadvantage of commercially available polysulfide mixtures is their unsatisfactory thermal stability for incorporation into thermoplastic polymers, eg. As polystyrene, and the proportion of low molecular weight, volatile components contained therein. This is associated with a partially occurring during their use in polymers at elevated temperature odor development, eg. B. in the extrusion or cutting of foam blocks.

The object of the invention is to provide a process for the preparation of polyphenol disulfides which exhibit good thermal stability and a low proportion of low molecular weight, readily volatile components and, during their use in polymers, no odor at elevated temperature.

It has been found that the reaction of mercaptophenols with an oxidizing agent, optionally in the presence of a base, leads to the formation of polyphenol disulfides, which have a higher proportion of dimeric and polymeric phenol disulfides compared to polyphenol disulfides of the prior art and are distinguished by improved performance properties ,

The object is therefore achieved by a process for the preparation of a polyphenol disulfide of the formula (I)

by reacting one or more mercaptophenols of the formula (II)

with one or more oxidizing agents, optionally in the presence of one or more bases, where the symbols R ¹ , R ² , R ³ , R ⁴ and n have the following meanings: R ¹ , R ² , R ³ are identical or different C ₁ -C 6 -alkyl, C ₂ -C 1 ₈ alkenyl, C ₂ -C ₈ alkynyl, C ₆ -C ₂ aryl, C3-Cio-cycloalkyl, C6-Ci2-aryl-Ci-CI8-alkyl, a heteroaryl, the N one or more hetero atoms from the group , O and S, 0- (C ₁ -C ₈ ) -alkyl, 0- (C ₂ -C ₈ ) -alkenyl, O- (C ₂ -C ₈ ) -alkynyl, O- (C ₆ -C ₂ ) aryl, 0- (C ₃ -Cio) cycloalkyl, _{_(C6-Ci2)} aryl (Ci-C ₈₎ -alkyl-0, S- (Ci-cis) alkyl, S- (C ₂ -C ₈₎ -alkenyl, S- (C ₂ -C ₈₎ -alkynyl, S- (C ₆ -C ₂₎ aryl, S- (C ₃ -Cio) - cycloalkyl, (C ₆ -C ₂₎ - Aryl- (Ci-Ci ₈ ) -alkyl-S, OH, F, Cl, Br or H;

R ⁴ is identical or different SH, Ci-C ₈ alkyl, C ₂ -C ₈ alkenyl, C ₂ -C ₈ alkynyl, C ₆ -C ₂ aryl, C3-Cio-cycloalkyl, C6-Ci ₂ - aryl-Ci-C ₈ alkyl, a heteroaryl group which contains one or more heteroatoms from the group N, O and S, 0- (Ci-C ₈₎ -alkyl, 0- (C ₂ -C ₈₎ alkenyl, O- (C ₂ -C ₈₎ -alkynyl, 0- _{_(C6-Ci2)} aryl, 0- (C ₃ -Cio) cycloalkyl, _{_(C6-Ci2)} aryl (Ci-Ci ₈ ) -Alkyl-O, S-

(Ci-Cis) alkyl, S- (C ₂ -C ₈₎ -alkenyl, S- (C ₂ -C ₈₎ -alkynyl, S- (C ₆ -C ₂₎ aryl, S- (C ₃ - Cio) - cycloalkyl, _{_(C6-Ci2)} aryl (Ci-C ₈₎ -alkyl-S, OH, F, Cl, Br or H; n is an integer from 0 to 1000, preferably an integer from 1 to 1000; with the proviso that R ⁴ in at least one of the one or more mercaptophenols of formula (II) SH is when n is greater than zero.

Polyphenol disulfide prepared in accordance with the present invention has a higher level of dimeric and polymeric phenolic disulfides, a lower level of monosulfide units, a lower level of polysulfide units, and a lower level of elemental sulfur compared to prior art polyphenol disulfides. The polyphenol disulfides prepared by the process according to the invention are characterized by a satisfactory thermal stability. Associated with this is also during their use in polymers at elevated temperature barely or not at all occurring odor development, eg. B. in the extrusion or cutting of foam blocks. In addition, the inventive method is preferably carried out at relatively low temperatures, which is advantageous from an economic and environmental point of view. Among other things, this also counteracts the sublimation of unreacted phenol.

In the context of the invention, the term "polyphenol disulfide of the formula (I)" comprises dimeric phenol disulfides and polymeric phenol disulfides.

A dimeric phenol disulfide according to the invention or a diphenol disulfide according to the invention is understood as meaning a compound of the formula (I) in which n is 0.

A polymeric phenol disulfide according to the invention is understood as meaning a compound of the formula (I) in which n is not 0 and where not more than 5%, preferably .about.3%, are particularly preferred. At least <1%, very particularly preferably <0.5% of the (n + 1) disulfide bridges contained in the formula (I) are replaced by a mono- or oligosulfide bridge. In the context of the invention, an oligosulfide bridge is understood as meaning a linear chain consisting of 3 or more, in particular 3 to 8, sulfur atoms.

Examples of polymeric phenol disulfides according to the invention are the compounds listed in the examples poly (tert-butylphenol disulfide) and poly (tert-pentylphenoldisulfid).

The symbols in the formula (I) preferably have the following meanings:

R ² is preferably identical or different C 1 -C 6 -alkyl, C 2 -C 18 -alkenyl, C 2 -C 18 -alkynyl, C 6 -C 12 -aryl, C 1 -C 10 -cycloalkyl, O- (C 1 -C ₈ ) -alkyl, O- (C ₂ -C ₈₎ alkenyl, 0- (C ₂ -C ₈₎ -alkynyl, 0- _(C6 - _Ci2) aryl, or 0- (C ₃ -Cio) cycloalkyl. R ¹ , R ³ are preferably identical or different H, Ci-Cis-alkyl, C2-Ci8-alkenyl, C2-C18-

Alkynyl, C ₆ -C ₂ aryl, Cs-Cio-cycloalkyl, 0- (Ci-C ₈₎ -alkyl, 0- (C ₂ -C ₈₎ alkenyl, 0- (C ₂ - C ₈₎ alkynyl , 0- _{_(C6-Ci2)} aryl, or 0- (C ₃ -Cio) cycloalkyl.

R ⁴ is preferably SH or H. n is preferably an integer from 0 to 500, preferably an integer from 1 to 500.

