The invention relates to a method of preparing sulfur- containing compounds, comprising reacting a sulfur compound with a compound containing unsaturated carbon-carbon bonds, to a method of preparing polymeric polysulfides, to a method of preparing heterocyclic sulfur compounds, and to a method of vulcanizing rubbers .

The addition of sulfur to unsaturated carbon-carbon bonds is a known and widely used reaction in the state of the art. R.T. Lalonde et al. in Org. Geochem. H (1987), 563-571 have described that highly activated compounds with unsaturated carbon-carbon bonds undergo a Michael addition in the presence of polysulfide as sulfur donor. These highly activated compounds are generally activated alkenes or alkynes, with an electron acceptor group being attached to one of the unsaturated carbon atoms. More particularly, the publication referred to relates to the Michael addition of conjugated ene carbonyls with polysulfide in alcoholic or aqueous alcoholic solutions. The Michael additions described occur at room temperature. Further, Vairavamurthy and Mopper in Nature 329 (1987) , 623-625 have described that the activated compounds described above also undergo a Michael addition when hydrogen sulfide is used. This addition took place under abiotic conditions in seawater. It has now been found that sulfur can add to non- activated compounds containing an unsaturated carbon-carbon bond, such as alkenes and alkynes, also under very mild conditions. Because of the lack of an activated α, β-conjugated system, however, this addition is not a Michael addition as with the known method.

The method of preparing sulfur-containing compounds is characterized, according to the invention, in that in a solvent one or more organic compounds with non-activated

unsaturated carbon-carbon bonds react with monohydrogen polysulfide .

Monohydrogen polysulfide can for instance be obtained by oxidation of monohydrogen monosulfide. In a preferred embodiment, however, monohydrogen polysulfide is formed in situ from monohydrogen sulfide in the presence of sulfur.

By the term "non-activated carbon-carbon bond" is meant a compound in which no electron acceptor group is attached to the carbon atoms which are attached to each other through the unsaturated bond referred to. More particularly, no highly electrophilic, electron sequestor groups, such as C=0, -COOH, -COOR and -CΝ, may be attached to these carbon atoms.

In the present description, "monohydrogen sulfide" is understood to mean a compound of the formula MHS, wherein M is, for instance, an univalent cation, for instance Νa ⁺ , K ⁺ , a quaternary ammonium ion, etc. Monohydrogen polysulfide can be represented by the formula MHS _X , wherein M can have the above meanings, while x varies between 2 and 10. Preferably, x varies between 2 and 8, most preferably from 2 to 6. It is not known, incidentally, what (mixture of) monohydrogen polysulfides exactly is formed in situ from monohydrogen sulfide and sulfur; it is certain, however, that the reagent does satisfy the formula given.

Without additional steps, -dihydrogen (poly) sulfides are not as such suitable for serving as reagent in the method according to the invention. When these compounds are used, the pH of the solutions is reduced too much. Further, no monohydrogen polysulfide is formed in situ then, but the equilibrium is on the side of H ₂ S and S. However, the use of dihydrogen (poly) sulfides is not excluded, for instance to form a monohydrogen (poly) sulfide in the reaction medium after neutralization by means of a base.

A preferred embodiment of the method according to the invention is characterized in that the reaction is performed at a pH varying between 6 and 10.

The method according to the invention can be carried out under mild conditions. Neither the pressure nor the reaction temperature need to be increased.

Generally, the sulfur addition proceeds very suitably in a solvent at neutral pH and at temperatures varying between -10°C and 80°C. Preferably, the addition takes place at temperatures between 0°C and 60°C, most preferably between 20°C and 50°C. The pressure in the reaction vessel will as a rule be equal to the outside pressure. In certain cases, however, increasing or reducing the pressure may give advantages. Catalysts are not requisite. The addition under these mild conditions is as a rule completed between 1 and 10 days, depending on the temperature and the concentration of sulfur donor . It is very surprising that this addition occurs under mild conditions. In fact, heretofore it had only been managed to add sulfur to alkenes or alkynes under considerably less mild conditions .

It is for instance known from an overview article of Hoffert and Wendtner in the Journal of the Institute of Petroleum 3_5 (1949), 171, that hydrogen sulfide adds to alkenes only under increased pressure and at temperatures above 150°C utilizing suitable catalysts, for instance strong acids. Further, reference can be made to Jones and Reid, J.A .Chem.Soc. ___Q. (1938) 2452.

In addition, it is known from W.A. Pryor, Mechanisms of Sulfur Reactions (1962) McGraw-Hill, Chapter 5 that a similar reaction of alkenes, alkadienes and allylic alcohols with elemental sulfur to organic sulfur compounds occurs only at temperatures above 100°C. The reaction products of these reactions typically consist of a nondescript mixture of sulfur containing compounds .