Preferred are polyphenol disulfides of the formula (I) in which all symbols have the preferred meanings.

The symbols in the formula (I) particularly preferably have the following meanings:

R ² is particularly preferably identical or different Ci-Ci6-alkyl or C6-Ci2-aryl. R ¹ , R ³ are particularly preferably identical or different H, Ci-Cis-alkyl, C3-Cio-cycloalkyl

R ⁴ is preferably SH or H. n is more preferably an integer from 0 to 250, preferably an integer from 1 to 250.

Particular preference is given to polyphenol disulfides of the formula (I) in which all symbols have the particularly preferred meanings.

Most preferably, the symbols in the formula (I) have the following meanings:

R ² is very particularly preferably identical or different C 1 -C 10 -alkyl or C 6 -C 12 -aryl. R ¹ , R ³ are most preferably H.

R ⁴ is very particularly preferably SH or H. n is very particularly preferably an integer from 0 to 150, preferably an integer from 1 to 150.

Very particular preference is given to polyphenol disulfides of the formula (I) in which all symbols have the very particularly preferred meanings.

Very particular preference is furthermore given to the compounds of the formula (I) listed in the Examples, poly (tert-butylphenol disulfide) and poly (tert-pentylphenol disulfide). Very particular preference is also given to the compounds di (tert-butylphenol) disulphide, di (tert-pentylphenol) disulphide, di (para-phenylphenol) disulphide and poly (para-phenylphenol disulphide):

Particularly preferably, the symbols in the formula (I) have the following meanings: R ² is particularly preferably identical or different t-C4Hg or t-CsHu.

R ¹ , R ³ are particularly preferably H. R ⁴ is particularly preferably SH or H. n is more preferably an integer from 3 to 100. Particular preference is given to polyphenol disulfides of the formula (I) in which all symbols have the particularly preferred meanings.

Also particularly preferred are the compounds of the formula (I), poly (tert-butylphenol disulfide) and poly (tert-pentylphenol disulfide) listed in the examples.

Another example of a compound of formula (I) is di (ortho-phenylphenol) disulphide:

According to one embodiment, the polyphenol disulfide of the formula (I) is di (tert-pentylphenol) disulphide, di (para-phenylphenol) disulphide or di (ortho-phenylphenol) disulphide:

In another embodiment, the polyphenol disulfide of formula (I) is di- (tert-pentylphenol) disulphide or di (para-phenylphenol) disulphide. According to a further embodiment, the polyphenol disulfide of the formula (I) is di- (orthophenylphenol) disulphide.

One or more, preferably one to three, more preferably one or two mercaptophenols of the formula (II) are used in the process according to the invention.

Preference is given to mercaptophenols of the formula (II) which lead to the formation of the preferred polyphenol disulfides of the formula (I). Particular preference is given to mercaptophenols of the formula (II) which lead to the formation of the particularly preferred polyphenol disulfides of the formula (I).

Very particular preference is given to mercaptophenols of the formula (II) which lead to the formation of the very particularly preferred polyphenol disulfides of the formula (I). Examples of very particularly preferred mercaptophenols of the formula (II) are 4- (tert-butyl) -2-mercaptophenol, 4- (tert-butyl) -2,6-dimercaptophenol, 4- (tert-pentyl) -2- mercaptophenol, 4- (tert-pentyl) -2,6-dimercaptophenol, 4- (phenyl) -2-mercaptophenol and 4- (phenyl) -2,6-dimercaptophenol.

Particular preference is given to mercaptophenols of the formula (II) which lead to the formation of the more particularly preferred polyphenol disulfides of the formula (I). These are the compounds 4- (tert-butyl) -2-mercaptophenol, 4- (tert-butyl) -2,6-dimercaptophenol, 4- (tert-pentyl) -2-mercaptophenol and 4- (tert-butyl) -2-mercaptophenol. tert-pentyl) -2,6-dimercaptophenol.

Another example of a mercaptophenol of formula (II) is 6- (phenyl) -2-mercaptophenol.

According to one embodiment, 4- (tert-pentyl) -2-mercaptophenol, 4- (phenyl) -2-mercaptophenol or 6- (phenyl) -2-mercaptophenol is used as mercaptophenol of the formula (II).

According to a further embodiment, 4- (tert-pentyl) -2-mercaptophenol or 4- (phenyl) -2-mercaptophenol is used as mercaptophenol of the formula (II).

In a particular embodiment, two mercaptophenols of the formula (II) are used, wherein R ^{4 is} SH in one of these two mercaptophenols and H in the other. In a very particular embodiment, two mercaptophenols of the formula (II) are used, wherein R ^{4 is} SH in one of these two Mercaptophenole and in the other H, and wherein the symbols R ¹ , R ² , R ³ and n in these two Mercaptophenolen the have the same meaning. If a mercaptophenol of the formula (II) with R ⁴ = SH and a mercaptophenol of the formula (II) where R ⁴ = H are used, the molar ratio of the mercaptophenol with R ⁴ = H to the mercaptophenol with R ⁴ = SH is preferably 1: 1 to 1: 10,000, more preferably 1: 1 to 1: 1000, most preferably 1: 1 to 1: 100. One or more, preferably one to three, more preferably one or two, oxidizing agents are used in the process according to the invention. Very particular preference is given to the use of one (1) oxidizing agent.

Suitable oxidizing agents are, for example, molecular oxygen, peroxides such as hydrogen peroxide, urea-hydrogen peroxide adduct, tert-butyl hydroperoxide or cumene hydroperoxide; Peroxymonosulphates such as sodium, potassium or ammonium peroxymonosulphates; Oxon; Peroxide isulfates such as sodium, potassium or ammonium peroxydisulfates; Peroxyborates such as sodium or potassium peroxyborate; Peroxycarbonates such as sodium or potassium peroxycarbonate; Periodic acid such as sodium or potassium periodate; Vanadium pentoxide or osmium tetroxide.

Preferred oxidants are molecular oxygen and hydrogen peroxide. A particularly preferred oxidizing agent is molecular oxygen.

Molecular oxygen is preferably used in the form of air.

The use of oxygen-depleted air is also preferred, the oxygen content being depleted to 15% by volume, preferably up to 7% by volume, particularly preferably up to 5% by volume.