The nature of the solvent is not essential to the practice of the method according to the invention. The sulfur addition can occur in polar as well as apolar, in protic as well as aprotic solvents. Preferably, however, an organic

solvent or a two-phase solvent system with a phase transfer agent. Optionally, a large excess of the organic unsaturated starting material can be used as solvent.

A preferred embodiment of the method according to the invention is characterized in that an aprotic polar solvent is used. Suitable aprotic solvents are dimethyl sulfoxide (DMSO) , hexamethylphosphoric acid triamide (HMPA) , Ν,Ν-dimethyl- acetamide (DMA), Ν-methylpyrrolidone (ΝMP), tetrahydrofuran (THF) and dimethylformamide (DMF) . Most preferably, DMF is used as solvent .

The addition according to the invention can also be carried out in alcoholic solvents, such as isopropanol and butanol. The reaction also occurs in ethanol, albeit much slower than in the aprotic solvents just mentioned. Suitable two-phase systems that can be utilized as medium for the addition according to the invention are mixtures of water with an organic solvent selected from, for instance, ethyl actetate or other esters, such as butyl acetate, isoamyl acetate, diethyl phthalate, acetone or other ketones, such as methylethyl ketone, and toluene. If the addition is carried out in such a two-phase system, typically a phase transfer agent is added. Preferably, this phase transfer agent is a quaternary ammonium salt such as tetrabutylammonium bromide or methyltrioctylammonium chloride. Other phase transfer agents, for instance cholines and polyamines, can also be employed. The method according to the invention gives the highest yield when it is carried out with exclusion of oxygen. The fact is that oxygen reacts very rapidly with monohydrogen polysulfide . What product is obtained by the use of the method according to the invention primarily depends on the starting compound which contains at least one unsaturated carbon-carbon bond. In addition, the concentration of this starting compound and of the sulfur donor are of importance . It should be noted that even at a high concentration of sulfur radicals, the addition follows the Markovnikov rule.

When alkenes are reacted with sodium hydrogen sulfide and flowers of sulfur for 1 to 10 days at temperatures between 20 and 50°C, polysulfides are obtained in accordance with the following reaction equation:

In the final product, y varies between 2 and x. Variable x will as a rule vary between 2 and 10 as mentioned above. When, for instance, 1-decene is started from, then, according to the invention, mainly 2, 2 ^• -didecylpolysulfide is obtained.

The mechanism of this reaction is not known. No radical chain reaction occurs, nor can a proton-catalyzed addition occur in view of the neutral pH of the medium. As mentioned above, the addition follows the Markovnikov rule.

In the above-specified reaction equation, an α-alkene is started from. If the double bond is present at a different position in the molecule, polysulfides with longer side chains are obtained. In this case the reaction typically proceeds slower.

It is also possible to start from alkadienes. Utilizing the method according to the invention, these compounds give polymeric sulfides according to the following reaction equation:

(π»l)

Accordingly, the invention also relates to a method of preparing polymeric polysulfides. This method is characterized in that alkadienes react with monohydrogen polysulfide.

Although all alkadienes can be started from, preferably α,ω- alkadienes are used.

Polymeric polysulfides possess a major advantage in that they can be broken down relatively rapidly, even in nature. This (bio) degradability is connected with the presence of many oxidizable and/or reducible polysulfide bridges.

In addition the starting materials are amply available. Further, the properties of the polymeric products can be adjusted very well . It has for instance been found that the ratio of HS ^~ ions to elemental sulfur affects the ratio of free thiol groups and polysulfide bridges. This makes it possible to regulate the degree of polymerization. The free thiol groups that are present in the polymers obtained by the use of the method according to the invention can possibly effect links between separate polymeric molecules.

If in the method according to the invention alkadienes with a chain of at most 3 carbon atoms between carbon atoms which are attached to another carbon atom through an unsaturated bond, react with monohydrogen polysulfide, heterocyclic sulfur compounds can be formed. In other words, if the starting material is alkadienes of which a part of the carbon chain can be represented by the formula -C=C- (C) _n -C=C-, wherein n is at most 3, heterocyclic products can be obtained. Depending on the process conditions and the starting materials, it is thus possible to form, for instance, thiolanes, thianes and dithiolanes, which can be readily converted, as desired, to unsaturated and aromatic heterocyclic sulfur compounds, such as thiophenes .

Further, it is possible to start from unsaturated compounds which have been substituted with non-activating groups, such as alkyl groups, hydroxyl groups, etc.

Finally, the invention relates to a method of vulcanizing rubber, which method is characterized in that rubber reacts with monohydrogen polysulfide.

As a rule, in this vulcanization the monohydrogen polysulfide reagent will be contacted with the rubber to be vulcanized in a suitable solvent .

The major advantage of this method is that now a method has been found which enables rubber to be vulcanized under very mild conditions. This enables a better control of the vulcanization process.

The method according to the invention will now be further explained in and by the following examples.