Oxygen enriched air can also be used. Usually, the mercaptophenol of the formula (II) and the oxidizing agent in a molar ratio of 1: 1 to 1: 1000, preferably 1: 1 to 1: 100, more preferably 1: 1 to 1: 10 reacted together.

By varying the molar ratio of mercaptophenol used of the formula (II) to the oxidizing agent used, the degree of oligomerization of the compound of the formula (I) can be influenced.

If molecular oxygen is used as the oxidant, it is generally used in excess.

If appropriate, the process according to the invention is carried out in the presence of one or more bases.

In one embodiment, the process according to the invention is carried out in the absence of a base.

In a preferred embodiment, the process according to the invention is carried out in the presence of one or more bases. Preferably one to three, especially preferred used one or two bases. Very particular preference is given to the use of one (1) base.

Suitable bases are, for example, hydroxides such as sodium hydroxide, potassium hydroxide, magnesium hydroxide, calcium hydroxide or ammonium hydroxides; Carbonates, such as lithium carbonate, sodium carbonate, potassium carbonate, cesium carbonate, magnesium carbonate, calcium carbonate, barium carbonate or ammonium carbonates; Phosphates such as sodium phosphate, potassium phosphate, calcium phosphate or ammonium phosphates; Acetates such as lithium acetate, sodium acetate, potassium acetate, calcium acetate, zinc acetate, copper acetate or ammonium acetates, amines such as triethylamine, diethylamine, Hünig's base, imidazole, N-methylimidazole, pyridine, lutidine or collidine.

The term ammonium encompasses NH ₄ ⁺ and alkylammonium ions, in particular NH ₄ ⁺ , tetramethyl, tetraethyl, tetrapropyl and tetrabutylammonium ions. A particularly preferred base is ammonium hydroxide.

The mercaptophenol of the formula (II) and the base are customarily reacted with one another in a molar ratio of 1: 0.1 to 1: 100, preferably 1: 1 to 1:10, particularly preferably 1: 1 to 1: 5.

By varying the molar ratio of mercaptophenol used of the formula (II) to the base used, the degree of oligomerization of the compound of the formula (I) can be influenced.

Preferably, the process according to the invention is carried out in the presence of a solvent.

Suitable solvents are, for example, water; Alcohols, such as methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, 2-methylpropanol or tert-butanol; dipolar aprotic solvents such as Ν, Ν-dimethylformamide (DMF), Ν, Ν-diethylformamide, N, N-

Dimethylacetamide (DMAC), Ν, Ν-dimethylpropionamide, N-methyl-2-pyrolidinone (NMP),

Ν, Ν, Ν ', Ν'-tetramethylurea, 1, 3-dimethyl-2-imidazolidinone (DMI), 1, 3-dimethyl-3,4,5,6-tetrahydro-2 (1 H) -pyrimidinone (DMPU ), Dimethylsulfoxide, diethylsulfoxide, sulfolane, acetonitrile, propionitrile, butyronitrile or benzonitrile; Esters such as methyl acetate, ethyl acetate, n-butyl acetate or isobutyl acetate; Ethers, such as diethyl ether, diisopropyl ether, dibutyl ether, methyl tert-butyl ether

(MTBE), tetrahydrofuran (THF), dioxane or 1,2-dimethoxyethane (DME); aromatic hydrocarbons such as benzene, toluene, ethylbenzene, cumene, xylenes or mesitylene; aliphatic hydrocarbons such as hexane or cyclohexane; Carbonates such as diethyl carbonate or ethylene carbonate.

It is also possible to use mixtures of these solvents.

A particularly preferred solvent is ethanol. In general, the process according to the invention is carried out at a temperature in the range from -10 to 150.degree. The relatively low temperatures (see US Pat. No. 3,968,062) counteract the sublimation of unreacted mercaptophenol of the formula (II).

Generally, the reaction is carried out in the process according to the invention over a period of 0.1 h to 10 d.

The process according to the invention is usually carried out at atmospheric pressure. In the context of the invention, atmospheric pressure is understood to mean a pressure in the range from 1003 to 1023 mbar, preferably 1013 mbar. The inventive method can also be carried out at elevated pressure, in particular when gaseous oxidants such as molecular oxygen are used as the oxidant.

The work-up of the reaction mixtures is carried out, for example, by filtration, aqueous work-up and / or distillation.

In general, the products are obtained as viscous oils or solids which, for example, under reduced pressure (preferably 10 ^-5 to 100 mbar, particularly preferably 10 ^-4 to 10 mbar) and / or at elevated temperature (preferably 120 to 250 ° C, particularly preferably 150 to 230 ° C) can be freed of residual solvent and / or unreacted mercaptophenol.

If the products are solids, the purification can also take place, for example, by recrystallization or trituration. Adhesive base can be used to remove products obtained as solids, for example by washing with C 1 -C 3 alcohols and / or water.

The products can also be purified, for example, by crystallization or precipitation.

The purification of the diphenol disulfides prepared according to the invention (compounds of the formula (I) where n = 0) can also be carried out by vacuum distillation.

The one or more mercaptophenols of the formula (I I) used in the process according to the invention are commercially available or can be synthesized on the basis of methods known from the literature.

In a particular embodiment, the mercaptophenols of the formula (II) are prepared by reacting polyphenol sulfides of the formula (I II)

with a reducing agent, e.g. Zinc in combination with hydrochloric acid, wherein the symbols R ¹ , R ² , R ³ and R ⁴ in the formula (III) have the same meaning as in the mercaptophenol of the formula (II), and wherein the symbols k, I and m in the formula (III) have the following meanings: k is an integer from 0 to 1000, preferably an integer from 1 to 1000;

I is an integer from 0 to 7;

m is an integer from 0 to 7.

Polyphenol sulfides of the formula (III) are known, for example, from US Pat. Nos. 3,968,062, AT 508,304 and WO 201 1/000019 and the non-prepublished international patent application

PCT / EP201 1/073958 known.

In a particular embodiment, mercaptophenols of the formula (II) where R ⁴ * H are prepared by a process comprising the following steps: i) reaction of a phenol of the formula (IV),

wherein the symbols R ¹ , R ² , R ³ and R ⁴ in the formula (IV) have the same meaning as in the

Mercaptophenol of the formula (II), with chlorosulfonic acid (CISO3H) to a phenolsulfonic acid of formula (V),

wherein the symbols R ¹ , R ² and R ³ in the formula (V) have the same meaning as in the mercaptophenol of the formula (II) wherein R ^{5 is} SO ³ H when R ⁴ in the formula (IV) is H, and wherein R ^{5 has} the same meaning as in the formula (IV) when R ⁴ in the formula (IV) is not H; and ii) reacting the phenolsulfonic acid of the formula (V) with a reducing agent to give the mercaptophenol of the formula (II) with R ⁴ * H.

Preference is given to using phenols of the formula (IV) where R ² * H.

In a preferred embodiment, phenols of formula (IV) are used with R ⁴ = H, whereby mercaptophenols of formula (II) are obtained with R ⁴ = SH.

In a particularly preferred embodiment, phenols of the formula (IV) with R ² * H and R ⁴ = H are used.

Phenols of the formula (IV) are either commercially available or can be synthesized on the basis of methods known from the literature.

Suitable reducing agents for the reaction of phenolsulfonic acids of the formula (V) to form mercapto-phenols of the formula (II) with R4 * H are, for example, molecular hydrogen in combination with a catalyst, e.g. Raney nickel, or hydrazine in combination with a catalyst, e.g. Raney nickel. Also suitable are, for example, iron in combination with hydrochloric acid, or zinc in combination with hydrochloric acid.

Another object of the invention is a Polyphenoldisulfid of formula (I), obtainable by the inventive method.

According to the invention, in such a polyphenol disulfide with n * 0 at most 5%, preferably -i 3%, more preferably <1%, most preferably <0.5% of the (n + 1) disulfide bridges contained in the formula (I) by a Mono or Oligosulfidbrücke replaced. The invention further provides a flame retardant system comprising i) at least one polyphenol disulfide of the formula (I) according to the invention; and ii) at least one halogen-free organic phosphorus compound having a phosphorus content in the range of 0.5 to 40% by weight, based on the phosphorus compound.

Another object of the invention is a polymer composition containing one or more polymers and the flame retardant system according to the invention.

The polyphenol disulfides prepared by the process according to the invention are suitable, for example, as flame retardant synergists in polymers. Preferred is the use in foams, in particular in polystyrene-based foams. The flame retardant synergist prepared according to the invention is used in admixture with one or more flameproofing agents and optionally with further synergists for the production of flame-retardant (or flame-retardant) polymers, in particular thermoplastic polymers. For this purpose, the flame retardants and synergists are preferably physically mixed with the corresponding polymer in the melt and then either finished ready as a polymer mixture and then further processed in a second process step together with the same or a different polymer. Alternatively, in the case of styrene polymers, the addition of the flame retardants and synergists before, during and / or after the preparation by suspension polymerization is preferred. Foamed or unfoamed styrene polymers, including ABS, ASA, SAN, AMSAN, SB and HIPS polymers, polyimides, polysulfones, polyolefins such as polyethylene and polypropylene, polyacrylates, polyetheretherketones, polyurethanes, polycarbonates, polyphenylene oxides, unsaturated polyester resins, Phenol resins, polyamides, polyethersulfones, polyether ketones and polyether sulfides, each individually or in mixture as polymer blends be used.

Preference is given to thermoplastic polymers, such as foamed or unfoamed styrene homopolymers and copolymers, in each case individually or in a mixture as polymer blends. According to the invention, the term styrene polymer encompasses polymers based on styrene, alpha-

Methylstyrene or mixtures of styrene and alpha-methylstyrene; The same applies to the styrene content in SAN, AMSAN, ABS, ASA, MBS and MABS (see below). Inventive styrene polymers are based on at least 50 parts by weight of styrene and / or alpha-methylstyrene monomers.

Preference is given as styrene polymers to glassy polystyrene (GPPS), toughened polystyrene (HIPS), anionically polymerized polystyrene or toughened polystyrene (A-IPS), styrene-alpha-methylstyrene copolymers, acrylonitrile-butadiene-styrene polymers (ABS), styrene-acrylonitrile Copolymers (SAN), acrylonitrile-alpha-methylstyrene copolymers (AMSAN), acrylonitrile-styrene-acrylic esters (ASA), methyl acrylate-butadiene-styrene (MBS), methyl methacrylate-acrylonitrile-butadiene-styrene (MABS) polymers or mixtures thereof or used with polyphenylene ether (PPE).

The abovementioned styrene polymers can be used to improve the mechanical properties or the thermal stability, if appropriate by using compatibilizers with thermoplastic polymers, such as polyamides (PA), polyolefins, such as polypropylene (PP) or polyethylene (PE), polyacrylates, such as polymethyl methacrylate (PMMA), polycarbonate ( PC), polyesters such as polyethylene terephthalate (PET) or polybutylene terephthalate (PBT), polyethersulfones (PES), polyether ketones or polyether sulfides (PES) or mixtures thereof in proportions of not more than 30 parts by weight, preferably in the range of 1 to 10 Parts by weight, based on 100 parts by weight of the polymer melt. Furthermore, mixtures in the abovementioned quantitative ranges are also possible with, for example, hydrophobically modified or functionalized polymers or oligomers, rubbers such as polyacrylates or polydienes, for example styrene-butadiene block copolymers or biodegradable aliphatic or aliphatic / aromatic copolyesters.

Preference is given to flame-retardant polymer foams, in particular based on styrene polymers, preferably EPS and XPS.

The expandable styrene polymers (EPS) can be prepared either by the extrusion process or by suspension polymerization in aqueous suspension in the presence of a flame retardant, the synergist of the invention and an organic blowing agent.

The flameproofed polymer foams preferably have a density in the range from 5 to 200 kg / m ³ , particularly preferably in the range from 10 to 50 kg / m ³ , and are preferably more than 80%, particularly preferably 90 to 100% closed-cell.

The invention is explained in more detail by the following examples. Examples A. Synthesis Examples

Example 1: Synthesis of 4- (t-butyl) -2-mercaptophenol and 4- (t-butyl) -2,6-dimercaptophenol

The synthesis of 4- (t-butyl) -2-mercaptophenol and 4- (t-butyl) -2,6-dimercaptophenol was carried out starting from poly (4- (t-butyl) -2,6-dimercaptophenol) by reduction with zinc and hydrochloric acid. Due to the manufacturing process, the reactants had different lengths

Sulfur bridges before, as indicated in Scheme 1. Monosulfur-bridged constituents of the educt are inert to the reducing agent and thus remained in the reduction. The amount of these constituents in the educt was determined to be 10% by means of ¹ H NMR spectroscopy. The purification and separation of the products was carried out by fractional distillation.