Example 1 A thermostated flask with a magnetic agitator was filled with 0.56 g (10 mmol) ΝaHS, 0.80 g (25 mmol) flowers of sulfur, 0.22 g (1.0 mmol) 1-hexadecene and 25 ml DMF. Then nitrogen gas was blown through until all of the air in and above the solution was replaced with nitrogen. Then the flask was shut off from the air.

Using a magnetic agitator, the contents of this flask were stirred for 24 h at 50°C. Then the contents of the flask were poured into 50 ml of a solution of 2.5 g sodium sulfite in water. In this solution the -free sulfur dissolved. Then the reaction products were extracted with hexane. The hexane was subsequently distilled off under reduced pressure.

The product contained 80% 2, 2 ' -dihexadecyl polysulfides, approximately 10% 1-hexadecene and approximately 10% other isomers of hexadecene and hexadecane. The product analysis was carried out with -R ΝMR spectroscopy (Varian VXR 400S (400MHz) , CDCI3) , with gas chromatography (Carlo Erba 5300, capillary silica column 25 m x 0.32 mm, coated with CP-Sil-5, carrier gas helium, injection temperature 70°C, gradient to 130°C at 20°C/min, gradient to 320°C at 4°C/min) , and by means of gas chromatography/mass spectrometry (HP 5480 coupled to a VG-70S mass spectrometer, 70 eV, mass range 40-900, cycle time 1.8 s) .

From the ΝMR spectrum it can be derived that the number of sulfur atoms in the polysulfide bridge in the desired product is mainly 3 and 4, whilst a small proportion of the product contains a disulfide bridge.

Example 2

The procedure according to Example 1 was repeated, but instead of 1.0 mmol hexadecene 1.0 mmol of another unsaturated compound was utilized. Starting from 1.0 mmol 1-decene, 80% 2,2'-didecyl polysulfides were obtained.

A mixture of a total of 1.0 mmol neo- , cis- and trans- 1, 3-phytadiene was stirred at 50°C for 5 days. Upon analysis 60% of the product mixture appeared to consist of isomers of dithiolanes .

If a similar mixture of 1, 3-phytadiene is stirred for 5 days at 20°C instead of 50°C, polymers of phytane units are found to be bonded, with polysulfide bridges formed at the 2,3- and 3, -positions . The yield of this reaction is 90%.

Example 3

A thermostated flask with a magnetic agitator was filled with 6 g (0.1 mol) ΝaHs.aq, 0.16 g (5 mmol) flowers of sulfur, 2.24 g (10 mmol) 1-hexadecene, -6.22 g (20 mmol) tetrabutylammonium bromide, 50 ml ethyl acetate and 10 ml water. Air is again replaced with nitrogen. Then stirring was done at 50°C for 10 days.

After completion of the reaction, the organic layer was separated from the aqueous layer. This organic layer was washed with water and then with diluted hydrochloric acid.

Then the solvent, the ethyl acetate, was distilled off under reduced pressure.

The product contained 65% 2, 2 '-dihexadecyl polysulfides, and 15% 2-hexadecane thiol.

Example 4

The procedure according to Example 3 was repeated, but instead of 10 mmol hexadecene 10 mmol of a different unsaturated compound was used.

Starting from 10 mmol 1-decene, 60% 2,2'-didecyl polysulfides and 20% 2-decane thiol were found, whilst 10 mmol 1, 13-tetradecadiene yielded 95% of a 1, 12-dimethyl- dodecamethylene polysulfide polymeric mixture .

A 1, 8-dimethyloctamethylene polysulfide polymeric mixture was obtained in a yield of 90%, starting from 1, 9-decadiene . An element analysis was performed on the polymeric mixture obtained and the result found was CιrjH2θS2,2-

A reaction mixture containing 1, 7-octadiene gave a high yield (95%) of 1, 6-dimethylhexamethylene polysulfide polymer, whilst in addition 3% 2,7-dimethyl thiepan was found. Shorter dienes yielded mainly small compounds in the reaction mixture according to this example. From 1,6- heptadiene, after distillation, 35% 2,6-dimethyl thiane and 15% 3, 7-dimethyl-l, 2-dithiepan were obtained. 1, 5-hexadiene yielded 46% 2, 5-dimethyl thiolane, whilst from 1, -pentadiene 40% 3, 5-dimethyl-l, 2-dithiolane was formed.

A mixture of a total of 10 mmol neo- , cis- and trans-1,3- phytadiene was stirred for 5 days at 20°C instead of 50°C. The product comprised 90% polymers of phytane units, with polysulfide bridges formed on the 2,3- and 3, 4-positions. A reaction mixture in which acethylphytol constituted the unsaturated starting material was stirred at a temperature of 50°C for 4 weeks. This compound yielded 38% 3-methyl-3- (4,8, 12-trimethyltridecyl) -1, 2-dithiolane .