Scheme 1: Synthesis of 4- (t-butyl) -2-mercaptophenol and 4- (t-butyl) -2,6-dimercaptophenol by reduction of poly (4- (t -butyl) -2,6-dimercaptophenol) with Zn / HCI; m, I = 0-5, n = 0-20.

Execution:

In a 2 L three-necked flask with reflux condenser, magnetic stirrer and thermometer, poly (4- (t-butyl) -2,6-dimercaptophenol) (50 g) in a mixture of toluene (150 mL) and ethanol (150 mL) was added at room temperature solved. There was a clear, yellow solution. To this was added, with vigorous stirring, half concentrated hydrochloric acid (600 ml), part of the educt precipitating as a viscous liquid and forming a turbid, yellow emulsion. The reaction mixture was added in portions over a period of 20 h zinc powder (50 g, 0.765 mol) with vigorous stirring and strong gas evolution. The temperature increased to 50 ° C. Before the addition of the next portion of zinc powder was waited until the evolution of gas had subsided. The end of the reaction was detected by complete decolorization of the reaction mixture. The stirrer was then stopped and waited for the phases to separate. The organic phase was separated and the aqueous phase extracted three more times with 50 ml of toluene. Then the combined organic extracts were dried over anhydrous MgSO ₄ and filtered. The filtrate was freed from the solvent under reduced pressure to give, as the crude product, a pale yellow, very viscous liquid (48.2 g, 96%). From this, the desired products were obtained as clear, colorless liquids by fractional distillation in vacuo over a Vigreux column.

1st fraction: 4- (t-butyl) -2-mercaptophenol, yield 6.3 g. . B.p. 61 ° C, 2 ^* 10 ^"3 mbar; ¹ H-NMR: δ (CDCIs) = 1:36 (s, 9H), 3.16 (s, 1 H), 6:36 (s, 1 H), 6.97 (d, J = 8.5 Hz, 1 H), 7.29 (dd, J = 8.5 Hz, J = 1.8 Hz, 1 H), 7.52 (d, J = 1.7 Hz, 1 H) ppm, ³ C-NMR: δ (CDCIs) = 31.36 (s, 3C), 34.02 (s, 1 C), 1 1 1 .33 (s, 1 C), 1 14.55 (s, 1 C), 127.30 (s, 1 C) , 131.95 (s, 1 C), 143.79 (s, 1 C), 153.51 (s, 1 C) ppm, HR EI: calculated for [Ci ₀ Hi ₄ OS] ⁺ 182.0765, found 182.0760 ,

2nd fraction: 4- (t-butyl) -2,6-dimercaptophenol, yield 25.7 g. . B.p. 83 ° C, 2 ^* 10 ^"3 mbar; ¹ H NMR: δ (CDCIs) = 1 .27 (s, 9H), 3:41 (s, 2H), 6:54 (s, 1 H), 7.27 (s , 2H) ppm; ³ C-NMR: δ (CDCIs) = 31.22 (s, 3C), 34.07 (s, 1C), 1 14.02 (s, 2C), 129.18 (s, 2C), 143.79 (s , 1 C), 153.51 (s, 1 C) ppm, HR EI: calculated for [Ci ₀ Hi ₄ OS ₂ ] ⁺ 214.0486, found 214.0517. Example 2: Synthesis of 2-mercapto-4- (t-pentyl) phenol and 2,6-dimercapto-4- (t-pentyl) phenol

The synthesis of 2-mercapto-4- (t-pentyl) phenol and 2,6-dimercapto-4- (t-pentyl) phenol (Scheme 2) was carried out in the same manner as in Example 1 for the t-butylphenol - Mercaptans described. The starting material used was poly (2,6-dimercapto-4- (t-pentyl) phenol) with a proportion of monosulfur bridges of 10% (determined by ¹ H-NMR spectroscopy).

Scheme 2: Synthesis of 2-mercapto-4- (t-pentyl) phenol and 2,6-dimercapto-4- (t -pentyl) phenol by reduction of poly (2,6-dimercapto-4- (t-pentyl ) phenol) with Zn / HCl; m, I = 0-5, n = 0-20.

From the crude product (48.9 g, 97.8%), the desired products were obtained by double fractional distillation in vacuo over a Vigreux column. Despite the second distillation step, traces of impurities remained.

1st fraction: 4- (t-butyl) -2-mercaptophenol, yield 5.8 g. . B.p. 72 ° C, 2 ^* 10 ^"3 mbar; ¹ H-NMR: δ (CDCIs) = 0.69 (t, J = 7.4 Hz, 3H), 1 .25 (s, 6H), 1 .61 ( q, J = 7.4Hz, 2H), 3.02 (s, 1H), 6.08 (s, 1H), 6.91 (d, J = 8.5Hz, 1H), 7.20 (dd, J = 8, 5 Hz, J = 1.5 Hz, 1 H), 7.40 (d, J = 1.5 Hz, 1 H) ppm, 3 C-NMR: δ (CDCIs) = 9.01 (s, 1 C), 28.42 (s , 2C), 36.72 (s, 1 C), 37.22 (s, 1 C), 1 10.95 (s, 1 C), 1 14.41 (s, 1 C), 128.07 (s, 1 C), 132.82 (s, 1 C), 142.04 (s, 1 C), 153.59 (s, 1 C) ppm, MS (ESI ⁺ ): m / z = 408 [2M + H ₂ 0] ⁺ .

2nd fraction: 4- (t-butyl) -2,6-dimercaptophenol, Yield 21, 3 g. . B.p. 101 ° C, 2 ^* 10 ^"1 mbar; ¹ H NMR: δ (CDCIs) = 0.68 (t, J = 7.4 Hz, 3H), 1.22 (s, 6H), 1:58 (q, J = 7.4Hz, 2H), 3.41 (s, 2H), 6.54 (s, 1H), 7.21 (s, 2H) ppm, ³ C-NMR: δ (CDCIs) = 9.03 (s, 1 C), 28.34 (s, 2C), 36.62 (s, 1C), 37.31 (s, 1C), 1.13.99 (s, 2C), 129.81 (s, 2C), 142.15 (s, 1C), 150.19 (s, 1 C) ppm HR-EI:

calculated for [CnHi ₆ OS ₂ ] ⁺ 228.0643, found 228.0664.

Example 3: Synthesis of poly (4- (t-butyl) -2,6-dimercaptophenol)

Oxidation of the purified t-butylphenol mercaptans with aqueous ammonium hydroxide solution and atmospheric oxygen allowed the preparation of pure poly (4- (t-butyl) -2,6-dimercaptophenol) (Scheme 3).

Scheme 3: Synthesis of poly (4- (t-butyl) -2,6-dimercaptophenol) by oxidation of 4- (t-butyl) -2-mercaptophenol and 4- (t-butyl) -2,6-dimercaptophenol with NH4OH and atmospheric oxygen. Execution:

In an open 250 mL magnetic stirring flask were charged 4- (t-butyl) -2-mercaptophenol (2.18 g, 0.012 mol) and 4- (t-butyl) -2,6-dimercaptophenol (15.63 g, 0.073 mol) mol) in ethanol (70 mL). After just a few minutes, a yellow color of the resulting solution appeared. To the reaction mixture was then added 25% aqueous ammonium hydroxide solution (15 g, 0.107 mol) and then the solution was stirred at high speed in air at room temperature for four days. During this time, a yellow precipitate fell out. After completion of the reaction, the mixture was cooled to 0 ° C. with an ice bath and stirred for a further 1 h at this temperature. It was then filtered and the residue washed with cold ethanol (50 mL) and cold water (50 mL) and dried at 150 ° C in vacuo. There was obtained 9.5 g (53%) of poly (4- (t-butyl) -2,6-dimercaptophenol) as a finely crystalline yellow solid. The mean chain length was determined by ¹ H NMR spectroscopy by comparing the integrals of the signals of the edge groups and the center pieces to n = 18. 3 (s, 9H), 7.40 (s, 2H), 8.65 (bs,

0 (s, 9H), 6.71 (d, J = 8.4 Hz, J = 2.4 Hz, 1 H), 7.47 (d, J =

Td (1%) = 276 ° C (see Table 1); EA: calculated for C200H242O20S38 (n = 18) [%] C 57.41; H 5.83; S 29,12; found C 57.38; H 5.50; S 28,41.

Example 4: Synthesis of poly (2,6-dimercapto-4- (t-pentyl) phenol)

The synthesis of poly (2,6-dimercapto-4- (t-pentyl) phenol) was carried out by oxidation of the purified t-pentylphenol mercaptans with aqueous ammonium hydroxide solution and

Atmospheric oxygen (Scheme 4), analogous to the synthesis of poly (4- (t-butyl) -2,6-dimercaptophenol) (see Example 3).

Scheme 4: Synthesis of poly (2,6-dimercapto-4- (t-pentyl) phenol) by oxidation of 2-mercapto-4- (t-pentyl) phenol and 2,6-dimercapto-4- (t- pentyl) phenol with NH 4 OH and atmospheric oxygen.

Execution:

In an open 250 ml magnetic driven flask were added 2-mercapto-4- (t-pentyl) phenol (7.7 g, 0.039 mol) and 2,6-dimercapto-4- (t-pentyl) phenol (31, 04 g , 0.136 mol) in ethanol (150 ml_). After just a few minutes, a yellow color of the resulting solution appeared. To the reaction mixture was then added 25% aqueous ammonium hydroxide solution (30 g, 0.214 mol), and then the solution was stirred at high speed in air at room temperature for four days. During this time, a yellow precipitate fell out. After completion of the reaction, the mixture was cooled to 0 ° C. with an ice bath and stirred for a further 1 h at this temperature. It was then filtered and the residue washed with cold ethanol (50 mL) and cold water (50 mL) and dried at 150 ° C in vacuo. There were obtained 23.3 g (60%) of poly (2,6-dimercapto-4- (t-pentyl) phenol) in the form of a finely crystalline yellow solid. The mean chain length was determined by ¹ H NMR spectroscopy by comparing the integrals of the signals of the edge groups and the center pieces (ratio 1: 1 1) to n = 22. H-NMR: δ (THF-d ₈ ) = 0.56 (t, J = 7.0 Hz, 3H), 1, 10 (s, 6H), 1 .47 (q, J = 7.3 Hz, 2H) , 7.35 (s, 2H), 8.66 (s, 1H) ppm; ³ C-NMR: δ (THF-d ₈ ) = 6.62, 25.83, 34.55, 35.42, 120.71, 127.14, 139.71, 150.25 ppm.

0.61 (m, 3H), 1.17 (s, 6H), 6.71

7 (dd, J = 8.1 Hz, J = 1.5 Hz,

Td (1%) = 292 ° C (see Table 1); EA: calculated for C264H338O24S46 (n = 22) [%] C, 59.04; H 6,34; S 27.46 found C 59.02; H 5.89; S 27,12. Table 1: Composition and temperature stability of inventively prepared

Poly-tert-alkylphenol disulfides (examples) and of comparative examples

a determines via 1 H-NMR spectroscopic analysis using DMSO-d6 as a solvent;

b determines fractional precipitation and subsequent analysis of the different fractions; Proportion stated in wt .-%, based on the product sample; ^c determined by thermogravimetric analysis (under N2, 20-800 ° C, heating rate 10 K / min); ^d determines total chlorine determination via tube combustion, subsequent difference formation with chloride value; Content given in wt .-%, based on the product sample.

A comparison (Table 1) of poly-tert-alkylphenol disulfide (B1 and B2) prepared according to the invention with poly-tert-alkylphenol disulfides of the prior art (V1 and V2) shows that the poly-tert-alkylphenol disulfide prepared according to the invention has a higher proportion dimeric and polymeric phenol disulfides and a lower proportion of monosulfide units and a has a lower content of elemental sulfur and covalently bound chlorine. In addition, poly-tert-alkylphenol disulfide prepared according to the invention is characterized by a higher temperature stability. As flame retardants and synergists

In the examples, the flame retardants FR1 to FR2 and the synergists S1 to S4 were used.

The flame retardants and synergists used in the examples were obtained commercially, according to literature rules or as described in point A Synthi Siert.

FR1: Available commercially as Disflamoll ^® TP from Lanxess.

FR2: Synthesized as described in WO201 1/083009.

S1: Vultac® TB7 from Arkema.

S2: Vultac® 3 from Arkema.

S3, S4: Synthesized according to Example 3 or 4.

C. Application examples

Unless otherwise stated, the fire behavior of the foam panels was determined at a foam density of 15 kg / m ^{3 in} accordance with DIN 4102.

In the comparative experiments, the commercial products Vultac® TB7 and Vultac® 3 (Arkema) were used.

Expandable styrene polymers (extrusion process):

7 parts by weight of n-pentane were dissolved in polystyrene 158K (Mw = 261,000 g / mol, Mn = 77,000 g / mol, determined by GPC, Rl detector, PS as standard) of BASF SE with a viscosity number of 98 ml / g mixed. After cooling the propellant-containing melt of originally 260 ° C to a temperature of 190 ° C, a polystyrene melt containing the Tabel le said flame retardant and optionally athermane compounds such as graphite, carbon black, etc., mixed via a side stream extruder into the main stream. The stated amounts in parts by weight relate to the total amount of polystyrene (100 parts by weight). The mixture of polystyrene melt, blowing agent and flame retardant was conveyed at 60 kg / h through a nozzle plate with 32 holes (diameter of the nozzle: 0.75 mm). With the help of pressurized underwater granulation, compact granules with a narrow size distribution were produced. The molecular weight of the granules was 220,000 g / mol (Mw) and 80,000 g / mol (Mn) (determined by GPC, Rl detector, PS as standard).

The granules were prefoamed by the action of flowing steam and, after 12 hours' storage, were further welded by a further treatment with steam in a closed mold to form foam blocks with a density of 15 kg / m ³ .

The foam blocks were subsequently by means of hot wire cut with a Styrofoam incandescent cutting machine from Charlotte Kaiser, 67071 Ludwigshafen, (power: 180 VA, primary: 230 V, secondary: 0 - 12 V, 15 A) in the corresponding standard test specimens, z. B. tailored to determine the fire behavior according to DIN 4102.

The results of the blank are summarized in Table 2.

Table 2: Odor development of inventive polymer composition in

Hot wire cut (examples) and comparative examples

The determination of the fire behavior of the foam panels was carried out after 72 hours of storage at a foam density of 15 kg / m ³ according to DIN 4102.

The firing results are summarized in Table 3. Table 3: Fire behavior of inventive polymer composition (examples) and of comparative examples

Extruded polystyrene foam sheets:

100 parts by weight of polystyrene 158K (Mw = 261,000 g / mol, Mn = 77,000 g / mol determined by GPC, Rl detector, PS as standard) of BASF SE with a viscosity number of 98 ml / g, 0.1 Parts by weight of talc as nucleating agent for controlling the cell size and the parts by weight of flame retardants and synergists indicated in the table are fed continuously to an extruder having an internal screw diameter of 120 mm. Through a mounted in the extruder inlet opening a blowing agent mixture of 3.25 parts by weight of ethanol and 3.5 parts by weight of CO2 is pressed in continuously continuously. The uniformly kneaded in the extruder at 200 ° C gel is passed through a calming zone and extruded after a residence time of 15 minutes with an outlet temperature of 105 ° C through a 300 mm wide and 1, 5 mm wide nozzle into the atmosphere. The foam is passed through a mold channel connected to the extruder to form a foamed sheet having a cross section of 650 mm x 50 mm and a density of 40 g / l.

The molecular weight of the polystyrene was 240,000 g / mol (Mw) and 70,000 g / mol (Mn) (determined by GPC, Rl detector, PS as standard). The extrusion results are summarized in Table 4.

Table 4: Odor development of inventive polymer composition in

Hot wire cut (examples) and comparative examples

Example flame retardant odor on extrusion (200 odor of extruded

(Parts by weight based on ° C) foam boards

Polystyrene) (storage time: 48 h at RT)

VB5 FR1 (1, 0) + S1 (2,0) significant / sulphurous significant / sulphurous

VB6 FR1 (1, 0) + S2 (2,0) significant / sulphurous significant / sulphurous

VB7 FR2 (0.75) + S1 (2.0) significant / sulphurous significant / sulphurous

VB8 FR2 (0.75) + S2 (2.0) significant / sulphurous significant / sulphurous

1 FR1 (1, 0) + S3 (2,0) low / sulphurous odorless

2 FR1 (1, 0) + S4 (2,0) low / sulphurous odorless

3 FR2 (0.75) + S3 (2.0) low / sulphurous odorless

4 FR2 (0.75) + S4 (2.0) low / sulphurous odorless The product was then mechanically cut into 1 cm thick plates. The fire behavior of the samples was tested after a storage time of 30 days according to DIN 4102.

The firing results of the examples are summarized in Table 5.

Table 5: Fire behavior of inventive polymer composition (examples) and of comparative examples

Example flame retardant system Brandtest (B2 according to DIN 4102)

(Parts by weight referred to / extinguishing time (s)

polystyrene)

VB5 FR1 (1.0) + S1 (2.0) passed / 14 s

VB6 FR1 (1.0) + S2 (2.0) passed / 10 s

VB7 FR2 (0.75) + S3 (2.0) passed / 9 s

VB8 FR2 (0.75) + S4 (2.0) passed / 11 s

1 FR1 (1.0) + S3 (2.0) passed / 12 s

2 FR1 (1.0) + S4 (2.0) passed / 10 s

3 FR2 (0.75) + S3 (2.0) passed / 11 s

4 FR2 (0.75) + S4 (2.0) passed / 11 s

Claims

claims

1 . Process for the preparation of a polyphenol disulfide of the formula (I)

with one or more oxidizing agents, if appropriate in the presence of one or more bases, where the symbols R ¹ , R ² , R ³ , R ⁴ and n have the following meanings:

R ¹ , R ² , R ³ are identical or different C 1 -C 6 -alkyl, C 2 -C 18 -alkenyl, C 2 -C 6 -alkynyl, C 6 -C 12 -aryl, C 3 -C 10 -cycloalkyl, C 6 -C 12 -aryl-C 1 -C 4 -alkyl Ci8-alkyl, a heteroaryl group containing one or more heteroatoms from the group N, O and S, 0- (Ci-Ci8) alkyl, O- (C ₂ -Ci ₈ ) alkenyl, 0- (C ₂ - Ci ₈₎ -alkynyl, 0- _{_(C6-Ci2)} aryl, 0- (C ₃ -Cio) cycloalkyl, (C ₆ -C ₂₎ - aryl (Ci-C ₈₎ -alkyl-0, S- (Ci-C ₈₎ -alkyl, S- (C ₂ -C ₈₎ -alkenyl, S- (C ₂ -C ₈₎ -alkynyl, S- (C ₆ - C ₂₎ aryl, S- ( C ₃ -Cio) cycloalkyl, _(C6 -Ci2) aryl (Ci-C ₈₎ -alkyl-S, OH, F, Cl, Br or H;

R ⁴ is identical or different SH, Ci-Cis alkyl, C ₂ -C ₈ alkenyl, C ₂ -C ₈ alkynyl, C ₆ -C ₂ - aryl, C3-Cio-cycloalkyl, C6-Ci2-aryl Ci-CI8-alkyl, a heteroaryl group which contains one or more heteroatoms from the group N, O and S, 0- (Ci-CI8) alkyl, 0- (C2-Ci ₈₎ alkenyl, 0- (C ₂ -C ₈₎ -alkynyl, 0- _{_(C6-Ci2)} aryl, 0- (C ₃ -Cio) cycloalkyl, (C ₆ -C ₂₎ - aryl (Ci-C ₈₎ -alkyl-0 , S- (Ci-C ₈₎ -alkyl, S- (C ₂ -C ₈₎ -alkenyl, S- (C ₂ -C ₈₎ -alkynyl, S- (C ₆ - C ₂₎ aryl, S- (C ₃ -Cio) cycloalkyl, _(C6 -Ci2) aryl (Ci-C ₈₎ -alkyl-S, OH, F, Cl, Br or H; n is an integer from 1 to 1000; with the proviso that R ⁴ in at least one of the one or more mercaptophenols of the formula (II) SH is.

The method of claim 1, wherein n is an integer of 3 to 1000.

A method according to claim 1 or 2, wherein the symbols R ¹ , R ² , R ³ , R ⁴ and n have the following meanings:

R, R ³ is H;

R ² is identical or different C 1 -C 10 -alkyl or C 6 -C 12 -aryl;

R ⁴ is the same or different SH or H;

n is an integer from 1 to 150.

The method according to any one of claims 1 to 3, wherein n is an integer of 3 to 100.

A process according to any one of claims 1 to 4, wherein the polyphenol disulfide of formula (I) is poly (tert-butylphenol disulfide) or poly (tert-pentylphenol disulfide).

A process according to any one of claims 1 to 5, wherein it is carried out in the presence of a base.

Process according to claim 6, wherein ammonium hydroxide is used as base.

Method according to one of claims 1 to 7, wherein molecular oxygen is used as oxidizing onsmittel.

Process for the preparation of a polyphenol disulfide of the formula (I)

by reacting one or more mercaptophenols of the formula (II)

R ¹ , R ² , R ³ are identical or different C 1 -C 6 -alkyl, C 2 -C 18 -alkenyl, C 2 -C 6 -alkynyl, C 1 -C 12 -aryl, C 3 -C 10 -cycloalkyl, C 6 -C 12 -aryl-C 1 -C 5 -alkyl Ci8-alkyl, a heteroaryl group containing one or more heteroatoms from the group N, O and S, 0- (Ci-Ci8) alkyl, O- (C ₂ -Ci ₈ ) alkenyl, 0- (C ₂ - Ci ₈₎ -alkynyl, 0- _{_(C6-Ci2)} aryl, 0- (C ₃ -Cio) cycloalkyl, (C ₆ -C ₂₎ - aryl (Ci-C ₈₎ -alkyl-0, S- (Ci-C ₈₎ -alkyl, S- (C ₂ -C ₈₎ -alkenyl, S- (C ₂ -C ₈₎ -alkynyl, S- (C ₆ - C ₂₎ aryl, S- ( C ₃ -Cio) cycloalkyl, _(C6 -Ci2) aryl (Ci-C ₈₎ -alkyl-S, OH, F, Cl, Br or H;

R ⁴ is identical or different SH, Ci-C ₈ alkyl, C ₂ -C ₈ alkenyl, C ₂ -C ₈ alkynyl, C ₆ -C ₂ - aryl, C3-Cio-cycloalkyl, C6-Ci2-aryl -Ci-Ci8-alkyl, a heteroaryl group containing one or more heteroatoms from the group N, O and S, 0- (Ci-Ci8) alkyl, 0- (C2-Ci ₈ ) alkenyl, 0- (C ₂ -C ₈₎ -alkynyl, 0- _{_(C6-Ci2)} aryl, 0- (C ₃ -Cio) cycloalkyl, (C ₆ -C ₂₎ - aryl (Ci-C ₈₎ alkyl 0, S- (Ci-C ₈₎ -alkyl, S- (C ₂ -C ₈₎ -alkenyl, S- (C ₂ -C ₈₎ -alkynyl, S- (C ₆ -

Ci ₂ ) -aryl, S- (C ₃ -Cio) -cycloalkyl, (C ₆ -C 12) -aryl (Ci-Ci ₈ ) -alkyl-S, OH, F, Cl, Br or H; n is an integer from 1 to 1000; with the proviso that R ⁴ in at least one of the one or more mercaptophenols of the formula (II) SH is when n is greater than 0, the polyphenol disulfide of the formula (I) being di- (tert-pentylphenol) disulphide, Di (para-phenylphenol) disulphide or di (ortho-phenylphenol) disulphide is:

10. The method according to claim 9, wherein it is carried out in the presence of a base.

1 1. Process according to claim 10, wherein ammonium hydroxide is used as base. 12. The method according to any one of claims 9 to 1 1, wherein molecular oxygen is used as the oxidizing agent.

13. Polyphenol disulfide of the formula (I) obtainable by the process according to one of claims 1 to 8.

14. Flame retardant system containing

i) at least one polyphenol disulfide of the formula (I) obtainable by the process according to one of claims 1 to 12; and

ii) at least one halogen-free organic phosphorus compound having a phosphorus content in the range of 0.5 to 40 wt .-%, based on the phosphorus compound.

A polymer composition containing one or more polymers and the flame retardant system of claim 14.